Deoxyribonucleic Acid Binding and Transcriptional Silencing by a Truncated c- erb Aβ1 Thyroid Hormone Receptor Identified in a Severely Retarded Patient with Resistance to Thyroid Hormone 1

Abstract

We describe the analysis of a thyroid hormone receptor (TR) beta causing resistance to thyroid hormone, the patient exhibiting hypothyroid symptoms (severe mental retardation, hypoactivity, obesity) and hyperthyroid symptoms (tachycardia, low serum cholesterol) and, additionally, relative early puberty, advanced bone age, and short stature. The patient was heterozygous, with a point mutation producing a premature stop-codon in TR beta-gene exon 10, resulting in a 28-amino acid carboxy-terminal deletion in the cognate TR beta (TR beta-EZ). T3 binding was abolished. Homodimer binding of TR beta-EZ to DR4- and F2-T3 response elements (TREs) was weaker, and to a palindromic TRE (PAL) was stronger than that of wild-type TR beta (TR beta-WT) in the absence of T3. T3 dissociated TR beta-WT, but not TR beta-EZ homodimer, from DR4, F2, and Pal. Heterodimerization of TR beta-EZ with retinoid x receptor beta was seen. TR beta-EZ repressed basal thymidine kinase-promotor activity, coupled to DR4, F2, or PAL. Silencing of basal gene transcription via PAL was weaker, and via DR4 and F2 was more pronounced, compared with TR beta-WT. TR beta-EZ had a strong dominant negative effect on TR beta-WT, attenuated in a TRE- and cell-specific manner by high T3 concentrations. Finally, the degree of TR beta-EZ homodimer-binding affinity to DNA did not correlate with the degree of transcriptional dominant negative activity.