Loss-of-function mutations in the thyrotropin receptor gene as a major determinant of hyperthyrotropinemia in a consanguineous community

Abstract

Context: Resistance to TSH (RTSH) is a condition of impaired responsiveness of the thyroid gland to TSH, characterized by elevated serum TSH, low or normal thyroid hormone levels, and hypoplastic or normal-sized thyroid gland.

Objectives: The aim of the study was to evaluate the clinical course and the genotype-phenotype relationship of RTSH caused by two different TSH receptor (TSHR) gene mutations in a consanguineous population.

Patients and methods: We conducted a clinical and genetic investigation of 46 members of an extended family and 163 individuals living in the same town. In vitro functional studies of the mutant TSHRs were also performed.

Results: Two TSHR gene mutations (P68S and L653V) were identified in 33 subjects occurring as homozygous L653V (five subjects), heterozygous L653V (20 subjects), heterozygous P68S (four subjects), and compound heterozygous L653V/P68S (four subjects). With the exception of one individual with concomitant autoimmune thyroid disease, all homozygotes and compound heterozygotes presented with compensated RTSH (high TSH with free T(4) and T(3) in the normal range). Only nine of 24 heterozygotes had mild hyperthyrotropinemia. The L653V mutation resulted in a higher serum TSH concentration and showed a more severe in vitro abnormality than P68S. Haplotype analysis predicted a founder of the L653V six to seven generations earlier, whereas the P68S is older. Cross-sectional and prospective longitudinal studies indicate that TSH and T(4) concentrations remain stable over time.

Conclusions: High frequency hyperthyrotropinemia in an Israeli Arab-Muslim consanguineous community is attributed to two inactivating TSHR gene mutations. Concordant genotype-phenotype was demonstrated clinically and by in vitro functional analysis. Retrospective and prospective studies indicate that in the absence of concomitant autoimmune thyroid disease, elevated TSH levels reflect stable compensated RTSH.

Figure 1

Pedigree of the extended family with RTSH. The shadings of symbols indicate ranges of serum TSH concentrations as explained in the legend on the upper right corner. Subjects were tested for the indicated polymorphic markers. The haplotypes are shown below each symbol, and different colors help to trace the inheritance of the different alleles harboring the mutations, as indicated in the legend on the bottom left. Predicted haplotypes are in squared brackets.

Figure 2

Serum TSH (upper graph), FT₄I (middle graph), and FT₃I (lower graph) concentrations in individual subjects grouped according to the genotype indicated on the bottom of the figure. Closed symbols represent members of the extended family. Open symbols represent mutation carriers identified by screening the population and the corresponding normal WT controls matched for age and gender. Mean values ± sd for each genotype group are shown on the left of data points. Asterisks above the sd bar indicate P values for difference with the group expressing only the WT alleles.

Figure 3

Serum TSH (upper graph) and FT₄I (lower graph) concentrations in individual subjects graphed according to age. Shaded areas are the respective normal ranges. Open symbols are homozygotes for the WT allele, and closed symbols are heterozygotes for L653V. Note that there is no trend of change with age for both parameters and in both genotypes.

Figure 4

Prospective longitudinal measurement of TSH and FT₄ in the five subjects homozygous for the L653V mutation. The evolution of TSH over time is displayed in the upper graph. Individuals are identified by different symbols, and age at the time of the first measurement is given in parentheses in the key. Note that regression analysis showed no significant upward or downward trend of TSH with time (slope) for any of the individuals. In the bottom graph, TSH values are correlated to those of FT₄. Note that only subject IV-6, with AITD, showed a significant (P < 0.02) reciprocal correlation between TSH and FT₄. This is indicated by the regression line (dashed) plotted by the least mean square method for the five pairs of TSH and FT₄ measurements in subject IV-6 over the 2.5 yr of observation.

Figure 5

In vitro functional analysis of the mutant compared with the WT TSHR. A, TSH dose response curves of a cotransfected CRE-driven reporter. B, Basal activity in transfections simulating the different genotypes observed in humans. C, Cell surface expression of the mutant receptors detected by FACS using three different antibodies. Results are expressed relative to those obtained with WT-TSHR.