Name of Student:
Courteney Lai
Research Supervisor: Dr. Keith Humphries and Dr. Sam Aparicio
Title of Presentation: Exploring the role of Mixed Lineage Leukemia 5 (MLL5), a Trithorax
protein and MLL family member, on leukemic stem cell function
Abstract
While MLL1 is widely known to be essential for hematopoietic development and for its
involvement in numerous gene translocations in hematological malignancies, the possible role
of its family member Mixed Lineage Leukemia 5 (MLL5) in normal and leukemic
hematopoiesis has emerged only recently. MLL5 was first identified in a commonly deleted
region of chromosome 7 in myeloid malignancies including myelodysplastic syndrome
(MDS), acute myeloid leukemia (AML), and therapy-associated leukemias. Further insights
into its role have come from the recent independent generation and analysis of Mll5 knockout
models in three labs, including our own. Work from our lab has revealed that mice lacking
Mll5 show defects in immunity and hematopoiesis as well as impaired repopulating
capabilities of hematopoietic stem cells (HSCs) isolated from Mll5tm1Apa homozygous (Mll5-/-)
mice. Furthermore, treatment of Mll5 -/- mice with the DNA methyltransferase inhibitor 5-aza2’-deoxycytidine (decitabine) resulted in a marked reduction in hematopoietic stem cells, thus
linking Mll5 to epigenetic regulation of HSC function. (Heuser, Yap et al. 2009)
To further explore the role of Mll5 in leukemic stem cell (LSC) function, we first assessed if
Mll5 was required for leukemic transformation using the potent combination of retrovirally
engineered co-expression of the transcription factors HOXA9 and MEIS1 in wildtype (+/+)
and -/- bone marrow cells. In both backgrounds, immortalized factor-dependent cell lines were
readily generated, showed similar growth kinetics and replating efficiency by CFC assay, and
had expected phenotypes of undifferentiated myeloid cells with low Gr-1 expression (3.63%
and 1.33% for the Mll5 +/+ and -/- lines, respectively), high Mac-1 expression (52.2% and
48.8%), and high cKit expression (84.1% and 65.4%). Furthermore, transplantation into sublethally irradiated wildtype recipient mice resulted in both myeloid reconstitution and
leukemia onset after 86 and 115 days, respectively (n=2, p=0.38). To determine if Mll5
modulates the function of LSCs, we are examining the sensitivity of these cells to decitabine in
vivo. For these studies, mice were transplanted with the Mll5 +/+ or -/- leukemic cell lines.
Eighteen days after transplant, these mice were treated with decitabine (1mg/kg by
subcutaneous injection, 6 doses every 2 days). Comparison of leukemic stem cell frequencies
after treatment is now in progress by secondary limit dilution transplant assay. Confirming
differential sensitivity of leukemic stem cells to decitabine, as in normal stem cells, would
provide a powerful new model to unravel the role of Mll5 in leukemic stem cell function and
suggest that methods to impair Mll5 function could enhance the therapeutic efficacy of the
epigenetic modifier decitabine.