Mutations in the COL4A4 gene in thin basement membrane disease

Association of Genetically Predicted Skipping of COL4A4 Exon 27 with Hematuria and Albuminuria

Background:

Hematuria is an established sign of glomerular disease and can be associated with kidney failure, but there has been limited scientific study of this trait.

Methods:

Here, we combined genetic data from the UK Biobank with predicted gene expression and splicing from GTEx kidney cortex samples (n = 65) in a transcriptome-wide association study (TWAS) to identify additional potential biological mechanisms influencing hematuria.

Results:

The TWAS using kidney cortex identified significant associations for 5 genes in terms of expression and 3 significant splicing events. Notably, we identified an association between the skipping of COL4A4 exon 27, which is genetically predicted by intronic rs11898094 (minor allele frequency 13%), and hematuria. Association between this variant was also found with urinary albumin excretion. We found independent evidence supporting the same variant predicting this skipping event in glomeruli-derived mRNA transcriptomics data (n = 245) from NEPTUNE. The functional significance of loss of exon 27 was demonstrated using the split NanoLuc-based α3α4α5(IV) heterotrimer assay, in which type IV collagen heterotrimer formation was quantified by luminescence. The causal splicing variant for this skipping event is yet to be identified.

Conclusions:

In summary, by integrating multiple data types, we identify a potential splicing event associated with hematuria and albuminuria.

The same heterozygous Col4A4 mutation triggered different renal pathological changes in Chinese family members

Background: Mutations in the collagen components of the glomerular basement membrane (GBM) often lead to hereditary glomerulonephritis. Previous studies have identified that autosomal dominant mutations of Col4A3, Col4A4 or Col4A5 are associated with thin basement membrane nephropathy (TBMN), Alport syndrome and other hereditary kidney diseases. However, the genetic mutations underlying other glomerulonephritis types have not been elucidated. Methods: In this study, we investigated a Chinese family with hereditary nephritis using the methods of genetic sequencing and renal biopsy. Genomic DNA was extracted from peripheral blood of the proband and her sister, and subsequently was performed genetic sequencing. They were found to have the similar mutation sites. Other family members were then validated using Sanger sequencing. The proband and her sister underwent renal puncture biopsies, and experienced pathologists performed PAS, Masson, immunofluorescence, and immunoelectron microscopic staining of the kidney tissue sections. Results: Through genetic sequencing analysis, we detected a novel heterozygous frameshift mutation c.1826delC in the COL4A4 (NM_000092.4) gene coding region, and 1 hybrid missense variation c.86G>A (p. R29Q) was also detected in the TNXB (NM_019105.6) gene coding region in several members of this Chinese family. Interestingly, we found that the same mutations caused different clinical features and distinct pathological changes in individual family members, which confirmed that pathological and genetic testing are crucial for the diagnosis and treatment of hereditary kidney diseases. Conclusion: In this study, we found a novel heterozygous mutation in Col4A4 and co-mutations of the TNXB gene in this Chinese family. Our study indicated that the same Col4A4 mutated variants produced different pathological and clinical changes in different family members. This discovery may provide novel insights into the study of hereditary kidney disease. In addition, new genetic biology techniques and renal biopsy of individual family members are essential.

A COL4A4-G394S Variant and Impaired Collagen IV Trimerization in a Patient with Mild Alport Syndrome

Background

Missense variants in COL4A genes are often found in patients with an Alport syndrome-like presentation, but their pathogenicity is not always clear. We encountered a woman with microscopic hematuria and proteinuria at 33 years of age with a diagnosis of thin basement membrane disease who was approaching end stage kidney disease at 59 years of age. We hypothesized that this patient's kidney disease was within the spectrum of Alport syndrome.

Methods

We used histologic, genetic, and biochemical approaches to investigate the mechanisms of kidney disease. By immunofluorescence, we investigated collagen IV chain composition of the glomerular basement membrane (GBM). We employed targeted sequencing to search for pathogenic variants in COL4A and other relevant genes. We utilized N- and C-terminal split NanoLuciferase assays to determine the effect of a novel COL4A4 variant of uncertain significance (VUS) on collagen IV heterotrimer formation in vitro. We transfected COL4A4 expression constructs with split NanoLuciferase fragment-fused COL4A3 and COL4A5 constructs into human embryonic kidney 293T cells. To assay for α3α4α5(IV) heterotrimer formation and secretion, we measured luminescence in cell lysates and culture supernatants from transfected cells.

Results

Immunostaining suggested that the collagen α3α4α5(IV) network was present throughout the patient's GBMs. DNA sequencing revealed a novel homozygous VUS: COL4A4 c.1180G>A (p. Gly394Ser). In the C-terminal split luciferase-based α3α4α5(IV) heterotrimer formation assays, luminescence levels for G394S were comparable to WT, but in the N-terminal tag assays, the extracellular luminescence levels for G394S were decreased by approximately 50% compared with WT.

Conclusions

Our cell-based assay provides a platform to test COL4 VUS and shows that G394S impairs assembly of the α3α4α5(IV) N-terminus and subsequent trimer secretion. These data suggest that the COL4A4-G394S variant is pathogenic and causes an atypical mild form of autosomal recessive Alport syndrome.

Prevalence Estimates of Predicted Pathogenic COL4A3-COL4A5 Variants in a Population Sequencing Database and Their Implications for Alport Syndrome

BACKGROUND

The reported prevalence of Alport syndrome varies from one in 5000 to one in 53,000 individuals. This study estimated the frequencies of predicted pathogenic COL4A3-COL4A5 variants in sequencing databases of populations without known kidney disease.

METHODS

Predicted pathogenic variants were identified using filtering steps based on the ACMG/AMP criteria, which considered collagen IV α3-α5 position 1 Gly to be critical domains. The population frequencies of predicted pathogenic COL4A3-COL4A5 variants were then determined per mean number of sequenced alleles. Population frequencies for compound heterozygous and digenic combinations were calculated from the results for heterozygous variants.

RESULTS

COL4A3-COL4A5 variants resulting in position 1 Gly substitutions were confirmed to be associated with hematuria (for each, P<0.001). Predicted pathogenic COL4A5 variants were found in at least one in 2320 individuals. p.(Gly624Asp) represented nearly half (16 of 33, 48%) of the variants in Europeans. Most COL4A5 variants (54 of 59, 92%) had a biochemical feature that potentially mitigated the clinical effect. The predicted pathogenic heterozygous COL4A3 and COL4A4 variants affected one in 106 of the population, consistent with the finding of thin basement membrane nephropathy in normal donor kidney biopsy specimens. Predicted pathogenic compound heterozygous variants occurred in one in 88,866 individuals, and digenic variants in at least one in 44,793.

CONCLUSIONS

The population frequencies for Alport syndrome are suggested by the frequencies of predicted pathogenic COL4A3-COL4A5 variants, but must be adjusted for the disease penetrance of individual variants and for the likelihood of already diagnosed disease and non-Gly substitutions. Disease penetrance may depend on other genetic and environmental factors.

