Background and purpose: MORC2 (microrchidia family CW-type zinc finger 2

MORC2) is a newly identified chromatin remodeling protein that plays key roles in DNA-based biological processes including gene transcription and DNA damage repair. However

its functional role in breast cancer development and progression remains unknown. ALDH1A3 (aldehyde dehydrogenase 1 family

member A3)

a member of the aldehyde dehydrogenases (ALDH) superfamily

is a putative breast cancer stem cell marker

but its regulatory mechanism in breast cancer is poorly characterized. This study aimed to investigate the effects of knockdown of endogenous MORC2 on the expression levels of ALDH1A3 and the breast cancer stem-like phenotype in MCF-7 cells. Methods: Human breast cancer MCF-7 cells were infected with negative control short hairpin RNAs (shNC) and specific shRNAs targeting human MORC2 (shMORC2)

followed by selection with puromycin to generate stable MORC2 gene knockdown cell lines. Western blot and real-time fluorescent quantitative polymerase chain reaction (RTFQ-PCR) were used to examine the protein and mRNA levels of ALDH1A3 in MCF-7 cells stably expressing shNC and shMORC2. Microsphere formation and fluorescence-activated cell sorting (FACS) assays were used to analyze the effects of knockdown of MORC2 on the breast cancer stem-like phenotype. Results: Western blot and RTFQ-PCR analyses revealed that the protein and mRNA levels of ALDH1A3 were significantly down-regulated in shMORC2 expressing cells as compared with shNC -transfected control cells. Moreover

mammosphere formation assay showed that knockdown of endogenous MORC2 in MCF-7 cells significantly reduced the ability of cells to form microspheres. Consistently

FACS assays demonstrated that shMORC2-transfected cells had a lower proportion of ALDH-positive stem cells as compared with shNC expressing cells. In contrast

knockdown of MORC2 did not significantly affect the CD44

+

CD24

-

stem cell population. Conclusion: MORC2 promotes a breast cancer stem-like phenotype through

at least in part

regulating ALDH1A3 expression.