EDITORIALS www.jasn.org UP FRONT MATTERS

Apolipoprotein L1 and the a broader block of genomic sequence that exhibits features such as linkage disequilibrium, suggesting maintenance Genetic Basis for Racial through evolutionary pressure. APOL1 variants associated with FSGS were not interrogated in the original genetic Disparity in Chronic Kidney mapping studies because these variants were not found in Disease databases available at that time through the international haplotype mapping (HapMap) project. Subsequently, data Susan E. Quaggin* and Alfred L. George, Jr.† produced by the 1000 Genomes project annotated APOL1 *Department of Medicine, St. Michael’s Hospital and University variants, including a compound missense allele (glycine-342/ Health Network, and The Samuel Lunenfeld Research Institute, methionine-384; designated G1) and an in-frame deletion † University of Toronto, Toronto, Ontario, Canada; and Division of (deletion of asparagine-388 and tyrosine-389; designated Genetic Medicine, Department of Medicine, and Institute for Inte- grative Genomics, Vanderbilt University, Nashville, Tennessee G2), which emerged as the critical markers for risk of disease within this interval. Moreover, G1 and G2 were mutually J Am Soc Nephrol 22: 1955–1958, 2011. exclusive, never being observed together on the same chro- doi: 10.1681/ASN.2011090932 mosomal copy. These advances helped demonstrate that cod- ing sequence variants within APOL1 accounted for a large Persons of African ancestry living in the United States carry a fraction of observed FSGS risk in African Americans. How- disproportionate burden of chronic kidney disease (CKD). ever, demonstrating associations with other renal disease Compared with Americans of European ancestry, African phenotypes was needed, as was clarity about the potential Americans have an approximately fourfold greater life time biologic contribution of APOL1 to pathogenesis of renal dis- risk of ESRD and, on average, require initiation of renal re- ease. placement therapy at a younger age. This group also has a In this issue of JASN, five new studies addressing the ge- unique risk of HIV-associated nephropathy (HIVAN). Un- netics, clinical importance, and biology of APOL1-associated raveling the basis for these racial disparities in CKD offers renal disease in African Americans will help advance our un- opportunities to understand its pathogenesis, to identify bio- derstanding of this problem.5–9 markers of risk, and to conceive of new treatments or preven- In two of the articles, investigators examined the associa- tive strategies. tion of APOL1 variants with HIVAN.5,6 In previous studies, Recent progress has been made in identifying genomic risk of HIVAN in African Americans was associated with the factors that explain the excessive chronic kidney disease aforementioned MYH9 variants, but in light of the new find- (CKD) risk in nondiabetic African Americans. In 2008, two ings implicating APOL1 coding alleles as the functional vari- groups demonstrated highly significant associations of mark- ants responsible for FSGS risk, it became important to deter- ers on human chromosome 22q with idiopathic focal seg- mine if HIVAN risk was genetically similar or whether MYH9 mental glomerulosclerosis (FSGS), HIV-associated nephrop- alleles were still relevant. Kopp et al.2 report results of a case- athy (HIVAN), and nondiabetic end-stage renal disease control genetic association study examining the importance (ESRD) in African Americans.1,2 The strongest association of APOL1 variants (G1, G2) to HIVAN risk by comparing was centered on genomic variants within MYH9 encoding a allele and genotype frequencies in African Americans with nonmuscle myosin heavy chain expressed in glomerular biopsy-proven HIV-associated collapsing glomerulopathy podocytes. Despite the attractiveness of MYH9 as a candidate compared with HIV-infected African Americans with nor- to explain the association of 22q with CKD, further studies mal renal function. The investigators demonstrated an aston- failed to identify plausible functional variants within this ishing odds ratio (OR) of 29 for HIVAN risk, conferred by gene. In 2010, two groups reported an even stronger genetic two APOL1 risk alleles. They also found that two APOL1 risk association of FSGS in African Americans with variants in alleles confer a similarly impressive OR of 16.9 for idiopathic APOL1 encoding apolipoprotein L1.3,4 MYH9 and APOL1 FSGS, as well as an earlier age of onset and more rapid pro- are separated by a mere 14,000 nucleotides and coexist within gression toward ESRD. HIVAN and FSGS risk conformed best to a recessive inheritance model, while cases heterozy-

