A cytosine-rich splice-regulatory determinant enforces functional processing of the human α-globin gene transcript.

The establishment of efficient and stable splicing patterns in terminally differentiated cells is critical to maintenance of specific functions throughout the lifespan of an organism. The human (h)α-globin gene contains three exons separated by two short introns. Naturally occurring α-thalassemia mutations that trigger aberrant splicing have revealed the presence of cryptic splice sites within the hα-globin gene transcript. How cognate (functional) splice sites are selectively utilized in lieu of these cryptic sites has remained unexplored. Here we demonstrate that the preferential selection of a cognate splice donor essential to functional splicing of the hα-globin transcript is dependent on the actions of an intronic Cytosine (C)-rich splice regulatory determinant and its interacting polyC binding proteins. Inactivation of this determinant by mutation of the C-rich element or by depletion of PolyC binding proteins triggers a dramatic shift in splice donor activity to an upstream, out of frame, cryptic donor. The essential role of the C-rich element in hα-globin gene expression is supported by its co-evolution with the cryptic donor site in primate species. These data lead us to conclude that an intronic C-rich determinant enforces functional splicing of the hα-globin transcript, thus acting as an obligate determinant of hα-globin gene expression.