Characterization of a major modifier locus for polycystic kidney disease (Modpkdr1) in the Han:SPRD(cy/+) rat in a region conserved with a mouse modifier locus for Alport syndrome

Molecular genetics of familial hematuric diseases

The familial hematuric diseases are a genetically heterogeneous group of monogenic conditions, caused by mutations in one of several genes. The major genes involved are the following: (i) the collagen IV genes COL4A3/A4/A5 that are expressed in the glomerular basement membranes (GBM) and are responsible for the most frequent forms of microscopic hematuria, namely Alport syndrome (X-linked or autosomal recessive) and thin basement membrane nephropathy (TBMN). (ii) The FN1 gene, expressed in the glomerulus and responsible for a rare form of glomerulopathy with fibronectin deposits (GFND). (iii) CFHR5 gene, a recently recognized regulator of the complement alternative pathway and mutated in a recently revisited form of inherited C3 glomerulonephritis (C3GN), characterized by isolated C3 deposits in the absence of immune complexes. A hallmark feature of all conditions is the age-dependent penetrance and a broad phenotypic heterogeneity in the sense that subsets of patients progress to added proteinuria or proteinuria and chronic renal failure that may or may not lead to end-stage kidney disease (ESKD) anywhere between the second and seventh decade of life. In addition to other excellent laboratory tools that assist the clinician in reaching the correct diagnosis, the molecular analysis emerges as the gold standard in establishing the diagnosis in many cases of doubt due to equivocal findings that complicate the differential diagnosis. Recent work led to the description of candidate genetic modifiers which confer a variable risk for progressing to chronic renal failure when co-inherited on the background of a primary glomerulopathy. Finally, more families are still waiting to be studied and more genes to be mapped and cloned that are responsible for other forms of heritable hematuric diseases. The study of such genes and their protein products will likely shed more light on the structure and function of the glomerular filtration barrier and other important glomerular components.

Genetic background of nonmutant Piebald-Virol-Glaxo rats does not influence nephronophthisis phenotypes

BACKGROUND

Nephronophthisis (NPHP), which affects multiple organs, is a hereditary cystic kidney disease (CKD), characterized by interstitial fibrosis and numerous fluid-filled cysts in the kidneys. It is caused by mutations in NPHP genes, which encode for ciliary proteins known as nephrocystins. The disorder affects many people across the world and leads to end-stage renal disease. The aim of this study was to determine if the genetic background of the nonmutant female Piebald-Virol-Glaxo (PVG/Seac(-/-)) rat influences phenotypic inheritance of NPHP from mutant male Lewis polycystic kidney rats.

METHODS

Mating experiments were performed between mutant Lewis polycystic kidney male rats with CKD and nonmutant PVG and Wistar Kyoto female rats without cystic kidney disease to raise second filial and backcross 1 progeny, respectively. Rats that developed cystic kidneys were identified. Systolic blood pressure was determined in each rat at 12 weeks of age using the tail and cuff method. After euthanasia, blood samples were collected and chemistry was determined. Histological examination of the kidneys, pancreas, and liver of rats with and without cystic kidney disease was performed.

RESULTS

It was established that the genetic background of nonmutant female PVG rats did not influence the phenotypic inheritance of the CKD from mutant male Lewis polycystic kidney rats. The disease arose as a result of a recessive mutation in a single gene (second filial generation, CKD = 13, non-CKD = 39, χ (2) = 0.00, P ≥ 0.97; backcross 1 generation, CKD = 67, non-CKD = 72, χ (2) = 0.18, P > 0.05) and inherited as NPHP. The rats with CKD developed larger fluid-filled cystic kidneys, higher systolic blood pressure, and anemia, but there were no extrarenal cysts and disease did not lead to early pup mortality.

CONCLUSION

The genetic background of the nonmutant PVG rats does not influence the genetic and phenotypic inheritance of CKD from mutant Lewis polycystic kidney rats. A single recessive mutation incapacitated the gene, which relaxed its functional constraints, and led to formation of multiple cysts in the kidneys of the homozygous mutant rats.

Inhibition of Comt with tolcapone slows progression of polycystic kidney disease in the more severely affected PKD/Mhm (cy/+) substrain of the Hannover Sprague-Dawley rat

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common human inherited diseases. Modifier genes seem to modulate the disease progression and might therefore be promising drug targets. Although a number of modifier loci have been already identified, no modifier gene has been proven to be a real modifier yet.

