Functional characterization of a novel mutation in TITF-1 in a patient with benign hereditary chorea

Abstract

Benign hereditary chorea (BHC) is an autosomal dominant disorder of early onset characterised by non progressive choreic movements with normal cognitive function occasionally associated with hypothyroidism and respiratory problems. Numerous pieces of evidence link BHC with TITF-1/NKX2.1 gene mutations. We studied a patient with a familial benign hereditary chorea and normal thyroid and respiratory function. Sequence analysis of TITF-1 revealed the presence of a heterozygous C > T substitution at nucleotide 532, predicted to change an arginine (CGA) with a stop codon (TGA) at position 178 (R178X). A functional analysis shows that the mutated TTF-1 is not binding DNA, nor activating the canonical thyroid target gene promoter or interfering with the ability of wild type TTF-1 to activate transcription. In addition, the mutated protein is predominantly cytoplasmic, rather than nuclear as in the case of the wild type TTF-1. Thus, we have identified a new mutation in the TTF-1 coding gene in a patient with benign hereditary chorea. The results show that the mutation leads to a haploinsufficiency of TITF-1 and opens the question of genotype/phenotype correlation.

Introduction

Benign hereditary chorea (BHC) (MIM 118700) is an autosomal dominant disorder with a highly variable phenotype [1], [2]. Early onset BHC, is characterised by choreic movements with little or no progression, normal cognitive function, although some patients reportedly demonstrated memory and learning difficulties [2], [3]. This form is due to mutations of the gene coding for the Thyroid Transcription Factor 1 (TTF-1), mapping on chromosome 14q13 [3], [4], [5]. Recently, a new locus for an adult onset form of BHC has been reported, mapping on chromosome 8q21.3–23 [6]. TTF-1 (also termed NKX2.1 and T/EBP) is a homeodomain-containing transcription factor initially identified in thyrocytes as a nuclear protein able to bind and activate the thyroglobulin (Tg) gene promoter [7], [8]. In mouse the gene is expressed in thyroid, lung bronchial epithelium and in specific areas of the forebrain during development [8]. Titf-1 knock-out mice are born dead and completely lack the thyroid gland, lung parenchyma and pituitary gland [9]. In the developing mouse brain, TTF-1 plays a role in the specification of the ventral cell fate in the forebrain [9]. In the telencephalon of the Titf-1^−/− mice a ventral to dorsal transformation of the pallidal primordium into a striatal anlage takes place. Furthermore, in these mice, TTF-1 mediates the tangential migration of striatal interneurons from the medial ganglionic eminence (precursor of globus pallidus), to the lateral ganglionic eminence (precursor of striatum) and then to the cortex. [10], [11]. Interestingly, some of the striatal interneurons, namely the cholinergic, calrettinin⁺ and parvalbumin⁺ ones, maintain the expression of Titf-1 into adulthood [12]. In the only pathological report of a BHC patient, Kleiner-Fisman et al. have shown a loss of striatal interneurons, supporting the hypothesis that TTF-1 mediates the tangential migration of striatal interneurons in humans as well [13].

Large TITF-1 deletions had been originally reported in infants with respiratory distress, primary hypothyroidism, delayed motor milestones and ataxia [14], [15]. Since then several other patients with congenital hypothyroidism, choreoathetosis, respiratory abnormalities and heterozygous TITF-1 point mutations or gene deletions have been described, and, accordingly, the term ‘Brain–Thyroid–Lung syndrome’ has been proposed for this clinical picture [16]. However, several TITF-1 point mutations or gene deletions have also been identified in patients with BHC in the absence of thyroid and lung abnormalities [3], [4], [17], [18]. In Table 1 we recapitulate the TITF-1 mutations and the associated phenotype that are reported in the literature.

Only two studies [18], [19] so far have reported functional analyses of TITF-1 point mutations, both found in patients with respiratory and/or thyroid abnormalities. We report here on the functional characterization of a novel heterozygous TITF-1 mutation, R178X found in a patient with BHC in the absence of clinical, biochemical or radiological evidence of any thyroid and lung involvement.