Missense mutations in the APOL1 gene are highly associated with end stage kidney disease risk previously attributed to the MYH9 gene

The New Epidemiology of HIV-Related Kidney Disease

HIV-related kidney disease has been associated with significant morbidity and mortality in the HIV population. It is clear that the epidemiology of HIV-related kidney disease has changed dramatically since the first case reports in 1984. During these early years, the predominant etiology of kidney disease in HIV was recognized as HIV-associated nephropathy (HIVAN), an aggressive form of kidney disease with a high rate of progression to end-stage renal disease (ESRD). Subsequently, with the widespread use of combination antiretroviral therapy (cART), there was a dramatic decrease in the incidence of ESRD attributed to HIV/AIDS. Although the incidence of HIV-related ESRD has plateaued in the last 15 years, the prevalence has continued to increase because of improved survival. Available prevalence estimates do not include HIV-infected individuals with comorbid ESRD, although there is growing evidence that the epidemiology of kidney disease in the HIV-infected population has changed. This article reviews the impact of risk factors such as race, diabetes mellitus, hypertension, hepatitis C virus coinfection, and the chronic use of cART on the changing epidemiology of HIV-related kidney disease. Additionally in this review, we propose potential areas of translational research that will help to further characterize HIV-related kidney disease in the 21^st century.

APOL1 risk variants predict histopathology and progression to ESRD in HIV-related kidney disease

With earlier institution of antiretroviral therapy, kidney diseases other than HIV-associated nephropathy (HIVAN) predominate in HIV-infected persons. Outcomes for these diseases are typically worse among those infected with HIV, but the reasons for this are not clear. Here, we examined the role of APOL1 risk variants in predicting renal histopathology and progression to ESRD in 98 HIV-infected African Americans with non-HIVAN kidney disease on biopsy. We used survival analysis to determine time to ESRD associated with APOL1 genotype. Among the 29 patients with two APOL1 risk alleles, the majority (76%) had FSGS and 10% had hypertensive nephrosclerosis. In contrast, among the 54 patients with one APOL1 risk allele, 47% had immune-complex GN as the predominant lesion and only 23% had FSGS. Among the 25 patients with no APOL1 risk allele, 40% had immune-complex GN and 12% had FSGS. In 310 person-years of observation, 29 patients progressed to ESRD. In adjusted analyses, individuals with two APOL1 risk alleles had a nearly three-fold higher risk for ESRD compared with those with one or zero risk alleles (P=0.03). In summary, these data demonstrate an association between APOL1 variants and renal outcomes in non-HIVAN kidney disease, suggesting a possible use for APOL1 genotyping to help guide the care of HIV-infected patients.

Association of filtered sodium load with medullary volumes and medullary hypoxia in hypertensive African Americans as compared with whites

BACKGROUND

African Americans develop hypertension earlier with more target manifestations than whites despite having a higher glomerular filtration rate (GFR) for any level of serum creatinine. STUDY DESIGN & PARTICIPANTS: This study tested the hypothesis that increased GFR and sodium reabsorption in African Americans is associated with increased metabolic work and medullary hypoxia in 49 nondiabetic patients with essential hypertension (29 whites and 20 African Americans) following a constant-sodium diet (150 mEq/d) and renin-angiotensin system blockade.

PREDICTORS

Ethnicity, age, measured GFR, sodium excretion, and body mass index.

OUTCOMES

We examined cortical and medullary volumes and blood flows using multidetector computed tomography and intrarenal deoxyhemoglobin (R2*) using blood oxygen level-dependent magnetic resonance.

RESULTS

Blood pressure and sodium excretion were similar, whereas African Americans were more obese and had higher iothalamate GFRs. Renal cortical volumes did not differ, but medullary volumes adjusted for body size and age were higher in African Americans (32.3 ± 11.2 vs 25.1 ± 7.4 cm(3)/m(2) body surface area; P < 0.001). Sodium reabsorption and blood flows were higher in African Americans. Basal cortical deoxyhemoglobin values were similar between ethnic groups, whereas medullary R2* was higher in African Americans (39.7 ± 5.1 vs 36.3 ± 6.5/s; P = 0.02), but decreased to levels similar to whites after furosemide treatment. Levels of the circulating isoprostane prostaglandin F(2α) were higher in African Americans and daily urinary prostaglandin F(2α) excretion in African Americans correlated directly with renal blood flow (R = 0.71; P < 0.01).

LIMITATIONS

Studies were limited to treated volunteers with normal kidney function without knowledge of prior nutrient intake.

CONCLUSIONS

These data show for the first time that increased sodium reabsorption in obese African American patients with hypertension was associated with enlarged medullary volumes, functional hypoxia related to solute reabsorption, and a direct relationship between blood flows and urinary isoprostane levels. Our results support a model of increased oxygen consumption and oxidative stress in African Americans that may accelerate hypertension and target-organ injury compared with white patients with essential hypertension.

Genetic association and gene-gene interaction analyses in African American dialysis patients with nondiabetic nephropathy

BACKGROUND

African Americans have increased susceptibility to nondiabetic nephropathy relative to European Americans.

STUDY DESIGN

Follow-up of a pooled genome-wide association study (GWAS) in African American dialysis patients with nondiabetic nephropathy; novel gene-gene interaction analyses.

SETTING & PARTICIPANTS

Wake Forest sample: 962 African American nondiabetic nephropathy cases, 931 non-nephropathy controls. Replication sample: 668 Family Investigation of Nephropathy and Diabetes (FIND) African American nondiabetic nephropathy cases, 804 non-nephropathy controls.

PREDICTORS

Individual genotyping of top 1,420 pooled GWAS-associated single-nucleotide polymorphisms (SNPs) and 54 SNPs in 6 nephropathy susceptibility genes.

OUTCOMES

APOL1 genetic association and additional candidate susceptibility loci interacting with or independently from APOL1.