METHODS

Gene expression profiling of two substrains of the Han:SPRD rat, namely PKD/Mhm and PKD/US, both harboring the same mutation, was conducted in 36-day-old animals. Catechol-O-methyltransferase (Comt) was identified as a potential modifier gene. A 3-month treatment with tolcapone, a selective inhibitor of Comt, was carried out in PKD/Mhm and PKD/US (cy/+) animals.

RESULTS

Comt is localized within a known modifier locus of PKD (MOP2). The enzyme encoding gene was found upregulated in the more severely affected PKD/Mhm substrain and was hence presumed to be a putative modifier gene of PKD. The treatment with tolcapone markedly attenuated the loss of renal function, inhibited renal enlargement, shifted the size distribution of renal cysts and retarded cell proliferation, apoptosis, inflammation and fibrosis development in affected (cy/+) male and female PKD/Mhm and PKD/US rats.

CONCLUSIONS

Comt has been confirmed to be the first reported modifier gene for PKD and tolcapone offers a promising drug for treating PKD.

Role of genetic modifiers in an orthologous rat model of ARPKD

Human data and animal models of autosomal recessive polycystic kidney disease (ARPKD) suggest that genetic factors modulate the onset and severity of the disease. We report here for the first time that ARPKD susceptibility is attenuated by introgressing the mutated Pkhd1 disease allele from the polycystic kidney (PCK) rat onto the FHH (Fawn-Hooded Hypertensive) genetic background. Compared with PCK, the FHH.Pkhd1 strain had significantly decreased renal cyst formation that coincided with a threefold reduction in mean kidney weights. Further analysis revealed that the FHH. Pkhd1 is protected from increased blood pressure as well as elevated plasma creatinine and blood urea nitrogen levels. On the other hand, liver weight and biliary cystogenesis revealed no differences between PCK and FHH.Pkdh1, indicating that genes within the FHH genetic background prevent the development of renal, but not hepatic, manifestations of ARPKD. Microarray expression analysis of kidneys from 30-day-old PCK rats revealed increased expression of genes previously identified in PKD renal expression profiles, such as inflammatory response, extracellular matrix synthesis, and cell proliferation genes among others, whereas the FHH.Pkhd1 did not show activation of these common markers of disease. This newly developed strain can serve as a tool to map modifier genes for renal disease in ARPKD and provides further insight into disease variability and pathophysiology.

Two non-invasive GFR-estimation methods in rat models of polycystic kidney disease: 3.0 Tesla dynamic contrast-enhanced MRI and optical imaging

BACKGROUND

The aim of this study was the assessment of kidney morphology and glomerular filtration rate (GFR) in rat models of polycystic kidney disease and a healthy control group of Sprague-Dawley rats (SD rats). The performance of two non-invasive GFR estimation methods-3.0 Tesla magnetic resonance imaging (MRI) and optical imaging were investigated. Data of GFR assessment was compared to surrogate markers of kidney function and renal histology.

METHODS

Optical imaging of GFR was performed transcutaneously in a small animal imaging system with the fluorescent renal marker fluorescein-isothiocyanate-labelled-sinistrin. Morphologic and dynamic renal imaging was done on a clinical 3.0T MR scanner. Renal perfusion analysis was performed with a two-compartment filtration model.

RESULTS

The healthy SD rats showed physiological levels of creatinine and urea, indicating normal kidney function. These parameters were elevated in the small animal groups of polycystic kidney disease. For the calculation of perfusion and filtration parameters of kidney function in MRI, a 2D turbo FLASH sequence was performed and allowed to distinguish between normal GFR of healthy rats and reduced GFR of rats with polycystic kidney disease. Also, MRI GFR varied among two different rat strains of polycystic kidney disease, according to their status of renal function impairment. Optical imaging GFR confirmed higher GFR values in healthy rats compared to ill rats but did not show different results among the two rat strains of polycystic kidney disease. For this reason, MRI and optical imaging GFR estimation presented an intra-method bias.

