Altered expression of fragile X mental retardation-1 (FMR1) in the thymus in autoimmune myasthenia gravis.

Predisposition to autoimmunity and inflammatory disorders is observed in patients with fragile X-associated syndromes. These patients have increased numbers of CGG triplets in the 5' UTR region of FMR1 (Fragile X Mental Retardation 1) gene, that affects its expression. FMR1 is decreased in the thymus of myasthenia gravis (MG) patients, a prototypical autoimmune disease. We thus analyzed the number of CGG triplets in FMR1 in MG, and explored the regulatory mechanisms affecting thymic FMR1 expression. We measured the number of CGGs using thymic DNA from MG and controls, but no abnormalities in CGGs were found in MG that could explain thymic decrease of FMR1. We next analyzed by RT-PCR the expression of FMR1 and its transcription factors in thymic samples, and in thymic epithelial cell cultures in response to inflammatory stimuli. In control thymuses, FMR1 expression was higher in males than females, and correlated with CTCF (CCCTC-binding factor) expression. In MG thymuses, decreased expression of FMR1 was correlated with both CTCF and MAX (Myc-associated factor X) expression. Changes in FMR1 expression were supported by western blot analyses for FMRP. In addition, we demonstrated that FMR1, CTCF and MAX expression in thymic epithelial cells was also sensitive to inflammatory signals. Our results suggest that FMR1 could play a central role in the thymus and autoimmunity. First, in relation with the higher susceptibility of females to autoimmune diseases. Second, due to the modulation of its expression by inflammatory signals that are known to be altered in MG thymuses.