From Gene to Clinic: The Role of APOL1 in Focal Segmental Glomerulosclerosis

Abstract

Apolipoprotein L1 (APOL1) genetic variations, notably the G1 and G2 alleles, have important roles in the pathophysiology of focal segmental glomerulosclerosis (FSGS) and other kidney problems, especially in people of African descent. This review summarizes current understanding about the genetic, molecular, and clinical features of APOL1-associated FSGS and investigates new therapeutic options. It reveals how APOL1 mutations generate kidney injury through mechanisms such as podocyte dysfunction, mitochondrial impairment, and dysregulated inflammatory networks. Recent treatment developments, such as small-molecule inhibitors like inaxaplin, antisense oligonucleotides, and novel interventions targeting lipid metabolism and inflammatory pathways, are being assessed for their capacity to address the specific issues presented by APOL1-associated nephropathy. We also address gaps in knowledge, such as the function of environmental triggers and the systemic consequences of APOL1 mutations, emphasizing the significance of targeted research.

1. Introduction

Focal segmental glomerulosclerosis (FSGS) is a kidney disease characterized by glomerular scarring, which leads to proteinuria and, eventually, kidney failure [1]. The causes of FSGS are numerous, including genetic predispositions, environmental variables, and secondary causes such as viral infections. Genetic variants are increasingly being recognized as important factors, particularly in cases of steroid-resistant nephrotic syndrome (SRNS) and FSGS of undetermined cause (FSGS-UC). A study examined the prevalence and types of genetic variants associated with FSGS in a cohort of 76 patients, 52 with FSGS-UC and 24 with SRNS-FSGS. To identify deleterious mutations, targeted exome sequencing was performed on 84 genes related with glomerulopathies. The findings revealed that pathogenic or possibly pathogenic variants were found in 35.5% of patients, indicating a major role for genetic factors in FSGS. Genetic variants were more prevalent in SRNS-FSGS (41.7%) than FSGS-UC (32.7%). Mutations in the collagen 4A (COL4A3-5) genes were identified as the most common, accounting for 29.3% of all cases. These changes disrupt the glomerular basement membrane, which contributes to disease progression. Nephrotic syndrome type 2 (NPHS2) mutations, which affect slit diaphragm function, were responsible for 16.2% of cases. Furthermore, two African-origin patients carried apolipoprotein L1 (APOL1) high-risk (HR) alleles (G1/G2), indicating the significance of these variants in specific populations. In the Chronic Kidney Disease in Children (CKiD) Cohort, the genetic underpinnings of primary FSGS were investigated in African American children [2]. Using genome-wide association analysis, over 680,000 single-nucleotide polymorphisms (SNPs) in 140 participants were explored, including 32 with biopsy-confirmed FSGS. The study identified 14 genetic loci significantly associated with pediatric FSGS, highlighting both established risk genes such as APOL1 and novel genes like growth factor receptor-bound protein 2 (GRB2), fibroblast growth factor receptor 4 (FGFR4), and integrin subunit beta 1 (ITGB1). A notable discovery was the strong association between APOL1 HR genotypes and FSGS, implying a role in disease vulnerability. The study also found an increase in immune regulation-related genetic variations such HLA-DRB1*11:01, as well as pathways involved in antigen presentation and T-cell receptor signaling. These findings underscore the importance of immune responses in FSGS development. Variants in genes associated with podocyte shape and kidney injury, such as GRB2 and ITGB1, shed light on the underlying disease mechanisms. Pediatric FSGS has several genetic and immunological components that may offer treatment possibilities. However, limitations such as a small sample size and the lack of an independent validation cohort underscore the need for additional research to corroborate these findings and better understand the disease’s molecular processes. This review intends to expand on previous publications’ core insights by providing a new perspective on the role of APOL1 in FSGS. First, this work delves deeply into the molecular mechanisms underlying APOL1-induced kidney injury, with a focus on lesser-known pathways such stimulator of interferon genes (STING) and Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling. These pathways are being examined for their role in amplifying the cytotoxic effects of APOL1 mutations, which will provide a fuller knowledge of the molecular mechanisms involved. Furthermore, this review evaluates new therapeutic discoveries such as small-molecule inhibitors, antisense oligonucleotides, and clustered regularly interspaced short palindromic repeats (CRISPR)-based gene-editing technologies. Another unique feature of this review is its emphasis on the combination of genetic and environmental factors in the progression of APOL1-associated nephropathy. It emphasizes the significance of “second hits”,; such as infections and chronic inflammation, in disease progression. Finally, this review identifies key gaps in our understanding of APOL1-associated kidney disease, such as the need for large-scale, diversified clinical trials. This review proposes actionable research objectives and advocates for precision medicine approaches, offering a forward-thinking synthesis that links fundamental insights to clinical applications. In combining these elements, this review offers a distinct and comprehensive perspective, advancing the field by integrating molecular insights and therapeutic innovations.

2. Genetic Insights

2.1. Structure and Function of APOL1

APOL1 is a protein encoded by the APOL1 gene on chromosome 22q12.3 [3]. It is a component of human plasma’s high-density lipoprotein (HDL) particles and has an important role in innate immunity, particularly in providing resistance to specific species of Trypanosoma brucei, which cause African sleeping sickness. By analyzing liver transplant recipients with differing APOL1 genotypes between donors and recipients, it was demonstrated that the liver is the main contributor to circulating APOL1 levels [4]. However, also other tissues contribute to a lesser extent, as residual amounts of native APOL1 persisted post-transplant. APOL1 possesses various functional domains that play distinct biological roles [5]. An N-terminal signal peptide plays the role of secretion signal for the protein, allowing it to be extracellular for the purpose of interacting with pathogens and taking part in lipid transport. The membrane addressing domain (MAD) is located immediately after the signal peptide and is involved in engagement with the lipid bilayer of membranes, facilitating membrane interaction and is pH sensitive regulation of the protein’s ability. This domain regulates the functioning of the protein within different compartments of the cell including membrane perforation in the target cells. The pore-forming domain (PFD) is essential for the trypanolytic function of APOL1. The protein is inserted into the cell membranes of Trypanosoma brucei and forms pores, leading to an imbalance in osmotic pressure within the parasite, ultimately causing it to burst, and thereby acting as a natural defense against trypanosomiasis. APOL1 also contains a Bcl-2 Homology 3 (BH3) domain, which is related with pro-apoptotic actions. This domain shows that APOL1 may have a role in programmed cell death, showing that it is involved in cellular processes other than pathogen defense. The serum resistance-associated (SRA) binding domain of APOL1 interacts with the SRA protein from Trypanosoma brucei rhodesiense. This relationship alters susceptibility to Trypanosome infection because certain APOL1 variants can resist neutralization by the SRA protein, allowing them to continue their trypanolytic function. The leucine zipper (ZIP) domain at the C-terminal end of APOL1 promotes coiled-coil interactions that are required for channel oligomerization and function, allowing trypanolytic activity [6]. Using site-directed mutagenesis and lipid bilayer tests, it was shown that changes in leucine residues within the ZIP domain disrupt oligomerization, hinder channel formation, and eliminate trypanolytic function. Acidic conditions favor membrane insertion and oligomerization, while neutral pH promotes channel opening.

Recent research has revealed the complex role of circulating APOL1, which is largely released by the liver [4], in worsening kidney injury. Aside from its local cytotoxic effects on renal cells, circulating APOL1 promotes systemic inflammation and endothelial dysfunction, which contribute to kidney disease [7]. Notably, APOL1 is expressed in many tissues, including the liver, kidney (podocytes and proximal tubular cells), and vascular endothelium [8].

