Copy Number Variation at the APOL1 Locus

The 14th International Podocyte Conference 2023: from podocyte biology to glomerular medicine

The 14th International Podocyte Conference took place in Philadelphia, Pennsylvania, USA from May 23 to 26, 2023. It commenced with an early-career researchers' meeting on May 23, providing young scientists with a platform to present and discuss their research findings. Throughout the main conference, 29 speakers across 9 sessions shared their insights on podocyte biology, glomerular medicine, novel technologic advancements, and translational approaches. Additionally, the event featured 3 keynote lectures addressing engineered chimeric antigen receptor T cell- and mRNA-based therapies and the use of biobanks for enhanced disease comprehension. Furthermore, 4 brief oral abstract sessions allowed scientists to present their findings to a broad audience. The program also included a panel discussion addressing the challenges of conducting human research within the American Black community. Remarkably, after a 5-year hiatus from in-person conferences, the 14th International Podocyte Conference successfully convened scientists from around the globe, fostering the presentation and discussion of crucial research findings, as summarized in this review. Furthermore, to ensure continuous and sustainable education, research, translation, and trial medicine related to podocyte and glomerular diseases for the benefit of patients, the International Society of Glomerular Disease was officially launched during the conference.

SMOC2 gene interacts with APOL1 in the development of end-stage kidney disease: A genome-wide association study

Background

Some but not all African-Americans (AA) who carry APOL1 nephropathy risk variants (APOL1) develop kidney failure (end-stage kidney disease, ESKD). To identify genetic modifiers, we assessed gene-gene interactions in a large prospective cohort of the REasons for Geographic and Racial Differences in Stroke (REGARDS) study.

Methods

Genotypes from 8,074 AA participants were obtained from Illumina Infinium Multi-Ethnic AMR/AFR Extended BeadChip. We compared 388 incident ESKD cases with 7,686 non-ESKD controls, using a two-locus interaction approach. Logistic regression was used to examine the effect of APOL1 risk status (using recessive and additive models), single nucleotide polymorphism (SNP), and APOL1^*SNP interaction on incident ESKD, adjusting for age, sex, and ancestry. APOL1 ^*SNP interactions that met the threshold of 1.0 × 10^-5 were replicated in the Genetics of Hypertension Associated Treatment (GenHAT) study (626 ESKD cases and 6,165 controls). In a sensitivity analysis, models were additionally adjusted for diabetes status. We conducted additional replication in the BioVU study.

Results

Two APOL1 risk alleles prevalence (recessive model) was similar in the REGARDS and GenHAT studies. Only one APOL1-SNP interaction, for rs7067944 on chromosome 10, ~10 KB from the PCAT5 gene met the genome-wide statistical threshold (P _interaction = 3.4 × 10^-8), but this interaction was not replicated in the GenHAT study. Among other relevant top findings (with P _interaction < 1.0 × 10^-5), a variant (rs2181251) near SMOC2 on chromosome six interacted with APOL1 risk status (additive) on ESKD outcomes (REGARDS study, P _interaction =5.3 × 10^-6) but the association was not replicated (GenHAT study, P _interaction = 0.07, BioVU study, P _interaction = 0.53). The association with the locus near SMOC2 persisted further in stratified analyses. Among those who inherited ≥1 alternate allele of rs2181251, APOL1 was associated with an increased risk of incident ESKD (OR [95%CI] = 2.27[1.53, 3.37]) but APOL1 was not associated with ESKD in the absence of the alternate allele (OR [95%CI] = 1.34[0.96, 1.85]) in the REGARDS study. The associations were consistent after adjusting for diabetes.

Conclusion

In a large genome-wide association study of AAs, a locus SMOC2 exhibited a significant interaction with the APOL1 locus. SMOC2 contributes to the progression of fibrosis after kidney injury and the interaction with APOL1 variants may contribute to an explanation for why only some APOLI high-risk individuals develop ESKD.

