Genetic influences on nicotinic α5 receptor (CHRNA5) CpG methylation and mRNA expression in brain and adipose tissue.

Jessica E Ramsay,C Harker Rhodes,Keerthi Thirtamara-Rajamani,Ryan M Smith

doi:10.1186/s41021-015-0020-x

Jessica E Ramsay, C Harker Rhodes + Show 2 more

Open Access

https://doi.org/10.1186/s41021-015-0020-x

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Abstract

IntroductionThe nicotinic α5 receptor subunit, encoded by CHRNA5, harbors multiple functional single nucleotide polymorphisms (SNPs) that affect mRNA expression and alter the encoded protein. These polymorphisms are most notably associated with drug-taking behaviors and cognition. We previously identified common SNPs in a distant regulatory element (DRE) that increase CHRNA5 mRNA expression in the human prefrontal cortex (PFC) and confer risk for nicotine dependence. Genome-wide epigenetic studies in PFC and adipose tissue find strong effects of the DRE SNPs on CpG methylation. However, it is unclear whether DRE SNPs influence CpG methylation en route to modulating CHRNA5 mRNA expression. It is also unclear whether these polymorphisms affect expression in other brain regions, especially those mediating drug-taking behaviors.ResultsBy measuring total and allelic CHRNA5 mRNA expression in human habenula and putamen autopsy tissues, we found that CHRNA5 DRE variants considerably increase mRNA expression by up to 3.5-fold in both brain regions. Our epigenetic analysis finds no association between CpG methylation and CHRNA5 mRNA expression in the PFC or adipose tissues.ConclusionsThese finding suggests the mechanisms responsible for the genetic modulation of CpG methylation and mRNA expression are independent despite the DRE SNPs being highly associated with both measures. Our findings support a strong association between the DRE SNPs and mRNA expression or CpG methylation in the brain and periphery, but the independence of the two measures leads us to conclude that environmental factors affecting CpG methylation do not appear to directly modulate gene expression.

Highlights

The nicotinic α5 receptor subunit, encoded by CHRNA5, harbors multiple functional single nucleotide polymorphisms (SNPs) that affect mRNA expression and alter the encoded protein
By measuring total and allelic CHRNA5 mRNA expression in human habenula and putamen autopsy tissues, we found that CHRNA5 distant regulatory element (DRE) variants considerably increase mRNA expression by up to 3.5-fold in both brain regions
These finding suggests the mechanisms responsible for the genetic modulation of CpG methylation and mRNA expression are independent despite the DRE SNPs being highly associated with both measures

Summary

Introduction

The nicotinic α5 receptor subunit, encoded by CHRNA5, harbors multiple functional single nucleotide polymorphisms (SNPs) that affect mRNA expression and alter the encoded protein. Previous studies found that allelic variation in the α5/α3/β4 neuronal nicotinic acetylcholine receptor (nAChR) subunit gene cluster on chromosome region 15q25.1 significantly increases risk for addiction to multiple classes of drugs [1,2,3,4,5,6,7,8,9], but confers a protective effect for cocaine addiction [8, 10]. Despite strong evidence for altered Chrna expression in the rodent habenula affecting addiction phenotypes, and the association of regulatory DRE SNPs with nicotine addiction, it is unknown whether the DRE SNPs affect CHRNA5 mRNA expression in the human habenula Evidence that they modulate expression in the PFC, amygdala, and nucleus accumbens [5, 6, 9, 23], suggests the DRE exerts influence in cortical and subcortical brain regions

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