Enhanced Levels of Wild-Type versus Mutant Thyroid Hormone Receptor βl Messenger RNA in Fibroblasts from Heterozygotes of Kindred S with Thyroid Hormone Resistance

Thyroid hormone resistance syndromes, which result from heterozygous mutations in the βl thyroid hormone receptor gene, are sometimes associated with adult short stature, but more frequently with delayed bone age (BA). Primary fibroblasts from young children with both delayed BA and short stature from a kindred A have been reported to overexpress the mutant allele. However, in fibroblasts from affected members of two different kindreds with thyroid hormone resistance, S and Mf, there were equal levels of mutant and wild-type β1 mRNA. We investigated the ontogeny of differential allelic expression using competitive reverse transcription with PCR (RT-PCR) to measure relative mRNA levels for β1 and S receptor in very young affected children of kindred S. Total RNA was prepared from fibroblasts of two patients (ages 3-0.5/12 and 1-4/12 years) with delayed BA but normal growth curves. Using PCR amplimers that create an Mlu-1 site in wild-type but not mutant cDNA products from the competitive RT, we quantitated mRNA levels. Normal β1 mRNA was present at nearly twice the level of the mutant mRNA in cells from these patients. Relative expression of the c-erbAβ alleles thus appeared to be increased during this period of somatic growth. The relative overexpression of the normal allele potentially counteracted the potent dominant negative effect of the S receptor during early childhood ameliorating a deleterious effect on linear growth.