CONCLUSIONS

Both non-invasive estimation methods of GFR, MRI and optical imaging, can differentiate between healthy rats and animals with limited kidney function. Furthermore, optical imaging, unlike MRI, seems to consider that disease progression with increase of renal polycystic deterioration does not correlate with decrease of GFR in the initial stage of compensatory hyperfiltration.

Cyst formation in the PKD2 (1-703) transgenic rat precedes deregulation of proliferation-related pathways

Background

Polycystic Kidney Disease is characterized by the formation of large fluid-filled cysts that eventually destroy the renal parenchyma leading to end-stage renal failure. Although remarkable progress has been made in understanding the pathologic mechanism of the disease, the precise orchestration of the early events leading to cyst formation is still unclear. Abnormal cellular proliferation was traditionally considered to be one of the primary irregularities leading to cyst initiation and growth. Consequently, many therapeutic interventions have focused on targeting this abnormal proliferation, and some have even progressed to clinical trials. However, the role of proliferation in cyst development was primarily examined at stages where cysts are already visible in the kidneys and therefore at later stages of disease development.

Methods

In this study we focused on the cystic phenotype since birth in an attempt to clarify the temporal contribution of cellular proliferation in cyst development. Using a PKD2 transgenic rat model (PKD2 (1-703)) of different ages (0-60 days after birth) we performed gene expression profiling and phenotype analysis by measuring various kidney parameters.

Results

Phenotype analysis demonstrated that renal cysts appear immediately after birth in the PKD2 transgenic rat model (PKD2 (1-703)). On the other hand, abnormal proliferation occurs at later stages of the disease as identified by gene expression profiling. Interestingly, other pathways appear to be deregulated at early stages of the disease in this PKD model. Specifically, gene expression analysis demonstrated that at day 0 the RAS system is involved. This is altered at day 6, when Wnt signaling and focal adhesion pathways are affected. However, at and after 24 days, proliferation, apoptosis, altered ECM signaling and many other factors become involved.

Conclusions

Our data suggest that cystogenesis precedes deregulation of proliferation-related pathways, suggesting that proliferation abnormalities may contribute in cyst growth rather than cyst formation.

Clinico-pathological correlations in 127 patients in 11 large pedigrees, segregating one of three heterozygous mutations in the COL4A3/ COL4A4 genes associated with familial haematuria and significant late progression to proteinuria and chronic kidney disease from focal segmental glomerulosclerosis

BACKGROUND

Heterozygous mutations in the COL4A3/ COL4A4 genes are currently thought to be responsible for familial benign microscopic haematuria and maintenance of normal long-term kidney function.

METHODS

We report on 11 large Cypriot pedigrees with three such mutations. A total of 236 at-risk family members were genetically studied, and 127 (53.8%) carried a heterozygous mutation. Clinico-pathological correlations were available in all of these patients. Renal biopsies in 21 of these patients all showed various stages of focal, segmental glomerulosclerosis (FSGS). Thirteen of these biopsies were also studied with EM and showed thinning of the glomerular basement membrane.

RESULTS

Mutation G1334E (COL4A3) was found in six pedigrees, mutation G871C (COL4A3) in four and mutation 3854delG (COL4A4) in one pedigree. Clinical and laboratory correlations in all 127 mutation carriers (MC) showed that microscopic haematuria was the only urinary finding in patients under age 30. The prevalence of 'haematuria alone' fell to 66% between 31 and 50 years, to 30% between 51 and 70 and to 23% over age 71. Proteinuria with CRF developed on top of haematuria in 8% of all MC between 31 and 50 years, to 25% between 51 and 70 years and to 50% over 71 years. Altogether 18 of these 127 MC (14%) developed ESRD at a mean age of 60 years. Two members with different mutations married, and two of their children inherited both mutations and developed adolescent, autosomal recessive Alport syndrome (ATS), confirming that these mutations are pathogenic.

CONCLUSIONS

Our data confirm for the first time a definite association of heterozygous COL4A3/COL4A4 mutations with familial microscopic haematuria, thin basement membrane nephropathy and the late development of familial proteinuria, CRF, and ESRD, due to FSGS, indicating that the term 'benign familial haematuria' is a misnomer, at least in this cohort. A strong hypothesis for a causal relationship between these mutations and FSGS is also made. Benign familial haematuria may not be so benign as commonly thought.

