Oestrogen receptor genetics: a needle that cuts through many haystacks?

Oestrogen is known to affect multiple aspects of human health, including complex traits such as cardiovascular disease, cancer, fracture risk, arthritis, behaviour and Alzheimer’s disease. Oestrogen receptor a (ESR1), one of two known oestrogen activated transcription factors, is expressed throughout many tissues. It can regulate gene expression by both oestrogen-dependent and oestrogenindependent mechanisms that result in direct or indirect activation of transcription of a wide range of genes. Most of the hundreds of reports of ESR1 genetics published since 1990 have been in areas of cancer biology, bone mineral density, or fracture risk. In the past 3 or 4 years, however, there have been a handful of interesting reports of association of ESR1 polymorphism with cardiovascular disease, its established risk factors, or in relation to response to hormone replacement therapy. This recent literature, which was both particularly intriguing and inconclusive among women, provided a strong rationale for the investigation by Lawlor et al. These authors carried out a cross-sectional analysis of 3404 women from the British Women’s Heart and Health Study. The women were in their 60s or 70s, from 23 towns across Britain; characterization included incident and prevalent coronary heart disease, its risk factors, and use of hormone replacement. Two single nucleotide polymorphisms from the first intron of ESR1 were genotyped in participant DNA samples. The polymorphisms, c454-397T.C and c454-351A.G, are a few hundred basepairs from the start of exon 2 and have sometimes been described by the name of the detecting restriction enzyme, PvuII or XbaI, or their reference ID numbers, rs2234693 and rs9340799, respectively. The ESR1 gene is large, encompassing some 300 kb of DNA, and includes 8 exons. The first intron of a gene, like the promoter, usually contains a larger number of regulatory sequences than other introns. Although an intronic polymorphism cannot directly alter the sequence of amino acids it can alter expression, splicing, or binding of regulatory proteins. One of the two studied ESR1 polymorphisms, which have been examined in many previous studies, may alter transcription factor binding and affect expression level of the ESR1 protein. However, association studies cannot prove that the relationship is causal and some of the hundreds of other polymorphisms that have been catalogued in ESR1, rather than the genotyped ones, may be the cause of any observed phenotypic variation. In contrast to some previous studies, the British Women’s Heart and Health Study provided no evidence of association of ESR1 genotypes or haplotypes with cardiovascular disease, its risk factors, or with response to hormone replacement; it did, however, identify evidence of a relationship between genotype and chance of taking hormone replacement. The authors tested but found no support for the possibility that this result was driven by differences in social class that might have affected hormone replacement use. From prior studies, the authors hypothesized that the T–A haplotype (made up of c454-397T and c454-351A) would be associated with phenotypes of decreased oestrogen response. This might correspond to more menopausal symptoms that would lead to greater use of hormone replacement. A corresponding trend was observed, with 10, 12, and 13% of individuals with 0, 1, or 2 copies of the T–A haplotype having been past users of hormone replacement (P 1⁄4 0.02). A similar pattern was observed among current hormone replacement users (P 1⁄4 0.05). Lawlor et al. note that although these results are not adjusted for multiple testing the consistency seen across both past and current users provides support that the association is real. An interesting consideration is how such an ESR1 association with hormone replacement use might underlie association of ESR1 genotypes with multiple other hormonedependent phenotypes. This would have implications for studies which either do not adjust for this variable, or fail to match the case and control populations appropriately. In the report by Lawlor et al., a greater percentage of hormone replacement users, than non-users, were homozygous for the T–A haplotype. Thus, if hormone replacement were to increase the frequency of cardiovascular disease, we might expect to observe a higher frequency of T–A homozygotes among post-menopausal women with cardiovascular disease. This has in fact been observed in the