Our study represents the first investigation of Bcl-2 influences on age-related changes in brain morphology in healthy participants over a wide age range. The regional GM volumes of the right cerebellum, bilateral lingual gyrus, right middle temporal gyrus, and right parahippocampal gyrus were inversely correlated with age. However, the downward slope of the age-related reduction in GM was steeper in the A-allele carriers than in G homozygotes. Our findings support the hypothesis that Bcl-2 polymorphism may influence aging processes in the brain, and that the G/G allelic variant confers partial protection against age-related decreases in brain volume.
Many neuropathological studies have shown that normal aging is characterized by a substantial and extensive loss of neurons in the cerebral cortex. Morphometric imaging studies have demonstrated that aging predominantly affects the GM, including cortical and deep GM structures and the cerebellum 
. We found an accelerated loss in regional GM volumes with aging, which is consistent with the findings of previous studies 
Bcl-2 has been shown to regulate neuronal cell death during normal development, and has also been implicated in many models of acute and chronic neurodegeneration 
. Bcl-2 expression in the brain is up-regulated in Parkinson disease 
and Alzheimer disease, with Bcl-2 expression increasing with increased disease severity 
. The over-expression of Bcl-2 inhibits neuronal cell death in vitro 
and in vivo 
. Tanabe et al. 
showed that endogenous Bcl-2 regulates neuronal cell survival in the central nervous system, and that Bcl-2 deficiency reduces neuronal viability under various adverse cellular conditions. Considering the anti-apoptotic properties of Bcl-2 in neurodegeneration, our findings support those of Machado-Vieira et al. 
, in which the Bcl-2 G/G genotype was associated with increased Bcl-2 mRNA and protein expression. Previous studies have observed that higher Bcl-2 expression may protect against dysfunctional calcium homeostasis in bipolar disorder patients 
. Because Bcl-2 expression in the brain changes with age and increased expression of Bcl-2 may prevent or delay neuronal death 
, our findings suggest a potential genetic effect of Bcl-2 rs956572 in brain aging.
In our study, the protective effect of the homozygous Bcl-2-G allele was limited to the right cerebellum, the bilateral lingual gyrus, the right middle temporal gyrus, and the right parahippocampal gyrus. Thus, these regions may be sensitive to Bcl-2 modulation during brain aging. We observed that the cerebellum was most significantly affected by the Bcl-2 genotype. The Bcl-2 protein is widely expressed during the development of the nervous system, but is principally retained in specific regions of the brain, including the cerebellum 
. Hochman et al. 
found that Bcl-2-knockout mice displayed increased susceptibility to cellular oxidative processes and a loss of neurons in the cerebellum, which suggest that neuronal viability in the cerebellum may be influenced by Bcl-2. Kaufmann et al. 
found that level of Bcl-2 expression was higher in the central nervous system of older rats, especially in the cerebellum, and increased oxidative stress has been observed in the cerebellum of aged animals 
. If the increased expression of Bcl-2 represents a response to age-related oxidative challenge and cerebellum is highly susceptible to this challenge 
, the higher level of Bcl-2 expression from the homozygous G allele may protect against the age-related loss of neurons in the cerebellum.
Our study also demonstrated that Bcl-2 polymorphism influences the GM volume in the bilateral lingual gyrus, the right middle temporal gyrus, and the right parahippocampal gyrus. These findings are consistent with two previous imaging analyses of the genetic effects of Bcl-2. Salvadore et al. 
reported that Bcl-2 rs956572 was associated with GM volume in the subcortical structures. Our prior study found that the Bcl-2 genotype could modulate GM volume in the lingual gyrus and middle temporal gyrus in elderly men 
. The distribution of Bcl-2 varies among these regions, and the level of Bcl-2 expression has been shown to be associated with neurotoxin-triggered apoptosis and cellular injury 
. During the development of the human central nervous system, Bcl-2 expression declines gradually at more advanced stages, and an inverse correlation between apoptosis and Bcl-2 expression occurs in the areas surrounding the lingual gyrus 
. Postmortem evidence supports apoptotic involvement in neuropsychiatric disorders, and low levels of Bcl-2 protein have been demonstrated in the middle temporal gyrus 
. Furthermore, the hippocampus is particularly vulnerable to oxidative stress during aging, and altered Bcl-2 expression has been reported in the hippocampal region of aged rat 
. Because the age-related changes in GM volume in these brain regions may be associated with Bcl-2 expression, differences in Bcl-2 expression levels among the Bcl-2 rs956572 allelic variants may influence the age-related rates of GM volume decline in these regions.
Based on our findings, the Bcl-2 rs956572 polymorphism has the most prominent effect on age-related GM volume reductions in the cerebellum. Significant interconnections of the cerebellum with the hippocampus and the occipital and temporal regions of the cerebral cortex have been implicated in the integration of sensory information, visuospatial organization, visual memory, procedural learning, and the control of behavior and motivation 
. Because the cerebellum may have extensive outgoing connections to these regions, Bcl-2 rs956572 polymorphism may indirectly modulate GM volume reduction in the lingual gyrus, the middle temporal gyrus, and the parahippocampal gyrus through direct impacts on the cerebellum.
In our study, the age-related reduction in GM volume in the frontal and parietal lobes were not associated with Bcl-2 genotype. Although Bcl-2 expression is widespread in all brain regions, the effect of Bcl-2 expression on the trajectory of maturation or degeneration during brain aging may vary considerably in the cortex 
. Analysis of post-mortem brain samples from patients with Alzheimer disease showed that the level of Bcl-2 expression were significantly higher in the cerebellum than in the frontal lobe 
. Therefore, the effect of the Bcl-2 genotype on age- or neuropsychiatric disease-related changes in regional GM volumes warrants further investigation.
The need for statistically sufficient sample sizes in imaging studies of genetic variation has become increasingly recognized. The relatively large and, by international standards, homogenous sample of participants that were reviewed in our study lend credibility to our findings, based on previously proposed recommendations regarding cohort sizes 
. However, the cross-sectional nature of our study design may represent a limitation to our findings. Prospective studies have demonstrated greater sensitivity for clarifying the GM volume changes in specific brain regions during the aging process 
. In addition, it is possible that, rather than having a direct effect of GM volume, the Bcl-2 rs956572 polymorphism may be in linkage disequilibrium with the truly associated allele. Such linkage likely varies among different populations, which would confound the generalization of findings based on a homogenous Chinese cohort, such as ours. Furthermore, the addition of a clinical control group with a psychiatric disorder, such as bipolar disorder, to future study designs may yield added knowledge of the dual role of Bcl-2 in aging and disease states.
In conclusion, our findings of the effects of Bcl-2 rs956572 polymorphism on age-related morphologic changes in the brain indicate that Bcl-2 G homozygosity confers a protective effect against age-related GM volume reduction in several brain regions, particularly in the cerebellum. Although the underlying molecular mechanisms remain unclear, our findings support the hypothesis that Bcl-2-related genetic factors play a critical role in the effects of aging in the brain.