Recessive, gain-of-function toxicity in an APOL1 BAC transgenic mouse model mirrors human APOL1 kidney disease

People of recent sub-Saharan African ancestry develop kidney failure much more frequently than other groups. A large fraction of this disparity is due to two coding sequence variants in the APOL1 gene. Inheriting two copies of these APOL1 risk variants, known as G1 and G2, causes high rates of focal segmental glomerulosclerosis (FSGS), HIV-associated nephropathy and hypertension-associated end-stage kidney disease. Disease risk follows a recessive mode of inheritance, which is puzzling given the considerable data that G1 and G2 are toxic gain-of-function variants. We developed coisogenic bacterial artificial chromosome (BAC) transgenic mice harboring either the wild-type (G0), G1 or G2 forms of human APOL1. Expression of interferon gamma (IFN-γ) via plasmid tail vein injection results in upregulation of APOL1 protein levels together with robust induction of heavy proteinuria and glomerulosclerosis in G1/G1 and G2/G2 but not G0/G0 mice. The disease phenotype was greater in G2/G2 mice. Neither heterozygous (G1/G0 or G2/G0) risk variant mice nor hemizygous (G1/-, G2/-) mice had significant kidney injury in response to IFN-γ, although the heterozygous mice had a greater proteinuric response than the hemizygous mice, suggesting that the lack of significant disease in humans heterozygous for G1 or G2 is not due to G0 rescue of G1 or G2 toxicity. Studies using additional mice (multicopy G2 and a non-isogenic G0 mouse) supported the notion that disease is largely a function of the level of risk variant APOL1 expression. Together, these findings shed light on the recessive nature of APOL1-nephropathy and present an important model for future studies.

APOL1 risk genotype in European steroid-resistant nephrotic syndrome and/or focal segmental glomerulosclerosis patients of different African ancestries

BACKGROUND

Apolipoprotein L1 (APOL1) risk variants are strongly associated with sporadic focal segmental glomerulosclerosis (FSGS) in populations with African ancestry. We determined the frequency of G1/G2 variants in steroid-resistant nephrotic syndrome (SRNS)/FSGS patients with African or French West Indies ancestry in France and its relationships with other SRNS genes.

METHODS

In a cohort of 152 patients (139 families), the APOL1 risk variants were genotyped by direct Sanger sequencing and pathogenic mutations were screened by next-generation sequencing with a panel including 35 SRNS genes.

RESULTS

The two risk allele [high-risk (HR)] genotypes were found in 43.1% (66/152) of subjects compared with 18.9% (106/562) in a control population (P < 0.0001): 33 patients homozygous for APOL1 G1 alleles, 4 homozygous for G2 and 29 compound heterozygous for G1 and G2. Compared with patients in the low-risk (LR) group, patients in the HR group were more likely to originate from the French West Indies than from Africa [45/66 (68.2%) versus 30/86 (34.9%); P < 0.0001]. There were more familial cases in the HR group [27 (41.5%) versus 8 (11.4%); P < 0.0001]. However, causative mutations in monogenic SRNS genes were found in only 1 patient in the HR group compared with 16 patients (14 families) in the LR group (P = 0.0006). At diagnosis, patients in the HR group without other mutations were more often adults [35 (53.8%) versus 19 (27.1%); P = 0.003] and had a lower estimated glomerular filtration rate (78.9 versus 98.8 mL/min/1.73 m2; P = 0.02).

CONCLUSIONS

The HR genotype is frequent in FSGS patients with African ancestry in our cohort, especially in those originating from the West Indies, and confer a poor renal prognosis. It is usually not associated with other causative mutations in monogenic SRNS genes.

Effect of a Single Apolipoprotein L1 Gene Nephropathy Variant on the Risk of Advanced Lupus Nephritis in Brazilians

OBJECTIVE

Apolipoprotein L1 gene (APOL1) G1 and G2 renal risk alleles (RRA) are associated with endstage renal disease in blacks with lupus nephritis (LN). The present study determined frequencies of APOL1 RRA in nonwhite Brazilian patients with LN and controls to assess association with renal outcomes.

METHODS

APOL1 RRA were genotyped in 222 healthy blood donors (controls) and 201 cases with LN from 3 outpatient clinics. Two single-nucleotide polymorphisms in the G1 (rs73885319 and rs60910145) and an indel for the G2 (rs71785313) variant were genotyped.

RESULTS

The frequency of APOL1 RRA in nonwhite Brazilian LN cases did not differ significantly from healthy controls, and few participants had 2 RRA. In the sample, 84.6% of LN cases and 84.2% of controls had 0 RRA, 13.4% and 15.3% had 1 RRA, and 2.0% and 0.4% had 2 RRA, respectively. LN cases with ≥ 1 APOL1 RRA had similar baseline characteristics and renal responses to treatment, yet faced higher risk for progressive chronic kidney disease (CKD) to an estimated glomerular filtration rate < 30 ml/min/1.73 m² compared to those with 0 RRA (11.2% with 0, 29.6% with 1; 50% with 2 RRA, p = 0.005). Although glomerular lesions and activity scores on initial kidney biopsy did not differ significantly between individuals based on APOL1 genotype, chronicity scores, tubular atrophy, and interstitial fibrosis were more severe in those with ≥ 1 RRA (p = 0.011, p = 0.002, p = 0.018, respectively).

