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Two novel mutations of the TSH-beta subunit gene underlying congenital central hypothyroidism undetectable in neonatal TSH screening.
CONTEXT: Patients with TSH-beta subunit defects and congenital hypothyroidism are missed by TSH-based neonatal screening.
OBJECTIVE: Our objective was to report the molecular consequences of a novel splice-junction mutation and a novel missense mutation in the TSH-beta subunit gene found in two patients with congenital central hypothyroidism and conventional treatment-resistant anemia.
RESULTS: Patient 1 had a homozygous G to A nucleotide change at the 5' donor splice site of exon/intron 2. This resulted in a silent change at codon 34 of the mature protein. In vitro splicing assays showed that the mutant minigene dramatically affected pre-mRNA processing, causing exon 2 to be completely skipped. The putative product from a new out-of-frame translational start point in exon 3 is expected to yield a nonsense 25-amino-acid peptide. In patient 2, sequence analysis 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 C88Y change. This cysteine residue is conserved among all dimeric pituitary and placental glycoprotein hormone-beta subunits. Data from in silico analysis confirmed that the C88Y mutation would affect subunit conformation. Indeed, two different bioinformatics approaches, PolyPhen and SIFT analysis, predicted C88Y to be a damaging substitution.
CONCLUSIONS: In isolated TSH deficiency, the exact molecular diagnosis is mandatory for diagnosis of isolated pituitary deficiency, delineation of prognosis, and genetic counseling. Moreover, diagnosis of central hypothyroidism should be considered in the face of severe infant anemia of uncertain etiology.
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