Association between brain-derived neurotrophic factor genetic polymorphism Val66Met and susceptibility to bipolar disorder: a meta-analysis

Associations of BDNF/BDNF-AS SNPs with Depression, Schizophrenia, and Bipolar Disorder

Brain-Derived Neurotrophic Factor (BDNF) is crucial for various aspects of neuronal development and function, including synaptic plasticity, neurotransmitter release, and supporting neuronal differentiation, growth, and survival. It is involved in the formation and preservation of dopaminergic, serotonergic, GABAergic, and cholinergic neurons, facilitating efficient stimulus transmission within the synaptic system and contributing to learning, memory, and overall cognition. Furthermore, BDNF demonstrates involvement in neuroinflammation and showcases neuroprotective effects. In contrast, BDNF antisense RNA (BDNF-AS) is linked to the regulation and control of BDNF, facilitating its suppression and contributing to neurotoxicity, apoptosis, and decreased cell viability. This review article aims to comprehensively overview the significance of single nucleotide polymorphisms (SNPs) in BDNF/BDNF-AS genes within psychiatric conditions, with a specific focus on their associations with depression, schizophrenia, and bipolar disorder. The independent influence of each BDNF/BDNF-AS gene variation, as well as the interplay between SNPs and their linkage disequilibrium, environmental factors, including early-life experiences, and interactions with other genes, lead to alterations in brain architecture and function, shaping vulnerability to mental health disorders. The potential translational applications of BDNF/BDNF-AS polymorphism knowledge can revolutionize personalized medicine, predict disease susceptibility, treatment outcomes, and guide the selection of interventions tailored to individual patients.

Treatment-induced mood switching in affective disorders

Many patients under treatment for mood disorders, in particular patients with bipolar mood disorders, experience episodes of mood switching from one state to another. Various hypotheses have been proposed to explain the mechanism of mood switching, spontaneously or induced by drug treatment. Animal models have also been used to test the role of psychotropic drugs in the switching of mood states. We examine the possible relationship between the pharmacology of psychotropic drugs and their reported incidents of induced mood switching, with reference to the various hypotheses of mechanisms of mood switching.

BDNF polymorphisms across the spectrum of psychiatric morbidity: A protocol for a systematic review and meta-analysis

BACKGROUND

Nowadays, psychiatric morbidities are more and more common, which imposes huge social and economic burden on all countries across the world. Mental illnesses are found to be related to genetics. Over the past few years, a large number of risk genes and loci related to psychiatric morbidities have been reported. The brain-derived neurotrophic factor (BDNF) is one of the main candidates in neuropsychiatric genetics. However, different studies have shown inconsistencies regarding effect modification of BDNF polymorphisms on psychiatric morbidities. Therefore, in the present study, we aim to qualitatively and quantitatively summarize the relationship between BDNF single nucleotide polymorphisms (SNPs) and various psychiatric morbidities through a meta-analysis.

METHODS

PubMed, Web of Science and Embase will be searched using a specified search strategy to identify relevant studies up to April 2020. The meta-analysis will be performed on (1) allele model, (2) dominant model, (3) recessive model, (4) homozygote, and (5) heterozygote model. Sensitivity analyses will be conducted to explore the impact of individual studies on the overall result by evaluating the odd ratios (ORs) with their corresponding 95% confidence intervals (CIs) before and after removing each of the studies from our meta-analysis. Chi-square test will be used to determine whether the observed allele or genotype frequencies in the controls are consistent with HWE. The statistical heterogeneity will be verified by I statistics. The fixed effects model is needed to estimate the ORs and 95% CIs when there was no heterogeneity between results of included studies (I < 50%); instead, the random effects model should be used when results of included studies showed significant heterogeneity (I > 50%). Publication bias will be evaluated with the use of Begg test and Egger test (P < .05 is considered statistically significant).

DISCUSSION

With this protocol, a methodology is established that explores the effect modification of BDNF polymorphisms on the association with psychiatric morbidities. Findings from this meta-analysis can provide significant insight into the etiology of psychiatric morbidities.

REGISTRATION

Open Science Framework (OSF) Preregistration. September 15, 2020. OSF.IO/QS7XT.

Brain-derived neurotrophic factor and mental disorders

Brain-derived neurotrophic factor (BDNF) is a neurotrophin that modulates neuroplasticity in the brain, and is one of the most widely investigated molecule in psychiatric disorders. The researches of BDNF emcompassed the advance of investigative techniques of past decades. BDNF researches ranged from protein quantilization, to RNA expression measurements, to DNA sequencing, and lately but not lastly, epigenetic studies. In this review, we will briefly address findings on BDNF protein levels, mRNA expression, Val66Met polymorphism, and epigenetic modifications, in schizophrenia, major depressive disorder (MDD), and bipolar disorder.

