COL4A3/COL4A4 mutations link familial hematuria and focal segmental glomerulosclerosis. glomerular epithelium destruction via basement membrane thinning?

Genetic Modifiers of Mendelian Monogenic Collagen IV Nephropathies in Humans and Mice

Familial hematuria is a clinical sign of a genetically heterogeneous group of conditions, accompanied by broad inter- and intrafamilial variable expressivity. The most frequent condition is caused by pathogenic (or likely pathogenic) variants in the collagen-IV genes, COL4A3/A4/A5. Pathogenic variants in COL4A5 are responsible for the severe X-linked glomerulopathy, Alport syndrome (AS), while homozygous or compound heterozygous variants in the COL4A3 or the COL4A4 gene cause autosomal recessive AS. AS usually leads to progressive kidney failure before the age of 40-years when left untreated. People who inherit heterozygous COL4A3/A4 variants are at-risk of a slowly progressive form of the disease, starting with microscopic hematuria in early childhood, developing Alport spectrum nephropathy. Sometimes, they are diagnosed with benign familial hematuria, and sometimes with autosomal dominant AS. At diagnosis, they often show thin basement membrane nephropathy, reflecting the uniform thin glomerular basement membrane lesion, inherited as an autosomal dominant condition. On a long follow-up, most patients will retain normal or mildly affected kidney function, while a substantial proportion will develop chronic kidney disease (CKD), even kidney failure at an average age of 55-years. A question that remains unanswered is how to distinguish those patients with AS or with heterozygous COL4A3/A4 variants who will manifest a more aggressive kidney function decline, requiring prompt medical intervention. The hypothesis that a subgroup of patients coinherit additional genetic modifiers that exacerbate their clinical course has been investigated by several researchers. Here, we review all publications that describe the potential role of candidate genetic modifiers in patients and include a summary of studies in AS mouse models.

A Novel Homozygous Mutation in the COL4A4 Gene (Gly1436del) Causing Alport Syndrome Exposed by Pregnancy: A Case Report and Review of the Literature

Background

Alport syndrome results from a hereditary defect of collagen IV synthesis. This causes progressive glomerular disease, ocular abnormalities, and inner ear impairment. Case Presentation. Herein, we present a case of Alport syndrome in a 28-year-old woman caused by a novel mutation (Gly1436del) in the COL4A4 gene that was not unveiled until her first pregnancy. Within the 29^th pregnancy week, our patient presented with massive proteinuria and nephrotic syndrome. Light microscopic examination of a kidney biopsy showed typical histological features of segmental sclerosis, and electron microscopy revealed extensive podocyte alterations as well as thickness of glomerular basement membranes with splitting of the lamina densa. One and a half years after childbirth, renal function deteriorated to a preterminal stage, whereas nephrotic syndrome subsided quickly after delivery.

Conclusion

This case report highlights the awareness of atypical AS courses and emphasizes the importance of genetic testing in such cases.

Whole-Exome Sequencing Solved over 2-Decade Kidney Disease Enigma

Chronic kidney disease of unknown etiology (CKDu) has been a problem in renal practice as indefinite diagnosis may lead to inappropriate management. Here, we report a 54-year-old father diagnosed with CKDu at 33 years old and his 8-year-old son with steroid-resistant nephrotic syndrome. Using whole-exome sequencing, both were found to be heterozygous for c.737G>A (p.Arg246Gln) in LMX1B. The diagnosis of LMX1B-associated nephropathy has led to changes in the treatment plan with appropriate genetic counseling. The previously reported cases with this particular mutation were also reviewed. Most children with LMX1B-associated nephropathy had nonnephrotic proteinuria with normal renal function. Interestingly, our pediatric case presented with steroid-resistant nephrotic syndrome at 8 years old and progressed to ESRD requiring peritoneal dialysis at the age of 15 years. Our report emphasized the need of genetic testing in CKDu for definite diagnosis leading to precise management.

Type IV Collagen Variants in CKD: Performance of Computational Predictions for Identifying Pathogenic Variants

Rationale & Objective

Pathogenic variants in type IV collagen have been reported to account for a significant proportion of chronic kidney disease. Accordingly, genetic testing is increasingly used to diagnose kidney diseases, but testing also may reveal rare missense variants that are of uncertain clinical significance. To aid in interpretation, computational prediction (called in silico) programs may be used to predict whether a variant is clinically important. We evaluate the performance of in silico programs for COL4A3/A4/A5 variants.

Study Design, Setting, & Participants

Rare missense variants in COL4A3/A4/A5 were identified in disease cohorts, including a local focal segmental glomerulosclerosis (FSGS) cohort and publicly available disease databases, in which they are categorized as pathogenic or benign based on clinical criteria.

