ESKD is a major and growing health concern worldwide, especially in the developing regions of the world, resulting in reduced duration and quality of life, and necessitating ongoing renal replacement therapy to sustain life. Population discrepancies in the incidence and prevalence rates of ESKD within the USA and other constituencies have motivated the successful search for and the recent identification of genetic susceptibility factors, which provide important insights in developing disease preventive measures. Specifically, African ancestry sequence variants in the MYH9 gene have been shown to confer major disease risk susceptibility for a number of non-diabetic etiologies of ESKD (4
). More recently, association between MYH9 and albuminuria was detected in hypertensive African Americans (13
), and association between other polymorphisms in the same gene with serum creatinine concentrations has been reported in Europeans (12
). Additionally, numerous rare exonic variants have already been reported to cause ESKD in the context of a set of syndromes (Giant Platelet Syndromes) with autosomal dominant inheritance (19
). These observations suggest a fundamental role of variants in the MYH9 gene in the pathogenesis of non-diabetic ESKD. The association of a set of common polymorphic variants, whose frequency differs among ancestral populations with more common forms of non-diabetic ESKD, points to the existence of one or more causative variants with which these associated SNPs are in LD. This raises the question of whether multiple common causative disease susceptibility variants within the same gene are responsible for the same ESKD phenotype, and whether they interact with each other. Conversely, it is important to establish whether the same causative variant confers risk due to shared ancestry among different populations.
The Hispanic American population demonstrates an intermediary pattern of ESKD susceptibility when compared with African and Europeans Americans (1
), but the pathogenetic basis for this observation remains unknown. The previously reported partial African ancestry among Hispanic Americans has led us to hypothesize that the same ESKD risk polymorphisms reported among African Americans might also explain the elevated susceptibility to kidney disease observed among Hispanic Americans. Our results show that ~30% of the global genetic ancestry observed among the Hispanic Americans in this study can be attributed to an African component confirming their partially shared ancestry with African Americans. Interestingly, local African ancestry measures for chromosome 22, where the MYH9 gene is located, were highly associated with ESKD in Hispanic Americans, whereas only marginally associated in African Americans. This observation suggests that analysis of the Hispanic American population according to local chromosome 22 ancestry estimators might better identify the subpopulations included under the self-identified designation of Hispanic American, at higher risk for ESKD. A further proof of the important role that the African ancestry component contributes to the higher susceptibility to ESKD observed among Hispanic Americans, is evident from the positive confirmation of the association between the reported E-1 risk haplotype and non-diabetic ESKD among Hispanic Americans after corrections for both global and local ancestries. Cumulatively, our results show that, similarly to the case of African Americans, the higher proportions of non-diabetic ESKD observed among Hispanic Americans in the current study are MYH9-associated. Therefore, the overall higher incidence rate of ESKD observed among Hispanic Americans when compared with European Americans is likely to be attributable at least in part, to the same African population common causative variant with which the E-1 risk polymorphisms are in strong LD. Likewise, the overall lower incidence of ESKD cases observed among Hispanic Americans when compared with African Americans is explained by the overall lower African ancestry component, and hence lower frequencies of the African MYH9 risk alleles among Hispanic Americans. Moreover, our findings and analyses are entirely consistent with the existence of an African ancestry causative variant with which the highly associated SNPs are in LD, and which is responsible for the excess disease phenotype risk. This inference is greatly strengthened by the finding that the very same African ancestry SNPs are the most highly associated with the corresponding relevant ESKD phenotypes in two different populations with varying genomic backgrounds and degrees of African ancestry admixture. It is likely that this African ancestry causative variant shared between the populations studied, is necessary but not sufficient for pathogenesis of the relevant ESKD phenotypes of interest and that epistatic interactions with other genomic regions and/or gene-by-environment interactions are additionally necessary for penetrance of the disease phenotype.
