A NOVEL MECHANISM FOR ISOLATED CENTRAL HYPOTHYROIDISM - INACTIVATING MUTATIONS IN THE THYROTROPIN-RELEASING-HORMONE RECEPTOR GENE

Isolated central hypothyroidism, characterized by insufficient TSH sec retion resulting in low levels of thyroid hormones, is a rare disorder . We report a boy in whom isolated central hypothyroidism was diagnose d at 9 yr of age. Complete absence of TSH and PRL responses to TRH led us to speculate that he had an inactivating mutation of the TRH recep tor gene. The patients' genomic DNA was isolated, an? the entire codin g region of the TRH receptor was amplified by the PCR and sequenced di rectly. Confirmation of the mutations and haplotyping of the family wa s performed using restriction enzymes. The biological activity of the wild-type and mutated TRH receptors was verified by evaluating the bin ding of labeled TRH and stimulation by TRH of total inositol phosphate accumulation in transfected HEK-293 and COS-1 cells. The patient was found to be a compound heterozygote, having inherited a different muta ted allele from each of the parents; both mutations were in the 5'-par t of the gene. Mutated receptors were unable to bind TRH and to activa te total inositol phosphate accumulation. Our report is the first desc ription of naturally occurring inactivating mutations of a G protein-c oupled receptor linked to the phospholipase C second messenger pathway . The prevalence and phenotypic spectrum of TRH receptor mutations in isolated central hypothyroidism remain to be established.