Tests Compared & Outcomes

All rare missense variants identified in the 4 disease cohorts were subjected to in silico predictions using 12 different programs. Comparisons between the predictions were compared with: (1) variant classification (pathogenic or benign) in the cohorts and (2) functional characterization in a randomly selected smaller number (17) of pathogenic or uncertain significance variants obtained from the local FSGS cohort.

Results

In silico predictions correctly classified 75% to 97% of pathogenic and 57% to 100% of benign COL4A3/A4/A5 variants in public disease databases. The congruency of in silico predictions was similar for variants categorized as pathogenic and benign, with the exception of benign COL4A5 variants, in which disease effects were overestimated. By contrast, in silico predictions and functional characterization classified all 9 pathogenic COL4A3/A4/A5 variants correctly that were obtained from a local FSGS cohort. However, these programs also overestimated the effects of genomic variants of uncertain significance when compared with functional characterization. Each of the 12 in silico programs used yielded similar results.

Limitations

Overestimation of in silico program sensitivity given that they may have been used in the categorization of variants labeled as pathogenic in disease repositories.

Conclusions

Our results suggest that in silico predictions are sensitive but not specific to assign COL4A3/A4/A5 variant pathogenicity, with misclassification of benign variants and variants of uncertain significance. Thus, we do not recommend in silico programs but instead recommend pursuing more objective levels of evidence suggested by medical genetics guidelines.

Mutations in Collagen Genes in the Context of an Isolated Population

Genetic studies of population isolates have great potential to provide a unique insight into genetic differentiation and phenotypic expressions. Galičnik village is a population isolate located in the northwest region of the Republic of North Macedonia, established around the 10th century. Alport syndrome-linked nephropathy with a complex inheritance pattern has been described historically among individuals in the village. In order to determine the genetic basis of the nephropathies and to characterize the genetic structure of the population, 23 samples were genotyped using a custom-made next generation sequencing panel and 111 samples using population genetic markers. We compared the newly obtained population data with fifteen European population data sets. NGS analysis revealed four different mutations in three different collagen genes in twelve individuals within the Galičnik population. The genetic isolation and small effective population size of Galičnik village have resulted in a high level of genomic homogeneity, with domination of R1a-M458 and R1b-U106* haplogroups. The study explains complex autosomal in cis digenic and X-linked inheritance patterns of nephropathy in the isolated population of Galičnik and describes the first case of Alport syndrome family with three different collagen gene mutations.

Variations of type IV collagen-encoding genes in patients with histological diagnosis of focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS), an important cause of end-stage kidney disease (ESKD), covers a spectrum of clinicopathological syndromes sharing a common glomerular lesion, based on an injury of podocytes caused by diverse insults to glomeruli. Although it is well expressed in many reports that the term FSGS is not useful and applicable to a single disease, particularly in genetic studies, FSGS continues to be used as a single clinical diagnosis. Distinguishing genetic forms of FSGS is important for the treatment and overall prognosis because secondary forms of FSGS, produced by rare pathogenic variations in podocyte genes, are not good candidates for immunosuppressive treatment. Over the past decade, several next generation sequencing (NGS) methods have been used to investigate the patients with steroid resistance nephrotic syndrome (SRNS) or FSGS. Pathogenic variants in COL4A3, COL4A4, or COL4A5 genes have been frequently identified in patients with histologic diagnosis of FSGS. The contribution of these mostly heterozygous genetic variations in FSGS pathogenesis and the clinical course of patients with these variations have not been well characterized. This review emphasizes the importance of appropriate approach in selection and diagnosis of cases and interpretation of the genetic data in these studies and suggests a detailed review of existing clinical variant databases using newly available population genetic data.

The use of electron microscopy in the diagnosis of focal segmental glomerulosclerosis: are current pathological techniques missing important abnormalities in the glomerular basement membrane?

Background: There is an increasing appreciation that variants of the COL4A genes may be associated with the development of focal segmental glomerulosclerosis (FSGS). On electron microscopy, such variants may produce characteristic changes within the glomerular basement membrane (GBM). These changes may be missed if glomerular lesions histologically diagnosed as FSGS on light microscopy are not subjected to electron microscopy.

Methods: We conducted a retrospective cohort analysis of all patients presenting to two hospitals who received a primary histological diagnosis of FSGS to see if these samples underwent subsequent electron microscopy. Each such sample was also scrutinised for the presence of characteristic changes of an underlying type IV collagen disorder

Results: A total of 43 patients were identified. Of these, only 30 underwent electron microscopy. In two samples there were histological changes detected that might have suggested the underlying presence of a type IV collagen disorder. Around one in three biopsy samples that had a histological diagnosis of FSGS were not subjected to electron microscopy.

