MOLECULAR-GENETICS OF HEREDITARY THYROID-DISEASES DUE TO A DEFECT IN THE THYROGLOBULIN OR THYROPEROXIDASE SYNTHESIS

1. Hereditary goiter and the various degrees of thyroid hypofunction a re the result of structural changes in the thyroglobulin (Tg) or thyro peroxidase (TPO) proteins, the inability to couple iodotyrosines or de fective iodination, impairing or substantially altering the synthesis of T4 and T3. 2. The first mutations in the Tg and TPO genes responsib le for human cases of dyshormonogenesis have been described. The mutat ion in two siblings with hereditary goiter and marked impairment of Tg synthesis was a cytosine to thymine transition creating a stop codon at position 1510. The point mutation is removed by the preferential ac cumulation of a 171-nt deleted Tg mRNA. In another subject, molecular studies revealed that exon 4 was missing from the major Tg transcript due to a cytosine to guanine transversion at position minus 3 in the a cceptor splice site of intron 3. 3. Genomic DNA studies identified a d uplication of a 4-base sequence in the eighth exon of the TPO gene. In terestingly, besides abolishing the enzymatic activity by disrupting t he reading frame of the messenger RNA and introducing stop codons, the GGCC duplication also unmasks a cryptic acceptor splice site in exon 9. 4. In conclusion, the identification of different molecular defects provided evidence that hereditary goiter associated with abnormal Tg or TPO synthesis is caused by heterogeneous genetic alterations.