Exome sequencing and characterization of 49,960 individuals in the UK Biobank

The UK Biobank is a prospective study of 502,543 individuals, combining extensive phenotypic and genotypic data with streamlined access for researchers around the world¹. Here we describe the release of exome-sequence data for the first 49,960 study participants, revealing approximately 4 million coding variants (of which around 98.6% have a frequency of less than 1%). The data include 198,269 autosomal predicted loss-of-function (LOF) variants, a more than 14-fold increase compared to the imputed sequence. Nearly all genes (more than 97%) had at least one carrier with a LOF variant, and most genes (more than 69%) had at least ten carriers with a LOF variant. We illustrate the power of characterizing LOF variants in this population through association analyses across 1,730 phenotypes. In addition to replicating established associations, we found novel LOF variants with large effects on disease traits, including PIEZO1 on varicose veins, COL6A1 on corneal resistance, MEPE on bone density, and IQGAP2 and GMPR on blood cell traits. We further demonstrate the value of exome sequencing by surveying the prevalence of pathogenic variants of clinical importance, and show that 2% of this population has a medically actionable variant. Furthermore, we characterize the penetrance of cancer in carriers of pathogenic BRCA1 and BRCA2 variants. Exome sequences from the first 49,960 participants highlight the promise of genome sequencing in large population-based studies and are now accessible to the scientific community.

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.

The COL4A3 and COL4A4 Digenic Mutations in cis Result in Benign Familial Hematuria in a Large Chinese Family

Mutations in the COL4A5 gene result in X-linked Alport syndrome, homozygous or compound heterozygous mutations in COL4A3 or COL4A4 are responsible for autosomal recessive Alport syndrome, and heterozygous mutations in COL4A3 or COL4A4 cause autosomal dominant Alport syndrome or benign familial hematuria. Recently, the existence of a digenic inheritance in Alport syndrome has been demonstrated. We here report heterozygous COL4A3 and COL4A4 digenic mutations in cis responsible for benign familial hematuria. Using bioinformatics analyses and pedigree verification, we showed that COL4A4 c.1471C>T and COL4A3 c.3418 + 1G>T variants in cis are pathogenic and co-segregate with the benign familial hematuria. This result suggests that COL4A3 and COL4A4 digenic mutations in cis mimicking an autosomal dominant inheritance should be considered as a novel inheritance pattern of benign familial hematuria, although the disease-causing mechanism remains unknown.

The triple helix of collagens - an ancient protein structure that enabled animal multicellularity and tissue evolution

The cellular microenvironment, characterized by an extracellular matrix (ECM), played an essential role in the transition from unicellularity to multicellularity in animals (metazoans), and in the subsequent evolution of diverse animal tissues and organs. A major ECM component are members of the collagen superfamily -comprising 28 types in vertebrates - that exist in diverse supramolecular assemblies ranging from networks to fibrils. Each assembly is characterized by a hallmark feature, a protein structure called a triple helix. A current gap in knowledge is understanding the mechanisms of how the triple helix encodes and utilizes information in building scaffolds on the outside of cells. Type IV collagen, recently revealed as the evolutionarily most ancient member of the collagen superfamily, serves as an archetype for a fresh view of fundamental structural features of a triple helix that underlie the diversity of biological activities of collagens. In this Opinion, we argue that the triple helix is a protein structure of fundamental importance in building the extracellular matrix, which enabled animal multicellularity and tissue evolution.

Efficient Targeted Next Generation Sequencing-Based Workflow for Differential Diagnosis of Alport-Related Disorders

Alport syndrome (AS) is an inherited type IV collagen nephropathies characterized by microscopic hematuria during early childhood, the development of proteinuria and progression to end-stage renal disease. Since choosing the right therapy, even before the onset of proteinuria, can delay the onset of end-stage renal failure and improve life expectancy, the earliest possible differential diagnosis is desired. Practically, this means the identification of mutation(s) in COL4A3-A4-A5 genes. We used an efficient, next generation sequencing based workflow for simultaneous analysis of all three COL4A genes in three individuals and fourteen families involved by AS or showing different level of Alport-related symptoms. We successfully identified mutations in all investigated cases, including 14 unpublished mutations in our Hungarian cohort. We present an easy to use unified clinical/diagnostic terminology and workflow not only for X-linked but for autosomal AS, but also for Alport-related diseases. In families where a diagnosis has been established by molecular genetic analysis, the renal biopsy may be rendered unnecessary.

A Novel COL4A4 Mutation Identified in a Chinese Family with Thin Basement Membrane Nephropathy

Thin basement membrane nephropathy (TBMN) is often attributable to mutations in the COL4A3 or COL4A4 genes that encode the α3 and α4 chains of type IV collagen, respectively, a major structural protein in the glomerular basement membrane. The aim of this study was to explore a new disease-related genetic mutation associated with the clinical phenotype observed in a Chinese Han family with autosomal dominant TBMN. We conducted a clinical and genetic study comprising seven members of this TBMN family. Mutation screening for COL4A3 and COL4A4 was carried out by direct sequencing. The RNA sequences associated with both proteins were also analyzed with reverse transcription PCR and TA cloning. The result showed that every affected patient had a novel heterozygous splicing mutation in COL4A4 (c.1459 + 1G > A), which led to the elimination of the entire exon 21 from the COL4A4 cDNA and resulted in the direct splicing of exons 20 and 22. This in turn caused a frameshift mutation after exon 20 in the open reading frame of COL4A4. In conclusion, we describe a novel splicing mutation in COL4A4 that results in TBMN. This analysis increases our understanding of TBMN phenotype-genotype correlations, which should facilitate more accurate diagnosis and prenatal diagnosis of TBMN.

Collagen type IV-related nephropathies in Portugal: pathogenic COL4A3 and COL4A4 mutations and clinical characterization of 25 families

Pathogenic mutations in genes COL4A3/COL4A4 are responsible for autosomal Alport syndrome (AS) and thin basement membrane nephropathy (TBMN). We used Sanger sequencing to analyze all exons and splice site regions of COL4A3/COL4A4, in 40 unrelated Portuguese probands with clinical suspicion of AS/TBMN. To assess genotype-phenotype correlations, we compared clinically relevant phenotypes/outcomes between homozygous/compound heterozygous and apparently heterozygous patients. Seventeen novel and four reportedly pathogenic COL4A3/COL4A4 mutations were identified in 62.5% (25/40) of the probands. Regardless of the mutated gene, all patients with ARAS manifested chronic renal failure (CRF) and hearing loss, whereas a minority of the apparently heterozygous patients had CRF or extrarenal symptoms. CRF was diagnosed at a significantly younger age in patients with ARAS. In our families, the occurrence of COL4A3/COL4A4 mutations was higher, while the prevalence of XLAS was lower than expected. Overall, a pathogenic COL4A3/COL4A4/COL4A5 mutation was identified in >50% of patients with fewer than three of the standard diagnostic criteria of AS. With such a population background, simultaneous next-generation sequencing of all three genes may be recommended as the most expedite approach to diagnose collagen IV-related glomerular basement membrane nephropathies.

