BMI1 Cooperates with H-RAS to Induce an Aggressive Breast Cancer Phenotype with Brain Metastases Mark J. Hoenerhoff 1 , Isabel Chu 1 , Dalit Barkan 1 , Zi-yao Liu 1 , Sonal Datta 2 , Goberdhan P. Dimri 2 , and Jeffery E. Green 1 1 Transgenic Oncogenesis Group, Laboratory of Cancer Biology and Genetics, National Cancer Institute, National Institutes of Health, Bethesda, Maryland, USA 2 Division of Cancer Biology and Department of Medicine, NorthShore University HealthSystem Research Institute, Feinberg School of Medicine, Northwestern University, Evanston, Illinois, USA Abstract BMI1 is a member of the polycomb group of transcription repressors that functions in stem cell maintenance and oncogenesis through inhibition of the INK4A/ARF tumour suppressor locus. Overexpression of BMI1 is associated with poor prognosis in several human cancers, including breast cancer. We have previously shown that BMI1 collaborates with H-RAS to induce transformation of MCF10A human mammary epithelial cells via dysregulation of multiple growth pathways independent of the INK4A/ARF locus. In this study, we demonstrate that BMI1 collaborates with H-RAS to promote increased proliferation, invasion, and resistance to apoptosis in vitro, and an increased rate of spontaneous metastases from mammary fat pad xenografts including novel metastases to the brain. Furthermore, in collaboration with H-RAS, BMI1 induced fulminant metastatic disease in the lung using a tail vein model of haematogenous spread through accelerated cellular proliferation and inhibition of apoptosis. Finally, we show that knockdown of BMI1 in several established breast cancer cell lines leads to decreased oncogenic behaviour in vitro and in vivo. In summary, BMI1 collaborates with H-RAS to induce an aggressive and metastatic phenotype with the unusual occurrence of brain metastasis, making it an important target for diagnosis and treatment of aggressive breast cancer. Keywords BMI1; H-RAS; breast cancer; metastasis INTRODUCTION Although metastasis remains the major cause of mortality in breast cancer patients (Vernon et al., 2007), mechanisms of metastatic progression and disease recurrence remain poorly understood. Significant improvements have been made in identifying patients at earlier stages of disease, but even patients with minimal disease at initial diagnosis have up to a 30% risk of recurrence many years after the apparently successful treatment of the primary disease (Brackstone et al., 2007; Fisher et al., 2001; Wallgren et al., 2003). Thus, it is critical that identifying underlying molecular mechanisms regulating metastatic processes in breast cancer be elucidated. Corresponding author: Jeffery E. Green, MD, Head, Transgenic Oncogenesis Group, Laboratory of Cancer Biology and Genetics, National Cancer Institute, National Institutes of Health, 37 Convent Drive, Building 37, Room 4054, Bethesda, MD, USA, Phone: (301) 435-5193, Email: jegreen@nih.gov. NIH Public Access Author Manuscript Oncogene. Author manuscript; available in PMC 2010 February 27. Published in final edited form as: Oncogene. 2009 August 27; 28(34): 3022–3032. doi:10.1038/onc.2009.165. NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript