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

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

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.     

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.     

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.     

Autosomal recessive Alport syndrome: linkage analysis and clinical features in two families.

BACKGROUND: Genetic heterogeneity is a well-known feature of Alport syndrome (AS). Most families with AS show an X-linked dominant pattern of inheritance but about 15% of families show an autosomal inheritance of the disease. Autosomal recessive AS may account for 10% of the total number of cases and is caused by mutations in the COL4A3 and COL4A4 genes. The clinical spectrum of this rare disorder has not been well clarified. METHODS: We present two families with AS. Two affected members of these families have entered end-stage renal disease (ESRD) in their 30s, and the other three are older than 15 years and have normal serum creatinine. Four of the five patients have deafness but none have ocular abnormalities. Two have been transplanted and have not suffered from anti-GBM antibody nephritis. Men and women are equally affected. We have performed linkage analysis for chromosome 2 with the following markers: D2S279, COL4A3/4 DNTR, COL4A4 RFLP Hae III. RESULTS: We demonstrate that both families, one of them consanguineous, are linked to the COL4A3/4 locus. CONCLUSIONS: We can conclude that the only significant difference between the X-linked and the autosomal recessive forms of AS lies in the fact that in the latter females are as affected as males; thus the idea that autosomal recessive AS causes ESRD during childhood must be discarded. Other clinical features such as age of deafness or the presence of post-transplant anti-GBM antibody nephritis show no differences between the entities. Thus an accurate familial study is mandatory in patients with AS, as the identification of the different patterns of inheritance may cause a great difference in genetic counselling. Linkage analysis is the only effective molecular diagnosis that can be performed nowadays.     

Autosomal-dominant Alport syndrome: natural history of a disease due to COL4A3 or COL4A4 gene

BACKGROUND: Alport syndrome is a clinically and genetically heterogeneous nephropathy. The majority of cases are transmitted as an X-linked semidominant condition due to COL4A5 mutations. In this form males are more severely affected than females. Less than 10% of cases are autosomal recessive due to mutation in either COL4A3 or COL4A4. In this rarer form, both males and females are severely affected. Only two cases of autosomal-dominant Alport syndrome have been reported, one due to a COL4A3 mutation and the other due to a COL4A4 mutation. Because of the paucity of the reported families, the natural history of autosomal-dominant Alport syndrome is mostly unknown. METHODS: Four families with likely autosomal-dominant Alport syndrome were investigated. COL4A3 and COL4A4 genes were analyzed by denaturing high-performance liquid chromatography (HPLC). Automated sequencing was performed to identify the underlying mutation. RESULTS: Two families had a mutation in the COL4A4 gene and two in the COL4A3. Accurate clinical evaluation of family members showed interesting results. Affected individuals (22 persons) had a wide range of phenotypes from end-stage renal disease (ESRD) in the fifth decade to a nonprogressive isolated microhematuria. Finally, three heterozygous individuals (90, 22 and 11 years old, respectively) were completely asymptomatic. CONCLUSION: This paper demonstrated that patients affected by autosomal-dominant Alport syndrome have a high clinical variability. Moreover, a reduced penetrance of about 90% (3 of 25) may be considered for the assessment of recurrence risk during genetic counseling of these families.     

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.     

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     

Improving Mutation Screening in Familial Hematuric Nephropathies through Next Generation Sequencing

Alport syndrome is an inherited nephropathy associated with mutations in genes encoding type IV collagen chains present in the glomerular basement membrane. COL4A5 mutations are associated with the major X-linked form of the disease, and COL4A3 and COL4A4 mutations are associated with autosomal recessive and dominant forms (thought to be involved in 15% and 1%–5% of the families, respectively) and benign familial hematuria. Mutation screening of these three large genes is time-consuming and expensive. Here, we carried out a combination of multiplex PCR, amplicon quantification, and next generation sequencing (NGS) analysis of three genes in 101 unrelated patients. We identified 88 mutations and 6 variations of unknown significance on 116 alleles in 83 patients. Two additional indel mutations were found only by secondary Sanger sequencing, but they were easily identified retrospectively with the web-based sequence visualization tool Integrative Genomics Viewer. Altogether, 75 mutations were novel. Sequencing the three genes simultaneously was particularly advantageous as the mode of inheritance could not be determined with certainty in many instances. The proportion of mutations in COL4A3 and COL4A4 was notably high, and the autosomal dominant forms of Alport syndrome appear more frequently than reported previously. Finally, this approach allowed the identification of large COL4A3 and COL4A4 rearrangements not described previously. We conclude that NGS is efficient, reduces screening time and cost, and facilitates the provision of appropriate genetic counseling in Alport syndrome.     

