Association of exposure to prenatal phthalate esters and bisphenol A and polymorphisms in the ESR1 gene with the second to fourth digit ratio in school-aged children: Data from the Hokkaido study

Highlights

• We examined ESR1 SNPs, finger length of 2D:4D, and prenatal phthalate/BPA exposure.

• rs2077647 AG/GG with high MEHP/ΣDEHP exposure resulted in feminized 2D:4D in boys.

• No significant differences were found among girls.

• ESR1 polymorphisms modified the effects of prenatal DEHP on mean 2D:4D among boys.

Abstract

Phthalates and bisphenol A (BPA) are estrogenic endocrine disruptors. Polymorphisms in the gene encoding estrogen receptor 1 (ESR1) may contribute to the ratio of the lengths of the second and fourth digits (2D:4D), which is considered an index of prenatal exposure to sex hormones. Thus, we investigated whether ESR1 polymorphisms modify the effects of prenatal exposure to phthalates and BPA on 2D:4D in a birth cohort. Maternal serum in the first trimester was used to determine prenatal exposure to these compounds. Six hundred twenty-three children (7 years of age) provided mean 2D:4D from photocopies and were genotyped for single nucleotide polymorphisms in ESR1, particularly PvuII (T > C, dbSNP: rs2234693), XbaI (A > G, dbSNP: rs9340799), and rs2077647 (A > G). The associations among compound exposure, mean 2D:4D, and ESR1 polymorphisms were assessed by multiple linear regression adjusted for potential cofounding factors. Boys with the AG/GG genotype at rs2077647 in the group exposed to high levels of mono(2-ethylhexyl) phthalate (MEHP) or Σ Di(2-ethylhexyl) phthalate (DEHP) showed feminized 2D:4D compared with boys with the AA genotype at rs2077647 who had low exposure to MEHP or ΣDEHP (MEHP: increase in mean 2D:4D of 1.51%, 95% confidence interval [CI]: 0.40–2.63; ΣDEHP: increase in mean 2D:4D of 1.37%, 95% CI: 0.25–2.49). No significant differences were found among girls. There were no associations between mean 2D:4D and metabolites other than MEHP or BPA. These data suggest that ESR1 polymorphisms modify the effects of prenatal exposure to DEHP on mean 2D:4D among boys.

Introduction

Endocrine disrupting chemicals (EDCs) adversely affect the endocrine system, leading to compromised hormone functions [1]. Phthalates and bisphenol A (BPA) are two commonly studied EDCs. Phthalates have been widely used as plasticizers for various plastic compounds, such as toys, indoor materials, personal care products, and medical devices [2], whereas BPA is applied in the production of polycarbonate plastics and epoxy resins used in many consumer products, such as the lining of metal cans and drinking containers [3]. Despite their short half-lives, these two chemicals are thought to be ubiquitously present in the human body owing to daily exposure [4], [5]. Phthalates and BPA are also known estrogenic EDCs [1], [6] and have been detected not only in maternal urine, blood, and breast milk [7], [8], [9], [10], [11] but also in amniotic fluids [12], [13]. Because of apparent placental transfer, prenatal exposure of these two chemicals may have reproductive toxicologic effects on sex differentiation [14], [15], [16].

The ratio of the lengths of the second and fourth digits in human hands (2D:4D) tends to be lower in males than in females [17]. This ratio is believed to be fixed by the 14th week of gestation, at which time it is negatively correlated with prenatal testosterone and positively with prenatal estrogen levels [17], [18]. Zheng and Cohn [19] reported that androgen receptors enhance chondrocyte proliferation in the fourth digit and are required to lower the 2D:4D, whereas estrogen receptors (ERs) inhibit chondrocyte proliferation in the fourth digit and are required to establish a relatively high 2D:4D. In humans, 2D:4D is used as a noninvasive retrospective index of prenatal exposure to sex hormones.

Prenatal exposure to EDCs alters sex hormones in the fetus [2], [20]. In humans, anogenital distance (AGD), another prenatal sexual dimorphic index, has been extensively used for evaluation of the effects of prenatal exposure to EDCs on sex differentiation [21], [22]. However, animal experiments have suggested that 2D:4D is more sensitive than AGD in the context of low environmental dose exposure to EDCs [23]. Furthermore, 2D:4D is fixed exclusively by prenatal exposure of sex hormones or compounds, whereas AGD is modulated by both prenatal and postnatal exposure [19], [23] (J.P. Cravedi 2005, unpublished data; see the discussion paragraph in Auger. J. 2013 [23]). These animal investigations suggest that 2D:4D may be superior to AGD for evaluating of the effects of prenatal exposure to EDCs on sex differentiation. However, no studies have described the associations between exposure to prenatal phthalates/BPA and human 2D:4D.

One of the mechanisms of action of phthalates and BPA involves the agonistic activities of ERα, which plays a key role in both reproductive and nonreproductive tissues, such as bone [24], [25], [26]. ERα is encoded by the estrogen receptor 1 (ESR1) gene [27]. There are two well-known and functional single nucleotide polymorphisms (SNPs) in ESR1, i.e., PvuII (T > C, dbSNP: rs2234693) and XbaI (A > G, dbSNP: rs9340799), along with the silent polymorphism rs2077647 (A > G). We previously reported the association between 2D:4D and these three ESR1 polymorphisms in school-aged children [28]. Genetic variation may play an important role in individual susceptibility to EDCs, and genetic polymorphisms modify the associations between phthalates or BPA and the human body [29], [30]. It is possible that ESR1 polymorphisms also affect the associations between exposure to prenatal phthalates/BPA and 2D:4D. One study investigated the contributions of ESR1 polymorphisms to the association between phthalate exposure and the development of leiomyoma in adult women [31]; however, no reports have described the associations among prenatal EDC exposure, ESR1 polymorphisms, and sex differentiation.

In this study, we aimed to clarify the effects of prenatal exposure to phthalates and BPA on sex differentiation in humans by evaluating the associations between prenatal exposure to phthalates/BPA and 2D:4D and investigating whether the ESR1 genotype alters the effects of prenatal phthalates/BPA on 2D:4D.