Building on the fundamental understanding of APOL1’s structure and multifunctional roles in innate immunity and cellular processes, it is critical to investigate how certain genetic variations of APOL1, namely the G1 and G2 alleles, affect its functional features. The next section digs into the distinct characteristics of the G1 and G2 alleles, stressing their pathogenic processes and the consequences for the progression of FSGS.

2.2. APOL1 Variants (G1 and G2)

Two coding variants of the APOL1 gene, known as G1 and G2, have been identified with significant relevance to human health [9]. The G1 variant contains two nonsynonymous SNPs that are in almost absolute linkage disequilibrium and that change amino acids S342G and I384M. G2 contains a six-base pair in-frame deletion resulting in the loss of amino acids N388 and Y389. These alterations occur in the C-terminal region of APOL1, which is essential for its interaction with SRA. These APOL1 risk alleles are more common in African Americans and other populations of western Sub-Saharan African descent, which correlates with a higher frequency of APOL1-associated nephropathies [10,11,12,13,14]. APOL1 HR genes impact about 13% of the African American population [15].

The G1 and G2 APOL1 variants have increased channel activity [6]. These mutations disrupt alternate protein conformations that could result in the increased formation of cytotoxic channels in renal cells. Thus, the pathogenic consequence of APOL1 mutations may be due to dysregulated channel activity. The G1 and G2 variants greatly improve potassium (K⁺) channel activity, with a twofold increase relative to the wild-type protein (G0) [16]. This rise in cation permeability is thought to play an important role in the development of APOL1-associated kidney diseases. Interestingly, the variations show no differences in chloride (Cl⁻) permease activity, indicating a selective shift in ion channel function. G1 and G2 variants have more efficient membrane association under conditions that promote cation channel activity, which most likely leads to their improved function. G1 and G2 variants show dose-dependent cytotoxicity, with even moderate expression levels producing significant cellular damage, which are exacerbated as expression levels increase [17]. In contrast, the G0 version exhibits no toxicity. Importantly, coexpression of G0 with G1 or G2 did not reduce the cytotoxic effects, indicating that renal risk variants operate via a dominant toxic gain-of-function mechanism. Transcriptome analysis demonstrated that these mutations alter canonical pathways involved in cellular stress responses, exacerbating their harmful effects.

Ubiquitin D (UBD), a ubiquitin-like protein involved in proteasome degradation, was discovered to interact directly with APOL1 and reduce its cytotoxicity [18]. Experiments showed that overexpression of UBD reduced APOL1-induced toxicity, while knockdown or inactivation of UBD exacerbated cell damage. This protective effect is mediated through UBD’s ability to promote the proteasomal degradation of toxic APOL1 proteins, thereby reducing their harmful accumulation. Accordingly, interestingly, individuals of African American descent had lower expression of UBD, which was associated with increased risk for FSGS in carriers of APOL1 HR variants. These results point to the possible role of UBD as a therapeutic target and offer a new approach to the prevention of APOL1-induced renal injury by enhancing the function or expression of UBD.

The p.N264K missense variant has been identified as a potent protective modifier within APOL1 HR genotypes containing the G2 allele [15]. The p.N264K variant mitigates the toxic effects of APOL1 G2, altering the protein’s conformation and reducing its cytotoxicity. Functional analyses demonstrated that individuals with G1/G2 or G2/G2 genotypes who also carry the p.N264K variant exhibit significantly reduced risks of FSGS and chronic kidney disease (CKD). This protective effect is limited to G2-containing genotypes and does not apply to G1/G1 genotypes, highlighting the variant’s allele-specific effects. The presence of the p.N264K variation significantly reduces risk, thereby reclassifying several HR genotypes as low risk. These findings have important therapeutic implications, notably in kidney transplantation, where p.N264K-positive donors with HR genotypes may increase the donor pool while lowering graft failure chances. Furthermore, adding p.N264K screening into clinical practice may improve CKD risk stratification and guide customized therapy regimens.

Using data from 90 patients in the Nephrotic Syndrome Study Network (NEPTUNE) cohort, including those with biopsy-confirmed FSGS or minimal change disease (MCD), associations between APOL1 risk alleles, glomerular APOL1 gene expression, and detailed kidney morphology traits were assessed through light and electron microscopy [19]. Among subjects with APOL1 HR genotypes, the dominant morphologic abnormalities in FSGS included decreased global mesangial hypercellularity, lower actin cytoskeleton condensation, and more tubular microcysts. These findings point to significant tubular injury as a feature of APOL1-related kidney disorders, particularly FSGS [20,21,22,23,24]. This is consistent with previous research demonstrating similar tubular alterations in other APOL1-associated diseases, including arterionephrosclerosis, HIV-associated nephropathy, and collapsing glomerulonephritis in lupus. The reduction in global mesangial hypercellularity shows that APOL1-associated FSGS has separate underlying processes from other forms of the disease. Mesangial hypercellularity, which is often associated with both good and bad clinical outcomes in a variety of glomerular disorders, appears to be less prevalent in the APOL1 HR genotype, possibly indicating distinct pathogenic mechanisms. Furthermore, APOL1-associated FSGS is identified by reduced actin cytoskeleton condensation in podocytes at the glomerular basement membrane. While cytoskeletal condensation is characteristic of minimal change disease) and FSGS, the reasons for its decreased prevalence in the APOL1 HR state remain unknown. Relationships that were specific to FSGS etiology conferred by APOL1 mutations were not observed in MCD [19]. Furthermore, higher glomerular APOL1 mRNA expression linked with the number of risk alleles but not with specific morphologic features. The study also discovered morphologic variations between pediatric and adult patients, with specific characteristics, such as cytoskeletal condensation, being more prominent in adults.

The pathogenic pathways generated by APOL1 mutations are examined in the next part to provide a full understanding of how these processes lead to kidney disease.

3. Pathogenic Mechanisms

3.1. Podocyte Injury

Podocytes are specialized epithelial cells that keep the kidney’s glomerular filtration barrier structurally and functionally intact [25]. Injury to podocytes is a critical step in the course of several glomerular disorders, including FSGS (Table 1). Understanding the mechanisms underlying podocyte dysfunction and loss is critical for creating targeted treatment methods. APOL1 risk polymorphisms, G1 and G2, exert their effects by increasing podocyte cytotoxicity via several routes [26]. APOL1 HR mutations have a significant impact on mitochondrial function, causing energy deficits that destabilize podocytes and make them more vulnerable to stress. These energy restrictions are linked to mitochondrial fragmentation, decreased oxidative phosphorylation, and an increase in reactive oxygen species (ROS) levels. Furthermore, these HR mutations increase inflammatory responses by interacting with inflammasome components, including NOD-like receptor pyrin domain-containing protein 12 (NLRP12). This connection increases Toll-like receptor (TLR) signaling and activates the NLRP3 inflammasome, resulting in the production of pro-inflammatory cytokines including IL-1β [27,28,29]. This causes a prolonged pro-inflammatory milieu, exacerbating cellular injury and contributing to the progression of kidney disease. A recent study found that inflammasome activation plays an important role in enhancing podocyte injury and has the potential to be a therapeutic target in APOL1-associated nephropathy [27].

APOL1 HR mutations impair ribosomal activity, resulting in translational dysregulation and decreased protein synthesis, endangering cellular homeostasis [30]. These mutations disrupt the normal operation of ribosomal machinery by affecting the elongation initiation factor 2 (EIF2) pathways, which are required for effective protein translation. This disruption impairs cellular repair and protein quality control, rendering cells, particularly podocytes, more vulnerable to stress. Translational deficits impede the production of essential structural and regulatory proteins required for podocyte stability and function. This dysfunction is crucial for the initiation and progression of APOL1-related kidney diseases because it increases cellular stress and vulnerability to damage in pathological conditions.

