Background. Wilms' tumor suppressor gene (WT1) is essential for normal podocyte function, and transforming growth factor (TGF)-beta contributes to focal segmental glomerulosclerosis (FSGS). We aimed to address whether TGF-beta affects WT1 expression in podocytes.

Methods. A human podocyte cell line treated with TGF-beta1 and kidneys in Alb/TGF-beta1-transgenic mice were analyzed for WT1 expression.

Results. In cultured podocytes, TGF-beta1 reduced WT1 protein expression determined by western blotting beginning at 8 h and decreased WT1 messenger RNA (mRNA) expression measured by quantitative reverse transcription–polymerase chain reaction beginning at 3 h. Knockdown of Smad4 by small hairpin (sh) RNA partially rescued the TGF-beta1-induced reduction of both WT1 protein and mRNA expressions in the cultured podocytes. TGF-beta1 did not alter luciferase activity of the reporter construct for a human WT1 promoter but reduced that for a human WT1 5′ enhancer construct, suggesting that TGF-beta1 may regulate WT1 expression by altering the 5′ enhancer activity. In the transgenic mice, WT1 protein expression in podocytes was decreased at 1 and 3 weeks of age, while glomeruloclerosis developed after 3 weeks.

Conclusion. TGF-beta1 reduces WT1 expression in cultured human podocytes and podocytes in mice before overt glomerulosclerosis begins. The effects are at least partially Smad4 dependent. Our findings identify a novel pathway linking TGF-beta1 to podocyte injury and FSGS. The WT1 reduction may be a useful marker for early podocyte injury.

Recently, an association was found between non-diabetic kidney disease in African Americans and two independent sequence variants in the APOL1 gene, encoding apolipoprotein L1. In this study we determined the frequency of APOL1 risk variants in patients with biopsy-proven HIV-associated nephropathy (HIVAN) and distinctive pathological characteristics potentially driven by those risk variants. Among 76 patients with HIVAN, 60 were successfully genotyped for APOL1 G1 and G2 polymorphisms. In this cohort, 37 had two risk alleles, 18 were heterozygous and 5 had neither risk variant. There were no differences in the pathological findings of HIVAN and the number of APOL1 risk alleles. Further, the progression to end stage kidney disease or death did not differ by the number of risk alleles. Median renal survival was 9.3 months in patients with none or one risk allele compared to 11.7 months in patients with two APOL1 risk alleles. Thus, our study suggests that although the majority of African American patients with HIVAN have two APOL1 risk alleles, other as yet unknown factors in the host including genetic risk variants and environmental or viral factors may influence the development of this disorder in those with none or one APOL1 risk allele.

CKD is a major public health problem in the developed and the developing world. The degree of proteinuria associated with renal failure is a generally well accepted marker of disease severity. Agents with direct antiproteinuric effects are highly desirable therapeutic strategies for slowing, or even halting, progressive loss of kidney function. We review progress on therapies acting further downstream of the renin–angiotensin–aldosterone system pathway (e.g., transforming growth factor-beta antagonism, endothelin antagonism) and on those acting independent of the renin–angiotensin–aldosterone system pathway. In all, we discuss 26 therapeutic targets or compounds and 2 lifestyle changes (dietary modification and weight loss) that have been used clinically for diabetic or nondiabetic kidney disease. These therapies include endogenous molecules (estrogens, isotretinoin), biologic antagonists (monoclonal antibodies, soluble receptors), and small molecules. Where mechanistic data are available, these therapies have been shown to exert favorable effects on glomerular cell phenotype. In some cases, recent work has indicated surprising new molecular pathways for some therapies, such as direct effects on the podocyte by glucocorticoids, rituximab, and erythropoietin. It is hoped that recent advances in the basic science of kidney injury will prompt development of more effective pharmaceutical and biologic therapies for proteinuria.