Conclusion: Renal biopsy samples that have a histological diagnosis of primary FSGS not subjected to subsequent electron microscopy may potentially miss ultrastructural changes in the GBM that could signify an underlying type IV collagen disorder as the patient’s underlying disease process. This could potentially affect both them and their families’ investigative and management decisions given potential for implications for transplant, heritability and different disease pathogenesis. This represents a gap in care which should be reflected upon and rectified via iterative standard care and unit-level quality assurance initiatives.

Collagens and elastin genetic variations and their potential role in aging-related diseases and longevity in humans

Collagens and elastin are 'building blocks' of tissues and extracellular matrix. Mutations in these proteins cause severe congenital syndromes. Adverse genetic variations may accelerate the aging process in adults contributing to premature morbidity, disability and/or mortality. Favorable variants may contribute to longevity and/or healthy aging, but this is much less studied. We reviewed the association between variation in the genes of collagens and elastin and premature aging, accelerated aging, age-related diseases and/or frailty; and the association between genetic variation in those and longevity and/or healthy aging in humans. A systematic search was conducted in MEDLINE and other online databases (OMIM, Genetics Home Reference, Orphanet, ClinVar). Results suggest that genetic variants lead to aging phenotypes of known congenital disease, but also to association with common age-related diseases in adults without known congenital disease. This may be due to the variable penetrance and expressivity of many variants. Some collagen variants have been associated with longevity or healthy aging. A limitation is that most studies had <1000 participants and their criterion for statistical significance was p < 0.05. Results highlight the importance of adopting a lifecourse approach to the study of the genomics of aging. Gerontology can help with new methodologies that operationalize biological aging.

Autosomal Recessive Alport Syndrome Unveiled by Pregnancy

Alport syndrome is a hereditary disease affecting Type IV collagen characterized by hematuria, progressive renal failure, sensorineural hearing loss, and ocular abnormalities. Most cases are X-linked and involve the COL4A5 gene with a minority of patients having autosomal recessive mutations in the COL4A3 or COL4A4 genes encoding the α3(IV) or α4(IV) chain respectively. Here, we describe the case of a 31-year-old woman who presented during pregnancy with hematuria and proteinuria and was diagnosed with autosomal recessive Alport syndrome (ARAS) post-partum. Her biopsy was notable for findings of segmental glomerulosclerosis with some collapsing features, in addition to thin basement membranes and rare "splitting". Genetic testing identified 2 novel mutations in the COL4A4 gene: a truncating frame shift mutation c.3861delinsCTC and a missense mutation c.4708G>A (p.Glu1570Lys), both of which we assert to be pathogenic. She had normal full-term delivery without complications. This case has several unique features including the relatively mild disease phenotype and the findings of glomerular scarring with collapsing features on renal biopsy. The successful pregnancy outcome and her clinical presentation add to the growing body of evidence that ARAS can have a variable phenotype.

Genotype-phenotype correlation and prognostic impact in Chinese patients with Alport Syndrome

BACKGROUND

Alport Syndrome (AS) is a progressive hereditary glomerular disease. It is often accompanied by sensorineural hearing loss and ocular abnormalities and can sometimes develop into end stage renal disease (ESRD), which is caused by mutations in the genes encoding the collagen type IV family of proteins.

METHODS

This study analyzed the association between the clinical data of seven AS families and genes and the disease progression of different mutation types, including COL4A3 (OMIM 120070)，COL4A4 (OMIM 120131), and COL4A5 (OMIM303630).

RESULTS

A total of six new pathogenic mutation sites, one complex heterozygous mutation at COL4A3, and a combined mutation of COL4A5 and INF2 (OMIM 610982) were identified in this study. It was revealed that the clinical manifestations of X-linked AS caused by mutations in the COL4A5 gene were more severe in males than in females. In addition, the difference in patient phenotype can be attributed to the location of gene mutations affecting the protein domain or functional domain. Our data suggested that the gene deletion and nonsense mutations had a high risk for progression to ESRD.

CONCLUSION

Our results revealed the spectrum of type IV collagen genes, which contribute to the enrichment of database resources and has important implications in the diagnosis, prognosis, and guiding treatment of AS.

Autosomal dominant form of type IV collagen nephropathy exists among patients with hereditary nephritis difficult to diagnose clinicopathologically

The study revealed that 69% of families with hereditary nephritis that was difficult to diagnose clinicopathologically had heterozygous mutations of COL4A3/A4 (TBMN/ADAS). The finding suggests the importance of genetic testing in appropriate patients.

