Discussion

Previous research has indicated an involvement of oestrogen receptor genes in the aetiology of depression in women. Notably, the strongest evidence was reported for an association of rs2234693 and rs9340799 with severe depressive symptoms and major depression in women. ^{Reference Li, Xie, Wang, Liu, Liao and Yin60,Reference Ryan and Ancelin61} However, these studies failed to distinguish between major depression and depression during the perinatal period. While all of these phases are characterised by changes in sex steroid hormones, they exhibit distinct patterns of hormonal fluctuations. ^{Reference Miller, Girgis and Gupta7,Reference Payne, Palmer and Joffe9,Reference Serrano and Warnock10,Reference Soares and Zitek36} The most pronounced fluctuations are observed during the perinatal period, and there is accumulating evidence suggesting that depression during this phase is distinguishable from major depression. ^{Reference Kundakovic and Rocks6,Reference Di Florio and Meltzer-Brody19–Reference Batt, Duffy, Novick, Metcalf and Epperson21} Therefore, we provided an overview of the published literature on the association between oestrogen receptor genes and perinatal depression symptoms, by including genetic, epigenetic and gene expression studies. Overall, our review revealed that a limited number of genetic variations in ESR1 and ESR2 were associated with perinatal depression symptoms. Most of the associations were found for variations in ESR1, of which the RFLP rs2234693 emerged as the most frequently reported variant to show a significant association with perinatal depression symptoms. Moreover, epigenetic modifications, including DNA methylation marks involved in oestrogen signalling and gene expression levels of ESR1 and ESR2, were found to be associated with perinatal depression symptoms.

When interpreting the associations between oestrogen receptor gene variants and perinatal depression symptoms, it is important to consider some mediating factors. One such factor is the specific genes and the function of their encoded receptor, since three different oestrogen receptor genes were investigated. We only report findings for ESR1 and ESR2, as we found no candidate gene studies investigating the association between GPER and perinatal depression symptoms and no GWASs with genome-wide significance for variations in GPER. Despite the fact that GPERs are membrane-bound oestrogen receptors, which are expressed in various brain areas and are important mediators of oestrogen’s rapid, non-genomic effects through the activation of various signal transduction pathways, their function in the brain and thus their implications for behaviour are still largely understudied. ^{Reference Marino, Galluzzo and Ascenzi51,Reference Hadjimarkou and Vasudevan106–Reference Sundermann, Maki and Bishop108} Consequently, no conclusions can be drawn regarding the impact of genetic variations in GPERs and their associated rapid signalling pathways, which may be implicated in the aetiology of perinatal depression symptoms. However, our review revealed that ESR1 was more frequently associated with perinatal depression symptoms than ESR2. On the one hand, this finding can be partially explained by the higher number of genetic variations studied in ESR1 compared to ESR2, and thus a potential publication bias. On the other hand, the distinct expression patterns of the encoded receptors indicate a more important role in emotional functions and moods for oestrogen receptor-α encoded by ESR1 than for oestrogen receptor-β encoded by ESR2. ^{Reference Osterlund and Hurd45}

