Absence of the α6(IV) chain of collagen type IV in Alport syndrome is related to a failure at the protein assembly level and does not result in diffuse leiomyomatosis

X-linked Alport syndrome is a progressive nephropathy associated with mutations in the COL4A5 gene. The kidney usually lacks the alpha 3-alpha 6 chains of collagen type IV, although each is coded by a separate gene. The molecular basis for this loss remains unclear. In canine X-linked hereditary nephritis, a model for X-linked Alport syndrome, a COL4A5 mutation results in reduced mRNA levels for the alpha 3, alpha 4, and alpha 5 chains in the kidney, implying a mechanism coordinating the production of these 3 chains. To examine whether production of alpha 6 chain is under the same control, we studied smooth muscle cells from this animal model. We determined the canine COL4A5 and COL4A6 genes are separated by 435 bp, with two first exons for COL4A6 separated by 978 bp, These two regions are greater than or equal to 78% identical to the human sequences that have promoter activity. Despite this potential basis for coordinated transcription of the COL4A5 and COL4A6 genes, the cub mRNA level remained normal in affected male dog smooth muscle while the alpha 5 mRNA level was markedly reduced. However, both alpha 5 and alpha 6 chains were absent at the protein level, Our results suggest that production of the alpha 6 chain is under a control mechanism separate from that coordinating the alpha 3-alpha 5 chains and that the lack of the alpha 6 chain in Alport syndrome is related to a failure at the protein assembly level, raising the possibility that the alpha 5 and alpha 6 chains are present in the same network. The lack of the alpha 6 chain does not obviously result in disease, in particular leiomyomatosis, as is seen in Alport patients with deletions involving the COL4A5 and COL4A6 genes.