Isolation of chromosome 18-specific brain transcripts as positional candidates for bipolar disorder.

Several studies have proposed the existence of susceptibility loci for bipolar disorder on chromosome 18. To identify possible candidate genes for this disease, we isolated brain-expressed transcripts by direct cDNA selection on chromosome 18-specific biotinylated cosmid clones. Longer cognate cDNA clones of the selected cDNAs were isolated from a normalized infant brain cDNA library. Physical mapping by PCR on a panel of somatic cell hybrids was conducted by the use of primers derived from partial sequences on either the 5' or 3' ends of the clones. In our initial analysis, 48 cDNA clones were found to be chromosome 18-specific, mapping to different subchromosomal regions. Sequence redundancy among these clones yielded 30 unique transcripts, five of which were represented in previously known genes. Further sequencing of the remaining 25 unique cDNA clones confirmed the absence of significant homology to known genes, indicating that these transcripts represented novel genes. Mapping with the use of a radiation hybrid panel positioned the brain cDNAs to within = 100 to 1100 kb from reference sequence tag sites (STSs) and assembled them into six high resolution linkage groups. The majority of the transcripts were found to cluster to discrete locations on 18p and 18q, previously hypothesized as susceptibility regions for bipolar disorder, identifying them as positional candidate genes.

Serotonin transporter (5-HTT) gene and bipolar affective disorder.

Interactions with antidepressants, as well as other biochemical evidence, implicate the serotonin transporter 5-HTT in the etiology of affective disorders. However, genetic studies have produced conflicting results concerning an association of 5-HTT with bipolar disorder. We examined a variable number tandem repeat in the regulatory region of this gene to investigate the possible contribution of 5-HTT to bipolar disorder susceptibility in a 22-pedigree series. By affected-sib-pair analysis and the transmission/disequilibrium test, we found no significant linkage or association of 5-HTT to bipolar disorder. During the course of this study, we adapted a PCR technique designed to amplify long templates to replicating long GC stretches with complex structure. We also refined the location of 5-HTT by radiation hybrid mapping, placing the locus between D17S1294 and SHGC11022 on 17q11.2.

Childhood-onset schizophrenia/autistic disorder and t(1;7) reciprocal translocation: identification of a BAC contig spanning the translocation breakpoint at 7q21.

Childhood-onset schizophrenia (COS) is defined by the development of first psychotic symptoms by age 12. While recruiting patients with COS refractory to conventional treatments for a trial of atypical antipsychotic drugs, we discovered a unique case who has a familial t(1;7)(p22;q21) reciprocal translocation and onset of psychosis at age 9. The patient also has symptoms of autistic disorder, which are usually transient before the first psychotic episode among 40-50% of the childhood schizophrenics but has persisted in him even after the remission of psychosis. Cosegregating with the translocation, among the carriers in the family available for the study, are other significant psychopathologies, including alcohol/drug abuse, severe impulsivity, and paranoid personality and language delay. This case may provide a model for understanding the genetic basis of schizophrenia or autism. Here we report the progress toward characterization of genomic organization across the translocation breakpoint at 7q21. The polymorphic markers, D7S630/D7S492 and D7S2410/D7S646, immediately flanking the breakpoint, may be useful for further confirming the genetic linkage for schizophrenia or autism in this region. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:749-753, 2000. Published 2000 Wiley-Liss, Inc.

Multiple transcriptional variants and RNA editing in C18orf1, a novel gene with LDLRA and transmembrane domains on 18p11.2.

C18orf1 is a novel brain-expressed transcript, mapping to 18p11.2. Upon further characterization, we found multiple and differentially expressed transcriptional variants. C18orf1 alpha 1, an 8.5-kb transcript, was predicted to code for a 306-amino-acid protein and a 7.1-kb 3'-untranslated region (UTR). This variant was encoded by at least six exons. Alternative transcripts included alpha 2, identical to alpha 1 but missing 18 residues, and N-terminal-truncated variants termed beta 1 and beta 2. A motif search suggested the presence of a transmembrane domain in both alpha and beta and a low-density lipoprotein receptor class A (LDLRA) domain in the alpha-specific N-terminal. In LDLR, LDLRA has been shown to be involved in binding Ca2+ and LDL, raising the possibility that C18orf1 might bind Ca2+ and an unknown ligand. We also present evidence of RNA editing in the 5'-UTR of beta 2, the first demonstration of this phenomenon in 5'-UTR.

A novel human myo-inositol monophosphatase gene, IMP.18p, maps to a susceptibility region for bipolar disorder.

