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) 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. 2) We are now investigating the role of genetic role of childhood-onset nephrotic syndrome in Indian and Black children in in South Africa. We find that APOL1 does not contribute to childhood nephrotic syndrome; however, we did find that sporadic steroid resistant nephrotic syndrome (SRNS) in Durban, South Africa, nearly a third 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. This study as been submitted for review. 3) 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 withwhites, 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. 4) 6) We assessed correlations between APOL1 profiles and renal histological features in a of individuals who experienced sudden death. Glomerular number and mean glomerular volume were measured by our collaborators in kidneys of blacks and whites 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 blacks. This study was published in the Journal of the American Society of Nephrologists. 5) Only 15% of carriers of two APOL1 high risk genotypes develop chronic kidney disease or progress to kidney failure, suggesting that other genetic or environmental factor is required for disease initiation and progression. In a collaborative study with investigators at NIDDK and the University of Virginia, we found that the GSTM1 null allele, which previously has been shown to associate with CKD progression to kidney failure or death, potentiates progression to ESRD in APOL1 high-risk participants. The GSTM1 gene product modulates oxidative stress. Having 0 or only 1 copy of the GSTM1 gene amplified the effect of the high-risk APOL1 genotypes in black patients with chronic kidney disease attributed to hypertension enrolled in the AASK clinical trial. We were unable to determine if carriage of 2 GSTM1 alleles would be protective because of limited numbers of individuals. Larger cohorts are needed to further explore the interactions between GSTM1 null and APOL1 high-risk genotypes (published in Journal of the American Society of Nephrology).
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