Characterization of a Novel Loss of Function Mutation of PAX8 in a Familial Case of Congenital Hypothyroidism with In-Place, Normal-Sized Thyroid

Abstract

Thyroid dysgenesis is the most common cause of congenital hypothyroidism, a relatively frequent disease affecting 1 in 3000-4000 newborns. Whereas most cases are sporadic, mutations in transcription factors implicated in thyroid development have been shown to cause a minority of cases transmitted as monogenic Mendelian diseases. PAX8 is one of these transcription factors, and so far, five mutations have been identified in its paired domain in patients with thyroid dysgenesis. We have identified a novel mutation of PAX8, in the heterozygous state, in a father and his two children both presenting with congenital hypothyroidism associated with an in-place thyroid of normal size at birth. In addition, one of the affected siblings displayed unilateral kidney agenesis. The mutation substitutes a highly conserved serine in position 54 of the DNA-binding domain of the protein (S54G mutation) by a glycine. Functional analyses of the mutant protein (PAX8-S54G) demonstrated that it is unable to bind a specific cis-element of the thyroperoxidase gene promoter in EMSAs and that it has almost completely lost the ability to act in synergy with Titf1 to transactivate transcription from the thyroglobulin promoter/enhancer. These results indicate that loss of function mutations of the PAX8 gene may cause congenital hypothyroidism in the absence of thyroid hypoplasia.

Two Cases of Thyroid Dysgenesis Caused by Different Novel PAX8 Mutations in the DNA Binding Region: In vitro studies reveal different pathogenic mechanisms.

“…This mutation is located in a highly conserved area of the gene, which encodes the DNA-binding domain of the PAX8 protein. Other mutations in this area of the gene have been described before and have been characterized at the molecular level (4,5,7,(9)(10)(11)(12)(13)(14)(15)17). In agreement with previously reported mutations (R31C, R31H, Q40P, R52P, S54G, H55Q, C57Y, L62R), S54R has lost its ability to bind to TPO promoter-binding sites.…”

“…Furthermore, PAX8 and TTF1 synergistically activate the promoter of human TG (3). So far, 13 different mutations in the PAX8 gene have been reported (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17), and the phenotypes of affected individuals vary considerably. Even within the same family, heterozygous individuals with PAX8 mutations can have no thyroid gland abnormality, have thyroid hypoplasia or TD (6), making it very difficult to identify individuals harboring a PAX8 gene mutation (1).…”

Thyroid-specific inactivation of KIF3A alters the TSH signaling pathway and leads to hypothyroidism

“…All affected people are heterozygous for the PAX8 mutation and transmission is autosomal dominant (Meeus et al 2004;Montanelli & Tonacchera 2010). Unexpectedly, no structural or functional thyroid defect have been previously reported in Pax8 Cre/+ mice, suggesting that, in this species, the presence of one functional Pax8 allele is sufficient for normal organogenesis and function (Amendola et al 2005).…”

“…In man, at least 10 different mutations have been identified in the PAX8 gene, leading to thyroid dysgenesis and congenital hypothyroidism (Meeus et al 2004;Montanelli & Tonacchera 2010). All affected people are heterozygous for the PAX8 mutation and transmission is autosomal dominant (Meeus et al 2004;Montanelli & Tonacchera 2010).…”

Novos aspectos da genética e dos mecanismos moleculares da morfogênese da tiróide para o entendimento da disgenesia tiroidiana

“…Dez mutações do gene PAX8 (oito missense, uma nonsense e uma deleção) já foram descritas em casos esporádicos e familiares com transmissão autossômica dominante de HC com DT, todas em heterozigose (Tabela 2) (40)(41)(42)(43)(44)(45)(46)(47)(48). O fenótipo tiroidiano é bastante variável (de casos leves àqueles com hipoplasia grave) entre indivíduos afetados da mesma família, o que indica penetrân-cia incompleta (41,45).…”