Aim

Type IV collagen nephropathies include Alport Syndrome and thin basement membrane nephropathy (TBMN), which are caused by mutations in COL4A3/A4/A5 genes. Recently, reports of patients with heterozygous mutations in COL4A3/A4 have been increasing. The clinical course of these patients has a wide variety, and they are diagnosed as TBMN, autosomal dominant Alport syndrome (ADAS), or familial focal segmental glomerular sclerosis. However, diagnosis, frequency and clinicopathological manifestation of them remains unclear. We tested COL4A3/A4/A5 genes in patients with hereditary nephritis that was difficult to diagnose clinicopathologically, and investigated who should undergo such testing.

Methods

We performed immunostaining for α5 chain of type IV collagen [α5 (IV)] in 27 patients from 21 families who fitted the following criteria: (i) haematuria and proteinuria (± renal dysfunction); (ii) family history of haematuria, proteinuria, and/or renal dysfunction (autosomal dominant inheritance); (iii) no specific glomerulonephritis; and (iv) thinning, splitting, or lamellation of the glomerular basement membrane (GBM) on electron microscopy. Then we performed genetic testing in 19 patients from 16 families who showed normal α5 (IV) patterns. We conducted a retrospective analysis of their clinicopathological findings.

Results

Among 16 families, 69% were detected heterozygous mutations in COL4A3/A4, suggesting the diagnosis of TBMN/ADAS. Twenty‐one percent of patients developed end stage renal disease. All patients showed thinning of GBM, which was accompanied by splitting or lamellation in seven patients.

Conclusion

A considerable fraction of patients with hereditary nephritis that is difficult to diagnose clinicopathologically have TBMN/ADAS. It is important to recognize TBMN/ADAS and perform genetic testing in appropriate patients.

A Novel Heterozygous Mutation of the COL4A3 Gene Causes a Peculiar Phenotype without Hematuria and Renal Function Impairment in a Chinese Family

Mutations in the COL4A3 gene are frequently reported to be associated with various types of hereditary nephropathy. COL4A3 encodes the α3 chain of type IV collagen, which is the main structural protein in the basement membrane. Mutations in this gene are always related to kidney performance, and deafness and ocular lesion have also been reported. In this study, using next-generation sequencing, we investigated the DNA of a family visiting a clinic for hearing loss. A new missense mutation was found in COL4A3 of 5 patients, c.3227C>T (p.P1076L). Based on these results, we predict that the mutation is pathogenic and leads to abnormal collagen IV. Here, we report for the first time on this autosomal dominant syndrome, characterized by hearing loss and eye abnormalities, but without renal damage, in all carriers. Since the oldest patient in the trial was less than 50 years old, however, we recommend that renal examination be reviewed regularly. Our results reveal expansion in the mutation spectrum of the COL4A3 gene and phenotypic spectrum of collagen IV disease. Our study suggests that next-generation sequencing is an economical and effective method and may help in the accurate diagnosis and treatment of these patients.

Identification of a Novel COL4A4 Variant in Compound-Heterozygous State in a Patient With Alport Syndrome and Histological Findings Similar to Focal Segmental Glomerulosclerosis (FSGS)

Alport syndrome (AS) is a rare and inherited renal disorder with an autosomal recessive mode of inheritance. AS patients usually manifest with hematuria and progressive renal disorder also occasionally accompanied by hearing loss and ophthalmic disease. Germline variants in collagen type IV α-4 (COL4A4) gene lead to autosomal recessive Alport syndrome. In the present study, we investigated a Chinese family with Alport syndrome. The index patient is a 24-year-old Chinese woman who has been suffering from proteinuria. Renal biopsy and renal pathology were performed and found focal segmental glomerulosclerosis (FSGS) like lesion in the index patient. The index patient also presented with binocular edema and blurred vision. However, binocular edema dissipated gradually without any further treatment. Unlikely, the index patient was not diagnosed with hearing impairment. Index patient's parents are phenotypically normal. Targeted next generation sequencing and Sanger sequencing was performed. A novel heterozygous single nucleotide insertion, c.4760_4761insC and a previously reported likely pathogenic variant, c.1323_1340delTGGCTTGCCTGGAGCACC in the COL4A4 gene were identified in the index patient. The novel heterozygous single nucleotide insertion (c.4760_4761insC) leads to a frameshift which eventually results in the formations of a truncated COL4A4 protein. In addition, the other heterozygous likely pathogenic variant, c.1323_1340delTGGCTTGCCTGGAGCACC, has been already identified with causing AS an autosomal recessive mode of inheritance. Sanger sequencing confirmed that these two variants were inherited in the index patient from her father and mother, respectively. These two variants were not found in 100 normal control individuals. In conclusion, our present finding emphasizes the significance of high throughput targeted next generation sequencing technology for rapid and cost-effective genetic screening which allows us easy and accurate clinical diagnosis of AS patients.

