Association of Brain‐Derived Neurotrophic Factor rs6265 G>A polymorphism and Post‐traumatic Stress Disorder susceptibility: A systematic review and meta‐analysis

Abstract Background Previous studies have shown that the brain‐derived neurotrophic factor (BDNF) rs6265 G > A polymorphism is closely related post‐traumatic stress disorder (PTSD) risk. However, the results were not consistent. We therefore conducted a meta‐analysis to explore the underlying relationships between BDNF rs6265 G > A polymorphism and PTSD risk. Materials and Methods Five online databases were searched, and all related studies were reviewed up to July 1, 2020. Odds ratios (ORs) and corresponding 95% confidence intervals (CIs) were calculated to examine the statistical power of each genetic model. In addition, heterogeneity, sensitivity accumulative analysis, and publication bias were examined to check the statistical power. Result Overall, 16 publications involving 5,369 subjects were included in this systematic review and 11 case‐control studies were analyses in meta‐analysis. The pooled results indicated an increasing risk of A allele mutations with PTSD risk. Moreover, the sequential subgroup analysis also demonstrated some similar situations in Asian populations and other groups. Conclusion Current meta‐analysis suggests that the BDNF rs6265 G > A polymorphism might be involved in PTSD susceptibility.

In recent decades, there has been increasing evidence that PTSD is caused by interactions between various neural and traumatic factors (Disner et al., 2018;Joshi et al., 2020). In addition to the requisite etiological factor of trauma exposure for its onset, the abnormal expression of and functional changes in some neurotransmitters and neurotrophins, such as dopamine, serotonin, and brain-derived neurotrophic factor (BDNF), are considered the most important factors that contribute to PTSD susceptibility (Miller et al., 2017;Rakofsky et al., 2012). BDNF is an important neurotrophic factor that participates in neuronal survival and growth-promotion in the central nervous system, particularly in the hippocampus Notaras & Buuse, 2020); alterations in BDNF levels are seen in the brain's fear circuit following trauma exposure (Burstein et al., 2018). BDNF expression, a potential biomarker for PTSD, is significantly lower in patients with PTSD than in healthy controls (Angelucci et al., 2014;Dell'Osso et al., 2009). However, a newly published meta-analysis indicated that BDNF levels were significantly higher in the PTSD group than in healthy controls (Mojtabavi et al., 2020). Some studies have speculated that this increase is accompanied by acute restoration or reconstruction of brain neurons in the early stages after a traumatic experience (Hauck et al., 2010;Matsuoka et al., 2013).
Animal-based research has also found over-expression of BDNF protein in the plasma and hippocampus of stressed rats relative to that in nonstressed controls in the compensatory stage (Faure et al., 2007;Zhang et al., 2014).
Rs6265 G > A is the most common single nucleotide polymorphism (SNP) locus in the 5′ promoter region of the BDNF gene, which is located on the short arm of chromosome 11p13. This gene variant involves a nucleotide substitution from guanine to adenine at position 196 in the BDNF coding region, resulting in a nonsynonymous amino acid alternation from valine (Val) to methionine (Met) in codon 66 of the BDNF prodomain . The Met allele exhibits abnormal intracellular trafficking and regulates the secretion of BDNF in comparison with the Val allele, which has always been suggested to be associated with lower BDNF release, resulting in a reduced release of activity-dependent dopamine when neurons are activated . In terms of PTSD, Met allele carriers exhibit increased activity in neural structures and appear to be more susceptible to disease development (Lonsdorf et al., 2015). To date, the rs6265 G > A polymorphism has been shown to be associated with many central nervous system diseases, such as Alzheimer's disease, Parkinson's disease, depression, and suicide (Aldoghachi et al., 2019;Brown et al., 2020;Wang et al., 2015). In 2006, Zhang et al. conducted the first case-control study on the association between the BDNF rs6265 G > A polymorphism and PTSD susceptibility and found no significant association in a US population (Zhang et al., 2006). Since then, many studies have been published but the association between BDNF rs6265 G > A polymorphism and PTSD susceptibility remains controversial. Considering the inconsistencies among published studies, we conducted this meta-analysis to further elucidate the association between BDNF rs6265 G > A polymorphism and PTSD susceptibility.

| MATERIAL S AND ME THODS
This meta-analysis was conducted according to the guidelines of the preferred reporting items for systematic reviews and meta-analyses (PRISMA) statement (Moher et al., 2009). All collected information was obtained from all published articles, and no ethical approval was necessary.

