There is strong evidence for a negative association between schizophrenia and rheumatoid arthritis (RA). However, the mechanism for this association is unknown. We hypothesize that these two diseases share susceptibility genes. Recently, extensive studies have identified some RA susceptibility genes, including NFKBIL1, SLC22A4, RUNX1, FCRL3 and PADI4, in the Japanese population. To assess whether polymorphisms in these RA susceptibility genes are implicated in vulnerability to schizophrenia, we conducted a two-stage case-control association study in Japanese subjects. In a screening population of 534 patients and 559 control subjects, we examined eight polymorphisms in RA susceptibility genes and found a potential association of padi4-94 in PADI4 with schizophrenia. However, we could not replicate this association in a confirmatory population of 2126 patients and 2228 control subjects. The results of this study suggest that these polymorphisms in RA susceptibility genes do not contribute to genetic susceptibility to schizophrenia.
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