III) We performed a GWAS of CUD cases against schizophrenia cases in the iPSYCH cohort (2,387 CUD cases and 1,789 individuals with schizophrenia (ICD10 F20) without a CUD diagnosis). Smoking rates among schizophrenia cases is in the range of 60-90%31, so comparable with the rates among CUD cases. By comparing the two groups with comparable smoking rates we would expect the association signal to disappear if smoking is the driving factor. The index variant rs56372821 was associated with CUD with P=1.68x10−5, OR=0.76. The observed strong association with only a slight increase in OR from 0.73 in the original CUD GWAS to 0.76 supports that the signal is primarily driven by CUD and not confounded by tobacco smoking.
