Identification of the Slynar gene (AY070435) and related brain expressed sequences as a candidate gene for susceptibility to affective disorders through allelic and haplotypic association with bipolar disorder on chromosome 12q24

OBJECTIVE

Three linkage studies of bipolar disorder have implicated chromosome 12q24.3, with significant lod scores of over 3.00. Several other linkage studies have found lod scores between 2.00 and 3.00. In order to identify which gene on this chromosome is responsible, the authors carried out tests of allelic association with bipolar disorder in order to fine map an affective disorder susceptibility gene.

METHOD

DNA samples from 681 bipolar disorder patients and 570 comparison subjects from Denmark and the United Kingdom were genotyped with markers close to the region at which the authors had found maximum linkage in previous studies.

RESULTS

Single marker allelic association was found with four markers in the Danish cohort. Seven markers in exactly the same region were then found to show significant allelic association in the U.K. cohort. Tests of haplotypic association were also significant, confirming the single marker allelic associations.

CONCLUSIONS

These positive fine mapping results validate earlier linkage studies and implicate a 278-kilobase region of chromosome 12 that contributes to the etiology of bipolar disorder. Several brain transcripts are transcribed from sequences in the region. The main candidate gene has no known function but is found in human brain cDNA and is homologous to a Macaque brain cDNA. Sequencing of expressed sequences and control regions in the area should identify etiological base pair changes that increase susceptibility to bipolar disorder.

Mutational screening and association study of glutamate decarboxylase 1 as a candidate susceptibility gene for bipolar affective disorder and schizophrenia

Recent evidence from postmortem studies suggests that GAD1 encoding the gamma-aminobutyric acid (GABA) synthetic enzyme GAD67 is a functional candidate susceptibility gene for both bipolar affective disorder (BPAD) and schizophrenia. Previous studies suggest linkage between D2S326 near GAD1 and BPAD. We systematically screened GAD1 exons, flanking intronic sequences, and the promoter sequence for polymorphisms in 16 BPAD patients and five controls from Denmark. We identified eight single nucleotide polymorphisms (SNPs) including two in the promoter sequence. An association study of SNPs covering GAD1 was performed in a Danish sample of 82 BPAD subjects and 120 controls and in a Scottish sample of 197 individuals with schizophrenia, 200 BPAD subjects and 199 controls. Linkage disequilibrium (LD) and haplotype frequencies were estimated from genotype data from eight SNPs. Strong pairwise LD was observed among all pairs of neighboring markers. In the Danish sample, we found weak association between BPAD and two promoter SNPs spaced 1 kb apart. Furthermore, one, two, and three loci haplotype analysis showed weak association with BPAD in the Danish sample. The results from the association studies indicate that promoter variants are of importance for the Danish BPAD cases and we cannot reject the hypothesis of GAD1 as a functional candidate gene for BPAD. No association was observed between BPAD or schizophrenia and any of the investigated SNPs in the Scottish sample set. Thus the results obtained from the Scottish sample suggest that the GAD1 gene variants do not play a major role in the predisposition to schizophrenia.

Further evidence for a bipolar risk gene on chromosome 12q24 suggested by investigation of haplotype sharing and allelic association in patients from the Faroe Islands

A number of studies have strongly suggested a susceptibility locus for bipolar affective disorder on chromosome 12q24. The present study investigates for a shared chromosomal segment among distantly related patients with bipolar affective disorder from the Faroe Islands, using 17 microsatellite markers covering 24 cM in the previously suggested region on chromosome 12q24. D12S342 showed possible allelic association to bipolar affective disorder (P-value using CLUMP below 0.01). Increased sharing among cases of two-marker haplotypes were suggested at D12S1614--D12S342 (P-values using CLUMP below 0.01), and D12S2075--D12S1675 (P-values using CLUMP around 0.001). The region of most interest is around 6 cM and bounded by markers D12S1614 and D12S1675 as suggested by the interesting two-marker haplotypes. This area contains the minimum interesting region between D12S342 and D12S1658 suggested by the previously reported haplotypes in the two Danish families with bipolar affective disorder.

Amphotropic murine leukemia virus entry is determined by specific combinations of residues from receptor loops 2 and 4

Pit2 is the human receptor for amphotropic murine leukemia virus (A-MuLV); the related human protein Pit1 does not support A-MuLV entry. Interestingly, chimeric proteins in which either the N-terminal or the C-terminal part of Pit2 was replaced by the Pit1 sequence all retained A-MuLV receptor function. A possible interpretation of these observations is that Pit1 harbors sequences which can specify A-MuLV receptor function when presented in a protein context other than Pit1, e.g., in Pit1-Pit2 hybrids. We reasoned that such Pit1 sequences might be identified if presented in the Neurospora crassa protein Pho-4. This protein is distantly related to Pit1 and Pit2, predicted to have a similar membrane topology with five extracellular loops, and does not support A-MuLV entry. We show here that introduction of the Pit1-specific loop 2 sequence conferred A-MuLV receptor function upon Pho-4. Therefore, we conclude that (i) a functional A-MuLV receptor can be constructed by combining sequences from two proteins each lacking A-MuLV receptor function and that (ii) a Pit1 sequence can specify A-MuLV receptor function when presented in another protein context than that provided by Pit1 itself. Previous results indicated a role of loop 4 residues in A-MuLV entry, and the presence of a Pit2-specific loop 4 sequence was found here to confer A-MuLV receptor function upon Pho-4. Moreover, the introduction of a Pit1-specific loop 4 sequence, but not of a Pit2-specific loop 4 sequence, abolished the A-MuLV receptor function of a Pho-4 chimera harboring the Pit1-specific loop 2 sequence. Together, these data suggest that residues in both loop 2 and loop 4 play a role in A-MuLV receptor function. A-MuLV is, however, not dependent on the specific Pit2 loop 2 and Pit2 loop 4 sequences for entry; rather, the role played by loops 2 and 4 in A-MuLV entry can be fulfilled by several different combinations of loop 2 and loop 4 sequences. We predict that the residues in loops 2 and 4, identified in this study as specifying A-MuLV receptor function, are to be found among those not conserved among Pho-4, Pit1, and Pit2.

Single amino acid insertion in loop 4 confers amphotropic murine leukemia virus receptor function upon murine Pit1

Pit1 is the human receptor for gibbon ape leukemia virus (GALV) and feline leukemia virus subgroup B (FeLV-B), while the related human protein Pit2 is a receptor for amphotropic murine leukemia virus (A-MuLV). The A-MuLV-related isolate 10A1 can utilize both Pit1 and Pit2 as receptors. A stretch of amino acids named region A was identified in Pit1 (residues 550 to 558 in loop 4) as critical for GALV and FeLV-B receptor function. We have here investigated the role of region A in A-MuLV and 10A1 entry. Insertion of a single amino acid in region A of mouse Pit1 resulted in a functional A-MuLV receptor, showing that region A plays a role in A-MuLV infection. Moreover, the downregulation of 10A1 receptor function by changes in region A of human Pit1 indicates that this region is also involved in 10A1 entry. Therefore, region A seems to play a role in infection by all viruses utilizing Pit1 and/or Pit2 as receptors.