Microarray-based approach identifies microRNAs and their target functional patterns in polycystic kidney disease

Background

MicroRNAs (miRNAs) play key roles in mammalian gene expression and several cellular processes, including differentiation, development, apoptosis and cancer pathomechanisms. Recently the biological importance of primary cilia has been recognized in a number of human genetic diseases. Numerous disorders are related to cilia dysfunction, including polycystic kidney disease (PKD). Although involvement of certain genes and transcriptional networks in PKD development has been shown, not much is known how they are regulated molecularly.

Results

Given the emerging role of miRNAs in gene expression, we explored the possibilities of miRNA-based regulations in PKD. Here, we analyzed the simultaneous expression changes of miRNAs and mRNAs by microarrays. 935 genes, classified into 24 functional categories, were differentially regulated between PKD and control animals. In parallel, 30 miRNAs were differentially regulated in PKD rats: our results suggest that several miRNAs might be involved in regulating genetic switches in PKD. Furthermore, we describe some newly detected miRNAs, miR-31 and miR-217, in the kidney which have not been reported previously. We determine functionally related gene sets, or pathways to reveal the functional correlation between differentially expressed mRNAs and miRNAs.

Conclusion

We find that the functional patterns of predicted miRNA targets and differentially expressed mRNAs are similar. Our results suggest an important role of miRNAs in specific pathways underlying PKD.

Genomic organization and mutation screening of the human ortholog of Pkdr1 associated with polycystic kidney disease in the rat

Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common inherited disorders in humans. Although disease-causing mutations have been found in two genes, PKD1 and PKD2, a small number of ADPKD families exist that are unlinked to either of these genes, suggesting involvement of a third, as yet unidentified PKD3 gene. Susceptibility to renal cyst formation in the (cy/+) rat is caused by a missense mutation in Pkdr1 encoding the novel protein SamCystin. To initiate studies of the human orthologous gene, we determined the location and the organization of human PKDR1. We genotyped microsatellite markers flanking the human ortholog in PKD families that either are unlinked to known PKD genes, or in which mutations have not yet been identified and carried out mutation analysis in PKD patients. We identified eight novel single nucleotide polymorphisms, including three leading to amino acid changes. These variants are unlikely to account for PKD in these patients, yet the screening of other affected populations may provide information about the involvement of PKDR1 as a modifier gene in cystic kidney disease.

Missense mutation in sterile alpha motif of novel protein SamCystin is associated with polycystic kidney disease in (cy/+) rat

Autosomal dominant polycystic kidney disease (PKD) is the most common genetic disease that leads to kidney failure in humans. In addition to the known causative genes PKD1 and PKD2, there are mutations that result in cystic changes in the kidney, such as nephronophthisis, autosomal recessive polycystic kidney disease, or medullary cystic kidney disease. Recent efforts to improve the understanding of renal cystogenesis have been greatly enhanced by studies in rodent models of PKD. Genetic studies in the (cy/+) rat showed that PKD spontaneously develops as a consequence of a mutation in a gene different from the rat orthologs of PKD1 and PKD2 or other genes that are known to be involved in human cystic kidney diseases. This article reports the positional cloning and mutation analysis of the rat PKD gene, which revealed a C to T transition that replaces an arginine by a tryptophan at amino acid 823 in the protein sequence. It was determined that Pkdr1 is specifically expressed in renal proximal tubules and encodes a novel protein, SamCystin, that contains ankyrin repeats and a sterile alpha motif. The characterization of this protein, which does not share structural homologies with known polycystins, may give new insights into the pathophysiology of renal cyst development in patients.

Modifier genes play a significant role in the phenotypic expression of PKD111See Editorial by Pei, p. 1630

BACKGROUND

Polycystic kidney disease type 1 (PKD1) is characterized by extreme variation in the severity and progression of renal and extrarenal phenotypes. There are significant familial phenotype differences; but it is not clear if this is due to differences in PKD1 mutations, differences in genetic background, or both.

METHODS

A total of 315 affected relatives (83 PKD1 families) without end-stage renal disease (ESRD) were evaluated for disease markers, including renal volume, creatinine clearance, proteinuria, liver cysts, and hypertension. Of these patients, 19% progressed to ESRD within 1 to 10 years after the initial examination. Nested analysis of variance was used to investigate interfamilial and intrafamilial differences in these phenotypes. Heritability analyses were used to estimate the effect of the genetic background on phenotypic variability. The age of onset of ESRD was also analyzed with an additional 389 family members from the same PKD1 families without clinical evaluation but with data on age of onset of ESRD (or age without ESRD).