In addition to the specific gene and the function of the receptor it encodes, the type of genetic variation must also be considered, with a broad spectrum of genetic variation having already been investigated. Most of the reviewed studies analysed SNPs, including RFLPs, and only a few studies examined STRs and haplotype combinations. Overall, 11 SNPs (including two RFLPs) and one STR were found to be associated with perinatal depression symptoms. ^{Reference Costas, Gratacòs, Escaramís, Martín-Santos, Diego and Baca-García76–Reference Zou, Zhang, Xie, Chen and Xia78,Reference El-Ibiary, Hamilton, Abel, Erdman, Robertson and Finley87,Reference Pinsonneault, Sullivan, Sadee, Soares, Hampson and Steiner88,Reference Jiang, Wei and Xie91,Reference Kiewa, Mortlock, Meltzer-Brody, Middeldorp, Wray and Byrne92} Seven of the SNPs did not remain significant after correction for multiple testing, indicating that the initial associations may be susceptible to false positives. ^{Reference Costas, Gratacòs, Escaramís, Martín-Santos, Diego and Baca-García76,Reference El-Ibiary, Hamilton, Abel, Erdman, Robertson and Finley87,Reference Pinsonneault, Sullivan, Sadee, Soares, Hampson and Steiner88} Regarding RFLPs, one of three studies analysing rs4986938 (Alu-l) and two of three studies analysing rs2234693 (Pvu-ll) found a significant association with perinatal depression symptoms ^{Reference Costas, Gratacòs, Escaramís, Martín-Santos, Diego and Baca-García76–Reference Zou, Zhang, Xie, Chen and Xia78,Reference Tan, Lim, Chua, Tan, Lee and Chen93} However, these findings were not adjusted for multiple testing, thus increasing the likelihood of false positive results. Notably, although these variants are, by definition, also SNPs, they are referred to as RFLPs because of their ability to disrupt the recognition sites of specific restriction enzymes, such as Pvu-II or Alu-I, which cut the DNA at these points. ^{Reference Maney55} Previous research linked both of these RFLPs to a risk of depression in women. ^{Reference Li, Xie, Wang, Liu, Liao and Yin60,Reference Ryan and Ancelin61,Reference Keyes, Agnew-Blais, Roberts, Hamilton, Vivo and Ranu109} However, as these investigations did not strictly distinguish between major depression and depression that occurs during reproductive transitions, for the time being, it is not possible to conclude whether rs4986938 and rs2234693 may represent a shared genetic factor for both major depression and perinatal depression or whether they are specific to perinatal depression symptoms. Regarding the STR thymine-adenine repeat, one study found a significant association with perinatal depression symptoms while another detected a trend for an association. ^{Reference Elwood, Murray, Bell, Sinclair, Kernohan and Stockdale58,Reference Ryan and Ancelin61} Notably, a previous study found that the thymine-adenine repeat was in strong linkage disequilibrium with the RFLP rs2234693. ^{Reference Becherini, Gennari, Masi, Mansani, Massart and Morelli110} Considering linkage disequilibrium, single variation analyses might be insufficient, as some alleles occur at different sites more frequently than would be randomly expected. ^{Reference Slatkin111} For instance, while some variants in linkage disequilibrium are highly correlated, one may serve as a marker, whereas another could directly be implicated in gene expression. Analysing a single variant can therefore miss subtle or combined effects that contribute to a phenotype. Therefore, analyses including haplotypes, a combination of frequently co-occurring alleles on a chromosome, provide more information on further genetic variations. ^{Reference Sundermann, Maki and Bishop108,Reference Clark112} This approach improves the robustness and reliability of results and enhances the power of association tests, particularly when multiple alleles are involved. ^{Reference Clark112,Reference Morris and Kaplan113} However, the reviewed haplotype combinations were not found to be significantly associated with perinatal depression symptoms, ^{Reference Costas, Gratacòs, Escaramís, Martín-Santos, Diego and Baca-García76,Reference Tan, Lim, Chua, Tan, Lee and Chen93} although this finding should be interpreted with caution as only four different haplotype combinations were examined, which were each tested only once. Overall, although various types of genetic variations were analysed across the included candidate gene studies, the majority of the specific variants were only examined in a limited number of studies. Given that genetic variations generally predict only small differences in the phenotype, ^{Reference Cannon and Keller114} a careful selection of the genetic variation to be investigated might be crucial to achieve more stable and reliable results. Moreover, genome-wide approaches may provide evidence for novel risk variants without the bias of prior hypotheses, thereby overcoming limitations of candidate gene approaches and leading to a more comprehensive understanding of the genetic architecture of perinatal depression symptoms. ^{Reference Tam, Patel, Turcotte, Bossé, Paré and Meyre115,Reference Duncan, Ostacher and Ballon116} Notably, both SNPs identified by the included GWASs represent novel risk variants, as they have not been investigated by the included candidate gene studies. ^{Reference Jiang, Wei and Xie91,Reference Kiewa, Mortlock, Meltzer-Brody, Middeldorp, Wray and Byrne92} However, GWASs are often underpowered, since a high level of significance is needed to account for multiple testing. ^{Reference Tam, Patel, Turcotte, Bossé, Paré and Meyre115} This limitation was also evident in the included GWASs, where the majority was underpowered. Therefore, conducting meta-analyses combining different cohorts has gained considerable interest in increasing sample size and thus statistical power. However, although 20 distinct cohorts were investigated in a recent meta-analysis of GWASs of perinatal depression, no SNP achieved genome-wide significance. ^{Reference Guintivano, Byrne, Kiewa, Yao, Bauer and Aberg74} To address the power limitation, Kiewa et al ^{Reference Kiewa, Mortlock, Meltzer-Brody, Middeldorp, Wray and Byrne92} employed another approach, using a two-step analysis. First, they identified genome-wide significant loci for a latent reproductive factor, including reproductive disorders such as endometriosis. Second, the loci identified were tested specifically for their association with perinatal depression symptoms, applying a significance threshold of p ≤ 0.05. This approach reduced the multiple testing burden compared to conducting a genome-wide analysis specifically for perinatal depression. In summary, although various genetic variations have been investigated, only a small number of them have been found to be significantly associated with perinatal depression symptoms. Moreover, the majority did not withstand correction for multiple testing or were not adjusted for multiple testing, increasing the risk of false positive results. Initial evidence suggests a more important role for RFLPs such as rs2234693 in perinatal depression symptoms than for other variations in oestrogen receptor genes. However, little is known about their role and the biological pathways that potentially result in perinatal depression symptoms. ^{Reference Sundermann, Maki and Bishop108} Moreover, it is important to note that the observed associations are based on correlations, which precludes any direct causal interpretations regarding the specific biological mechanisms that potentially explain these associations. Consequently, it may also be that these variations do not affect oestrogen signalling directly but are in linkage disequilibrium with another functional variation, which may in turn explain the significant associations with perinatal depression symptoms.

