BRCA1: The chromatin connection

Understanding how BRCA1 functions as a tumor suppressor may ultimately lead to strategies to combat breast and ovarian cancers arising in individuals with germline BRCA1 mutations. Many reports have suggested that BRCA1 is involved in a broad range of cellular activities that include transcriptional control and DNA repair. In a recent study by Bochar et al., researchers isolated the main BRCA1-containing protein complex from cells, and found that BRCA1 is a component of the multiprotein human chromatin remodeling complex, SWI/SNF. BRCA1 interacted directly with the BRG1 protein component of SWI/SNF, and this interaction mediated the ability of BRCA1 to function as a transcriptional coactivator. These findings indicate involvement of chromatin-remodeling complexes in the pathways regulated by BRCA1 and also point to other components of the SWI/SNF complex as potential genes whose mutations may result in breast and ovarian cancers.

Leila Alland

Transcriptional dynamics of the estrogen receptor

Estrogen is important in many physiological and pathological processes, including breast cancer. Tamoxifen is a selective estrogen response modulator (SERM) that functions as both an estrogen agonist and antagonist that is used in the chemoprevention of breast cancer. The biological actions of estrogen and SERMs are mediated by estrogen receptors a and b, which modulate transcription through the recruitment of a variety of cofactors. Shang et al. studied the dynamics of cofactor recruitment to target genes by liganded estrogen receptor in response to estrogen or tamoxifen in MCF-7 breast cancer cells. Using chromatin immunoprecipitation assays to detect endogenous promoter-bound transcription factors, they found that in response to estrogen, estrogen receptors recruited many coactivators to estrogen-responsive promoters. This occurred in a cyclic way, as there were substantial changes in the coactivators bound to the promoter over time. In contrast, tamoxifen induced the recruitment of estrogen receptors and an associated co-repressor complex to the promoter, indicating that tamoxifen may be actively involved in gene repression. These results also support a model in which the ability of SERMs to act as an agonist or antagonist of estrogen may be influenced by differences in cellular cofactor levels.

Leila Alland

Tamoxifen trial

In the 10 years after diagnosis of breast cancer, women with BRCA1 or BRCA2 mutations have a 35% risk of developing cancer in the other breast. Prophylactic mastectomy and chemoprevention are the most common strategies for preventing this reoccurrence of breast cancer. Tamoxifen, a drug that blocks estrogen receptor function, is known to protect against contralateral breast cancer in the general population. However, it was not known whether it protects against breast cancer associated with the BRCA1 or BRCA2 mutations. Narod et al. report that tamoxifen reduces the risk of developing contralateral breast cancer by 50% in carriers of BRCA1 and BRCA2 mutations. The authors made this discovery after studying the history of tamoxifen use in 209 women with bilateral breast cancer and 384 women with unilateral disease, both associated with these mutations. In women who used tamoxifen for 2-4 years, the risk of contralateral breast cancer was reduced by 75%. However, the study does not show whether tamoxifen slows the growth of existing tumors or reduces the development of new ones, and tamoxifen side effects include an increased risk of endometrial cancer.

Kris Novak

Phase III for HER2

An antibody against HER2 holds promise for improving metastatic breast cancer chemotherapy. HER2 is a tyrosine kinase receptor that dimerizes with another member of the epidermal growth factor receptor family to initiate signaling pathways that lead to cell division. This receptor is overexpressed in 25-30% of breast cancers, and trastuzumab, a humanized monoclonal antibody that blocks HER2 function, slows tumor growth in vitro and in vivo. Phase I and II clinical trials showed that the antibody is safe, causes regression of HER2-overexpressing metastatic breast tumors and is most effective when given in combination with chemotherapy. Slamon et al. report the results of a phase III trial comparing the outcomes of weekly intravenous trastuzumab plus first-line chemotherapy with chemotherapy alone. In a trial of 469 women with HER2-overexpressing metastatic breast cancer, they found that patients receiving the combination therapy had significantly higher response rates, longer time to disease progression and better survival rates than patients receiving chemotherapy alone. The trial, however, also demonstrated some risks of this approach, as the incidence of cardiotoxicity was significantly higher among patients who received the dual therapy. The basis of the cardiotoxicity remains unclear, and future studies are required to determine whether these side effects arise from administration of the antibody itself or a toxic combination of trastuzumab and specific chemotherapeutic agents.

Kris Novak

A functional role for BRCA2

It is now well established that the inheritance of mutations in the breast cancer susceptibility genes BRCA1 and BRCA2 increases the risk of developing breast and ovarian cancer. Although these genes are classic tumor suppressor genes, in that loss of both alleles is required for the initiation of malignancy, information on which cellular pathways are affected by alterations in the BRCA genes is limited, and the function of the BRCA2 protein remains a mystery. A recent study by Marmorstein et al. sheds some light on the latter question.

