The genetics of bipolar disorder

Methodological considerations for gene expression profiling of human brain

Gene expression profiles of postmortem brain tissue represent important resources for understanding neuropsychiatric illnesses. The impact(s) of quality covariables on the analysis and results of gene expression studies are important questions. This paper addressed critical variables which might affect gene expression in two brain regions. Four broad groups of quality indicators in gene expression profiling studies (clinical, tissue, RNA, and microarray quality) were identified. These quality control indicators were significantly correlated, however one quality variable did not account for the total variance in microarray gene expression. The data showed that agonal factors and low pH correlated with decreased integrity of extracted RNA in two brain regions. These three parameters also modulated the significance of alterations in mitochondrial-related genes. The average F-ratio summaries across all transcripts showed that RNA degradation from the AffyRNAdeg program accounted for higher variation than all other quality factors. Taken together, these findings confirmed prior studies, which indicated that quality parameters including RNA integrity, agonal factors, and pH are related to differences in gene expression profiles in postmortem brain. Individual candidate genes can be evaluated with these quality parameters in post hoc analysis to help strengthen the relevance to psychiatric disorders. We find that clinical, tissue, RNA, and microarray quality are all useful variables for collection and consideration in study design, analysis, and interpretation of gene expression results in human postmortem studies.

[Genes related to phospholipid metabolism as risk factors related to bipolar affective disorder]

The studies of genetic epidemiology provides consistent evidence of genetic factors having a major role on the risk for the bipolar affective disorder, although, vulnerability genes have not yet been identified in unequivocal form. The authors show that phospholipids play an important role in the cellular signalling processes, besides this, some studies with mood-stabilisers neurochemistry suggest that these drugs act in the phospholipase regulated signalling views. They conclude that analysis of gene variants that code enzymes of the phospholipids metabolism as potential susceptibility genes can extend the knowledge concerning the risk factors and the physiopatological mechanisms underling this mood disturbance.

Basic concepts in the study of diseases with complex genetics

Most diseases run in families--this is also true of virtually all psychiatric disorders. Twin and adoption studies have shown that most psychiatric disorders have a genetic component, yet very few genetic factors are known, as is true for most disorders with a complex genetic origin. Here I review, for nongeneticists, some of the basic terminology and concepts used when studying complex genetic diseases, with examples from psychiatric genetics. This review is intended to help in the understanding and critical evaluation of reports on genetics of psychiatric illnesses in the literature.

Neuropsychological performance of monozygotic twins discordant for bipolar disorder

BACKGROUND

A paradigm that involves cognitive assessment of monozygotic (MZ) twins discordant for a neuropsychiatric disorder (here bipolar illness) allows for the examination of both disease-specific impairments (in the comparison of affected to unaffected twins) and risk factors (in the comparison of unaffected twins to normal twins).

METHODS

Neuropsychological functions were evaluated in seven MZ twin pairs discordant for bipolar illness and seven pairs of normal MZ twins in an attempt to highlight cognitive abilities associated with manifestations of disease and genetic risk factors. At the time of testing, 3 of the affected twins were euthymic, 2 had depressive symptoms, and 2 had manic symptoms; all were receiving medication. All twins receive neuropsychological tests to evaluate intelligence, attention, visuospatial skills, language, learning and memory, and problem solving.

RESULTS

Statistical analyses revealed that the affected twins were significantly impaired as compared to the unaffected (and normal) twins on some measures of visuospatial functioning and some verbal memory measures. In contrast to a sample of MZ twins discordant for schizophrenia studied previously, the cognitive impairments we observed in bipolar twins were mild in nature and fairly circumscribed. The unaffected twins performed significantly worse than normal controls on a Brown-Petersen memory task, verbal list learning, and overall Wechsler Memory Quotient.

CONCLUSIONS

These data suggest that while some visuospatial deficits and verbal memory deficits may be features of bipolar disorder related to disease parameters, mild attenuations in overall memory or retrieval function may be related to genetic factors associated with the illness.

Nonparametric simulation-based statistical analyses for bipolar affective disorder locus on chromosome 21q22.3

Straub et al. [1994: Nat Genet 8:291-296] reported a candidate bipolar affective disorder (BAD) locus on chromosome 21q22.3. As a replication study, we analyzed 12 Australian BAD pedigrees for the presence of excess allele sharing and cosegregation with the putative chromosome 21q22.3 BAD locus, using six microsatellite markers. The nonparametric simulation-based statistic SimAPM produced positive results for the marker PFKL (P < 0.001) and D21S198 (P = 0.007). PFKL also demonstrated linkage (P < 0.001) when analyzed using the more conservative statistic, SimIBD. Comparable results were obtained when using the original APM statistic (P = 0.02 for D21S198). However, other nonparametric analyses such as GENEHUNTER and model-free linkage (MFLINK) analysis did not yield significant results. Combined LOD scores for the 12 families were strongly negative for all six markers under six genetic models. Two-point and multipoint analyses of individual families revealed one family, family 17, with maximal LOD scores greater than 1.41 for the 10.5-cM region between PFKL and D21S198. This report provides additional support for the suggestive linkage of a susceptibility locus for BAD on chromosome 21q22.3.