RESULTS

The strongest GWAS associations included 2 noncoding APOL1 SNPs, rs2239785 (OR, 0.33; dominant; P = 5.9 × 10(-24)) and rs136148 (OR, 0.54; additive; P = 1.1 × 10(-7)) with replication in FIND (P = 5.0 × 10(-21) and 1.9 × 10(-05), respectively). rs2239785 remained associated significantly after controlling for the APOL1 G1 and G2 coding variants. Additional top hits included a CFH SNP (OR from meta-analysis in the 3,367 African American cases and controls, 0.81; additive; P = 6.8 × 10(-4)). The 1,420 SNPs were tested for interaction with APOL1 G1 and G2 variants. Several interactive SNPs were detected; the most significant was rs16854341 in the podocin gene (NPHS2; P = 0.0001).

LIMITATIONS

Nonpooled GWASs have not been performed in African American patients with nondiabetic nephropathy.

CONCLUSIONS

This follow-up of a pooled GWAS provides additional and independent evidence that APOL1 variants contribute to nondiabetic nephropathy in African Americans and identified additional associated and interactive nondiabetic nephropathy susceptibility genes.

APOL1 genetic variants in focal segmental glomerulosclerosis and HIV-associated nephropathy

Trypanolytic variants in APOL1, which encodes apolipoprotein L1, associate with kidney disease in African Americans, but whether APOL1-associated glomerular disease has a distinct clinical phenotype is unknown. Here we determined APOL1 genotypes for 271 African American cases, 168 European American cases, and 939 control subjects. In a recessive model, APOL1 variants conferred seventeenfold higher odds (95% CI 11 to 26) for focal segmental glomerulosclerosis (FSGS) and twenty-nine-fold higher odds (95% CI 13 to 68) for HIV-associated nephropathy (HIVAN). FSGS associated with two APOL1 risk alleles associated with earlier age of onset (P = 0.01) and faster progression to ESRD (P < 0.01) but similar sensitivity to steroids compared with other subjects. Individuals with two APOL1 risk alleles have an estimated 4% lifetime risk for developing FSGS, and untreated HIV-infected individuals have a 50% risk for developing HIVAN. The effect of carrying two APOL1 risk alleles explains 18% of FSGS and 35% of HIVAN; alternatively, eliminating this effect would reduce FSGS and HIVAN by 67%. A survey of world populations indicated that the APOL1 kidney risk alleles are present only on African chromosomes. In summary, African Americans carrying two APOL1 risk alleles have a greatly increased risk for glomerular disease, and APOL1-associated FSGS occurs earlier and progresses to ESRD more rapidly. These data add to the evidence base required to determine whether genetic testing for APOL1 has a use in clinical practice.

A systems view of genetics in chronic kidney disease

A tight interplay of genetic predisposition and environmental factors define the onset and the rate of progression of chronic renal disease. We are seeing a rapid expansion of information about genetic loci associated with kidney function and complex renal disease. However, discovering the functional links that bridge the gap from genetic risk loci to disease phenotype is one of the main challenges ahead. Risk loci are currently assigned to a putative context using the functional annotation of the closest genes via a guilt-by-proximity approach. These approaches can be extended by strategies integrating genetic risk loci with kidney-specific, genome-wide gene expression. Risk loci-associated transcripts can be assigned a putative disease-specific function using gene expression coregulation networks. Ultimately, genotype-phenotype dependencies postulated from these associative approaches in humans need to be tested via genetic modification in model organisms. In this review, we survey strategies that employ human tissue-specific expression and the use of model organisms to identify and validate the functional relationship between genotype and phenotype in renal disease. Strategies to unravel how genetic risk and environmental factors orchestrate renal disease manifestation can be the first steps toward a more integrated, holistic approach urgently needed for chronic renal diseases.

APOL1 variants increase risk for FSGS and HIVAN but not IgA nephropathy

A chromosome 22q13 locus strongly associates with increased risk for idiopathic focal segmental glomerulosclerosis (FSGS), HIV-1-associated nephropathy (HIVAN), and hypertensive ESRD among individuals of African descent. Although initial studies implicated MYH9, more recent analyses localized the strongest association within the neighboring APOL1 gene. In this replication study, we examined the six top-most associated variants in APOL1 and MYH9 in an independent cohort of African Americans with various nephropathies (44 with FSGS, 21 with HIVAN, 32 with IgA nephropathy, and 74 healthy controls). All six variants associated with FSGS and HIVAN (additive ORs, 1.8 to 3.0; P values 3 × 10(-2) to 5 × 10(-5)) but not with IgA nephropathy. In conditional and haplotype analyses, two APOL1 haplotypes accounted for virtually all of the association with FSGS and HIVAN on chromosome 22q13 (haplotype P value = 5.6 × 10(-8)). To assess the role of MYH9 deficiency in nephropathy, we crossbred Myh9-haploinsufficient mice (Myh9(+/-)) with HIV-1 transgenic mice. Myh9(+/-) mice were healthy and did not demonstrate overt proteinuria or nephropathy, irrespective of the presence of the HIV-1 transgene. These data further support the strong association of genetic variants in APOL1 with susceptibility to FSGS and HIVAN among African Americans.

Population-based risk assessment of APOL1 on renal disease

Case-control studies suggest that African Americans with genetic variants in both copies of APOL1 have increased risk for hypertension-attributable ESRD and focal segmental glomerulosclerosis. Here, we tested these risk variants in the Dallas Heart Study to ascertain the prevalence of APOL1-associated renal disease in a large population-based study and to estimate the contribution of APOL1 risk variants to disparities in renal disease. We determined the genotype of 1825 African Americans and 1042 European Americans. Among participants without diabetes, we identified microalbuminuria in 2.3% of European Americans, 6.0% of African Americans with no or one APOL1 risk allele, and 16.5% of African Americans with two risk alleles. In addition, the proportions of participants with estimated GFR < 60 ml/min per 1.73 m(2) was 1.5% for nondiabetic European Americans, 1.7% for African Americans with no or one APOL1 risk allele, and 6.7% for African Americans with two risk alleles. The APOL1 genotype did not associate with any differences in rates of CKD for study participants with diabetes. Our data suggest that more than 3 million African Americans likely have the high-risk genotype and are at markedly increased risk for nondiabetic CKD. In contrast, African Americans without the risk genotype and European Americans appear to have similar risk for developing nondiabetic CKD.

