A novel splice variant involving the 5’ untranslated region of thyroid hormone receptor beta1 (TRbeta1)

The thyroid hormone receptor β (TRβ) gene generates two different proteins by use of a different promoter (β1 and β2). We now report a novel short TRβ1 RNA splice variant in humans lacking 35 nucleotides at the 3’ end of the noncoding exon 1 due to an alternative 5’ splice donor site. This short variant was first identified in sequences of cDNA obtained from cultured human fibroblasts. Both variants were found in human fibroblasts, brain, pituitary, adrenal gland, placenta, muscle, thyroid and lymphocytes. These TRβ1 variants possess splice donor sites with a sequence score slightly favoring the TRβ1 long variant. Variant-specific real-time polymerase chain reaction (PCR) showed that their relative proportions were equal except in pituitary and muscle, in which the long form was 3- and 5-fold in excess. T3 treatment of fibroblasts grown in thyroid hormone depleted medium did not affect the absolute or relative expression of the two variants. Furthermore, the expression level in fibroblasts from patients with resistance to thyroid hormone with or without TRβ gene mutations was not different to that in fibroblasts from normal controls.