Summary of methods used to detect possible genetic defects in psychiatric disorders

Summary of methods used to detect possible genetic defects in psychiatric disorders The genetic basis of late onset Alzheimer’s disease conforms to the common disease–common variant hypothesis which states that genetic susceptibility is attributable to common variants present in the population at a high frequency. Significant associations have been demonstrated between several common polymorphisms such as APO E4 and Alzheimer’s disease, already referred to. Individuals differ from each other at many positions across the genome and a variation at a particular nucleotide is called a polymorphism. It has been calculated that approximately one nucleotide base in every 1000–2000 nucleotides differs between chromosomes. If, as seems likely, the common disease–common variant hypothesis applies to schizophrenia and bipolar disorder then it may be essential to undertake association studies which follow the inheritance of a gene within astudies are an attractive means to understanding complex psychiatric diseases because they have the power to identify causal associations between a particular allele and a heterogeneous disease and do not require the collection of large pedigrees containing multiple affected individuals. However, for such a method to be applied, the polymorphisms used in the assessments must have been identified. This necessitates the collection of a large group of polymorphisms to form a library of potential disease-causing alleles. Traditionally techniques for the detection of polymorphisms have involved gel-based screening methods or direct DNA sequencing of numerous individuals. Such methods are laborious and time-consuming. Microarray technology has revolutionized the approach by developing DNA microchips that contain a high density of oligonucleotides which are capable of rapidly detecting variations in nucleotide sequences. Microarrays consist of microchips of DNA attached to the surface of a solid support that may vary according to the density and the form, or size, of the DNA on the surface. There are different commercially available forms of microarray chips and the presence of many independent DNA molecules on the chip surface allows hybridization of many different species simultaneously and for the results to be similarly detected on a microscope slide. By labelling the final products formed with a fluorescent dye, the resulting hybridization pattern can be detected by a confocal microscope. The major question which arises now that the technology for rapid screening has become available is which genes should be chosen? If the possible cause of the disease is known then the answer is clear. However, for most of the major psychiatric disorders the causes are unknown although neurochemical, neurodevelopmental, autoimmune and environmental factors are probably involved. For this reason, association studies in schizophrenia and bipolar disorder have focused on genes related to the dopaminergic and serotonergic systems. Unfortunately, polymorphisms related to these genes have not convincingly been found to be associated with schizophrenia or bipolar disorder. However, several other lines of evidence suggest that schizophrenia may be a neurodevelopmental disorder in which the frontal association cortex, an area responsible for several important hig-level functions, has been shown to be malfunctional. Thus schizophrenia may result from a disorder of neuronal migration. If so, the discovery of polymorphisms in genes that regulate cortical development could prove to be a useful area of investigation. This approach has already identified the gene dsh (dishevelled, named after a gene identified in the fruit fly, Drosophila) in the mouse which, when absent, leads to a reduction in the startle response. Prepulse inhibition, the reduction in the startle that the animal shows to a previous stimulus, has been shown to be diminished in patients with schizophrenia. While it is premature to propose that dsh deficient mice are a model for schizophrenia, such findings do suggest thatsuch genes may play an important role in the early differentiation of the brain. During the normal development of the brain, a large number of genes are activated and become involved at different times and have different functions. In the early pattern formation of the brain, during the development of the neural tube, the homeotic genes (the so-called hox family genes) encode for transcriptional factors that bind to DNA and thereby regulate the expression of other genes. This process is believed to convey information on dorsoventral positioning to the cells of the developing neural tube. However, there is evidence that a mutation in a homeobox gene is associated with gross brain defects which makes it unlikely that a mutation in the early genes could be responsible for the subtle changes found in schizophrenia. There is evidence that the cortex of the brain of schizophrenic patients contains neurons that are in abnormal positions when compared to nonschizophrenic individuals. This suggests that the migration of neurons may be abnormal during the developmental period. The neuronal cell adhesion molecule (NCAM) is an immunoglobulin that mediates adhesion between neurons, thereby exerting a key role in morphogenesis, differentiation and the migration of neurons. NCAM is encoded by a single gene that undergoes alternative splicing to generate several alleles which exhibit different spatial and temporal patterns of expression in vertebrates. The NCAM gene is located on chromosome 11 and two polymorphisms are known to occur in the NCAM gene region. However linkage analysis of 71 families has failed to confirm that an abnormality in the NCAM gene occurs in schizophrenia. During brain development, neurons send out numerous dendrites in which growth factors play a prominent part. The neurotrophins comprise a family of structurally and functionally related growth factors that include nerve growth factor (NGF), brain derived neurotrophic factor (BDNF) and the neurotrophins 3 and 4/5. These peptides cause neuronal growth, increase the size of the body of the neuron and maintain the survival of the neurons of the dopaminergic, serotonergic and glutaminergic systems. As a decrease of 20–30% in BDNF mRNA has been reported to occur in the hippocampi of schizophrenic patients, it seems possible that a defect in this growth factor could be responsible for the occurrence of smaller neurons in these patients. The BDNF gene is located on chromosome 11 and preliminary findings from 72 nuclear families suggest that the frequency of the A2 allele is significantly higher, and that of the A1 allele significantly lower, in schizophrenic patients. The NT-3 gene, located on chromosome 12, and the promoter region of this gene contain a highly polymorphic marker yielding at least 11 dinucleotide repeat alleles. Of the casecontrolled association studies undertaken so far, abnormal alleles have been reported in two of the five studies.Lastly, studies on the different polymorphic forms of the synapsins, that organize the mobilization of neurotransmitter vesicles thereby regulating neurotransmitter release, could account for some of the subtle changes in neurotransmission that occur in schizophrenia. However, to date linkage analysis studies have failed to reveal any positive associations between the various polymorphisms of the synapsin gene and schizophrenia. population rather than within families. Association studies test if a polymorphism is more frequently found in those with the disease (called ‘‘cases’’) than in normal individuals (controls). In such tests, the transmission of a polymorphism from a heterozygous parent to an affected offspring is followed. If the polymorphisms are not associated with the disease, then the rate of transmission from parent to affected offspring in a population will be 50%. Significant deviations from this predicted transmission rate indicate a possible association with the disease.