Promoter Hypermethylation of Estrogen Receptor Alpha Gene Is Correlated to Estrogen Receptor Negativity in Iranian Patients with Sporadic Breast Cancer

Headnote

Abstract

Objective: Breast Cancer is the most common cancer in Iranian women. Breast tumors are classified based on the estrogen receptor alpha (ERa) expression status into ER negative and ER positive tumors. ER negative tumors tend to have worse prognosis and less likely to respond to endocrine therapy. Aberrant méthylation of gene promoter is one of the mechanisms for gene silencing in breast tumors. Because of its reversible nature, promoter méthylation is a good target for new therapeutic strategies. We aimed to evaluate the frequency of this epigenetic event in ERa gene and its association to clinicopathological features in Iranian breast cancer patients.

Materials and Methods: In this case control study the patient series consisted of 100 spo- radic primary breast cancer cases (51 ER negative and 49 ER positive tumors). None of the participants had chemo or radiotherapy before surgery. In breast tumors ERa promoter méthylation were assessed with méthylation specific polymerase chain reaction (MSP).

Data was collected on clinicopathological features of the patients. Correlation between ERa méthylation and clinicopathological characteristics of the patients was investigated by Pearson Chi-Square and Fisher's exact test.

Results: ERa méthylation was detected in 98% of ER negative and 65% of ER positive breast tumors. A strong correlation was found between ERa méthylation and ER negativ- ity in tumors (p<0.0001). Also, ERa méthylation has associated to progesterone receptor negativity (p<0.008) and double receptor negative status (p<0.0001) in breast tumors.

Conclusion: ERa méthylation occurs with high frequency in the breast tumors of Iranian breast cancer patients and may play a considerable role in pathogenesis of ERa negative tumors as a poor prognosis and more aggressive category. The reversible nature of DNA méthylation may provide new therapeutic possibilities in ER negative breast tumors.

Keywords: Estrogen Receptor, Méthylation, Breast Cancer

Cell Journal(Yakhteh), Vol 1 4, No 2, Summer 201 2, Pages: 1 02-1 09

Introduction

Breast Cancer, as the most common cancer in Iranian women, affects women at least one dec- ade younger than their counterparts in developed countries (1). The highest frequency of this malig- nancy has been observed in the 40-49 years old age group (2).

It is commonly accepted that estrogen and its receptor have an important role in the pathogen- esis of breast malignancies. Estrogen receptor alpha (ERa) expression classifies breast tumors into ER negative and ER positive cancers. About 40% of breast tumors are ER negative (3). ER negative tumors have poor prognosis in comparison to ER positive tu- mors. This type of tumor is more prevalent in younger patients and is not responsive to endocrine therapies.

Considering to the challenging nature of ERa negative tumors treatment and their innate poor prognosis, clarification of the molecular mecha- nisms that control expression of ERa is essential. This knowledge may enable us to modify the situa- tion as such to restore sensitivity to endocrine thera- pies which provide us new opportunities for thera- peutic options for ERa negative breast tumors (4).

Despite of many studies on the mechanisms of negativity of ER in breast tumors, many details still need to be clarified (5,6). The loss of ERa expression in breast cancer may result from different underlying causes, such as structural changes within the gene or transcriptional silencing (7). Abnormalities, such as point mutations, deletions, loss of heterozygosity or polymorphisms within the gene have not shown to be frequent enough to explain ER negativity phenotype (7, 8). In breast tumors such as other types of cancer, epigenetic alterations are common and related to gene expression modification (9). ft has been shown that tumor suppressor genes promoter méthylation gives growth advantage to malignant cells (10). Because of the potential reversible nature of epigenetic gene silencing, epigenetic mechanisms have been under in- tense investigations in the recent years (5, 7, 11, 12). Regarding to evidences which have been resulted from several in vitro and in vivo studies, it has been shown that the inhibitors of DNA methyltransferase and histone deacetylase enzymes can reactivate ERa gene transcription in ERa negative cells. These epi- genetic therapies could restore response to endocrine therapy in non responsive ER negative cells (13, 14). These promising landscapes have encouraged re- searchers to focus on the relationship between ERa negative phenotype and ERa promoter méthylation. However, heterogeneity of the cellular population in breast tumors,, differences in methodological ap- proaches, and variation of the studied populations (e.g. environmental exposures and ethnicity) have resulted in various reported frequencies for ERa méthylation and its relevance to clinicopathological parameters (7, 11, 12).

