Inhibition of growth hormone (GH) secretion by a mutant GH-I gene product in neuroendocrine cells containing secretory granules: An implication for isolated GH deficiency inherited in an autosomal dominant manner

Isolated GH deficiency (IGHD) type II is a disease inherited in an autosoma l dominant manner. Although point mutations at the donor splice site of int ron 3 of the GH-I gene have been identified in patients, the mechanism of h ow such mutations result in severe GH deficiency is unclear. Recently, we i dentified two mutations in Japanese patients with IGHD type II, G to A subs titutions at the first (mutA) and fifth (mutE) nucleotides of intron 3. Mes senger ribonucleic acids skipping exon 3 were transcribed from both mutant GH-I genes. We studied in this report the synthesis and secretion of GH enc oded by the mutant GH-I genes and tested whether inhibition of wild-type GH secretion by mutant products could be demonstrated in cultured cell lines. A metabolic labeling study in COS-1 cells revealed that a mutant GH with a reduced molecular mass was synthesized from the mutant messenger ribonuclei c acid and retained in the cells for at least 6 h, On the other hand, the w ild-type GH was rapidly secreted into the medium. Coexpression of mutant an d wild-type GH did not result in any inhibition of wild-type GPI secretion in COS-1 or HepG2 cells. However, coexpression of mutant GH resulted in sig nificant inhibition of wild-type GH secretion in somatotroph-derived MtT/S cells as well as in adrenocorticotroph-derived AtT-20 cells, without affect ing cell viability. We conclude that the dominant negative effect of mutant GH on the secretion of wildtype GH is at least in part responsible for the pathogenesis of IGHD type II. Our results also suggest that neuroendocrine cell type-specific mechanisms, including intracellular storage of the secr etory proteins, are involved in the inhibition.