APOL1 localization in normal kidney and nondiabetic kidney disease

In patients of African ancestry, genetic variants in APOL1, which encodes apolipoprotein L1, associate with the nondiabetic kidney diseases, focal segmental glomerulosclerosis (FSGS), HIV-associated nephropathy (HIVAN), and hypertensive nephropathy. Understanding the renal localization of APOL1 may provide clues that will ultimately help elucidate the mechanisms by which APOL1 variants promote nephropathy. Here, we used immunohistology to examine APOL1 localization in normal human kidney sections and in biopsies demonstrating either FSGS (n = 8) or HIVAN (n = 2). Within normal glomeruli, APOL1 only localized to podocytes. Compared with normal glomeruli, fewer cells stained for APOL1 in FSGS and HIVAN glomeruli, even when expression of the podocyte markers GLEPP1 and synaptopodin appeared normal. APOL1 localized to proximal tubular epithelia in normal kidneys, FSGS, and HIVAN. We detected APOL1 in the arteriolar endothelium of normal and diseased kidney sections. Unexpectedly, in both FSGS and HIVAN but not normal kidneys, the media of medium artery and arterioles contained a subset of α-smooth muscle actin-positive cells that stained for APOL1. Comparing the renal distribution of APOL1 in nondiabetic kidney disease to normal kidney suggests that a previously unrecognized arteriopathy may contribute to disease pathogenesis in patients of African ancestry.

Polymorphisms in MYH9 are associated with diabetic nephropathy in European Americans

BACKGROUND

Polymorphisms in the non-muscle myosin IIA gene (MYH9) are associated with focal segmental glomerulosclerosis (FSGS) and non-diabetic end-stage renal disease (ESRD) in African Americans and FSGS in European Americans. We tested for association of single nucleotide polymorphisms (SNPs) in MYH9 with T2DM-ESRD in European Americans; additionally, three APOL1 gene variants were evaluated.

METHODS

Fifteen MYH9 SNPs and two APOL1 SNPs plus a 6-bp deletion were genotyped in 1963 European Americans, 536 cases with T2DM-ESRD and 1427 non-nephropathy controls (467 with T2DM and 960 without diabetes).

RESULTS

Comparing T2DM-ESRD cases with the 467 T2DM non-nephropathy controls, single variant associations trending toward significance were detected with SNPs rs4821480, rs2032487 and rs4281481 comprising part of the major MYH9 E1 risk haplotype [P-values 0.053-0.055 recessive, odds ratio (OR) 6.08-6.14]. Comparing T2DM-ESRD cases to all 1427 non-nephropathy controls, we confirmed evidence of association in these three SNPs as well as in the fourth E1 SNP (rs3752462) (P-values 0.017-0.035, OR 1.41-3.72). APOL1 G1/G2 nephropathy risk variants were rare in individuals of European American heritage, present in 0.28% of chromosomes in T2DM-ESRD cases and 0.32% of controls.

CONCLUSIONS

MYH9 SNPs rs4821480, rs2032487, rs4281481 and rs3752462 are associated with T2DM-ESRD susceptibility in European Americans. The APOL1 risk variants are not present at appreciable frequency in this cohort with T2DM-ESRD. Therefore, polymorphisms in MYH9 appear to influence nephropathy risk in this sample.

Absence of APOL1 risk variants protects against HIV-associated nephropathy in the Ethiopian population

BACKGROUND

Susceptibility to end-stage kidney disease (ESKD) among HIV-infected Americans of African ancestral heritage has been attributed to APOL1 genetic variation. We determined the frequency of the APOL1 G1 and G2 risk variants together with the prevalence of HIV-associated nephropathy (HIVAN) among individuals of Ethiopian ancestry to determine whether the kidney disease genetic risk is PanAfrican or restricted to West Africa, and can explain the previously reported low risk of HIVAN among Ethiopians.

METHODS

We studied a cohort of 338 HIV-infected individuals of Ethiopian ancestry treated in one Israeli and one Ethiopian center. We sought clinical evidence for HIVAN (serum creatinine >1.4 mg/dl or proteinuria >30 mg/dl in a spot urine sample). Genetic analyses included the genotyping of the APOL1 G1 and G2 variants, and a panel of 33 genomic ancestry-informative markers. Statistical analysis compared clinical and genetic indices for HIV-infected individuals of Ethiopian ancestry and overall Ethiopians to those reported for HIV-infected African-Americans, overall African-Americans, West Africans and non-Africans.

FINDINGS

Three (0.8%) of 338 HIV-infected patients of Ethiopian ancestry showed clinical criteria compatible with renal impairment. Two of these 3 patients also have severe poorly controlled diabetes mellitus. The third nondiabetic patient underwent renal biopsy which ruled out HIVAN. This absence of clinically apparent HIVAN was significantly different from that reported for African-Americans. The APOL1 G1 and G2 risk variants were found, respectively, in 0 and 2 (heterozygote state) of the 338 HIV-infected individuals. Global ancestry and the frequencies of the APOL1 G1 and G2 variants are not statistically different from their frequencies in the general Ethiopian population, but are significantly and dramatically lower than those observed among HIV-infected African-Americans, African-Americans and West Africans.

INTERPRETATION

The coinciding absence of HIVAN and the APOL1 risk variants among HIV-infected individuals of Ethiopian ancestry support a Western rather than Pan-African ancestry risk for ESKD, and can readily explain the lack of HIVAN among individuals of Ethiopian ancestry.

Apolipoprotein L1 nephropathy risk variants associate with HDL subfraction concentration in African Americans

BACKGROUND

Coding variants in the apolipoprotein L1 gene (APOL1) are strongly associated with non-diabetic nephropathy in African Americans. ApoL1 proteins associate with high-density lipoprotein (HDL) particles in the circulation. Plasma HDL particle subclass concentrations were compared in 73 African Americans based on APOL1 genotypes to detect differences potentially contributing to renal disease.