Transcriptomic analysis of human podocytes was used to study the molecular pathways driving podocyte injury as they relate to differentiation state and APOL1 genotypes [31]. Using conditionally immortalized human podocyte cell lines generated from urine, differentiation drastically altered podocyte transcriptome profiles, upregulating podocyte-specific genes and extracellular matrix-related pathways. These changes indicate increased podocyte function after differentiation. Also, differentiation was associated with reduced proliferation, evidenced by the downregulation of tRNA-derived fragment-1 (tRF-1). APOL1 HR genotypes were shown to disrupt ribosomal and translational pathways, leading to possible podocyte dedifferentiation and injury. Key miRNAs, especially miR-629-3p and miR-1285-3p, were identified as putative regulators of APOL1 expression and thus may be involved in the pathogenic effects of the HR mutations. The study also included single-cell RNA sequencing of urinary podocytes from FSGS patients, confirming previous results of dysregulated pathways in APOL1 HR genotypes, including the elongation initiation factor 2 pathway and chondrial dysfunction. These shared insights from in vitro and patient-derived data highlight the importance of the identified pathways. The study emphasizes the influence of APOL1 HR variations and differentiation on podocyte transcriptomes, with translational regulation and small RNA expression being important areas of dysregulation. These findings pave the way for the identification of novel therapy targets and biomarkers in podocyte-related illnesses. However, the study is hampered by its in vitro nature, use of male-derived cells, and lack of genotypic diversity among the cell lines, necessitating more clinical validation.

Another major cause of podocyte dysfunction is disruption of the actin cytoskeleton [32]. The dynamic modulation of the actin cytoskeleton is critical for preserving the foot process architecture, which supports the glomerular filtration barrier. APOL1 risk variants have been reported to disrupt actomyosin organization, possibly via suppressing APOL3 activities, disrupting the cytoskeletal structure, and causing foot process effacement, a characteristic of podocyte injury.

Damage to podocytes is also associated with endoplasmatic reticulum (ER) stress caused by the accumulation of misfolded proteins resulting from their very high demand for protein synthesis [33,34]. Severe and sustained ER stress may induce apoptotic pathways leading to podocyte loss. Variations in APOL1 have been expected to increase this ER stress even more, further promoting the risk of apoptosis. Similarly, autophagic failure contributes to podocyte injury since autophagy is required for the degradation of damaged organelles and the maintenance of cellular homeostasis. Disruption of this mechanism, which may be impacted by APOL1 risk alleles, causes the accumulation of toxic cellular debris, increasing podocyte sensitivity.

Table 1. Pathogenic Mechanisms of Apolipoprotein L1 (APOL1) Variants.

Mechanism	Description	Cellular Effects	References
Podocyte Dysfunction	Cytotoxic ion channel activity	Proteinuria; podocyte detachment	[26,32]
Mitochondrial Dysfunction	Disruption of mitochondrial homeostasis	Energy deficits; oxidative stress	[35,36]
ER Stress	Induction of unfolded protein response	Cellular apoptosis	[34,37]
Lipid Dysregulation	Altered lipid metabolism in podocytes	Impaired membrane dynamics	[35]
Inflammation	Activation of innate immune pathways (e.g., STING, JAK-STAT)	Chronic inflammatory signaling	[27,38,39]

Abbreviations: APOL1; Apolipoprotein L1; ER: Endoplasmatic Reticulum; STING: Stimulator of Interferon Genes; JAK-STAT: Janus Kinase-Signal Transducer and Activator of Transcription.

Table 1. Pathogenic Mechanisms of Apolipoprotein L1 (APOL1) Variants.

Mechanism	Description	Cellular Effects	References
Podocyte Dysfunction	Cytotoxic ion channel activity	Proteinuria; podocyte detachment	[26,32]
Mitochondrial Dysfunction	Disruption of mitochondrial homeostasis	Energy deficits; oxidative stress	[35,36]
ER Stress	Induction of unfolded protein response	Cellular apoptosis	[34,37]
Lipid Dysregulation	Altered lipid metabolism in podocytes	Impaired membrane dynamics	[35]
Inflammation	Activation of innate immune pathways (e.g., STING, JAK-STAT)	Chronic inflammatory signaling	[27,38,39]

Abbreviations: APOL1; Apolipoprotein L1; ER: Endoplasmatic Reticulum; STING: Stimulator of Interferon Genes; JAK-STAT: Janus Kinase-Signal Transducer and Activator of Transcription.

Other major causes of podocyte injury include mitochondrial dysfunction [35]. Mitochondria are needed to support the high energy requirements of podocytes. Impaired mitochondrial activity, as demonstrated in the presence of APOL1 mutations, causes insufficient energy generation and increased oxidative stress, thus impairing podocyte survival. Expression of APOL1 G1 and G2 variants in human podocytes leads to increased cellular triglyceride content and mitochondrial dysfunction, accompanied by compensatory elevation of oxidative phosphorylation complexes. APOL1 risk variants exacerbate podocyte injury by increasing inflammatory stress, which may further compromise mitochondrial function [26]. Metabolomics investigation of HEK293 cells conditionally expressing the APOL1 G0 (reference), G1, and G2 variants revealed severe abnormalities in mitochondrial metabolic pathways associated with these risk variants [36]. APOL1 G1 and G2 mutations disturb the tricarboxylic acid (TCA) cycle, accumulating upstream metabolites such as citrate and acetyl-CoA while depleting downstream products such as succinate and fumarate. The disturbed energy metabolism, a consequence of reduced activity of mitochondrial complex I, is partially relieved by enhancing fatty acid β-oxidation in G1 and G2 variants. This is reflected in reduced levels of long-chain acylcarnitine and raised levels of acetyl-CoA. This in turn results in the worsening of oxidative stress as was reflected in increased values of oxidative markers, in addition to indicating a reduced availability of glutathione-related metabolites. Furthermore, one-carbon metabolism and transsulfuration pathways were altered, with increased levels of intermediates but decreased cysteine, indicating poorer antioxidant production. Downregulation of critical complex I subunits (NDUFV2 and NDUFS1) emphasizes the mitochondrial dysfunction induced by these variations, which results in reduced electron transport chain efficiency and a loss of mitochondrial membrane potential. G1 and G2 variations cause considerable mitochondrial fragmentation in kidney cells, including human proximal tubule cells and the HEK293 Tet-on cell line. This fragmentation is facilitated by the increased activity of dynamin-related protein 1 (DRP1), a protein required for mitochondrial fission. Furthermore, BCL2 antagonist/killer 1 (BAK1), a mitochondrial outer membrane protein, was identified as an important upstream regulator of APOL1. Loss of APOL1 and its harmful effects were decreased in BAK1 knockout conditions, which again highlighted its importance in mitochondria dysfunction. Therapeutically, mitochondrial division inhibitor 1 (Mdivi-1) also mitigated mitochondrial fragmentation, restored mitochondrial membrane potential, and enhanced cell survival in APOL1 risk allele-expressing cells. Both G1 and G2 variants increased the phosphorylation of DRP1 at Ser616, which caused excessive mitochondrial fission and impaired mitochondrial integrity. The disturbed mitochondrial dynamics compromises energy production and enhances cellular vulnerability to injury.

