A single amino acid change in the acetylcholinesterase-like domain of thyroglobulin causes congenital goiter with hypothyroidism in the cog/cog mouse: A model of human endoplasmic reticulum storage diseases

Abstract

Newly synthesized thyroglobulin (Tg), the major secretory glycoprotein of the thyroid gland, folds and homodimerizes in the endoplasmic reticulum (ER) before its export to the site of iodination, where it serves as the precursor for thyroid hormone synthesis. In families with defective Tg export, affected individuals suffer from a thyroidal ER storage disease characterized by a distended thyrocyte ER containing misfolded Tg, along with induced ER molecular chaperones. Inherited as an autosomal recessive trait, deficient Tg causes congenital hypothyroidism in newborns that, if untreated, results in goiter along with serious cognitive and growth defects. Recently, a similar phenotype has been observed in inbred cog͞cog mice, although the precise molecular defect has remained undefined. Here, we have isolated and cloned a full-length 8.5-kb Tg cDNA from cog͞cog mice and unaffected isogenic AKR͞J mice. Comparison of the complete sequences reveals that cog͞cog mice express a Leu-2263 3 Pro missense mutation in the acetylcholinesterase-homology domain of Tg. Heterologous expression studies in COS cells indicate that cog Tg exhibits a severe defect in exit from the ER. Site-directed mutagenesis of cog Tg to convert the single amino acid back to Leu-2263 restores normal Tg secretion. We conclude that the cog mutation in Tg is responsible for this ER storage disease that causes thyroid dyshormonogenesis.

Delineation of Five Thyroglobulin T Cell Epitopes with Pathogenic Potential in Experimental Autoimmune Thyroiditis

“…The complete mTg sequence (19) was scanned for the presence of two A k -binding motifs, a heptamer motif A and a pentamer motif B (Table I), by using the algorithm described by Altuvia et al (20). This computerized method was developed following a compilation of an extended database of helper T cell sites and takes into account physical-chemical and structural properties of peptides (dictated by the primary amino acid sequence) that may be responsible for binding to MHC class II Ags.…”

“…In this study, we proceeded to do a systematic search of the complete mTg sequence (19) for the detection of dominant and/or additional EAT-causing T cell epitopes by using an algorithm (20) that searches for A k -binding motifs within a protein sequence. This algorithm takes into account the physicochemical characteristics and structural properties of amino acids within motifs that are shared among immunogenic A k -binding peptides.…”

The Acetylcholinesterase Homology Region Is Essential for Normal Conformational Maturation and Secretion of Thyroglobulin

“…By contrast, in thyrocytes of cog/cog mice, there is activation of the unfolded protein response resulting in induced expression of multiple ER molecular chaperones, secondary to a temperature-sensitive defect in Tg folding and dimerization (20). The Tg from cog/cog mice fails to homodimerize and cannot be efficiently exported from the ER (22), instead undergoing a slow process of ER-associated degradation (23). Consequently, in cog/cog mice, there is greatly diminished iodination of Tg resulting in insufficient thyroid hormone synthesis (24 -26) and congenital goitrous hypothyroidism.…”

“…Consequently, in cog/cog mice, there is greatly diminished iodination of Tg resulting in insufficient thyroid hormone synthesis (24 -26) and congenital goitrous hypothyroidism. The cog Tg secretion defect is associated with a single L2263P substitution that falls within the AChE homology domain (22). Interestingly, authentic AChE also undergoes homodimerization (27)(28)(29)(30), which has been reported to enhance its efficiency of export in the secretory pathway (31).…”

A novel compound heterozygous mutation in the thyroglobulin gene resulting in congenital goitrous hypothyroidism with high serum triiodothyronine levels

“…Recently, the ACHE homology domain has been shown to be essential for the normal secretion of thyroglobulin (Park and Arvan 2004). A missense mutation in this domain in congenital goitrous hypothyroidism cog/cog mice leads to the retention of thyroglobulin within the endoplasmic reticulum (Kim et al 1998). There are also reports of missense mutations in this domain in humans and in rdw rats (Caron et al 2003;Hishinuma et al 2000).…”