We previously showed that kidney dysfunction/interstitial fibrosis by folate predisposes mice to sepsis mortality (normal/sepsis: 15%; folate/sepsis: 90%); agents that increased survival in normal septic mice were ineffective in the two-stage model. We used a recently characterized 5/6 nephrectomy (Nx) mouse model of progressive chronic kidney disease (CKD) to study how CKD impacts sepsis and acute kidney injury (AKI) induced by cecal ligation-puncture (CLP). CKD intensified sepsis severity (by kidney and liver injury, cytokines, and spleen apoptosis). Accumulation of HMGB1, VEGF, TNF-α, IL-6, or IL-10 was increased in CKD or sepsis alone and to a greater extent in CKD-sepsis, and only part of this effect could be explained by decreased renal clearance. Surprisingly, we found splenic apoptosis in CKD, even in the absence of sepsis. Although sFLT-1 effectively treated sepsis, it was ineffective against CKD-sepsis. Conversely, a single dose of HMGB1-neutralizing antiserum, administered 6h after sepsis alone was ineffective; however, CKD/sepsis was attenuated by anti-HMGB1. Splenectomy transiently decreased circulating HMGB1 levels, which reversed the effectiveness of anti-HMGB1 treatment on CKD/sepsis. We conclude that progressive CKD increases sepsis severity, in part, by reducing renal clearance of cytokines; CKD-induced splenic apoptosis and HMGB1 could be important common mediators for both CKD and sepsis.

Background

Conditionally immortalized podocytes are valuable research tools but are difficult to efficiently transfect and do not provide graded transgene expression.

Methods

Conditionally immortalized mouse podocyte cell lines were established employing a tetracycline-inducible system. Glomerular cells, isolated from transgenic mice bearing two transgenes, NPHS2-reverse tetracycline-controlled transactivator, rtTA (A transgene) and H2-Kb-thermosensitive SV40 T, ts58A (I transgene), were cloned. One clone (AI podocytes) expressing WT1 and synaptopodin was transfected with pBI-EGFP (enhanced green fluorescent protein, G transgene) and separately with ptTS-Neo (transcriptional suppressor, T transgene) to produce stable transformants, AIG podocytes and AIT podocytes.

Results

AIG podocytes expressed EGFP at 33 and 37°C after doxycycline treatment, and retained podocin and rtTA mRNA expression and temperature-sensitive growth regulation. AIT podocytes, transiently transfected with luciferase-BI-EGFP (LG transgene), showed reduced background expression of EGFP and luciferase in the absence of doxycycline. In AITLG podocytes, generated by stable transfection of AIT podocytes with the LG transgene, luciferase expression was tightly regulated by doxycycline in a time- and concentration-dependent manner both at 33 and 37°C, although background expression was not entirely eliminated. These podocytes retained temperature-sensitive growth regulation and expression of podocyte differentiation markers.

Conclusion

Mouse podocytes expressed tetracycline-induced transgenes efficiently while retaining differentiation markers.

Objective

Metabolic complications of HIV pose challenges for health maintenance among young adults who acquired HIV in early childhood.

Materials/Methods

Between 7/2004–7/2009 we evaluated 47 HIV-infected subjects who acquired HIV in early life. Participants completed glucose tolerance testing, insulin, lipid, urine albumin and creatinine determinations and DXA scans. Longitudinal data were available for 39 subjects; duration of follow-up was 26.4±16.8 months.

Results

At baseline, participants were 17.1±3.9y and duration of antiretroviral therapy was 12.7±3.4y. CD4 count was 658±374 cells/mm3 and 55% had undetectable viral load. Impaired glucose tolerance (IGT) was present in 15%; 33% had insulin resistance (HOMA-IR>4.0). Further, 52% had triglycerides ≥150 mg/dL, 36% had HDL-c <40 mg/dl, 18% had LDL-c ≥130 mg/dL and 25% had total cholesterol ≥200 mg/dL. Microalbuminuria was present in 15% of participants and was inversely correlated with CD4% (p=0.001). During follow-up more than one third remained insulin resistant; lipid parameters tended to improve. There were significant increases in BMI (p=0.0002), percent leg fat (p=0.008) and trunk fat (p=0.002).