Published online ahead of print. Publication date available at www.jasn.org. gous for only one APOL1 risk allele had marginal or no asso- ciation with kidney disease. Papeta et al.6 report qualitatively Correspondence: Dr. Alfred L. George, Jr, Division of Genetic Medicine, 529 Light Hall, Vanderbilt University, 2215 Garland Avenue, Nashville, TN 37232- similar findings for recessive genetic association models in a 0275. Phone: 615-936-2660; Fax: 615-936-2661; E-mail: al.george@ smaller HIVAN case-control study (OR ϭ 10.9), whereas ge- vanderbilt.edu netic association determined using an additive genetic effect Copyright © 2011 by the American Society of Nephrology model was also significant but less robust. The study by Pa-

J Am Soc Nephrol 22: 1955–1962, 2011 ISSN : 1046-6673/2211-1955 1955 EDITORIALS www.jasn.org

peta et al. also demonstrates that APOL1 variants are not asso- single allele carriers differs from the other studies that impli- ciated with IgA nephropathy. cate recessive genetic mechanisms in renal disease susceptibil- Together, these studies provide compelling evidence that ity, and this may imply that the rate of progression to ESRD can APOL1 confers genetic risk for HIVAN in African Americans. be independently influenced by APOL1 variants. Further evidence against a significant biologic contribution of The high carrier frequency of APOL1 renal disease risk al- MYH9 is provided by the study by Papeta et al., with dem- leles in African Americans has evolutionary origins by way of onstration of absent nephropathy in the offspring of mice natural selection for a trait protective against infection with generated by crossing HIV-1 transgenic mice, in a genetic subspecies of the protozoan parasite Trypanosoma brucei that background protective against nephropathy, with Myh9 causes sleeping sickness endemic to sub-Saharan Africa. Apo- hemizygous mice. Haploinsufficiency for Myh9 did not pro- lipoprotein L1 lyses trypanosomes by causing osmotic swelling mote overt albuminuria or glomerular histopathology, and of parasite lysosomes through a pore-mediated mechanism therefore the plausibility that this gene is the biologic culprit and renders humans resistant to infection.11,12 However, one for HIVAN seems less likely.6 However, Myh9 might contrib- trypanosome subspecies responsible for African sleeping sick- ute to other forms of CKD, as suggested by increased suscep- ness (T. b. rhodesiense) produces a virulence factor—serum tibility to doxorubicin glomerulopathy in podocyte-specific resistance associated-factor (SRA)—that neutralizes APOL1 Myh9 knockout mice.10 by binding to its C-terminus. However, both APOL1 risk vari- Although case-control association studies have become the ants (G1, G2) alter amino acid residues directly within the foundation for new discoveries in the modern genomic era, C-terminal SRA binding site thus preserving lytic activity and such studies have important liabilities that can skew results, conferring T. b. rhodesiense resistance to heterozygous carri- including ascertainment bias and mismatched control sub- ers.3 This heterozygote advantage is reminiscent of malaria re- jects. To thwart this concern for the recent discoveries regard- sistance conferred by ␤-hemoglobin mutations in sickle cell ing APOL1, Friedman and colleagues examined whether vari- disease. In both situations, the selective advantage of parasite ants in this gene associate with two proxies of CKD, resistance bestowed on heterozygous carriers creates disease- microalbuminuria and reduced estimated GFR, in the Dallas prone homozygous carriers in the population. For perspective, Heart Study, a large population-based cohort with a predom- the allele frequency of hemoglobin-S among African Ameri- inance of African American participants.8 Consistent with the cans is 5 to 10%, and this is dwarfed by the combined allele prior case-control association studies, nondiabetic carriers of frequency of APOL1 G1 and G2 risk alleles (37% in the Dallas two APOL1 variants had approximately 3 times higher rates of Heart Study8), but sickle cell disease in homozygous hemoglo- microalbuminuria and reduced GFR (Ͻ60 ml/min/1.73 m2) bin-S carriers exhibits complete penetrance. than subjects with 0 to 1 variant alleles. In further analyses, How do APOL1 variants predispose to CKD? Renal lesions rates of microalbuminuria and reduced GFR were not different associated with APOL1 risk alleles, including FSGS and HI- between nondiabetic African Americans with 0 to 1 APOL1 VAN, are characterized by glomerular podocyte dysfunction. risk alleles and nondiabetic subjects of European ancestry. The Rare Mendelian forms of FSGS and congenital nephrotic syn- latter observation provides an important perspective on the drome have been associated with mutations in several genes relative level of CKD risk among genotype-defined groups. Fu- encoding podocyte-expressed proteins.13,14 In light of evidence ture longitudinal studies using this population could be very that predisposition of CKD associated with APOL1 variants is useful for determining the predictive value of APOL1 genotype recessive, it is plausible that human APOL1 may contribute to in assessing ESRD risk, which is essential information for ex- glomerular structure and/or function, with injury being the ploiting these findings in clinical practice. result of loss of function. Given the population allele frequency of APOL1 variants, As a first step in determining the function of APOL1 in the the number of African Americans who carry two copies of the kidney and glomerulus, Madhavan and colleagues provide the risk alleles likely exceeds three million. The public health im- evidence for intrarenal APOL1 protein expression.7 Similar to pact of this large population of potentially at-risk individuals is many other genes linked to FSGS, these investigators demon- magnified further by the observed accelerated progression to strate that APOL1 is expressed in podocytes. Additionally, they ESRD. To further quantify the evidence for earlier onset ESRD found APOL1 expression in proximal tubular cells, which may in APOL1 variant carriers, Kanji et al.9 investigated the age at be relevant to the tubulointerstitial injury that is prominent in hemodialysis initiation for nondiabetic African Americans HIVAN. In biopsies from patients with HIVAN or FSGS, with ESRD participating in an observational cohort study, the podocyte and tubular expression of APOL1 is reduced and de Accelerated Mortality on Renal Replacement (ArMORR) novo expression is observed in renal arterioles, most likely in study. The data indicate that subjects who carried 1 to 2 APOL1 vascular smooth muscle cells. The latter finding is intriguing, risk alleles exhibited a significantly younger age at initiation of given the association of arteriolar lesions with ESRD in African hemodialysis compared with noncarriers. Specifically, carriers Americans due to FSGS. Future studies are needed to validate of1or2APOL1 G1 alleles initiated dialysis 6 and Ͼ10 yr these findings and to determine whether renal APOL1 expres- earlier, respectively, than noncarriers. Findings for the less fre- sion varies by genotype. quent G2 allele were not conclusive. The effect observed for The mechanism by which APOL1 variants predispose to

