Journal of the National Cancer Institute Advance Access originally published online on November 13, 2007
JNCI Journal of the National Cancer Institute 2007 99(22):1655-1657; doi:10.1093/jnci/djm230
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
© The Author 2007. Published by Oxford University Press.
EDITORIALS |
Estrogen Receptors in BRCA1-Mutant Breast Cancer: Now You See Them, Now You Don’t
Correspondence to: V. Craig Jordan, OBE, PhD, DSc, Fox Chase Cancer Center, 333 Cottman Ave, Philadelphia, PA 19111-2497 (e-mail: v.craig.jordan{at}fccc.edu).
Estrogen receptor (ER) protein is expressed in estrogen target tissues (1,2). The binding of exogenous estrogen to ER orchestrates many important responses throughout a woman's body to maintain the optimal homeostasis for successful reproduction. Without estrogen, there would be no human race. However, estrogen is also involved in the development and growth of breast and endometrial cancers and, as a result, has recently earned a bad reputation in women's health (3,4).
The measurement of ER expression in breast tumors was originally used to identify which women were likely to respond to endocrine ablation therapy (5). Patients whose tumor expressed no ER were unlikely to respond to endocrine ablative surgery, whereas patients whose tumors had a detectable level of ER had improved chances of responding to ablative surgery (6). However, during the early 1970s ER was recognized as a therapeutic target for improving treatment rather than as a predictive test to recommend short-term palliation from endocrine ablative surgery (7,8). The antiestrogen tamoxifen was reinvented from being a failed contraceptive to the first targeted therapy in breast cancer (7,8). This conceptual shift led to the current recognition that the ER is perhaps the most important target identified thus far in cancer medicine. Hundreds of thousands of breast cancer patients’ lives have been improved and lengthened with the application of long-term adjuvant tamoxifen therapy (9). Although the aromatase inhibitors are now improving response rates and the side-effect profile of long-term adjuvant therapy in postmenopausal women, tamoxifen remains the antiestrogenic treatment of choice for premenopausal women and those high-risk women who choose to reduce their chances of developing breast cancer (10).
Despite the prominence of the ER as a target in breast cancer, many aspects concerning its origins and its efficacy as a therapeutic target have remained a mystery. Questions about how ER synthesis and regulation are accomplished, whether ER-negative breast cancers are derived from ER-positive breast cancers, and whether ER expression can be regenerated in ER-negative breast cancers have remained central issues in endocrinology and cancer biology for the past 40 years.
In this issue of the Journal, Hosey et al. (11) provide a fascinating insight into these issues by presenting a unifying hypothesis for the regulation of ER synthesis in breast cancer. They approached these questions by integrating prior clinical observations that have shown that BRCA1-mutant breast cancers express little ER compared with spontaneous breast tumors (12) and then deployed breast cancer cell lines, nucleic acid transfection technology, chromatin precipitation assays, and, most importantly, the power of short-interfering RNA technology to knock down expression of BRCA1. They found that BRCA1 is a central player in the regulation of ER synthesis in breast cancer.
Overall, the current success by Hosey et al. (11) in answering the questions about ER regulation is best summarized by a statement taken from the book Trilobite! by Richard Fortey (13): "Central ... is the notion of science as a web of knowledge where the apparently peripheral can suddenly become pivotal." Hosey et al. (11) have answered questions that could not have been answered 15 years ago. For example, the identification of the BRCA1 gene (14) and its mutations in familial breast cancer initially appeared to be unrelated to the ER, but the finding that breast tumors occur early during the premenopausal years of a woman's life and may have a hormonal component to their growth control (15,16) but, paradoxically, are ER-negative (12) provided a crucial piece of information necessary to solve the riddle of ER regulation. The question then became "what does a BRCA1 mutation have to do with the ER system?"
A connection between BRCA1 expression and ER has already been made by others. For example, Rosen's group (17,18) has demonstrated that the transient transfection of the wild-type BRCA1 gene into MCF-7 breast cancer cells inhibits signaling by the ER complex (17) and that BRCA1 protein interacts directly with ER (18). More recently, Rosen's group has shown that the repression of ER activity by BRCA1 is mediated through phosphatidylinositol-3 kinase signaling (19), which increases ER phosphorylation at serine 167 located in the activating function-1 domain of ER. All of these studies are interesting, but none directly addresses what a BRCA1 mutation has to do with the ER system.
