Association and linkage studies between bipolar affective disorder and the polymorphic CAG/CTG repeat loci ERDA1, SEF2-1B, MAB21L and KCNN3

de Paula V, Pires T, Michelon L, Camilo C, Esteban N, Pereira A, Schalling M, Vallada H. Shorter telomere length and suicidal ideation in familial bipolar disorder

Bipolar Disorder (BD) has recently been related to a process of accelerated aging, with shortened leukocyte telomere length (LTL) in this population. It has also been observed that the suicide rate in BD patients is higher than in the general population, and more recently the telomere length variation has been described as shorter in suicide completers compared with control subjects. Objectives The aim of the present study was to investigate if there is an association between LTL and BD in families where two or more members have BD including clinical symptomatology variables, along with suicide behavior. Methods Telomere length and single copy gene ratio (T/S ratio) was measured using quantitative polymerase chain reaction in a sample of 143 relatives from 22 families, of which 60 had BD. The statistical analysis was performed with a polygenic mixed model. Results LTL was associated with suicidal ideation (p = 0.02) as that there is an interaction between suicidal ideation and course of the disorder (p = 0.02). The estimated heritability for LTL in these families was 0.68. In addition, covariates that relate to severity of disease, i.e. suicidal ideation and course of the disorder, showed an association with shorter LTL in BD patients. No difference in LTL between BD patients and healthy relatives was observed. Conclusion LTL are shorter in subjects with familial BD suggesting that stress related sub-phenotypes possibly accelerate the process of cellular aging and correlate with disease severity and suicidal ideation.

Unstable repeat expansion in major psychiatric disorders: two decades on, is dynamic DNA back on the menu?

For a period in the mid-1990s, soon after the discovery of the involvement of trinucleotide repeat expansions in fragile-X syndrome (both A and E), Huntington's disease, myotonic dystrophy, and a number of hereditary ataxias, there was a clear sense that this new disease mechanism might provide answers for psychiatric disorders. Given the then failures to replicate initial genetic linkage findings for schizophrenia (SCZ) and bipolar disorder (BD), a greater emphasis was placed on the role of complex and non-Mendelian mechanisms, and repeat instability appeared to have the potential to provide adequate explanations for numerous apparently non-Mendelian features such as anticipation, incomplete penetrance, sporadic occurrence, and nonconcordance of monozygotic twins. Initial molecular studies using a ligation-based amplification method (repeat expansion detection) appeared to support the involvement of CAG•CTG repeat expansion in SCZ and BD. However, subsequent studies that dissected the large repeats responsible for much of the positive signal showed that there were three main loci where CAG•CTG repeat expansion was occurring (on 13q21.33, 17q21.33-q22, and 18q21.2). None of the expansions at these loci appeared to segregate with SCZ or BD, and research into repeat expansions in psychiatric illness petered out in the early 2000s. The 13q expansion occurs within a noncoding RNA and appears to be associated with spinocerebellar ataxia 8 (SCA8), but with a still unexplained dichotomy in penetrance - either very high or very low. The 17q expansion occurs within an intron of the carbonic anhydrase-like gene, CA10. The 18q expansion is located within an intron of the TCF4 gene. Mutations in TCF4 are a known cause of Pitt-Hopkins syndrome. Also, pertinently, genome-wide association studies have shown a well-replicated association between TCF4 and SCZ. Two decades on, in 2016, it appears to be an appropriate juncture to reflect on what we have learned, and, with the arrival of newer technologies, whether there is any mileage to be made in revisiting the unstable DNA hypothesis for psychiatric illness.

Genome wide association study identifies variants in NBEA associated with migraine in bipolar disorder

BACKGROUND

Migraine is a common comorbidity among individuals with bipolar disorder, but the underlying mechanisms for this co-occurrence are poorly understood. The aim of this study was to investigate the genetic background of bipolar patients with and without migraine.

METHODS

We performed a genome-wide association analysis contrasting 460 bipolar migraneurs with 914 bipolar patients without migraine from the Bipolar Genome Study (BiGS).

RESULTS

We identified one genome-wide significant association between migraine in bipolar disorder patients and rs1160720, an intronic single nucleotide polymorphism (SNP) in the NBEA gene (P=2.97 × 10(-8), OR: 1.82, 95% CI: 1.47-2.25), although this was not replicated in a smaller sample of 289 migraine cases.

