Serotonin transporter mRNA levels are associated with the methylation of an upstream CpG island

Abstract

The serotonin reuptake transporter (5HTT) is thought to be the principal regulator of serotonergic activity and epigenetic effects at this locus are thought to be important moderators of vulnerability to neuropsychiatric illness. In attempt to understand the basis of this regulation, several gene polymorphisms that affect 5HTT mRNA levels have been described. But to date, no clear mechanism linking these polymorphisms to vulnerability to epigenetic effects have been described. In this communication, we describe a CpG island in the 5 0 region of the 5HTT gene that contains an alternative exon 1 and possible promoter for 5HTT. We then confirm the existence of this transcript and ascertain the methylation status of this CpG island in 49 lymphoblast cell lines and analyze the relationship between methylation and 5HTT mRNA levels. We demonstrate that methylation at this CpG island is associated with decreased levels of 5HTT mRNA, but that this effect is evident only when 5HTTLPR genotype is taken into account. We suggest that these findings have significant implications for the understanding of the role of this locus in behavioral illness. INTRODUCTIONThe serotonin transporter (5HTT) located at 17q11 is thought to be a major regulator of serotonergic neurotransmission and thereby vulnerability to behavioral illness. In efforts to determine whether genetic variation at 5HTT contributes variation in serotonin reuptake, a large number of studies have been conducted. To a large extent, these studies have failed to demonstrate a significant role for coding variants in regulating serotonin re-uptake. In contrast, several studies have demonstrated that non-coding variants have a significant role in regulating serotonin transport by altering 5HTT mRNA levels. The best characterized of these non-coding variants, the 5HTTLPR polymorphism, has effects on 5HTT mRNA levels in lymphoblasts Bradley et al., 2005], in platelet serotonin reuptake [Greenberg et al., 1999;Nobile et al., 1999] and platelet serotonin content [Hanna et al., 1998]. Other non-coding variants including the recently reported Long G polymorphism [Hu et al., 2005] and an intron 2 polymorphism denoted as the STin2 variable number tandem repeat (VNTR) [Hranilovic et al., 2004] also may have effects on serotonin mRNA production, although these effects are less well established.Given the importance of this gene in regulating serotonergic neurotransmission, at least several hundred studies have examined the role of the genetic variation in 5HTT, in particular, the 5HTTLPR polymorphism, in mediating vulnerability to neuropsychiatric illness such as depression, schizophrenia, and autism. The results of these studies have been mixed, with many studies inferring that the 5HTTLPR polymorphism may have effect on vulnerability to a broad range of behavioral illnesses [Conroy et al., 2004;Gorwood, 2004;Lotrich and Pollock, 2004;Savitz and Ramesar, 2004]. However, with the exception of alcoholism and schizophrenia [Fan and Sklar, 2005;Lasky-Su et al., 2005], meta-analyses of...

Gender differences in the genetic and environmental determinants of adolescent depression

“…For example, it is possible that including childhood maltreatment as an "E" variable in our models may have enabled us to detect depression-related G X E interactions among adolescent females consistent with previous reports in the literature [16] ; in addition, evidence is emerging that epigenetic mechanisms may also play a role in regulating expression of the serotonin transporter gene [41,42] , with one report indicating a nearly significant (p=0.07), higher methylation level of SLC6A4 in those without a lifetime history of major depression when compared to those who have at some point suffered from the disorder [41] . Investigations focused on G X E interactions using the 5-HTTLPR locus are thus inherently complex and will likely require multiple levels of measurement at both the molecular and environmental levels in order to improve our identification of etiological risk factors for increased depressive symptoms.…”

Prospects for Epigenetic Epidemiology

“…Increasing methylation of a CpG island in the serotonin transporter gene (5HTT), for example, is associated with decreasing levels of gene expression, but this effect is evident only when the 5HTTLPR genotype (length of an upstream DNA repeat) is included (40).…”

Variants at serotonin transporter and 2A receptor genes predict cooperative behavior differentially according to presence of punishment

“…Of equal interest is recent molecular evidence for genotypeor haplotype-specific epigenetic regulation at HTR2A (27,28) and SLC6A4 (29,30). These results, in concert with those from gene by environment studies (31,32), lend increasing support to the hypothesis that, rather than conferring susceptibility to psychopathology, polymorphisms at SLC6A4 and HTR2A enable increased plasticity of the serotonergic system in response to the social environment (31, 32) (SI Text: Haplotype Classification).…”