Whole exome sequencing reveals novel COL4A3 and COL4A4 mutations and resolves diagnosis in Chinese families with kidney disease

Background

Collagen IV-related nephropathies, including thin basement membrane nephropathy and Alport Syndrome (AS), are caused by defects in the genes COL4A3, COL4A4 and COL4A5. Diagnosis of these conditions can be hindered by variable penetrance and the presence of non-specific clinical or pathological features.

Methods

Three families with unexplained inherited kidney disease were recruited from Shanghai, China. Whole exome sequencing (WES) was performed in the index case from each family and co-segregation of candidate pathogenic mutations was tested by Sanger sequencing.

Results

We identified COL4A4 missense variants [c.G2636A (p.Gly879Glu) and c.C4715T (p.Pro1572Leu)] in the 21-year-old male proband from family 1, who had been diagnosed with mesangial proliferative nephropathy at age 14. COL4A4 c.G2636A, a novel variant, co-segregated with renal disease among maternal relatives. COL4A4 c.C4715T has previously been associated with autosomal recessive AS and was inherited from his clinically unaffected father. In family 2, a novel COL4A3 missense mutation c.G2290A (p.Gly997Glu) was identified in a 45-year-old male diagnosed with focal segmental glomerulosclerosis and was present in all his affected family members, who exhibited disease ranging from isolated microscopic hematuria to end stage renal disease (ESRD). In family 3, ESRD occurred in both male and females who were found to harbor a known AS-causing COL4A5 donor splice site mutation (c.687 + 1G > A). None of these variants were detected among 100 healthy Chinese individuals.

Conclusion

WES identified 2 novel and 2 known pathogenic COL4A3/COL4A4/COL4A5 mutations in 3 families with previously unexplained inherited kidney disease. These findings highlight the clinical range of collagen IV-related nephropathies and resolved diagnostic confusion arising from atypical or incomplete clinical/histological findings, allowing appropriate counselling and treatment advice to be given.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2369-15-175) contains supplementary material, which is available to authorized users.

A new mutation in the COL4A3 gene responsible for autosomal dominant Alport syndrome, which only generates hearing loss in some carriers

Bilateral sensorineural hearing loss is a characteristic feature of Alport syndrome, which is always linked to renal manifestations so they have a parallel evolution and prognosis, and deafness helps to identify the renal disease. We report a family that suffers an autosomal dominant Alport syndrome caused by a previously undescribed mutation in the COL4A3 gene, in which several members have hearing impairment as the only clinical manifestation, suggesting that in this family deafness can occur independent of renal disease. This mutation is also present in a patient with anterior lenticonus, an observation only found in families with recessive and sex-linked Alport disease.

COL4A3/COL4A4 mutations and features in individuals with autosomal recessive Alport syndrome

Alport syndrome is an inherited disease characterized by hematuria, progressive renal failure, hearing loss, and ocular abnormalities. Autosomal recessive Alport syndrome is suspected in consanguineous families and when female patients develop renal failure. Fifteen percent of patients with Alport syndrome have autosomal recessive inheritance caused by two pathogenic mutations in either COL4A3 or COL4A4. Here, we describe the mutations and clinical features in 40 individuals including 9 children and 21 female individuals (53%) with autosomal recessive inheritance indicated by the detection of two mutations. The median age was 31 years (range, 6-54 years). The median age at end stage renal failure was 22.5 years (range, 10-38 years), but renal function was normal in nine adults (29%). Hearing loss and ocular abnormalities were common (23 of 35 patients [66%] and 10 of 18 patients [56%], respectively). Twenty mutation pairs (50%) affected COL4A3 and 20 pairs affected COL4A4. Of the 68 variants identified, 39 were novel, 12 were homozygous changes, and 9 were present in multiple individuals, including c.2906C>G (p.(Ser969*)) in COL4A4, which was found in 23% of the patients. Thirty-six variants (53%) resulted directly or indirectly in a stop codon, and all 17 individuals with early onset renal failure had at least one such mutation, whereas these mutations were less common in patients with normal renal function or late-onset renal failure. In conclusion, patient phenotypes may vary depending on the underlying mutations, and genetic testing should be considered for the routine diagnosis of autosomal recessive Alport syndrome.

Impact of gene polymorphisms of interleukin-18, transforming growth factor-β, and vascular endothelial growth factor on development of IgA nephropathy and thin glomerular basement membrane disease

Background

We investigated the effects of gene polymorphisms on the development of IgA nephropathy and thin glomerular basement membrane (GBM) disease by analyzing polymorphisms in the interleukin (IL)-18, transforming growth factor (TGF)-β, and vascular endothelial growth factor (VEGF) genes in Korean patients.

Methods

This study included 146 normal individuals and 69 biopsy-proven IgA nephropathy and 44 thin GBM disease patients. The gene polymorphisms −607 A/C and −137 G/C in IL-18, −509C/T and T869C in TGF-β, and −2578C/A and 405C/G in VEGF were investigated in DNA extracted from peripheral blood.

Results

The frequencies of the IL-18 −607CC genotype (43.5% vs. 21.2%, P=0.002, P corrected=0.012) and the VEGF 405 GG genotype (37.7% vs. 21.2%, P=0.002, P corrected=0.012) were significantly increased in the IgA nephropathy group compared with the control group, whereas no significant differences in genotype frequency were observed between the thin GBM disease and control groups. However, there were no significant differences in genotype and allele frequencies between the IgA nephropathy and thin GBM disease groups.

Conclusion

This study did not show any statistically significant differences of six selected gene polymorphisms of the IL-18, TGF-β, and VEGF genes between IgA nephropathy and thin GBM disease. Additional extensive studies are required to clarify the potential role of gene polymorphism to discriminate IgA nephropathy and thin GBM disease without renal biopsy.

Genotype-phenotype correlations in 17 Chinese patients with autosomal recessive Alport syndrome

Autosomal recessive Alport syndrome (ARAS) results from mutations in the COL4A3 or COL4A4 gene. We analyzed the genotype and phenotype of 17 unrelated Chinese patients with ARAS. Clinical data were reviewed. All coding exons of COL4A3 and COL4A4 genes were PCR-amplified and sequenced from genomic DNA. We identified pathologic mutations in all patients, giving a mutation detection rate of 100%, with 82% in COL4A3 gene and 18% in COL4A4 gene. Sixteen novel mutations in COL4A3 gene and four novel mutations in COL4A4 gene were identified. Furthermore, a previously reported in-frame deletion mutation (40_63del24) in exon 1 of the COL4A3 gene was found in four patients in our study. A single 40_63del24 mutation in COL4A3 seems to result in mild or no renal manifestations, whereas the homozygous state of 40_63del24 in COL4A3 gene or compound heterozygous mutation of 40_63del24 plus another nonsense or frameshift mutation in COL4A3 gene seems to result severe ARAS with hearing loss. Half of the probands' parents had hematuria with or without mild proteinuria. Therefore, we recommend that ARAS be considered when a patient has a positive family history of hematuria, and screening for COL4A3 mutations firstly may be an efficient strategy for molecular diagnosis of ARAS.

