Genetic epidemiology of bipolar disorder

Genetic contributions to bipolar disorder: current status and future directions

Bipolar disorder (BD) is a highly heritable mental disorder and is estimated to affect about 50 million people worldwide. Our understanding of the genetic etiology of BD has greatly increased in recent years with advances in technology and methodology as well as the adoption of international consortiums and large population-based biobanks. It is clear that BD is also highly heterogeneous and polygenic and shows substantial genetic overlap with other psychiatric disorders. Genetic studies of BD suggest that the number of associated loci is expected to substantially increase in larger future studies and with it, improved genetic prediction of the disorder. Still, a number of challenges remain to fully characterize the genetic architecture of BD. First among these is the need to incorporate ancestrally-diverse samples to move research away from a Eurocentric bias that has the potential to exacerbate health disparities already seen in BD. Furthermore, incorporation of population biobanks, registry data, and electronic health records will be required to increase the sample size necessary for continued genetic discovery, while increased deep phenotyping is necessary to elucidate subtypes within BD. Lastly, the role of rare variation in BD remains to be determined. Meeting these challenges will enable improved identification of causal variants for the disorder and also allow for equitable future clinical applications of both genetic risk prediction and therapeutic interventions.

Serotonin as a drug receptor for Li+: a computational study

Although Li+ is widely used to treat bipolar disorder, its antidepressant pharmacological mechanism of action remains unelucidated. Herein, based on molecular modelling studies, we present the novel hypothesis that one possible receptor for Li+ is serotonin (5-hydroxytryptamine, 5-HT), a small molecule neurotransmitter rather than a large macromolecular protein. The resulting Li+/5-HT drug–receptor complex subsequently interacts with “upstream” macromolecular receptors such as the 5-HT1A receptor differently than 5-HT alone, producing an enhanced antidepressant effect. The notion that a neurotransmitter could itself be a receptor for a therapeutic is an interesting receptor cascade concept. Using molecular mechanics and semi-empirical and ab initio levels of theory, the potential interactions between Li+ and 5-HT and between Li+ and 5-HIAA (5-hydroxyindoleacetic acid, a 5-HT metabolite) were examined. Molecular dynamics simulations were then used to examine how Li+ affects the binding of 5-HT to its target 5-HT1A antidepressant receptor. The results of these calculations suggest that Li+ can interact with 5-HT and in such a way that it modifies the ligand–protein interactions.

Bipolar disorder and its relation to major psychiatric disorders: a family-based study in the Swedish population

OBJECTIVES

Bipolar disorder (BPD) shares genetic components with other psychiatric disorders; however, uncertainty remains about where in the psychiatric spectra BPD falls. To understand the etiology of BPD, we studied the familial aggregation of BPD and co-aggregation between BPD and schizophrenia, depression, anxiety disorders, attention-deficit hyperactivity disorder, drug abuse, personality disorders, and autism spectrum disorders.

METHODS

A population-based cohort was created by linking several Swedish national registers. A total of 54,723 individuals with BPD were identified among 8,141,033 offspring from 4,149,748 nuclear families. The relative risk of BPD in relatives and the co-occurrence of other psychiatric disorders in patients with BPD and their relatives were compared to those of matched-population controls. Structural equation modeling was used to estimate the heritability and tetrachoric correlation.

RESULTS

The familial risks for relatives of BPD probands were 5.8-7.9 in first-degree relatives, and decreased with genetic distance. Co-occurrence risks for other psychiatric disorders were 9.7-22.9 in individuals with BPD and 1.7-2.8 in full siblings of BPD probands. Heritability for BPD was estimated at 58%. The correlations between BPD and other psychiatric disorders were considerable (0.37-0.62) and primarily due to genetic effects. The correlation with depression was the highest (0.62), and was 0.44 for schizophrenia.

CONCLUSIONS

The high familial risks provide evidence that genetic factors play an important role in the etiology of BPD, and the shared genetic determinants suggest pleiotropic effects across different psychiatric disorders. Results also indicate that BPD is in both the mood and psychotic spectra, but possibly more closely related to mood disorders.

Investigating the genetic variation underlying episodicity in major depressive disorder: suggestive evidence for a bipolar contribution

BACKGROUND

Highly recurrent major depressive disorder (MDD) has reportedly increased risk of shifting to bipolar disorder; high recurrence frequency has, therefore, featured as evidence of 'soft bipolarity'. We aimed to investigate the genetic underpinnings of total depressive episode count in recurrent MDD.