CONCLUSION

Although initial kidney lesions and treatment responses were similar, a single APOL1 RRA in nonwhite Brazilians with LN was associated with increased risk of advanced CKD and possibly more tubulointerstitial damage.

Dilemmas and challenges in apolipoprotein L1 nephropathy research

PURPOSE OF REVIEW

The purpose of this mini-review is to highlight some unresolved questions and controversies in the evolving story of apolipoprotein L1 (APOL1) nephropathy.

RECENT FINDINGS

We highlight studies that introduce complexity in unraveling the mechanisms whereby APOL1 risk variant alleles cause disease. These include studies which support a possible protective role for the APOL1 GO nonrisk ancestral allele, and studies which explore the initiating events that may trigger other downstream pathways mediating APOL1 cellular injury. We also review studies that reconcile the perplexing findings regarding APOL1 anionic or cationic conductance, and pH dependency, and also studies that attempt to characterize the 3-dimensional structure of APOL1 C-terminal in APOL1 variants, as well as that of the serum resistance-associated protein. We also attempt to convey new insights from in-vivo and in-vitro models, including studies that do not support the differential toxicity of APOL1 renal risk variants and recapitulate the clinical variability of individuals at genotypic risk.

SUMMARY

Along with major progress that had been achieved in the field of APOL1 nephropathy, controversies and enigmatic issues persist. It remains to be determined which of the pathways which have been demonstrated to mediate cell injury by ectopically expressed APOL1 risk variants in cellular and organismal models are relevant to human disease and can pave the way to potential therapy.

Apolipoprotein L1 nephropathies: 2017 in review

PURPOSE OF REVIEW

To review publications relating to apolipoprotein L1 (APOL1) renal risk variants published 2017.

RECENT FINDINGS

The study of APOL1 variants continues to be highly active; 24 articles published in 2017 were selected to highlight. These include clinical studies of kidney disease, kidney transplantation, hypertension, cardiovascular disease, and genetic diversity. Laboratory studies included APOL1 association with vesicle-associated membrane soluble N-ethylmaleimide-sensitive factor activating protein receptor protein and with soluble urokinase-type plasminogen activator receptor, mitochondrial dysfunction, endolysosomal dysfunction, and inflammasome activation.

SUMMARY

Our understanding of the role of APOL1 genetic variants and the mechanisms for renal toxicity continues to deepen. It is not yet clear which pathways are most relevant to human disease, and so, the most relevant drug targets remain to be defined.

APOL1 Nephropathy: A Population Genetics and Evolutionary Medicine Detective Story

Common DNA sequence variants rarely have a high-risk association with a common disease. When such associations do occur, evolutionary forces must be sought, such as in the association of apolipoprotein L1 (APOL1) gene risk variants with nondiabetic kidney diseases in populations of African ancestry. The variants originated in West Africa and provided pathogenic resistance in the heterozygous state that led to high allele frequencies owing to an adaptive evolutionary selective sweep. However, the homozygous state is disadvantageous and is associated with a markedly increased risk of a spectrum of kidney diseases encompassing hypertension-attributed kidney disease, focal segmental glomerulosclerosis, human immunodeficiency virus nephropathy, sickle cell nephropathy, and progressive lupus nephritis. This scientific success story emerged with the help of the tools developed over the past 2 decades in human genome sequencing and population genomic databases. In this introductory article to a timely issue dedicated to illuminating progress in this area, we describe this unique population genetics and evolutionary medicine detective story. We emphasize the paradox of the inheritance mode, the missing heritability, and unresolved associations, including cardiovascular risk and diabetic nephropathy. We also highlight how genetic epidemiology elucidates mechanisms and how the principles of evolution can be used to unravel conserved pathways affected by APOL1 that may lead to novel therapies. The APOL1 gene provides a compelling example of a common variant association with common forms of nondiabetic kidney disease occurring in a continental population isolate with subsequent global admixture. Scientific collaboration using multiple experimental model systems and approaches should further clarify pathomechanisms further, leading to novel therapies.