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.     

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.     

Mutations in the COL4A4 gene in thin basement membrane disease

BACKGROUND: Patients with thin basement membrane disease (TBMD) are often from families where hematuria segregates with the COL4A3 and COL4A4 genes. These genes also are affected in autosomal recessive Alport syndrome. The aim of this study was to demonstrate COL4A4 mutations in TBMD. METHODS: Forty-eight unrelated individuals with TBMD who had no family members with autosomal recessive Alport syndrome were examined for COL4A4 mutations. The diagnosis of TBMD had been confirmed by renal biopsy (43/48, 90%) or by a family history of hematuria but without a renal biopsy (5/48, 10%). The 47 coding exons of COL4A4 were screened for mutations with the methods of enzyme mismatch cleavage or single stranded conformational polymorphism (SSCP) analysis, and exons that demonstrated electrophoretic abnormalities were sequenced. RESULTS: Nine variants that altered the coding sequences were identified. These were nonsense and frameshift mutations that resulted in stop codons (N = 3), and glycine (N = 3) and non-glycine missense variants (N = 3). Four intronic variants and three neutral polymorphisms were also detected. In total, four variants were considered 'pathogenic' principally because they resulted in stop codons or were not present in non-hematuric normal subjects. Three variants were considered 'possibly pathogenic' but two of these were each present in one of 46 non-hematuric normal subjects. CONCLUSIONS: Pathogenic COL4A4 mutations were demonstrated in three of the nine (33%) families in whom hematuria segregated with the COL4A3/COL4A4 locus. Two stop codons (R1377X and 2788/91delG) and a glycine substitution (G960R) resulted in hematuria in all 16 members who were tested from these three families. The S969X mutation described here in TBMD for the first time, as well as the R1377X mutation, also occur in autosomal recessive Alport syndrome.     

Successful renal transplantation in a family with a novel mutation in COL4A3 gene and autosomal recessive Alport syndrome

Alport syndrome (AS) is an inherited disorder of basement membranes caused by mutations affecting specific proteins of the type IV collagen family, presenting with nephropathy and extrarenal manifestations such as sensorineural deafness and ocular anomalies. Ten percentage to 15% of the patients with AS have autosomal recessive (ARAS) due to mutation in either COL4A3 or COL4A4 gene. We report a novel mutation in the COL4A3 gene in an Indian family with ARAS. The above‐mentioned genetic anomaly was a missense variation in exon 26 of the COL4A3 gene (chr2:228137797G>A; c.1891G>A) that resulted in the amino acid substitution of Arginine for Glycine at codon 631 (p.Gly631Arg) that was present in the heterozygous state in the asymptomatic parents and homozygous state in the male offspring who presented with early‐onset end‐stage renal disease, lenticonus and hearing loss. The patient (male offspring) underwent successful renal transplantation with his mother as a donor.     

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.     

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.

     

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.     

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.     

Characterization of the peripheral retinopathy in X-linked and autosomal recessive Alport syndrome.

BACKGROUND: Alport syndrome is an inherited disease resulting in kidney failure, hearing loss and ocular abnormalities. Alport syndrome is however often unrecognized, and the aim of this study was to characterize the associated but rarely described peripheral retinopathy and determine whether its demonstration was diagnostically helpful. METHODS: Index cases were diagnosed with Alport syndrome on renal biopsy in themselves or a family member. Inheritance and affected status were determined using microsatellite markers at the COL4A5 and COL4A3/COL4A4 loci, respectively. Participants' eyes were dilated, and examined with direct and indirect ophthalmoscopy, and slit lamp biomicroscopy by an expert ophthalmologist who was unaware of the patients' disease status. RESULTS: Ten males and nine females with X-linked Alport syndrome and seven with autosomal recessive disease were studied. Of the 26 patients, 16 had central retinopathy (62%), and 19 patients had peripheral retinopathy (74%). The peripheral changes occurred in both males and females with X-linked and autosomal recessive Alport syndrome, and were more common when renal failure, hearing loss, lenticonus and the central changes were present, but were also noted in 3 X-linked carriers with normal renal function. CONCLUSIONS: The peripheral retinopathy occurs in X-linked and autosomal recessive Alport syndrome even when the central retinopathy is absent. Careful retinal examination and photography that includes the periphery is a safe and inexpensive method that may help in the diagnosis of Alport syndrome especially in carriers of X-linked disease.     

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.