1956 Journal of the American Society of Nephrology J Am Soc Nephrol 22: 1958, 2011 www.jasn.org EDITORIALS

FSGS and HIVAN is not known. Additional experiments are Kopp JB, Pays E, Pollak MR: Association of trypanolytic ApoL1 variants needed to determine if circulating and/or intracellular APOL1 with kidney disease in African Americans. Science 329: 841–845, 2010 4. Tzur S, Rosset S, Shemer R, Yudkovsky G, Selig S, Tarekegn A, Bekele is important for renal function and how variant forms might E, Bradman N, Wasser WG, Behar DM, Skorecki K: Missense muta- contribute to kidney dysfunction. A higher incidence of trans- tions in the APOL1 gene are highly associated with end stage kidney planted kidney graft failure occurring in organs procured from disease risk previously attributed to the MYH9 gene. Hum Genet 128: donors carrying two APOL1 risk alleles suggests that intrinsic 345–350, 2010 renal expression may be important.15 The protein consists of 5. Kopp JB, Nelson GW, Sampath K, Johnson RC, Genovese G, An P, Friedman D, Briggs W, Dart R, Korbet S, Mokrzycki MH, Kimmel PL, specific functional domains, including a pore-forming domain Limou S, Ahuja TS, Berns JS, Fryc J, Simon EE, Smith MC, Trachtman in the N-terminal region, C-terminal SRA interaction domain, H, Michel DM, Schelling JR, Vlahov D, Pollak M, Winkler CA: APOL1 as discussed above, and a membrane-addressing domain.16 In genetic variants in focal segmental glomerulosclerosis and HIV-asso- addition to trypanolytic membrane pore-forming activity, ciated nephropathy. J Am Soc Nephrol 22: 2129–2137, 2011 APOL1 may contribute to lipid metabolism, vascular function, 6. Papeta N, Kiryluk K, Patel A, Sterken R, Kacak N, Snyder HJ, Imus PH, Mhatre AN, Lawani AK, Julian BA, Wyatt RJ, Novak J, Wyatt CM, Ross 16–20 and autophagy. The latter function is intriguing in view of MJ, Winston JA, Klotman ME, Cohen DJ, Appel GB, D’Agati VD, recent data demonstrating the importance of autophagy and Klotman PE, Gharavi AG: APOL1 variants increase risk for FSGS and autophagic flux for maintaining podocyte health.21 HIVAN but Not IgA nephropathy. J Am Soc Nephrol 22: 1991–1996, Animal models could also be valuable for determining dis- 2011 ease mechanisms, but the options are limited, as only humans 7. Madhavan SM, O’Toole JF, Konieczkowski M, Ganesan S, Bruggeman LA, Sedor JR: APOL1 localization in normal kidney and nondiabetic and some non-human primates express APOL1 natively, thus kidney disease. J Am Soc Nephrol 22: 2119–2128, 2011 precluding the possibility of employing knockout mouse or rat 8. Friedman DJ, Kozlitina J, Genovese G, Jog P, Pollak MR: Population- models. Transgenic mice overexpressing wild-type human based risk assessment of APOL1 on renal disease. J Am Soc Nephrol APOL1 have helped determine the biology of trypanosome re- 22: 2098–2105, 2011 sistance,22 and together with transgenic animals carrying spe- 9. Kanji Z, Powe CE, Wenger JB, Huang C, Ankers E, Sullivan DA, Collerone G, Powe NR, Tonelli M, Bhan I, Bernhardy AJ, DiBartolo S, cific APOL1 risk alleles, might provide one approach to eluci- Friedman D, Genovese G, Pollak MR, Thadhani R: Genetic variation in date mechanisms responsible for renal injury. Investigating APOL1 associates with younger age at hemodialysis initiation. JAm mechanisms of susceptibility in APOL1-associated renal dis- Soc Nephrol 22: 2091–2097, 2011 ease should also consider potential triggering mechanisms or 10. Johnstone DB, Zhang