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Cell of origin/Leukemia Initiating Cell in infant Pro-B MLL-AF4+ ALL

Final Report Summary - MODELING MLL-AF4 ALL (Cell of origin/Leukemia Initiating Cell in infant Pro-B MLL-AF4+ ALL)

The mixed lineage leukemia (MLL, also known as KMT2A) gene is frequently rearranged in human acute leukemia and it can fuse with more than 135 fusion partners. MLL-AF4, MLL-ENL, and MLL-AF9 are the most frequent MLL translocations and are generally associated with poor disease outcome in infant, pediatric, adult, and therapy-induced acute leukemias (t-AL). Acute lymphoblastic leukemia (ALL) is the most common cancer in children and although survival rates have improved during the last decades, some subgroups still have a very poor prognosis. In particular, ALL cases with MLL rearrangements show poor survival in < 1-year-old infants. Further research is necessary to better understand the etiology, pathogenesis, cell-of-origin and possible secondary events of infant MLL-r leukemias.

The project aimed to investigate infant leukemia generation, specially the mechanisms responsible for its short latency. Firstly, we attempted to recreate infant MLL-AF4+ pro-B ALL using lentiviral transduction to express MLL-AF4 or AF4-MLL in hematopoietic stem and progenitor cells (HSPC) at different developmental stages: aorta-gonada mesonephros (AGM), fetal liver (FL) and neonatal bone marrow (BM). However, this was not feasible due to large size of fusion protein constructs. Secondly, we generated mouse models by inducing the MLL-ENL rearrangement in the endogenous MLL locus, comparing the leukemogenic capacity of HSPC from different ontogenic stages. Using MLL-ENL translocator model (MLL-LoxP/ENL-LoxP mouse model) carrying a ubiquitous tamoxifen-inducible Cre recombinase transgene (b-actin-CreERT2), we isolated concrete hematopoietic cell populations (E14.5 fetal liver cells or adult Lin- BM cells) and induce in vitro fusion protein expression by adding tamoxifen to hematopoietic cell cultures. MLL-ENL-expressing cells induced for 3 days were transplanted into myelo-ablated newborn pups and followed up to see if leukemia developed. However, no signs of disease were detected by peripheral blood analysis in any of these animals, probably due to insufficient MLL-ENL expression level. Finally, since MLL rearrangement can be secondary to anti-tumoral drug administration, such as DNA topoisomerase II (TOP2) inhibitors, we studied if maternal exposure to TOP2 inhibitors during pregnancy is sufficient to break MLL and recreate MLL-r leukemia in offspring mice. Although MLL-r leukemia frequently occurs in patients with t-AL after exposure to TOP2 inhibitors, administration of etoposide into wild-type mice during pregnancy did not provoke MLL breaks nor generated leukemia, even in mice with DNA damage response (DDR) deficient background (PARP KO). In conclusion, we need better models of infant MLL leukemia to fully understand the initiation mechanisms of this rapid-onset disease and develop a rational basis for novel therapies.