Conclusions

IGT, insulin resistance, dyslipidemia and microalbuminuria are common among young adults with HIV. Long-term exposure to therapy may translate into substantial persistent metabolic risk.

Rapidly progressive glomerulonephritis (RPGN) is a clinical a morphological expression of severe glomerular injury. Glomerular injury manifests as a proliferative histological pattern (“crescents”) with accumulation of T cells and macrophages, and proliferation of intrinsic glomerular cells. We show de novo induction of heparin-binding epidermal growth factor-like growth factor (HB-EGF) in intrinsic glomerular epithelial cells (podocytes) from both mice and humans with RPGN. HB-EGF induction increases phosphorylation of the EGFR/ErbB1 receptor in mice with RPGN. In HB-EGF-deficient mice, EGFR activation in glomeruli is absent and the course of RPGN is improved. Autocrine HB-EGF induces a phenotypic switch in podocytes in vitro. Conditional deletion of the Egfr gene from podocytes of mice alleviates the severity of RPGN. Pharmacological blockade of EGFR also improves the course of RPGN, even when started 4 days after the induction of experimental RPGN. This suggests that targeting the HB-EGF/EGFR pathway could also be beneficial for treatment of human RPGN.

Background

Trophoblast glycoprotein (Tpbg), a 72-kDa transmembrane glycoprotein, is known to regulate the phenotypes of epithelial cells by modifying actin organization and cell motility. Recently, a microarray study showed that Tpbg is upregulated in Thy1 glomerulonephritis (Thy1 GN). We hypothesized that Tpbg regulates cytoskeletal rearrangement and modulates phenotypic alteration in podocytes under pathological conditions.

Methods

We examined Tpbg expression in Thy1 GN and Tpbg function in mouse podocytes.

Results

We demonstrated that Tpbg is upregulated in the injured podocytes of Thy1 GN. In vitro, immunofluorescence studies revealed that Tpbg colocalized with the focal adhesion protein, vinculin, in parallel with stress fiber formation. This colocalization was observed even when actin filaments were depolymerized with cytochalasin D. Tpbg localization at focal adhesions was induced by dominant-active RhoA and suppressed by the ROCK1 inhibitor Y-26732. In addition, transforming growth factor-β increased Tpbg expression at focal adhesions concurrently with rearrangement of stress fibers. Stress fiber formation was suppressed in differentiated podocytes transfected with full-length Tpbg. Furthermore, knockdown of Tpbg using small interfering RNA decreased podocyte motility.

Conclusion

Our findings suggest a novel role of Tpbg in the phenotypic alteration of injured podocytes, and we accordingly propose a new mechanism of glomerular injury in glomerulonephritis.

Inhibitors for protein-protein interactions are challenging to design, in part due to the unique and complex architectures of each protein’s interaction domain. Most approaches to develop inhibitors for these interactions rely on rational design, which requires prior structural knowledge of the target and its ligands. In the absence of structural information, a combinatorial approach may be the best alternative to finding inhibitors of a protein-protein interaction. Current chemical libraries, however, consist mostly of molecules designed to inhibit enzymes. In this manuscript, we report the synthesis and screening of a library based on an N-acylated polyamine (NAPA) scaffold that we designed to have specific molecular features necessary to inhibit protein-protein interactions. Screens of the library identified a member with favorable binding properties to the HIV viral protein R (Vpr), a regulatory protein from HIV that is involved in numerous interactions with other proteins critical for viral replication.

OBJECTIVE

The pathogenesis of diabetic nephropathy is complex and involves activation of multiple pathways leading to kidney damage. An important role for altered lipid metabolism via sterol regulatory element binding proteins (SREBPs) has been recently recognized in diabetic kidney disease. Our previous studies have shown that the farnesoid X receptor (FXR), a bile acid-activated nuclear hormone receptor, modulates renal SREBP-1 expression. The purpose of the present study was then to determine if FXR deficiency accelerates type 1 diabetic nephropathy in part by further stimulation of SREBPs and related pathways, and conversely, if a selective FXR agonist can prevent the development of type 1 diabetic nephropathy.

