Why some people develop major depressive disorder and others do not is a complex and not well-understood process. Several factors have been discussed to contribute to depression, among them:
Genetic variation: Individuals carrying one or two copies of a specific risk allele on one or more “depression gene/s” have a higher risk of developing depression.
Environmental influences: Negative life events such as trauma, negligence, or abuse increase the risk of developing depression.
Gene-by-environment interactions: Negative life events only lead to depression in individuals with a specific genetic set-up that makes them risk-prone to develop depression.
The gene most commonly associated with depression is the serotonin transporter gene SLC6A4 (Bleys et al., 2018). Serotonin is a neurotransmitter affecting multiple physiological processes and cognitive brain functions, among them mood and emotions, which is why it has been linked to mood disorders such as depression. Indeed, low serotonin levels have been associated with depressed mood (Jenkins et al., 2016), and selective serotonin reuptake inhibitors (SSRIs) are the most commonly prescribed antidepressants. SSRIs block the reuptake of serotonin during cellular communication in the brain, making more serotonin available, and thus in theory helping to reduce depression.
Along these lines, the idea that the serotonin transporter gene could affect depression risk or severity intuitively made sense. Specifically, many scientists focused on the so-called 5-HTTLPR polymorphism in the promoter region of the serotonin transporter gene to research the effects of this gene on depression. Genetic polymorphism means that at a specific location in the genome, different people might have slight variations in their DNA which could affect how well the protein that the gene produces could do its job. In the case of the 5-HTTLPR polymorphism, there is a short allele (s) and a long allele (l). Already back in the 1990s, researchers showed that people with two or one short alleles have a higher chance of developing depression than those with two long alleles, as the short allele leads to reduced expression of the serotonin transporter (Collier et al., 1996).
This initial study sparked interest in the 5-HTTLPR polymorphism, but not all empirical works could find a clear association. In 2003, a surprising finding seemingly resolved this controversy. In a widely cited study, Caspi and colleagues were able to show that the effects of 5-HTTLPR polymorphism genotype on depression were moderated by a so-called gene-by-environment interaction (Caspi et al., 2003). This means that the genotype would only have an effect if individuals were also subjected to specific environmental conditions. Specifically, the scientists found that individuals reacted differently to highly stressful life events, depending on the 5-HTTLPR genotype. People with at least one short allele on the 5-HTTLPR polymorphism developed more depressive symptoms if they experienced a highly stressful life event than people with two long alleles. However, without a stressful life event, the genotype did not have an effect on the probability to develop depression.
This study further increased the interest in the 5-HTTLPR polymorphism and its relation to depression, leading to more studies on this topic. However, a problem of many of these studies was that their sample sizes were comparably small for genetic studies, potentially leading to erroneous results and overblown effects.
Almost a decade ago, Risch and co-workers (Risch et al., 2009) conducted a so-called meta-analysis, a statistical integration of empirical studies. They analyzed 14 studies on the 5-HTTLPR polymorphism and its relation to depression and on whether this relation was influenced by stressful life events as had been suggested by Caspi et al. (2003). Their result was clear: While more stressful life events led to a higher risk of depression, there was no effect of the 5-HTTLPR genotype on depression and no gene-by-environment interaction effect between genotype and stressful life events.
Despite this finding, hundreds of studies on the 5-HTTLPR polymorphism and depression have been published since 2009 (the scientific search engine PubMed lists more than 800 hits for the search term “5-HTTLPR depression” as of early May 2019). A new study recently published by Richard Border and colleagues in The American Journal of Psychiatry (Border et al., 2019) aimed to resolve the controversy about whether or not the 5-HTTLPR genotype affects depression and whether there is a gene-by-environment interaction between this genotype and stressful life events once and for all. To avoid the statistical problems of previous studies, they obtained data from several large genetic datasets available to researchers, leading to a sample size of several hundred thousand individuals. The results of the analysis were clear as well: There was no statistical evidence for a relation between the 5-HTTLPR polymorphism and depression, and there was also no evidence that traumatic life events or adverse socioeconomic conditions might show a gene-by-environment interaction with this genotype.
This, of course, does not mean that there is no relationship between serotonin and depression (there clearly is, as shown by the treatment success of SSRIs), but it lends further support to an emerging insight in psychiatry genetics: Mental illness is a highly complex process that is likely influenced by a large number of genetic and non-genetic effects. As such, it is unlikely that single genetic variations such as the 5-HTTLPR polymorphism have a huge impact on whether or not an individual develops depression or any other form of mental illness. Future psychiatry genetic studies will need to take this complexity into account by analyzing genetic variation across the whole genome and epigenome and relating it to mental illness (Meier & Deckert, 2019).
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Bleys D, Luyten P, Soenens B, Claes S. (2018). Gene-environment interactions between stress and 5-HTTLPR in depression: A meta-analytic update. J Affect Disord, 226, 339-345.
Border R, Johnson EC, Evans LM, Smolen A, Berley N, Sullivan PF, Keller MC. (2019). No Support for Historical Candidate Gene or Candidate Gene-by-Interaction Hypotheses for Major Depression Across Multiple Large Samples. Am J Psychiatry, 176, 376-387.
Caspi A, Sugden K, Moffitt TE, Taylor A, Craig IW, Harrington H, McClay J, Mill J, Martin J, Braithwaite A, Poulton R. (2003). Influence of life stress on depression: moderation by a polymorphism in the 5-HTT gene. Science, 301, 386-389.
Collier DA, Stöber G, Li T, Heils A, Catalano M, Di Bella D, Arranz MJ, Murray RM, Vallada HP, Bengel D, Müller CR, Roberts GW, Smeraldi E, Kirov G, Sham P, Lesch KP. (1996). A novel functional polymorphism within the promoter of the serotonin transporter gene: possible role in susceptibility to affective disorders. Mol Psychiatry, 1, 453-460.
Jenkins TA, Nguyen JC, Polglaze KE, Bertrand PP. (2016). Influence of Tryptophan and Serotonin on Mood and Cognition with a Possible Role of the Gut-Brain Axis. Nutrients, 8, pii: E56.
Meier SM, Deckert J. (2019). Genetics of Anxiety Disorders. Curr Psychiatry Rep, 21, 16.
Risch N, Herrell R, Lehner T, Liang KY, Eaves L, Hoh J, Griem A, Kovacs M, Ott J, Merikangas KR. (2009). Interaction between the serotonin transporter gene (5-HTTLPR), stressful life events, and risk of depression: a meta-analysis. JAMA, 301, 2462-2471.