Worsened Outcome in Patients with Membranous Nephropathy Carrying Apolipoprotein L1 High-Risk Genotype

Membranous nephropathy is a type of kidney disease that is characterized by the thickening of the basement membranes in the glomeruli. This thickening is caused by the accumulation of immune complexes.^1,2 The immune complexes can be seen under a light microscope as subepithelial immunoglobulin-containing deposits. Immune complexes activate the C5b–C9 complement components, which form a membrane attack complex on the glomerular epithelial cells and contribute to the pathology of the disease. Membranous nephropathy is caused by autoimmune reactions against certain antigens in the kidneys, such as phospholipase A2 receptor (PLA2R) antibodies.³ This antibody is responsible for approximately 60%–70% of primary membranous nephropathy cases, with an additional 3%–5% caused by antibodies against thrombospondin type 1 domain-containing 7A; however, many additional antigens have been discovered.⁴ Testing for anti-PLA2R can be helpful in diagnosing and treating the disease as well as predicting the risk of disease progression and the likelihood of spontaneous remission by tracking the titer level over time.

A recent genome-wide association study (GWAS) investigated the genetic factors that may contribute to the development of primary membranous nephropathy in more than 3000 individuals.⁵ The study identified two previously unknown genetic loci (nuclear factor kappa beta subunit 1 and interferon regulatory factor 4) that are associated with membranous nephropathy and provided more information about the PLA2R locus, which has previously been linked to the disease. The study also found that three specific human leukocyte antigen alleles have different effects on the risk of developing membranous nephropathy depending on the individual's ethnicity. The results of the GWAS showed that these genetic risk loci explain a significant portion of the risk of developing membranous nephropathy, with 32% of the risk explained in East Asians and 25% in Europeans. It is possible that the high frequencies of membranous nephropathy risk alleles may be because membranous nephropathy often occurs after the peak reproductive age, allowing the risk alleles to escape purifying selection or because the alleles may have protective effects against common infections.

Apolipoprotein L1 (APOL1) G1 and G2 are variations of the APOL1 gene that are more commonly found in certain populations of West Africa, where they may have evolved as a protective mechanism against trypanosomiasis, a parasitic disease transmitted by the tsetse fly.⁶ These variations have been linked to a higher risk of developing kidney disease in people of African ancestry. The APOL1 G1 and G2 variants are associated with different types of kidney disease, including collapsing FSGS, HIV-associated FSGS, coronavirus disease 2019 (COVID-19)–associated FSGS, and hypertensive kidney disease. The presence of the high-risk APOL1 genotype is associated with a 2–100 times higher risk of kidney failure, but the lifetime risk of developing the disease is only around 20%. A two-hit model has been proposed to explain this excess kidney disease risk, with the first hit being the underlying genetic susceptibility conferred by the APOL1 gene and the second hit being an inflammatory trigger, such as COVID-19 or HIV, that increases APOL1 expression, mostly by increased interferon release⁷ (Figure 1). In vitro studies have shown that APOL1-induced toxicity is highly dependent on the genotype and dose and that APOL1 transcription is regulated by STAT and interferon regulatory factor transcription factors. The expression of APOL1 is higher after cytokines, such as interferon and tumor necrosis factor, administration.

Figure 1.

The model for APOL1-associated kidney disease development. The genotype represents the first hit. Higher APOL1 expression by COVID or infection represents a second hit, and the cell type expression and podocytes, endothelial cells, and liver cells represent the third hit. The exact mechanism by which APOL1 contributes to the development of diseases is not yet fully understood, but it has been proposed that APOL1 may disrupt a process called mitophagy, which is the selective degradation of mitochondria. This disruption may lead to the release of cytosolic mitochondrial DNA, which can then activate inflammation pathways such as the inflammasome (NLRP3) and the cytosolic nucleotide sensing pathway (STING). APOL1, apolipoprotein L1; cGAS, cyclic GMP-AMP synthase; COVID, coronavirus disease.

