Based on our current understanding, it is believed that the leukemic clones are perpetuated by a rare population of leukemia stem cells (LSC). These LSCs share many characteristics with normal hematopoietic stem cells (HSCs), such as phenotype, self-renewal potential, a quiescent cell cycle status and enhanced drug resistance. Whereas cytotoxic drugs eradicate actively cycling leukemic blasts, they often do not target LSCs embedded in the bone marrow niche. Recent studies indicate that the bone marrow stromal cells and the hematopoietic microenvironment in the bone marrow dictate stem cell quiescence, lack of motility and expression of anti-apoptotic proteins which leads to LSC survival and ultimately results in refractory leukemic disease or relapse. The disruption of specific LSC–stroma cell interactions might drive this leukemia-maintaining cell population into cell cycle and thus render them vulnerable to chemotherapy or targeted therapies.
In order to identify genes regulated upon leukemia–stroma cell interaction we performed global gene expression screens in stromal feeder layer cells and leukemic blasts co-cultured for different time periods. In comparison to mono-cultured blasts or normal HSCs, several differentially regulated genes such as receptors, ligands, extracellular matrix components or known proto-oncogenes were identified in the gene expression analysis of co-cultured leukemic blasts. We currently explore the role of several candidate genes and the associated signaling pathways. Promising target genes are further evaluated in vivo in murine leukemia models. In addition we are performing a comparative functional screen using a genome-scale CRISPR-Cas9 knockout library in leukemic blasts mono-cultured or co-cultured with stromal cells aiming to identify genes involved in stroma-mediated drug resistance.