RESULTS

There were significant phenotype differences between patients with the same mutation and different genetic backgrounds. The phenotypic variation between patients with different mutations and different genetic backgrounds was not significantly greater than the variation between patients with the same mutation and different genetic backgrounds. However, when the 389 family members were included, both the mutation and modifier genes had significant effects on the age of onset of ESRD. Inherited differences in genetic background were estimated to account for 18% to 59% of the phenotypic variability in PKD1 disease markers in patients prior to ESRD and in the subsequent progression to ESRD (43% heritability) in the 315 patients who were clinically evaluated.

CONCLUSION

Modifier loci in the genetic background are important factors in inter- and intrafamilial variability in the phenotypic expression of PKD1. The extreme intrafamilial phenotype differences are consistent with the hypothesis that one or a few modifier genes have a major effect on the progression and severity of PKD1.

A Mendelian locus on chromosome 16 determines susceptibility to doxorubicin nephropathy in the mouse

The development of kidney disease is influenced by both genetic and environmental factors. Searching for models of glomerulopathy that display strong gene-environment interaction, we examined the determinants of anthracycline-induced nephropathy, a classic, strain-dependent experimental model applied to rodents in the past four decades. We produced three crosses derived from mice with contrasting susceptibility to doxorubicin (DOX) nephropathy and, surprisingly, we found that this widely studied model segregates as a single-gene defect with recessive inheritance. By genome-wide analysis of linkage, we mapped the trait locus to chromosome 16A1-B1 (DOXNPH locus) in all three crosses [peak logarithm of odds (lod) score of 92.7, P = 1 x 10(-65)]; this interval represents a susceptibility locus for nephropathy. Gene expression analysis indicated that susceptibility alleles at the DOXNPH locus are associated with blunted expression of protein arginine methyltransferase 7 (Prmt7) on chromosome 8, a protein previously implicated in cellular sensitivity to chemotherapeutic agents (lod = 12.4, P = 0.0001). Therefore, Prmt7 expression serves as a molecular marker for susceptibility to DOX nephropathy. Finally, increased variation in the severity of kidney disease among affected mice motivated a second genome-wide search, identifying a locus on chromosome 9 that influences the severity and progression of nephropathy (DOXmod, peak lod score 4.3, P = 0.0018). These data provide genetic and molecular characterization of a previously unrecognized Mendelian trait. Elucidation of DOX nephropathy may simultaneously provide insight into the pathogenesis of renal failure and mechanisms of cytotoxicity induced by chemotherapeutic agents.

Familial hematuria due to type IV collagen mutations: Alport syndrome and thin basement membrane nephropathy

PURPOSE OF REVIEW

Recent molecular genetic studies have shown that mutations in type IV collagen account for a significant proportion of patients with persistent glomerular hematuria. This review will discuss the implications of these findings for the diagnosis and management of persistent glomerular hematuria.

RECENT FINDINGS

Type IV collagen mutations are associated with a continuum of disease severity. Heterozygous mutations typically cause isolated, nonprogressive hematuria. Mutations in both alleles of the autosomal type IV collagen genes, or hemizygous mutations in the X-linked gene encoding the alpha 5 chain of type IV collagen, result in progressive renal disease that is often associated with sensorineural deafness (Alport syndrome). Animal models of Alport syndrome have begun to provide insights into the pathogenesis of end-stage renal disease in Alport syndrome, with potentially important implications for therapy.

SUMMARY

Recognition that glomerular hematuria frequently has a genetic basis is important for accurate genetic counseling, early identification of individuals at risk for end-stage renal disease development, and institution of therapies to delay the onset of ESRD.