Common variants in the COL4A4 gene confer susceptibility to lattice degeneration of the retina

Lattice degeneration of the retina is a vitreoretinal disorder characterized by a visible fundus lesion predisposing the patient to retinal tears and detachment. The etiology of this degeneration is still uncertain, but it is likely that both genetic and environmental factors play important roles in its development. To identify genetic susceptibility regions for lattice degeneration of the retina, we performed a genome-wide association study (GWAS) using a dense panel of 23,465 microsatellite markers covering the entire human genome. This GWAS in a Japanese cohort (294 patients with lattice degeneration and 294 controls) led to the identification of one microsatellite locus, D2S0276i, in the collagen type IV alpha 4 (COL4A4) gene on chromosome 2q36.3. To validate the significance of this observation, we evaluated the D2S0276i region in the GWAS cohort and in an independent Japanese cohort (280 patients and 314 controls) using D2S0276i and 47 single nucleotide polymorphisms covering the region. The strong associations were observed in D2S0276i and rs7558081 in the COL4A4 gene (Pc = 5.8 × 10(-6), OR = 0.63 and Pc = 1.0 × 10(-5), OR = 0.69 in a total of 574 patients and 608 controls, respectively). Our findings suggest that variants in the COL4A4 gene may contribute to the development of lattice degeneration of the retina.

The role of molecular genetics in diagnosing familial hematuria(s)

Familial microscopic hematuria (MH) of glomerular origin represents a heterogeneous group of monogenic conditions involving several genes, some of which remain unknown. Recent advances have increased our understanding and our ability to use molecular genetics for diagnosing such patients, enabling us to study their clinical characteristics over time. Three collagen IV genes, COL4A3, COL4A4, and COL4A5 explain the autosomal and X-linked forms of Alport syndrome (AS), and a subset of thin basement membrane nephropathy (TBMN). A number of X-linked AS patients follow a milder course reminiscent of that of patients with heterozygous COL4A3/COL4A4 mutations and TBMN, while at the same time a significant subset of patients with TBMN and familial MH progress to chronic kidney disease (CKD) or end-stage kidney disease (ESKD). A mutation in CFHR5, a member of the complement factor H family of genes that regulate complement activation, was recently shown to cause isolated C3 glomerulopathy, presenting with MH in childhood and demonstrating a significant risk for CKD/ESKD after 40 years old. Through these results molecular genetics emerges as a powerful tool for a definite diagnosis when all the above conditions enter the differential diagnosis, while in many at-risk related family members, a molecular diagnosis may obviate the need for another renal biopsy.

Novel corneal phenotype in a patient with alport syndrome

PURPOSE

To report the clinical and histopathologic findings of an unusual keratopathy, which may represent a new corneal dystrophy in a patient with Alport syndrome (ATS).

METHODS

A 59-year-old woman with longstanding diagnosis of autosomal recessive ATS was evaluated for progressively decreasing vision in the left eye. She had anterior lenticonus and cataract and central corneal stromal opacification with significant thinning and flattening bilaterally. She underwent penetrating keratoplasty and cataract extraction with posterior chamber intraocular lens implantation. We describe the light microscopic and ultrastructural findings from the cornea.

RESULTS

Histopathology of the corneal button revealed marked stromal thinning with decreased keratocytes. The endothelial cells were attenuated and focally lost. Immunohistochemical stains for cytokeratin were positive, findings consistent with posterior polymorphous dystrophy (PPMD). Transmission electron microscopy showed necrosis and a marked loss of keratocytes. Multilayering of the endothelium was consistent with PPMD, but mature desmosomes and microvilli were absent. In vivo confocal microscopy on the fellow eye showed linear hyporeflective bands at the level of Descemet's membrane consistent with PPMD. In addition, there were fine linear changes in the deep stroma and diffuse hyperreflectivity of the mid and superficial stroma with lack of identifiable keratocytes throughout.

CONCLUSIONS

We believe this to be the first reported case to demonstrate some histopathologic features of PPMD in ATS. However, the clinical, histopathologic, and ultrastructural characteristics are not typical of PPMD. This may represent a new phenotypic expression of PPMD or may be a distinct clinicopathologic dystrophy associated with ATS.

Phenotype/Genotype Correlations in the Ultrastructure of Monogenetic Glomerular Diseases

Electron microscopy defined classic patterns of hereditary glomerular disease long before genetics revealed an underlying specific mutation. Genetic analysis is now easier to perform in clinical practice but an earlier optimism that genetics would predict disease severity and phenotype is challenged. The classic paradigm is Alport nephritis in which only a subset of mutations may predict glomerular abnormalities and disease severity. Interpretation of ultrastructural pathology of monogenetic diseases like Alport nephritis is complicated when the proband is the first family member to be diagnosed or there is discrepancy between clinical presentation and ultrastructural changes. In this review the authors have selected a dozen cases representative of common monogenetic glomerular diseases as a platform to discuss the utility of diagnostic electron microscopy in the era of molecular genetics. The emphasis is on genotype/glomerular phenotype correlations.

The R229Q mutation in NPHS2 may predispose to proteinuria in thin-basement-membrane nephropathy

Thin-basement-membrane nephropathy (TBMN) is characterized by persistent dysmorphic hematuria, and the presence of proteinuria is a risk factor for renal impairment. TBMN is often due to mutations in the COL4A3 and COL4A4 genes, and this study determined whether additional mutations in genes encoding other structures in the glomerular filtration barrier contributed to the development of proteinuria. Fifty-six unrelated individuals with TBMN including 18 (32%) with proteinuria > or = 300 mg/L and ten (18%) with proteinuria > or = 500 mg/L were studied. Deoxyribonucleic acid (DNA) was screened for NPHS2 mutations and variants (R138Q and P375L) using single-stranded conformational analysis (SSCA) and for the R229Q mutation by sequencing. DNA was also screened for ACTN4 mutations. R229Q was more common in patients with TBMN and proteinuria > or = 500 mg/L (p < 0.05), and a possible NPHS2 mutation (671G>A, R224H) was identified in one patient with proteinuria 700 mg/L. No other NPHS2 variants correlated with proteinuria, and no ACTN4 mutations were found. Individuals with TBMN and R229Q are carriers of the autosomal recessive forms of both Alport syndrome and familial focal segmental glomerulosclerosis (FSGS). The early demonstration of R229Q in individuals with TBMN may indicate those at increased risk of proteinuria and renal impairment.

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

The recent description of multiple gene defects in hereditary podocytopathies and in hereditary glomerular basement membrane diseases has dramatically improved the current state of our knowledge on the renal glomerular filtration barrier. Recently described mutations in collagen IV and laminin in patients with hematuria and severe nephrotic syndrome add to other experimental data supporting the hypothesis that the glomerular basement membrane (GBM) may also have a significant role in protein filtration, a function previously attributed exclusively to the podocytes. Collagen IV heterozygous mutations were thought to cause only a mild form of renal disease (thin basement membrane nephropathy--TBMN). However, data from our laboratory show that many patients who carry such mutations may later on in life develop focal and segmental glomerulosclerosis, on top of the TBMN and the microscopic hematuria, a situation that frequently progresses to chronic renal failure or even end-stage renal disease. The role of unknown modifier genes may explain the heterogeneity of symptoms in TBMN and other glomerular diseases and in particular the selected development of chronic renal failure. The molecular communication between GBM and podocytes may also be a key factor in the search for these major genetic modifiers while their understanding may improve novel drug design for glomerular diseases.

