Polymorphisms of the 5-HTTLPR Gene: Associations to ERN and Depression
Mentor: Michael J. Larson, Psychology and Neuroscience
Our aim with this study of polymorphisms in the serotonin transporter gene (5-HTTLPR) was to investigate the association between performance monitoring capabilities (i.e., detecting errors in performance using the error-related negativity [ERN] component of the scalp recorded event-related potential [ERP]) against three variations of gene 5-HTTLPR allelic pairs: homozygous short (S/S), heterozygous short-long (S/L), and homozygous long (L/L). The serotonin transporter gene is associated with our ability to cope with stress and regulate serotonin, which affects mood, social behavior, appetite, and sleep (Adam, Doane, Zinbarg, Mineka, Craske, & Griffith, 2010). Previous studies (Barnes, Dean, Nandam, O’Connell, & Bellgrove, 2011) have investigated the relationship between increased-amplitude ERN and the S/S and S/L polymorphism when compared to the L/L allelic pair. Some findings have has associated the short allele with a decreased ability to cope with stress while others have negated that relationship (Starr, Hammen, Brennan, Najman, 2012; Barnes, Dean, Nandam, O’Connell, & Bellgrove, 2011; de Brujin, Sabbe, Hulstiijn, Ruigt, & Verkes, 2006; Fallgatter, Herrmann, Roemmler, Ehlis, et. Al 2004). Our study was important because we made two key changes: we controlled for a L/L SNP variation that frequently behaves like the short allele (Caspi, Sugden, Moffitt, et. Al 2003) and we used a sample size that would help us achieve more conclusive results. Our hypothesis was that while accounting for the long A/G SNP variation, we would test the relationship between short/long allelic pairs in the 5-HTTLPR polymorphism and an increase in performance monitoring when a short allele is present when compared to homozygous long alleles to attempt to demonstrate that there would be no significant difference between S/S, S/L, and L/L allelic variations and increased ERN.
We initially recruited 308 participants (160 younger adults and 148 older adults [50+]). Several were excluded for the following reasons: 22 for having fewer than 6 usable error trials, 8 for having more than 200 error trials (i.e., didn’t understand the task), and 21 for being unable to complete the EEG. This left us with a total sample size of 257 (106 older adults and 151 younger adults).
Data collected include saliva (collection via DNA Genotek and isolation via the Qiagen QIAamp DNA Mini Kit where 5HTTLPR alleles were isolated using qPCR to determine allelic length) and ERN amplitude data obtained during a modified version of the Eriksen Flanker Task (which was used to measure cognitive control functioning and is known to reliably elicit ERN response) while participants wore 128-channel electrode nets.
Our sample consisted of 69 younger females (45.7% female) and 60 older females (56.6% female). Of the total sample (both males and females), 64 participants had short homozygous pairs – 25%, 124 participants had SL heterozygous pairs – 48%, 61 participants had long homozygous pairs – 24%, and 8 participants were without recall. 78 participants had Lg (expressed as short) alleles – 30%, 116 participants had heterozygous LgL alleles – 45%, 55 had homozygous LaLa alleles – 21%, and 8 participants were again without recall.
We used 3-way ANOVAs to compare age groups (older, younger), genotype (L/L, S/L, S/S), and accuracy (correct, incorrect while doing the modified Flanker Task). The main effect of accuracy had a value of p < .001 with error trials being more negative (mean = -0.11microvolts) than correct trials (mean = 1.95 microvolts). The main effect of age group approached significance (p = .056) with a larger ERN amplitude for younger adults than older adults. Lastly for 5HT groups, the main effect of group approached significance (p = .056) – individuals with a short allele or Lg had more negative ERN than the homozygous L/L participants. There were no significant interactions among the three groups.
Discussion & Conclusion
Overall our results showed the following: ERN amplitude is smaller with age (i.e., older adults had smaller ERN amplitude than younger adults), ERN amplitude was trend level (though not quite p < .05) more negative for short alleles than for long alleles, and ERN amplitude did not interact with age. The first finding suggests that there is quicker and more accurate recognition of mistakes by younger adults than by older adults; this finding is not necessarily new but we did find new evidence to support it. The second finding is the data that we were initially seeking when we began this study – when compared against ERN we found that individuals with short alleles trended more negative than those with long alleles. Our findings did not have a p < .05, but there was a trend toward significance. We hope that our publication will provoke further study of this relationship. In summary, our findings are important because our study is the first to boast such a large sample size, control for age, and one of only two studies that coded the Lg SNP as an SS allelic pair. We plan to continue cleaning the data and will soon submit them for publication in a scholarly journal. We hope that our findings promote further study of the connection between ERN and 5-HTTLPR allelic lengths and that Meta-analysis will ensue.
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