Genetic vulnerability

Introduction The current state of schizophrenia genetics might seem at first glance to be at odds with family, twin and adoption studies pointing towards a high genetic liability, with a point estimate of 81% (Sullivan, Kendler & Neale, 2003), because no single gene appears to carry by itself a major risk effect (Harrison & Weinberger, 2005). The molecular aetiology of schizophrenia remains enigmatic, pointing towards the involvement of multiple genes with small effects across diverse populations (acting like quantitative trait genes), aetiopathological heterogeneity, or the combination of both. Complex inheritance patterns are suggested where detrimental and protective gene effects interact and may define a threshold determining if the phenotype schizophrenia is expressed under certain environmental influences, and during critical developmental periods. Genes are turned on and off during critical developmental periods and may interact with particular environmental influences. We can think of those genes as the basic instruction blocks of human development and maintenance that encode protein products at the molecular level that will finally direct an organism’s phenotypic characteristics (Prathikanti & Weinberger, 2005). In contrast to Mendelian disorders (e.g. cystic fibrosis), major psychosis does not behave in such a way, where one mutated gene and its product impacts on a range of developmental processes and finally translates into abnormal phenotypic characteristics that are defined as one particular ‘disease’. In the case of relatively common disorders such as schizophrenia or bipolar disorder, inheritance is more complex, and it is likely that single genetic variations do not substantially alter the protein structure. Susceptibility genes appear to increase the probability of illness, and they do so by acting in conjunction with other gene variations and environmental factors, until they finally manifest on a functional (clinical) level.