RESEARCH DESIGN AND METHODS

Insulin deficiency and hyperglycemia were induced with streptozotocin (STZ) in C57BL/6 FXR KO mice. Progress of renal injury was compared with nephropathy-resistant wild-type C57BL/6 mice given STZ. DBA/2J mice with STZ-induced hyperglycemia were treated with the selective FXR agonist INT-747 for 12 weeks. To accelerate disease progression, all mice were placed on the Western diet after hyperglycemia development.

RESULTS

The present study demonstrates accelerated renal injury in diabetic FXR KO mice. In contrast, treatment with the FXR agonist INT-747 improves renal injury by decreasing proteinuria, glomerulosclerosis, and tubulointerstitial fibrosis, and modulating renal lipid metabolism, macrophage infiltration, and renal expression of SREBPs, profibrotic growth factors, and oxidative stress enzymes in the diabetic DBA/2J strain.

CONCLUSIONS

Our findings indicate a critical role for FXR in the development of diabetic nephropathy and show that FXR activation prevents nephropathy in type 1 diabetes.

The specialized epithelial cell of the kidney, the podocyte, has a complex actin-based cytoskeleton. Dynamic regulation of this cytoskeleton is required for efficient barrier function of the kidney. Podocytes are a useful cell type to study the control of the actin cytoskeleton in vivo, because disruption of components of the cytoskeleton results in podocyte damage, cell loss, and a prototypic injury response called focal segmental glomerulosclerosis (FSGS). Searching for actin regulatory proteins that are expressed in podocytes, we identified a RhoA-activated Rac1 GTPase-activating protein (Rac1-GAP), Arhgap24, that was upregulated in podocytes as they differentiated, both in vitro and in vivo. Increased levels of active Rac1 and Cdc42 were measured in Arhgap24 knockdown experiments, which influenced podocyte cell shape and membrane dynamics. Consistent with a role for Arhgap24 in normal podocyte functioning in vivo, sequencing of the ARHGAP24 gene in patients with FSGS identified a mutation that impaired its Rac1-GAP activity and was associated with disease in a family with FSGS. Thus, Arhgap24 contributes to the careful balancing of RhoA and Rac1 signaling in podocytes, the disruption of which may lead to kidney disease.

Context

Etiologic factors and pathways leading to altered podocyte phenotype are clearly numerous and involve the activity of different cellular function.

Objective

To focus on recent discoveries in podocyte biology and genetics and their relevance to these human glomerular diseases, named podocytopathies.

Data Sources

Genetic mutations in genes encoding for proteins in the nucleus, slit diaphragm, podocyte cytoplasm, and cell membrane are responsible for podocyte phenotype and functional abnormalities. Podocyte injury may also derive from secondary stimuli, such as mechanical stress, infections, or use of certain medications. Podocytes can respond to injury in a limited number of ways, which include (1) effacement, (2) apoptosis, (3) arrest of development, and (4) dedifferentiation. Each of these pathways results in a specific glomerular morphology: minimal change nephropathy, focal segmental glomerulosclerosis, diffuse mesangial sclerosis, and collapsing glomerulopathy.

Conclusions

Based on current knowledge of podocyte biology, we organized etiologic factors and morphologic features in a taxonomy of podocytopathies, which provides a novel approach to the classification of these diseases. Current and experimental therapeutic approaches are also discussed.

Genetic variation in MYH9, encoding non-muscle heavy chain IIA, has been recognized for over a decade as the cause of an autosomal dominant syndrome characterized by macrothrombocytopenia, neutrophil inclusions, and glomerular pathology. More recently, genetic variation in the MYH9 region on chromosome 22 has been associated with chronic kidney disease in African-descent individuals. A better understanding of the disease mechanisms responsible for glomerular injury in autosomal dominant MYH9 syndromes will lead to fuller appreciation of the role of this gene in glomerular biology.

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.