Within the broad susceptibility region for bipolar disorder on the pericentromeric portion of chromosome 18, the highest allele sharing in our 22-pedigree series has been found in markers mapping to 18p11.2. Studies by other investigators on independently ascertained pedigrees have also shown increased sharing in this region, making 18p11.2 a plausible site for a candidate gene search. We found expressed sequence tags (ESTs) mapping within this area that are homologous to the myo-inositol-1-phosphate phosphohydrolase (myo-inositol monophosphatase: IMP) gene of Xenopus laevis. Since IMP has been proposed to be the potential target of lithium, a drug commonly used for the treatment of bipolar disorder, we proceeded to characterize the cognate transcript. Northern blot analysis detected a major transcript of 1.5 kb with abundant expression in adult and fetal tissues, but minimal expression in whole brain. In subcortical brain regions, however, substantial levels of transcript were evident, most prominently in the caudate. We have isolated and sequenced the full-length cDNA. The deduced amino acid sequence revealed approximately 54% identity with an existing human IMP, which we found mapped to chromosome 8, and IMP of other species. The sequence also included motifs characteristic of the IMP gene family. To provide a more precise location of this gene, mapping with a panel of radiation hybrids (RH) was conducted. Multipoint RH analysis placed the gene between GNAL and D18S71 within the 18p11.2 region. We, therefore, designated this novel gene as IMP.18p. The physical position and possible function suggest that IMP.18p is an important candidate gene for bipolar disorder.

Genomic structure and novel variants of myo-inositol monophosphatase 2 (IMPA2).

Recently, we cloned the human myo-inositol monophosphatase 2 (IMPA2) cDNA and established its map location to chromosome 18p11.2, a region previously implicated in bipolar disorder. Because the myo-inositol monophosphatase enzyme has been shown to be inhibited by lithium, an effective therapeutic agent for bipolar disorder, IMPA2 is a plausible positional and functional candidate gene. To permit comprehensive screening for variants we characterized the genomic structure and isolated the potential promoter of IMPA2. The gene was found to encode eight exons spanning;27 kb. The proximal 1-kb 5' flanking region did not contain an obvious TATA box but multiple potential binding sites for Sp1 and consensus motifs for AP2 and other transcription factors were evident. Sequencing of the coding region and splice junctions in unrelated bipolar disorder patients detected novel variants. A missense mutation in exon 2, His76Tyr, was found in one patient. His76 is evolutionarily conserved and replacement with Tyr introduces a potential site for phosphorylation. The other polymorphisms included an RsaI polymorphism, IVS1-15G>A, and a T --> C silent mutation in the third nucleotide of codon 53 in exon 2. By Fisher's exact test the silent mutation showed a trend for association (P = 0.051) with bipolar disorder suggesting that further scrutiny of this gene is warranted.

An integrated physical map of 18p11.2: a susceptibility region for bipolar disorder.

The reported linkage between bipolar disorder and a large pericentric portion of chromosome 18 has been replicated in an independent study. Further examination of this region showed that 18p11.2 had the greatest allele sharing in our pedigrees and increased sharing in other independently ascertained pedigree series permitting refinement of the region of significance. To facilitate positional cloning of a susceptibility gene, we used a combination of mapping reagents, including a subchromosomal somatic cell hybrid panel, a contig of clones in yeast artificial chromosomes (YAC), and a radiation hybrid (RH) panel, to construct a high resolution physical map of the region including sequence tag sites (STSs) and expressed sequence tags (ESTs). This approach generated the interlocus distance and order of 15 STSs and 16 ESTs including four novel transcripts, with an average of approximately 200 kb between loci, over a approximately 6-Mb region. This high resolution integrated map will be an important tool in providing loci for contig construction, and positional candidates for mutation screening.

A high-density genome scan detects evidence for a bipolar-disorder susceptibility locus on 13q32 and other potential loci on 1q32 and 18p11.2.

Bipolar disorder is a severe mental illness characterized by mood swings of elation and depression. Family, twin, and adoption studies suggest a complex genetic etiology that may involve multiple susceptibility genes and an environmental component. To identify chromosomal loci contributing to vulnerability, we have conducted a genome-wide scan on approximately 396 individuals from 22 multiplex pedigrees by using 607 microsatellite markers. Multipoint nonparametric analysis detected the strongest evidence for linkage at 13q32 with a maximal logarithm of odds (lod) score of 3.5 (P = 0. 000028) under a phenotype model that included bipolar I, bipolar II with major depression, schizoaffective disorder, and recurrent unipolar disorder. Suggestive linkage was found on 1q31-q32 (lod = 2. 67; P = 0.00022) and 18p11.2 (lod = 2.32; P = 0.00054). Recent reports have linked schizophrenia to 13q32 and 18p11.2. Our genome scan identified other interesting regions, 7q31 (lod = 2.08; P = 0. 00099) and 22q11-q13 (lod = 2.1; P = 0.00094), and also confirmed reported linkages on 4p16, 12q23-q24, and 21q22. By comprehensive screening of the entire genome, we detected unreported loci for bipolar disorder, found support for proposed linkages, and gained evidence for the overlap of susceptibility regions for bipolar disorder and schizophrenia.