LIMITATIONS

Our study is based on self-reported migraine.

CONCLUSIONS

NBEA encodes neurobeachin, a scaffolding protein primarily expressed in the brain and involved in trafficking of vesicles containing neurotransmitter receptors. This locus has not previously been implicated in migraine per se. We found no evidence of association in data from the GWAS migraine meta-analysis consortium (n=118,710 participants) suggesting that the association might be specific to migraine co-morbid with bipolar disorder.

Molecular and cellular basis of small--and intermediate-conductance, calcium-activated potassium channel function in the brain

Small conductance calcium-activated potassium (SK or K_Ca2) channels link intracellular calcium transients to membrane potential changes. SK channel subtypes present different pharmacology and distribution in the nervous system. The selective blocker apamin, SK enhancers and mice lacking specific SK channel subunits have revealed multifaceted functions of these channels in neurons, glia and cerebral blood vessels. SK channels regulate neuronal firing by contributing to the afterhyperpolarization following action potentials and mediating I_AHP, and partake in a calcium-mediated feedback loop with NMDA receptors, controlling the threshold for induction of hippocampal long-term potentiation. The function of distinct SK channel subtypes in different neurons often results from their specific coupling to different calcium sources. The prominent role of SK channels in the modulation of excitability and synaptic function of limbic, dopaminergic and cerebellar neurons hints at their possible involvement in neuronal dysfunction, either as part of the causal mechanism or as potential therapeutic targets.

Genética do transtorno bipolar

O Transtorno bipolar (TB) possui alta prevalência na população mundial e causa perdas significativas na vida dos portadores. É uma doença cuja herança genética se caracteriza por mecanismos complexos de transmissão envolvendo múltiplos genes. Na tentativa de identificar genes de vulnerabilidade para o TB, várias estratégias de investigação genética têm sido utilizadas. Estudos de ligação apontam diversas regiões cromossômicas potencialmente associadas ao TB, cujos marcadores ou genes podem ser candidatos para os estudos de associação. Genes associados aos sistemas monoaminérgicos e vias de sinalização intracelulares são candidatos para investigação da etiologia genética do TB. Novas técnicas de mapeamento de expressão gênica em tecidos especializados apontam para novos genes cujas mutações possam ser responsáveis pelo aparecimento da doença. Em virtude da complexidade do modo de transmissão do TB e de sua heterogeneidade fenotípica, muitas dificuldades são encontradas na determinação desses genes de vulnerabilidade. Até o momento, há apenas resultados preliminares identificando alguns genes associados à vulnerabilidade para desenvolver o TB. Entretanto, a compreensão crescente dos mecanismos epigenéticos de controle da expressão gênica e a abordagem dimensional dos transtornos mentais podem colaborar nas investigações futuras em genética psiquiátrica.

Novel CAG/CTG repeat expansion mutations do not contribute to the genetic risk for most cases of bipolar disorder or schizophrenia

The possible presence of anticipation in bipolar affective disorder and schizophrenia has led to the hypothesis that repeat expansion mutations could contribute to the genetic etiology of these diseases. Using the repeat expansion detection (RED) assay, we have systematically examined genomic DNA from 100 unrelated probands with schizophrenia and 68 unrelated probands with bipolar affective disorder for the presence of CAG/CTG repeat expansions. Our results show that 28% of the probands with schizophrenia and 30% of probands with bipolar disorder have a CAG/CTG repeat in the expanded range, but that each expansion could be explained by one of three nonpathogenic repeat expansions known to exist in the general population. We conclude that novel CAG/CTG repeat expansions are not a common genetic risk factor for bipolar disorder or schizophrenia.

Studies of offspring of parents with bipolar disorder

Children and adolescents who are the biological offspring of individuals with bipolar disorder (BD) (bipolar offspring) represent a population rich in potential for revealing important aspects in the development of BD. Multiple cross-sectional assessments of psychopathology in bipolar offspring have confirmed high incidences of BD, as well as mood and behavioral disorders, and other psychopathology in this population. Longitudinal studies of offspring have begun to shed light on precursors of BD development. Other assessments of bipolar offspring have included dimensional reports of psychiatric and psychosocial functioning, temperament assessments, and descriptions of family environments and parenting styles. Neurobiological studies in bipolar offspring are just beginning to yield findings that may be related to the underlying neuropathophysiology of BD. The future holds promise for longitudinal studies of bipolar offspring incorporating all of these facets, including genetic analyses, to further elucidate the factors involved in the evolution of BD.