J, George B, Leon C, Gachet C, Wong H, Parekh whether variant alleles represent intrinsic time bombs. The R, Holzman LB: Podocyte-specific deletion of Myh9 encoding non- muscle myosin heavy chain 2A predisposes mice to glomerulopathy. robust associations of APOL1 with HIVAN reported in this Mol Cell Biol 31: 2162–2170, 2011 issue of JASN suggest that viral infection or inflammation 11. Vanhamme L, Paturiaux-Hanocq F, Poelvoorde P, Nolan DP, Lins L, might provide local or systemic triggers that potentiate glo- Van Den AJ, Pays A, Tebabi P, Van XH, Jacquet A, Moguilevsky N, merular injury in patients carrying two risk-associated vari- Dieu M, Kane JP, De BP, Brasseur R, Pays E: Apolipoprotein L-I is the ants. A wealth of new information in this issue should stimu- trypanosome lytic factor of human serum. Nature 422: 83–87, 2003 12. Perez-Morga D, Vanhollebeke B, Paturiaux-Hanocq F, Nolan DP, Lins late the next wave of discoveries. L, Homble F, Vanhamme L, Tebabi P, Pays A, Poelvoorde P, Jacquet A, Brasseur R, Pays E: Apolipoprotein L-I promotes trypanosome lysis by forming pores in lysosomal membranes. Science 309: 469–472, DISCLOSURES 2005 13. Pollak MR: Inherited podocytopathies: FSGS and nephrotic syndrome None. from a genetic viewpoint. J Am Soc Nephrol 13: 3016–3023, 2002 14. Tryggvason K, Patrakka J, Wartiovaara J: Hereditary proteinuria syn- dromes and mechanisms of proteinuria. N Engl J Med 354: 1387– 1401, 2006 REFERENCES 15. Reeves-Daniel AM, DePalma JA, Bleyer AJ, Rocco MV, Murea M, Adams PL, Langefeld CD, Bowden DW, Hicks PJ, Stratta RJ, Lin JJ, 1. Kao WH, Klag MJ, Meoni LA, Reich D, Berthier-Schaad Y, Li M, Coresh Kiger DF, Gautreaux MD, Divers J, Freedman BI: The APOL1 gene J, Patterson N, Tandon A, Powe NR, Fink NE, Sadler JH, Weir MR, and allograft survival after kidney transplantation. Am J Transplant 11: Abboud HE, Adler SG, Divers J, Iyengar SK, Freedman BI, Kimmel PL, 1025–1030, 2011 Knowler WC, Kohn OF, Kramp K, Leehey DJ, Nicholas SB, Pahl MV, 16. Pays E, Vanhollebeke B, Vanhamme L, Paturiaux-Hanocq F, Nolan DP, Schelling JR, Sedor JR, Thornley-Brown D, Winkler CA, Smith MW, Perez-Morga D: The trypanolytic factor of human serum. Nat Rev Parekh RS: MYH9 is associated with nondiabetic end-stage renal dis- Microbiol 4: 477–486, 2006 ease in African Americans. Nature Genet 40: 1185–1192, 2008 17. Vanhollebeke B, Pays E: The function of apolipoproteins L. Cell Mol 2. Kopp JB, Smith MW, Nelson GW, Johnson RC, Freedman BI, Bowden Life Sci 63: 1937–1944, 2006 DW, Oleksyk T, McKenzie LM, Kajiyama H, Ahuja TS, Berns JS, Briggs 18. Monajemi H, Fontijn RD, Pannekoek H, Horrevoets AJ: The apolipo- W, Cho ME, Dart RA, Kimmel PL, Korbet SM, Michel DM, Mokrzycki protein L gene cluster has emerged recently in evolution and is MH, Schelling JR, Simon E, Trachtman H, Vlahov D, Winkler CA: MYH9 expressed in human vascular tissue. Genomics 79: 539–546, 2002 is a major-effect risk gene for focal segmental glomerulosclerosis. 19. Wan G, Zhaorigetu S, Liu Z, Kaini R, Jiang Z, Hu CA: Apolipoprotein Nature Genet 40: 1175–1184, 2008 L1, a novel Bcl-2 homology domain 3-only lipid-binding protein, 3. Genovese G, Friedman DJ, Ross MD, Lecordier L, Uzureau P, Freed- induces autophagic cell death. J Biol Chem 283: 21540–21549, man BI, Bowden DW, Langefeld CD, Oleksyk TK, Uscinski Knob AL, 2008 Bernhardy AJ, Hicks PJ, Nelson GW, Vanhollebeke B, Winkler CA, 20. Zhaorigetu S, Wan G, Kaini R, Jiang Z, Hu CA: ApoL1, a BH3-only

J Am Soc Nephrol 22: 1958, 2011 1957 EDITORIALS www.jasn.org

lipid-binding protein, induces autophagic cell death. Autophagy 4: 1079–1082, 2008 See related articles, “APOL1 Variants Increase Risk for FSGS and HIVAN but Not 21. Hartleben B, Godel M, Meyer-Schwesinger C, Liu S, Ulrich T, Kobler S, IgA Nephropathy,” on pages 1991–1996; “Genetic Variation in APOL1 Associ- Wiech T, Grahammer F, Arnold SJ, Lindenmeyer MT, Cohen CD, ates with Younger Age at Hemodialysis Initiation,” on pages 2091–2097; “Pop- Pavenstadt H, Kerjaschki D, Mizushima N, Shaw AS, Walz G, Huber TB: ulation-based Risk Assessment of APOL1 on Renal Disease,” on pages 2098– Autophagy influences glomerular disease susceptibility and maintains 2105; “APOL1 Localization in Normal Kidney and Nondiabetic Kidney Disease,” podocyte homeostasis in aging mice. J Clin Invest 120: 1084–1096, on pages 2119–2128; and “APOL1 Genetic Variants in Focal Segmental Glo- merulosclerosis and HIV-Associated Nephropathy,” on pages 2129–2137. 2010 22. Samanovic M, Molina-Portela MP, Chessler AD, Burleigh BA, Raper J: Trypanosome lytic factor, an antimicrobial high-density lipoprotein, ameliorates Leishmania infection. PLoS Pathog 5: e1000276, 2009

1958 Journal of the American Society of Nephrology J Am Soc Nephrol 22: 1958, 2011