Familial bipolar disorder: preliminary results from the Otago Familial Bipolar Genetic Study

OBJECTIVE

This paper outlines the methodologies used, and preliminary descriptive data collected, on a cohort of familial bipolar disorder (BPD) probands and first-degree relatives taking part in a descriptive and genetic study into familial BPD in New Zealand.

METHOD

Fifty-five bipolar probands and 67 first-degree relatives were interviewed using the modified Diagnostic Interview for Genetic Studies (DIGS) and Family Interview for Genetic Studies (FIGS). Data was also collated from other sources. Blood samples were taken for DNA genomic analysis.

RESULTS

New Zealand families in which BPD segregates proved willing participants in this familial based genetic research. The methodologies used were acceptable. High rates of comorbidity were found in probands (27.3% met DSM-IV criteria for panic disorder/sub-threshold panic disorder; 12.7% for phobic disorder; 1.8% for obsessive-compulsive disorder; 9.1% for alcohol-related disorders and 7.3% for an eating disorder) and relatives (major depression 34.3%; panic disorder/sub-threshold panic disorder 12.0%; phobias 11.9% and alcohol-related disorders 11.9%). The polarity of index BPD illness was related to age of onset and frequency of comorbidity. Suicidal behaviour was common.

CONCLUSIONS

Psychiatric genetic research in New Zealand families is highly feasible. Emerging trends in the familial transmission of BPD include high rates of comorbidity, illness patterns based on polarity of index episode and frequent suicidal behaviour. Such trends will be delineated further as numbers accrue, perhaps enabling identification of more homogenous phenotypic subgroups than currently produced by diagnostic schemes.

A 'sticky' interhemispheric switch in bipolar disorder?

Despite years of research into bipolar disorder (manic depression), its underlying pathophysiology remains elusive. It is widely acknowledged that the disorder is strongly heritable, but the genetics are complex with less than full concordance in monozygotic twins and at least four susceptibility loci identified. We propose that bipolar disorder is the result of a genetic propensity for slow interhemispheric switching mechanisms that become 'stuck' in one or the other state. Because slow switches are also 'sticky' when compared with fast switches, the clinical manifestations of bipolar disorder may be explained by hemispheric activation being 'stuck' on the left (mania) or on the right (depression). Support for this 'sticky' interhemispheric switching hypothesis stems from our recent observation that the rate of perceptual alternation in binocular rivalry is slow in euthymic subjects with bipolar disorder (n = 18, median = 0.27 Hz) compared with normal controls (n = 49, median = 0.60 Hz, p < 0.0005). We have presented evidence elsewhere that binocular rivalry is itself an interhemispheric switching phenomenon. The rivalry alternation rate (putative interhemispheric switch rate) is robust in a given individual, with a test-retest correlation of more than 0.8, making it suitable for genetic studies. The interhemispheric switch rate may provide a trait-dependent biological marker for bipolar disorder.

Molecular biology and the major psychoses

OBJECTIVE

To highlight the potential role of molecular biological studies in examining the expression of genes of interest in brain tissue to elucidate the pathophysiological basis of the major psychoses.

METHOD

To review the principles underlying the available techniques for expression studies.

RESULTS

Detection of messenger RNA by in situ hybridisation and quantitation by Northern analysis are powerful tools to detect abnormalities in gene expression in brain tissue.

CONCLUSION

The availability of simple techniques to examine the expression of RNA and protein products of individual genes, including examination at the level of individual cells, offers a clear opportunity to define the molecular basis of the major psychoses.

Exclusion of close linkage of bipolar disorder to the Gs-alpha subunit gene in nine Australian pedigrees

Growing evidence suggests that guanine nucleotide binding proteins (G proteins) may be involved in both the pathogenesis and treatment of bipolar affective disorder. Both overactive G proteins and increased levels of the alpha subunit of the stimulatory form (Gs-alpha) have been demonstrated in peripheral leucocytes of manic patients while an increase of Gs-alpha subunit levels has also been found in a postmortem study of bipolar disorder. The function of Gs and Gi alpha subunits has now been shown to be affected by lithium. The present study aimed to determine whether bipolar affective disorder was linked to the Gs-alpha subunit gene which has been mapped to chromosomal region 20q13.2. Linkage analysis utilized the PCR amplification of a portion of the Gs-alpha gene that contains a dinucleotide repeat (CA repeat) polymorphism. Linkage of bipolar disorder and recurrent depression to the Gs-alpha subunit gene was tested using a series of autosomal dominant and recessive models with varying penetrance levels. Additionally, linkage was examined using a series of levels of definitions of affective illness. Close linkage to the Gs-alpha subunit gene was strongly excluded using each model and definition. Thus, our study indicates that a genetic defect in the Gs-alpha subunit gene is unlikely to be the cause of bipolar disorder.