Candidate gene studies in psychiatric illness

Abstract

Schizophrenia, bipolar disorder and major depression are common, heritable neuropsychiatric conditions and yet the source of the inherited risk remains largely unknown. This thesis focuses on two complementary strategies for identifying and characterising the genetic component of these illnesses: homozygosity mapping in consanguineous pedigrees, and genetic and neurobiological investigations of candidate genes identified by the analysis of structural chromosomal abnormalities carried by patients with psychiatric diagnoses. In a family of a cousin marriage, five of six offspring presented with a rare combination of schizophrenia, sensori-neural hearing impairment and epilepsy. Two loci were located on chromosomes 22q13 and 2p24-25 where a series of markers were homozygous by descent (HBD). Five further HBD loci were identified in a second, related family where four of five offspring had hearing loss. However, there was no overlap of the HBD intervals in the two families, and sequencing coding regions of candidate genes failed to identify causative mutations. A second study investigated the candidate gene ABCA13 identified at a breakpoint region on chromosome 7 in a patient with schizophrenia who carried a complex chromosomal rearrangement. Re-sequencing exons encoding the highly conserved functional domains identified eight potentially pathogenic, rare coding variants. Case control association studies involving cohorts of schizophrenia, bipolar disorder and major depression revealed significant associations of these variants with all three clinical phenotypes, and follow-up in relatives displayed familial inheritance patterns. Disruption of ABCA13, expressed in human hippocampus and frontal cortex, implicates aberrant lipid biology as a pathological pathway in mental illness. A third study focused on GRIK4, a candidate gene previously reported disrupted in a patient with schizophrenia who carried a chromosome abnormality. A deletion in the 3’UTR of GRIK4, encoding the kainate receptor subunit KA1, was identified as a protective factor for bipolar disorder. Using post mortem human brain tissue from control subjects, KA1 protein expression patterns were characterized in the hippocampal formation, amygdala, frontal cortex and cerebellum. KA1 expression was found significantly increased in subjects with the protective allele, supporting the hypothesis that reduced glutamatergic neurotransmission is a risk factor in major psychiatric illnesses. Together, these novel discoveries define aspects of the genetic contribution to mental illness, implicate specific dysfunctional processes and suggest new directions for research in the quest to find rationally based treatments and preventative strategies for some of the most common and disabling psychiatric disorders.