THRB splice site variants lead to exon 4 skipping and TRβ1 gain-of-function syndrome.

BACKGROUND

Heterozygous c.283+1G>A and c.283G>A variants in the THRB gene, encoding for thyroid hormone receptor (TR)β1 and β2, lead to autosomal dominant macular dystrophy (ADMD). We report the detailed clinical characterization of two first-degree relatives with ADMD, heterozygous for THRB c.283+1G>A, and an unrelated ADMD patient with a novel variant, c.283G>C. The genomic and molecular consequences of both variants were studied.

METHODS

gDNA and mRNA were obtained from leukocytes. Clinical characterization included biochemistry, bone density and body composition, ECG, echocardiography, ultrasound, audiometry and color-vision. In vitro assays investigated TR function and DNA binding.

RESULTS

The patients manifested no resistance to thyroid hormone beta (RTHβ) and had normal FT4 and TSH. Detailed studies in two patients showed no goiter, tachycardia, hypercholesterinemia or hepatic steatosis. Hearing was not impaired. Both had impaired color vision and reduced bone density. RT-PCR from all three patients revealed skipping of exon 4 exclusive to TRβ1, producing a deletion of 87 amino acids in the N-terminal domain (TRβ1ΔNTD). In vitro, DNA-binding affinity of TRβ1ΔNTD to DR4-TRE with or without RXRα was comparable to TRβ1WT. Surprisingly, TRβ1ΔNTD was transcriptionally twice more active than TRβ1WT with a similar EC50 for T3, demonstrating gain-of-function of TRβ1ΔNTD. THRA expression in leukocytes was increased by 3-fold compared to unrelated controls and different from RTHβ patients.

CONCLUSION

These THRB splice site variants produce TRβ1 exon 4 skipping, resulting in a gain-of-function mutant, TRβ1ΔNTD. This explains the dominant ADMD phenotype devoid of RTHβ and suggests a TRβ1 gain-of-function syndrome.