Molecular basis of inherited growth hormone resistance in childhood

The growth hormone receptor (GHR), a member of the cytokine receptor superfamily that gives rise to a soluble and circulating counterpart (GHBP), is the main target of Laron syndrome (LS), a severe autosomal recessive dwarfism characterized by complete GH insensitivity. Genetic and mutation analyses have attested to the high molecular heterogeneity of this syndrome, and, to date, more than 30 different GHR mutations including deletion, frameshift, nonsense, missense and splicing defects have been described. However, among them, missense mutations are of particular interest in potentially providing critical information on the structure-function relationship of the GHR and related molecules. The study of the recently described forms of atypical LS is now very promising. These patients display detectable plasma GH binding activity associated with complete or partial GH insensitivity. Molecular analysis of such a phenotype with positive GHBP and complete GH insensitivity has revealed the existence of a missense mutation abolishing receptor homodimerization, thereby providing in vivo evidence for the critical role of the dimerization process in the growth-promoting action of GH. Similarly, mutations in the cytoplasmic region, which are expected to be associated with normal GH binding activity, should contribute to the identification of other functionally important domains. Partial GH insensitivity syndromes may theorically encompass a wide range of distinct phenotypes with variable degrees of GH resistance. Missense GHR mutations and a quantitative GHR mRNA defect have been identified in some cases belonging to this heterogeneous group. Interestingly, exclusion of linkage between the Laron phenotype and the GHR locus was demonstrated in one affected family. This latter situation may indicate the existence of other genes controlling GHR expression or required at different steps of the signal transduction pathway. In this regard, the availability of a possible animal model for LS should offer new prospects in the identification of GH-inducible genes.