COL4A3/COL4A4 mutations producing focal segmental glomerulosclerosis and renal failure in thin basement membrane nephropathy

Mutations in the COL4A3/COL4A4 genes of type IV collagen have been found in approximately 40% of cases of thin basement membrane nephropathy, which is characterized by microscopic hematuria and is classically thought to cause proteinuria and chronic renal failure rarely. Here we report our observations of 116 subjects from 13 Cypriot families clinically affected with thin basement membrane nephropathy. These families first came to our attention because they segregated microscopic hematuria, mild proteinuria, and variable degrees of renal impairment, but a dual diagnosis of focal segmental glomerulosclerosis (FSGS) and thin basement membrane nephropathy was made in 20 biopsied cases. Molecular studies identified founder mutations in both COL4A3 and COL4A4 genes in 10 families. None of 82 heterozygous patients had any extrarenal manifestations, supporting the diagnosis of thin basement membrane nephropathy. During follow-up of up to three decades, 31 of these 82 patients (37.8%) developed chronic renal failure and 16 (19.5%) reached end-stage renal disease. Mutations G1334E and G871C were detected in seven and three families, respectively, and were probably introduced by founders. We conclude that these particular COL4A3/COL4A4 mutations either predispose some patients to FSGS and chronic renal failure, or that thin basement membrane nephropathy sometimes coexists with another genetic modifier that is responsible for FSGS and progressive renal failure. The findings presented here do not justify the labelling of thin basement membrane nephropathy as a benign condition with excellent prognosis.

A novel mutation of COL4A3 presents a different contribution to Alport syndrome and thin basement membrane nephropathy

BACKGROUND

Alport syndrome (AS) and thin basement membrane nephropathy (TBMN) are heterogeneous renal hereditary diseases. Mutations of COL4A3 and COL4A4 genes were reported to be the underlying pathogenicity in both diseases. However, the mechanism of the same mutation causing totally different clinical processes and outcomes in AS and TBMN is still not clear.

SUBJECTS AND METHODS

Mutations of all coding exons of COL4A3 and COL4A4 were screened in a patient with autosomal recessive Alport syndrome (ARAS) of a Chinese Han consanguineous family by means of PCR and direct sequencing. Furthermore, the identified mutation was validated by restriction endonuclease AvaII in all 20 members in his family, as well as 46 patients with TBMN, 2 patients with AS from another two families, and 50 healthy controls.

RESULTS

A novel missense mutation (3725G>A, G1242D) in exon 42 of COL4A3 was identified in the proband in the homozygous form. This pathogenic mutation was demonstrated in all carriers who presented with hematuria or mild proteinuria in the heterozygous form, whereas it was not detected in others whose urinalysis was normal within the family. In addition, 10 polymorphisms, including 1 non-glycine missense variant and 9 neutral polymorphisms, were detected in COL4A3/COL4A4.

CONCLUSION

The novel mutation (3725G>A, G1242D) of COL4A3 was the underlying pathogenic role in the homozygous form in ARAS and in the heterozygous form in TBMN within an identical family. The result provided a potentially useful clue for the functional investigation of COL4A3 in these two hereditary glomerular disorders.

Do mutations in COL4A1 or COL4A2 cause thin basement membrane nephropathy (TBMN)?

Thin basement membrane nephropathy (TBMN) is the commonest cause of persistent glomerular haematuria and often presents in childhood. Only 40% of affected individuals have mutations identified in the COL4A3 and COL4A4 genes, but mutations in the genes for other COL4A isoforms also result in thinned membranes in humans (COL4A5) and mice (COL4A1). This study examined whether COL4A1/COL4A2 represented a further genetic locus for TBMN. Nine families with TBMN in whom haematuria did not segregate with COL4A3/COL4A4, were examined for linkage to COL4A1/COL4A2 using five micro-satellite markers. In addition, index cases from these families plus a further 14 unrelated individuals with TBMN that was not due to COL4A3 or COL4A4 mutations (n=23) were screened for mutations in each of the 52 exons of COL4A1 and the 47 exons of COL4A2 using single stranded conformational analysis (SSCA). DNA samples that demonstrated bandshifts were sequenced. Haplotype analysis demonstrated that haematuria segregated with the COL4A1/COL4A2 locus in only two small families (2/9, 22%). No definite COL4A1 or COL4A2 mutations were identified in the 23 unrelated individuals with TBMN although novel polymorphisms were demonstrated. This study indicates that COL4A1/COL4A2 does not represent a further major genetic locus for TBMN.

Nine novel COL4A3 and COL4A4 mutations and polymorphisms identified in inherited membrane diseases

Both thin basement membrane nephropathy (TBMN) and autosomal recessive Alport syndrome result from mutations in the COL4A3 and COL4A4 genes, and this study documents further mutations and polymorphisms in these genes. Thirteen unrelated children with TBMN and five individuals with autosomal recessive Alport syndrome were examined for mutations in the 52 exons of COL4A3 and the 47 coding exons of COL4A4 using single-stranded conformation polymorphism (SSCP) analysis. Amplicons producing different electrophoretic patterns were sequenced, and mutations were defined as variants that changed an amino acid but were not present in 50 non-hematuric normals. Three further novel mutations were identified. These were IVS 22-5 T>A in the COL4A3 gene in a consanguineous family with autosomal recessive Alport syndrome, and R1677C and R1682Q in the COL4A4 gene. In addition, six novel polymorphisms (G455G, I462I, G736G and IVS 38-8 G>A in COL4A3, and L658L and A1577A in COL4A4) were demonstrated.Many different COL4A3 and COL4A4 mutations cause TBMN and autosomal recessive Alport syndrome. The identification of polymorphisms in these genes is particularly important to enable diagnostic laboratories to distinguish mutations from uncommon normal variants.

Sixteen novel mutations identified in COL4A3, COL4A4, and COL4A5 genes in Slovenian families with Alport syndrome and benign familial hematuria

Alport syndrome (ATS) and benign familial hematuria (BFH) are type IV collagen inherited disorders. Mutations in COL4A5 are generally believed to cause X-linked ATS, whereas mutations in COL4A3 and COL4A4 genes can be associated with the autosomal-recessive and -dominant type of ATS or BFH. In view of the wide spectrum of phenotypes, an exact diagnosis is sometimes difficult to achieve. This study involved screening each exon with boundary intronic sequences of COL4A3, COL4A4, and COL4A5 genes by optimized polymerase chain reaction-single-stranded conformational polymorphism analysis in 17 families with ATS and in 40 families diagnosed as having BFH. Twelve different mutations were found in the COL4A5 gene in ATS patients, comprising nine missense mutations, a splice site mutation, a mutation causing frameshift, and a nonsense mutation. One of the missense mutations (p.G624D) was present not only in one family with ATS but also in five families with suspected BFH. Three heterozygous mutations in the COL4A3 gene (two missense and one frameshift) and four heterozygous mutations in COL4A4 (two splice site, one in-frame deletion, and one missense) were identified in patients with BFH. Sixteen mutations are to the best of our knowledge new and private.