Acute kidney injury due to thin basement membrane disease mimicking Deferasirox nephrotoxicity: a case report

BACKGROUND

Although the renal toxicity of Deferasirox, an oral iron chelator, has been reported to be mild, there have been reports of acute interstitial nephritis or Fanconi syndrome due to this agent. Thin basement membrane disease (TBMD) is a hereditary disease characterized primarily by hematuria, with gross hematuria also observed in about 7% of cases. We herein report a case of TBMD that presented with acute kidney injury and gross hematuria during treatment with Deferasirox.

CASE PRESENTATION

The patient was a 63-year-old man who had been diagnosed with myelodysplastic syndrome 6 years ago. He had started taking Deferasirox at 125 mg due to post-transfusion iron overload 6 months ago. Deferasirox was then increased to 1000 mg three months ago. When the serum creatinine level increased, Deferasirox was reduced to 500 mg three weeks before hospitalization. Although the serum creatinine level decreased once, he developed a fever and macroscopic hematuria one week before hospitalization. The serum creatinine level increased again, and Deferasirox was stopped four days before hospitalization. He was admitted for the evaluation of acute kidney injury and gross hematuria. Treatment with temporary hemodialysis was required, and a kidney biopsy was performed on the eighth day of admission. Although there was no major abnormality in the glomeruli, the leakage of red blood cells into the Bowman's space was observed. Erythrocyte cast formation was observed in the tubular lumen, which was associated with acute tubular necrosis. The results of an electron microscopic study were compatible with TBMD.

CONCLUSION

Although Deferasirox is known to be nephrotoxic, gross hematuria is relatively rare. When we encounter a case of acute kidney injury with gross hematuria during treatment with Deferasirox, TBMD should be considered as a possible cause of gross hematuria.

A functional variant in NEPH3 gene confers high risk of renal failure in primary hematuric glomerulopathies. Evidence for predisposition to microalbuminuria in the general population

Background

Recent data emphasize that thin basement membrane nephropathy (TBMN) should not be viewed as a form of benign familial hematuria since chronic renal failure (CRF) and even end-stage renal disease (ESRD), is a possible development for a subset of patients on long-term follow-up, through the onset of focal and segmental glomerulosclerosis (FSGS). We hypothesize that genetic modifiers may explain this variability of symptoms.

Methods

We looked in silico for potentially deleterious functional SNPs, using very strict criteria, in all the genes significantly expressed in the slit diaphragm (SD). Two variants were genotyped in a cohort of well-studied adult TBMN patients from 19 Greek-Cypriot families, with a homogeneous genetic background. Patients were categorized as “Severe” or “Mild”, based on the presence or not of proteinuria, CRF and ESRD. A larger pooled cohort (HEMATURIA) of 524 patients, including IgA nephropathy patients, was used for verification. Additionally, three large general population cohorts [Framingham Heart Study (FHS), KORAF4 and SAPHIR] were used to investigate if the NEPH3-V353M variant has any renal effect in the general population.

Results and conclusions

Genotyping for two high-scored variants in 103 TBMN adult patients with founder mutations who were classified as mildly or severely affected, pointed to an association with variant NEPH3-V353M (filtrin). This promising result prompted testing in the larger pooled cohort (HEMATURIA), indicating an association of the 353M variant with disease severity under the dominant model (p = 3.0x10^-3, OR = 6.64 adjusting for gender/age; allelic association: p = 4.2x10^-3 adjusting for patients’ kinships). Subsequently, genotyping 6,531 subjects of the Framingham Heart Study (FHS) revealed an association of the homozygous 353M/M genotype with microalbuminuria (p = 1.0x10^-3). Two further general population cohorts, KORAF4 and SAPHIR confirmed the association, and a meta-analysis of all three cohorts (11,258 individuals) was highly significant (p = 1.3x10^-5, OR = 7.46). Functional studies showed that Neph3 homodimerization and Neph3-Nephrin heterodimerization are disturbed by variant 353M. Additionally, 353M was associated with differential activation of the unfolded protein response pathway, when overexpressed in stressed cultured undifferentiated podocyte cells, thus attesting to its functional significance. Genetics and functional studies support a “rare variant-strong effect” role for NEPH3-V353M, by exerting a negative modifier effect on primary glomerular hematuria. Additionally, genetics studies provide evidence for a role in predisposing homozygous subjects of the general population to micro-albuminuria.

