Objective: Mutations in the GH-releasing hormone (GHRH) receptor (GHRHR) gene (GHRHR) cause autosomal recessive familial isolated GH deficiency (IGHD). We searched for GHRHR mutations in two siblings with IGHD type IB and a history of parental consanguinity.
Design: We analyzed peripheral genomic DNA of an index patient. After identifying a novel mutation in the GHRHR, we performed functional studies in order to confirm that the mutation causes receptor malfunction.
Methods: The entire GHRHR was analyzed in the index case by denaturing gradient gel electrophoresis. Abnormally migrating bands were isolated and sequenced. The mutated area was then sequenced in all family members whose DNA was available. The newly found mutation was inserted into a GHRHR cDNA. Wild-type and mutant cDNAs were expressed into CHO cells and the cyclic AMP (cAMP) response to GHRH was measured. In order to determine whether the mutant receptor was properly expressed on the cell membrane surface, CHO cells were transfected with wild-type or mutant GHRHR cDNA containing a FLAG epitope tag in the extracellular N-terminus.
Results: Both patients were homozygous for a new missense mutation in codon 176, corresponding to the second transmembrane domain of the receptor protein that replaces alanine with valine (A176V). The mother and three unaffected siblings were heterozygous for the mutation; DNA from the father was not available. Cells expressing the A176V receptor had a significantly reduced cAMP response to GHRH, despite appropriate expression on the cell surface.
Conclusions: We describe two siblings with IGHD due to a new mutation in the GHRHR that disrupts GHRH signaling and leads to GHRH resistance.