While Mendelian inheritance provides a framework for understanding how single-gene traits are transmitted over generations, the majority of complex disorders, including perinatal depression symptoms, do not follow simple Mendelian patterns and involve multiple factors. ^{Reference Cannon and Keller114} Notably, none of the reviewed studies investigated potential gene–environment interactions with oestrogen receptor genes in perinatal depression symptoms. One study additionally examined the influence of environmental factors and found that a distressed partner relationship, unplanned pregnancy and a self-reported history of depression were predictors of perinatal depression. ^{Reference El-Ibiary, Hamilton, Abel, Erdman, Robertson and Finley87} However, one study examined potential gene–gene interactions with oestrogen receptor genes in perinatal depression symptoms and found that three out of seven potential gene–gene interactions with the thymine-adenine repeat were associated with perinatal depression symptoms, including the dopamine transporter gene (SLC6A3) and two genes involved in serotonergic signalling (HTR2A and SLC6A4). ^{Reference Pinsonneault, Sullivan, Sadee, Soares, Hampson and Steiner88} Although these data were not adjusted for multiple testing, meaning that false positive results cannot be ruled out, the initial evidence suggests that various gene–gene interactions with oestrogen receptor genes may be involved in perinatal depression symptoms.

Besides genetic factors, it is also crucial to consider epigenetic modifications, as they have the potential to regulate gene transcription. ^{Reference Attwood, Yung and Richardson56} Only a limited number of studies (n = 5) were found to examine epigenetic modifications of oestrogen receptor genes in relation to perinatal depression symptoms. The majority investigated these modifications through epigenome-wide DNA methylation analysis. ^{Reference Guintivano, Arad, Gould, Payne and Kaminsky81,Reference Nakamura, Nakatochi, Kunimoto, Okada, Aleksic and Toyama83,Reference Lund, Kyläniemi, Pettersson, Kaukonen, Konki and Scheinin101,Reference Lapato, Roberson-Nay, Kirkpatrick, Webb, York and Kinser102} Notably, none of the reviewed studies investigated methylation levels using a targeted approach. Initial evidence from the reviewed studies suggests that DNA methylation marks in regions involved in oestrogen signalling, rather than in oestrogen receptor genes directly, may be associated with perinatal depression symptoms. However, the ability to directly compare these studies is limited, since there is only a small number of studies, which employed varying methodologies, including the analysis of distinct tissue types. Given the tissue-specific nature of epigenetic modifications, findings from peripheral blood may not be applicable to other tissues. ^{Reference Bakulski, Halladay, Hu, Mill and Fallin117,Reference Smith, Kilaru, Klengel, Mercer, Bradley and Conneely118} Moreover, although one study found an association between microRNAs targeting ESR1 and ESR2 and perinatal depression symptoms, ^{Reference Tang, Liu, Chen and Wang95} the findings were not adjusted for multiple testing, meaning that false positive results cannot be ruled out. Therefore, it is not yet possible to draw definitive conclusions regarding the involvement of epigenetic modifications of oestrogen receptor genes in the aetiology of perinatal depression symptoms. Nevertheless, initial evidence points to an involvement of epigenetic modifications related to oestrogen signalling in perinatal depression symptoms.

