Regulation of the paired type IV collagen genes COL4A5 and COL4A6 Role of the proximal promoter region

Tissue-specific expression patterns of the paired type IV collagen genes CO L4A5 and COL4A6 form the basis for organ involvement in X-linked Alport syn drome, a disorder in which these genes are mutated. We investigated the pro ximal promoter region of COL4A5 and COL4A6 using glomerular visceral epithe lial cells, in which COL4A5 alone is transcribed; keratinocytes, in which t he genes are co-transcribed; and additional model cell Lines. By RNase prot ection assays, the intergenic region is 292 base pairs, Transcription start sites for two 5' splice variants of COL4A6 are 1 kilobase apart. Transient transfections with reporter gene constructs revealed that the minimal prom oters for COL4A5 and COL4A6 are within 100 base pairs of their respective t ranscription start sites and are functionally distinct. In further transfec tion, gel shift and footprinting assays, we defined a bidirectional positiv e regulatory element, which functions in several cell types, but not in glo merular visceral epithelial cells selectively transcribing COL4A5. The exis tence of separate promoters for COL4A5 and COL4A6 permits fine control over their expression. Activation through the bidirectional element can bring a bout co-expression of the genes, exploiting their paired arrangement. Featu res of the proximal promoter region frame its roles in a hierarchy regulati ng type IV collagen gene expression.