A novel heterozygous COL4A4 missense mutation in a Chinese family with focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is the most common glomerular histological lesion associated with high-grade proteinuria and end-stage renal disease. Histologically, FSGS is characterized by focal segmental sclerosis with foot process effacement. The aim of this study was to identify the disease-causing mutation in a four-generation Chinese family with FSGS. A novel missense mutation, c.1856G>A (p.Gly619Asp), in the collagen type IV alpha-4 gene (COL4A4) was identified in six patients and it co-segregated with the disease in this family. The variant is predicted to be disease-causing and results in collagen IV abnormalities. Our finding broadens mutation spectrum of the COL4A4 gene and extends the phenotypic spectrum of collagen IV nephropathies. Our study suggests that exome sequencing is a cost-effective and efficient approach for identification of disease-causing mutations in phenotypically complex or equivocal disorders. Timely screening for COL4A3/COL4A4 mutations in patients with familial FSGS may help both accurately diagnose and treat these patients.

Identification of 47 novel mutations in patients with Alport syndrome and thin basement membrane nephropathy

BACKGROUND

Alport syndrome (ATS) is a progressive hereditary nephropathy characterized by hematuria and proteinuria. It can be associated with extrarenal manifestations. In contrast, thin basement membrane nephropathy (TBMN) is characterized by microscopic hematuria, is largely asymptomatic, and is rarely associated with proteinuria and end-stage renal disease. Mutations have been identified in the COL4A5 gene in ATS and in the COL4A3 and COL4A4 genes in ATS and TBMN. To date, more than 1000 different mutations in COL4A5, COL4A3, and COL4A4 are known.

METHODS

In this study mutational analysis by exon sequencing and multiplex ligation-dependent probe amplification was performed in a large European cohort of families with ATS and TBMN.

RESULTS

Molecular diagnostic testing of 216 individuals led to the detection of 47 novel mutations, thereby expanding the spectrum of known mutations causing ATS and TBMN by up to 10 and 6%, respectively, depending on the database. Remarkably, a high number of ATS patients with only single mutations in COL4A3 and COL4A4 were identified. Additionally, three ATS patients presented with synonymous sequence variants that possible affect correct mRNA splicing, as suggested by in silico analysis.

CONCLUSIONS

The results of this study clearly broaden the genotypic spectrum of known mutations for ATS and TBMN, which will in turn now facilitate future studies into genotype-phenotype correlations. Further studies should also examine the significance of single heterozygous mutations in COL4A3 and COL4A4 and of synonymous sequence variants associated with ATS.

A novel FN1 variant associated with familial hematuria: TBMN?

OBJECTIVE

Thin basement membrane nephropathy (TBMN), an autosomal dominant inherited condition in general, is characterized clinically by persistent hematuria and pathologically by thinning of glomerular basement membrane. TBMN is occasionally accompanied with proteinuria, hypertension and renal impairment in some cases. The aim of this study is to explore the genetic defect in a Chinese pedigree with familial hematuria.

DESIGN AND METHODS

A four-generation Chinese Han pedigree with familial hematuria was recruited. Exome sequencing was conducted in the proband diagnosed as TBMN, followed by verification in the proband and other family members with Sanger sequencing.

RESULTS

A novel missense variant, c.4616C>G (p.S1539C), in the fibronectin 1 gene (FN1), was identified, and it co-segregated with the disease condition in the family. It was not observed in 100 normal controls.

CONCLUSIONS

A missense variant in the FN1 gene is possibly responsible for familial hematuria or TBMN in this family, which may broaden the phenotype and mutation spectrums of the FN1 gene. A male patient in this family progressed to end-stage renal disease requiring kidney transplantation, supporting that familial hematuria or TBMN may not always be as benign as generally thought. The findings may have new implications for clinical monitoring and genetic counseling of the family, and may also help understand the pathogenesis.

Carriers of Autosomal Recessive Alport Syndrome with Thin Basement Membrane Nephropathy Presenting as Focal Segmental Glomerulosclerosis in Later Life

