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Brain-derived neurotrophic factor (BDNF) may play a role in modulating memory function and there is growing evidence that the BDNF V166M polymorphism may influence episodic memory in humans. However, previous association studies examining this polymorphism and working memory are inconsistent. The current study examined this association in a large sample of adolescent twin-pairs and siblings (785 individuals from 439 families). A range of measures (event-related potential, general performance and reaction time) was obtained from a delayed-response working-memory task and total association was examined using the quantitative transmission disequilibrium tests (QTDT) program. Analyses had approximately 93–97% power (α= 0.05) to detect an association accounting for as little as 2% of the variance in the phenotypes examined. Results indicated that the BDNF V166M polymorphism is not associated with variation in working memory in healthy adolescents.
Brain-derived neurotrophic factor (BDNF) belongs to a family of highly conserved polypeptide growth factors and is an important component in the normal development of the central nervous system (CNS) and in modifying CNS structure and function in adults (Bibel & Barde, 2000; Huang & Reichardt 2001). In humans, it may play a role in the development of neurological conditions such as epilepsy (Scharfman 2005) and in psychiatric disorders such as schizophrenia and bipolar disorder (Fanous et al. 2005; Neves-Pereira et al. 2002).
Extensive studies in rats indicate that BDNF is essential for some forms of learning and memory (Yamada et al. 2002) and suggest an involvement in working memory. For instance, Mizuno et al. (2000) reported that infusion of antisense BDNF oligonucleotide (which was associated with a significant reduction of BDNF mRNA and protein levels in the hippocampus) impaired spatial memory formation, retention and/or recall in rats. Furthermore, frontal cortex BDNF levels are reported to correlate negatively with the number of working-memory errors in aged rats, but not young rats (Bimonte et al. 2003), and in Ts65D mice (an animal model of Down syndrome) (Bimonte-Nelson et al. 2003).
A growing number of studies suggest a role for BDNF in the function of human memory. Egan et al. were the first to examine the V166M polymorphism in the 5′ pro-region of the human BDNF protein in relation to human memory and hippocampal function (Egan et al. 2003) – this polymorphism has also been widely studied for a range of neurological and psychiatric conditions (e.g. Hall et al. 2003; Karamohamed et al. 2005). Egan and colleagues showed that relative to Val, the Met allele was associated with a qualitatively different hippocampal response, as assayed with functional magnetic resonance imaging during performance of an N-back working-memory task. More specifically, a robust and reliable hippocampal deactivation is normally produced in these tasks (Meyer-Lindenberg et al. 2001) and was observed in Val/Val individuals, but an abnormal pattern of increased activity was observed in Val/Met individuals. A subsequent study by Hariri et al. (2003) examined hippocampal and cortical function during the encoding and retrieval of novel, complex scenes – a task previously associated with increased hippocampal activation (Stern et al. 1996). They found hippocampal activity to be reduced in Met-allele carriers compared to Val/Val individuals during both encoding and retrieval. In addition to these observations of disrupted hippocampal function, reduced grey matter volumes for the hippocampus and dorsolateral prefrontal cortex (both regions known to subserve working memory (Fuster 1997)), have been reported for Met-allele carriers compared to Val homozygotes (Bueller et al. 2006; Pezawas et al. 2004; Szeszko et al. 2005).
Egan et al. also examined the association between the V166M polymorphism and a range of memory phenotypes (episodic, semantic, working) (Egan et al. 2003). They found an association with episodic memory [as assessed with the WMS but not the CVLT (Table 1)], such that Met/Met individuals performed poorly compared to Val-allele carriers. This was found in controls alone and also in their entire sample, which comprised individuals with schizophrenia, their siblings and controls. However, they found no association between the BDNF genotype and either semantic or working memory.
