Human GH protein consists of four -helices and contains two disulfide bridges. Isolated growth hormone deficiency type II (IGHD II) is mainly caused by heterozygous splice site mutations of GH-1 leading to the disruption of one disulfide bridge (Cys53-Cys165) and to the loss of amino acids (aa) 32-71 which comprise the complete loop between -helices 1 and 2. The mutant GH protein exerts a dominant-negative effect on wild-type (wt) GH secretion by unclear mechanisms.

For the study of the structure-function relationship of GH mutants concerning the dominant-negative effect, expression vectors harboring mutated GH cDNAs were transiently cotransfected with a vector encoding wtGH (pwtGH) into GH₄C₁-cells. Plasmids encoding either -galactosidase, luciferase or IGFBP-2 were cotransfected with pwtGH and either of the GH mutants.

Compared with the control transfection with pwtGH, GH secretion was mildly decreased by coexpressing wtGH and different GH point mutants with isolated disruption of the disulfide bridge Cys53-Cys165. Similar results were observed with GH mutants deleted in aa 32-46 or 32-52. Deletion of more aa (32-53, 32-63, 32-69, 32-71) ascendingly decreased GH secretion and content of GH in parallel with the increasing length of the deleted stretch. An inhibitory dose-dependent effect of del32-69GH and del32-71GH on the activity/amount of coexpressed -galactosidase, luciferase, and IGFBP-2 was found while mRNA levels were unaffected. Hence, the extent of deletion played the major role in expression of the dominant-negative effect. The inhibitory effect of GH mutants on heterologously expressed non-GH proteins suggests that the dominant-negative effect is not limited to GH and not even to proteins of the regulated secretory pathway but may depend on expression levels.