METHODS

HDL subclass concentrations were measured using nuclear magnetic resonance spectroscopy in African American first-degree relatives of patients with non-diabetic end-stage renal disease. Participants had estimated glomerular filtration rates (GFRs) > 80 mL/min and lacked albuminuria. Additive effects of the number of APOL1 risk variants on natural logarithm-transformed HDL subclass concentrations were computed.

RESULTS

Participants were 58.9% female with mean ± SD age 47.2 ± 13.3 years and GFR 92.4 ± 18.8 mL/min. The numbers with 2, 1 and 0 APOL1 nephropathy risk variants, respectively, were 36, 17 and 20. Mean ± SD medium-sized HDL concentrations were significantly lower for each additional APOL1 risk variant (2 versus 1 versus 0 risk variants: 9.0 ± 5.6 versus 10.1 ± 5.5 versus 13.1 ± 8.2 μmol/L, respectively; P = 0.0222 unadjusted; P = 0.0162 triglyceride- and ancestry adjusted).

CONCLUSIONS

Lower medium-sized HDL subclass concentrations are present in African Americans based on increasing numbers of APOL1 nephropathy risk variants. Potential mechanistic roles of altered medium HDL concentrations on APOL1-associated renal microvascular diseases should be evaluated.

MYH9 and APOL1 are both associated with sickle cell disease nephropathy

Renal failure occurs in 5-18% of sickle cell disease (SCD) patients and is associated with early mortality. At-risk SCD patients cannot be identified prior to the appearance of proteinuria and the pathobiology is not well understood. The myosin, heavy chain 9, non-muscle (MYH9) and apolipoprotein L1 (APOL1) genes have been associated with risk for focal segmental glomerulosclerosis and end-stage renal disease in African Americans. We genotyped 26 single nucleotide polymorphisms (SNPs) in MYH9 and 2 SNPs in APOL1 (representing the G1 and G2 tags) in 521 unrelated adult (18-83 years) SCD patients screened for proteinuria. Using logistic regression, SNPs were evaluated for association with proteinuria. Seven SNPs in MYH9 and one in APOL1 remained significantly associated with proteinuria after multiple testing correction (P < 0·0025). An MYH9 risk haplotype (P = 0·001) and the APOL1 G1/G2 recessive model (P < 0·0001) were strongly associated with proteinuria, even when accounting for the other. Glomerular filtration rate was negatively correlated with proteinuria (P < 0·0001), and was significantly predicted by an interaction between MYH9 and APOL1 in age-adjusted analyses. Our data provide insight into the pathobiology of renal dysfunction in SCD, suggesting that MYH9 and APOL1 are both associated with risk.

Genetics of kidney failure and the evolving story of APOL1

Chronic kidney disease (CKD) results from a wide array of processes that impair the kidney's ability to perform its major functions. As many as 20 million Americans suffer from CKD and nearly a half million from end-stage renal disease, but there are also examples of centenarians with adequate renal function. Family-based and genome-wide studies suggest that genetic differences substantially influence an individual's lifetime risk for kidney disease. One emerging theme is that evolution of genes related to host defense against pathogens may limit kidney longevity. The identification of these genetic factors will be critical for expanding our understanding of renal development and function as well as for the design of novel therapeutics for kidney disease.

Clinical and plasma proteomic markers correlating with chronic kidney disease after liver transplantation

Chronic kidney disease (CKD) occurs frequently after liver transplantation (LT) and is associated with significant morbidity and mortality. Thus, there is a pressing need to identify characteristics and biomarkers diagnostic of CKD to enable early diagnosis allowing preemptive interventions, as well as mechanistic insights into the progression from kidney injury to irreversible kidney failure. We analyzed 342 patients who had baseline glomerular filteration rate (GFR) >60 at the time of LT and are now >3 years post-LT. Risk factors for post-LT CKD were compared between three different groups defined by current GFR: >90 (n = 40), 60-90 (n = 146) and <60 (n = 156) mL/min. Age, cyclosporine use and pre-LT GFR were independently associated with new onset CKD. A subset (n = 64) without viral/immune disease or graft dysfunction underwent multianalyte plasma proteomic evaluations for correlation with CKD. Plasma proteomic analysis of two independent cohorts, test (n = 22) and validation (n = 42), identified 10 proteins highly associated with new onset CKD. In conclusion, we have identified clinical characteristics and a unique plasma proteomic signature correlating with new onset CKD after LT. These preliminary results are currently being validated in a prospective, multicenter study to determine if this signature precedes the onset of CKD and resolves with early interventions aimed at preserving kidney function.

Novel susceptibility locus at 22q11 for diabetic nephropathy in type 1 diabetes

Background

Diabetic nephropathy (DN) affects about 30% of patients with type 1 diabetes (T1D) and contributes to serious morbidity and mortality. So far only the 3q21–q25 region has repeatedly been indicated as a susceptibility region for DN. The aim of this study was to search for new DN susceptibility loci in Finnish, Danish and French T1D families.

Methods and Results

We performed a genome-wide linkage study using 384 microsatellite markers. A total of 175 T1D families were studied, of which 94 originated from Finland, 46 from Denmark and 35 from France. The whole sample set consisted of 556 individuals including 42 sib-pairs concordant and 84 sib-pairs discordant for DN. Two-point and multi-point non-parametric linkage analyses were performed using the Analyze package and the MERLIN software. A novel DN locus on 22q11 was identified in the joint analysis of the Finnish, Danish and French families by genome-wide multipoint non-parametric linkage analysis using the Kong and Cox linear model (NPL_pairs LOD score 3.58). Nominal or suggestive evidence of linkage to this locus was also detected when the three populations were analyzed separately. Suggestive evidence of linkage was found to six additional loci in the Finnish and French sample sets.

Conclusions

This study identified a novel DN locus at chromosome 22q11 with significant evidence of linkage to DN. Our results suggest that this locus may be of importance in European populations. In addition, this study supports previously indicated DN loci on 3q21–q25 and 19q13.