METHODS

Our primary sample included 1966 MDD cases with negative family history of bipolar disorder from the RADIANT studies. Total episode count was adjusted for gender, age, MDD duration, study and center before being tested for association with genotype in two separate genome-wide analyses (GWAS), in the full set and in a subset of 1364 cases with positive family history of MDD (FH+). We also calculated polygenic scores from the Psychiatric Genomics Consortium MDD and bipolar disorder studies.

RESULTS

Episodicity (especially intermediate episode counts) was an independent index of MDD familial aggregation, replicating previous reports. The GWAS produced no genome-wide significant findings. The strongest signals were detected in the full set at MAGI1 (p=5.1×10(-7)), previously associated with bipolar disorder, and in the FH+ subset at STIM1 (p=3.9×10(-6) after imputation), a calcium channel signaling gene. However, these findings failed to replicate in an independent Munich cohort. In the full set polygenic profile analyses, MDD polygenes predicted episodicity better than bipolar polygenes; however, in the FH+ subset, both polygenic scores performed similarly.

LIMITATIONS

Episode count was self-reported and, therefore, subject to recall bias.

CONCLUSIONS

Our findings lend preliminary support to the hypothesis that highly recurrent MDD with FH+ is part of a 'soft bipolar spectrum' but await replication in larger cohorts.

Imunologia do transtorno bipolar

OBJETIVO: Pesquisas recentes têm implicado fatores imunes na patogênese de diversos transtornos neuropsiquiátricos. O objetivo do presente trabalho é revisar os trabalhos que investigaram a associação entre transtorno bipolar e alterações em parâmetros imunes. MÉTODOS: Artigos que incluíam as palavras-chave: "bipolar disorder", "mania", "immunology", "cytokines", "chemokines", "interleukins", "interferon" e "tumor necrosis factor" foram selecionados em uma revisão sistemática da literatura. As bases de dados avaliadas foram MedLine e Scopus, entre os anos de 1980 e 2008. RESULTADOS: Foram identificados 28 trabalhos que estudaram alterações imunes em pacientes com transtorno bipolar. Seis artigos investigaram genes relacionados à resposta imune; cinco, autoanticorpos; quatro, populações leucocitárias; 13, citocinas e/ou moléculas relacionadas à resposta imune e seis, leucócitos de pacientes in vitro. CONCLUSÕES: Embora haja evidências na literatura correlacionando o transtorno bipolar a alterações imunes, os dados não são conclusivos. O transtorno bipolar parece estar associado a níveis mais elevados de autoanticorpos circulantes, assim como à tendência à ativação imune com produção de citocinas pró-inflamatórias e redução de parâmetros anti-inflamatórios.

Family environment in families with versus families without parental bipolar disorder: a preliminary comparison study

OBJECTIVES

To compare family environmental characteristics of families with at least one bipolar parent and families with parents without any Axis I disorder.

METHODS

Family environment of 24 families with at least one parent with bipolar disorder (BPD) and 27 families with healthy parents (healthy families, HF) were assessed using the Family Environment Scale (FES). We compared FES subscale scores between BPD and HF. We also compared FES normative scores with scores of BPD families.

RESULTS

Seventeen (71%) of the 24 BPD families had at least one child with a mood disorder and one (3.7%) of the 27 HF had a child with a mood disorder. Families with BPD reported lower cohesion (p = 0.009) and expressiveness (p = 0.03) scores compared with HF, after controlling for group socioeconomic status differences. Bilineal BPD families had higher cohesion scores than unilineal BPD families (p = 0.05). We found no significant differences in any subscales between BPD families with (n = 9) versus without (n = 15) children with BPD. Compared with normative FES data BPD families reported lower cohesion (C) (p = 0.02) and independence (IND) (p = 0.004) scores and higher conflict (CON) (p = 0.02), intellectual-cultural orientation (ICO) (p = 0.05), moral-religious emphasis (MRE) (p < 0.001) and control (CTL) (p < 0.001) scores.

CONCLUSION

Our results suggest that members of BPD families may have difficulty communicating effectively with one another, suggesting that interventions aimed at improving this may be beneficial to patients with BPD. Results suggest that the impact of different lifestyles in different communities may influence FES scores, and thus it is important to use demographically matched comparison groups.