Rac1b is the only known splice variant of the small GTPase Rac1 with an in-frame insertion of an additional 19 amino acids. It was previously shown that the genetic deletion of Rac1 impairs mammary alveologenesis during pregnancy, post-weaning tissue remodelling of mammary gland during involution, mammary epithelial stem cell (MaSC) maintenance in nulliparous glands, and breast tumourigenesis. However, it is still unclear whether these phenotypes are caused by the loss of function of Rac1 or its splice variant Rac1b. Here, I have investigated the variant specific functions of Rac1b during mammary gland development and breast tumourigenesis by utilising a knockout mouse line that specifically lacks only the Rac1b variant and a knock-in mouse line serving as the surrogate reporter for Rac1b expression that has been generated by using CRISPR gene editing. The data obtained in this project demonstrate that the loss of Rac1b function has no obvious effects on mammary gland development and on the self-renewal of MaSCs, suggesting that it is Rac1, but not Rac1b, that is indispensable for normal mammary gland development. In contrast, in the mouse model of HER2+ breast cancer, MMTV-Neu-IRES-Cre (NIC), Rac1b is expressed by a substantial subpopulation of breast cancer stem cells (BCSCs), which require Rac1b function for maintaining their BCSC ability. Importantly, the Rac1b-null primary breast cancer cell lines display increased chemosensitivity to doxorubicin treatment compared with Rac1b-proficient primary cell lines. Taken together, my findings suggest that Rac1b is dispensable for normal mammary gland development, while it is required for BCSC maintenance and chemoresistance of breast tumour cells. Thus, Rac1b could be a clinically relevant molecular target for the development of BCSC-targeting therapies to improve the outcomes of currently available chemotherapies.
The functional roles of Rac1b in mammary gland development and breast tumourigenesis
Chen, F. (Author). 1 Aug 2023
Student thesis: Phd