The current findings from two independent samples suggest that gender has a strong influence on the association among 5-HTTLPR frequency, chronic environmental stress and depressive symptoms in non-patient populations. Specifically, our study suggests that in females the 5-HTTLPR genotype associated with less transcriptional activity (s/s) may increase an individual’s susceptibility to depression under stressful life conditions, whereas the genotype associated with increased activity (l/l) may do so in males. While the interaction between 5-HTTLPR genotype and stress to predict depression has been documented previously e.g., (Caspi et al. 2003
; Grabe et al. 2005
; Manuck et al. 2004
; Moffitt et al. 2005
), the role of gender has received scant attention. Effects of stress to increase the impact of 5-HTTLPR genotype on depressive symptoms are strong in both studies—effects sizes (d), 0.78–0.94—whether the exposure to stressful life circumstances is current (being a caregiver in Study 1) or in the past (low father’s education in Study 2).
This study expands upon a growing literature showing that 5-HTTLPR genotype can have an opposite effect on behavioral characteristics in men and women. In a sample of 128 females, participants carrying the s allele had significantly higher
ratings on a measure of subthreshold depression, as compared with those who carried the ll genotype (Gonda et al. 2005
). Although not present in males, Eley et al. (2004)
found a significant 5-HTTLPR × environmental stress interaction in females such that the proportion of depressed subjects who were carrying the s allele, coupled with high environmental stress, was higher than the proportion of non-s allele carriers and those with low environmental stress. Flory et al. have shown that anxiety was significantly lower
among males with the short variant of 5-HTTLPR, whereas, no association was present in females (Flory et al. 1999
). Likewise, in prior work on another sample we found that men, but not women, with the s variant of the 5-HTTLPR reported lower
Anxiety (Brummett et al. 2003
). In another study of 374 females, stressful life events interacted with the 5-HTTLPR genotype to predict symptoms of depression such that those with the s allele had significantly higher depression ratings, as compared to those with the l allele (Jacobs et al. 2006
). Similarly, in a predominately female sample, lower expressing 5-HTTLPR alleles were associated with increased severity of major depression in those with moderate to severely stressful life events (Zalsman et al. 2006b
). Additionally, results from a general population study indicate that for females, but not males, the s allele was associated with perceived mental and physical distress in unemployed participants as compared to those who were employed (Grabe et al. 2005
). Finally, in a sample of 81 boys and 119 girls, Sjoberg et al. (2006)
found results strikingly similar to those of the present study. Specifically, their findings suggest that females homozygous for the 5-HTTLPR s allele, in conjunction with the stressor of family conflict, are more likely to develop depression than girls with no family conflict, or the presence of the l allele; whereas, for males the s allele in conjunction with a traumatic background serves to protect an individual from depression. Thus, a similar pattern of gender X 5-HTTLPR X environmental stress effects on depression, and/or perceived stress, has now been demonstrated in 4 independent samples. The present results do, however, contrast with those of Caspi et al. (2003)
who have shown in a sample of both males and females, that the s allele interacts with environmental stress to increase probability of depression later in life.
Females have higher rates of depression than males (Piccinelli and Wilkinson 2000
) and a number of findings suggest that sex differences in serotonergic function may contribute to the enhanced risk in women. Selective serotonin re-uptake inhibitors (SSRI) efficacy is higher in females than in males (Kornstein et al. 2000), and conversely, the emergence of depressive symptoms after tryptophan depletion is significantly greater in women (Booji et al. 2002
; Moreno et al. 2006
). One meta analysis has even suggested that tryptophan depletion causes depressive symptoms only in women (Jans et al. 2007
). The specific sex differences in serotonergic function which contribute to vulnerability to depressive symptoms are not clear, although there are some plausible candidates. Rates of central nervous system (CNS) serotonin synthesis are approximately 50% higher in males than in females (Nishizawa et al. 1997
). Our studies and others (Jonsson et al. 2000
; Williams et al. 2003
) have shown that CSF 5 HIAA is higher in women than in men, although this could reflect either enhanced serotonin neuron firing or enhanced metabolism. Both 5-HT1A
receptor density is lower in the brains of women than men (Biver et al. 1996
; Costes et al. 2005
), although such a difference could reflect either lower receptor expression or lower serotonin release to compete with radioligand for binding. How these characteristics combine to produce serotonergic “vulnerability” that leads to depression is not clear, but the consistency of the sex differences is suggestive.
The initial findings of Lesch et al. (1996)
, that polymorphisms in the serotonin transporter may be associated with normal variation in personality traits, has prompted an exciting area of research. One ongoing controversy about the mechanism by which the 5-HTTLPR influences behavior is the relative importance of neurodevelopmental effects that influence the development of serotonergic circuitry for current transporter expression. Reports vary from the s/s genotype exhibiting more (van Dyck et al. 2004
), less (Heinz et al. 2000
) or no difference (Shioe et al. 2003
; Willeit et al. 2001
) from s/l or l/l genotypes in serotonin transporter binding. A pair of exciting recent studies from one research group have particular relevance for the present study. In the first study, they confirmed their previous finding that overall 5-HTTLPR genotype is not associated with serotonin transporter expression in human brain, even when considering the recent triallelic categorization of genotypes (Parsey et al. 2006
). However, this same laboratory reported a strong association between the low-expressing 5-HTTLPR genotype and increased vulnerability to depression in persons experiencing high levels of stressful life events. (Zalsman et al. 2006a
). Although they did not report a sex difference in this association, 71% of their depressed subjects were female and this association was detected with patients describing either early life and current stress, as in the present study. It is tempting to speculate the neurodevelopmental events as well as transporter expression may play a critical role in the association we report here among sex, stressful life events and depression vulnerability.