Pattern of cortical thinning associated with the BDNF Val66Met polymorphism in Parkinson's disease

The brain-derived neurotrophic factor (BDNF) val⁶⁶met polymorphism has been suggested to modulate cognitive deterioration in Parkinson's disease (PD). In particular, the val/val genotype has been recently suggested to increase the risk of cognitive decline in this population. However, to date, little is known about the underlying brain alterations responsible for this association. Here, in a cohort of 93 early PD patients with preserved cognition from the Parkinson's Progression Markers Initiative (PPMI), we found that BDNF val/val patients experience an increased cortical atrophy rate with respect to met carriers in frontal and posterior-cortical regions (p<0.05, corrected). Additionally, BDNF val/val PD patients showed lower I¹²³-ioflupane SPECT DAT uptake in the contralateral caudate region (p=0.017) than met carriers, suggesting an increased striatal dopaminergic degeneration, which represents the pathological hallmark of PD. None of these observations were found in a sample of 38 healthy control (HC) subjects of comparable age and gender. We also observed an interaction effect on brain structure between the BDNF and APOE genotypes: cortical atrophy was associated with harboring the apoliprotein E (APOE) ε4 allele only in BDNF val/met subjects (both in HC and PD groups). Overall, these findings suggest that harboring the BDNF val/val genotype in PD leads to a set of cortical and subcortical brain alterations that could promote cognitive decline in this population.

The impact of BDNF Val66Met polymorphism on cognition in Bipolar Disorder: A review: Special Section on "Translational and Neuroscience Studies in Affective Disorders" Section Editor, Maria Nobile MD, PhD. This Section of JAD focuses on the relevance of translational and neuroscience studies in providing a better understanding of the neural basis of affective disorders. The main aim is to briefly summaries relevant research findings in clinical neuroscience with particular regards to specific innovative topics in mood and anxiety disorders

BACKGROUND

Converging lines of evidence suggest that Brain-Derived Neurotrophic Factor (BDNF) may play a central role in the pathogenesis of Bipolar Disorder (BD), thus representing a valid biomarker of the disease. A common genetic variation in the BDNF gene, the Val66Met, is associated with reduced maturation and secretion of BDNF and therefore it has been related to specific mood, cognitive and neuroanatomical alterations in BD. However, so far, only a handful of studies have investigated the association between Val66Met polymorphism and cognitive functioning in BD.

METHODS

We performed a bibliographic search on PUBMED of all genetic studies investigating Val66Met modulation on cognitive performances in BD subjects. The inclusion criteria were met by nine studies, including a total amount of 897 BD subjects and 803 healthy controls.

RESULTS

From the analysis of the existing literature emerged that a) Val allele in BD adults, but not in BD adolescents, was associated with better performances in selective cognitive domains including executive functions, verbal learning and memory; b) Met allele may negatively modulate the association between childhood trauma and performances in memory, verbal ability and verbal fluency tasks; c) Met allele may also negatively regulate structural abnormalities in cognitive cerebral structures; d) Val/Met carriers showed greater improvements in cognitive functions compared to Val/Val and Met/Met carriers.

LIMITATIONS

Few genetic studies exploring the impact of Val66Met on cognition in BD.

CONCLUSIONS

Val66Met polymorphism likely modulates cognitive functions in BD patients with complex gene-environment interactions and through potential modulations of cerebral structures. Further and larger genetic studies are required in order to detect association between BDNF polymorphism, BDNF levels, brain abnormalities and cognition in BD.

Critical Issues in BDNF Val66Met Genetic Studies of Neuropsychiatric Disorders

Neurotrophins have been implicated in the pathophysiology of many neuropsychiatric diseases. Brain-derived neurotrophic factor (BDNF) is the most abundant and widely distributed neurotrophin in the brain. Its Val66Met polymorphism (refSNP Cluster Report: rs6265) is a common and functional single-nucleotide polymorphism (SNP) affecting the activity-dependent release of BDNF. BDNF Val66Met transgenic mice have been generated, which may provide further insight into the functional impact of this polymorphism in the brain. Considering the important role of BDNF in brain function, more than 1,100 genetic studies have investigated this polymorphism in the past 15 years. Although these studies have reported some encouraging positive findings initially, most of the findings cannot be replicated in following studies. These inconsistencies in BDNF Val66Met genetic studies may be attributed to many factors such as age, sex, environmental factors, ethnicity, genetic model used for analysis, and gene–gene interaction, which are discussed in this review. We also discuss the results of recent studies that have reported the novel functions of this polymorphism. Because many BDNF polymorphisms and non-genetic factors have been implicated in the complex traits of neuropsychiatric diseases, the conventional genetic association-based method is limited to address these complex interactions. Future studies should apply data mining and machine learning techniques to determine the genetic role of BDNF in neuropsychiatric diseases.

