Role of DNA methyltransferases in maintaining mammary stem/progenitor and cancer stem cells

Abstract

Breast cancer is the leading cause of cancer death in women worldwide and it affects one in eight women in western countries. Like other human cancers, breast cancer also consists of cellular hierarchy, and heterogeneous. However, the cancer cell of origin and how a normal self-renewal pathway turns into abnormal self-renewal signaling are not known. DNA methylation provides a potential epigenetic mechanism for the cellular memory and heterogeneity, which needed to preserve the tumorigenic potential through repeated cell divisions. Further DNA methylation plays an essential role in stem/progenitor cell maintenance and provides a potential epigenetic mechanism for maintaining cellular memory and heterogeneity during self-renewal. However, the specific role of DNMTs in maintaining mammary stem cells (MaSC) and cancer stem cell (CSC) in a constantly replenishing organ, like mammary glands, is not yet known. Here, we show that Dnmt1 is essential for mammary gland development and indispensable for terminal end bud development and that mammary-gland specific Dnmt1 deletion in mice leads to significant reduction in mammary stem/progenitor cell formation. Moreover, Dnmt1 deletion almost completely abolishes Neu-Tg- and C3(1)-SV40-Tg- driven mammary tumor formation. The reduced tumor incidence observed in Dnmt1 deleted mouse is associated to significant reduction in cancer stem cell formation. These observations were recapitulated using pharmacological inhibitors of DNMTs in Neu-Tg mice in vivo. Further, we show that there is a substantial increase in DNMT1 expression when mammary stem/progenitor cells turn into tumor initiating cells. Using genome-scale methylation approach, we found that hypermethylation of genes involved in development and cell commitment pathways impart immortality and autonomous growth to the cancer stem cells. Moreover, our study provides evidence that stem cells, in addition to luminal progenitor cells, are susceptible for genetic and epigenetic modification and associated with chemotherapeutic resistance. Thus, combination of DNMT and HDAC inhibitors can be used as a therapeutic strategy to block mammary tumor formation and to overcome drug resistance by inhibiting CSCs. These findings improve our understanding of abnormal self-renewal associated with cell of origin, and highlight novel methylation markers that have the potential to serve as useful diagnostic tools and therapeutic targets in early detection of breast cancer.