Hosey et al. (11) took a direct approach to this question. They used three breast cancer cell lines: HCC1937 cells (20), which are homozygous for the BRCA1 5382insC mutation (which causes the last 34 amino acids of the BRCA1 protein to be missing) and are essentially ER negative, and the two ER-positive cell lines, MCF-7 (21) and T47D, which have different ER regulatory systems (22). Simply stated, Hosey et al. (11) showed that transfection of the wild-type BRCA1 gene into HCC1937 cells reactivates ER production and that the knockdown of BRCA1 expression with short-interfering RNAs in ER-positive cells eliminates expression of ER. They provide convincing evidence that BRCA1 protein directly regulates the synthesis of ER through binding to the ESR1 promoter and that the ubiquitous transcription factor Oct-1 also plays an important role in the regulation of ER expression. Finally, Hosey et al. (11) demonstrate that knockdown of BRCA1 expression in ER-positive cells abrogates the growth inhibitory response of the cells to the pure antiestrogen drug fulvestrant (23,24). They nicely show that expression of exogenous ER in BRCA1-depleted cells reactivates fulvestrant sensitivity. However, it would have been interesting to examine the effects of BRCA1 expression on the sensitivity of the cells to tamoxifen, a more clinically relevant antiestrogen drug. Fulvestrant is usually used as a second- or third-line antihormone therapy and is not really used to treat premenopausal patients, i.e., patients who tend to carry BRCA1 mutations. The fact that tamoxifen substantially enhances the development of mammary tumors in BRCA1 co/co MMTV-CRE/p53+/– mice and is more estrogen-like in cells with no full-length BRCA1 knockdown (25) suggests that this valuable observation should be pursued because of its clinical relevance.
Despite the large size of BRCA1, many mutations that alter the functions of the BRCA1 protein have been identified across the entire gene. The 5382insC mutation in the HCC1937 cells used by Hosey et al. (11), which is located in the terminal transactivation domain of BRCA1, and the 185delAG mutation are the two most common mutations found in the Ashkenazi Jewish population. Mutations for the BRCA1 gene occur with a combined frequency of about 100x higher in Ashkenazi Jews than in an unselected white population (26,27). Because 185delAG and 5382insC are the most severe mutations (i.e., they are associated with more aggressive, ER-negative breast cancers), the decision by Hosey et al. (11) to study a cell line that has the 5382insC mutation was a wise one. However, it is possible that other mutations in the BRCA1 gene may explain why some BRCA1 mutant breast tumors remain ER positive and actually respond to tamoxifen treatment (16). This possibility would be interesting to test.
On the basis of their results, Hosey et al. (11) developed a plausible model to explain the formation of an ER-negative tumor through 1) the loss of ER expression after the wild-type BRCA1 allele is lost by a mechanism involving loss of heterozygosity and 2) the loss of BRCA1 expression in sporadic tumors by mechanisms involving loss of heterozygosity and epigenetic inactivation. Their model can now be rigorously investigated and validated so that the mystery of ER regulation can be settled once and for all.
In summary, the study by Hosey et al. (11) exemplifies the "notion of science as a web of knowledge where the apparently peripheral can suddenly become central" (13). The results of Hosey et al. (11) provide justifiable optimism that the current technology can be used to solve biologic questions. However, this is only one of the lessons to be learned from the advance made by Hosey et al. (11). The other lessons are that models are needed to solve mechanisms in biology and that there needs to be an integrated approach with different medical disciplines to address current research problems in biology and medicine. The discovery of mutations in the BRCA1 gene was clearly peripheral to the discovery of a plausible mechanism to explain the regulation of ER synthesis. The use of a breast cancer cell line (20) that was derived from a BRCA1 mutation carrier was critical for the demonstration that wild-type BRCA1 plays a role in ER synthesis. Perhaps most importantly, however, it is the financial investment in individual nondirected research that has provided the most powerful tools for investigators to solve problems. For example, Fire et al. (28) and Mello (29) studied the development of Caenorhabditis elegans, a transparent worm, and made the unanticipated discovery that a certain form of RNA would silence or interfere with the expression of genes. This discovery created and commercialized short-interfering RNAs for the whole human genome that ultimately allowed Hosey et al. (11) to silence genes selectively. They switched off ER synthesis by silencing the BRCA1 gene in two widely used ER-positive cell lines MCF-7 and T47D. Now you see the ER and now you don’t. We do not live simply in interesting times; we live in exciting times.
