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.     

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.     

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.

     

Evidence that NPHS2-R229Q predisposes to proteinuria and renal failure in familial hematuria

Background  

Familial hematuria (FH) is associated with at least two pathological entities: thin basement membrane nephropathy (TBMN), caused by heterozygous COL4A3/COL4A4 mutations, and C3 nephropathy caused by CFHR5 mutations. It is now known that TBMN patients develop proteinuria and changes of focal segmental glomerulosclerosis when biopsied. End-stage kidney disease (ESKD) is observed in 20% of carriers, at ages 50–70. A similar progression is observed in CFHR5 nephropathy. Recent evidence suggests that NPHS2-R229Q, a podocin polymorphism, may contribute to proteinuria in TBMN and to micro-albuminuria in the general population.     

The clinical features of thin basement membrane nephropathy

Thin basement membrane nephropathy (TBMN) is a common, lifelong condition affecting the kidneys that is characterized by microscopic glomerular hematuria, minimal or no proteinuria, and normal renal function. It often is discovered incidentally, and usually has an excellent prognosis. Many cases are familial and show autosomal-dominant inheritance. The defining characteristic is a glomerular basement membrane (GBM) that is thinned to about half its normal thickness on ultrastructural examination of the renal biopsy specimen. However, occasionally patients with TBMN develop marked proteinuria or renal impairment. It is unclear whether individuals with TBMN and impaired renal function represent part of the spectrum of TBMN associated with heterozygous COL4A3 or COL4A4 mutations, or if their disease is caused by mutations of other genes, or whether it is caused by a second coexistent renal lesion or is misdiagnosed Alport syndrome.     

The R229Q mutation in <Emphasis Type="Italic">NPHS2</Emphasis> 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 ≥ 300 mg/L and ten (18%) with proteinuria ≥ 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 ≥ 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.     

Type-IV collagen related diseases

Alport syndrome (ATS) is a progressive inherited glomerulonephritis accounting for 1-2% of all patients who start renal replacement therapy, with an estimated gene frequency of approximately 1 in 5000. ATS is a genetically heterogeneous disease, commonly inherited as an X-linked semi-dominant trait, caused by mutations in COL4A5, on the X-chromosome, and only rarely (less than 10% of cases) caused by the COL4A3 or the COL4A4 gene on chromosome 2q. In the X-linked form females are generally less affected than males, microhematuria being the only sign present throughout life, although approximately 30% can progress to end-stage renal disease. It became evident in recent years that mutations in the COL4A3 or the COL4A4 gene can give rise not only to autosomal recessive ATS syndrome, in which males and females are severely affected, but also to an autosomal dominant form, where the clinical progression towards impaired renal function can be very slow and also to benign familial hematuria (BFH) in which renal function is preserved.     

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.     

Nine novel <Emphasis Type="Italic">COL4A3</Emphasis> and <Emphasis Type="Italic">COL4A4</Emphasis> 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.     

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.     

The genetics of thin basement membrane nephropathy

The diagnosis of thin basement membrane nephropathy (TBMN) usually is made on the basis of the clinical features or the glomerular membrane ultrastructural appearance. Only now are we beginning to understand the genetics of TBMN and the role of diagnostic genetic testing. The similarity of clinical and glomerular membrane features first suggested TBMN might represent the carrier state for autosomal-recessive Alport syndrome. This was confirmed subsequently by the demonstration that 40% of families with TBMN have hematuria that segregates with the corresponding locus ( COL4A3/COL4A4 ), and identical mutations occur in both conditions. To date, about 20 COL4A3 and COL4A4 mutations have been shown in TBMN, and these mainly are single nucleotide substitutions that are different in each family. The families in whom hematuria does not appear to segregate with the COL4A3/COL4A4 locus cannot all be explained by de novo mutations, and nonpenetrant or coincidental hematuria. This suggests a further TBMN locus. In patients with persistent hematuria, testing for COL4A3 and COL4A4 mutations to diagnose TBMN is problematic because of the huge size of these genes, their frequent polymorphisms, and the likelihood of a further gene locus. It is far more practicable to perform genetic testing to exclude or confirm X-linked Alport syndrome because this condition is the major differential diagnosis of TBMN and has a very different prognosis.     

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.     

薄基底膜肾病并发局灶节段性肾小球硬化症一家系的COL4A3和COL4A4基因突变分析

目的 探讨薄基底膜肾病(TBMN)合并局灶节段性肾小球硬化症(FSGS)的遗传学机制.方法 对一病理学诊断为TBMN合并FSGS患者及其家系的COL4A3和COL4A4基因突变,应用与COL4A3和COL4A4基因连锁的微卫星标记连锁分析方法进行分析.PCR扩增COIAA3和COL4A4全部98个外显子后,直接测序筛查突变.同时测序排除已为公认的FSGS相关基因NPHS1、NPHS2、WT1、TRPC6、ACTN4、CD2AP突变导致FSGS的可能.结果 微卫星标记连锁分析显示此家系与COL4A3和COL4A4基因连锁.直接测序在此家系中发现疾病患者COL4A4基因1214位的鸟嘌呤突变为腺嘌呤,导致Ⅳ型胶原α4链第405位甘氨酸突变为谷氨酸,并且发现COL4A3基因一多态性IVS1-4C＞T.此多态性随疾病分布,可能与致病相关.未发现FSGS相关基因的突变.结论 此家系是在TBMN的基础上发生FSGS.Ⅳ型胶原α4链突变及随疾病分布的基因多态性是否导致TBMN合并FSGS或使其易感性增加尚待更多家系进一步研究.     

