Génétique des schizophrénies : mise en perspective des schizophrénies à début précoce et autres pathologies du développement

Abstract

Schizophrenia (SCZ) is a severe brain disorder characterized by hallucinations, delusions, flat and/or inappropriate affect and cognitive impairment. The lifetime risk is about 0.5% with heritability of 65–85%. The prevalence of early-onset schizophrenia (defined here as before 15 years of age) has not been well studied, but is likely to be 5–10% of all cases. The rarity of early-onset SCZ has made it difficult to study. We focus on genetic studies of adults with schizophrenia, highlighting results for early-onset schizophrenia where available. Prior to the past 5years, studies failed to find replicable association or linkage between SCZ and specific genes when appropriate statistical corrections for multiple testing were used. Many false positive results were probably reported using the candidate gene approach. Recently, the development of single nucleotide polymorphism (SNP) “chips” has permitted large genome-wide association study (GWAS) analyses that suggest that across all age groups, a proportion of genetic risk can be attributed to a large number of common SNP, each with a very small effect on risk (odds ratios of 1.1 or less). The greatest known genetic effect is conferred by the 1.5–3 Mb 22q.11.2 deletions, which occurs in ∼ 1/4000–1/6000 births with SCZ developing in 20–30% of carriers. Large SNP and aCGH microarray studies have now identified associations between SCZ and other rare, large copy number variations (CNV, insertions and deletions) with high odds ratios (5–10), including deletions of 1q21, 2p16.3 (neurexin-1 gene), 3q29 and 15q13.3, and duplications of 16p11.2. Some of these CNV are also associated with autism or other developmental disorders as well as epilepsy or intellectual deficiency, suggesting some overlap in the mechanisms that contribute to risks of these disorders. Based on preliminary data from larger-scale analyses in progress, approximately 1–2% of cases carry a CNV that has been clearly associated with SCZ (ORs 4–12). Whole exome and genome sequencing studies of large adult samples will be the next steps to identify rarer SCZ-associated mutations, including point mutations and smaller as well as rarer CNV. Genetic findings are beginning to contribute to an understanding of biological mechanisms of SCZ risk and may lead to new approaches to treatment.