Nephrotoxicity of HAART

Highly active antiretroviral therapy (HAART) and other medical therapies for HIV-related infections have been associated with toxicities. Antiretroviral therapy can contribute to renal dysfunction directly by inducing acute tubular necrosis, acute interstitial nephritis, crystal nephropathy, and renal tubular disorders or indirectly via drug interactions. With the increase in HAART use, clinicians must screen patients for the development of kidney disease especially if the regimen employed increases risk of kidney injury. It is also important that patients with chronic kidney disease (CKD) are not denied the best combinations, especially since most drugs can be adjusted based on the estimated GFR. Early detection of risk factors, systematic screening for chronic causes of CKD, and appropriate referrals for kidney disease management should be advocated for improved patient care. The interaction between immunosuppressive therapy and HAART in patients with kidney transplants and the recent endorsement of tenofovir/emtricitabine by the Centers for Disease Control (CDC) for preexposure prophylaxis bring a new dimension for nephrotoxicity vigilance. This paper summarizes the common antiretroviral drugs associated with nephrotoxicity with particular emphasis on tenofovir and protease inhibitors, their risk factors, and management as well as prevention strategies.

Genetics of proteinuria: an overview of gene mutations associated with nonsyndromic proteinuric glomerulopathies

Heritable causes of proteinuria are rare and account for a relatively small proportion of all cases of proteinuria affecting children and adults. Yet, significant contributions to understanding the mechanistic basis for proteinuria have been made through genetic and molecular analyses of a small group of syndromic and nonsyndromic proteinuric disorders which are caused by mutations encoding structural components of the glomerular filtration barrier. Technological advances in genomic analyses and improved accessibility to mutational screening at clinically approved laboratories have facilitated diagnosis of proteinuria in the clinical setting. From a clinical standpoint, it may be argued that a genetic diagnosis mitigates exposure to potentially ineffective and harmful treatments in instances where a clear genotype-phenotype correlation exists between a specific gene mutation and treatment nonresponsiveness. However, cautious interpretation of risk may be necessitated in cases with phenotypic heterogeneity (eg, variability in clinical or histological presentation). This review summarizes gene mutations which are known to be associated with proteinuric glomerulopathies in children and adults.

Advances in our understanding of the pathogenesis of HIV-1 associated nephropathy in children

Childhood HIV-1 associated nephropathy (HIVAN) is a clinical and renal histological disease characterized by heavy proteinuria associated with focal and segmental glomerular sclerosis and/or mesangial hyperplasia in combination with microcystic tubular dilatation. These lesions lead to renal enlargement and rapid progression to kidney failure. Children of African ancestry have a unique susceptibility to developing HIVAN. It is estimated that approximately 300,000 HIV-infected children living in the sub-Saharan Africa could develop HIVAN if they do not receive appropriate antiretroviral therapy. This article discusses recent developments and controversies related to the pathogenesis of childhood HIVAN. The role of host genetic factors, including the newly identified variants in the APOL1 gene, is discussed in the context of previous studies that established the pathological paradigm for HIVAN, and our current understanding of the functional genomics analysis. Hopefully, these advances will provide new research opportunities to generate better treatments for children with HIVAN.

Plasma from a case of recurrent idiopathic FSGS perturbs non-muscle myosin IIA (MYH9 protein) in human podocytes

The MYH9 gene encodes a non-muscle myosin IIA heavy chain (NMMHC-IIA) expressed in podocytes. Heterozygous MYH9 mutations cause a set of overlapping syndromes characterized by variable degrees of deafness, morphologic abnormalities of platelets and focal segmental glomerulosclerosis (FSGS) with progressive renal dysfunction. Similar glomerular lesions are seen in a variety of nephropathies, including an idiopathic form of FSGS in children which recurs in renal allografts, implying a circulating factor that affects glomerular podocyte biology. It is unknown whether NMMHC-IIA is perturbed in the idiopathic form of FSGS. We describe a pediatric patient with typical idiopathic FSGS, in whom proteinuria recurred within hours of deceased donor renal transplantation but who responded to plasmapheresis. We demonstrate in vitro that plasmapheresis effluent from our patient rapidly decreased cultured podocyte levels of the phosphorylated myosin light chain (MLC) that mediates NMMHC-IIA binding to actin and induced dispersion of NMMHC-IIA from its usual position along actin stress fibers. FSGS plasma also caused dispersion of slit diaphragm proteins (nephrin and podocin) and vinculin-positive focal adhesion complexes. Our observations suggest that the putative circulating factor in idiopathic FSGS disrupts normal NMMHC-IIA function in podocytes and might contribute to the pathogenesis of recurrent FSGS in other children.

Renal disease in HIV-infected individuals

PURPOSE OF REVIEW

Highly active antiretroviral therapy (HAART) has resulted in a marked decrease in AIDS-related conditions and death. With improved survival, cardiovascular disease, hepatic, renal disease, and non-AIDS-related cancers represent an increasing burden for HIV-infected individuals.

RECENT FINDINGS

HIV-associated nephropathy (HIVAN), acute renal injury, HAART, and comorbid conditions such as hepatitis C, hypertension, and diabetes are among the multiple causes of renal disease. In HIVAN there is incomplete understanding of the interaction of the virus with renal cells and the host genetics leading to susceptibility to this form of renal dysfunction. There is agreement that a baseline estimated glomerular filtration should be obtained and that renal function should be monitored during antiretroviral therapy. There is, however, no agreement as to the most accurate method of estimating GFR. Renal transplantation has emerged as a feasible and successful modality of management of end-stage renal disease (ESRD) in HIV-infected individuals.

SUMMARY

Kidney disease represents an increasing concern in the care of HIV-infected persons, although there are questions remaining regarding the pathophysiology of HIVAN. Transplantation, however, can be carried out safely in infected persons with ESRD.