| Inclusion and exclusion criteria
The following criteria were used to identify relevant studies: (a) only case-control and cohort studies were selected; (b) studies on the association between BDNF rs6265 G > A polymorphism and PTSD susceptibility were selected, and subsequent meta-analyses were conducted with studies in which the P value of the Hardy-Weinberg

| Data extraction and quality evaluation
Two authors (Hu and Wu) independently reviewed all included studies and extracted the following information: the name of the first author, publication year, country and subject ethnicity, control design (healthy control or controls with traumatic exposures but without a PTSD diagnosis [PTSD − ]), genotyping method, sample sizes of cases and controls, frequency information for the genotype distribution of the case and control groups, traumatic factors, assessment of the HWE in the control group, age distribution, and diagnostic criteria.

| Statistical analysis
Crude odds ratios (ORs) and 95% confidence intervals (CIs) were calculated to examine the association between the BDNF rs6265 G > A polymorphism and PTSD susceptibility. GG + GA). Heterogeneity among the included studies was examined using Cochran's Q and I 2 tests (Huedo-Medina et al., 2006). A random effects model was adopted when I 2 > 40%; otherwise, a fixed effects model was adopted (DerSimonian, 1996;Mantel & Haenszel, 1959). All statistical analyses, including cumulative analysis, sensitivity analysis, publication biases, and subgroup analysis, were conducted with studies that satisfied the HWE criterion. Subgroup analyses were conducted based on differences in ethnicity, control design, traumatic factors, and sex differences.
Cumulative meta-analysis and sensitivity analysis were conducted to explore the tendency and verify the stability of results according to the shifting of dates. Potential publication biases were detected using Egger's linear regression test and Begg's funnel plots (Begg & Mazumdar, 1994;Egger et al., 1997). All statistical analyses were conducted using STATA version 14.0 (Stata Corporation, College Station, TX, USA). A value of p <.05 (two-sided) was considered statistically significant.

| Study characteristics
The selection process is illustrated in Figure 1. First, 355 potential case-control studies were identified using a detailed search strategy.

| Meta-analysis
After selecting studies according to their HWE status, two studies and three studies were removed because the P value of the HWE test was less than 0.05, or unavailable, respectively. Eleven studies involving 1,228 PTSD patients and 2,613 controls were included in the meta-analysis (Bruenig et al., 2016;Dretsch et al., 2016;Guo et al., 2019;Heon-Jeong et al., 2006;Hori et al., 2020;Li et al., 2016;Lyoo et al., 2011;Pivac et al., 2012;Qi et al., 2020;Valente et al., 2011;Zhang et al., 2006).   ( Figure 3 for A versus. G model). Accumulative analysis was also performed and showed a progressively increasing effect on PTSD risk ( Figure 4 for A versus. G model).

| Publication bias
Publication bias was evaluated, and funnel plots did not demonstrate any significant asymmetry ( Figure 5