Recent trends on the role of epigenomics, metabolomics and noncoding RNAs in rationalizing mood stabilizing treatment

Mood stabilizers are the cornerstone in treatment of mood disorders, but their use is characterized by high interindividual variability. This feature has stimulated intensive research to identify predictive biomarkers of response and disentangle the molecular bases of their clinical efficacy. Nevertheless, findings from studies conducted so far have only explained a small proportion of the observed variability, suggesting that factors other than DNA variants could be involved. A growing body of research has been focusing on the role of epigenetics and metabolomics in response to mood stabilizers, especially lithium salts. Studies from these approaches have provided new insights into the molecular networks and processes involved in the mechanism of action of mood stabilizers, promoting a systems-level multiomics synergy. In this article, we reviewed the literature investigating the involvement of epigenetic mechanisms, noncoding RNAs and metabolomic modifications in bipolar disorder and the mechanism of action and clinical efficacy of mood stabilizers.

BDNF and BMI effects on brain structures of bipolar offspring: results from the global mood and brain science initiative

OBJECTIVE

To compare brain-derived neurotrophic factor (BDNF) levels between offspring of individuals with bipolar disorders (BD) and healthy controls (HCs) and investigate the effects of BDNF levels and body mass index (BMI) on brain structures.

METHOD

Sixty-seven bipolar offspring and 45 HCs were included (ages 8-28). Structural images were acquired using 3.0 Tesla magnetic resonance imaging. Serum BDNF levels were measured using enzyme-linked immunosorbent assay. Multivariate and univariate analyses of covariance were conducted.

RESULTS

Significantly higher BDNF levels were observed among bipolar offspring, relative to HCs (P > 0.025). Offspring status moderated the association between BDNF and BMI (F₁ =4.636, P = 0.034). After adjustment for relevant covariates, there was a trend for a significant interaction of group and BDNF on neuroimaging parameters (Wilks'λ F_56,94 =1.463, P = 0.052), with significant effects on cerebellar white matter and superior and middle frontal regions. Brain volume and BDNF were positively correlated among HCs and negatively correlated among bipolar offspring. Interactions between BDNF and BMI on brain volumes were non-significant among HCs (Wilks'λ F_28,2 =2.229, P = 0.357), but significant among bipolar offspring (Wilks'λ F_28,12 =2.899, P = 0.028).

CONCLUSION

Offspring status and BMI moderate the association between BDNF levels and brain structures among bipolar offspring, underscoring BDNF regulation and overweight/obesity as key moderators of BD pathogenesis.

Biological hypotheses and biomarkers of bipolar disorder

The most common mood disorders are major depressive disorders and bipolar disorders (BD). The pathophysiology of BD is complex, multifactorial, and not fully understood. Creation of new hypotheses in the field gives impetus for studies and for finding new biomarkers for BD. Conversely, new biomarkers facilitate not only diagnosis of a disorder and monitoring of biological effects of treatment, but also formulation of new hypotheses about the causes and pathophysiology of the BD. BD is characterized by multiple associations between disturbed brain development, neuroplasticity, and chronobiology, caused by: genetic and environmental factors; defects in apoptotic, immune-inflammatory, neurotransmitter, neurotrophin, and calcium-signaling pathways; oxidative and nitrosative stress; cellular bioenergetics; and membrane or vesicular transport. Current biological hypotheses of BD are summarized, including related pathophysiological processes and key biomarkers, which have been associated with changes in genetics, systems of neurotransmitter and neurotrophic factors, neuroinflammation, autoimmunity, cytokines, stress axis activity, chronobiology, oxidative stress, and mitochondrial dysfunctions. Here we also discuss the therapeutic hypotheses and mechanisms of the switch between depressive and manic state.

