Unequal homologous crossing over resulting in duplication of 36 base pairs within exon 47 of the COL4A5 gene in a family with Alport syndrome

Meta-analysis of gross insertions causing human genetic disease: novel mutational mechanisms and the role of replication slippage

Although gross insertions (>20 bp) comprise <1% of disease-causing mutations, they nevertheless represent an important category of pathological lesion. In an attempt to study these insertions in a systematic way, 158 gross insertions ranging in size between 21 bp and approximately 10 kb were identified using the Human Gene Mutation Database (www.hgmd.org). A careful meta-analytical study revealed extensive diversity in terms of the nature of the inserted DNA sequence and has provided new insights into the underlying mutational mechanisms. Some 70% of gross insertions were found to represent sequence duplications of different types (tandem, partial tandem, or complex). Although most of the tandem duplications were explicable by simple replication slippage, the three complex duplications appear to result from multiple slippage events. Some 11% of gross insertions were attributable to nonpolyglutamine repeat expansions (including octapeptide repeat expansions in the prion protein gene [PRNP] and polyalanine tract expansions) and evidence is presented to support the contention that these mutations are also caused by replication slippage rather than by unequal crossing over. Some 17% of gross insertions, all >or=276 bp in length, were found to be due to LINE-1 (L1) retrotransposition involving different types of element (L1 trans-driven Alu, L1 direct, and L1 trans-driven SVA). A second example of pathological mitochondrial-nuclear sequence transfer was identified in the USH1C gene but appears to arise via a novel mechanism, trans-replication slippage. Finally, evidence for another novel mechanism of human genetic disease, involving the possible capture of DNA oligonucleotides, is presented in the context of a 26-bp insertion into the ERCC6 gene.

A large tandem duplication within the COL4A5 gene is responsible for the high prevalence of Alport syndrome in French Polynesia

A large tandem duplication within the COL4A5 gene is responsible for the high prevalence of Alport syndrome in French Polynesia. Background. The prevalence of X-linked Alport syndrome, a progressive inherited nephropathy associated with mutations in the type IV collagen gene COL4A5, is remarkably high in French Polynesia. Methods. A vast clinical, genealogic, and molecular study was undertaken in Polynesia, based on public records, patients' interviews, linkage analysis, and mutation screening. Results and Conclusions. We show that the high frequency of Alport syndrome in this region is due to a founder mutation that occurred onto a common haplotype shared by affected and unaffected individuals, the presence of which precludes indirect molecular diagnosis. We have characterized the mutation as a tandem duplication of 35 COL4A5 exons, resulting in a approximately 65% increase in the length of the collagenous domain of the alpha 5(IV) chain, which is still able to assemble into type IV collagen network as shown by immunofluorescence analysis. That mutation is associated with severe and highly penetrant ocular symptoms and with uniformly thin glomerular basement membrane (GBM) in male adult patients. However, the rate of progression of the renal disease is very variable from one male patient to another, demonstrating the importance of strong modifier factors. Our results suggest that the 20% to 50% of "missing"COL4A5 mutations in X-linked Alport syndrome may be rearrangements similar to that reported here, which was not detectable by sequencing of either individual COL4A5 exons or overlapping cDNA fragments. Finally, we provide the basis for a polymerase chain reaction (PCR) assay that accurately identifies female carriers and allows adequate genetic counseling in this population.

The clinical spectrum of type IV collagen mutations

Clinical manifestations of type IV collagen mutations can vary from the severe, clinically and genetically heterogeneous renal disorder, Alport syndrome, to autosomal dominant familial benign hematuria. The predominant form of Alport syndrome is X-linked; more than 160 different mutations have yet been identified in the type IV collagen alpha 5 chain (COL4A5) gene, located at Xq22-24 head to head to the COL4A6 gene. The autosomal recessive form of Alport syndrome is caused by mutations in the COL4A3 and COL4A4 genes, located at 2q35-37. Recently, the first mutation in the COL4A4 gene was identified in familial benign hematuria. This paper presents an overview of type IV collagen mutations, including eight novel COL4A5 mutations from our own group in patients with Alport syndrome. The spectrum of mutations is broad and provides insight into the clinical heterogeneity of Alport syndrome with respect to age at renal failure and accompanying features such as deafness, leiomyomatosis, and anti-GBM nephritis.