Furthermore, gene expression levels also warrant careful consideration, as they may offer crucial information about the functional and biological implications of the genetic variants and epigenetic modifications. While none of the genetic and epigenetic studies investigated whether these (epi-)genetic variations affected gene expression levels, a small number of studies (n = 8) investigated gene expression levels of oestrogen receptor genes in relation to perinatal depression symptoms. Four studies reported an association between mRNA levels of ESR1 and perinatal depression symptoms. ^{Reference Mehta, Newport, Frishman, Kraus, Rex-Haffner and Ritchie80,Reference Guleria, Suneja, Sharma, Banerjee, Bhatia and Chaudhary82,Reference Guintivano, Aberg, Clark, Rubinow, Sullivan and Meltzer-Brody89,Reference Pan, Xu, Zhang, Su, Chen and Li90,Reference Redei, Ciolino, Wert, Yang, Kim and Clark103} However, the findings from one of these studies did not withstand correction for multiple testing, ^{Reference Guintivano, Aberg, Clark, Rubinow, Sullivan and Meltzer-Brody89} and findings from another study were not adjusted for multiple testing, ^{Reference Guleria, Suneja, Sharma, Banerjee, Bhatia and Chaudhary82} indicating a higher risk of false positive results. Similarly for mRNA levels of ESR2, although two studies reported an association with perinatal depression symptoms, one of these did not withstand correction for multiple testing and the other did not adjust for multiple testing. ^{Reference Guintivano, Aberg, Clark, Rubinow, Sullivan and Meltzer-Brody89,Reference Redei, Ciolino, Wert, Yang, Kim and Clark103} Moreover, conflicting results have been reported, with both up- and downregulation of mRNA levels. Given that gene expression levels of oestrogen receptors are known to be tissue specific, ^{Reference Grandien, Berkenstam and Gustafsson119} comparisons between the studies are challenging and interpretations may be limited in terms of generalisability, because of the use of distinct tissue types across the reviewed studies. Furthermore, although blood is suitable for gene expression analysis, variations in collection, storage and extraction may differentially affect transcription profiles. ^{Reference Gautam, Donohue, Hoke, Miller, Srinivasan and Sowe120} Although binding to oestrogen receptors can induce both genomic and non-genomic effects, ^{Reference Björnström and Sjöberg121,Reference Thiebaut, Vlaeminck-Guillem, Trédan, Poulard and Le Romancer122} the studies did not examine the specific biological mechanisms implicated in the manifestation of distinct gene expression levels, meaning that no causal conclusions about the implicated pathways can be drawn. Nevertheless, initial evidence indicates that gene expression levels of ESR1 and ESR2 are associated with perinatal depression symptoms.

In addition, when interpreting the findings of this review, it is essential to take potential confounding factors into account. Notably, 15 studies accounted for confounding variables, most frequently age, followed by genetic ancestry. Although a previous study found no direct association between genetic ancestry and perinatal depression, ^{Reference Guintivano, Sullivan, Stuebe, Penders, Thorp and Rubinow123} it is possible that differences in genetic variations may reflect population-specific differences rather than behavioural differences. This limitation is particularly relevant to the candidate gene approach, since this approach is less effective in accounting for subtle genetic ancestry differences compared to genome-wide approaches. ^{Reference Duncan, Ostacher and Ballon116}

The reviewed studies exhibited numerous methodological differences regarding the assessment of perinatal depression symptoms, which may account for some of the inconclusive findings. First, the reviewed studies employed a wide range of assessment time points, spanning from 11–14 weeks of pregnancy to 12 months postpartum. These differences reflect the current inconsistencies regarding the distinction between major depression and perinatal depression, which can largely be attributed to varying definitions of the time of symptom onset and how the postpartum period is defined. ^{Reference Johann and Ehlert20} Research has suggested distinct genetic factors contributing to depression during pregnancy and postpartum, with a greater impact after birth. ^{Reference Figueiredo, Parada, Araujo, Silva and Del-Ben124,Reference Samuelsen, Ystrom, Gjerde and Eilertsen125} However, it was not possible to compare the findings regarding onset during pregnancy and onset postpartum because of the limited number of studies investigating each time period, as well as the heterogeneous study designs. As such, it cannot be ruled out that oestrogen receptor gene variants associated with perinatal depression symptoms differ depending on whether onset occurs during pregnancy or postpartum. Second, the reviewed studies employed a wide range of self-report instruments and clinical interviews to assess perinatal depression symptoms, most of which were only employed in one study and were not specifically designed for use during the perinatal period. Given that some depressive symptoms are associated with common physiological symptoms during the perinatal period, their differentiation may be challenging. ^{Reference Halbreich and Karkun126,Reference Marcus127} However, the most widely used screening tool was the EPDS, which was specifically designed for use during the perinatal period by excluding normative physiological perinatal symptoms associated with depressive symptoms, ^{Reference Cox, Holden and Sagovsky105} although notably, the studies applied various cut-off values for the EPDS, ranging from 8 to 20. ^{Reference Costas, Gratacòs, Escaramís, Martín-Santos, Diego and Baca-García76,Reference Zou, Zhang, Xie, Chen and Xia78,Reference Mehta, Grewen, Pearson, Wani, Wallace and Henders79,Reference Guleria, Suneja, Sharma, Banerjee, Bhatia and Chaudhary82,Reference Nakamura, Nakatochi, Kunimoto, Okada, Aleksic and Toyama83,Reference El-Ibiary, Hamilton, Abel, Erdman, Robertson and Finley87,Reference Pinsonneault, Sullivan, Sadee, Soares, Hampson and Steiner88,Reference Tang, Liu, Chen and Wang95} Although a cut-off value of 11 has been found to maximise both sensitivity and specificity for detecting depression in pregnant and postpartum women, other cut-off values may be used to prioritise either sensitivity or specificity. ^{Reference Levis, Negeri, Sun, Benedetti and Thombs128} Moreover, the majority of the GWASs employed a retrospective assessment of perinatal depression symptoms using the EPDS, which may introduce recall bias and thus affect accuracy of the reported symptoms. ^{Reference Kiewa, Meltzer-Brody, Milgrom, Guintivano, Hickie and Whiteman96–Reference Byrne, Carrillo-Roa, Penninx, Sallis, Viktorin and Chapman98,Reference Pouget, Taylor, Dennis, Grigoriadis, Oberlander and Frey104} Overall, the assessment of perinatal depression symptoms varies widely, which makes it difficult to compare the findings and may explain some of the inconsistencies.

