Objectives: All known (>230) different mutations in thyroid hormone receptor β (TRβ) causing Resistance to Thyroid Hormone β (RTHβ), localise to three clusters within its hormone binding domain. Here, we report phenotypes and molecular studies in an unique family with RTHβ due to a mutation in the DNA binding domain (DBD) of TRβ.
Methods: We ascertained clinical and biochemical features in four children and their parents from this family. Following identification of the THRB variant by next generation sequencing, functional properties of the TRβ mutant were assessed in vitro. We studied expression of T3-regulated target genes (KLF9, HR) and lipid metabolism in mutation-containing hepatocytes derived from inducible pluripotent stem cells (iPSCs) of patients, generated ex vivo.
Results: A male child (P1, age 4.7yrs) and his sister (P2, age 17.5yrs) with intellectual disability, elongated facies, winged scapulae, epiphyseal dysgenesis (P1), sensorineural hearing loss, goiter, tachycardia and abnormal thyroid function tests (P1: FT4 45pmol/l (RR 10.5-21), FT3 20pmol/l (RR 4-8.1), TSH 4.95mU/l (RR 0.35-5.5; P2: FT4 31pmol/l, FT3 11pmol/l, TSH1.95mU/l) were homozygous for an aminoacid substitution (Arg158Leu, R158L) at a highly conserved residue in the TRβ DBD. In contrast, their parents and two other siblings, all heterozygous for R158L TRβ, had normal thyroid function tests and no clinical abnormalities. Although their cellular expression and nuclear translocation is preserved, R158L mutant TRβ1 and TRβ2 exhibit negligible T3-dependent activation or repression of reporter genes (DR+4 TKLUC, TSHαLUC). Loss of binding of R158L TRβ1 and TRβ2 to DNA correlates with reduced thermal stability of the mutant DBD in vitro. Correlating with clinical phenotypes, hepatocytes derived from homozygous, R158L mutant TRβ iPSCs, exhibit loss of target gene (KLF9, HR) expression and excess lipid accumulation, whereas target gene expression in heterozygous R158L mutant TRβ hepatocytes is preserved.
Conclusions: Receptor haploinsufficiency, due to a heterozygous, loss-of-function mutation in the TRβ DNA binding domain, does not cause RTHβ. Homozygosity for this receptor defect is associated with RTHβ and additional phenotypes that mirror features observed in the first recorded cases of the disorder with homozygous deletion of THRB (Refetoff et al 1967,