Improved understanding of the mechanisms in- volved in ER negative breast tumors may permit improved therapeutic choices in this poor prog- nosis and more aggressive type of breast cancer. Thus, in the present study we aimed to evaluate the prevalence of epigenetic silencing of ERa via pro- moter méthylation in Iranian patients with breast cancer and its association to ER negativity of tu- mors and other clinicopathological characteristics.

Materials and Methods

Patients

In this case control study the studied population con- sisted of 100 sporadic primary breast cancer patients referred to the Cancer Institute of Tehran University of Medical Sciences. This study was approved by the local Ethical Committee at Tarbiat Modares Univer- sity. Written consent was taken from all patients en- rolled in the study. Primary breast tumor tissues were provided by the Iran National Tumor Bank. Our inclu- sion criteria were sporadic and primary breast cancer. Patients who had recurrent breast tumor, had chemo or radiotherapy before surgery, or showed breast and/ or ovarian cancers in first or second degree relatives were excluded from the study. Tumor samples were obtained through a surgical resection, then an expert pathologist perfonned rapid macro dissection of sam- ples and transferred tumor tissues to liquid nitrogen reservoir, immediately.

Clinical and pathological data collection

The main clinicopathological features of the study population were collected. These features included age at diagnosis, menopausal status, tumor size, histological type, grade, lymph node involvement, stage and immunohistochemistry (1HC) panel (ER, PR and her2). To overcome inter laboratory varia- tions, a pathologist decided the histological type, grade and immunohistochemistry evaluations, then another expert rechecked and approved them. ER and PR were considered negative when nuclear staining of tumor cells was less than 1%. In tumors which complete and intense membrane staining were determined in the more than 10% of tumor cells, Her2 over expression was considered positive.

DNA extraction and bisulphite modification

Genomic DNA was extracted from the breast tu- mors stored in liquid nitrogen tank. DNA was ob- tained by High pure PCR template preparation kit (Roche, Gennany). Quantity and quality of the ex- tracted DNA were evaluated by a spectrophotometer (Nano drop 2000, Thenno Scientific, USA). For each sample, 1 pg of genomic DNA was used for sodium bisulphite modification as previously described (15).

Méthylation specific PCR

Two regions of ERa promoter in CpG Island were studied by methylation-specific PCR (MSP). These regions were selected based on the previous studies. ER3 and ER5 regions were included in the 5'UTR of proximal promoter of ERa gene (7,12,16). Two sep- arate MSP reactions were carried out for each region, like one with primers specified for the methylated sequence (M) and the other with primers for the un- methylated sequence (U). Primer pairs and PCR con- dition were previously described (7). MSP reaction was optimized in our lab with 1 /PCR buffer, 6.7 mM MgCL,, and 1.25 mM dNTP per 25 pi reaction vol- umes. After initial dénaturation, 2.5 units Taq DNA polymerase (Cinaclone, Iran) was added to each re- action (manual hot start). Positive MSP control for methylated specific primers was DNA extracted from MDA-MB-231 cell line. It was shown in the previ- ous works that this DNA was totally methylated in 5'UTR of ERa (7). Negative MSP control for meth- ylated specific primers was the extracted DNA from MCF-7 cell line which was shown in the previous works to be totally unmethylated in our target regions in 5'UTR of ERa (7). No DNA control was used in both methylated and unmethylated MSP reactions as the blank to show absence of template contamination. Untreated DNA was used as the control of specificity of reactions. MSP amplification products were elec- trophoresed on a 2% agarose gel with 1XTBE buffer, stained with ethidium bromide and visualized under a UV light. A tumor was considered methylated if a band was visualized in the MSP reaction with meth- ylated primers.

