A six-year longitudinal population-based cohort for the extended psychosis phenotype: An epidemiological study of the gene-environment interactions (TürkSch)

Introduction Both genetic and environmental factors play a role in the extended psychosis phenotype which covers psychotic experiences, symptoms and disorders. Objectives The respective contributions of genetic and environmental factors over time remain largely unknown. Aims To describe the objectives and design of a multistage study. Methods The TurkSch (Izmir mental health survey for gene-environment interaction in psychoses) is a prospective-longitudinal study consisted of several data collection stages to screen extended psychosis phenotype in a general population sample, and to assess individual, familial, genetic and neighbourhood level variables. Results The study aimed to assess the prevalence of psychotic experiences and symptoms in Izmir-Turkey (stage I, cross-sectional; n : 4011), the socioeconomic deprivation and the social capital of neighbourhoods in a separate sample (stage II, cross-sectional; n : 5124) in 2008. A nested case-control study (stage III) recruited individuals with psychotic outcomes and healthy controls from stage I, and included blood sampling for gene-environment interaction and clinical reappraisal as well. After 6 years, follow-up study (stage IV) was set to assess the mental health outcomes with a focus on extended psychosis phenotype, environmental exposures of the eligiable sample ( n : 2192) from the stage I, and to collect blood samples for further genetic analysis. On both stages, Composite International Diagnostic Interview was used by clinically trained interviewers, and was able to provide broad assessment of psychotic experiences, experience-related disabilities, help-seeking and health care utilization. Conclusions The TurkSch has a unique study design and yields data of high quality in the Turkish population, with a specific focus on psychosis.