NOTES
Dr V. C. Jordan is supported by the Department of Defense Breast Program under Center of Excellence award number BC050277, R01 GM067156; Fox Chase Cancer Center (FCCC) Core Grant, National Institutes of Health P30 CA006927; and the Weg Fund of FCCC. Views and opinions of and endorsements by the author(s) do not reflect those of the US Army or the Department of Defense.
REFERENCES
(1) Jensen EV, Jacobson HI. Basic guides to the mechanism of estrogen action. Recent Prog Horm Res (1962) 18:387–414.[ISI]
(2) Toft D, Gorski J. A receptor molecule for estrogens: isolation from the rat uterus and preliminary characterization. Proc Natl Acad Sci USA (1966) 55:1574–81.
(3) Rossouw JE, Anderson GL, Prentice RL, LaCroix AZ, Kooperberg C, Stefanick ML, et al. Risks and benefits of estrogen plus progestin in healthy postmenopausal women: principal results from the Women's Health Initiative randomized controlled trial. JAMA (2002) 288:321–33.
(4) Chlebowski RT, Hendrix SL, Langer RD, Stefanick ML, Gass M, Lane D, et al. Influence of estrogen plus progestin on breast cancer and mammography in healthy postmenopausal women: the Women's Health Initiative randomized trial. JAMA (2003) 289:3243–53.
(5) McGuire WL, Carbone PP, Sears ME, Escher GC. Estrogen receptors in human breast cancer: an overview. In: Estrogen receptor in human breast cancer—McGuire WL, Carbone PP, Volmer EP, eds. (1975) New York: Raven Press. 1–7.
(6) Jensen EV, Block GE, Smith S, Kyser K, DeSombre ER. Estrogen receptors and breast cancer response to adrenalectomy. Natl Cancer Inst Monogr (1971) 34:55–70.[Medline]
(7) Jordan VC. Tamoxifen: a most unlikely pioneering medicine. Nat Rev Drug Discov (2003) 2:205–13.[CrossRef][ISI][Medline]
(8) Jordan VC, Brodie AMH. Development and evolution of therapies targeted to the estrogen receptor for the treatment and prevention of breast cancer. Steroids (2007) 72:7–25.[CrossRef][ISI][Medline]
(9) Early Breast Cancer Trialists’ Collaborative Group. Effects of chemotherapy and hormonal therapy for early breast cancer on recurrence and 15-year survival: an overview of the randomised trials. Lancet (2005) 365:1687–1717.[CrossRef][ISI][Medline]
(10) Fisher B, Costantino JP, Wickerham DL, Cecchini RS, Cronin WM, Robidoux A, et al. Tamoxifen for the prevention of breast cancer: current status of the National Surgical Adjuvant Breast and Bowel Project P-1 study. J Natl Cancer Inst (2005) 97:1652–62.
(11) Hosey AM, Gorski JJ, Murray MM, Quinn JE, Chung WY, Stewart GE, et al. Molecular basis for estrogen receptor 
deficiency in BRCA1-linked breast cancer. J Natl Cancer Inst (2007) 99:1683–94.
(12) Verhoog LC, Brekelmans CT, Seynaeve C, van den Bosch LM, Dahmen G, van Geel AN, et al. Survival and tumour characteristics of breast-cancer patients with germline mutations of BRCA1. Lancet (1998) 351:316–21.[CrossRef][ISI][Medline]
(13) Fortey R. Tribolite! Eyewitness to evolution. (2000) London: Harper Collins.