Expression of APOL1 risk mutations in podocytes impairs lipid homeostasis, leading to intracellular lipid buildup [35]. This lipotoxicity causes mitochondrial malfunction and oxidative stress, which promotes inflammatory responses and fibrosis in the glomeruli. APOL1 mutations may affect cholesterol efflux pathways, resulting in lipid buildup in podocytes. This buildup is linked to mitochondrial dysfunction, which is defined by decreased mitochondrial membrane potential and increased generation of reactive oxygen species (ROS). The resultant oxidative stress damages podocytes and creates an inflammatory milieu, which contributes to glomerular injury. More precisely, the APOL1 G1 and G2 variations promote lipid accumulation in renal tissues, notably triglycerides and cholesterol esters. Renal impairment is significantly associated with such a lipid imbalance. Also, similar lipid imbalances are characteristic for human podocytes that express these risk variants. Therefore, APOL1 variations have a direct impact on podocyte lipid homeostasis. Mitochondrial structural abnormalities also characterize the podocytes that had G1 and G2 variations, with manifestations including vacuolization and changes in the cristae shape that impaired oxidative phosphorylation (OXPHOS) and generation of ATP. Despite increased production of OXPHOS complexes as a compensatory response, mitochondrial function was still reduced. Importantly, the study found that lipid-modifying drugs increased mitochondrial function and ATP production in podocytes carrying APOL1 risk alleles, indicating novel therapeutic methods for APOL1-associated nephropathy.

Autophagy is an essential cellular process that restores podocyte homeostasis by digesting damaged organelles and proteins [40]. Podocytes, the specialized epithelial cells of the glomerulus, have high basal levels of autophagy, which is required to maintain their complex structure and function. This process begins with the creation of autophagosomes, which sequester cytoplasmic components before combining with lysosomes to breakdown and recycle. Autophagy disruption in podocytes can result in the accumulation of cellular debris, which contributes to podocyte damage and glomerular disease. Indeed, recent investigations have shown that podocyte-specific ablation of autophagy-related genes such as Atg5 causes glomerulopathy with proteinuria and glomerulosclerosis, highlighting the importance of autophagy in podocytes. APOL1 risk variations, including the G1 and G2 alleles, have been linked to abnormal autophagic processes in podocytes, aggravating podocyte dysfunction [37]. Studies on transgenic mice with APOL1 risk mutations have shown that they can impair autophagic flow, resulting in podocyte damage [26]. For instance, studies in Drosophila nephrocytes, a model for mammalian podocytes, have demonstrated that APOL1-G2 induces cell death through inhibition of the autophagy-lysosome pathway. APOL1 risk alleles might enhance inflammatory signaling in podocytes as an additional pathway, influencing autophagic processes. Testing the effects of APOL1 risk mutations in a model of APOL1 transgenic mice revealed that this gene indeed enhances inflammatory responses and contributes to podocyte injury via connections between APOL1-driven inflammation and disrupted autophagy. The interaction of autophagy and other cellular degradation systems, such as the ubiquitin-proteasome system (UPS), is also essential for podocyte function [33]. Disruptions in the equilibrium of these pathways can cause proteostatic stress and podocyte damage. Recent research has shown that autophagy and UPS play important roles in sustaining podocyte proteostasis and in the pathophysiology of podocyte damage.

Podocyte loss in APOL1-mediated nephropathy is caused by defects in the cytoskeleton, inflammation, ribosomal dysregulation, and mitochondrial dysfunction. Podocyte loss, and subsequent glomerular scarring, are central to the progression of kidney diseases, including proteinuria and eventual ESKD. These interwoven networks show the multiple mechanisms that drive APOL1-mediated kidney failure, which has a profound impact on clinical outcomes. Surprisingly, autophagy and mitochondrial failure appear to play a critical role in linking inflammatory signals, metabolic stress, and podocyte structure. Therapies that target inflammation, metabolic stress, and cytoskeletal integrity may provide a more comprehensive approach to disease management. Despite tremendous progress, large research gaps persist. To guide targeted therapies, we must first understand the molecular links between APOL1 mutations and their various actions, such as autophagy and EIF2 signaling. Understanding the relationship between cytoskeletal dynamics and mitochondrial dysfunction, especially in the setting of actin alteration, is crucial for creating treatments to improve podocyte health. Moreover, the role of environmental and genetic modifiers, such as additional ‘hits,’ in the pathogenesis of podocyte fragility remains largely unknown. Finally, studying the role of short RNAs and miRNAs in the modulation of APOL1 expression offers promising opportunities for the development of specific therapies and biomarkers.

APOL1 risk variations have far-reaching repercussions, even though podocyte injury is a major contributor in APOL1-mediated nephropathy. APOL1-driven processes also affect non-podocyte cells, such as endothelial and tubular epithelial cells, which can have serious consequences for renal and overall health. These extrarenal symptoms highlight the diverse role of APOL1 mutations in causing kidney disease. The next part investigates the involvement of non-podocyte cell damage, with a focus on endothelial dysfunction, immune cell activation, and systemic inflammation in worsening the pathogenic consequences of APOL1 mutations.

3.2. Non-Podocyte Injury

Emerging data indicate that APOL1 risk variants play important roles in non-podocyte cells and systemic processes [41]. In endothelial cells, APOL1 mutations cause mitochondrial stress, inhibit autophagic processes, and enhance vascular inflammation during inflammation. The outcomes of these events are the severe outcomes such as increased risk of sepsis and AKI. Other immunological cells that express APOL1, including T cells and macrophages, are implicated in modified immunological responses. Examples include the accumulation of cholesterol by macrophages expressing APOL1 risk variants to enhance renal inflammation and tissue injury. In the setting of kidney transplantation, the expression of APOL1 variants in recipient immune cells is associated with higher rates of rejection and shortened graft survival. The expression of APOL1 mutations in placental cells has significant implications for pregnancy problems. Variations in APOL1 have been linked to preeclampsia, most likely due to a malfunction in the trophoblast cells that are essential for placental development. Experimental models back up this claim, indicating that risk mutations relate to unfavorable pregnancy outcomes such as hypertension and fetal growth limitation. Triggering factors, or “second hits,” have a huge effect on enhancing the outcome of APOL1 variants. Infections, such as those due to HIV and SARS-CoV-2, are well documented to cause APOL1-driven kidney injury. Collapsing glomerulopathy, a more serious form of FSGS, often appears among APOL1 risk carriers following such infections. Furthermore, chronic inflammatory situations typified by high interferon (IFN) levels can increase APOL1 expression, exacerbating kidney injury. The interaction of genetic and environmental variables also influences disease risk. APOL1 risk mutations are necessary for disease development, but they are insufficient on their own. Additional modifiers, such as specific genetic loci or environmental exposures, serve as “third hits”, altering the likelihood and severity of disease progression.

APOL1 mutations can activate and injure endothelial cells, worsening glomerular inflammation and contributing to fibrosis [26,42]. Endothelial cells play an important role in vascular homeostasis, and their malfunction can cause increased vascular permeability and leukocyte adhesion, which promotes inflammation. Studies have shown that APOL1 risk mutations increase the expression of adhesion molecules and pro-inflammatory cytokines in endothelial cells, resulting in greater monocyte adherence and endothelial damage. Endothelial dysfunction leads to the inflammatory environment within the glomerulus, which promotes fibrosis and the progression of FSGS.