Table 1. Studies investigating associations between the BDNF V166M polymorphism and memory phenotypes
|Study||Sample (Age)||Memory phenotype (Test/Test Battery)*||Finding|
|Egan et al. (2003)||203 with schizophrenia||Episodic (WMS)||Met/Met associated with worse scores for episodic memory obtained from WMS, but not CVLT. No association found with semantic or working memory.|
| ||305 siblings||Episodic (CVLT)|| |
| ||133 healthy controls||Semantic (WCST)|| |
| ||(18–60 years)||Working (WCST)|| |
|Hariri et al. (2003)||64 healthy participants (30.9 ± 1.3 years Val/Val, 30.3 ± 1.6 years Val/Met & Met/Met)||Episodic (encode/retrieve fMRI paradigm)||Met allele associated with an increased number of recognition errors.|
|Rybakowski et al. (2003)||54 with bipolar (18–72 years, mean 46 years)||Working (WCST)||Met allele associated with significantly worse scores.|
|Strauss et al. (2004)||63 with major depression or dysthymic disorder (18.4 ± 2.5 years)||Episodic (WMS)||No association found.|
| ||Episodic (VPALT)|| |
|Dempster et al. (2005)||92 with schizophrenia 114 healthy relatives (17–85 years)†||Episodic (WMS)||Met allele associated with significantly worse score in relatives.|
|Tan et al. (2005)||108 with schizophrenia||Episodic (WMS)||Met/Met associated with significantly worse scores.|
|Rybakowski et al. (2006)||111 with bipolar (18–72 years, mean 43 years)||Working (WCST)||Met allele associated with significantly worse scores in bipolar patients for three of five subtests. No differences found for schizophrenia or control groups.|
| ||129 with schizophrenia (18–65 years, mean 27 years)|| |
| ||92 healthy controls (19–58 years, mean 31 years)|| |
|Echeverria et al. (2005)||194 male dentists||Episodic (BEES)||Met allele associated with 1 Poorer episodic memory in males but not females, and 2 Reduced working memory performance in females but not males – although result was in expected direction.|
| ||233 female dental assistants (all with chronic low-level mercury exposure)||Working (BEES)|| |
Overall, the evidence suggests that the human V166M polymorphism may influence episodic memory performance but with fewer studies, and with the largest study reporting a lack of association (Egan et al. 2003), the evidence for an effect on working memory is less convincing. The present study therefore examined the association between the V166M polymorphism and working memory in a large sample, with greater power than previous studies to detect association in healthy individuals. Furthermore, in addition to measures of overall performance and reaction time, P300 and slow-wave event-related potential (ERP) measures of brain function (all obtained from a delayed-response working-memory task) were examined.
It has been suggested that the hippocampus may be an important generator of the P300 (Fushimi et al. 2005; Halgren et al. 1980; Nakajima et al. 1995; Okada et al. 1983). Thus, as the BDNF V166M polymorphism has been associated with hippocampal function during an N-back working-memory task (Egan et al. 2003), it may also be associated with variation in the P300 elicited during a delayed-response working-memory task. Similarly, the V166M polymorphism has been associated with variation in grey matter volume of the dorsolateral prefrontal cortex (Pezawas et al. 2004) and this may be reflected in P300 and slow-wave measures recorded over the prefrontal brain region. Therefore, it was hypothesized that the P300 and the prefrontal slow wave may be associated with the V166M polymorphism. ERP measures have generally shown little association with behavioural working-memory performance (Hansell et al. 2005) and studies have generally found no association between behavioural measures of working memory and the V166M polymorphism in healthy individuals (Egan et al. 2003; Rybakowski et al. 2006). Consequently, it was hypothesized that no association would be found between the behavioural measures of working memory and the BDNF V166M polymorphism.
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The present study examined the association between the V166M polymorphism and working memory in healthy adolescents and had considerable power to do so (approximately 93–97% power to detect an association accounting for 2% of the variance of the phenotypes examined). A range of measures obtained from a delayed-response working-memory task, including both event-related potential and behavioural performance measures, were examined. No associations were found.
A finding of no association was consistent with that hypothesized for the non-ERP behavioural measures, which was based upon previous findings (Egan et al. 2003; Rybakowski et al. 2006). However, a lack of association was contrary to that hypothesized for the ERP measures, in particular, for the P300 measures and for slow wave recorded over the prefrontal brain regions.