Differential effects of MYH9 and APOL1 risk variants on FRMD3 Association with Diabetic ESRD in African Americans

Single nucleotide polymorphisms (SNPs) in MYH9 and APOL1 on chromosome 22 (c22) are powerfully associated with non-diabetic end-stage renal disease (ESRD) in African Americans (AAs). Many AAs diagnosed with type 2 diabetic nephropathy (T2DN) have non-diabetic kidney disease, potentially masking detection of DN genes. Therefore, genome-wide association analyses were performed using the Affymetrix SNP Array 6.0 in 966 AA with T2DN and 1,032 non-diabetic, non-nephropathy (NDNN) controls, with and without adjustment for c22 nephropathy risk variants. No associations were seen between FRMD3 SNPs and T2DN before adjusting for c22 variants. However, logistic regression analysis revealed seven FRMD3 SNPs significantly interacting with MYH9-a finding replicated in 640 additional AA T2DN cases and 683 NDNN controls. Contrasting all 1,592 T2DN cases with all 1,671 NDNN controls, FRMD3 SNPs appeared to interact with the MYH9 E1 haplotype (e.g., rs942280 interaction p-value = 9.3E⁻⁷ additive; odds ratio [OR] 0.67). FRMD3 alleles were associated with increased risk of T2DN only in subjects lacking two MYH9 E1 risk haplotypes (rs942280 OR = 1.28), not in MYH9 E1 risk allele homozygotes (rs942280 OR = 0.80; homogeneity p-value = 4.3E⁻⁴). Effects were weaker stratifying on APOL1. FRMD3 SNPS were associated with T2DN, not type 2 diabetes per se, comparing AAs with T2DN to those with diabetes lacking nephropathy. T2DN-associated FRMD3 SNPs were detectable in AAs only after accounting for MYH9, with differential effects for APOL1. These analyses reveal a role for FRMD3 in AA T2DN susceptibility and accounting for c22 nephropathy risk variants can assist in detecting DN susceptibility genes.

The population genetics of chronic kidney disease: insights from the MYH9-APOL1 locus

Many rare kidney disorders exhibit a monogenic, Mendelian pattern of inheritance. Population-based genetic studies have identified many genetic variants associated with an increased risk of developing common kidney diseases. Strongly associated variants have potential clinical uses as predictive markers and may advance our understanding of disease pathogenesis. These principles are elegantly illustrated by a region within chromosome 22q12 that has a strong association with common forms of kidney disease. Researchers had identified DNA sequence variants in this locus that were highly associated with an increased prevalence of common chronic kidney diseases in people of African ancestry. Initial research concentrated on MYH9 as the most likely candidate gene; however, population-based whole-genome analysis enabled two independent research teams to discover more strongly associated mutations in the neighboring APOL1 gene. The powerful evolutionary selection pressure of an infectious pathogen in West Africa favored the spread of APOL1 variants that protect against a lethal form of African sleeping sickness but are highly associated with an increased risk of kidney disease. We describe the data sources, process of discovery, and reasons for initial misidentification of the candidate gene, as well as the lessons that can be learned for future population genetics research.

The APOL1 gene and allograft survival after kidney transplantation

Coding variants in the apolipoprotein L1 gene (APOL1) are strongly associated with nephropathy in African Americans (AAs). The effect of transplanting kidneys from AA donors with two APOL1 nephropathy risk variants is unknown. APOL1 risk variants were genotyped in 106 AA deceased organ donors and graft survival assessed in 136 resultant kidney transplants. Cox-proportional hazard models tested for association between time to graft failure and donor APOL1 genotypes. The mean follow-up was 26.4 ± 21.8 months. Twenty-two of 136 transplanted kidneys (16%) were from donors with two APOL1 nephropathy risk variants. Twenty-five grafts failed; eight (32%) had two APOL1 risk variants. A multivariate model accounting for donor APOL1 genotype, overall African ancestry, expanded criteria donation, recipient age and gender, HLA mismatch, CIT and PRA revealed that graft survival was significantly shorter in donor kidneys with two APOL1 risk variants (hazard ratio [HR] 3.84; p = 0.008) and higher HLA mismatch (HR 1.52; p = 0.03), but not for overall African ancestry excluding APOL1. Kidneys from AA deceased donors harboring two APOL1 risk variants failed more rapidly after renal transplantation than those with zero or one risk variants. If replicated, APOL1 genotyping could improve the donor selection process and maximize long-term renal allograft survival.

Expert opinion on pharmacotherapy of kidney disease in HIV-infected patients

INTRODUCTION

Human immunodeficiency virus (HIV) infection is associated with the development of a wide spectrum of kidney diseases. HIV-associated nephropathy (HIVAN) is the most common cause of chronic kidney disease (CKD) in HIV-infected individuals and predominantly affects patients of African ancestry. HIVAN is a leading cause of end-stage renal disease (ESRD) among African-Americans.

AREAS COVERED

An overview of the spectrum of kidney disease in patients with HIV is given. Current pharmacologic interventions to treat kidney disease in HIV are discussed. This review will enhance knowledge regarding the most common causes of kidney disease in HIV-infected patients. An understanding of the principles related to pharmacotherapy in HIV-infected patients with kidney disease will also be gained.

EXPERT OPINION

Kidney disease is an important cause of morbidity and mortality in HIV-infected patients. The most common cause of chronic kidney disease in this population is HIV-associated nephropathy, which is caused by viral infection of the renal epithelium. Several medications that are commonly used in HIV-infected patients can have adverse effects on the kidneys and the doses of many antiretroviral medications need to be adjusted in patients with impaired renal function.