Murine models of polycystic kidney disease: molecular and therapeutic insights

Numerous murine (mouse and rat) models of polycystic kidney disease (PKD) have been described in which the mutant phenotype results from a spontaneous mutation or engineering via chemical mutagenesis, transgenic technologies, or gene-specific targeting in mouse orthologs of human PKD genes. These murine phenotypes closely resemble human PKD, with common abnormalities observed in tubular epithelia, the interstitial compartment, and the extracellular matrix of cystic kidneys. In both human and murine PKD, genetic background appears to modulate the renal cystic phenotype. In murine models, these putative modifying effects have been dissected into discrete factors called quantitative trait loci and genetically mapped. Several lines of experimental evidence support the hypothesis that PKD genes and their modifiers may define pathways involved in cystogenesis and PKD progression. Among the various pathway abnormalities described in murine PKD, recent provocative data indicate that structural and/or functional defects in the primary apical cilia of tubular epithelia may play a key role in PKD pathogenesis. This review describes the most widely studied murine models; highlights the data regarding specific gene defects and genetic modifiers; summarizes the data from these models that have advanced our understanding of PKD pathogenesis; and examines the effect of various therapeutic interventions in murine PKD.

A Major Gene Locus Links Early Onset Albuminuria with Renal Interstitial Fibrosis in the MWF Rat with Polygenetic Albuminuria

ABSTRACT. The development of renal interstitial fibrosis (RIF) represents an important step in the progression of chronic proteinuric nephropathies. The Munich Wistar Frömter (MWF) rat represents a valuable model to study the progression in proteinuric renal disease. MWF animals demonstrate a significant increase of urinary albumin excretion (UAE) and RIF compared with the spontaneously hypertensive rat (SHR) with low UAE. The aim of this study was to analyze the genetic basis and the relation between UAE and RIF by genetic linkage and quantitative trait loci (QTL) mapping analysis. The authors generated a backcross population between MWF and SHR including 215 male animals. UAE was determined in young backcross animals at 8 wk, and at 14 and 24 wk of age, respectively. RIF was evaluated by Sirius red staining of kidney sections and quantified by computer-assisted image analysis at 24 wk. Total genome scan analysis identified in total eight QTL linked to UAE and a major locus on chromosome 6. At this locus, homozygosity for the MWF allele exhibited a strong effect on UAE levels (threefold elevation) and displayed significant linkage already at 8 wk (logarithm of odds [LOD] = 4.3) with increasing significance at 14 and 24 wk (LOD = 7.8 and 10.1, respectively). In addition, this was the only QTL that was linked to the amount of RIF (P= 0.0009, LOD = 2.4). These data establish a genetic link between early onset albuminuria and progression of RIF at the QTL on RNO6. This study demonstrates the power of genetic linkage analysis for the dissection of physiologic pathways involved in renal disease progression.

Genetic loci contribute to the progression of vascular and cardiac hypertrophy in salt-sensitive spontaneous hypertension

OBJECTIVE

The salt-sensitive Dahl rat and the spontaneously hypertensive rat develop comparable spontaneous hypertension on a low-salt diet, whereas only the salt-sensitive Dahl rat strain develops a striking increase in blood pressure and cardiovascular hypertrophy on a high-salt diet. We set out to identify quantitative trait loci (QTLs) contributing to the progression of salt-induced organ damage in hypertension by studying an F2 population derived from both strains.

METHODS AND RESULTS

We determined systolic blood pressure (SBP), vascular aortic hypertrophy (AH), cardiac left ventricular (LV) hypertrophy (LVH), and LV fibrosis in 230 male F2-animals on a high-salt diet. A strong correlation between AH and LVH was found (r=0.58, P<0.0001), and genome-wide QTL mapping detected suggestive or significant QTLs in overlapping chromosomal fragments for AH and LVH on chromosomes 1, 3, and 19, respectively. A significant influence of SBP on the extent of LVH and AH was evident at all QTLs, although significant linkage to SBP (together with LVH) was only found on chromosome 9. No QTLs for LV fibrosis were detected.

CONCLUSIONS

This study demonstrates a strong correlation between AH and LVH in salt-sensitive hypertension and identifies QTLs contributing to the progression of cardiovascular hypertrophy in this condition.

Modifier genes and protective alleles in humans and mice

Interest in modifier genes is growing rapidly because of their ability to modulate the phenotype of individuals with monogenic and multigenic traits and diseases. A neglected class of modifiers is protective alleles that can suppress disease in otherwise susceptible individuals. Together these modifier genes and protective alleles provide important glimpses into the molecular and cellular basis for the functional networks that provide robustness and homeostasis in complex biological systems.