Anticipation and repeat expansion in bipolar disorder

Anticipation is the phenomenon whereby a disease becomes more severe and/or presents with earlier onset as it is transmitted down through generations of a family. The only known mechanism for true anticipation is a class of mutations containing repetitive sequences exemplified by the pathogenic trinucleotide repeat. Studies of bipolar disorder (BPD) are consistent with the presence of anticipation and, by inference, the possibility that trinucleotide repeats contribute to this disorder, although it is possible that these data are the result of methodological problems. On the assumption that anticipation in BPD may be real, several surveys of the genome of BPD probands for large trinucleotide repeats have been conducted, as have studies of many repeat-containing candidate genes. No pathogenic triplet repeat has yet been unambiguously implicated.

CAG-repeat length in exon 1 of KCNN3 does not influence risk for schizophrenia or bipolar disorder: a meta-analysis of association studies

Schizophrenia and bipolar disorder both show some evidence for genetic anticipation. In addition, significant expansion of anonymous CAG repeats throughout the genome has been detected in both of these disorders. The gene KCNN3, which codes for a small/intermediate conductance, calcium-regulated potassium channel, contains a highly polymorphic CAG-repeat array in exon 1. Initial evidence for association of both schizophrenia and bipolar disorder with increased CAG-repeat length of KCNN3 has not been consistently replicated. In the present study, we performed several meta-analyses to evaluate the pooled evidence for association with CAG-repeat length of KCNN3 derived from case-control and family-based studies of both disorders. Each group of studies was analyzed under two models, including a test for direct association with repeat length, and a test for association with dichotomized repeat-length groups. No evidence for a linear relationship between disease risk and repeat length was observed, as all pooled odds ratios approximated 1.0. Results of dichotomized allele-group analyses were more variable, especially for schizophrenia, where case-control studies found a significant association with longer repeats but family-based studies implicated shorter alleles. The results of these meta-analyses demonstrate that the risks for both schizophrenia and bipolar disorder are largely, if not entirely, independent of CAG-repeat length in exon 1 of KCNN3. This study cannot exclude the possibility that some aspect of this polymorphism, such as repeat-length disparity in heterozygotes, influences risk for these disorders. Further, it remains unknown if this polymorphism, or one in linkage disequilibrium with it, contributes to some distinct feature of the disorder, such as symptom severity or anticipation.

Triplet repeats and bipolar disorder

Anticipation, the phenomenon of a disease becoming more severe or having earlier onset as it is transmitted down the generations, was originally described in families with psychiatric illness but was thought due to ascertainment bias and became forgotten. Interest was rekindled when a number of neurodegenerative disorders that show this phenomenon, were found to be due to a novel form of mutation--unstable triplet repeats showing intergenerational expansion. Some recent studies of anticipation are consistent with its occurrence in bipolar disorder but are still associated with methodological problems making interpretation difficult. A number of case-control studies employing the repeat expansion detection (RED) technique have found longer repeats in bipolar probands but other studies have found no such association. Despite a large number of studies examining the role of various repeat containing candidate genes, a pathogenic triplet repeat has yet to be found for bipolar disorder. It is likely that the controversy surrounding anticipation and the existence of triplet repeats will only finally be resolved with the demonstration of such a mutation in the aetiology of bipolar disorder.

Studies of anticipation in bipolar affective disorder

Anticipation refers to the increase in disease severity or decrease in age of onset in successive generations. The concept evolved from the theories and dogma of degeneration that were pervasive in psychiatry and medicine in the late 19th century and into the early 20th century. The term was set aside with the criticism of geneticist Lionel Penrose, who argued that anticipation was the result of ascertainment biases. The renewed interest in anticipation followed the identification of its molecular genetic basis in the form of unstable trinucleotide repeats. Subsequently, several diseases have been studied clinically for the presence of anticipation. Although anticipation has been identified in many diseases, including bipolar disorder, only diseases showing a pattern of progressive neurodegeneration have been associated with unstable trinucleotide repeats. This review summarizes the research on anticipation in bipolar disorder and other secular trends in the patterns of the illness such as the cohort effect. The changing nature of bipolar disorder is likely to be a result of combined influences from several genes, some of which are likely to be in a state of flux, as well as environmental or cultural forces that converge to give the clinical picture of anticipation.