Schizophrenia is caused by a combination of genetic and environmental factors, as first evidenced by twin studies. Extensive efforts have been made to identify the genetic roots of schizophrenia, including large genome-wide association studies, but these yielded very small effect sizes for individual markers. In this study, we aimed to assess the relative contribution of genome-wide association study-derived genetic versus environmental risk factors to crucial determinants of schizophrenia severity: disease onset, disease severity, and socioeconomic measures.
In this parallel analysis, we studied 750 male patients from the Göttingen Research Association for Schizophrenia (GRAS) dataset (Germany) with schizophrenia for whom both genome-wide coverage of single-nucleotide polymorphisms and deep clinical phenotyping data were available. Specifically, we investigated the potential effect of schizophrenia risk alleles as identified in the most recent large genome-wide association study versus the effects of environmental hazards (ie, perinatal brain insults, cannabis use, neurotrauma, psychotrauma, urbanicity, and migration), alone and upon accumulation, on age at disease onset, age at prodrome, symptom expression, and socioeconomic parameters.
In this study, we could show that frequent environmental factors become a major risk for early schizophrenia onset when accumulated (prodrome begins up to 9 years earlier; p=2·9×10-10). In particular, cannabis use-an avoidable environmental risk factor-is highly significantly associated with earlier age at prodrome (p=3·8×10-20). By contrast, polygenic genome-wide association study risk scores did not have any detectable effects on schizophrenia phenotypes.
These findings should be translated to preventive measures to reduce environmental risk factors, since age at onset of schizophrenia is a crucial determinant of an affected individual's fate and the total socioeconomic cost of the illness.