State-dependent increase in the levels of neurotrophin-3 and neurotrophin-4/5 in patients with bipolar disorder: A meta-analysis

Bipolar disorder (BD) is one of the most serious psychiatric disorders in the world, but its pathophysiology is still unclear. Regulation of neurotrophic factors have been thought to play a role in this process. There have been inconsistent findings regarding the differences in blood neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5) between patients with BD and healthy controls (HCs). The aim of the current meta-analysis is to examine the changes in the levels of NT-3 and NT-4/5 in BD patients at different affective states. Eight articles (including 465 BD patients and 353 HCs) were included in the analysis, and their results were pooled by using a random effects model. We found the levels of both NT-3 (p = 0.0046) and NT-4/5 (p = 0.0003) were significantly increased in BD patients, compared to HCs. Through subgroup analysis, this increase persisted only in patients in depressed state (p = 0.0038 for NT-3 and p = 0.0001 for NT-4/5), but not in manic or euthymic state. In addition, we found the differences in NT-3 and NT-4/5 were significantly associated with the duration of illness, but not by the mean age or female proportion. Our results suggest a state-dependent increase in NT-3 and NT-4/5 levels in patients with BD. Further studies are needed to examine dynamic changes of these neurotrophins in BD patients along the disease course.

Association between obesity and the brain-derived neurotrophic factor gene polymorphism Val66Met in individuals with bipolar disorder in Mexican population

BACKGROUND

The brain-derived neurotrophic factor (BDNF) has been considered as an important candidate gene in bipolar disorder (BD); this association has been derived from several genetic and genome-wide studies. A polymorphic variant of the BDNF (Val66Met) confers some differences in the clinical presentation of affective disorders. In this study, we evaluated a sample population from Mexico City to determine whether the BDNF (rs6265) Val66Met polymorphism is associated with the body mass index (BMI) of patients with BD.

METHODS

This association study included a sample population of 357 individuals recruited in Mexico City. A total of 139 participants were diagnosed with BD and 137 were classified as psychiatrically healthy controls (all individuals were interviewed and evaluated by the Diagnostic Interview for Genetic Studies). Genomic DNA was extracted from peripheral blood leukocytes. The quantitative polymerase chain reaction (qPCR) assay was performed in 96-well plates using the TaqMan Universal Thermal Cycling Protocol. After the PCR end point was reached, fluorescence intensity was measured in a 7,500 real-time PCR system and evaluated using the SDS v2.1 software, results were analyzed with Finetti and SPSS software. Concerning BMI stratification, random groups were defined as follows: normal <25 kg/m(2), overweight (Ow) =25.1-29.9 kg/m(2), and obesity (Ob) >30 kg/m(2).

RESULTS

In the present work, we report the association of a particular BMI phenotype with the presence of the Val66Met allele in patients with BD (P=0.0033 and odds ratio [95% confidence interval] =0.332 [157-0.703]), and correlated the risk for valine allele carriers with Ow and Ob in patients with BD.

CONCLUSION

We found that the methionine allele confers a lower risk of developing Ow and Ob in patients with BD. We also confirmed that the G polymorphism represents a risk of developing Ow and Ob in patients with BD. In future studies, the haplotype analysis should provide additional evidence that BDNF may be associated with BD and BMI within the Mexican population.

Brain-derived neurotrophic factor genetic polymorphism (rs6265) is protective against chemotherapy-associated cognitive impairment in patients with early-stage breast cancer

Background

Brain-derived neurotrophic factor (BDNF), a neurotrophin that regulates neuronal function and development, is implicated in several neurodegenerative conditions. Preliminary data suggest that a reduction of BDNF concentrations may lead to postchemotherapy cognitive impairment. We hypothesized that a single nucleotide polymorphism (rs6265) of the BDNF gene may predispose patients to cognitive impairment. This study aimed to evaluate the effect of BDNF gene polymorphism on chemotherapy-associated cognitive impairment.

Methods

Overall, 145 patients receiving chemotherapy for early-stage breast cancer (mean age: 50.8 ± 8.8 y; 82.1% Chinese) were recruited. Patients' cognitive functions were assessed longitudinally using the validated Functional Assessment of Cancer Therapy–Cognitive Function (v.3) and an objective computerized tool, Headminder. Genotyping was performed using Sanger sequencing. Logistic regression was used to evaluate the association between BDNF Val66Met polymorphism and cognition after adjusting for ethnicity and clinically important covariates.

Results

Of the 145 patients, 54 (37%) reported cognitive impairment postchemotherapy. The Met/Met genotype was associated with statistically significant lower odds of developing cognitive impairment (odds ratio [OR] = 0.26; 95% CI: 0.08–0.92; P = .036). The Met carriers were less likely to experience impairment in the domains of verbal fluency (OR = 0.34; 95% CI: 0.12–0.90; P = .031) and multitasking ability (OR = 0.37; 95% CI: 0.15–0.91; P = .030) compared with the Val/Val homozygote. No associations were observed between Headminder and the BDNF Val66Met polymorphism.

Conclusions

This is the first study to provide evidence that carriers of the BDNF Met allele are protected against chemotherapy-associated cognitive impairment. Further studies are required to validate the findings.