Collagen IV nephropathies (COL4Ns) comprise benign familial microscopic hematuria, thin basement membrane nephropathy (TBMN), X-linked Alport syndrome (AS) and also autosomal recessive and dominant AS. Apart from the X-linked form of AS, which is caused by hemizygous mutations in the COL4A5 gene, the other entities are caused by mutations in the COL4A3 or COL4A4 genes. The diagnosis of these conditions used to be based on clinical and/or histological findings of renal biopsies, but it is the new molecular genetics approach that revolutionised their investigation and proved particularly instrumental, especially, in many not so clear-cut cases. More recently, the spectrum of COL4N has expanded to include late onset focal segmental glomerulosclerosis (FSGS) that develops on top of TBMN in later life. Also, other reports showed that some patients with a primary diagnosis of familial FSGS proved to have variants in COL4 genes. In the presence of a renal biopsy picture of FSGS and in the absence of either electron microscopy studies or molecular genetic studies that point to TBMN and COL4N, the patient and his family may be mistakenly diagnosed with hereditary FSGS leading to unnecessary further investigations, erroneous family counselling and improper corticosteroid treatment. TBMN is a frequent finding in the general population, and according to several recent reports, it may be the underlying cause and the explanation for many familial and sporadic cases of late-onset FSGS with non-nephrotic proteinuria. This is an important new finding that needs widespread recognition. It is anticipated that the molecular genetic analysis with next generation sequencing will certainly offer timely correct diagnosis.

Evidence for activation of the unfolded protein response in collagen IV nephropathies

Thin-basement-membrane nephropathy (TBMN) and Alport syndrome (AS) are progressive collagen IV nephropathies caused by mutations in COL4A3/A4/A5 genes. These nephropathies invariably present with microscopic hematuria and frequently progress to proteinuria and CKD or ESRD during long-term follow-up. Nonetheless, the exact molecular mechanisms by which these mutations exert their deleterious effects on the glomerulus remain elusive. We hypothesized that defective trafficking of the COL4A3 chain causes a strong intracellular effect on the cell responsible for COL4A3 expression, the podocyte. To this end, we overexpressed normal and mutant COL4A3 chains (G1334E mutation) in human undifferentiated podocytes and tested their effects in various intracellular pathways using a microarray approach. COL4A3 overexpression in the podocyte caused chain retention in the endoplasmic reticulum (ER) that was associated with activation of unfolded protein response (UPR)-related markers of ER stress. Notably, the overexpression of normal or mutant COL4A3 chains differentially activated the UPR pathway. Similar results were observed in a novel knockin mouse carrying the Col4a3-G1332E mutation, which produced a phenotype consistent with AS, and in biopsy specimens from patients with TBMN carrying a heterozygous COL4A3-G1334E mutation. These results suggest that ER stress arising from defective localization of collagen IV chains in human podocytes contributes to the pathogenesis of TBMN and AS through activation of the UPR, a finding that may pave the way for novel therapeutic interventions for a variety of collagenopathies.

A novel TRPC6 mutation in a family with podocytopathy and clinical variability

BACKGROUND

Mutation in several podocyte-specific genes have been noted to result in phenotypic heterogeneity. Herein, we report a novel, autosomal dominant TRPC6 mutation in a family with disease ranging from asymptomatic minimal change disease to end-stage kidney disease.

CASE PRESENTATION

A 35 year old woman developed asymptomatic, nephrotic range proteinuria during pregnancy that did not resolve after delivery. Her mother had end-stage kidney disease of unknown etiology and her brother had asymptomatic proteinuria. Kidney biopsy revealed minimal change disease in both the proband and her brother. Genetic testing was performed in the proband and mother, revealing a novel frameshift mutation in TRPC6, D873fsX878. The proband continues to have subnephrotic range proteinuria and normal creatinine but her brother has since developed progressive chronic kidney disease.

CONCLUSIONS

The current case report underscores the heterogeneity of disease in podocytopathies and related genes. Genetic testing of podocyte genes is useful in order to understand the pathophysiologic processes underlying these overlapping diseases.

Epistatic role of the MYH9/APOL1 region on familial hematuria genes

Familial hematuria (FH) is explained by at least four different genes (see below). About 50% of patients develop late proteinuria and chronic kidney disease (CKD). We hypothesized that MYH9/APOL1, two closely linked genes associated with CKD, may be associated with adverse progression in FH. Our study included 102 thin basement membrane nephropathy (TBMN) patients with three known COL4A3/COL4A4 mutations (cohort A), 83 CFHR5/C3 glomerulopathy patients (cohort B) with a single CFHR5 mutation and 15 Alport syndrome patients (cohort C) with two known COL4A5 mild mutations, who were categorized as “Mild” (controls) or “Severe” (cases), based on renal manifestations. E1 and S1 MYH9 haplotypes and variant rs11089788 were analyzed for association with disease phenotype. Evidence for association with “Severe” progression in CFHR5 nephropathy was found with MYH9 variant rs11089788 and was confirmed in an independent FH cohort, D (cumulative p value = 0.001, odds ratio = 3.06, recessive model). No association was found with APOL1 gene. Quantitative Real time PCR did not reveal any functional significance for the rs11089788 risk allele. Our results derive additional evidence supporting previous reports according to which MYH9 is an important gene per se, predisposing to CKD, suggesting its usefulness as a prognostic marker for young hematuric patients.