Author Summary

African Americans have high rates of kidney disease attributed to type 2 diabetes mellitus. However, approximately 25% of patients are misclassified and have non-diabetic kidney disease on renal biopsy. The APOL1-MYH9 gene region on chromosome 22 is powerfully associated with non-diabetic kidney diseases in African Americans. Therefore, we tested for interactions between single nucleotide polymorphisms across the genome with APOL1 and MYH9 non-diabetic nephropathy risk variants in African Americans with presumed diabetic nephropathy. Markers in FRMD3, a gene associated with type 1 diabetic nephropathy in Caucasians, appeared to interact with MYH9; however, increased nephropathy risk was seen in diabetic cases lacking two MYH9 risk haplotypes, and protective effects were seen in those with two MYH9 risk haplotypes. Stratified analyses based on the chromosome 22 nephropathy risk haplotypes demonstrated that FRMD3 variants were associated with diabetic nephropathy risk in cases without two MYH9 (or APOL1) risk haplotypes. It appears that African Americans with diabetes and kidney disease who are not chromosome 22 nephropathy risk variant homozygotes are enriched for the presence of diabetic nephropathy and FRMD3 risk alleles. This genetic dissection ultimately allowed for detection of the FRMD3 diabetic nephropathy gene association in a subset of cases enriched for this disorder.

The remnant kidney model in C57BL6 mice does not develop progressive chronic kidney disease (CKD). In this study we modified the model to mimic features of human CKD and to define accelerants of disease progression using three strains of mice. Following the procedure there was a progressive increase in albuminuria, progressive loss in renal function, severe glomerulosclerosis and interstitial fibrosis, hypertension, cardiac fibrosis, and anemia by 4 weeks in CD-1 mice and by 12 weeks in 129S3 mice. In contrast, even after 16 weeks, the C57BL/6 mice with a remnant kidney had modestly increased albuminuria without increased blood pressure and without developing CKD or cardiac fibrosis. The baseline blood pressure, determined by radiotelemetry in conscious animals, correlated with CKD progression rates in each strain. Administering angiotensin II overcame the resistance of C57BL/6 mice to CKD following renal mass reduction displaying high blood pressure and albuminuria, severe glomerulosclerosis, and loss of renal function by 4 weeks. Decreasing blood pressure with olmesartan but not hydralazine in CD-1 mice with a remnant kidney reduced CKD progression and cardiac fibrosis. C57BL/6 mice with a remnant kidney and DOCA-salt hypertension developed modest CKD. Each strain had similar degrees of interstitial fibrosis in three different normotensive models of renal fibrosis. Thus reducing renal mass in CD-1 or 129S3 mice mimics many features of human CKD. Angiotensin II can convert the C57BL/6 strain from CKD-resistant to susceptible in this disease model.

Summary

Genetic variation in MYH9, encoding nonmuscle myosin IIA heavy chain, has been associated recently with increased risk for kidney disease. Previously, MYH9 missense mutations have been shown to cause the autosomal-dominant MYH9 (ADM9) spectrum, characterized by large platelets, leukocyte Döhle bodies, and, variably, sensorineural deafness, cataracts, and glomerulopathy. Genetic testing is indicated for familial and sporadic cases that fit this spectrum. By contrast, the MYH9 kidney risk variant is characterized by multiple intronic single nucleotide polymorphisms, but the causative variant has not been identified. Disease associations include human immunodeficiency virus-associated collapsing glomerulopathy, focal segmental glomerulosclerosis, hypertension-attributed end-stage kidney disease, and diabetes-attributed end-stage kidney disease. One plausible hypothesis is that the MYH9 kidney risk variant confers a fragile podocyte phenotype. In the case of hypertension-attributed kidney disease, it remains unclear if the hypertension is a contributing cause or a consequence of glomerular injury. The MYH9 kidney risk variant is strikingly more common among individuals of African descent, but only some will develop clinical kidney disease in their lifetime. Thus, it is likely that additional genes and/or environmental factors interact with the MYH9 kidney risk variant to trigger glomerular injury. A preliminary genetic risk stratification scheme, using 10 single nucleotide polymorphisms, may estimate lifetime risk for kidney disease. Nevertheless, at present, no role has been established for genetic testing as part of personalized medicine, but testing should be considered in clinical studies of glomerular diseases among populations of African descent. Such studies will address critical questions pertaining to MYH9-associated kidney disease, including mechanism, course, and response to therapy.

