CONGENITAL HYPOTHYROIDISM DUE TO MUTATIONS IN THE SODIUM IODIDE SYMPORTER - IDENTIFICATION OF A NONSENSE MUTATION PRODUCING A DOWNSTREAM CRYPTIC 3'-SPLICE-SITE/

A 12-yr-old hypothyroid girl was diagnosed at birth as athyreotic beca use her thyroid gland could not be visualized by isotope scanning. Goi ter development due to incomplete thyrotropin suppression, a thyroidal radioiodide uptake of < 1%, and a low saliva to plasma ratio of 2.5 s uggested iodide (I-) transport defect, mRNA isolated from her thyroid gland and injected into Xenopus oocytes failed to increase I- transpor t. Sequencing of the entire Na+/I- symporter (NIS) cDNA revealed a C t o G transversion of nucleotide (nt) 1146 in exon 6, resulting in a Gln 267 (CAG) to Glu (GAG) substitution, This missense mutation produces an NIS with undetectable I- transport activity when expressed in COS-7 cells, Although only this missense mutation was identified in thyroid and lymphocyte cDNA, genotyping revealed that the proposita and her u naffected brother and father were heterozygous for this mutation. Howe ver, amplification of cDNA with a primer specific for the wild-type nt 1146 yielded a sequence lacking 67 nt. Genomic DNA showed a C to G tr ansversion of nt 1940 producing a stop codon as well as a new downstre am cryptic 3' splice acceptor site in exon 13, responsible far the 67 nt deletion, frameshift, and premature stop predicting an NIS lacking 129 carboxy-terminal amino acids, This mutation was inherited from the mother and present in the unaffected sister. Thus, although the propo sita is a compound heterozygote, because of the very low expression ( < 2.5%) of one mutant allele, she is functionally hemizygous for an NI S without detectable bioactivity.