(14) Futreal PA, Liu Q, Shattuck-Eidens D, Cochran C, Harshman K, Tavtigian S, et al. BRCA1 mutations in primary breast and ovarian carcinomas. Science (1994) 266:120–2.
(15) Rebbeck TR, Levin AM, Eisen A, Snyder C, Watson P, Cannon-Albright L, et al. Breast cancer risk after bilateral prophylactic oophorectomy in BRCA1 mutation carriers. J Natl Cancer Inst (1999) 91:1475–9.
(16) Narod SA, Brunet JS, Ghadirian P, Robson M, Heimdal K, Neuhausen SL, et al. Tamoxifen and risk of contralateral breast cancer in BRCA1 and BRCA2 mutation carriers: a case-control study. Hereditary Breast Cancer Clinical Study Group. Lancet (2000) 356:1876–81.[CrossRef][ISI][Medline]
(17) Fan S, Wang JA, Yuan R, Ma Y, Meng Q, Erdos MR, et al. BRCA1 inhibition of estrogen receptor signaling in transfected cells. Science (1999) 284:1354–6.
(18) Fan S, Ma YX, Wang C, Yuan RQ, Meng Q, Wang JA, et al. Role of direct interaction of BRCA1 inhibition of estrogen receptor activity. Oncogene (2001) 20:77–87.[CrossRef][ISI][Medline]
(19) Ma Y, Hu C, Riegel AT, Fan S, Rosen EM. Growth factor signaling pathways modulate BRCA1 repression of estrogen receptor-a activity. Mol Endocrinol (2007) 21:1905–23.
(20) Tomlinson GE, Chen TT, Stastny VA, Virmani AK, Spillman MA, Tonk V, et al. Characterization of a breast cancer cell line derived from a germ-line BRCA1 mutation carrier. Cancer Res (1998) 58:3237–42.
(21) Levenson AS, Jordan VC. MCF-7: the first hormone-responsive breast cancer cell line. Cancer Res (1997) 57:3071–8.
(22) Pink JJ, Jordan VC. Models of estrogen receptor regulation by estrogens and antiestrogens in breast cancer cell lines. Cancer Res (1996) 56:2321–30.
(23) Howell A. Pure oestrogen antagonists for the treatment of advanced breast cancer. Endocr Relat Cancer (2006) 13:689–706.
(24) Lower EE, Esparaz BT, Garnett SA, Wade JL 3rd. Evaluation of fulvestrant in clinical practice: use of an electronic data registry. Clin Breast Cancer (2007) 7:565–9.[ISI][Medline]
(25) Jones LP, Li M, Halama ED, Ma Y, Lubet RA, Grubbs CJ, et al. Promotion of mammary development by tamoxifen in a mouse model of BRCA-1 mutation-related breast cancer. Oncogene (2005) 24:3554–62.[CrossRef][ISI][Medline]
(26) Struewing JP, Hartge P, Wacholder S, Baker SM, Berlin M, McAdams M, et al. The risk of cancer associated with specific mutations of BRCA1 and BRCA2 among Ashkenazi Jews. N Engl J Med (1997) 336:1401–8.
(27) Ford D, Easton DF, Peto J. Estimates of the gene frequency of BRCA1 and its contribution to breast and ovarian cancer incidence. Am J Hum Genet (1995) 57:1457–62.[ISI][Medline]
(28) Fire A, SiQun X, Montgomery MK, Kostas SA, Driver SE, Mellow CC. Potent and specific genetic interference by double-stranded RNA in Caenorhabditis elegans. Nature (1998) 391:806–11.[CrossRef][Medline]
(29) Mello CC. Return to the RNAi world: rethinking gene expression and evolution [Nobel lecture]. Angew Chem Int Engl (2006) 46:6985–94.
Related Articles in JNCI
CiteULike 
Connotea 
Del.icio.us What's this?
Deficiency in BRCA1-Linked Breast Cancer
J Natl Cancer Inst 2007 99: 1683-1694.
J Natl Cancer Inst 2007 99: 1653.
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||