Background

Alb/TGF-β1 transgenic mice overexpress active transforming growth factor-β1 (TGF-β1) in the liver, leading to increased circulating levels of the cytokine and progressive renal fibrosis. This study was designed to explore if exogenous all-trans retinoic acid (tRA) prevents renal fibrosis in this animal model.

Methods

The retinoid profile in kidney and liver of wild-type and Alb/TGF-β1 transgenic mice was examined by high-performance liquid chromatography and slow-release pellets containing different amounts of tRA were implanted subcutaneously to treat the Alb/TGF-β1 transgenic mice, starting at 1 week of age; mice were sacrificed 2 weeks later.

Results

Kidneys of 3-week-old wild-type mice had abundant tRA, which was completely absent in kidneys of the transgenic mice. Low doses of tRA (6–10.7 mg/kg/day) failed to affect renal fibrosis although it tended to suppress the mRNA expression of some molecular markers of fibrosis and retinal dehydrogenase 2 (RALDH2), a gene encoding a key tRA-synthesising enzyme. These tendencies disappeared, mortality tended to increase and RALDH2 and connective tissue growth factor (CTGF) mRNAs significantly increased in the medium-dose group (12.7–18.8 mg/kg/day). High doses (20.1–27.4 mg/kg/day) showed even higher toxicity with increased renal fibrosis and significant mortality.

Conclusions

Alb/TGF-β1 transgenic mice are characterised by depletion of endogenous renal tRA. Exogenous tRA dose-dependently increases mortality and kidney fibrosis, which is associated with dose-dependent regulation of renal RALDH2 and CTGF mRNA expression.

Importance of the field

Many chronic diseases of various etiologies universally lead to fibrosis and organ dysfunction. Despite many advances in medicine in recent years, options to slow the progression of fibrotic diseases have remained limited. The recent availability of pirfenidone, an anti-fibrotic and anti-inflammatory investigational agent, thus offers a new hope for treating progressive fibrotic diseases.

Areas covered in this review

This review provides concise review of the available data regarding mechanism and pharmacokinetics of pirfenidone and preclinical and clinical data regarding efficacy and safety in fibrotic diseases of the kidney. It also reviews results of clinical trials involving pirfenidone in other fibrotic diseases.

What the reader will gain

The review will provide in-depth review of pirfenidone with a renal focus.

Take home message

Because many of the available clinical trials have been small and/or uncontrolled, conclusive evidence regarding efficacy and safety of pirfenidone is lacking, particularly in patients with renal or hepatic dysfunction. Larger studies are needed both to better understand long-term efficacy and safety of this medication in various patient populations.

Until recently knowledge of genetic causes of glomerular disease was limited to certain rare or uncommon inherited diseases, and to a genes, either rare or with small effect, identified in candidate gene studies. These genetic factors accounted for only a very small fraction of kidney disease. However, the striking differences in frequency of many forms of kidney disease between African Americans and European Americans, which could not be completely explained by cultural or economic factors, pointed to a large unidentified genetic influence. Since FSGS and HIV-associated collapsing glomerulopathy (HVAN) have striking racial disparities, we performed an admixture mapping study to identify contributing genetic factors. Admixture mapping identified genetic variants in the non-muscle myosin gene MYH9 as having an extreme influence on both FSGS and HIVAN, with odds ratios from 4 to 8 and attributable fractions of 70–100%. Previously identified, rare inherited MYH9 disorders point to a mechanism by which MYH9 variation disrupts the actin-myosin filaments responsible for maintaining the structure of podocytes, the cells that provide one of three filtration barriers in the glomeruli. MYH9 variation has a smaller but still highly significant effect on non-diabetic kidney disease, and a weaker but significant effect on diabetic kidney disease; it is unclear whether underlying cryptic FSGS is responsible for the MYH9 association with these diseases. The strong predicted power of MYH9 variation for disease indicates a clear role for genetic testing for these variants in personalized medicine, for assessment of genetic risk, and potentially for diagnosis.

