Autoimmunity and Klinefelter's syndrome: when men have two X chromosomes.

Similar to other autoimmune diseases, systemic lupus erythematosus (SLE) predominately affects women. Recent reports demonstrate excess Klinefelter's among men with SLE and a possible under-representation of Turner's syndrome among women with SLE as well as a case report of a 46,XX boy with SLE. These data suggest that risk of SLE is related to a gene dose effect for the X chromosome. Such an effect could be mediated by abnormal inactivation of genes on the X chromosome as has been demonstrated for CD40L, or by genetic polymorphism as has been demonstrated for Xq28. On the other hand, a gene dose effect could also be mediated by a gene without an SLE-associated polymorphism in that a gene that avoids X inactivation will have a higher level of expression in persons with two X chromosomes.