This is the first systematic review to provide an overview of the current literature on the association among genetic variations, epigenetic modifications and gene expression levels of oestrogen receptor genes and perinatal depression symptoms. However, some limitations need to be addressed. Despite 29 studies being included, only a small number of studies investigated each category, with 16 studies investigating genetic variations, five studies examining epigenetic modifications and eight studies analysing gene expression levels of oestrogen receptor genes. Moreover, although 16 genetic studies were identified for inclusion, nine of these employed a candidate gene approach, whereby each genetic variation was only analysed in a limited number of studies, with the majority being tested in only one study. Furthermore, the genetic studies conducting a genome-wide approach were mostly underpowered. In addition, the broad inclusion criteria in terms of study design restricted potential comparisons between the studies. Accordingly, we were unable to compare the findings for subgroups with symptom onset during pregnancy and symptom onset postpartum. Moreover, as all of the included studies employed a correlational design, we cannot draw causal conclusions regarding how the investigated markers may ultimately affect perinatal depression symptoms. Another limitation of this review lies in its primary focus on oestrogen receptor genes. As other sex steroid hormones such as progesterone also fluctuate during the perinatal period and are also associated with mood and behaviour, the sensitivity of their receptors may also be implicated in perinatal depression symptoms. ^{Reference Sundström-Poromaa, Comasco, Sumner and Luders129} Since progesterone and oestrogen can exert both synergistic and antagonistic effects within the same neurotransmitter systems, a complex interplay underlying perinatal depression symptoms cannot be ruled out and warrants further investigation. ^{Reference Del Río, Alliende, Molina, Serrano, Molina and Vigil43}

To summarise, initial evidence supports the involvement of oestrogen receptor genes in perinatal depression symptoms, through a potential inter-individual modulation of the oestrogen receptors’ sensitivity to oestrogen. Genetic variations such as rs2234693 in ESR1, along with epigenetic modifications, including DNA methylation marks involved in oestrogen signalling and gene expression levels of ESR1 and ESR2, may serve as future biomarkers for perinatal depression symptoms. However, the current empirical evidence is insufficient to establish these biomarkers for clinical use, and the correlational nature of the results restricts definitive conclusions about the specific biological mechanisms underlying these associations. In view of the present findings, we recommend that future research should not only replicate these findings in larger samples, with an emphasis on reducing methodological inconsistencies and exploring potential interactions, but should also conduct further functional investigations to uncover the specific biological pathways through which oestrogen receptor genes may contribute to perinatal depression symptoms. Future research also needs to explore the potential involvement of GPER in perinatal depression symptoms in more detail. Since non-genomic oestrogen signalling may be crucial in modulating the rapid intracellular responses to oestrogen fluctuations, a consideration of both the genomic and non-genomic effects of oestrogen may provide vital insights into how hormonal changes affect the brain and mood regulation during the perinatal period. Given the complexity of perinatal depression and its aetiology, we recommend that future studies should also employ more comprehensive and integrative approaches, such as multi-omics, to simultaneously analyse data on various genetic, epigenetic, hormonal, environmental and psychosocial factors. Such approaches will be critical in moving towards a better understanding of the aetiology of perinatal depression symptoms and in establishing reliable biomarkers.