Divergent patterns of genic copy number variation in KCNIP1 gene reveal risk locus of type 2 diabetes in Chinese population

Copy number variation (CNV) has emerged as another important genetic marker in addition to SNP for understanding etiology of complex disease. Kv channel interacting protein 1 (KCNIP1) is a Ca²⁺-dependent transcriptional modulator that contributes to the regulation of insulin secretion. Previous genome-wide CNV assay identified the KCNIP1 gene encompassing a CNV region, however, its further effect and risk rate on type 2 diabetes (T2D) have rarely been addressed, especially in Chinese population. The current study aims to detect and excavate genetic distribution profile of KCNIP1 CNV in Chinese T2D and control populations, and further to investigate the associations with clinical characteristics. Divergent patterns of the KCNIP1 CNV were identified (p < 0.01), in which the copy number gain was predominant in T2D, while the copy number normal accounted for the most in control group. Consistently, the individuals with copy number gain showed significant risk on T2D (OR = 4.550, p < 0.01). The KCNIP1 copy numbers presented significantly positive correlations with fasting plasma glucose and glycated hemoglobin in T2D. For OGTT test, the T2D patients with copy number gain had remarkably elevated glucose contents (60, 120, 180-min, p < 0.05 or p < 0.01) and diminished insulin levels (60, 120-min, p < 0.05) than those with copy number loss and normal, which suggested that the KCNIP1 CNV was correlated with the glucose and insulin action. This is the first CNV association study of the KCNIP1 gene in Chinese population, and these data indicated that KCNIP1 might function as a T2D-susceptibility gene whose dysregulation alters insulin production.

APOL1 variants change C-terminal conformational dynamics and binding to SNARE protein VAMP8

APOL1 variants in African populations mediate resistance to trypanosomal infection but increase risk for kidney diseases through unknown mechanisms. APOL1 is expressed in glomerular podocytes and does not vary with underlying kidney disease diagnoses or APOL1 genotypes, suggesting that the kidney disease-associated variants dysregulate its function rather than its localization or abundance. Structural homology searches identified vesicle-associated membrane protein 8 (VAMP8) as an APOL1 protein interactor. VAMP8 colocalizes with APOL1 in the podocyte, and the APOL1:VAMP8 interaction was confirmed biochemically and with surface plasmon resonance. APOL1 variants attenuate this interaction. Computational modeling of APOL1's 3-dimensional structure, followed by molecular dynamics simulations, revealed increased motion of the C-terminal domain of reference APOL1 compared with either variant, suggesting that the variants stabilize a closed or autoinhibited state that diminishes protein interactions with VAMP8. Changes in ellipticity with increasing urea concentrations, as assessed by circular dichroism spectroscopy, showed higher conformational stability of the C-terminal helix of the variants compared with the reference protein. These results suggest that reference APOL1 interacts with VAMP8-coated vesicles, a process attenuated by variant-induced reduction in local dynamics of the C-terminal. Disordered vesicular trafficking in the podocyte may cause injury and progressive chronic kidney diseases in susceptible African Americans subjects.

Does copy number variation of APOL1 gene affect the susceptibility to focal segmental glomerulosclerosis?

Background: APOL1 risk variants (G1 and G2) are associated with increased susceptibility to focal segmental glomerulosclerosis (FSGS) in African population. However, the two risk mutations were not found in Chinese FSGS patients. In this study, we explored the association between the copy number variation (CNV) of APOL1 gene and FSGS.

Methods: APOL1 copy number variations were detected by quantitative real-time PCR with TaqMan probes and compared between 133 FSGS patients and 123 controls. The association between CNV of APOL1 gene and clinical parameters was also investigated.

Results: The distribution of APOL1 CNV did not show significant difference between FSGS patients and controls. The creatinine and proteinuria in the high copy number group (CN ≥ 3) were higher than the other two groups, but the difference was not significant (p > .05). The FSGS pathological types were different among the three groups.

Conclusion: There was no significant difference in the distribution of APOL1 gene copy variants between FSGS patients and normal controls, and there was no significant correlation between the APOL1 gene CNV and the FSGS patients’ clinical manifestations. APOL1 CNVs may be not associated with susceptibility to FSGS.

The evolving science of apolipoprotein-L1 and kidney disease

PURPOSE OF REVIEW

There are evolving epidemiological and biological data to support an association between the gene encoding apolipoprotein-L1 (APOL1) and progressive chronic kidney disease (CKD) among African-Americans.