One of the areas of ongoing research in the field is the phenotypic heterogeneity that is associated with the high-risk APOL1 genotype. This genotype has been linked to several different types of diseases, including sepsis, hypertension, hypertensive kidney disease, pre-eclampsia, HIV-associated nephropathy, and COVID-associated nephropathy, in addition to providing protection against trypanosomiasis. It is thought that the specific cell type in which the APOL1 gene is expressed may play a role in the development of certain diseases. For example, expression of the APOL1 gene in liver cells may be important for protection against trypanosomiasis, whereas expression in podocytes may be important for the development of glomerular disease. Similarly, expression of the APOL1 gene in endothelial cells may be important for the development of vascular diseases such as sepsis and coronavirus disease–associated vascular disease.^8,9

The exact mechanism by which APOL1 contributes to the development of diseases is not yet fully understood, but it has been proposed that APOL1 may disrupt a process called mitophagy, which is the selective degradation of mitochondria. This disruption may lead to the release of cytosolic mitochondrial DNA, which can then activate inflammation pathways such as the inflammasome and the cytosolic nucleotide sensing pathway. Studies in mice have shown that genetic deletion or pharmacological inhibition of inflammasome and cytosolic nucleotide sensing pathway can protect against the negative effects of APOL1 in endothelial cells and podocytes^8,9 (Figure 1). This suggests that targeting these pathways may be a potential approach for reducing the health disparities associated with APOL1 in sepsis and COVID-19.

In this issue of CJASN, Chen et al. studied 690 patients with membranous nephropathy, including 118 Black patients.¹⁰ Of these Black patients, 16 (14%) had the high-risk APOL1 genotype (two risk alleles), and 102 (86%) had the low-risk APOL1 genotype (zero or one risk alleles). These groups were compared with 572 non-Black patients with membranous nephropathy. The researchers analyzed histology data from biopsies of Black patients and found that those with the high-risk APOL1 genotype had a higher prevalence of global sclerosis and segmental sclerosis compared with those with the low-risk genotype. In addition, a higher proportion of high-risk patients had FSGS lesions (71.4%) compared with low-risk patients. These findings suggest that the APOL1 genotype may be associated with certain histological features of membranous nephropathy in Black patients.

In their study, Chen et al. found that among patients with membranous nephropathy, those with the high-risk APOL1 genotype had a higher risk of developing kidney failure during the follow-up period compared with those with the low-risk APOL1 genotype and non-Black patients. The high-risk APOL1 group also had a faster time to develop kidney failure compared with the other groups. In addition, the high-risk APOL1 group had significantly worse eGFR trajectories compared with the other groups after the first year of diagnosis. The eGFR slope was much worse in the high-risk APOL1 group (−16 ml/min per 1.73 m² per year) compared with the group with one APOL1 risk allele (−3 ml/min per 1.73 m²), the group with no APOL1 risk alleles (−4 ml/min per 1.73 m²), and the non-Black group (−2 ml/min per 1.73 m²). These differences in eGFR slope remained significant even after adjusting for age, sex, and baseline eGFR.

There are several limitations to the study by Chen et al. that should be considered. One significant limitation is the small sample size, with only 16 patients having the high-risk APOL1 genotype. It is unclear if the patients were chosen randomly or if there is bias in the sample selection because it is possible that patients with more severe disease may have been more likely to undergo biopsy. In addition, it is worth noting that membranous nephropathy is more commonly reported in Europeans, and it is not clear whether the cases analyzed in this study were pure membranous cases or if they were confounded by FSGS, which can sometimes be misdiagnosed. These limitations should be considered when interpreting the findings of the study.

In summary, this study is an important step toward the goal of molecular precision nephrology because it shows that genetic and antibody-based diagnosis of PLA2R antibodies can be useful for diagnosing membranous nephropathy. In particular, genotyping that takes into account the APOL1 high-risk genotype in Black Americans can improve diagnostic accuracy. Future studies should further examine the prognostic value of these diagnostic markers and use this information to develop genotype-based precision therapies. This approach has the potential to improve the management and treatment of membranous nephropathy and other kidney diseases and to ultimately improve patient outcomes.

Disclosures

K. Susztak reports consultancy agreements with AstraZeneca, GSK, Jnana, Novo Nordisk, Pfizer, and Ventus; ownership interest in Jnana; research funding from AstraZeneca, Bayer, Boehringer Ingelheim, Calico, Genentech, Gilead, GSK, Jnana, Lilly, Maze, Merck, Novartis, Novo Nordisk, Regeneron, Variant Bio, and Ventus; honoraria from AstraZeneca, Bayer, Jnana, Maze, and Pfizer; serving on the editorial boards of Cell Metabolism, eBioMedicine, JASN, Journal of Clinical Investigation, Kidney International, and Med; and serving in an advisory or leadership role for Jnana and Pfizer.

Funding

NIDDKD grants DK076077, DK105821, and DK132630.

Author Contributions

K. Susztak conceptualized the study, provided supervision, was responsible for visualization, wrote the original draft, and reviewed and edited the manuscript.