Background

Retinoic acid is the bioactive derivative of vitamin A, which plays an indispensible role in kidney development by activating retinoic acid receptors. Although the location, concentration and roles of endogenous retinoic acid in post-natal kidneys are poorly defined, there is accumulating evidence linking post-natal vitamin A deficiency to impaired renal concentrating and acidifying capacity associated with increased susceptibility to urolithiasis, renal inflammation and scarring. The aim of this study is to examine the presence and the detailed localization of endogenous retinoic acid activity in neonatal, young and adult mouse kidneys, to establish a fundamental ground for further research into potential target genes, as well as physiological and pathophysiological roles of endogenous retinoic acid in the post-natal kidneys.

Methodology/Principal Findings

RARE-hsp68-lacZ transgenic mice were employed as a reporter for endogenous retinoic acid activity that was determined by X-gal assay and immunostaining of the reporter gene product, β-galactosidase. Double immunostaining was performed for β-galactosidase and markers of kidney tubules to localize retinoic acid activity. Distinct pattern of retinoic acid activity was observed in kidneys, which is higher in neonatal and 1- to 3-week-old mice than that in 5- and 8-week-old mice. The activity was present specifically in the principal cells and the intercalated cells of the collecting duct system in all age groups, but was absent from the glomeruli, proximal tubules, thin limbs of Henle's loop and distal tubules.

Conclusions/Significance

Endogenous retinoic acid activity exists in principal cells and intercalated cells of the mouse collecting duct system after birth and persists into adulthood. This observation provides novel insights into potential roles for endogenous retinoic acid beyond nephrogenesis and warrants further studies to investigate target genes and functions of endogenous retinoic acid in the kidney after birth, particularly in the collecting duct system.

SUMMARY

The glomerular microvasculature is particularly susceptible to injury in thrombotic microangiopathy, but the mechanisms by which this occurs are unclear. We report the cases of six patients who were treated with bevacizumab, a humanized monoclonal antibody against vascular endothelial growth factor (VEGF), in whom glomerular disease characteristic of thrombotic microangiopathy developed. To show that local reduction of VEGF within the kidney is sufficient to trigger the pathogenesis of thrombotic microangiopathy, we used conditional gene targeting to delete VEGF from renal podocytes in adult mice; this resulted in a profound thrombotic glomerular injury. These observations provide evidence that glomerular injury in patients who are treated with bevacizumab is probably due to direct targeting of VEGF by antiangiogenic therapy.

Background. Although MYH9 is strongly associated with biopsy-proven idiopathic and HIV-associated focal segmental glomerulosclerosis (FSGS) and clinically diagnosed ‘hypertension-associated’ end-stage renal disease (ESRD) in African Americans, its role in type 2 diabetes mellitus (T2DM)-associated ESRD is unclear.

Methods. To assess whether MYH9 was associated with T2DM-ESRD, 751 African Americans with T2DM-ESRD, 227 with T2DM lacking nephropathy and 925 non-diabetic non-nephropathy controls were genotyped for 14 MYH9 SNPs. Association analyses used SNPGWA and Dandelion.

Results. Comparing T2DM-ESRD cases with non-diabetic controls, single SNP associations were detected with 8 of 14 SNPs, gender- and admixture-adjusted P-values 0.047–0.005 [recessive model, odds ratio (OR) range 1.30–1.55]. The previously associated MYH9 E1 and L1 haplotypes were associated with T2DM-ESRD (E1: OR 1.27, 95% CI 1.04–1.56, P = 0.021 recessive and L1: OR 1.43, 95% CI 1.09–1.87, P = 0.009 dominant). Contrasting the 751 T2DM-ESRD cases with 227 T2DM non-nephropathy controls revealed that E1 haplotype SNPs rs4821480, rs2032487 and rs4821481 were associated with kidney failure (OR 1.38–1.40 recessive, all P < 0.048). Among E1 and L1 risk homozygotes, respectively, mean (SD) diabetes duration prior to renal replacement therapy was 16.6 (9.7) and 16.4 (10.0) years, and 65% had diabetic retinopathy.

