Molecular Genetics of Schizophrenia and Related Intermediate Phenotypes in a Founder Population

Schizophrenia is a severe mental disorder characterized by positive and negative symptoms, such as psychosis and anhedonia, as well as cognitive deficits. Schizophrenia affects about 0.5 to 1% of population worldwide, with equal prevalence between the genders and different residential environments. That schizophrenia constitutes a significant burden for both the affected individuals and for the whole societies has encouraged its active research over the past decades. At present, schizophrenia is considered a complex disorder of neurodevelopmental origin with both genetic and environmental factors contributing to its onset. Despite the established strong genetic component, any major genetic determinant has remained unidentified. The current hypothesis for schizophrenia liability comprises multiple genetic variants with small individual effects in conjunction with environmental factors. These combinations of genetic variants may differ substantially between the affected individuals, especially between the families not sharing the same ancestry. On the other hand, the same variants may cause different disorders depending on their combinations. Although a number of candidate genes for schizophrenia have been highlighted, only very few schizophrenia patients are likely to share identical genetic liability. Close relatives of schizophrenia patients have an elevated risk for both schizophrenia and so-called schizophrenia spectrum disorders. This favors the use of family samples in genetic studies of schizophrenia. Furthermore, samples collected from isolated populations with increased genetic homogeneity may assist in identification of predisposing variants. This study is based on the nation-wide schizophrenia family sample of the National Institute for Health and Welfare. This wide series, collected from the relatively isolated Finnish population with limited genetic variation, represents one of the largest and most well-characterized familial series in the world. In the first part of this study, we investigated the roles of the Dystrobrevin binding protein 1 (DTNBP1), Neuregulin 1 (NRG1), and V-akt murine thymoma viral oncogene homolog 1 (AKT1) genes in the background of schizophrenia in Finland. Although these genes are associated with schizophrenia liability in several populations, any significant association with clinical diagnostic information of schizophrenia remained absent in our sample of 441 schizophrenia families. Therefore, our study provides no support for any major role of these genes behind schizophrenia in the Finnish population. In the second part of this study, we first replicated schizophrenia linkage on the long arm of chromosome 7 in 352 schizophrenia families. In the following association analysis, we utilized additional clinical disorder features and intermediate phenotypes— endophenotypes—in addition to diagnostic information from altogether 290 neuropsychologically assessed schizophrenia families. An intragenic short tandem repeat (STR) allele of the regional Reelin (RELN) gene, supposed to play a role in the background of several neurodevelopmental disorders, showed significant association with poorer cognitive functioning and more severe schizophrenia symptoms. Importantly, the effect of the risk allele on cognition was replicated in an independent subsample, and interestingly, its effect was stronger among the individuals affected with psychosis in the whole sample. Although any significant association with the clinical diagnoses remained absent, this risk allele was significantly more prevalent among the individuals affected with schizophrenia spectrum disorders. Our results support the involvement of RELN in schizophrenia liability, and especially, its role as a genetic modifier of the disorder features. The wide spectrum of additional diagnostic information available in altogether 293 schizophrenia families was further utilized in the last part of this study. We have previously identified linkage of schizophrenia and its cognitive endophenotypes on the long arms of chromosomes 2, 4, and 5. Here, we selected altogether 104 functionally relevant candidate genes from the linked regions, and performed association analysis of clinical diagnostic categories, clinical disorder features, and several endophenotypic traits representing the central cognitive functions impaired in schizophrenia. Our approach allowed identification of several promising associations, of which especially interesting are the Verb-a erythroblastic leukemia viral oncogene homolog 4 (ERBB4) gene, showing association with the severity of schizophrenia symptoms and impairments in traits related to verbal abilities, and the Glutamate receptor, ionotropic, AMPA 1 (GRIA1) gene, showing association with the severity of schizophrenia symptoms. This study supports the view that due to the heterogeneity of the disorder, sole clinical diagnostic information may be insufficient for detection of all predisposing variants for schizophrenia, and supposedly, for psychiatric disorders overall. Our results extend the previous evidence that the genetic risk for schizophrenia is at least partially mediated via the effects of the candidate genes and their combinations on relevant brain systems, resulting in alterations in different disorder domains, such as the cognitive deficits. Therefore, these results encourage the use of detailed disorder-related features and intermediate factors to extract maximal information from the study material in the search for specific risk variants.