GPR50 is an orphan G protein-coupled receptor (GPCR) located on Xq28, a region previously implicated in multiple genetic studies of bipolar affective disorder (BPAD). Allele frequencies of three polymorphisms in GPR50 were compared in case-control studies between subjects with BPAD (264), major depressive disorder (MDD) (226), or schizophrenia (SCZ) (263) and ethnically matched controls (562). Significant associations were found between an insertion/ deletion polymorphism in exon 2 and both BPAD (P ¼ 0.0070), and MDD (P ¼ 0.011) with increased risk associated with the deletion variant (GPR50 D502-505 ). When the analysis was restricted to female subjects, the associations with BPAD and MDD increased in significance (P ¼ 0.00023 and P ¼ 0.0064, respectively). Two other single-nucleotide polymorphisms (SNPs) tested within this gene showed associations between: the female MDD group and an SNP in exon 2 (P ¼ 0.0096); and female SCZ and an intronic SNP (P ¼ 0.0014). No association was detected in males with either MDD, BPAD or SCZ. These results suggest that GPR50 D502-505 , or a variant in tight linkage disequilibrium with this polymorphism, is a sex-specific risk factor for susceptibility to bipolar disorder, and that other variants in the gene may be sex-specific risk factors in the development of schizophrenia. Bipolar affective disorder (BPAD) is a severe psychiatric disorder affecting approximately 1% of the world's population, and shows no difference in lifetime prevalence between male and female subjects. Twin and adoption studies have demonstrated a strong genetic component, with a concordance in BPAD between monozygotic twins of 60%. 1 Major depressive disorder (MDD) has a lifetime prevalence of 17% with women twice as likely as men to develop the disorder. 2 Estimates of the heritability of MDD vary, but a meta-analysis of studies gives a point estimate of heritability of liability to MDD of 0.37. 3 Schizophrenia (SCZ), as with BPAD, has an estimated frequency of 1% in the population, but heritability estimates suggest that it has a larger genetic component than either BPAD or MDD, with monozygotic twins giving a point estimate of comorbidity of 0.81 in another recent meta-analysis. 4 Despite the strong genetic component in these major psychiatric disorders, there are also strong environmental influences.Linkage to Xq28 has been studied many times in BPAD. Two loci, colour-blindness (CB), and glucose-6-phosphate dehydrogenase (G6PD), have been detected through linkage and association in more than one population. 5 Most significant are LOD scores of 8.1 and 7.35 between CB and BPAD in the American and Belgian populations, respectively, although reanalysis of much of the data from positive linkage results on the X chromosome has resulted in much reduced evidence for linkage and suggestions of ascertainment bias. 6-9 Several more recent studies have again renewed interest in the distal end of Xq, although the region implicated in these studies (Xq24-28) is larger than that depicted in the earlier reports. 5,[...

Schizophrenia and bipolar disorder are leading causes of morbidity across all populations, with heritability estimates of approximately 80% indicating a substantial genetic component. Population genetics and genome-wide association studies suggest an overlap of genetic risk factors between these illnesses but it is unclear how this genetic component is divided between common gene polymorphisms, rare genomic copy number variants, and rare gene sequence mutations. We report evidence that the lipid transporter gene ABCA13 is a susceptibility factor for both schizophrenia and bipolar disorder. After the initial discovery of its disruption by a chromosome abnormality in a person with schizophrenia, we resequenced ABCA13 exons in 100 cases with schizophrenia and 100 controls. Multiple rare coding variants were identified including one nonsense and nine missense mutations and compound heterozygosity/homozygosity in six cases. Variants were genotyped in additional schizophrenia, bipolar, depression (n > 1600), and control (n > 950) cohorts and the frequency of all rare variants combined was greater than controls in schizophrenia (OR = 1.93, p = 0.0057) and bipolar disorder (OR = 2.71, p = 0.00007). The population attributable risk of these mutations was 2.2% for schizophrenia and 4.0% for bipolar disorder. In a study of 21 families of mutation carriers, we genotyped affected and unaffected relatives and found significant linkage (LOD = 4.3) of rare variants with a phenotype including schizophrenia, bipolar disorder, and major depression. These data identify a candidate gene, highlight the genetic overlap between schizophrenia, bipolar disorder, and depression, and suggest that rare coding variants may contribute significantly to risk of these disorders.

Linkage disequilibrium maps of the TRAX/DISC region. Maps were generated using Haploview v2.05 (http:// www.broad.mit.edu/mpg/haploview/) under the analysis criteria solid spine of LD40.