The hippocampal formation is thought to be a major generator of the P300 (e.g. Okada et al. 1983) and Met-allele carriers are reported to have reduced hippocampal volumes (e.g. Bueller et al. 2006) and abnormal hippocampal function during an N-back working-memory task (Egan et al. 2003). However, the results indicate that any BDNF-related hippocampal deficits did not affect the generation of the P300 component during a delayed-response task. This may be because the P300 has multiple generators (e.g. Halgren et al. 1998) and/or because BDNF-related differences in hippocampal structure and function are not sufficient to affect P300 generation, at least in healthy adolescents. Similarly, any BDNF-related differences in grey matter volumes in the dorsolateral prefrontal cortex (Pezawas et al. 2004) did not appear to influence ERPs recorded over the prefrontal regions.
Consistent findings of an association between the V166M polymorphism and episodic but not working memory may reflect a greater reliance on hippocampal function for episodic versus working memory. A large literature indicates that the hippocampus plays a critical role in episodic memory (Squire et al. 2004). For instance, a recent study by Kramer et al. (2005) examined episodic memory in a sample of neurodegenerative patients and normal older controls, and showed that hippocampal volume predicted recall and recognition memory accuracy, but that frontal lobe, anterior temporal lobe and posterior cortex volumes did not.
In contrast, while the hippocampus is considered critical to the formation of memory networks in general (Fuster 1997), the prefrontal cortex appears to be of vital importance to working-memory function and its role has been widely studied in this context (Fuster 2001). Van Asselen et al. (2006) recently showed the importance of both the prefrontal cortex and the hippocampus to working memory. They examined working memory in stroke patients and healthy controls and found that performance was impaired by damage to the right dorsolateral prefrontal cortex and the bilateral hippocampal formation, in addition to the right posterior parietal cortex. Interestingly, patients with bipolar disorder, for whom an association between the V166M polymorphism and working memory has been found (Rybakowski et al. 2003, 2006), are reported to show impaired prefrontal and hippocampal function during memory-related encoding (Deckersbach et al. 2006). It is also interesting to note that there is some evidence to suggest that the effect of the BDNF V166M genotype may be greater in patient groups than in healthy volunteers (for influence on volume of the hippocampal formation see Szeszko et al. 2005; and for influence on working memory and executive functions see Rybakowski et al. 2006).
A limitation of the current study may be its focus on adolescents. The limited age range provides insight into the genotype/phenotype association in adolescents, but these insights may not be transferable to other age groups. Working memory tasks engage the prefrontal cortex (Fuster 2001) and the human prefrontal cortex is not functionally or structurally mature until early adulthood (Giedd et al. 1999; Hudspeth & Pribram 1992; Levin et al. 1991; Sowell et al. 1999). Furthermore, Webster et al. have shown that in relation to total mRNA, BDNF mRNA levels in the dorsolateral prefrontal cortex are relatively low in adolescents compared to young adults, adults, and aged individuals (Webster et al. 2002). In addition, during adolescence BDNF may be more involved in regulating neuronal morphology and synaptic pruning than in the maintenance of connectivity and synaptic plasticity as in the mature cortex (Webster et al. 2002). A further limitation of the study may be that the delayed-response task is not a standard task used to measure working memory at a behavioural level.
In conclusion, of studies examining the association between the BDNF V166M genotype and memory, this is the first to examine the following:
Phenotypes from a working-memory delayed-response task
Event-related potential measures of memory function, and
Data from a large sample of healthy adolescents.
Furthermore, the study had considerably more power to examine this association in healthy individuals than previous studies. For all measures examined (electrophysiological, general performance and reaction time) no association was found. Therefore, while the loss of efficiency in CNS function associated with the Met allele (Chen et al. 2004) may affect longer-term memory processes underlying episodic memory, in healthy adolescents this loss of efficiency does not appear to affect the shorter-term processes underlying working memory.
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We thank Marlene Grace, Ann Eldridge and Kathleen Moore for twin recruitment and data collection, Megan Campbell for preparation of DNA samples, Troy Dumenil for SNP typing, and the twins and their families for their participation. Phenotype collection was supported by the Australian Research Council (A79600334 (N.M.), A79801419 (G.G., N.M., L.G., M.W.), A79906588 (N.M., G.S., G.G., L.G., M.W.), DP0212016 (N.M., M.W., G.G., L.G.) and the Human Frontier Science Program [RG0154.1998-B (D.B., N.M., J.A.)].