The podocytopathies, including FSGS and collapsing glomerulopathy, involve genetic and acquired forms of podocyte injury10
, and podocyte injury is proposed to play a central role in the pathogenesis of other progressive kidney diseases, including diabetic and hypertensive nephrosclerosis25
. Over ten genes have been associated with FSGS; most are associated with disease forms with mendelian inheritance or with evidence for new mutations26
. With regard to sporadic, non-familial FSGS, associations have been identified with WT1
), but polymorphisms in these genes explain only a small portion of the disease burden.
We have used MALD to identify genetic variation in MYH9 as an important contributing factor to the development of idiopathic FSGS, HIV-associated FSGS (collapsing glomerulopathy) and hypertensive ESKD. The MALD scan revealed a single peak centered on MYH9 on chromosome 22q13.1 that showed strong association of African chromosomal ancestry with FSGS in African Americans. Fine mapping and functional considerations further implicated MYH9. MYH9 polymorphism does not differentially affect progression from FSGS to ESKD, indicating that the MYH9 association with FSGS is a general association for this disease, and is not limited to ESKD due to FSGS.
An independent study29
of an African American population composed of both diabetic and nondiabetic individuals with ESKD also found a MALD association with a similar, overlapping region of chromosome 22. As this study found a very similar peak on chromosome 22, and had overlapping collaborators with our study, we shared our finding of a strong association of MYH9
variants with FSGS. Kao et al.
explain the admixture signal with the three MYH9
intron 23 SNPs that had the strongest FSGS associations in our study, and replicate our findings of an association of nondiabetic ESKD and the MYH9
MYH9 risk alleles are more frequent among African Americans, and protective alleles are more frequent among European Americans (), providing a genetic explanation for the disparities in risk for FSGS and hypertensive ESKD in these populations.
Nonmuscle myosin, like muscle myosin, is assembled from separately coded heavy and light chains, and binds to actin to perform intracellular motor functions30
. Mutations in MYH9
have been associated with four autosomal dominant clinical syndromes: May-Hegglin, Sebastian, Fechtner and Epstein31
. All four syndromes involve autosomal dominant macrothrombocytopenia, with the variable accompaniment of sensorineural deafness, cataracts, neutrophil Döhle-like bodies and glomerular disease31
. There are few histologic descriptions of MYH9
-associated glomerular disease. One individual with a MYH9
mutation, undergoing kidney biopsy early in the disease course, had normal light microscopy and electron microscopic evidence of focal podocyte foot process effacement and loss of podocyte slit diaphragms, indicating podocyte injury32
. The Myh9
knockout mouse shows embryonic lethality, whereas heterozgyous mice have apparently normal phenotype other than hearing loss with incomplete penetrance33
. Myosin IIA has been localized to podocytes and possibly mesangial cells22,32
. Other mutations affecting podocyte proteins that interact with the actin cytoskeleton, including alpha-actinin-4 (ref. 34
), CD2-associated protein35
, have been associated with podocyte injury and FSGS in humans or experimental animals, suggesting the requirement for an intact actin cytoskeleton to maintain normal podocyte cytoarchitecture and filtration barrier function.