Statistical analysis

Correlation between ERa méthylation and clin- icopathological characteristics of the patients was investigated by Pearson Chi-Square and Fisher's exact test using SPSS version 13. The value of p<0.05 was considered significant and confidence intervals quoted were at the 95% level.

Results

Méthylation frequency of ERa promoter in sporadic breast carcinomas

The méthylation status of ERa promoter in ER3 and ER5 regions was determined in the study population using the MSP method. In analyzed 100 tumors, 71% were methylated in ER3 region and 56% in ER5 region. Tumors were classified as methylated if their bisulfate treated DNA had amplification with either one or both methylated primers. According to this definition, overall ERa méthylation was detected in 88 tumors (88%). Breast tumors methylated in ER3 region were 1.3 times more than ER5 region.

Correlation of ERa promoter méthylation and ER negativity by IHC

To determine if there is a correlation between ER méthylation and loss of ER protein expression in tu- mors (based on IHC results), we compared méthyla- tion frequency between ER negative and ER positive tumors. According to IHC results, 51% of samples were negative for ER expression. A strong correla- tion was found between ERa méthylation in ER3 region and ER negativity in tumors. It was observed that 90% of ER negative versus 51% of ER posi- tive cases were methylated in the mentioned region (pO.0001). Frequency of méthylation in ER5 re- gion was lower than ER3 region in both ER negative (69%) and ER positive (43%) tumors. Overall, ERa méthylation in both regions was significantly higher in ER negative tumors than ER positive cases. As we observed, 61 % of ER negative tumors versus 28% of ER positive tumors were methylated in both regions synchronously (p<0.0001).

ERa promoter méthylation and correlation with clinicopathologic features

The clinicopathological characteristics of the 100 primary breast tumors are described in the table 1. There were significant correlations be- tween estrogen receptor negativity in tumors and méthylation in ER3 and ER5 regions in ER promoter. No correlation was found between the followings: ERa méthylation status, age, menopausal status, tumor size, grade, nodal in- volvement, TNM stage and Her2 status. How- ever, there was a significant correlation be- tween méthylation in ER3 region and negative status of progesterone receptor by IHC in tu- mors (p<0.008). In addition, presence of double receptor negativity by IHC in tumors (ER nega- tive/PR negative status) showed a significant correlation with ER3 méthylation (pcO.OOOl).

Discussion

Breast cancer affects Iranian patients at least one decade earlier than western patients (1,2, 17, 18). Estrogen receptor alpha negative tu- mors are more prevalent in young patients with a worse prognosis than ER positive tumors (3). Different mechanisms, including: absence of specific transcription factor, presence of repres- sors, and epigenetic changes in CpG Island are potentially associated with ER negative pheno- type (5, 7, 19). Previous studies have shown that epigenetic silencing of ERa is a major mecha- nism for ER negative phenotypes in breast can- cer cells (7, 11) and the reversible nature of this mechanism may provide new therapeutic av- enues for ER negative breast tumors (13, 20).

In the present study, méthylation status of two important regions of the ERa 5'CpG Island with respect to ER expression was investigated. Previous studies have shown that ER promoter méthylation has association with lack of ER ex- pression in breast tumors (7, 12, 21-23). On the other hand, in a recent population based on case- control study on 200 breast tumors, it has been sug- gested that the relationship between DNA méthyla- tion in ER promoter and ER protein expression is weak. So, these researchers have concluded that ER méthylation is unlikely to represent a major mechanism of receptor silencing (24). Because of the reversible nature of DNA méthylation, if there is a major mechanism for ER silencing in breast tumors, it will potentially provide new options for ER negative tumors treatment in the future. Thus, we aimed to evaluate this relationship in our study population and reveal the strong correlation be- tween ERa negativity (based on IHC results) and ERa méthylation in concordance with the previ- ous works (7, 12, 21, 23, 25, 26). It seems that ER méthylation can be an important mechanism for ER silencing at least in Iranian breast cancer pa- tients.

Méthylation frequency of ERa gene in the present study was higher in comparison to the previous investigations in the different study populations (Table 2).

