Diabetes and other autoimmune endocrine diseases
Failure of tolerance to the thyrotropin receptor (TSHR) is the crucial event in Graves' disease (GD) pathogenesis. TSHR gene is associated to GD through non-coding SNPs rs179247 and rs12101255 SNPs. Two non-mutually excluding mechanisms have been proposed to explain the association: 1) Differential TSHR soluble isoforms production and 2) allele specific reduction of TSHR expression in thymus leading to peripheral and central tolerance failure respectively. To discern among them, we measured:
1) The effect of the above SNPs in the expression of the full-length transcripts (flTSHR) and isoform transcripts (ST4 and ST5) by qPCR in 39 thymus and 49 thyroid samples. There was no significant effect of GD‑associated SNPs on isoform expression neither in thymic nor in thyroid. Interestingly, the expression level of flTSHR in thymus was much above the predicted, being approximately 20% of thyroid expression; ST4 expression was similar.
2) Allele-specific TSHR expression by NGS of gDNA and cDNA obtained from 19 thymi and 8 thyroids heterozygous for rs179247 that confirmed thymic unbalanced transcription and that the TSHR protective G allele is preferentially transcribed as reported (Colobran, 2011).
These results argue against the effect of rs179247 and rs12101255 allele on TSHR soluble isoform production by the thyroid but confirm their effect on TSHR thymic expression. The unexpected finding of abundant TSHR short isoforms in the thymus, and more precisely, in DP thymocytes rather than in thymic epithelial cells, suggests new mechanisms by which tolerance to TSHR may be lost.