Myeloperoxidase Mediated Protection in Breast Cancer

Institution: The Burnham Institute for Medical Research
Investigator(s): Wanda Reynolds, Ph.D. -
Award Cycle: 2010 (Cycle 16) Grant #: 16IB-0111 Award: $286,500
Award Type: IDEA
Research Priorities
Biology of the Breast Cell>Pathogenesis: understanding the disease

Initial Award Abstract (2010)

Metastatic spread of cancer cells is the principle cause of death for breast cancer patients. Recent epidemiological studies link a high expressing genotype for myeloperoxidase (MPO) to a twofold reduction in risk of recurrent breast cancer in patients undergoing chemotherapy. MPO is an enzyme present in white blood cells that produces oxidants, including bleach, which function to kill invading microbes. The observation that patients with high expressing MPO genotypes are at less risk for cancer recurrence suggests that MPO oxidants help to kill residual tumor cells. Our recent findings show that MPO is aberrantly expressed in breast cancer cells, leading to the hypothesis that MPO oxidants could directly kill tumor cells, and this could reduce the risk of tumor recurrence.

We propose to test this hypothesis, and verify the epidemiological studies, using a mouse model for breast cancer that carries the human MPO gene. We propose to treat the mice with chemotherapy cyclophosphamide (CPA) and tamoxifen, as was used in the human clinical studies, and determine if MPO enhances the tumoricidal activities of CPA. This will be accomplished by cross-breeding mice carrying a gene which makes them susceptible to breast cancer to our mouse strain carrying the human MPO gene.

MPO expression in tumor cells, as well as in tumor associated macrophages (TAM) and neutrophils, will be monitored along with markers of apoptosis, MPO-dependent oxidation, and biomarkers of tumor progression such as Neu, estrogen receptor (ER), or cyclin D1 (cell cycle regulation). Next, CPA and tamoxifen will be injected or added to the water/food supplied to the mice. The mice will be monitored for tumor growth and metastasis, and at the end of the experiment, tumor size and grade/histology will be analyzed.

The finding that MPO is expressed at high levels in some human breast carcinoma cells is a novel and potentially important discovery. MPO expression is normally restricted to bone marrow myeloid precursors. The aberrant expression in breast cancer cells is likely to impact survival of the cancer cells, and this could lead to novel therapeutics aimed at enhancing MPO expression and oxidant generation, thereby enhancing the efficacy of chemotherapy.

Final Report (2012)

Our studies are predicated on epidemiological studies showing that the high expressing -463GG MPO genotype is protective against recurrent breast cancer in women undergoing chemotherapy following reductive surgery. The implication is that MPO generated oxidants potentiate the effects of chemotherapeutic agents in the killing of cancer cells through oxidative damage to DNA or proteins. We used the PyMT mouse model crossed to human MPO transgenic mice to test this hypothesis.

Our findings have shown that MPO is expressed in mammary cancer cells in the PyMT model carrying the huMPO transgene. The MPO-463 G allele is expressed in up to 80% of the tumor cells in the MPO-PyMT model while the MPO-463A allele is expressed at low levels in a few tumor cells. This provides evidence that this specific polymorphism underlies the difference in expression in mammary tumors. Our studies have shown that the MPOG allele results in slower tumor growth in MPOG-PyMT mice than in MPOA-PyMT or PyMT controls, and this expression correlates with greater oxidative damage including nitrotyrosine. The MPO enzyme expressed in tumor cells is enzymatically active and correctly processed. We created stable cell lines from the MPOG-PyMT and MPOAPyMT tumors and showed by FACS analysis that cells with higher MPO expression exhibited higher levels of activated caspase-3, providing evidence that MPO oxidants promote apoptosis in the tumor cells. These studies provide evidence that aberrant expression of MPO in breast cancer cells can facilitate apoptosis.