Recent data analysis revealed common features in APOL1-mediated non-podocyte damage, including mitochondrial failure, inflammation, and disruption of cellular homeostasis. These interrelated processes offer a unified framework for comprehending the systemic and renal symptoms of APOL1 mutations. The linkages between endothelial dysfunction and immune cell activation demonstrate the systemic impact of APOL1 mutations beyond the kidney, emphasizing the intricate interplay between genetic predisposition and environmental variables. Despite progress in understanding the significance of APOL1 in non-podocyte cells, numerous gaps remain. First, the molecular processes underpinning APOL1-mediated dysfunction in endothelial and immune cells must be further investigated. It is unknown how much circulating versus tissue-specific APOL1 contributes to systemic inflammation and kidney damage. Furthermore, the impact of genetic modifiers and “third hits” in APOL1 disease outcomes need further exploration. Current animal and in vitro models frequently fail to capture the complexities of APOL1-associated systemic processes, which limits translational potential. Future research should focus on developing humanized models and advanced organoid systems to study APOL1’s systemic effects. Single-cell transcriptomic methods could provide cell-specific information about APOL1 activity in immunological and endothelial cells. Therapeutic approaches that prioritize endothelium repair, inflammasome activation, and mitochondrial function may be able to prevent non-podocyte damage. Furthermore, developing precision medicine for APOL1-associated disorders requires an integrated approach that considers genetic, environmental, and epigenetic factors. By filling in these gaps, we can gain a better understanding of the overall impacts of APOL1 risk variations on the body and create comprehensive plans to reduce their harmful effects on kidney and overall health.

To further understand mitochondrial dysfunction, inflammation, and dysregulated autophagy, we must look at the major cellular pathways that allow APOL1-mediated toxicity. The next section delves into important signaling pathways such as IFN, STING, and JAK-STAT, which regulate the molecular processes that cause damage to podocytes and non-podocytes and give information on prospective treatment targets.

3.3. Cellular Pathways

3.3.1. Interferon Pathway

Cytokines, such as IFNs, strictly regulate APOL1. Type I IFNs (such as IFN-α and IFN-β) significantly increase APOL1 expression [43,44]. IFNs, which are essential for the innate immune response, drive APOL1 transcription by activating IFN-stimulated response elements (ISREs) in the promoter region. This increase is particularly pronounced during immunological responses and chronic inflammatory situations, implying that IFNs contribute to kidney tissue damage (Figure 1).

Chronic activation of the type I IFN pathway has been strongly associated with renal tissue destruction in a variety of kidney diseases, including FSGS [45]. Persistent IFN signaling can promote inflammation and apoptosis, making renal cells more vulnerable to damage. This overexpression may increase the cytotoxic capacity of APOL1, resulting in cellular malfunction and mortality in individuals carrying APOL1 HR variants (G1 and G2). Specifically, podocytes, the specialized epithelial cells critical for glomerular filtration, are particularly susceptible to APOL1-mediated damage under conditions of sustained interferon exposure. A human podocyte cell model carrying the APOL1 G2/G2 HR genotype, which exhibited altered features, including upregulation of CD2-associated protein (CD2AP), alteration of cytoskeleton, reduction of autophagic flux, and increased permeability under continuous perfusion, indicated that podocytes with APOL1 risk variants are particularly susceptible to injury under inflammatory conditions.

In systemic autoimmune disorders such as systemic lupus erythematosus (SLE), elevated levels of type I IFNs are associated with increased APOL1 expression in renal tissues [46]. This points to a clear relationship between systemic inflammatory responses and APOL1-mediated kidney disease. In addition, experimental models have shown that type I IFNs act on renal endothelial cells by inducing the production of adhesion molecules and pro-inflammatory cytokines, creating a vicious cycle of inflammation, endothelial activation, and renal tissue injury [47]. In FSGS, chronic type I IFN signaling seems to enhance the effects of APOL1 mutations on podocyte health [48,49]. Mechanistic investigations have revealed that IFN stimulation stimulates APOL1 expression, which induces mitochondrial dysfunction, disrupts cytoskeletal integrity, and increases susceptibility to apoptotic signals. These alterations collectively contribute to podocyte detachment and loss, characteristic of progressive glomerulosclerosis.

In contrast, several therapeutic strategies directed to the type I IFN pathway may serve as potential protective measures for APOL1-related kidney injury [39]. Among them, inhibitors of Janus kinase (JAK)-signal transducer and activator of transcription (STAT) have demonstrated interference in IFN signaling and resultant pathways, including the overexpression of APOL1 [49]. A recent study revealed that the administration of baricitinib reduced COVID-19-driven cytokine-mediated APOL1 expression in cultured human kidney cells, raising hope regarding possible future applications in cytokine-driven APOL1-mediated podocytopathy. Monoclonal antibodies to the IFN-α receptor may decrease systemic interferon levels and reduce kidney injury in susceptible individuals.

3.3.2. STING Pathway

Recent research has shed light on the significance of the STING pathway in innate immunity [50]. When activated by cyclic GMP-AMP synthase (cGAS) in response to cytosolic DNA, STING recruits and activates TANK-binding kinase 1 (TBK1), which phosphorylates interferon regulatory factor 3 (IRF3). This cascade results in the synthesis of type I IFNs and pro-inflammatory cytokines, which are required for immune surveillance. However, disruption of this system can cause pathological inflammation.

Recent studies have highlighted the role of the STING pathway in the pathophysiology of FSGS, especially concerning APOL1 HR variants [51]. Activation of the STING pathway has been linked to increased APOL1 expression in renal cells. Nucleosomal double-stranded DNA stimulates APOL1 expression in human podocytes by activating the cGAS/IFI16-STING signaling pathway. The importance of NLRP3 and STING in APOL1-associated podocytopathy suggests that inhibiting these pathways could reduce podocyte damage [27].

STING activation in podocytes has been linked to mitochondrial malfunction, which is characteristic of APOL1-related toxicity [27,35]. Mitochondrial DNA release, which is frequently caused by APOL1-induced stress, can act as a ligand for cGAS, activating the STING pathway and resulting in a feed-forward cycle of inflammation and cell injury. This loop contributes to podocyte death and separation, both of which are critical to the course of FSGS.

Recent studies have highlighted how important STING-driven inflammation is in promoting renal injury and FSGS [27]. In models, the inhibition of STING pathways was associated with an upregulated production of type I IFNs, reduced APOL1 expression, and resulted in improved podocyte injury along with glomerulosclerosis. For example, one study found that administering the STING inhibitor C176 dramatically improved kidney function in animals harboring the APOL1 risk mutation G2 in podocytes.

The STING pathway’s effects are not limited to podocytes [27,52]. In the presence of APOL1 mutations, renal endothelial and tubular cells show enhanced STING activity. Activation enhanced endothelial dysfunction and tubulointerstitial inflammation, accelerating the fibrotic course of FSGS. Additionally, STING activation was associated with the migration of immune cells into the kidney, promoting the local inflammatory environment and thereby accelerating the tissue destruction further.

In fact, there has now been active interest in the pursuit of STING-pathway small molecule inhibitors for their potential clinical use in both FSGS and related kidney diseases associated with sustained inflammatory reactions [53,54]. Results derived from STING inhibition provided an attenuated IFN-signaling resultant increase of lesser renal inflammatory forms concerning disease models featuring the spectrum between lupus nephritis to FSGS, thus providing another evidence to treat APOL1 nephropathies.

3.3.3. JAK-STAT Pathway

The JAK-STAT pathway is an important modulator of cytokine signaling, influencing inflammation, immunology, and cellular stress responses [38]. Dysregulation of this system has been linked to a variety of chronic inflammatory and autoimmune disorders, including APOL1-mediated FSGS. APOL1 expression is upregulated by pro-inflammatory cytokines, including IFNs (IFN-α, IFN-β, and IFN-γ), through the activation of the JAK-STAT pathway [38,39]. IFN stimulation leads to phosphorylation and activation of JAKs, which, in turn, phosphorylate STAT proteins. Phosphorylated STATs dimerize and translocate to the nucleus, where they bind to ISREs in the APOL1 promoter region, enhancing its transcription. This regulation is particularly significant during chronic inflammatory states, where prolonged cytokine signaling amplifies APOL1 expression, exacerbating kidney injury.