Studies of ovarian steroid modulation of serotonergic function in non-human primate brain further support the existence of functionally important sex differences in females that could influence the emergence of depressive symptoms. In female Macaques, estrogen administration has been shown to decrease mRNA levels of the 5-HT1A
autoreceptor, monoamine oxidase A and the 5-HTT and increase mRNA of tryptophan hydroxylase in dorsal raphe and hypothalamic nuclei (Bethea et al. 2002
; Gundlah et al. 2002
; Pecins-Thompson and Bethea 1999
) Using protein levels to index expression, estrogen treatment was found to increase tryptophan hydroxylase 2 and 5-HTT expression and decrease 5-HT1A
receptor expression (Lu and Bethea 2002
; Smith et al. 2004
). While more research will be required to fully understand effects of ovarian steroids on serotonergic functions, there are clearly effects on serotonin synthesis, its removal from the synapse, its metabolism and also its stimulation of pre and postsynaptic receptor populations that strongly support a role for ovarian steroids in regulating almost every gene, its mRNA and protein that controls extracellular serotonin.
Research in rodent models also has documented sex differences in the effects of variation in serotonin transporter function on serotonergic function that could influence the emergence of depressive symptoms. In serotonin transporter knockout mice, the 5-HT1A
autoreceptor is desensitized more extensively in females than males (Bouali et al. 2003
; Li et al. 2000
). Similarly, in 5-HT1B
knockout mice, females exhibit greater disinhibition of serotonin release which manifests as less depressive behavior than 5-HT1B
knockout males. These findings suggest that the presynaptic 5-HT1A
autorecpetors contribute differentially more to regulation of extracellular serotonin in intact females than in intact males.
Finally it is important to consider the possibility that sex differences in effects of 5-HTTLPR interactions with stress on depression could reflect sex differences in stress reactivity that affect serotonergic function. In humans, an extensive literature supports the contribution of early life stressors to the development of depression, an association that is particularly strong in women who have been sexually or physically abused (Heim et al. 2004
). Similarly, female macaques who carry the 5-HTTLPR s allele and who have been exposed to early adversity (peer rearing) exhibit lower cortisol responses to stress, a pattern that has been associated with certain stress-related neuropsychiatric disorders (Barr et al. 2004
). More research will be required to determine how these mechanisms might account for the opposite effects of the 5-HTTLPR s allele on depressive symptoms that we observe in men vs. women exposed to stressful life circumstances.
Several factors should be noted with respect to the present findings. In addition to moderation by environmental stress, we hypothesized a main effect for 5-HTTLPR with respect to depressive symptoms that was not supported in either study. This highlights the importance of continued examination of potential moderators of genetic effects. Although a significant 3-way interaction was observed with respect to gender × genotype × stressor, this finding resulted under conditions of limited power. Importantly, our inability to detect a similar 3-way interaction with respect to race may have been due to the fact that the present samples were underpowered with regard to detection of potential race effects. Finally, it should be noted that the allele frequencies differed somewhat between the two present studies, a factor that was likely influenced by the differences in racial composition. Thus, these results are to be interpreted with caution prior to replication in additional samples.
Recent research (Hu et al. 2006
) has revealed the existence of a common single base substitution (A → G) within the 5-HTTLPR L allele, with the rarer (10–15% in Caucasians, 24% in African Americans) LG
allele showing reduced transcriptional efficiency, comparable to that of the S allele, while the LA
allele is about twice as transcriptionally efficient as the S or LG
alleles. The presence of the less functional LG
allele within the LL or LS subjects of our study would dilute the effects of the more active LA
allele, making it harder to find an association between 5-HTTLPR genotype and depressive symptoms. It is likely, therefore, that our findings represent a conservative estimate of the impact of 5-HTTLPR genotype on depressive symptoms in persons exposed to stressful life circumstances.
The present study contributes to an emerging literature showing that the 5-HTTLPR genotype and stress interact to affect depression vulnerability by demonstrating that this association is moderated by gender. Our findings, in two independent samples, combine with those of Sjoberg et al. (2006)
to suggest that—whether the stressor is early in life or in adulthood—in women, the 5-HTTLPR s/s genotype confers increased risk of depression following exposure to stressful life circumstances, while in men the l/l genotype confers increased risk. It will be important to pursue this finding by exploring the relative importance of current versus developmental contributions of 5-HTTLPR genotype.
The present findings may inform efforts to develop more effective approaches to prevention and treatment. Kaufman et al. (2004)
reported, for example, that the presence of positive social support reduces the risk of depression in maltreated children with the 5-HTTLPR s/s genotype. The current findings suggest that, to maximize the chances for success, a clinical trial to test the efficacy of an intervention that increases social support in preventing depression in persons exposed to stress should target women with the s/s genotype and men with the l/l genotype.