Conclusions. Genetic dissection of T2DM-associated ESRD reveals that MYH9 underlies a portion of this clinically diagnosed disorder in African Americans. It is likely that a subset of African Americans with T2DM and coincident nephropathy have primary MYH9-related kidney disease (e.g. FSGS or global glomerulosclerosis), although renal biopsy studies need to be performed.

MYH9 was recently identified as renal susceptibility gene (OR 3–8, p<10−8) for major forms of kidney disease disproportionately affecting individuals of African descent. The risk haplotype (E-1) occurs at much higher frequencies in African Americans (≥60%) than in European Americans (<4%), revealing a genetic basis for a major health disparity. The population distributions of MYH9 risk alleles and the E-1 risk haplotype and the demographic and selective forces acting on the MYH9 region are not well explored. We reconstructed MYH9 haplotypes from 4 tagging single nucleotide polymorphisms (SNPs) spanning introns 12–23 using available data from HapMap Phase II, and by genotyping 938 DNAs from the Human Genome Diversity Panel (HGDP). The E-1 risk haplotype followed a cline, being most frequent within sub-Saharan African populations (range 50–80%), less frequent in populations from the Middle East (9–27%) and Europe (0–9%), and rare or absent in Asia, the Americas, and Oceania. The fixation indexes (FST) for pairwise comparisons between the risk haplotypes for continental populations were calculated for MYH9 haplotypes; FST ranged from 0.27–0.40 for Africa compared to other continental populations, possibly due to selection. Uniquely in Africa, the Yoruba population showed high frequency extended haplotype length around the core risk allele (C) compared to the alternative allele (T) at the same locus (rs4821481, iHs = 2.67), as well as high population differentiation (FST(CEU vs. YRI) = 0.51) in HapMap Phase II data, also observable only in the Yoruba population from HGDP (FST = 0.49), pointing to an instance of recent selection in the genomic region. The population-specific divergence in MYH9 risk allele frequencies among the world's populations may prove important in risk assessment and public health policies to mitigate the burden of kidney disease in vulnerable populations.

Background

MYH9 is a podocyte-expressed gene encoding nonmuscle myosin IIA that is associated with idiopathic and human immunodeficiency virus-associated focal segmental glomerulosclerosis (FSGS) and hypertensive end-stage renal disease in African Americans.

Methods

Four single nucleotide polymorphisms comprising the major MYH9 E1 risk haplotype were tested for association with estimated glomerular filtration rate (eGFR) and urine albumin:creatinine ratio (ACR) in 2,903 HyperGEN participants (1,458 African Americans (AA) in 895 families and 1,445 European Americans (EA) in 859 families) to determine the role of MYH9 in subclinical nephropathy. Association analyses employed general linear models in unrelated probands and generalized estimating equations in families. Adjustment was performed for age, sex, diabetes, BMI, medications, and mean arterial pressure separately in each race.

Results

Mean (SD) eGFR and ACR were 74.3 (16.0) ml/min/1.73 m2 and 20.3 (119.9) mg/g in EA, and 88.6 (20.9) ml/min/1.73 m2 and 76.8 (394.5) mg/g in AA (both p < 0.0001 across ethnicities). Urine ACR was associated with rs3752462 (p = 0.01) and rs4821481 (p = 0.05) in unrelated AA and with rs4821481 (p = 0.03), rs2032487 (p = 0.04) and the E1 3224 haplotype (p = 0.013) in AA families. Single nucleotide polymorphisms and the haplotype were not associated with ACR in EA or with eGFR in either ethnic group.

Conclusions

MYH9 variants are associated with albuminuria in hypertensive AA. The strength of the association was weaker than that in FSGS and hypertensive end-stage renal disease. MYH9 risk variants appear to be associated with primary FSGS with secondary hypertension, although nephrosclerosis may develop in response to hypertension in subjects homozygous for the MYH9 E1 risk haplotype.