In the context of FSGS, overactivation of the JAK-STAT pathway increases the cytotoxic effects of APOL1 HR variants (G1 and G2) in podocytes [38,39]. These variations impair mitochondrial function, increase oxidative stress, and stimulate inflammatory cytokine production, all of which are exacerbated by JAK-STAT signaling. It has also been demonstrated that interference with this pathway may mitigate the downstream consequences of APOL1 overexpression, including podocyte injury and proteinuria. Therapeutic inhibition of the JAK-STAT system has also shown promise in preclinical and early clinical studies [55]. Baricitinib was tested in Phase 2 JUSTICE for proteinuria treatment in APOL1-mediated renal disorders by the selective inhibition of JAK1/2. Treatment with baricitinib effectively reduced the pathogenic burden on podocytes by inhibiting cytokine-induced APOL1 overexpression and hence preserved glomerular function. These results emphasized JAK-STAT inhibitors as one of the targeted therapeutic methodologies for APOL1-related kidney diseases.

Further, JAK-STAT participates in APOL1-associated endothelial and tubular cell dysfunction [27]. The activation of the pathway promotes vascular inflammation, increases leukocyte adhesion, and weakens the integrity of the endothelial barrier, leading to an acceleration of renal fibrosis and an acceleration toward end-stage kidney disease (ESKD). Ongoing research is looking into the interactions between JAK-STAT signaling and other inflammatory pathways, such as the cGAS-STING and NLRP3 inflammasome pathways, to identify synergistic therapeutic targets. Combining JAK-STAT inhibitors with additional anti-inflammatory drugs may give a more complete strategy to addressing the multifactorial damage seen in APOL1-mediated nephropathy.

Understanding all these pathways is crucial for translating molecular discoveries into useful treatment therapies. The next section explores the therapeutic implications of these findings, with an emphasis on risk stratification, genetic screening, and the development of treatment techniques to lessen the impact of APOL1-associated kidney disease.

4. Clinical Implications

Risk stratification based on the existence of the APOL1 allele has been a focus in nephrology, with the goal of identifying those who are more likely to develop progressive kidney disease. Genetic screening for APOL1 risk variations is recommended for people of African ancestry, especially those who have a family history of kidney disease or have been diagnosed with FSGS. Screening should also be undertaken for living kidney donors of African heritage to determine the hazards to both the donor and the receiver. Pre-test counseling is required to ensure that patients understand the meaning of their screening results and the related risks.

Recent investigations have found a substantial link between APOL1 HR alleles and poor renal outcomes, particularly in people of African heritage. Carriers of two APOL1 risk alleles have a much higher chance of acquiring kidney disorders such FSGS (Odds ratio = 17, 95% CI: 11–26) [56] and ESKD (Odds ratio = 7.3, 95% CI: 5.6–9.5) [57]. This increased risk appears to be attributable to a gain-of-function effect of the APOL1 variants, which may confer resistance to Trypanosoma brucei infection but also result in heightened kidney damage through mechanisms such as podocyte injury, mitochondrial dysfunction, and inflammation [58]. Notably, people with two APOL1 risk alleles have a lifetime risk of developing FSGS of roughly 4%, compared to 0.2% for those without these alleles [59]. On top of that, the APOL1 risk allele has also been associated with faster rates of disease progression and poor renal outcomes [38].

4.1. ARIC Study

The Atherosclerosis Risk in Communities (ARIC) study looked into the link between APOL1 HR alleles and racial differences in renal outcomes among the elderly (

Table 2. Key Studies on APOL1-associated Nephropathy.

Study Name	Population Characteristics	Key Findings	Implications	References
NEPTUNE cohort	African American children with nephropathy	APOL1 HR variants linked to faster eGFR decline	Highlights need for early intervention	[61]
CKiD Cohort	Children with CKD, including APOL1 variant carriers	APOL1 HR associated with faster progression of CKD	Underscores importance of genetic risk stratification	[2]
ARIC Study	Older adults, APOL1 genotype association with CKD outcomes	APOL1 HR linked to increased risk of ESKD and cardiovascular events	Emphasizes systemic impact of APOL1 variants	[60]
DUPLEX Trial	Adults with biopsy-proven FSGS	Sparsentan reduced proteinuria significantly	Supports dual endothelin–angiotensin blockade	[62]
Genetic Epidemiology Study	Global population; APOL1 variant frequencies	G1 and G2 alleles prevalent in African ancestry	Emphasizes health disparities in CKD	[12,15]

Abbreviations: APOL1, Apolipoprotein L1; HR, high risk; NEPTUNE: Nephrotic Syndrome Study Network; CKiD: Chronic Kidney Disease in Children; ARIC: Atherosclerosis Risk in Communities; FSGS: Focal Segmental Glomerulosclerosis; CKD: Chronic Kidney Disease; eGFR: Estimated Glomerular Filtration Rate; ESKD: End-Stage Kidney Disease.

) [60]. The APOL1 HR genotype was characterized as having two copies of the risk alleles G1/G1, G1/G2, or G2/G2, which was strongly related with a considerably elevated risk of ESKD in Black people. However, when controlling for baseline kidney function and albuminuria, this correlation became statistically insignificant, indicating that these variables play an important role in regulating the relationship between APOL1 genotype and ESKD. Even after controlling for socioeconomic, clinical, and genetic factors such as APOL1 status, Black participants in the study had considerably higher incidences of CKD and ESKD than other groups. Other causes driving the high incidence in Black communities would involve unmeasured socioeconomic determinants of health or environmental exposures. In contrast, surprisingly, APOL1 HR genotypes were not associated with higher hospitalization rates, all-cause mortality, or incident CKD among those who had normal kidney function. These data emphasize the significance of APOL1 in disease progression rather than beginning. The impact of APOL1 HR genotypes was greatest in people who already had impaired kidney function, underlining its involvement in accelerating kidney deterioration. While APOL1 HR genotypes contribute to kidney disease development, addressing the broader spectrum of social determinants of health and taking into account other genetic or environmental risk factors are critical for reducing the excess burden of kidney disease in Black populations.

4.2. NEPTUNE Study

The Nephrotic Syndrome Study Network (NEPTUNE) cohort study investigated low birth weight (LBW), prematurity, and clinical outcomes in nephrotic syndrome in relation to APOL1 HR allele function [63]. LBW and preterm birth have been identified as risk factors for CKD and nephrotic syndrome because these conditions are believed to relate to reduced endowment and increased vulnerability of the nephrons to insults. This investigation sheds light on the interplay between these perinatal factors and genetic predispositions, specifically APOL1 risk alleles. APOL1 HR variants were more prevalent in the LBW and prematurity group. APOL1 HR alleles were linked to a faster decline in kidney function, as measured by estimated glomerular filtration rate (eGFR). Participants with APOL1 risk alleles showed more dramatic disease progression, including a higher decrease of renal function over time. Interestingly, the presence of APOL1 HR alleles explained just a portion of the link between LBW/prematurity and eGFR decline. Even after accounting for APOL1 status, LBW and preterm each contributed to a faster deterioration in kidney function. This shows that, while APOL1 mutations increase the risk, other variables associated with LBW and preterm, including as nephron loss and early-life environmental exposures, also play important roles in disease progression. The findings highlight the importance of integrating genetic testing and detailed birth history into the clinical management of nephrotic syndrome. Screening for APOL1 risk variations in people with a history of LBW or preterm can assist identify those who are more likely to have rapid kidney deterioration. This technique enables individualized therapies to halt disease development and improve patient outcomes. Finally, the study emphasizes the crucial interplay between genetic and prenatal variables in nephrotic syndrome. APOL1 HR mutations, when paired with LBW or preterm, contribute to a more severe course of kidney disease.