[Pathological peculiarities of chronic kidney disease in patient from sub-Saharan Africa. Review of data from the Democratic Republic of the Congo]

Chronic kidney disease (CKD) is a major global public health problem. But kidney involvement is more common and appears more severe in Africa than in developed countries. The likely causes of end stage renal disease (ESRD) or CKD stage 3 and above in developed countries are diabetes, hypertension and less frequently glomerular diseases. In contrast, in decreasing order in Africa are glomerulopathies, hypertension and diabetes. The reasons for this preponderance of glomerular diseases are not fully known but may be linked to the persistence or reemergence of tropical diseases. This study reviews the kidney involvements more associated with common tropical diseases including HIV/AIDS. The most common HIV/AIDS lesion is a specific focal and segmental glomerulosclerosis (FSGS) termed HIV-associated nephropathy (HIV-AN). Renal complications of tropical parasites are heterogenous. Various glomerulopathies like FSGS occur during various filariasis infections. Schistosoma mansoni is responsible for membranoproliferative glomerulonephritis and amyloidosis. Human African trypanosomiasis is associated with cryoglobulinemic membranoproliferative glomerulonephritis. The Plasmodium malariae is mainly responsible for membranoproliferative glomerulonephritis. Acute patterns (acute tubular necrosis or acute postinfectious glomerulonephritis) are observed during Plasmodium falciparum infection. Several other viral, bacterial or mycobacterial infections like leprosy and tuberculosis still prevalent in Africa can also affect the kidney. Sickle cell disease is responsible for a variety of renal injuries. In conclusion, kidney lesions linked to tropical diseases partly explain the peculiar pattern of CKD of the black race and play a significant role in the current outbreak of the CKD in Subsaharan Africa.

The MYH9/APOL1 region and chronic kidney disease in European-Americans

Polymorphisms in the MYH9 and adjacent APOL1 gene region demonstrate a strong association with non-diabetic kidney disease in African-Americans. However, it is not known to what extent these polymorphisms are present in other ethnic groups. To examine the association of genetic polymorphisms in this region with chronic kidney disease (CKD; estimated glomerular filtration rate <60 ml/min/1.73 m(2)) in individuals of European ancestry, we examined rs4821480, an MYH9 single-nucleotide polymorphism (SNP) recently identified as associated with kidney disease in African-Americans, in 13 133 participants from the Framingham Heart Study (FHS) and Atherosclerosis Risk in Communities (ARIC) Study. In addition, we further interrogated the MYH9/APOL1 gene region using 282 SNPs for association with CKD using age-, sex- and center-adjusted models and performed a meta-analysis of the results from both studies. Because of prior data linking rs4821480 and kidney disease, we used a P-value of <0.05 to test the association with CKD. In the meta-analysis, rs4821480 (minor allele frequency 4.45 and 3.96% in FHS and ARIC, respectively) was associated with higher CKD prevalence in participants free of diabetes (odds ratio 1.44; 95% confidence interval 1.15-1.80; P = 0.001). No other SNPs achieved significance after adjusting for multiple testing. Results utilizing directly genotyped data confirmed the results of the primary analysis. Recently identified APOL1 risk variants were also directly genotyped, but did not account for the observed MYH9 signal. These data suggest that the MYH9 polymorphism rs4821480 is associated with an increased risk of non-diabetic CKD in individuals of European ancestry.

Podocyte-specific deletion of Myh9 encoding nonmuscle myosin heavy chain 2A predisposes mice to glomerulopathy

Genome-wide association studies linked single-nucleotide polymorphisms (SNPs) at the MYH9 locus to chronic kidney disease among African-Americans, particularly glomerular diseases such as HIV nephropathy and idiopathic focal and segmental glomerulosclerosis (FSGS). However, these MYH9 SNPs are intronic, and despite extensive sequencing, a causal variant remains elusive. To investigate the role of MYH9 in kidney disease, we selectively deleted Myh9 from mouse podocytes and found that mutant C57BL/6 mice did not develop renal insufficiency or proteinuria compared to control littermates, even when the mice were aged for 9 months. To explain the surprisingly normal phenotype, we considered genetic redundancy with the paralog Myh10 in podocytes, but we found that Myh10 was not expressed in podocytes in Myh9-deficient or control mice. We tested whether Myh9 podocyte deletion predisposed mice to glomerulopathy in response to injury by doxorubicin hydrochloride (Adriamycin), and we found that Myh9 podocyte-deleted mice developed proteinuria and glomerulosclerosis, while control mice were resistant. In summary, Myh9 podocyte deletion in C57BL/6 mice results in susceptibility to experimental doxorubicin hydrochloride glomerulopathy. We review evidence that MYH9 dysfunction in humans results in similar susceptibility and place our data, the first examination of Myh9 kidney disease in experimental animals, in the context of recent findings in human kidney disease, including the role of APOL1.

The search for host genetic factors of HIV/AIDS pathogenesis in the post-genome era: progress to date and new avenues for discovery

Though pursuit of host genetic factors that influence the pathogenesis of HIV began over two decades ago, progress has been slow. Initial genome-level searches for variations associated with HIV-related traits have yielded interesting candidates, but less in the way of novel pathways to be exploited for therapeutic targets. More recent genome-wide association studies (GWAS) that include different phenotypes, novel designs, and that have examined different population characteristics suggest novel targets and affirm the utility of additional searches. Recent findings from these GWAS are reviewed, new directions for research are identified, and the promise of systems biology to yield novel insights is discussed.

Effects of race on albuminuria and risk of cardiovascular and kidney disease

The literature describing the relationship between urinary protein excretion and risk of cardiovascular disease and renal disease is rapidly proliferating. Several studies have demonstrated racial differences in the relationship between albuminuria and associated disorders. The purpose of this article is to summarize the effects of race on the relationship between albuminuria and renal and cardiovascular disease risk, propose explanatory hypotheses, and suggest directions for future investigation.

Polymorphisms in the nonmuscle myosin heavy chain 9 gene (MYH9) are associated with the progression of IgA nephropathy in Chinese

BACKGROUND

IgA nephropathy (IgAN) is the leading cause of end-stage renal disease (ESRD) in China considering different compositions of ESRD causes in different ethnicities. A recent genome-wide association study (GWAS) indicated that the MYH9 gene was significantly associated with non-diabetic ESRD in African-Americans and also influenced kidney function in Europeans. Thus, in the present study, we aim to clarify whether MYH9 confers a shared mechanism among different causes of ESRD and to seek possible further insight into our understanding of IgAN by applying GWAS data from ESRD to IgAN.

METHODS

One thousand one hundred and sixteen Chinese, including 527 patients with renal biopsy-proven IgAN and 589 healthy controls, were enrolled in the present study. Four single neucleotide polymorphisms (SNPs) (rs3752462, rs4821480, rs11089788 and rs2413396) reported to be associated with ESRD with the most significance were genotyped by TaqMan assay or a restriction fragment length polymorphism assay for a further case-control study.