| D ISCUSS I ON
PTSD is a serious disorder that occurs after experiencing unusual psy- Many studies have shown that the occurrence of PTSD is often accompanied by damage to brain tissue, which in turn leads to neuronal cell dysfunction (Nampiaparampil, 2008). BDNF is distributed across multiple brain regions and plays a key role in neurophysiological processes, such as neuroprotection, maturation, repair, and maintenance of neurons. Several studies have shown that the expression levels and protein activity of BDNF are important for neurophysiological regulation (Ji et al., 2015). BDNF production and activity can be genetically determined and controlled using mutated sequence regions in its gene. In mammalian, BDNF is critically involved in synaptic plasticity and is implicated in hippocampusdependent learning and memory (Bramham & Messaoudi, 2005;Hariri et al., 2003); memory abnormalities are considered a core feature of PTSD, and patients with PTSD always present with a negative memory bias relative to healthy controls (Itoh et al., 2019). The abnormal involuntary recovery of traumatic memories, including invasive thoughts, flashbacks, and nightmares, often causes great mental pain. Recent studies have indicated that this BDNF polymorphism could dramatically alter the intracellular trafficking and packaging of pro-BDNF, subsequently regulating the secretion of mature peptides . Individuals with the Met allele always present with a lower level of hippocampal N-acetyl aspartate compared to those with the Val allele. In knock-in mice, extinction learning was impaired in Met allele carriers compared to non-Met allele carriers (Soliman et al., 2010); there was a similar finding in patients with PTSD with the Met allele when compared to those with the Val allele (Felmingham et al., 2018). A human study by Horri et al. revealed that PTSD patients with the Met allele had significantly worse memory performance than controls, indicating that the rs6265 polymorphism could be involved more in core memory abnormalities than general memory dysfunction in PTSD (Hori et al., 2020).
To date, the rs6265 G > A locus has been suggested to correlate with a notably lower serum level, and negative effects that alter the  (Pivac et al., 2012).
Moreover, Li et al., Dretsch et al., and other researchers also reported an elevated risk of PTSD (Dretsch et al., 2016;Li et al., 2016). In contrast, Jin et al. found that the GG genotype may play a critical role in the occurrence of PTSD (Jin et al., 2019). In addition, Bruenig et al.
and others did not find any significant association between the BDNF s6265 G > A polymorphism and PTSD risk (Bruenig et al., 2016).
How can we reach a more precise conclusion regarding the relationship between BDNF rs6265 G > A polymorphism and PTSD risk with these current inconsistent results? To the best of our knowledge, meta-analysis is the most valuable method for resolving the current confusion due to the shortage of samples. In this meta-analysis, we examined the correlation between the BDNF rs6265 G > A polymorphism and PTSD susceptibility, based on 11 publications that met the inclusion criteria. In our meta-analysis, we comprehensively summarized the current evidence regarding the association between BDNF rs6265 G > A polymorphism and PTSD susceptibility. According to the pooled data, there was a significant correlation between the A mutation and an increased risk of PTSD. Subsequently, a subgroup analysis was conducted and a similar elevated risk was observed with this variant of PTSD susceptibility, especially in the Asian population and PTSD − groups. Our results indicate that that the mutation from  nine studies were included, but the results obtained were based on different genetic models. Moreover, only one genetic model was examined to determine the association between BDNF rs6265 G > A polymorphism and PTSD susceptibility, without any subgroup analysis or quantitative assessment (Bountress et al., 2017). Therefore, we conducted this meta-analysis to gain a better insight into the trends in the results from earlier publications.
To our knowledge, this systematic review and meta-analysis included all current publications that assessed the association between the BDNF rs6265 G > A polymorphism and PTSD susceptibility. There were some limitations to this study. First, the quantitative analysis was conducted with only 11 publications; the other studies were eliminated because of a lack of data or because the P value of the genotype distribution deviated from the HWE. Therefore, the inference based on a small sample size might be biased, leading to deviation in the pooled results. Second, the BDNF rs6265 G > A polymorphism was the only locus examined in this meta-analysis, and the interactive effects of different SNPs and other environmental or lifestyle factors were not assessed simultaneously. Third, most studies were based on Caucasian or Asian participants, and the current results may not be applicable to all populations. Finally, only reports published in Chinese or English were included in this meta-analysis, which may have resulted in a language bias. Despite these shortcomings, some positive aspects were found to enhance the quality of our study: (a) More studies were included than in previous analyses; (b) all five genetic models were examined in general and subgroups to explore the potential relationship between rs6265 G > A polymorphism and PTSD; (c) the statistics examined were taken from studies that satisfied the HWE for the genotype distribution in controls; (d) no significant heterogeneity was found, indicating a fair consistency among all included studies; and (e) no significant publication bias was found using Egger's test and Begg's funnel plots.

| CON CLUS ION
Our results from this meta-analysis suggest that the BDNF rs6265 G > A polymorphism is associated with PTSD susceptibility in Asian people. Further studies are needed to assess the association between this SNP and PTSD.

ACK N OWLED G M ENT
This study was supported by the Hubei Province health and family planning scientific research project (No. WJ2019F135).

CO N FLI C T S O F I NTE R E S T
The authors declare no competing financial interests.

AUTH O R CO NTR I B UTI O N
HXY, WYL, LXX, and ZL conceived the study. HXY and WYL searched the databases and extracted the data. HXY and CCH analyzed the data. HXY, LXX, and ZL wrote the draft of the paper. All the authors approved the final manuscript.

E TH I C A L A PPROVA L
All collected information was obtained from all published articles, and no ethical approval was necessary.

PE E R R E V I E W
The peer review history for this article is available at https://publo ns.com/publo n/10.1002/brb3.2118.

DATA AVA I L A B I L I T Y S TAT E M E N T
All data collected or analyzed for this study are available from the corresponding author upon reasonable request to any researchers.

R E FE R E N C E S
Albert, K., Pruessner, J., & Newhouse, P. (2015). Estradiol levels modulate brain activity and negative responses to psychosocial stress F I G U R E 5 Funnel plot analysis to detect publication bias for A versus. G model of BDNF rs6265 G > A polymorphism. Circles represent the weight of the studies