The differences between ERa méthylation frequencies in various studies may be due to variation in ethnicity and carcinogenic expo- sures of populations studied. Also, technical issues in MSP conditions, such as: annealing; temperature; cycles number and selection of tar- gets regions in CpG Islands may influence on final frequency. We have shown presence of ER méthylation in a sizable fraction of ER positive cases. This observation maybe correlated to cel- lular heterogeneity in breast tumors. Classifica- tion of breast tumors based on their ER status into ER negative and ER positive groups may cause partial loss of information (27). ER posi- tivity in breast tumors is a dynamic phenotype and over the natural course of cancer progres- sion, Estrogen Receptor can be lost and many ER positive tumors become ER negative (6). Resistance to endocrine therapy in a signifi- cant fraction of ER positive breast tumors and recurrence of many ER positive tumors as ER negative ones emphasize again that this group is more heterogeneous than expected. Presence of ERa méthylation in ER positive tumors is a manifestation of this heterogeneity and may contribute to endocrine therapy resistance or re- currence. More investigations needed to clarify the contribution of ERa méthylation in endo- crine therapy resistance and recurrence in ER positive breast tumors.

Investigation of clinicopathological asso- ciation with ERa méthylation showed that this phenomenon is not an age-related event in our breast cancer patients (Table 1). The relation- ship between age and ERa méthylation in dif- ferent studies was controversial. In one study on 193 Australian patients, ERa méthylation was associated with younger patient (22). In other studies there was no correlation between ERa méthylation and age in breast tumors (11, 12, 21, 23, 28).

In our study, we did not see any correlations between ERa méthylation, tumor size and can- cer stage in concordance to other investigations (12, 21-23). But in one study, the comparison of DCIS (Ductal Carcinoma In Situ), invasive ductal carcinoma and metastatic lesions showed an increasing trend in ERa méthylation with malignant progression (25). The significant correlation between méthylation in ER3 region and progesterone receptor status in tumors was not unexpected due to the fact that progesterone receptor expression is under ER control. This relationship was not independent of ER status of tumor because this association was observed only in ER negative/PR negative tumors (not in ER positive/PR negative cases). Also, we did not see any correlation between progesterone status and méthylation in ER5 region of ER promoter. It is known that the impacts of méth- ylation in various CpGs in a CpG Island are not equal and méthylation in some regions is more critical in gene silencing. It may be due to less influence of méthylation in the ER5 region on ER gene expression. Although, finding underly- ing mechanism of this observation needs more investigations.

Conclusion

Our result showed that méthylation of ERa is a prevalent epigenetic phenomenon in Iranian breast cancer patients. Since 98% of ER nega- tive tumors had méthylation in ER3, ER5 or both regions, it seems that ERa was a major target of méthylation in our population stud- ied. Therefore, the role of ERa méthylation in the etiology of ER negative phenotype, which might be regarded as a common phenomenon in Iranian patients, should be investigated fur- ther. Higher prevalence of ER méthylation in Iranian patients may be due to environmental exposures or carcinogenic lifestyles which must investigate in the future surveys. Al- though DNA méthylation in promoter region of a gene has equal impact with a mutation, epigenetic modifications are potentially re- versible, so they are good targets for new ther- apeutic strategies.

Acknowledgements

This study was a part of a PhD thesis in Medi- cal Genetics delivered by the first author and supported by a grant from faculty of Medical Sciences of Tarbiat Modares University.

The authors are grateful to Dr. Forouzan- deh Fereidooni, Dr. Zahra Mostakhdemine Hosseini, Dr. Fatemeh Kamali and Mr. Ah- mad Joulaie for their valuable collaborations in gathering clinicopathological information and to Dr. Nateghi for statistical analysis. The authors declare that they have no conflict of interest.

Sidebar

Citation: Izadi P, Noruzinia M, Karimipoor M, Karbassian MH, Akbari MT. Promoter hypermethylation of estrogen receptor alpha gene is correlated to estrogen receptor negativity in iranian patients with sporadic breast cancer. Cell J. 2012; 14(2): 102-109.

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