一例女性Fabry病并发薄基底膜肾病及其家系调查

目的 了解具有两种遗传性疾病,即Fabry病并发薄基底膜肾病（TBMN）的临床病理和基因突变特点以及家系患病情况。 方法 总结分析本院收治的1例41岁女性Fabry病并发TBMN患者的临床病理特征和基因突变情况,同时对家系成员进行调查及相关检测。 结果 先证者呈现典型的Fabry病的肾外临床表现,包括皮疹、神经痛、眩晕、耳鸣、肥厚型心肌病等,同时亦有蛋白尿、镜下血尿及高血压等肾脏受累表现;肾活检光镜下病理改变为局灶性节段性肾小球硬化（FSGS）,部分足细胞空泡变性;电镜下肾小球脏层上皮细胞胞质内多数髓磷脂小体形成,肾小球基底膜（GBM）弥漫性变薄,厚度为（216±31） nm。家系调查及基因突变检测显示先证者女儿除有典型Fabry病肾外表现外,亦有以血尿为主的肾脏症状。先证者的1个妹妹仅表现为镜下血尿。先证者及其女儿α-半乳糖苷酶 A（α-Gal A）活性分别为33和75活性单位（正常参考值为100~500活性单位）,且2人均携带新发现的GLA基因突变——1208ins21 bp及COL4A3基因多态性——c:3627 G>A（p:M1209I）。仅表现为镜下血尿的先证者的妹妹仅携带COL4A3基因的c:3627 G>A（p:M1209I）多态性,α-Gal A活性正常,无GLA基因突变。 结论 对于Fabry肾病患者呈现血尿,尤其是表现为家族性血尿时,应考虑并认真排除并发TBMN的可能。     

Autosomal dominant Alport syndrome linked to the type IV collage &agr;3 and &agr;4 genes (COL4A3 and COL4A4)

Background: Alport syndrome is a hereditary nephritis that may lead to end-stage renal disease (ERSD) in young adult life and is often associated with sensorineural deafness and/or ocular abnormalities. The majority of families are X-linked due to mutations in the COL4A5 gene at X122. Autosomal forms of the disease are also recognized with recessive disease, having been shown to be due to mutations in the COL4A3 and COL4A4 genes on chromosome 2. Familial benign haematuria has also been mapped to this region in some families.Subjects and methods: We describe a large family with autosomal dominant Alport syndrome in which males and females are equally severely affected and one member with a mild sensorineural deafness reached ESRD aged 35 years. Renal biopsy in four affected patients demonstrated characteristic thickened and split glomerular basement membranes on electron-microscopy. Results: Genetic linkage analysis using markers on chromosome 2q demonstrated co-segregation of the disease with the markers D2S351 and D2S401 with a maximum lod score of 3.4 at zero recombination. Linkage to the COL4A4 gene was confirmed using an intragenic COL4A4 polymorphism. Mutation analysis has revealed a missense Leu36Pro mutation in exon 5 of the adjacent COL4A3 gene in the unaffected mother, which may lead to a more severe phenotype in affected family members carrying this mutation. Conclusion: Mutations in the COL4A3 and COL4A4 cause a spectrum of glomerular basement membrane disease ranging from autosomal recessive Alport syndrome to autosomal dominant Alport syndrome and familial benign haematuria.     

COL4A3/COL4A4 mutations: from familial hematuria to autosomal-dominant or recessive Alport syndrome

COL4A3/COL4A4 mutations: From familial hematuria to autosomal-dominant or recessive Alport syndrome. BACKGROUND: Mutations of the type IV collagen COL4A5 gene cause X-linked Alport syndrome (ATS). Mutations of COL4A3 and COL4A4 have been reported both in autosomal-recessive and autosomal-dominant ATS, as well as in benign familial hematuria (BFH). In the latter conditions, however, clinical features are less defined, few mutations have been reported, and other genes and non-genetic factors may be involved. METHODS: We analyzed 36 ATS patients for COL4A3 and COL4A4 mutations by polymerase chain reaction-single strand conformational polymorphism (PCR-SSCP) and direct sequencing. Sporadic patients who had tested negative for COL4A5 mutations were included with typical cases of autosomal recessive ATS to secure a better definition of the phenotype spectrum. RESULTS: We identified seven previously undescribed COL4A3 mutations: in two genetic compounds and three heterozygotes, and one in COL4A4. In agreement with the literature, some of the mutations of compound heterozygotes were associated with microhematuria in healthy heterozygous relatives. The mutations of heterozygous patients are likely dominant, since no change was identified in the second allele even by sequencing, and they are predicted to result in shortened or abnormal chains with a possible dominant-negative effect. In addition, both genes showed rare variants of unclear pathogenicity, and common polymorphisms that are shared in part with other populations. CONCLUSIONS: This study extends the mutation spectrum of COL4A3 and COL4A4 genes, and suggests a possible relationship between production of abnormal COL IV chains and dominant expression of a continuous spectrum of phenotypes, from ATS to BFH.     

