Mark D. Fossey

Validity of the distinction between primary and secondary substance use disorder in patients with bipolar disorder: data from the first 1000 STEP-BD participants.

The validity of a primary/secondary substance use disorder (SUD) distinction was evaluated in the first 1000 patients enrolled in the Systematic Treatment Enhancement Program for Bipolar Disorder. Patients with primary SUD (n = 116) were compared with those with secondary SUD (n = 275) on clinical course variables. Patients with secondary SUD had fewer days of euthymia, more episodes of mania and depression, and a greater history of suicide attempts. These findings were fully explained by variations in age of onset of bipolar disorder. The order of onset of SUDs was not linked to bipolar outcomes when age of onset of bipolar disorder was statistically controlled. The primary/secondary distinction for SUD is not valid when variations in the age of onset of the non-SUD are linked to course characteristics.

Serotonin Gene Polymorphisms and Bipolar I Disorder: Focus on the Serotonin Transporter

The pathogenesis of bipolar disorder may involve, at least in part, aberrations in serotonergic neurotransmission. Hence, serotonergic genes are attractive targets for association studies of bipolar disorder. We have reviewed the literature in this field. It is difficult to synthesize results as only one polymorphism per gene was typically investigated in relatively small samples. Nevertheless, suggestive associations are available for the 5HT2A receptor and the serotonin transporter genes. With the availability of extensive polymorphism data and high throughput genotyping techniques, comprehensive evaluation of these genes using adequately powered samples is warranted. We also report on our investigations of the serotonin transporter, SLC6A4 (17q11.1‐q12). An insertion/deletion polymorphism (5HTTLPR) in the promoter region of this gene has been investigated intensively. However, the results have been inconsistent. We reasoned that other polymorphism/s may contribute to the associations and the inconsistencies may be due to variations in linkage disequilibrium (LD) patterns between samples. Therefore, we conducted LD analyses, as well as association and linkage using 12 polymorphisms, including 5HTTLPR. We evaluated two samples. The first sample consisted of 135 US Caucasian nuclear families having a proband with bipolar I disorder (BDI, DSM IV criteria) and available parents. For case‐control analyses, the patients from these families were compared with cord blood samples from local Caucasian live births (n = 182). Our second, independent sample was recruited through the Systematic Treatment Enhancement Program for Bipolar Disorder (STEP‐BD, 545 cases, 548 controls). No significant associations were detected at the individual polymorphism or haplotype level using the case‐control or family‐based analyses. Our analyses do not support association between SLC6A4 and BDI families. Further studies using sub‐groups of BDI are worthwhile.

Linkage disequilibrium mapping of the chromosome 6q21–22.31 bipolar I disorder susceptibility locus†‡

We previously reported genome‐wide significant evidence for linkage between chromosome 6q and bipolar I disorder (BPI) by performing a meta‐analysis of original genotype data from 11 genome scan linkage studies. We now present follow‐up linkage disequilibrium mapping of the linked region utilizing 3,047 single nucleotide polymorphism (SNP) markers in a case–control sample (N = 530 cases, 534 controls) and family‐based sample (N = 256 nuclear families, 1,301 individuals). The strongest single SNP result (rs6938431, P = 6.72 × 10−5) was observed in the case–control sample, near the solute carrier family 22, member 16 gene (SLC22A16). In a replication study, we genotyped 151 SNPs in an independent sample (N = 622 cases, 1,181 controls) and observed further evidence of association between variants at SLC22A16 and BPI. Although consistent evidence of association with any single variant was not seen across samples, SNP‐wise and gene‐based test results in the three samples provided convergent evidence for association with SLC22A16, a carnitine transporter, implicating this gene as a novel candidate for BPI risk. Further studies in larger samples are warranted to clarify which, if any, genes in the 6q region confer risk for bipolar disorder.