4.3. CKiD Study

In the CKiD study, the influence of APOL1 genetic risk variants on kidney and cardiovascular health in children with FSGS was explored [61]. Among African American children, 24% carried APOL1 HR genotypes, which were strongly associated with FSGS, accounting for 89% of cases in HR individuals. Children with APOL1 HR developed FSGS at a later median age (12 years, IQR: 9.5–12.5) than those with low-risk genotypes (5.5 years, IQR: 2.5–11.5). Cardiovascular problems were more common in the HR group, including left ventricular hypertrophy (53% vs. 12%) and uncontrolled hypertension (52% vs. 33%). Additionally, obesity was significantly more common in HR children (48% vs. 19%). Despite similar GFRs, the HR group exhibited distinct renal and cardiovascular profiles.

Analyzing 104 children from the NEPTUNE and CKiD cohorts, those with APOL1 HR genotypes had a later disease onset but experienced a more aggressive progression, including faster declines eGFR [64]. These children also had a considerably greater prevalence of FSGS, a severe kind of kidney disease, than those with LR genotypes. Furthermore, HR genotypes were associated with an increased risk of preterm birth, indicating that developmental vulnerabilities are linked to genetic determinants.

4.4. CureGN Study

In the Cure Glomerulonephropathy (CureGN) cohort, 650 participants were enrolled from different sites in a study on the association of APOL1 gene variants with the development of kidney disease in patients with FSGS [65]. Subjects were stratified based on the presence of APOL1 risk alleles: those with two alleles were considered HR, while those with one or no alleles were considered LR. The major research outcome was the rate of deterioration in eGFR. APOL1 HR genotypes were significantly related with accelerated disease progression, with a 2.75-fold increase in the likelihood of rapid eGFR reduction compared to low-risk patients.

In the early phases of arterionephrosclerosis, the APOL1 genotype causes more vascular than glomerular damage [66]. Severe arteriosclerosis in the renal interlobular arteries was closely related with aging and hypertension, and African Americans experienced more vascular injury than Whites. This effect was stronger among people with two APOL1 risk alleles, who had more intimal thickening of the arteries for the same amount of hypertension, a link that became apparent after the age of 35. The study also found that age and hypertension had a greater influence on glomerulosclerosis and cortical fibrosis than the APOL1 genotype. Importantly, no significant vascular abnormalities were seen before the age of 35, indicating that the identified modifications were acquired.

4.5. APOLLO Study

The APOL1 Long-term Kidney Transplantation Outcomes Network (APOLLO) study is a large-scale observational initiative designed to better understand the effects of APOL1 genetic risk variants (G1 and G2) on kidney transplant outcomes [67]. These variations are closely linked to outcomes after transplantation. The aim of APOLLO is to determine whether a kidney donor carrying APOL1 HR genotypes affects the allograft survival and renal function of transplant recipients. Long-term health consequences for living donors with HR genes are also assessed. To achieve these goals, APOLLO is enrolling approximately 2600 deceased-donor transplant recipients and living donors at numerous transplant sites across the United States. Participants are tested for APOL1, and their clinical outcomes are tracked over time, resulting in a strong dataset for studying genotype-outcome connections. APOLLO distinguishes itself by emphasizing community interaction through a Community Advisory Council (CAC). The CAC, comprising individuals of African ancestry, ensures that the study design, communication, and consent processes address the needs and concerns of affected communities. This approach fosters trust and encourages participation from populations disproportionately impacted by APOL1-associated kidney disease. The study’s findings are expected to transform kidney transplantation practices by incorporating APOL1 genetic data into donor evaluation processes. In that way, APOLLO hopes to come out with better ways of donor selection, lessen organ waste rates, and optimize the allocation of organs. This study also aimed at using evidence-based genetic insights for race-based selection criteria like the Kidney Donor Risk Index and thereby enhances equity in outcomes after transplantation. Despite logistical problems due to its size and complexity, APOLLO marks an important step toward reducing health inequities in renal disease. Its findings will help to advance customized treatment, increase transplant survival, and ensure safer practices for living kidney donors, particularly among African Americans. This landmark study emphasizes the importance of genetic and community-based initiatives in reducing inequities and increasing transplant outcomes.

The findings of these studies emphasize the critical role of APOL1 HR polymorphisms in the development and progression of FSGS. They also highlight that socioeconomic, environmental, and genetic considerations should be taken into account when considering and stratifying risk. Early diagnosis and selective therapy have gained significance due to the evidence that APOL1 function is particularly critical during the disease. These notwithstanding, there remain a few research gaps despite the phenomenal advances. The molecular pathways that connect APOL1 mutations to both vascular injury and systemic inflammation remain to be explored. Further investigation is required to better understand the association between genetic risk and environmental exposures, including infections and in utero exposures. Screening techniques themselves should also be improved in order to enhance reach and sensitivity in disadvantaged populations.

5. Treatment

5.1. Baricitinib

The Phase 2 Baricitinib Study, also referred to as the JUSTICE trial, is a landmark clinical study that aims to establish the efficacy and safety of baricitinib in treating kidney diseases associated with APOL1 HR genotypes [55]. Targeting JAK1/2, the Janus kinase inhibitor acts on the JAK-STAT signaling system, which has been implicated in increased APOL1 expression and resultant kidney injury (

Table 3. Comparison of Therapeutic Approaches for APOL1-Associated Nephropathy.

Therapeutic Approach	Mechanism of Action	Advantages	Limitations	References
Small-Molecule Inhibitors	Inhibit APOL1-mediated ion channel activity	Oral administration; promising preclinical data	Limited long-term data; early clinical stage	[68,69,70]
Antisense Oligonucleotides	Reduce APOL1 mRNA expression	High specificity; targeted therapy	Requires frequent administration; high cost	[71]
Gene-Editing Technologies	Correct APOL1 genetic variants via CRISPR	Potential for permanent correction	Ethical concerns; early-stage research	[43,69]
JAK-STAT Pathway Inhibitors	Modulate inflammatory signaling pathways	Addresses downstream effects of APOL1	Non-specific action on immune signaling	[38,39,55]
Lipid Metabolism Modulators	Restore lipid homeostasis disrupted by APOL1	Broad metabolic benefits	Indirect action on APOL1 mechanisms	[35,72]

Abbreviations: APOL1, Apolipoprotein L1; ASOs: Antisense Oligonucleotides; CRISPR: Clustered Regularly Interspaced Short Palindromic Repeats; JAK-STAT: Janus Kinase-Signal Transducer and Activator of Transcription.

). The JUSTICE study uses a randomized, double-blind, placebo-controlled design to assure reliable and unbiased outcomes. It includes 75 African American volunteers, 50 with hypertension-associated-CKD and 25 with FSGS, all between the ages of 18 and 70 who are proven APOL1 HR carriers. For six months, participants are randomized 2:1 to receive either baricitinib or a placebo. The trial’s primary focus is on two endpoints: efficacy, which is determined by the percentage change in urine albumin-to-creatinine ratio (UACR), and safety, which is defined by the incidence of clinically severe anemia. The study’s rationale is based on the discovery that JAK-STAT signaling drives the pathogenic effects of APOL1 HR genotypes by producing podocyte damage and proteinuria, which are hallmarks of APOL1-mediated kidney disorders. By blocking this route, baricitinib is thought to diminish proteinuria, providing early signs of therapeutic success. The JUSTICE trial is a critical step in meeting the unmet demand for targeted treatments in APOL1-mediated renal disorders. By evaluating baricitinib’s potential to attenuate kidney damage and improve clinical outcomes, the study seeks to pave the way for future Phase 3 trials and, ultimately, FDA approval. Its innovative design and community-focused approach may serve as a model for future clinical trials targeting underrepresented populations and addressing health disparities. The estimated primary completion date is 31 March 2026.