Do mutations in <Emphasis Type="Italic">COL4A1</Emphasis> or <Emphasis Type="Italic">COL4A2</Emphasis> 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.     

Segregation of hematuria in thin basement membrane disease with haplotypes at the loci for Alport syndrome

BACKGROUND: Inherited hematuria is common and is usually attributed to thin basement membrane disease (TBMD). The aim of this study was to determine how often hematuria in families with TBMD segregated with haplotypes at the chromosomal loci for autosomal recessive and X-linked Alport syndrome (COL4A3/COL4A4 and COL4A5, respectively). METHODS: The families of 22 individuals with TBMD on renal biopsy and with urinary glomerular red blood cell (RBC) counts of more than 50,000/mL were studied using phase-contrast microscopy of the urine and DNA microsatellite markers. Eighteen families had at least two members with hematuria. RESULTS: Hematuria segregated with or was consistent with segregation at the COL4A3/COL4A4 locus in eight (36%) families (P < 0.05 in 5 of these) and at the COL4A5 locus in four (18%) families (P < 0.05 in 2). The lack of segregation in the other 10 (45%) families may have occurred because of incomplete penetrance of the hematuria, de novo mutations, coincidental hematuria in other family members, or the presence of a novel gene locus. In four different families, three of which had hematuria that segregated with the COL4A3/COL4A4 locus, four family members with the hematuria haplotype had spouses with coincidental hematuria (4 of 29, 14%). However, none of their four offspring who had also inherited the hematuria haplotype had the clinical features of autosomal recessive Alport syndrome. CONCLUSIONS: Hematuria in families with TBMD commonly segregates with the COL4A3/COL4A4 locus and thus results from mutations in the same genes as autosomal recessive Alport syndrome. Sometimes TBMD may be confused with the carrier state for X-linked Alport syndrome. However, nearly half of the families in this study had hematuria that did not segregate with the loci for either autosomal recessive or X-linked Alport syndrome.     

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.     

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.Alport syndrome is an inherited renal disease characterized by hematuria, progressive renal failure, hearing loss, and ocular abnormalities. Alport commented in 1927 that the occurrence of hematuria and hearing loss in a pedigree was not coincidental but represented a clinical syndrome, and that the more severe disease in male individuals was consistent with X-linked inheritance.¹ We now understand that nearly 85% of patients have X-linked disease due to a pathogenic mutation in the COL4A5 gene, and the remaining individuals usually have autosomal recessive inheritance with two pathogenic mutations in either the COL4A3 or COL4A4 gene.Alport syndrome is usually suspected when the typical clinical features are present. Diagnostic features² include a positive family history, a lamellated glomerular basement membrane (GBM),³ high tone sensorineural hearing loss, and lenticonus and macular flecks on ophthalmoscopy.⁴ However, these features do not distinguish between X-linked and autosomal inheritance. The possibility of autosomal recessive disease is often overlooked, but its recognition is important because the genetic implications are different for the patient and other family members. Affected male individuals with X-linked disease, but few female individuals, eventually develop renal failure and the disease is transmitted from one generation to another. With autosomal recessive inheritance, male and female individuals are equally likely to be affected; renal failure tends to occur in only one generation except in the presence of multiple consanguinity. In our previous report of 206 patients referred for molecular testing of COL4A5, the pathogenic mutation detection rates in families fulfilling none, one, two, three, or four diagnostic criteria were 0%, 18%, 64%, 89%, and 81%, respectively. Autosomal recessive inheritance was suspected to account for the families meeting four diagnostic criteria in whom no pathogenic COL4A5 mutation was detected.⁵Nearly 300 pathogenic mutations have been described in the COL4A3 and COL4A4 genes (Leiden Open Variation Database; https://grenada.lumc.nl/LOVD2/COL4A/home.php?action=switch_db), but many of these are from patients with thin basement membrane nephropathy (TBMN). There are few reports describing two pathogenic mutations in individuals with autosomal recessive Alport syndrome.^6–16 Even fewer studies have examined how mutations may determine clinical features.Here we describe genetic mutations and clinical features in 40 patients in whom two pathogenic mutations were identified in the COL4A3 or COL4A4 gene, consistent with the diagnosis of autosomal recessive Alport syndrome. In many cases, the mutations were demonstrated to be in trans, which is on different chromosomes, confirming autosomal recessive inheritance. Testing examined the entire coding region and splice sites of both COL4A3 and COL4A4 using unidirectional fluorescent Sanger DNA sequencing, analyzed using Mutation Surveyor software. For detecting point mutations in the regions screened, this approach has an analytical sensitivity and specificity of >99%.¹⁷