Congenital goiter with hypothyroidism caused by a 5 ' splice site mutationin the thyroglobulin gene

Ln this work we have extended our initial molecular studies of a consanguin eous family with two affected goitrous siblings (H.S.N. and Ac.S.N.) with d efective thyroglobulin (Tg) synthesis and secretion because of a homozygoti c deletion of a fragment of 138 nucleotides (nt) in the central region of t he Tg mRNA, identified previously in H.S.N. In order to identify the intron /exon boundaries and to analyze the regions responsible for pre-mRNA proces sing corresponding to a 138 nt deletion, we performed a screening of a huma n genomic library. The intron/exon junction sequences were determined from one positive clone by sequencing both strands of the DNA template. The resu lts showed that the deletion mapped between positions 5549 and 5686 of the Tg mRNA and corresponded to exon 30. The positions of the exon limits diffe red by three nucleotides from the previously reported data obtained from di rect sequencing of the deleted reverse transcriptase-polymerase chain react ion fragment from H.S.N. These variations are because the intron/exon junct ions in this region were not available at the time when the deletion was fi rst described. The deletion does not affect the reading frame of the result ing mRNA and is potentially fully translatable into a Tg polypeptide chain that is shortened by 46 residues. The same 138 nt deletion was observed in reverse transcriptase-polymerase chain reaction studies performed in the th yroid tissues from Ac.S.N. Genomic DNA analysis showed that a G to T transv ersion was observed at position +1 in the donor site of intron 30. Both aff ected patients (H.S.N. and Ac.S.N.) are homozygous for the mutation whereas the normal sister (At.S.N.) had a normal allele pattern. The functional co nsequences of the deletion are related to structural changes in the protein molecule that either could modify the normal routing of the translation pr oduct through. the membrane system of the cell or could impair the coupling reaction. Probably the mutant Tg polypeptide might be functionary active i n the production of thyroid hormone, because in the presence of a normal io dine ingestion (similar to 150 mug/day), Ac.S.N. was able to maintain norma l serum levels of total triiodothyronine (T-3) associated with relatively l ow serum total thyroxine (T-4) with normal somatic development without sign s of brain damage.