Two Novel Mutations in the TSH-Beta Subunit Gene Underlying Congenital Central Hypothyroidism

BAQUEDANO, MS; CIACCIO, M; DUJOVNE, N; HERZOVICH, V; LONGUEIRA, Y; BACHELOR, Y; WARMAN, M; RIVAROLA, M; BELGOROSKY, A

San Diego

Congreso; The Endocrine Society´s 92nd Annual Meeting (ENDO 2010); 2010

  Isolated TSH deficiency caused by TSH-beta subunit defects is a rare cause of non-goitrous congenital hypothyroidism. Patients are born with low thyroid hormones and TSH serum levels. We are reporting the molecular consequences of a novel splice-junction mutation and a novel missense mutation in the TSH-â subunit gene, found in two non-related patients with congenital central hypothyroidism and severe conventional-treatment-resistant anemia, not detected by routine TSH-based neonatal screening. Patient 1 was found to be homozygous for a G to A nucleotide change at the 5´ donor splice site of exon/intron 2 at cDNA position 162 bp, the last base of exon 2 (c162G>A; the A of the ATG of the initiator Met codon is denoted nucleotide +1). This resulted in a silent change R34R in the mature protein. In vitro splicing assays showed that the mutant minigene dramatically affected pre-mRNA processing, causing exon 2 to be completely skipped and predicting a new out-of-frame translational start point in exon 3, and the production of a nonsense 25 amino acid peptide. Sequence analysis of TSHbeta-subunit gene of patient 2 revealed a compound heterozygosis for the already reported 313delT (C105Vfs114X) mutation and for a second novel mutation in exon 3, substituting G for A at cDNA nucleotide position 323, resulting in a change of cysteine to tyrosine at codon 88 (C88Y). This codon change results in the loss of one of the 12 cysteine residues conserved among all dimeric pituitary and placental glycoprotein hormone beta subunits. Data from in silico analysis confirmed that the C88Y mutation would be predicted to affect the subunit conformation, and either enhance TSH-beta subunit intracellular degradation or diminish its ability to interact with the alfa-subunit. Indeed, two different bioinformatics approaches, PolyPhen and SIFT analysis predicted C88Y to be a damaging substitution. In conclusion, we are reporting two novel homozygous and compound heterozygous mutations of the TSH-beta subunit gene as a cause of isolated congenital central hypothyroidism. Isolated TSH deficiency is not detected by routine TSH-based neonatal screening. The exact molecular diagnosis is mandatory for the delineation of prognosis, for genetic counseling, and for the avoidance of provocation tests of pituitary function in the newborn period or early in life. Finally the diagnosis of central hypothyroidism should be considered in any case with severe anemia of uncertain etiology