Complement component 4 genes contribute sex-specific vulnerability in diverse illnesses

Many common illnesses differentially affect men and women for unknown reasons. The autoimmune diseases lupus and Sjogren9s syndrome affect nine times more women than men1,2, whereas schizophrenia affects men more frequently and severely3-5. All three illnesses have their strongest common-genetic associations in the Major Histocompatibility Complex (MHC) locus, an association that in lupus and Sjogren9s syndrome has long been thought to arise from HLA alleles6-13. Here we show that the complement component 4 (C4) genes in the MHC locus, recently found to increase risk for schizophrenia14, generate 7-fold variation in risk for lupus (95% CI: 5.88-8.61; p