Thin basement membrane nephropathy: diffuse and segmental types

CONTEXT

Diffuse thinning of the glomerular basement membrane (GBM) is the ultrastructural diagnostic criterion of thin basement membrane nephropathy (TBMN). However, there is no consensus regarding what diagnosis should be made if the attenuation is segmental.

OBJECTIVE

To develop a diagnostic approach to TBMN in cases with segmental GBM thinning.

DESIGN

We compared the diagnostic sensitivities of 2 methods used for the quantitative expression of GBM width in a consecutive series of renal biopsies from 26 patients (median age, 36 years; range, 15 to 59 years) with dysmorphic hematuria (a variable degree of proteinuria in 19 patients), a thin GBM, and absence of other renal disease. The harmonic GBM width was determined from orthogonal intercepts, and the actual width in the thinnest loops was obtained by direct measurement. The GBM attenuation was categorized into diffuse or segmental types by conventional inspection.

RESULTS

Segmental TBMN accounted for one third of the series. In neither type did the male patients have a higher harmonic mean GBM width than the female patients. Focal-global glomerulosclerosis was more common in diffuse TBMN. The laborious orthogonal intercept method proved insensitive for the verification of segmental TBMN, whereas the much simpler direct measurement technique captured all the cases.

CONCLUSIONS

A considerable number of patients with TBMN display segmental GBM attenuation. Because the routine criterion excludes these cases from the diagnosis, we propose to define TBMN as a clinicopathologic entity of dysmorphic hematuria and a diffusely or segmentally thinned GBM confirmed by the direct measurement technique.

Renal transplantation in patients with Alport syndrome

For pediatric kidney transplant physicians, two aspects of Alport syndrome set the disease apart from other causes of terminal renal failure. First, an understanding of the genetics of Alport syndrome is needed to make appropriate decisions regarding potential related kidney donors to Alport patients requiring renal transplantation. Second, renal transplantation for Alport syndrome may be complicated by post-transplant anti-GBM nephritis, a problem that is nearly unique to this disease. This review discusses these aspects of Alport syndrome and attempts to provide rational recommendations for clinicians.

[From Alport syndrome to benign familial hematuria: clinical and genetic aspect]

Alport syndrome (AS) is a hereditary glomerulonephritis variably associated with neural hearing loss and ocular abnormalities. The prevalence of the disease is estimated at approximately 1 in 50,000 live births. AS arises from mutations in genes encoding alpha chains constituting type IV collagen. In 85% of patients, the disease results from mutations in the COL4A5 gene located on X chromosome. In the hemizygous male, persistent microhematuria is present from early life, then proteinuria and renal insufficiency occur with time, leading to end-stage renal failure before age 40. In the heterozygous female, clinical manifestations vary from completely healthy state to end-stage renal failure, most often reached after the age of 40. In 15% of patients, the disease results from mutations in either the COL4A3 or the COL4A4 gene, both located on chromosome 2. When both alleles are mutated (autosomal recessive form), the phenotype is constantly severe, resembling that of the hemizygous male in the X-linked form. In the heterozygous individual, the clinical spectrum vary from the absence of any manifestation to the development of proteinuria - the so-called autosomal-dominant AS -, and even renal insufficiency, sometimes reaching end-stage (after the age of 40) through the most frequently encountered phenotype, i.e. a persistently isolated microhematuria, accounting for the so-called benign familial hematuria (or healthy carrier state). The determinants of the phenotype remain largely unknown, so that it may be risky to predict renal prognosis in the individual with a single COL4A3/A4 mutation and an isolated microhematuria at the time of examination.

Chronic renal failure and shortened lifespan in COL4A3+/- mice: an animal model for thin basement membrane nephropathy

A heterozygous mutation in autosomal Alport genes COL4A3 and COL4A4 can be found in 20 to 50% of individuals with familial benign hematuria and diffuse glomerular basement membrane thinning (thin basement membrane nephropathy [TBMN]). Approximately 1% of humans are heterozygous carriers of mutations in the autosomal Alport genes and at risk for developing renal failure as a result of TBMN. The incidence and pathogenesis of renal failure in heterozygous COL4A3/4 mutation carriers is still unclear and was examined further in this study using COL4A3 knockout mice. In heterozygous COL4A3(+/-) mice lifespan, hematuria and renal function (serum urea and proteinuria) were monitored during a period of 3 yr, and renal tissue was examined by light and electron microscopy, immunohistochemistry, and Western blot. Lifespan of COL4A3(+/-) mice was found to be significantly shorter than in healthy controls (21.7 versus 30.3 mo). Persistent glomerular hematuria was detected starting in week 9; proteinuria of > 0.1 g/L started after 3 mo of life and increased to > 3 g/L after 24 mo. The glomerular basement membrane was significantly thinned (167 versus 200 nm in wild type) in 30-wk-old mice, coinciding with focal glomerulosclerosis, tubulointerstitial fibrosis, and increased levels of TGF-beta and connective tissue growth factor. The renal phenotype in COL4A3(+/-) mice resembled the clinical and histopathologic phenotype of human cases of TBMN with concomitant progression to chronic renal failure. Therefore, the COL4A3(+/-) mouse model will help in the understanding of the pathogenesis of TBMN in humans and in the evaluation of potential therapies.

Thin glomerular basement membrane nephropathy: incidence in 3471 consecutive renal biopsies examined by electron microscopy

CONTEXT

Thin glomerular basement membrane (GBM) nephropathy is often equated with benign familial hematuria, although it may occur sporadically and may not always be benign. Thin GBM nephropathy is reported to occur in at least 1% of the population, although its reported incidence varies considerably in different studies because there are presently no uniform criteria for its diagnosis by electron microscopy (EM).

OBJECTIVE

To determine the incidence of thin GBM nephropathy in a large sample of renal biopsies using a basic methodology that can easily be applied in any diagnostic EM laboratory.

DESIGN

Direct measurements of GBM thickness were made from electron micrographs at 3 specified points along each of 10 randomly selected glomerular capillaries to determine an average GBM thickness for each of 50 males and 50 females, ages 9 to 80 years, with minimal-change nephropathy or acute interstitial nephritis, without hematuria. The means of the average GBM thickness values were 330 +/- 50 (SD) nm in the males and 305 +/- 45 nm in the females; normal ranges for each sex were defined as being within 2 SD of these means. The average GBM thickness was then similarly determined for renal biopsies with suspected thin GBMs examined from January 2000 to December 2004; a total of 3471 renal biopsies were examined by EM during this period.

SETTING

Academic medical center renal pathology/EM laboratory.

RESULTS

Excluding biopsies with immunoglobulin A nephropathy, which is known to be frequently associated with thin GBMs, and biopsies with Alport syndrome, 67 biopsies (1.9% of total) had an average GBM thickness below the sex-specific normal range. Of these, 37 biopsies were performed specifically because of hematuria and had an average GBM thickness of 121 to 215 nm (mean, 185 +/- 20 nm). The remaining 30 (0.9%) biopsies, with average GBM thicknesses of 143 to 227 nm (mean, 190 +/- 20 nm), represent cases of incidentally discovered thin GBM nephropathy.