RECENT FINDINGS

Individuals with two APOL1 risk alleles are at greater risk of incident albuminuria, CKD, and progression to end-stage renal disease despite optimal blood pressure management and use of angiotensin-converting enzyme inhibitors. These variants also appear to influence outcomes in donor and recipients in kidney transplantation. Recent studies have also variably shown a potential role of APOL1 variants in cardiovascular disease. A number of studies have addressed genetic and environmental factors such as HIV but most do not modify the course of APOL1-related kidney disease. Although the exact mechanism remains unclear, functional studies have demonstrated the effect of APOL1 and related protein on innate immunity and cytotoxicity.

SUMMARY

APOL1 is an important genetic risk factor for kidney disease among African-Americans. With approximately one in 10 African-Americans at risk, further studies are warranted to identify underlying biological mechanisms and other potential modifiers leading to CKD. Moreover, studies that clarify the association of APOL1 variants with cardiovascular disease, independent of CKD, are also needed.

Human genetics of nephrotic syndrome and the quest for precision medicine

PURPOSE OF REVIEW

In this review, we take a combined membrane biologist's and geneticist's view of the podocyte, to examine how genetics have informed our understanding of membrane receptors, channels, and other signaling molecules affecting podocyte health and disease.

RECENT FINDINGS

An integral part of the kidney, the glomerulus, is responsible for the kidney's filter function. Within the glomerulus, the podocyte is a unique cell serving a critically important role: it is exposed to signals from the urinary space in Bowman's capsule, it receives and transmits signals to/from the basement membrane upon which it elaborates, and it receives signals from the vascular space with which it also communicates, thus exposed to toxins, viruses, chemicals, proteins, and cellular components or debris that flow in the blood stream. Our understanding of how podocytes perform their important role has been largely informed by human genetics, and the recent revolution afforded by exome sequencing has brought a tremendous wealth of new genetic data to light.

SUMMARY

Genetically defined, rare/orphan podocytopathies, as reviewed here, are critically important to study as they may reveal the next generation targets for precision medicine in nephrology.

History of Nephrotic Syndrome and Evolution of its Treatment

The recognition, evaluation, and early treatment of nephrotic syndrome in infants and children originate from physicians dating back to Hippocrates. It took nearly another 1000 years before the condition was described for its massive edema requiring treatment with herbs and other remedies. A rich history of observations and interpretations followed over the course of centuries until the recognition of the combination of clinical findings of foamy urine and swelling of the body, and measurements of urinary protein and blood analyses showed the phenotypic characteristics of the syndrome that were eventually linked to the early anatomic descriptions from first kidney autopsies and then renal biopsy analyses. Coincident with these findings were a series of treatment modalities involving the use of natural compounds to a host of immunosuppressive agents that are applied today. With the advent of molecular and precision medicine, the field is poised to make major advances in our understanding and effective treatment of nephrotic syndrome and prevent its long-term sequelae.

Re-Sequencing of the APOL1-APOL4 and MYH9 Gene Regions in African Americans Does Not Identify Additional Risks for CKD Progression

BACKGROUND

In African Americans (AAs), APOL1 G1 and G2 nephropathy risk variants are associated with non-diabetic end-stage kidney disease (ESKD) in an autosomal recessive pattern. Additional risk and protective genetic variants may be present near the APOL1 loci, since earlier age ESKD is observed in some AAs with one APOL1 renal-risk variant, and because the adjacent gene MYH9 is associated with nephropathy in populations lacking G1 and G2 variants.

METHODS

Re-sequencing was performed across a ∼275 kb region encompassing the APOL1-APOL4 and MYH9 genes in 154 AA cases with non-diabetic ESKD and 38 controls without nephropathy who were heterozygous for a single APOL1 G1 or G2 risk variant.

RESULTS

Sequencing identified 3,246 non-coding single nucleotide polymorphisms (SNPs), 55 coding SNPs, and 246 insertion/deletions. No new coding variations were identified. Eleven variants, including a rare APOL3 Gln58Ter null variant (rs11089781), were genotyped in a replication panel of 1,571 AA ESKD cases and 1,334 controls. After adjusting for APOL1 G1 and G2 risk effects, these variations were not significantly associated with ESKD. In subjects with <2 APOL1 G1 and/or G2 alleles (849 cases; 1,139 controls), the APOL3 null variant was nominally associated with ESKD (recessive model, OR 1.81; p = 0.026); however, analysis in 807 AA cases and 634 controls from the Family Investigation of Nephropathy and Diabetes did not replicate this association.

CONCLUSION

Additional common variants in the APOL1-APOL4-MYH9 region do not contribute significantly to ESKD risk beyond the APOL1 G1 and G2 alleles.