A monoclonal antibody marker for Alport syndrome identifies the Alport antigen as the α5 chain of type IV collagen

The nephropathy of Alport syndrome is associated with unique abnormalities of glomerular basement membranes and is caused in many families by mutations in the X-chromosomal gene COL4A5, which encodes the α5 chain of type IV collagen. We have previously reported that Alport epidermal and glomerular basement membranes fail to bind a monoclonal antibody Mab A7, that reacts with normal epidermal and glomerular basement membranes and that this abnormality is unique to Alport syndrome. The molecule in normal tissues that reacts with Mab A7 was termed the 'Alport antigen'. In the present study we used recombinant carboxy-terminal noncollagenous (NC1) domains of the α1, α2, α3, α4 and α5 chains of type IV collagen to determine the molecular identity of the Alport antigen. Mab A7 was found to bind specifically to the NC1 domain of the α5 chain of type IV collagen, by ELISA and immunoblotting studies. This finding provides a molecular explanation for the utility of Mab A7 as a marker for the Alport basement membrane defect. Mab A7 can identify the Alport basement membrane defect in those patients in whom COL4A5 mutations prevent incorporation of α5(IV) into basement membranes.