CONCLUSIONS

Based on the frequency of incidentally discovered cases and taking into account excluded cases and biopsies (eg, with diabetic nephropathy) in which diagnosis of incidental thin GBM nephropathy by EM is not possible, the incidence of thin GBM nephropathy in our population is estimated to be between 1% and 2%. Diseases most often associated with incidental thin GBM nephropathy were focal segmental glomerulosclerosis (10 cases) and minimal-change nephropathy (5 cases).

Persistent familial hematuria in children and the locus for thin basement membrane nephropathy

This study examined how often children with persistent familial hematuria were from families where hematuria segregated with the known genetic locus for the condition known as benign familial hematuria or thin basement membrane nephropathy (TBMN) at COL4A3/COL4A4. Twenty-one unrelated children with persistent familial hematuria as well as their families were studied for segregation of hematuria with haplotypes at the COL4A3/COL4A4 locus for benign familial hematuria and at the COL4A5 locus for X-linked Alport syndrome. Eight families (38%) had hematuria that segregated with COL4A3/COL4A4, and four (19%) had hematuria that segregated with COL4A5. At most, eight of the other nine families could be explained by disease at the COL4A3/COL4A4 locus if de novo mutations, non-penetrant hematuria or coincidental hematuria in unaffected family members was present individually or in combination. This study confirms that persistent familial hematuria is not always linked to COL4A3/COL4A4 (or COL4A5) and suggests the possibility of a further genetic locus for benign familial hematuria. This study also highlights the risk of excluding X-linked Alport syndrome on the basis of the absence of a family history or of kidney failure.

Childhood thin GBM disease: review of 22 children with family studies and long-term follow-up

Thin glomerular basement membrane (GBM) disease is generally known to have a good renal prognosis, although renal insufficiency has sometimes been reported and the overlap with Alport syndrome implies that a good prognosis cannot be guaranteed. In order to shed light on long-term prognosis of thin GBM disease we have retrospectively evaluated 22 children with persistent haematuria and biopsy-proven thin GBM. Mean follow up was 7 years (range 2-17 years), mean age at onset was 7 years (range 1.5-15). Biopsies were performed a mean of 3.8 years after detection of hematuria. The light microscopy (LM) and immunofluorescence (IF) findings were essentially unremarkable in all of the children, while electron microscopy (EM) showed thinning of the GBM in all cases and no changes characteristic of Alport syndrome. The family history was positive for renal disease in 17 (77.3%) patients with hematuria in 8 (36.3%) families, and hematuria with renal failure (RF) or deafness in 9 (40.9%). It was completely negative for renal disease in 4 (18.2%) and unavailable in 1 (4.5%). Four patients (18%) showed a decline in renal function after 6, 8, 9 and 12 years of follow-up, and 1 of these also developed hearing impairment. None developed hypertension. Our study suggests that thin GBM disease is not always benign and a child with thin GBM should never be assigned such a prognosis, especially if there is a family history of renal impairment or deafness, where careful follow-up is needed due to the risk of late onset renal failure.

Familial hematurias: what we know and what we don't

Over the past 30 years we have learned a great deal about the molecular genetics and natural history of familial forms of hematuria. Our enhanced understanding of these conditions has yet to generate effective therapies for Alport syndrome, the form of familial hematuria associated with end-stage renal disease. This review briefly presents the current state of knowledge about familial hematuria and argues for the organization of clinical therapeutic trials in Alport syndrome.

Autosomal-dominant familial hematuria with retinal arteriolar tortuosity and contractures: A novel syndrome

BACKGROUND

Autosomal-dominant forms of hematuria have been mostly related to mutations in the COL4A3/COL4A4 genes. Patients with thin basement membrane (BM) disease do not have extrarenal manifestations, while those with Alport syndrome often present with hearing loss, anterior lenticonus, and dot-and-fleck retinopathy.

METHODS

We performed a phenotypic study and a candidate gene approach in a four-generation family presenting with autosomal-dominant hematuria associated with extrarenal manifestations. Renal biopsy was analyzed for determination of BM thickness and expression of chains of type IV collagen. Linkage to 18 candidate genes/loci was investigated using polymorphic microsatellite markers.

RESULTS

In all affected patients, hematuria without proteinuria was associated with muscular contractures and retinal arterial tortuosities responsible for retinal hemorrhages. Cardiac arrhythmia, Raynaud phenomena, and brain MRI abnormalities were also observed. Despite the presence of red cells in tubule sections, no glomerular abnormalities were found by electron microscopy. Expression of type IV collagen chains and glomerular BM thickness was normal. We searched for a molecular defect affecting either BM or angiogenesis. Linkage analyses of genes encoding BM components (COL4A3/COL4A4, COL6A1, COL6A2, COL6A3, FBLN1), and angiogenic factors or their receptors (VHL, ANPT1, ANPT2, TIE, TEK, NOTCH2, NOTCH3, NOTCH4, DLL4, JAG1, JAG2) and of the facio-sapulo-humeral dystrophy and 3q21 loci failed to show segregation of the disease with those gene loci.

CONCLUSION

We have identified a new inherited hematuria syndrome associated with retinal vessel tortuosities and contractures. We recommend performing a fundus examination in patients with familial hematuria and episodes of visual impairment, as well as a urinary analysis in patients with retinal arterial tortuosity or congenital muscular contractures.

Thin glomerular basement membrane disease: clinical significance of a morphological diagnosis--a collaborative study of the Italian Renal Immunopathology Group

BACKGROUND

Thin glomerular basement membrane disease (TBMD) is a nephropathy defined by diffuse thinning of the glomerular basement membrane (GBM) at electron microscopy examination, without the alterations of Alport's syndrome (ATS). It is known that many patients with TBMD have a type IV collagen disorder and that the disease occasionally may be progressive. This study investigated 51 patients with the morphological diagnosis of TBMD lacking any sign of ATS, with the aim of defining the prevalence of type IV collagen mutations and the course of the disease.

METHODS

Patients were investigated as follows: (a) clinical picture and family investigation; (b) renal biopsy findings; (c) immunohistochemical study of renal tissue for collagen IV alpha-chains; (d) pedigree reconstruction and molecular investigations in genes encoding type IV collagen chains, when DNA samples were available; and (e) follow-up data.

RESULTS

Renal biopsy analysis revealed no light microscopy changes in 27 patients and minimal abnormalities in the remainder. Global glomerular sclerosis was found in seven cases and superimposed mesangial immunoglobulin-A deposits in four. Normal staining of GBM for alpha(IV) chains was observed in all but one patient, where alpha5(IV) was absent and molecular investigation revealed a COL4A5 mutation. Five out of 25 cases had a mutation in the COL4A3/COL4A4 genes. Eight out of 38 patients followed up for 12-240 months (21%) showed signs of disease progression or hypertension.

