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Scientific studies have found that numerous brain areas show altered activity in patients suffering from depression, and this has encouraged advocates of various theories that seek to identify a biochemical origin of the disease, as opposed to theories that emphasize psychological or situational causes. Several theories concerning the biologically based cause of depression have been suggested over the years, of which the most prominent and widely researched is the monoamine hypothesis. == Genetic factors == In contrast to other psychiatric disorders such as autism and schizophrenia, genetic factors involved in depression have been more difficult to identify. In 2003 ''Science'' published an influential study of Avshalom Caspi et al. who found that a gene-environment interaction (GxE) may explain why life stress is a predictor for depressive episodes in some individuals, but not in others, depending on an allelic variation of the serotonin-transporter-linked promoter region (5-HTTLPR). Soon after, the results were replicated by Kenneth Kendler's group, raising hopes in the psychiatric genetics community.〔http://www.esi-topics.com/nhp/2006/september-06-KennethSKendler.html〕 By 2007 there were 11 replications, 3 partial replication and 3 non-replications of this proposed GxE. However, two of the largest studies were negative. Two 2009 meta-analyses were also negative; one included 14 studies, the other just five, owing to different study selection criteria. A 2010 review of studies in this area found 17 replications, 8 partial replications (interaction only in females or only with one of several types of adversity), and 9 non-replications (no interaction or an interaction in the opposite direction). It also found a systematic relationship between the method used to assess environmental adversity and the results of the studies; all studies using objective indicators or structured interviews to assess stress replicated the gene–environment interaction fully or partially, whereas all non-replications relied on brief self-report measures of adversity. This review also found that both 2009 meta-analyses were significantly biased toward negative studies. Other hypothesized genomic influences are BDNF polymorphisms, but the replications studies have been mixed and insufficient as of 2005 for a meta-analysis. Studies also indicate an association of BDNF to suicidal behavior. However, findings from the gene-environment interactions studies suggest that the current BDNF models of depression are too simplistic. A 2008 study found interactions (biological epistasis) in the signaling pathways of the BDNF and the serotonin transporter; the BDNF Val66Met allele, which was predicted to have reduced responsitivity to serotonin, was found to exercise protective effects in individuals with the short 5-HTTLPR allele that is otherwise believed to predispose individuals to depressive episodes after stressful events. Thus, the BDNF-mediated signalling involved in neuroplastic responses to stress and antidepressants is influenced by other genetic and environmental modifiers.〔 The largest genome-wide study to date failed to identify variants with genome-wide significance in over 9000 cases. Recently, the first genetics study has been published with positively identified two variants with genome-wide association with major depressive disorder. This study, conducted in Chinese Han woman, identified two variants in intronic regions near SIRT1 and LHPP. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Biology of depression」の詳細全文を読む スポンサード リンク
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