STRUCTURAL AND FUNCTIONAL DIFFERENCES IN THE DIO1 GENE IN MICE WITH INHERITED TYPE-1 DEIODINASE DEFICIENCY

The type 1 deiodinase (D1) provides the major portion of the circulati ng T3 in vertebrates, In C3H and certain other inbred mice, liver and kidney D1 activity is 5- to 10-fold lower than in the common phenotype , C57, The lower D1 levels are paralleled by a decreased normal-sized diol mRNA and hyperthyroxinemia, Low activity cosegregates with a rest riction fragment length variant (RFLV) in both inbred and recombinant strains, indicating it is due to differences in the diol gene, The exo nic structure and the deduced amino acid sequences are identical for b oth strains and highly homologous to that of the rat, The RFLV is due to an approximately 150-base pair expansion of repetitive sequences in the second intron of the C3H gene, but this segment does not differen tially affect the transient expression of a human GH gene, The promote r and 5'-flanking regions of the C3H and C57 diol genes are very simil ar and are GC rich without TATA or CCAAT boxes. However, functional as says of 1.5-kilobase 5'-flanking diol-CAT constructs showed 2- to 3-fo ld higher activity of the C57-CAT constructs, Deletion mutants showed that sequences between -705 and -162 were the cause of this. In this r egion, the only major difference between the two genes is a 21-base pa ir insert containing five CTG repeats in the C3H promoter, This differ ence also cosegregates with low D1 activity and the intron RFLV in fou r other mouse strains. The correlation of the CTG repeat insert with b oth in vitro and in vivo expression and the absence of other significa nt sequence differences in the 5'-flanking region argue that this is t he major explanation for the impaired expression of the diol gene and the resulting hyperthyroxinemia of the C3H mouse.