Bi-phasic expression of Heterochromatin Protein 1 (HP1) during breast cancer progression: Potential roles of HP1 and chromatin structure in tumorigenesis.

Epigenetics in cancer prognosis and therapy is gaining recognition in recent years. Breast cancer is a genetic disease harboring numerous genetic mutations, including tumor suppressor BRCA1 and BRCA2 mutations. However, the functions of BRCA1 in cancer cells are also altered by non-genetic mechanisms, including DNA methylation and chromatin structure. Therefore, identification of epigenetic markers for breast cancer is very important for early diagnosis and effective therapy. This review focuses on recent findings on the roles of Heterochromatin protein 1 (HP1) in BRCA1 functions and breast cancer progression. We previously showed that BRCA1 function and breast cancer progression are frequently associated with HP1 expression level and potentially with chromatin structure. Herein, we suggest that bi-phasic expression of HP1 during breast cancer progression indicates dual roles of HP1 in tumorigenesis. Exploiting differential HP1 expression in tumors could lead to effective cancer therapy. Re-setting the chromatin structure may be a critical step for high-efficiency cancer therapy for many breast cancer patients.