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Palb2 ? A New Hereditary Cancer Gene: Implications For Treatment
Career Catalyst Research
BACKGROUND: This proposal focuses on investigating how mutations in the PALB2 gene (partner and localizer of BRCA2), a recently identified hereditary breast cancer gene, cause breast cancer. PALB2 is a BRCA2-interacting protein; up to 50% of BRCA2 is associated with PALB2 and more than 50% of PALB2 forms a complex with BRCA2. Association of BRCA2 with PALB2 appears to be essential for BRCA2 anchorage to nuclear structures and for its function in double strand break repair by homologous recombination. Data from our previous work and from other groups provide confirmation that PALB2 mutations predispose to breast cancer, and we have recently identified a founder PLAB2 mutation in the French Canadian population. However, the mechanism of carcinogenesis in is not understood ? we and others have seen no evidence of LOH and unpublished work by our group has shown no evidence of PALB2 allele inactivation by methylation in PALB2-related breast cancer tumors.
HYPOTHESES: 1) Breast cancer in PALB2 mutation carriers is caused either by a haploinsufficiency effect or the second allele is inactivated by a mechanism other than LOH or methylation 2) PALB2-related tumors may have a specific chromosome copy number and immunohistochemical phenotype which could provide further clues regarding the process of breast carcinogenesis. 3) Detailed functional characterization of LCLs and fibroblast cells lines from PALB2 mutation carriers will give a valuable insight into DNA repair related carcinogenesis and potentially identify new therapeutic avenues in hereditary, and possibly also sporadic breast cancer.
OBJECTIVES: 1) To screen for germline PALB2 mutations in familial and sporadic breast cancer cases with the aim of identifying further PALB2 mutation carriers. 2) To characterize PALB2-related tumors using immunohistochemical and molecular techniques to look for consistent differences between other hereditary or sporadic cancers. 3) To study the response of cell lines from PALB2 mutation carriers to genotoxic agents in terms of survival and chromosomal stability. 4) To investigate the effect of PALB2 mutations on telomere structure and function.
METHODS: We have lymphoblastoid cell lines (LCLs) from three individuals with different PALB2 mutations and from a further three unrelated individuals with the same PALB2 mutation. In addition we have fibroblast and lymphoblast cell lines from two first-degree relatives of one of the PALB2 mutation carriers who do not carry the PALB2 mutation which, given the close genetic background, will serve as excellent controls. We will identify further PALB2 cases by screening familial and unselected breast cancer cases. The functional work will focus on comparing the cellular response to agents which damage DNA by different mechanisms in cell lines from PALB2 mutation carriers compared to controls. We will use UV and gamma radiation, bleomycin (double strand breaks), mitomycin C (interstrand cross-links) and platinum agents (inter- and intrastrand cross-links). All agents will be tested in the presence or absence of PARP inhibitors. The following techniques will be used: 1) Bleomycin Assay 2) Survival Assay-Adherent cells. 3) Survival Assay-Lymphocytes. 4) Comet Assay-DNA repair. Using high resolution deconvolution microscopy, we will elucidate the three-dimensional (3D) organization of telomeres and chromosomes in the interphase nuclei of cells from PALB2 heterozygotes and controls. We will analyze the mRNA-expression profiles of cells treated with selected genotoxic agents to define possible specific targets associated with altered biological responses to DNA damage.
SIGNIFICANCE: The detailed study of PALB2 function in model cell systems and tumors will shed new evidence on mechanisms of breast cancer predisposition and potentially identify new therapeutic avenues in hereditary and sporadic breast cancer.
Project Summary: Around 5-10% of all breast cancers are caused by genetic defects (mutations) which can be passed on from one generation to the next. Typical families with such mutations might have three or four affected female relatives often diagnosed at less than 50 years of age or with cancer in both breasts. The two most important breast cancer genes, BRCA1 and BRCA2 were identified in the mid-1990s. More recently several other genes that, when mutated, predispose to f breast cancer have been identified. One of these is a gene called PALB2 which was linked to breast cancer by us and others earlier this year. PALB2 works closely with BRCA2 and in this project we plan to study PALB2 in detail as we believe this will provide important clues in understanding how breast cancer is caused.
Previous Research: Our group was among the first to link PALB2 with breast cancer and we have shown that breast tumors in women with PALB2 mutations may have a particular molecular ?signature?. We have identified a specific PALB2 mutation which we have repeatedly found in the French Canadian population, suggesting that it has probably been passed down multiple generations in this group.
Project Description: We will identify more breast cancer cases which are due to PALB2 mutations and investigate how these breast cancers differ to other breast cancers. We will grow cells from individuals with PALB2 mutations and use different techniques to see how these cell lines respond after being subjected to radiation and chemicals which mimic chemotherapy. Specifically we will look at the ends of the chromosomes in these cells using 3D imaging technology as we believe this is where differences will be most easy to measure.
Clinical Impact: In view of its involvement in repairing damaged DNA and close relationship with BRCA2, it is clear that PALB2 plays an important role in breast cancer. Using a number of different approaches, detailed insight will be gained into the function of this newly discovered gene, how defects in PALB2 cause breast cancer and how PALB2 could be used as a potential target for new treatments.