Chronic kidney disease (CKD), affecting over 26 million Americans, frequently leads to kidney failure. More than 100,000 individuals develop end stage kidney disease (ESKD) annually and nearly 500,000 receive kidney transplants or are ongoing dialysis patients at an annual cost of $30 billion dollars. Previously, we used admixture mapping to localize a region on chromosome 22 associated with focal segmental glomerulosclerosis (FSGS) and HIV-associated nephropathy (HIVAN). Subsequently, we and others showed that APOL1 coding variants within this region comprising 2 missense variants in absolute linkage disequilibrium (G1 allele) and an in frame 6 base pair deletion (G2 allele) were responsible for the association, with OR of 7, 19, and 27 for hypertensive ESKD, focal segmental glomerulosclerosis (FSGS), and HIV-associated nephropathy (HIVAN), respectively. ApoL1 provides protection against infection with Trypanosoma brucei brucei. The APOL1 risk alleles emerged recently in sub-Saharan Africa, but are found in other regions of the world as a result of the African Diaspora. The combined frequencies of G1 and G2 alleles are approximately 35% in African Americans. These alleles explain nearly all the excess risk of kidney disease in African Americans, thus providing a genetic basis for a major global health disparity. We have continued our studies of APOL1 to determine if the risk variants are associated with other non-renal or renal phenotypes that show racial disparities, such as papillary renal cancer and cardiovascular disease, in collaborative studies with intramural and extramural investigators. Accomplishments 1) We have entered into collaborations with researchers at the University of Chicago and Wayne State University to investigate the role of APOL1 variants on development of hypertension, chronic kidney disease, and kidney survival in the living donor. We found that more than half of former donors with kidney failure carried two APOL1 risk alleles (57%) compared to 13% in the general population. This result is confounded by the fact that the majority of living donors were first-degree relatives of the recipients and therefore more likely to carry APOL1 risk variants. It was reported by others that amongst African Americans with ESRD and 2 risk alleles, 23% had a first degree relative with ESRD. 2) About 20% of patients with primary FSGS or HIV-associated nephropathy carry 1 or 0 copies of APOL1 G1 or G2, suggesting that additional factors in APOL1 might contribute to disease. In an exhaustive study of more than 2000 people world wide, including over a 1000 case and controls for FSGS and HIV-associated nephropathy (HIVAN), we showed that no other common variants lysed trypanosomes and no additional common or rare variants were associated with FSGS or HIVAN using association and burden tests. The immediate relevance of this study is that there is no clinical utility in sequencing APOL1 in patients with FSGS or HIVAN who do not carrying G1 or G2 renal risk variants. This data was published in Kidney International. 3) In a international study with researchers in South Africa we have shown that APOL1 variants are strongly associated with HIVAN, a rapidly progressive kidney disease (OR 89).This manuscript was published in the Journal of the American Society of Nephrology. We are now investigating the role of APOL1 in pediatric HIV disease in collaboration with investigators in South Africa. 4) We have shown that in black children, but not Asian children, with sporadic steroid resistant nephrotic syndrome (SRNS) in Durban, South Africa, more than 20% are homozygous for a single mutation in the podocin gene (NPHS2). This study indicates that for the 30% of children with nephrotic syndrome carrying two copies of the NPHS2 variant, a precision diagnosis of steroid resistant focal segmental glomerulosclerosis can be made, abrogating the need for a renal biopsy or ineffective and potentially harmful steroid therapy. 5) We investigated the role of APOL1 on kidney function and albuminuria, a predictor of chronic kidney disease and cardiovascular disease in the Coronary Artery Risk Development in Young Adults study. We examined associations of APOL1 with incident albuminuria and kidney function decline among 3030 young adults with preserved glomerular filtration rate. The incidence rate per 1000 person-years for albuminuria, a predictor of chronic kidney disease and cardiovascular disease over 15 years follow-up was 15.6 for APOL1 high-risk genotypes, 7.8 for low-risk blacks, and 3.9 for whites. Compared with whites, the odds ratio for incident albuminuria was 5.71 ) for high-risk blacks and 2.32 for low-risk blacks. Adjustment for risk factors attenuated the difference between low-risk blacks and whites. APOL1 low-risk blacks also had a faster yearly decline in kidney function compared with whites, but this difference was attenuated after adjustment for risk factors and socioeconomic position. We found that blacks with two APOL1 risk alleles had the highest risk for albuminuria and kidney decline in young adulthood, whereas disparities between low-risk blacks and whites were related to differences in traditional risk factors. This study was published in Journal of the American Society of Nephrology. 6) We assessed correlations between APOL1 profiles and renal histological features in subjects without renal disease. Glomerular number and mean glomerular volume were measured by our collaborators in kidneys of African-American without renal disease, undergoing autopsies in Jackson, Mississippi. We found that carriage of APOL1 variants was associated with significant reductions in glomerular number and increases in glomerular volume with increasing age. Regression analysis predicted an annual average loss of 8834 glomeruli per single kidney over the first 38 years of adult life in African Americans with two risk alleles. These findings indicate that APOL1 risk alleles are associated with exaggerated age-related nephron loss, probably decaying from a larger pool of smaller glomeruli in early adult life, along with enlargement of the remaining glomeruli. The observations suggest a mechanism of accentuated susceptibility to kidney disease in APOL1-positive African Americans.This study was published in the Journal of the American Society of Nephrologists. 6) To better define the phenotype of APOL1-associated nephropathy we investigated children and young adults enrolled in the FSGS Clinical Trial which followed 138 children and young adults who were randomized to cyclosporin or mycophenolate mofetil plus pulse oral dexamethasone with a primary outcome of proteinuria remission. We found that two APOL1 risk alleles were present in 27 subjects, of whom four subjects did not self-identify as African American, and 23 of 32 (72%) self-identified African Americans. Individuals with the APOL1 risk genotype tended to present at an older age and had significantly lower baseline kidney function, more segmental glomerulosclerosis and total glomerulosclerosis, and more tubular atrophy/interstitial fibrosis. There were differences in renal histology, particularly more collapsing variants in those with the risk genotype. APOL1 risk genotype did not affect response to either treatment regimen. Individuals with the risk genotype were more likely to progress to ESRD. In conclusion, APOL1 risk genotypes are common in African-American subjects with primary FSGS and may also be present in individuals who do not self-identify as African American. APOL1 risk status was associated with lower kidney function, more glomerulosclerosis and interstitial fibrosis, and greater propensity to progress to ESRD. Consistent with other studies, we found that APOL1 risk genotype did not influence proteinuria responses to cyclosporin or mycophenolate mofetil/dexamethasone.
Showing the most recent 10 out of 71 publications
