TARGOVNIK Hector Manuel
Congenital goitre with hypothyroidism caused by a novel compound heterozygous mutations in the thyroglobulin gene.
Lugar: Malden; Año: 2010 vol. 72 p. 716 - 716
Congenital hypothyroidism because of thyroglobulin (TG) deficiency is an autosomal recessive disease with a prevalence of 1:40,000–1:100,000 live births and characterized by goitre, lower serum TG, elevated-serum TSH with simultaneous low levels of circulating thyroid hormones and a negative perchlorate discharge test. Some patients might have normal to high triiodothyronine levels. Untreated patients with TG abnormalities can have an abnormal growth and development as well as mental retardation. Early diagnosis and treatment with l-thyroxine, however, as a consequence of the neonatal screening programs has resulted in normal development in nearly all cases. Human TG is encoded by a large single copy gene, 270-kb long, located on chromosome 8q24 with 48 exons and encoding a 8,5-kb mRNA transcript. The preprotein monomer is composed of a 19 amino acid signal peptide followed by a 2749 residue polypeptide. Mutations in the human TG gene are associated with congenital goitre with hypothyroidism or euthyroidism and endemic and nonendemic simple goitre. To date, forty-two mutations have been identified and characterized in the human TG. We report here a new case of congenital goitrous hypothyroidism in a French family caused by mutations in the TG gene. The index patient II-1 was the first child born from healthy and unrelated parents. She was born at term (gestational age: 39 weeks) with a body weight of 3050 g, a body height of 49,5 cm and a cranial circumference of 34,5 cm. At the age of 8 days, neonatal screening was positive for TSH (243 ìIU/ml; reference values: 0,5–5), and thus the newborn was referred to a paediatric endocrinologist. Clinical examination was normal. Hormonal tests confirmed the diagnosis of hypothyroidism: TSH 533 ìIU/ml, Free T4: 2,32 pmol/l (reference values: 10,3–23,2 pmol/l), Free T3: 2,2 pmol/l (reference values: 2,6–5,4 pmol/l). Searches for antithyroid peroxydase and antiTG antibodies were negatives. Serum TG was undetectable (i.e. <0,07 ng/ml). Scintigraphy showed a normally located thyroid gland with a goitre which was confirmed by ultrasound imaging. The right lobe measured 8,8 · 6,1 · 24 mm and the left lobe 9,5 · 5,9 · 20,5 mm. The baby was treated with l-thyroxine drops (35 ìg per day). Her psychomotor development and growth were normal. Written informed consent was obtained from the parents and the research project was approved by the Institutional Review Board. Screening by single-strand conformational polymorphism of the TG gene5 showed two abnormal patterns of migration in the index patient that were not detected in the normal subjects (exons 22 and 27). Sequence analysis revealed that the index patient was heterozygous for a previously documented nonsense mutation due to a cytosine to thymine transition at nucleotide 4588 in exon 22 (c.4588C > T, father’s mutation) in one allele and for a novel also cytosine to thymine transition at nucleotide position 5386 in exon 27 (c.5386C > T, mother’s mutation) in the other allele. The c.4588C > T and c.5386C > T mutations resulted in premature stop codons at amino acids 1511 [p.R1511X] and 1777 [p.Q1777X], respectively. We have previously identified the p.R1511X mutation in associated with a p.R277X mutation in members of two unrelated families with history of congenital goitre. Interestingly, the p.R1511X mutation is removed from the transcripts by exon skipping using a alternative splicing normally present in concentrations below the detection limits and there is a preferential accumulation in the goiter of a TG mRNA lacking exon 22.1–3 Skipping of mutated exon 22 in the premRNA restores the reading frame allowing translation to reach the normal stop codon. In contrast, our recent results exclude an alternative splicing of exon 7 that could eliminate the p.R277X mutation and restore the normal reading frame.5 The excision of exon 22 in the TGmRNAresults in an in-frame deletion of 57 amino acid residues, which is localized in the TG Type 1-11 repeat motif. Whereas the p.Q1777X mutation is located in the TG Type 3-b1 repetitive region.1 An interesting but unsolved question because of the absence of mRNA analysis, as the thyroid tissue and leukocytes from patient II-1 were unavailable, is if a splicing mechanism by exon skipping or activation of cryptic splice sites might restore the normal reading frame disrupted by the mutation p.Q1777X. On the other hand, exon 27 is flanked by two large introns (intron 26: 7233 bp; intron 27: 12,064 bp)1 which impedes the analysis by minigenes technology. However, our RT-PCR and bioinformatic results and the comparative analysis with the information obtained in our previous studies with the p.R277X and p.R1511X mutations support the hypothesis that the p.Q1777X does not interfere with the definition and processing of the exon 27. Similarity to that observed with the exon 75 and in contrast to exon 22,1–3 exon 27 may not be subject to a non pre-existing alternative splicing event which argues against the role of the skipping of exon 27 as mechanism involved in the elimination of the p.Q1777X mutation. A second explanation is based on the observation that the p.Q1777X mutation does not eliminate any exonic splicing enhancer elements and is located near the end of the exon 27, whereas the p.R1511X eliminates a binding site recognized by the protein SC35 and is located internally in the exon 22. On the contrary, our results demonstrate that the p.Q1777X creates a new binding site detected by the protein SRp40 suggesting that the mutation increases high-scoring SR proteinbinding motifs present in the exon 27 sequences. The functional consequences of this new mutation could be the elimination of the acetylcholinesterase-like region and the carboxyl-terminal hormonogenic domain. In conclusion, we here describe a patient in whom a new compound heterozygous constellation, p.R1511X/p.Q1777X, was identified as the underlying molecular defect for a dyshormonogenetic goitre.