Through association with human RAD51, BRCA2 is believed to involved in DNA damage-response pathways. To determine the molecular basis for this function, Marmorstein's team isolated BRCA2 from HeLa cells and identified its components by mass spectrometry. They discovered that the BRCA2 complex contains a 35-kilodalton DNA-binding protein called BRCA2-associated factor (BRAF35). Analysis of the BRAF35 RNA expression in developing mouse embryos showed that the protein is mainly expressed during mitosis. In addition, the BRAF35-BRCA complex is associated with the early phases of mitotic cell cycle progression, and treatment with antibodies to each protein stopped the cell cycle at G2. This discovery could be exploited to develop therapeutics that stop cell division and combat the growth of BRCA cancers. Discovery of the BRAF35 DNA-binding protein supports the proposed function of BRCA2 in DNA repair and recombination, as BRAF35 may be responsible for targeting BRCA to sites of DNA damage.

Karen Birmingham

Pinpointing breast cancer with adenoviral vectors

One of the biggest challenges facing cancer therapy remains the ability to develop treatments that kill tumor cells but leave normal cells unaffected. Gene therapy is one strategy that may rise to this challenge, as the use of promoters enables therapeutic genes to be targeted specifically to cancer cells. An example of this strategy is the development by Kurihara et al. of an adenoviral construct targeting MUC1-positive breast cancer cells.

Mucins are large glycoproteins that form a protective layer along the lumens of the organs of the gastrointestinal and reproductive tracts. The MUC1 antigen is overexpressed in a number of human carcinomas, and as many as 80% of primary breast tumors express high levels of MUC1.

Kurihara's team created a replication-competent adenoviral vector in which the DF3/MUC1 gene promoter drives expression of E1A. Using a xenograft model of human breast cancer in female nude mice, the team showed that the vector selectively replicates in MUC1-positive breast cancer cells because the DF3/MUC1 promoter is upstream of E1A. Injection of the construct and lysis of the replicating virus caused tumors to shrink, and by 4 weeks after injection, tumors were barely palpable.

The therapeutic activity of the vector was then enhanced by inserting a gene encoding tumor necrosis factor (TNF), as this cytokine is known to have antitumor activity. Treatment of mice with this TNF construct not only stabilized tumor progression after a single injection but also caused tumor regression. It is hoped that such a therapy could be developed as a new and highly specific method for treating patients with MUC1-positive breast cancer.

Karen Birmingham

Control of tumor metastasis

The primary cause of death in human breast cancer is metastasis, the process by which cells from the primary tumor enter the bloodstream and give rise to secondary tumors, most commonly in the bone, lungs, liver and brain. Felding-Habermann and colleagues have now identified a surface molecule that, in its activated form, mediates an essential step in the metastatic process, the adherence of individual tumor cells to blood vessels during flow conditions. The molecule in question is integrin avb3, a member of the integrin family of cell adhesion molecules. Expression of activated avb3 caused cultured tumor cells to adhere to platelets in 'flow' conditions, and metastasis-derived tumor cell lines and freshly isolated metastatic human breast tumor cells express activated avb3. Finally, expression of activated but not non-activated avb3 in tumor cells strongly promoted their metastasis in a mouse model of breast cancer. These findings indicate that activation of the adhesion molecule integrin avb3 is an essential step in the development of metastases.

Charlotte Wang

Genetic susceptibility to breast cancer

Epidemiologic studies have long suggested genetic background as a risk factor for the development of breast cancer, and have in fact identified several genes that have a specific function, most notably BRCA1 and BRCA2. Now Feigelson and colleagues have studied the allele distribution for two genes involved in estrogen synthesis, CYP17 and HSD17B1, and find that women with certain 'susceptibility' alleles for these genes have a significantly higher risk of developing advanced breast cancer.

The gene CYP17 encodes the cytochrome p450c17a enzyme, which is essential in the synthesis of all steroid hormones; HSD17B1 encodes the enzyme that catalyzes the final step in the synthesis of active estrogen (estradiol). The authors found that the effect of these alleles on susceptibility to breast cancer was most easily detected in non-obese women not on hormone replacement therapy (fat can be a source of estrogen, as is hormone replacement). These results tend to confirm that circulating levels of estrogen contribute to susceptibility to breast cancer, and constitute the first step in developing a multi-gene model for susceptibility to advanced and/or aggressive breast cancers.

Charlotte Wang