Genes and environments in schizophrenia: The different pieces of a manifold puzzle

Genetic research targeting schizophrenia has undergone tremendous development during recent years. Supported by recently developed high-throughput genotyping technologies, both rare and common genetic variants have been identified that show consistent association with schizophrenia. These results have been replicated by independent studies and refined in meta-analyses. The genetic variation uncovered consists of common alleles, i.e. single nucleotide polymorphisms (SNPs) conveying small effects (odds ratios below 1.1) on disease risk. The source of rare variants is copy number variations (CNVs), only detectable in a small proportion of patients (3-5% for all known CNVs) with schizophrenia, furthermore extremely rare de novo mutations captured by next generation sequencing, the most recent technological advancement in the field. Despite these findings, the search for the genetic architecture underlying schizophrenia continues since these variants explain only a small proportion of the overall phenotypic variance. Gene-environment interactions provide a compelling model for resolving this paradox and interpreting the risk factors of schizophrenia. Epidemiologically proven risk factors, such as prenatal infection, obstetric complications, urbanicity, cannabis, and trauma have been demonstrated to interact with genetic risk, giving rise to higher prevalence rates or more severe symptomatology in individuals with direct or indirect genetic predisposition for schizophrenia. Further research will have to explain how the different forms of genetic variation interact and how environmental factors modulate their effects. Moreover, the challenging question lying ahead of us is how genetic and environmental factors translate to molecular disease pathways. New approaches, including animal studies and in vitro disease modeling, as well as innovative real-world environment assessment methods, will help to understand the complex etiology of schizophrenia.