Familial C3 glomerulopathy associated with CFHR5 mutations: clinical characteristics of 91 patients in 16 pedigrees

BACKGROUND AND OBJECTIVES

Complement factor H and related proteins (CFHR) are key regulators of the alternative complement pathway, where loss of function mutations lead to a glomerulopathy with isolated mesangial C3 deposits without immunoglobulins. Gale et al. (12) reported on 26 patients with the first familial, hematuric glomerulopathy caused by a founder mutation in the CFHR5 gene in patients of Cypriot descent living in the United Kingdom. CFHR5 nephropathy is clinically characterized by continuous microscopic hematuria whereas some patients present with additional episodes of synpharyngitic macrohematuria, associated with infection and pyrexia. A subgroup of patients, particularly men, develop additional proteinuria, hypertension, and chronic renal disease or ESRD.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We herewith expand significantly on the study by Gale et al., reporting on histologic, molecular, and clinical findings in 91 patients from 16 families with the same founder mutation.

RESULTS

Eighty-two patients (90%) exhibited microscopic hematuria; 51 (62%), exhibited only microscopic hematuria, whereas the remaining 31 additionally had proteinuria (38%); 28 proteinuric patients developed chronic renal failure (CRF). Among carriers of CFHR5 mutation aged >50 years, 80% of the men and 21% of the women developed CRF; 18 developed ESRD (14 men [78%], 4 women [22%]).

CONCLUSIONS

The diagnosis of CFHR5-related, isolated C3 glomerulopathy was established in 2009 using newly described mutation analysis after decades of follow-up with unclear diagnoses, occasionally confused with IgA nephropathy. This larger patient cohort establishes the clinical course, significant variable expressivity, and marked gender difference regarding the development of CRF and ESRD.

X-linked Alport syndrome in Hellenic families: phenotypic heterogeneity and mutations near interruptions of the collagen domain in COL4A5

The X-linked Alport syndrome (ATS) is caused by mutations in COL4A5 and exhibits a widely variable expression. Usually ATS is heralded with continuous microhematuria which rapidly progresses to proteinuria, hypertension and chronic or end-stage renal disease (ESRD) by adolescence, frequently accompanied by sensorineural deafness and ocular complications. Milder forms of ATS also exist. We studied 42 patients (19M, 23F) of nine Hellenic families suspected clinically of X-linked ATS who presented with marked phenotypic heterogeneity. We identified mutations in COL4A5 in six families. Two males with nonsense mutation E228X reached ESRD by ages 14 and 18. Frameshift mutation 2946delT followed the same course with early onset renal involvement and deafness. However, two males with the milder missense mutation G624D, reached ESRD after 39 years and one patient showed thin basement membrane nephropathy (TBMN). Another 5/8 affected males with missense mutation P628L also developed ESRD between 30 and 57 years, while three exhibit only mild chronic renal failure (CRF). The data support previous findings that certain mutations are associated with milder phenotypes and confirm that mutation G624D may be expressed as TBMN with familial hematuria. Similar conclusions apply for missense mutation P628L. Interestingly, mutations G624D and P628L are near the 12th natural interruption of COL4A5 triple helical domain, which may explain the milder phenotype.

Novel heterozygous COL4A3 mutation in a family with late-onset ESRD

Thin basement membrane nephropathy (TBMN) and Alport syndrome (ATS) are genetically heterogeneous conditions characterized by structural abnormalities in the glomerular basement membrane (GBM). TBMN presents with hematuria, minimal proteinuria, and normal renal function. Although TBMN is an autosomal dominant disease (COL4A3 and COL4A4), ATS can be inherited X-linked (COL4A5), autosomal recessive, or autosomal dominant (both COL4A3 and COL4A4). The clinical course of TBMN is usually benign, whereas ATS typically results in end-stage renal disease (ESRD). Nevertheless, there is a broad spectrum of clinical phenotypes caused by mutations in COL4A3 or COL4A4. We report an Italian family who presented with hematuria and mild proteinuria. Mutational analysis showed a novel heterozygous mutation p.G291E in exon 15 of the COL4A3 gene. Many different mutations in COL4A3 and COL4A4 that cause TBMN have already been identified, but most genetic variability in these genes has been found to cause autosomal ATS. A valid genotype-phenotype correlation for TBMN or ATS is not yet known. Therefore, it is important to identify new mutations by direct sequencing to clarify their clinical importance, to assess the prognosis of the disease, and to avoid renal biopsy.

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.