Isolated growth hormone deficiency type II caused by a point mutation that alters both splice site strength and splicing enhancer function

A heterozygous single base mutation in the human growth hormone (GH) gene (GH‐1) was identified in a family presenting with isolated GH deficiency type II (IGHD II). Affected individuals have a guanine to adenine transition at the first nucleotide of exon 3 (E3+1 G→A) that results in exon skipping and production of a dominant‐negative 17.5‐kDa isoform. We show that the mechanistic basis for exon skipping is due to the unique position of this mutation because it weakens the 3′ splice site and simultaneously disrupts a splicing enhancer located within the first seven bases of exon 3. A G→T mutation at this same position not only affects splicing but also results in a premature stop codon for those transcripts that include exon 3. Thus, mutations that alter the first nucleotide of exon 3 illustrate the various mechanisms by which changes in sequence can cause disease: splice site selection, splicing enhancer function, messenger RNA decay, missense mutations, and nonsense mutations. For IGHD II, only exon skipping leads to production of the dominant‐negative isoform, with increasing skipping correlating with increasing disease severity.