Maternal embryonic leucine zipper kinase is upregulated and required in mammary tumor-initiating cells in vivo.

Maternal Embryonic Leucine zipper Kinase (MELK) is expressed in several developing tissues, in the adult germ line, and in adult neural progenitors. MELK expression is elevated in aggressive undifferentiated tumors correlating with poor patient outcome in human breast cancer. To investigate the role of MELK in mammary tumorigenesis in vivo we used a MELK-GFP reporter mouse, which allows prospective isolation of MELK expressing cells based on GFP fluorescence. We found that in the normal mammary gland, cells expressing high levels of MELK were enriched for proliferating cells, expressing markers of mammary progenitors. The isolation of cells with high levels of MELK in mammary tumors from MMTV-Wnt1/MELK-GFP bitransgenic mice resulted in a significant enrichment of tumorsphere formation in culture and tumor initiation after transplantation into mammary fat pads of syngeneic mice. Furthermore, using lentiviral delivery of MELK-specific shRNA and limiting dilution cell transplantations we demonstrated that MELK function is required for mammary tumorigenesis in vivo. Our findings identify MELK as potential target in breast tumor initiating cells.