Detection of thyroglobulin mRNA as truncated isoform(s) in mouse thymus

Recent studies employing reverse transription-polymerase chain reaction (RT-PCR) have demonstrated the intrathymic presence of mRNA for various autoantigens, including thyroglobulin (Tg). Deliberations on the mechanisms of central tolerance usually assume that this approach detects intact mRNA transcripts that can be translated to express the whole autoantigen in the thymus. In the present study, we tested this assumption using mRNA transcripts of mouse Tg which encode at least 13 pathogenic peptides, scattered over a large (8.5 kb) sequence. We found that mRNA encoding 11 out of these 13 Tg peptides was present in both the thyroid and the thymus of CBA/J mice, with no apparent temporal fluctuations in expression from birth to 12 weeks of age. Interestingly, detection of these sequences was also demonstrable in the liver and kidney, but not in muscle. However, mRNA encoding two pathogenic peptides (amino acids 1-12 and amino acids 1579-1591) was detected intrathyroidally but not in the other tissues. Further analysis by RT-PCR showed that Tg mRNA transcripts in the thymus, liver and kidney lack segments within the 1-915 bp and 961-5013 bp regions, spANNing exons 1-7 and 9-22, respectively. These data strongly suggest that certain known and perhaps other, as yet unmapped, pathogenic T-cell epitopes of Tg cANNot be encoded by the truncated isoform(s) of intrathymic Tg mRNA. These findings also imply that central tolerance to endogenous Tg produced by thymic epithelial cells may be incomplete.