Our extension samples revealed that MYH9
was also associated with the common clinical syndrome of hypertensive nephrosclerosis in African Americans, but not with diabetic nephropathy. Individuals with hypertensive ESKD typically present with advanced nephropathy and secondarily elevated blood pressure with resultant left ventricular hypertrophy and retinal vascular changes. Some of these individuals may have occult glomerular diseases, often FSGS or global glomerulosclerosis presenting in non-nephrotic forms, as they do not typically undergo renal biopsy37,38
. Renal biopsy studies typically reveal the presence of focal or global glomerulosclerosis in clinically diagnosed hypertensive nephrosclerosis39,40
. Notably, the renal microvasculature changes that are typically attributed to hypertension fail to correlate with measured blood pressure, suggesting that factors other than hypertension caused nephropathy. In practice, cases are not often held to the strict diagnostic clinical criteria as in these reports, and hypertensive nephrosclerosis is a diagnosis of exclusion in many nondiabetic forms of nephropathy. It is possible that undiagnosed FSGS in those labeled with hypertensive nephrosclerosis contributed to the association, and studies involving biopsy-proven hypertensive nephrosclerosis are needed to clarify this issue. Alternatively, individuals with hypertension and with particular MYH9
alleles may be more susceptible to podocyte injury and progressive glomerular disease. The lack of demonstrated association of MYH9
with diabetic ESKD suggests that the underlying mechanisms leading to impaired kidney function in diabetics may be distinct from those of FSGS or hypertensive ESKD. It is possible that the podocyte is not the primary target of the metabolic abnormalities that are believed to cause diabetic nephropathy, or alternatively that those abnormalities are quantitatively much more important than MYH9
variation. Nevertheless, because of the disease burden, it should be determined whether MYH9
variation influences the rate of progressive kidney function loss in diabetic kidney disease.
Genetic studies of glomerular diseases such as FSGS are limited by the number of subjects available—typically hundreds rather than the thousands recommended for genome-wide association studies. Small sample sizes are generally unworkable for genome-wide association studies because of the need for statistical power to overcome correcting for the current 1,000,000 SNPs typed, and because the associations observed generally have modest odds ratios. However, there is a strong argument to be made for using admixture mapping for gene discovery in diseases such as FSGS with large ancestry-driven disparities that are not fully explained by non-genetic factors, because the gene or genes underpinning the disease may have strong effects and/or very frequent alleles, as observed in this study, and because the smaller number of markers needed simplifies the statistical issues.
A limitation of our study is that we have not yet identified the causal sequence variation in MYH9 that is associated with FSGS. Initial resequencing of the 40 exons and intron-exon junctions of MYH9 in 14 African Americans with FSGS and haplotype E-1 showed no obvious causal variation (data not shown). We hypothesize that the functional variation occurs in regulatory elements or splice-site determinants, and that these influence RNA expression or protein structure in podocytes. Further studies are needed to identify causal variants and to provide an understanding of what additional genetic or environmental factors must interact with MYH9 variants to cause podocyte injury leading to kidney failure.
Recently, results of genome-wide scans and MALD scans have been reported with generally modest odds ratios for the genotype-phenotype associations. These weak effects have been attributed to the complex nature of the diseases involving the interaction of multiple genetic and environmental factors. The present study using a MALD approach has securely identified a main effect gene that is involved in two histologically distinct forms of FSGS: idiopathic FSGS, which is characterized by depletion of podocytes, and HIV-1–associated FSGS (collapsing glomerulopathy), which is characterized by podocyte proliferation. Extrapolating to population data from our case-control study, the fractions of sporadic or HIV-associated FSGS among African Americans that can be explained41
by this haplotype are 4.7% and 12%, respectively. The large frequency difference for haplotype E-1 (60% in African Americans versus 4% in European Americans) combined with the extremely high FSGS susceptibility conferred by this factor (OR = 5.0 for carriers, 95% CI = 3.5–7.1, recessive model; Supplementary Table 2
) provides a plausible cause for the higher incidence of FSGS, HIV-associated nephropathy and hypertensive nephrosclerosis among African Americans. For individuals not carrying haplotype E-1, the extrapolated population frequency of sporadic FSGS is very similar between African Americans (0.06%; 95% CI = 0.03–0.1%) and European Americans (0.05%; 95% CI = 0.04–0.06%), suggesting that the two populations have comparable background levels of susceptibility to FSGS in the absence of MYH9
genetic susceptibility factors, and that MYH9
factors may account for 66–100% of the disparity in susceptibility to FSGS. The identification of MYH9
genetic variation as a risk factor for FSGS and hypertensive ESKD underscores the importance of the actin cytoskeleton in podocyte biology and as a potential pharmacologic target.