Glomerular Expression of Type IV Collagen Chains in Normal and X-Linked Alport Syndrome Kidneys

摘要

Alport syndrome is an inherited nephropathy characterized by alterations of the glomerular basement membrane because of mutations in type IV collagen genes. COL4A5 mutations, causing X-linked Alport syndrome, frequently result in the loss of the 5 chains of type IV collagen in basement membranes. This is associated with the absence of the 3(IV) and 4(IV) chains and increased amounts of 1(IV) and 2(IV) in glomerular basement membranes. The mechanisms resulting in such a configuration are still controversial and are of fundamental importance for understanding the pathology of the disease and for considering gene therapy. In this article we studied, for the first time, type IV collagen expression in kidneys from X-linked Alport syndrome patients, using in situ hybridization and immunohistochemistry. We show that, independent of the type of mutation and of the level of COL4A5 transcription, both COL4A3 and COL4A4 genes are actively transcribed in podocytes. Moreover, using immunofluorescence amplification, we were able to demonstrate that the 3 chain of type IV collagen was present in the podocytes of all patients. Finally, the 1(IV) chain, which accumulates within glomerular basement membranes, was found to be synthesized by mesangial/endothelial cells. These results strongly suggest that, contrary to what has been found in dogs affected with X-linked Alport syndrome, there is no transcriptional co-regulation of COL4A3, COL4A4, and COL4A5 genes in humans, and that the absence of 3(IV) to 5(IV) in glomerular basement membranes in the patients results from events downstream of transcription, RNA processing, and protein synthesis.