Four new cases of congenital secondary hypothyroidism due to a splice site mutation in the thyrotropin-beta gene: phenotypic variability and founder effect.

Isolated TSH deficiency is a rare cause of congenital hypothyroidism. We here report four children from two consanguineous Turkish families with isolated TSH deficiency. Affected children who were screened at newborn age had an unremarkable TSH result and a low serum TSH level at diagnosis. Age at diagnosis and clinical phenotype were variable. All affected children carried an identical homozygous splice site mutation (IVS2 + 5 G--> A) in the TSHbeta gene. This mutation leads to skipping of exon 2 and a loss of the translational start codon without ability to produce a TSH-like protein. However, using specific monoclonal antibodies, we detected a very low concentration of authentic, heterodimeric TSH in serum, indicating the production of a small amount of correctly spliced TSH mRNA. By genotyping all family members with polymorphic markers at the TSHbeta locus, we show that the mutation arose on a common ancestral haplotype in three unrelated Turkish families indicating a founder mutation in the Turkish population. These results suggest that this TSHbeta mutation is among the more common TSHbeta gene mutations and stress the need for a biochemical and molecular genetic workup in children with symptoms suggestive of congenital hypothyroidism, even when the neonatal TSH screening is normal.

Congenital secondary hypothyroidism caused by exon skipping due to a homozygous donor splice site mutation in the TSHbeta-subunit gene.

Isolated TSH deficiency as a cause for congenital hypothyroidism is relatively uncommon. Even more rare is the identification of mutations in the TSHbeta gene, only four of which have been identified. We here report a 4-month-old girl with isolated TSH deficiency born to consanguineous parents. Sequencing of the TSHbeta-subunit gene revealed a homozygous G to A transition at position +5 of the donor splice site of intron 2. TSHbeta gene transcript could not be obtained from fibroblasts or white blood cells by illegitimate amplification. Thus, to investigate further the mechanism leading to TSH deficiency in this patient, we used an in vitro exon-trapping system. The mutation at position +5 of the donor splicing site produced a skip of exon 2. The putative product of translation from a downstream start site is expected to yield a severely truncated peptide of 25 amino acids. Surprisingly, a missense substitution affecting the 14th amino acid of the signal peptide (SigP A14T) was found in one allele of the mother and brother. SigP 14T is polymorphic with a frequency of 1.8% and has no functional consequence.