5.2. Sparsentan

The Sparsentan Phase 3 DUPLEX Study explored baseline characteristics and genetic influences in FSGS, with a particular emphasis on the role of APOL1 HR genotypes [73]. APOL1 HR genotypes were identified in 4% of the evaluated population in the DUPLEX trial [74]. This phase 3 multicenter, double-blind, randomized experiment comprised 371 biopsy-confirmed FSGS patients aged 8 to 75 years. Participants were randomized to either sparsentan or irbesartan for a 108-week treatment duration. The results revealed that sparsentan reduced proteinuria substantially more than irbesartan. At 36 weeks, partial remission of proteinuria occurred in 42% of sparsentan patients and in 26% in the irbesartan group. This reduction remained constant over the entire 108-week period. The variation in the slope of eGFR did not differ significantly between groups, with a difference of merely 0.3 mL per minute per 1.73 m² per year. Safety profiles were similar for both therapies, with comparable rates of adverse events occurring [62]. Sparsentan is a first-in-class dual endothelin type A (ETA) receptor antagonist and angiotensin II type 1 (AT1) receptor blocker designed to treat FSGS and other glomerular diseases. It decreases proteinuria by targeting these two important pathways, which may halt the progression of chronic kidney disease. The ETA receptor is implicated in processes like vasoconstriction, inflammation, and fibrosis, all of which cause kidney injury. The AT1 receptor, a component of the renin-angiotensin-aldosterone system, is also involved in inflammation, fibrosis, and high blood pressure. Sparsentan provides better nephroprotection than single-pathway inhibitors because of its dual inhibition mechanism. Studies suggest that sparsentan not only reduces proteinuria more effectively than traditional angiotensin receptor blockers like irbesartan but also preserves kidney function by protecting podocytes and promoting endothelial health [75].

5.3. Diacylglycerol O-Acyltransferase 2 Inhibitors

Targeting lipid metabolism can mitigate the harmful effects of APOL1 risk variants, which are strongly associated with FSGS [72]. The mechanisms underlying the cytotoxic effects of APOL1 risk variants are not fully understood. Inhibiting diacylglycerol O-acyltransferase 2 (DGAT2) enhanced lipid droplet formation in kidney cells expressing APOL1 risk variants, which significantly reduced cytotoxicity. Lipid droplets have been found to function as protective structures, sequestering hazardous lipids and inhibiting their detrimental interactions with cellular components, particularly mitochondria. This resulted in better mitochondrial activity and decreased cell death. Pharmacological inhibition of DGAT2 has emerged as a viable technique for mitigating the cytotoxic consequences of APOL1 mutations, identifying a new therapeutic target. These findings imply that inhibiting DGAT2 could improve lipid droplet production and act as a preventive strategy against APOL1-associated kidney diseases.

5.4. Small-Molecule Inhibitors

Small-molecule inhibitors are designed to directly prevent the harmful effects of APOL1 risk variants, namely their ion channel function [69]. These drugs are intended to inhibit the improper pore-forming activity of APOL1 G1 and G2 variants in kidney cells. Early-stage clinical trials have yielded promising outcomes with drugs such inaxaplin (VX-147), which suppresses APOL1’s abnormal ion channel activity. A Phase 2A clinical trial involving 16 people with biopsy-proven FSGS found a 30–60% reduction in proteinuria among the 13 participants who finished the study [70]. The treatment was well tolerated, with just minor side effects and no need to discontinue therapy [76]. These findings put inaxaplin on the path to being a game-changer in precision medicine, targeting a specific genetic etiology of CKD. The reduction in proteinuria was associated with slower disease development, indicating possible long-term benefits for patients. However, the study’s small sample size and geographic variety underline the importance of bigger randomized controlled studies. Future studies should involve a varied population, particularly from countries with a high prevalence of APOL1 mutations, and focus on outcomes such as steady eGFR and complete proteinuria remission.

5.5. Antisense Oligonucleotides

Antisense oligonucleotides (ASOs) are synthetic nucleic acid sequences that attach to APOL1 mRNA, preventing translation into the deadly protein [71]. This is an efficient way of reducing APOL1 expression in renal cells. Preclinical animals with low levels of APOL1 protein showed improved mitochondrial activity and decreased proteinuria after ASO therapy. ASOs, which specifically target APOL1 expression in renal tissues, offer a promising method for reducing systemic off-target effects. AZD2373, an ASO that inhibits APOL1 protein synthesis, has completed a Phase 1 trial. In this investigation of healthy male volunteers of West African descent, repeated doses of AZD2373 were safe and well-tolerated, resulting in a dose- and time-dependent drop in plasma APOL1 levels. Based on these positive Phase 1 results, AstraZeneca plans to start Phase 2 trials in 2025. If successful, AZD2373 could be the first precision treatment for APOL1-mediated kidney disease (AMKD).

5.6. CRISPR-Cas9 Technology

CRISPR-associated protein 9 (CRISPR-Cas9) technology has the ability to fix APOL1 pathogenic mutations at the DNA level [69]. Precise editing of G1 and G2 variations back to the non-risk G0 allele will eliminate APOL1’s deleterious gain-of-function traits while maintaining its physiological roles. Although still in experimental stages, this approach could provide a one-time, long-lasting solution for individuals with HR genotypes, potentially preventing disease onset or progression. Research involving zebrafish embryos indicated that translational suppression or CRISPR/Cas9 genome editing of APOL1 led to podocyte loss and glomerular filtration defects [43]. Complementation with the APOL1-G0 allele rescued the phenotype, whereas the G1 or G2 alleles did not.

6. Conclusions

This review underlines the important role of genetic variations in APOL1, especially G1 and G2 alleles, in the genesis and development of FSGS. The specific mechanisms through which these variations cause renal injury underpin the need for focused treatment approaches. Recent breakthroughs, such as small-molecule inhibitors, antisense oligonucleotides, and gene-editing technologies, offer intriguing strategies to alleviate the negative effects of APOL1 variations. Furthermore, new therapies targeting lipid metabolism, mitochondrial dysfunction, and inflammatory pathways, such as the JAK-STAT and STING pathways, have the potential to significantly alter treatment paradigms.

Yet despite these successes, several notable knowledge gaps persist, the research of which is very essential. First, the specific molecular pathways through which APOL1 mutations cause podocyte injury and other renal cell dysfunctions must be further elucidated. The interaction between the genetic predisposition and “second hits” like diseases and environmental stressors should be understood in detail for identification and prevention. Second, while recent therapeutic advances appear promising, long-term efficacy and safety in varied groups must be verified. Clinical trials should be more geographically and demographically diverse, particularly in areas where APOL1 mutations are prevalent. Furthermore, protective modifiers such as the p.N264K variation, which may play important roles in disease risk modulation, could open the way for more precise risk assessment and targeted therapy. Third, translating genetic knowledge into routine clinical applications is difficult. The implementation of APOL1 variant genetic screening in routine clinical care in nephrology will necessitate clear guidelines, solid infrastructure, physician education, and relevant patient education to enable proper counseling. Finally, there is a significant gap in knowledge of the broader systemic impacts of the APOL1 variations themselves, such as effects on cardiovascular health, immune response, and pregnancy outcome.

Closing these knowledge gaps will, therefore, allow the development of detailed, personalized care plans to improve outcomes in those individuals at risk of APOL1-associated nephropathy. Overcoming these challenges, along with present progresses, will bring the specialty closer to the delivery of precision medicine solutions that not only will slow the progression of a disease but even avoid its start and decrease associated complications. This dual focus on treatment and prevention will be crucial in the reduction of burden from APOL1 nephropathy and the advancement of health equity for disproportionately affected populations.