Growth hormone deficiency and splicing fidelity - Two serine/arginine-rich proteins, ASF/SF2 and SC35, act antagonistically

The majority of mutations that cause isolated growth hormone deficiency type II are the result of aberrant splicing of transcripts encoding human growth hormone. Such mutations increase skipping of exon 3 and encode a 17.5- kDa protein that acts as a dominant negative to block secretion of full- length protein produced from unaffected alleles. Previously, we identified a splicing regulatory element in exon 3 ( exonic splicing enhancer 2 ( ESE2)), but we had not determined the molecular mechanism by which this element prevents exon skipping. Here, we show that two members of the serine/ arginine- rich (SR) protein superfamily (ASF/ SF2 and SC35) act antagonistically to regulate exon 3 splicing. ASF/ SF2 activates exon 3 inclusion, but SC35, acting through a region just downstream of ESE2, can block such activation. These findings explain the disease- causing mechanism of a patient mutation in ESE2 that creates a functional SC35- binding site that then acts synergistically with the downstream SC35 site to produce pathological levels of exon 3 skipping. Although the precedent for SR proteins acting as repressors is established, this is the first example of a patient mutation that creates a site through which an SR protein represses splicing.