Acute Lymphoblastic Leukemia (ALL) is the most common childhood cancer. Disease relapse following treatment still occurs in a significant minority of children and the majority of adult patients. The inability to further intensify current treatments due to dose limiting toxicities of chemotherapeutic agents demands the development of new agents. One exciting new treatment, is the mTOR inhibitor everolimus. Preclinical studies using everolimus, while promising, revealed that resistance can emerge following prolonged treatment in vivo.
This study uses ALL xenografts that have developed resistance to everolimus by long-term exposure ¬in vivo. This unique resource, combined with proteomic and transcriptome sequencing technology, allows a global approach to analyse the complex biological mechanisms behind the development of resistance to everolimus in ALL.
The expression of RNA and protein, the cell cycle distribution of everolimus resistant xenografts as well as the Kaplan Meier survival curves was vastly different between the two ALL xenografts analysed in this study. This indicates that resistance to everolimus is likely to have developed through different mechanisms. The cell cycle distribution of everolimus resistant ALL xenografts also differed depending on the tissues from which they were isolated. Leukemia cells may home to different tissue specific microenvironments that express specific factors that support ALL growth and survival to varying degrees. Furthermore, while individual genes were dissimilar between the two xenografts, there was a common regulation in pathways involved in cellular adhesion and the cytoskeleton. Proteomic sequencing identified 3 proteins possibly involved in everolimus resistance; PDLIM1, Vimentin and Stathmin-1. These proteins are involved with the cytoskeleton and may have a role in the adhesion, migration and cell cycle, yet their exact role in the development of resistance to everolimus is yet to be confirmed.
We were unable to correlate the possible mechanisms of resistance identified in the murine model to ALL patients after acute everolimus exposure. We identified a decrease in the expression of the oncogenic micro-RNA, miR-21, though, this was likely due to the immunosuppressive effects of everolimus and did not correlate to patient outcome.