CONCLUSIONS

This study confirms that a considerable proportion of patients with TBMD have a type IV collagen disorder and that this lesion is not always benign. Thus, families should be investigated carefully whenever possible and patients and affected relatives should be examined periodically for signs of disease progression.

Nephrin gene (NPHS1) in patients with minimal change nephrotic syndrome (MCNS)

BACKGROUND

Minimal change nephrotic syndrome (MCNS) is a major problem in pediatric nephrology. While the pathogenesis of MCNS is not known, the latest discoveries in the genetic diseases indicate that glomerular epithelial cells (podocytes) and the slit diaphragm play a primary role in development of proteinuria. Because nephrin is known to be a major component of the slit diaphragm, we analyzed the structure of nephrin gene (NPHS1) in patients with MCNS of different severity.

METHODS

Clinical data and DNA samples were collected from 25 adults who had biopsy-proven MCNS in childhood. A direct sequencing was performed to all 29 exons of the NPHS1 gene. The significance of the findings was evaluated by similar analysis of DNA samples from 25 healthy control patients.

RESULTS

The analysis of NPHS1 revealed no specific MCNS-associated mutation. However, 5 of the 25 MCNS patients had heterozygous allelic variants leading to nonconservative amino acid substitutions not previously reported (G879R; R800C; T294I; A916S). One of the five patients also had the Fin-major mutation, and two had new, conservative amino acid substitutions (S786N; A342G). Three of the five patients were classified as steroid sensitive, one was an early nonresponder, and one patient showed clear resistance to steroid treatment. Six known polymorphic changes in NPHS1 were also found, three of them leading to amino acid changes. The number of allelic variants was high both in MCNS patients and control patients (mean 3.0 and 2.6).

CONCLUSION

The results suggest that genetic changes in nephrin may have a pathogenetic role in some patients with MCNS.

COL4A3 mutations and their clinical consequences in thin basement membrane nephropathy (TBMN)

BACKGROUND

Thin basement membrane nephropathy (TBMN) is often caused by mutations in the COL4A3 and COL4A4 genes.

METHODS

We examined 62 unrelated individuals diagnosed with TBMN by renal biopsy (N= 49, 79%) or a positive family history of hematuria but without a biopsy (N= 13, 21%) for mutations in the COL4A3 gene and the COL4A3/COL4A4 promoter. All 52 exons of COL4A3 as well as the COL4A3/COL4A4 promoter were screened with single-stranded conformational polymorphism (SSCP) analysis at 4 degrees C and at room temperature. Amplicons that demonstrated electrophoretic abnormalities were sequenced.

RESULTS

Seven mutations were demonstrated in seven patients: G532C and G584C in exon 25, G596R in exon 26, G695R in exon 28, and IVS 2224 - 11C>T, IVS 2980 + 1G>A and IVS 3518 - 7C>G. No mutations were found in the COL4A3/COL4A4 promoter. Four novel polymorphisms or variants (P116T in exon 6, P690P in exon 27, and G895G and A899A in exon 33) were also demonstrated. In addition, P1109S and Q1495R, which had been described previously but whose status was unclear, were shown to be polymorphisms. All seven mutations described here were associated with hematuria. While one mutation (2980 + 1G>A) was found in an individual who also had proteinuria, none of her family members with the same mutation had increased urinary protein. None of the patients with these seven mutations had renal impairment. Hematuria was completely penetrant in families with the G532C, G584C, G596R, and IVS 2980 + 1G>A mutations but not with the G695R and IVS 3518 - 7C>G mutations.

CONCLUSION

COL4A3 mutations are common in TBMN.

Autosomal recessive Alport's syndrome and benign familial hematuria are collagen type IV diseases

BACKGROUND

Alport's syndrome (AS) is a genetically heterogeneous renal hereditary disease. Mutations in collagen type IV genes have been described to be responsible for X-linked (COL4A5), autosomal recessive, and autosomal dominant AS (COL4A3/COL4A4). Moreover, at least 40% of benign familial hematuria (BFH) cases cosegregate with the COL4A3/COL4A4 loci, following a dominant pattern of inheritance. Therefore, it has been suggested that BFH may represent the carrier state for autosomal recessive AS.

METHODS

We report a mutational study of the COL4A3 and COL4A4 genes in 14 AS and 2 BFH families. When possible, linkage analysis has been performed to confirm the pattern of inheritance. One affected proband from each family underwent mutation screening by single-strand conformation polymorphism/heteroduplex analysis.

RESULTS

We identified 13 mutations within the COL4A3 gene and 2 mutations within the COL4A4 gene, 9 of which are first reported here. We also detected 14 polymorphisms within the COL4A3 gene and 15 polymorphisms within the COL4A4 gene, 7 of them not previously described. In 2 of our AS families, we found mutations previously reported for BFH, and we characterized a novel mutation shared by an AS and a BFH family.

CONCLUSION

Collagen type IV nephropathy is an entity in itself, and phenotypic manifestations of COL4A3/COL4A4 mutations may range from monosymptomatic hematuria (BFH) to severe renal failure (AS), depending on the gene dosage. In 3 of our families, we genetically confirmed that BFH represents the carrier state for autosomal recessive AS.

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.

Thin basement membrane nephropathy

Thin basement membrane nephropathy. Thin basement membrane nephropathy (TBMN) is the most common cause of persistent glomerular bleeding in children and adults, and occurs in at least 1% of the population. Most affected individuals have, in addition to the hematuria, minimal proteinuria, normal renal function, a uniformly thinned glomerular basement membrane (GBM) and a family history of hematuria. Their clinical course is usually benign. However, some adults with TBMN have proteinuria >500 mg/day or renal impairment. This is more likely in hospital-based series of biopsied patients than in the uninvestigated, but affected, family members. The cause of renal impairment in TBMN is usually not known, but may be due to secondary focal segmental glomerulosclerosis (FSGS) or immunoglobulin A (IgA) glomerulonephritis, to misdiagnosed IgA disease or X-linked Alport syndrome, or because of coincidental disease. About 40% families with TBMN have hematuria that segregates with the COL4A3/COL4A4 locus, and many COL4A3 and COL4A4 mutations have now been described. These genes are also affected in autosomal-recessive Alport syndrome, and at least some cases of TBMN represent the carrier state for this condition. Families with TBMN in whom hematuria does not segregate with the COL4A3/COL4A4 locus can be explained by de novo mutations, incomplete penetrance of hematuria, coincidental hematuria in family members without COL4A3 or COL4A4 mutations, and by a novel gene locus for TBMN. A renal biopsy is warranted in TBMN only if there are atypical features, or if IgA disease or X-linked Alport syndrome cannot be excluded clinically. In IgA disease, there is usually no family history of hematuria. X-linked Alport syndrome is much less common than TBMN and can often be identified in family members by its typical clinical features (including retinopathy), a lamellated GBM without the collagen alpha3(IV), alpha4(IV), and alpha5(IV) chains, and by gene linkage studies or the demonstration of a COL4A5 mutation. Technical difficulties in the demonstration and interpretation of COL4A3 and COL4A4 mutations mean that mutation detection is not used routinely in the diagnosis of TBMN.