RESULTS

None of the four SNPs was associated with the susceptibility to IgAN or clinical and pathological characters at the time of renal biopsy. However, estimated glomerular filtration rate decline rate was associated with rs11089788 in the dominant model (P = 0.021). Cox regression showed that rs11089788 (hazard ratio, 3.95; 95% confidence interval, 1.23-12.63; P = 2.1 × 10(-2)) was an independent predictive factor for renal survival.

CONCLUSIONS

Based on a large Chinese IgAN cohort, we found an association between rs11089788 and prognosis of IgAN, adding to the mounting evidence of MYH9 as an important gene in IgAN to ESRD.

Hereditary kidney diseases: highlighting the importance of classical Mendelian phenotypes

A Mendelian inheritance underlies a nonnegligible proportion of hereditary kidney diseases, suggesting that the encoded proteins are essential for maintenance of the renal function. The identification of genes involved in congenital anomalies of the kidney and in familial forms of nephrotic syndrome significantly increased our understanding of the renal development and kidney filtration barrier physiology. This review will focus on the classical phenotype and clinical heterogeneity observed in the monogenic forms of these disorders. In addition, the role of susceptibility genes in kidney diseases with a complex inheritance will also be discussed.

Admixture mapping of end stage kidney disease genetic susceptibility using estimated mutual information ancestry informative markers

BACKGROUND

The question of a genetic contribution to the higher prevalence and incidence of end stage kidney disease (ESKD) among African Americans (AA) remained unresolved, until recent findings using admixture mapping pointed to the association of a genomic locus on chromosome 22 with this disease phenotype. In the current study we utilize this example to demonstrate the utility of applying a multi-step admixture mapping approach.

METHODS

A multi-step case only admixture mapping study, consisted of the following steps was designed: 1) Assembly of the sample dataset (ESKD AA); 2) Design of the estimated mutual information ancestry informative markers (n = 2016) screening panel 3); Genotyping the sample set whose size was determined by a power analysis (n = 576) appropriate for the initial screening panel; 4) Inference of local ancestry for each individual and identification of regions with increased AA ancestry using two different ancestry inference statistical approaches; 5) Enrichment of the initial screening panel; 6) Power analysis of the enriched panel 7) Genotyping of additional samples. 8) Re-analysis of the genotyping results to identify a genetic risk locus.

RESULTS

The initial screening phase yielded a significant peak using the ADMIXMAP ancestry inference program applying case only statistics. Subgroup analysis of 299 ESKD patients with no history of diabetes yielded peaks using both the ANCESTRYMAP and ADMIXMAP ancestry inference programs. The significant peak was found on chromosome 22. Genotyping of additional ancestry informative markers on chromosome 22 that took into account linkage disequilibrium in the ancestral populations, and the addition of samples increased the statistical significance of the finding.

CONCLUSIONS

A multi-step admixture mapping analysis of AA ESKD patients replicated the finding of a candidate risk locus on chromosome 22, contributing to the heightened susceptibility of African Americans to develop non-diabetic ESKD, and underscores the importance of using mutual information and multiple ancestry inference approaches to achieve a robust analysis, using relatively small datasets of "affected" only individuals. The current study suggests solutions to some limitations of existing admixture mapping methodologies, such as considerations regarding the distribution of ancestry information along the genome and its effects on power calculations and sample size.

The apolipoprotein L1 (APOL1) gene and nondiabetic nephropathy in African Americans

Mapping by admixture linkage disequilibrium (LD) detected strong association between nonmuscle myosin heavy chain 9 gene (MYH9) variants on chromosome 22 and nondiabetic nephropathy in African Americans. MYH9-related variants were posited to be the probable, but not necessarily the definitive, causal variants as a result of impressive statistical evidence of association, renal expression, and a role in autosomal dominant MYH9 disorders characterized by progressive glomerulosclerosis (Epstein and Fechtner syndromes). Dense mapping within MYH9 revealed striking LD patterns and racial variation in risk allele frequencies, suggesting population genetic factors such as selection may be operative in this region. Genovese and colleagues examined large chromosomal regions adjacent to MYH9 using genome-wide association methods and non-HapMap single nucleotide polymorphisms identified in Yoruba from the 1000 Genomes project. Statistically stronger associations were detected between two independent sequence variants in the Apolipoprotein L1 gene (APOL1) and nondiabetic nephropathy in African Americans, with odds ratios of 10.5 in idiopathic FSGS and 7.3 in hypertension-attributed ESRD. These kidney disease risk variants likely rose to high frequency in Africa because they confer resistance to trypanosomal infection and protect from African sleeping sickness. Risk variants in MYH9 and APOL1 are in strong LD, and the genetic risk that was previously attributed to MYH9 may reside, in part or in whole, in APOL1, although more complex models of risk cannot be excluded. This association likely explains racial disparities in nondiabetic nephropathy as a result of the high prevalence of risk alleles in individuals of African ancestry.

Recent developments in HIV-related kidney disease

Although kidney disease has been a recognized complication of HIV infection since the beginning of the HIV epidemic, its epidemiology, underlying causes and treatment have evolved in developed countries where HAART has been widely available. HIV-associated nephropathy and HIV immune complex-mediated kidney disease were the prominent renal diagnoses in the earlier period of the HIV epidemic. While HIV immune complex-mediated kidney disease remains a common finding among HIV-infected individuals with kidney disease, the incidence of HIV-associated nephropathy has been diminishing in developed countries. The role of the metabolic effects of long-term HAART exposure and nephrotoxicity of certain antiretroviral medications on the development and progression of chronic kidney disease is now of increasing concern. The long-term clinical implications of acute kidney injury among HIV-infected persons are increasingly recognized. Kidney disease in HIV-infected persons continues to be a major risk factor for morbidity and mortality in this patient population; therefore, early recognition and treatment of kidney disease are imperative in lessening the impact of kidney disease on the health of HIV-infected individuals. This review focuses on recent developments and ongoing challenges in the understanding, diagnosis and management of HIV-related kidney disease.