Identification of a Repressor in the First Intron of the Human alpha 2(I) Collagen Gene (COL1A2)

The human and mouse genes that code for the alpha 2 chain of collagen I (COL1A2 and Col1a2, respectively) share a common chromatin structure and nearly identical proximal promoter and far upstream enhancer sequences. Despite these homologies, species-specific differences have been reported regarding the function of individual cis-acting elements, such as the first intron sequence. In the present study, we have investigated the transcriptional contribution of the unique open chromatin site in the first intron of COL1A2 using a transgenic mouse model. DNase I footprinting identified a cluster of three distinct areas of nuclease protection (FI1-3) that span from nucleotides +647 to +760, relative to the transcription start site, and which contain consensus sequences for GATA and interferon regulatory factor (IRF) transcription factors. Gel mobility shift and chromatin immunoprecipitation assays corroborated this last finding by documenting binding of GATA-4 and IRF-1 and IRF-2 to the first intron sequence. Moreover, a short sequence encompassing the three footprints was found to inhibit expression of transgenic constructs containing the COL1A2 proximal promoter and far upstream enhancer in a position-independent manner. Mutations inserted into each of the footprints restored transgenic expression to different extents. These results therefore indicated that the unique open chromatin site of COL1A2 corresponds to a repressor, the activity of which seems to be mediated by the concerted action of GATA and IRF proteins. More generally, the study reiterated the existence of species-specific difference in the regulatory networks of the mammalian alpha 2(I) collagen coding genes.