The Power of Molecular Genetics in Establishing the Diagnosis and Offering Prenatal Testing: The Case for Alport Syndrome

Most Alport cases, 85%, are caused by mutations in the X-linked gene, COL4A5 that encodes the α5 chain of type IV collagen, the most abundant structural protein in the glomerular basement membrane (GBM). The remaining 15% of cases are caused by autosomal recessive mutations in the genes that encode the α3 and α4 chains of the type IV collagen, COL4A3/COL4A4 1,2. Thin basement membrane nephropathy (TBMN) is reportedly also a genetically heterogeneous condition, caused by heterozygous COL4A3/COL4A4 mutations in about 40-50% of the cases. No other responsible genes have been identified as yet. Collagen type IV, as all collagens, is a trimer formed by combinations of three of the six alpha chains, α1-α6. Genes COL4A1 and COL4A2 map to chromosome 13q34, COL4A3 and COL4A4 map to chromosome 2q36–q37 and COL4A5 and COL4A6 map to Xq22-23. All six genes are encoded in nearly 50 exons and close to 1600 aminoacids, and consist of an Nterminal 7S domain, a C-terminal non-collagenous (NC1) domain and a large collagenous domain in between, containing the characteristic Gly-X-Y repeat, common to all collagens. All six alpha chains contain 22-26 natural interruptions of the Gly-X-Y repeats, spread throughout their central collagenous domain. These are presumably regions of specific function and/or ligand binding sequences, of significance to its structural role in the basement membrane. Although there are many possible combinations among the six alpha chains, only three are biologically compatible and found in basement membranes. These are composed of α1α1α2, α3α4α5 and α5α5α6. Type IV collagen participates in forming networks interacting with additional important components of the basement membranes, such as laminin and nidogen 3,4. As a component of the glomerular filtration barrier, along