Molecular genetics of affective disorders: Positional cloning and gene-based association approaches

In order to identify genes involved in the etiology of bipolar disorder (BP) a positional cloning study was initiated in the candidate region on chromosome 18q21.33--q23. We constructed a physical map using Yeast Artificial Chromosomes (YACs). The minimum YAC tiling path consisted of 3 YACs on the basis of which we estimated the maximal candidate region at 4.8 Mbp. Next, the 3 YACs were subcloned in an exon-trap cosmid vector. The cosmid library was used to screen for CAG/CTG repeat sequences. Four CAG/CTG repeats had previously been identified by CAG/CTG YAC fragmentation and were confirmed by cosmid library screening. One polymorphic CAG/CTG repeat was identified in the 5'UTR of the cytoplasmic antiproteinase 2 (CAP2) gene, a positional and functional candidate gene for BP disorder. No expansions and no significant allele and genotype differences were detected for this polymorphic repeat in a BP association study.;We defined the genomic structure of CAP2 and performed an exhaustive mutation screening. The gene is composed of 7 exons spanning 17.1 kb. We identified 5 coding and 1 non-coding single nucleotide polymorphisms (SNPs). Genetic association analysis of all 6 SNPs in a BP case-control sample, revealed a significant difference for the synonymous SNP in exon 7 at codon 314. BP patients had a higher frequency of the T allele compared to controls (p = 0.03). Additional haplotype and linkage disequilibrium analyses did not provide supportive evidence that CAP2 is a susceptible gene for BP disorder.;Extensive studies have indicated that the hypothalamic-pituitary-adrenal (HPA) axis is dysregulated in patients with severe depression. We constructed a gene based SNP-map for the corticotropin-releasing hormone receptor 2 gene (CRHR2) consisting of 5 SNPs: 1 coding SNP, 2 intronic SNPs and 2 SNPs in the 5' upstream regulatory region. We compared allele and genotype frequencies for all 5 SNPs in a Belgian unipolar disorder (UP) and matched control sample. The coding SNP showed allelic and genotypic association with borderline significance (p = 0.04). However, a replication study in a BP sample of Belgian origin and a Swedish UP sample did not support association.