A MODEL to STUDY the Function of NPM-MLF1 IN MYELODYSPLASIA.

Abstract 1571

Myelodysplastic syndromes (MDS) are bone marrow disorders characterized by ineffective haematopoiesis and peripheral cytopenia(s) with frequent evolution to acute myeloid leukemia (AML). Apoptosis is significantly deregulated in early MDS whereas advanced MDS is characterized by deregulation of DNA damage response. As MDS proceeds to AML, the ratio of apoptosis to proliferation decreases, resulting in clonal outgrowth of abnormal cells.

The t(3;5)(q25;q34) translocation, creating the NPM-MLF1 fusion, has been found as a sole cytogenetic abnormality in MDS. It is recurrent, with poor prognosis but the precise mechanism through which NPM-MLF1 induces malignant transformation remains unknown. We aimed to model this disease in vitro and in vivo by expressing NPM-MFL1 in mouse bone marrow hematopoietic progenitor cells (HPCs) and analyzing any changes in HPC self-renewal and response to DNA damage.

NPM-MLF1 did not impair haematopoiesis in vitro and in vivo. FLT3/ITD was frequently associated with NPM mutant in AML patients; however, NPM-MLF1 did not collaborate with FLT3/ITD in our system. To recapitulate NPM hemizygosity in t(3;5)-MDS patients, we have expressed NPM-MLF1 in HPCs derived from Npm^+/− mice. A transient increase in the self-renewal of the NPM-MLF1-expressing Npm^+/− HPCs was seen. These cells did not exhibit enhanced proliferation as confirmed by growth curve and analysis of DNA synthesis. Interestingly, unlike control cells, NPM-MLF1-expressing Npm^+/− HPCs showed prolonged self-renewal ability in vitro, and an elevated expression of c-Myc, Hoxa9, Hoxa10 and Meis1 genes. In addition to altering HPC self-renewal, NPM-MLF1 was also found to modulate their DNA damage response. This study suggests that the ability of NPM-MLF1 to maintain HPC self-renewal and impaired DNA damage responses may favour the accumulation and outgrowth of the aberrant HPCs, contributing to the abnormal haematopoiesis.