Project description:Pre-leukemic stem cells (pre-LSCs) provide a reservoir of cells that evolve to acute leukemia and cause relapse following chemotherapy. We postulated that quiescence of pre-LSCs is an important mechanism of therapeutic resistance. Using a doxycycline-inducible H2B-GFP transgene in a mouse model of T-cell acute lymphoblastic leukemia, we show that long-term self-renewal, clonal evolution and resistance to chemo-radiation are intrinsically linked to restricted cell cycle. We show that restricted cell cycle of pre-LSCs requires the presence of the CDK inhibitor p21, with absence of p21 leading to chemosensitivity and clonal extinction by loss of asymmetric cell division and terminal differentiation. These results provide a model to identify and test strategies to eradicate an important source of clonal evolution and leukemic relapse.
Project description:<p>Although multi-agent combination chemotherapy is curative in a significant fraction of childhood acute lymphoblastic leukemia (ALL) patients, 20% of cases relapse and most die due to chemo-refractory disease. Here we used whole-exome and whole-genome sequencing to analyze the mutational landscape and pattern of clonal evolution at relapse in pediatric ALL cases. These analyses showed that ALL relapses originate from a common ancestral precursor clone of the diagnosis and relapsed populations and frequently harbor mutations implicated in chemotherapy resistance. RAS-MAPK pathway activating mutations in NRAS, KRAS and PTPN11 were present in 24/55 (44%) cases in our series. Notably, while some cases showed emergence of RAS mutant clones at relapse, in others, RAS mutant clones present at diagnosis were replaced by RAS wild type populations. Mechanistically, functional dissection of mouse and human wild type Kras and mutant Kras (Kras G12D) isogenic leukemia cells demonstrated induction of methotrexate resistance, but also improved response to vincristine, in mutant Kras- expressing lymphoblasts. These results identify chemotherapy driven selection as a central mechanism of leukemia clonal evolution and pave the road for the development of tailored personalized therapies for the treatment of relapsed ALL. </p>