Oncogenic and microenvironmental signals drive cell type-specific apoptosis resistance in juvenile myelomonocytic leukemia

Juvenile myelomonocytic leukemia (JMML) is caused by constitutively activated RAS signalling and is characterized by increased proliferation and predominant myelomonocytic differentiation of hematopoietic cells. Using Mx-Cre;PTPN11D61Y mice, which reflect human JMML, we show that RAS pathway activation affects apoptosis signalling through cell type-dependent regulation of BCL-2 family members. Apoptosis resistance observed in monocytes and granulocytes was mediated by overexpression of the anti-apoptotic and down-regulation of the pro-apoptotic members of the BCL-2 family. Two anti-apoptotic proteins, BCL-XL and MCL-1, were directly regulated by the oncogenic RAS signalling but in addition were influenced by microenvironmental signals. While BCL-XL and BCL-2 were required for the survival of monocytes, MCL-1 was essential for neutrophils. Interestingly, stem and progenitor cells expressing the oncogenic PTPN11 mutant did not display any increased apoptosis resistance. BCL-XL inhibition was the most effective in killing myeloid cells in vitro but was not sufficient to completely resolve myeloproliferation in vivo.