In this regard, the observation of higher ORs for risk genotypes in Hispanic Americans compared with African Americans warrants consideration. Since these associations are obtained using logistic regression—this is not an expected effect of lower levels of African ancestry and lower disease prevalence among the Hispanic Americans. One possible explanation that motivates further study is that the African genomic background may contain as yet unidentified or untested ESKD risk variants at other loci but with lesser effect, and which are less frequent in the European genomic background. Thus, in the Hispanic American ESKD cases the lower frequency of observed African-risk variants of MYH9 may be acting on a mostly ‘protective’ European genomic background, in contrast to African Americans where the African-risk variants of MYH9 are acting on the background of a higher genomic risk for ESKD. Therefore, the relative effect of risk MYH9 variant in Hispanic Americans would be more prominent than in African Americans. Differences in ESKD risk allele frequencies at multiple loci within a population could arise through a variety of mechanisms, including selective effects. This would motivate the search for additional proposed ESKD risk loci (20
The confirmation of the association between the previously reported E-1 haplotype and non-diabetic ESKD among Hispanic Americans lends further support to the role of the MYH9 in the phenotype of non-diabetic ESKD, but does not facilitate progress towards identifying a risk causing mutation. Additional genotyping within the MYH9 gene to a total of 42 SNPs within the MYH9 gene, does however both strengthen the association of the gene with the disease phenotype, and also provides potential insights pertaining to the region containing the causative mutation and its mode of inheritance. In this regard, our results yielded a total of 15 significantly associated SNPs, of which the 10 most significant clustered into three sets (Fig. ). These results clearly show that the S-1 SNPs (rs5750250, rs2413396, rs5750248) and F-1 SNPs (rs16996674, rs16996677, rs11912763), have a stronger association with non-diabetic ESKD when compared with the SNPs reported to comprise the E-1 haplotype. Of interest, these SNPs are found on both the 5′ and 3′ sides of the E-1 haplotype SNPs, spanning a region of 42 kb. The S-1 SNPs which are the most highly associated, are clustered in a smaller region of only 5.5 kb between intron 13 to intron 15 and like the E-1 SNPs have the strongest association in the recessive inheritance mode. The F-1 SNPs show a significant association with disease risk under a dominant or additive inheritance mode, even though they are located furthest 5′ and 3′ in relation to the SNPs comprising the E-1 and S-1 risk haplotypes, which demonstrate a significant disease risk association in the recessive or additive inheritance mode. This finding may suggest that even one copy of the causative mutation may already confer some of the increased risk for this disease phenotype. Stronger association of the E-1 and S-1 SNPs in the recessive mode may reflect the fact that in the presence of two risk alleles, the likelihood of recombination between the causative mutation variant and the E-1 or S-1 risk variant occurring on both parental chromosomes is greatly reduced. Thus, it will be difficult to draw inferences about the physical location of the causative variant based only on the strength of association in a recessive inheritance mode, without a more complete phylogenetic resolution or functional studies of candidate causative mutations at the molecular and mechanistic levels.
Some potential limitations of the study relate primarily to classification of subjects in the various disease status categories. Thus, it is as yet unclear whether the MYH9 associations relate primarily to FSGS, in which case inclusion of hypertensive ESKD without a biopsy diagnosis of FSGS, would weaken any inferences of association with FSGS per se. Conversely, inclusion of subjects with undiagnosed FSGS among those with a medical record diagnosis of diabetic nephropathy, or inclusion of early chronic kidney disease patients among the controls would certainly not mitigate, and also only weaken any inferences of positive associations. Clearly, the greater accuracy regarding the state and underlying etiology of chronic kidney disease would only render the estimates of strengths of association more secure and accurate. Unfortunately, clinical practice, in which kidney biopsy is often not indicated or carried out prior to initiation of renal replacement therapy, leaves the etiology as presumed, solely on the basis of clinical background.
The potential clinical relevance of the results, presented previously and herein, with respect to clinical and public health warrants some consideration. Thus questions regarding optimization in kidney transplant donation can be considered, and thresholds for instituting antiretroviral therapy in HIV-infected individuals might be influenced by consideration of MYH9 genotypic risk for HIVAN. The current considerations for a physician counseling a patient with normal kidney function and a genetic risk profile for chronic kidney disease at the MYH9 gene are complex, since the actual disease-risk causative variant is as yet unknown, and as only a minority of individuals bearing the MYH9 risk haplotypes will eventually develop ESKD likely reflecting the contribution of epistatic genomic as well as environmental factors (26
). Of note, the current results comparing MYH9 risk allele effects in African and Hispanic Americans highlight the contribution of differing population genomic backgrounds in MYH9 associated ESKD risk. Nevertheless, it seems reasonable for a physician to include the status of the MYH9 risk haplotype as one of the many considerations used in ESKD risk assessment.
In summary, our results demonstrate that the same MYH9 risk haplotypes are associated with non-diabetic ESKD in both African and Hispanic Americans as a result of their partially shared African ancestry. The finding of this association within the Hispanic American population, in which the overall global African ancestry is significantly lower when compared with African Americans, predicts a functional impact of the MYH9 local African ancestry to ESKD susceptibility, and adds very strong support for the existence and hence motivates the search for a causative disease phenotype risk variant. We have further identified additional SNPs in the MYH9 gene, grouped in clusters designated as S-1 and F-1 that show the strongest associations reported to date with ESKD in both African and Hispanic Americans. Moreover, the finding of significant independent associations in both dominant and additive inheritance modes for the F-1 SNPs, and recessive and additive inheritance modes for the S-1 SNPs, means that the search for the causative mutation should take into account the possibility that a single copy may confer a functional effect. Taken together, these findings strongly motivate the search for a functional African ancestry causative MYH9 variant, which will be facilitated by a deeper understanding of the phylogenetic history of the locus.