Characterization of the putative CALM/AF10 collaborator Meis1 in leukemia development

Chromosomal translocations are common in human leukemias. Detailed
studies of chromosomal translocation have been useful in
understanding the pathogenesis and identifying therapeutic targets
in hematologic malignancies. Some translocations result in the
formation of fusion genes. These fusion proteins play an important
role in leukemogenesis. The t(10;11)(p12;q14) translocation is rare
but recurring and results in the formation of the CALM/AF10 fusion
protein. Patients with this translocation have a bad prognosis. To
understand how CALM/AF10 leads to leukemia, various mouse models
have been established. In a murine bone marrow retroviral
transduction and transplantation model Deshpande et al. (2006)
showed that mice expressing CALM/AF10 in their bone marrow cells
developed an acute myeloid leukemia with a penetrance of 100% and a
short latency period of 110 days. Using a transgenic mouse model,
in which CALM/AF10 was under the control of Vav promoter, Peter
Aplan and colleagues demonstrated that only 40% to 50% of mice
developed leukemia after a long latency of 10 to 12 months. Two
classical transgenic CALM/AF10 models were established in our group
using the immunoglobulin heavy chain enhancer/promoter
(IgH-CALM/AF10) and proximal murine LcK promoter (pLck-CALM/AF10)
to drive CALM/AF10 expression. These transgenic mice did not show
any leukemic phenotype even after an observation period of 15
months. Taken together these studies strongly suggest that
additional collaborating factors are required for the CALM/AF10
fusion gene to induce leukemia. Meis1, a Hox cofactor, is known to
collaborate with several Hox genes and Hox fusion genes such as
HOXA9 and NUP98-HOXD13. In these studies, Meis1 played a critical
role in accelerating the development of leukemia. It could also be
shown that MEIS1 is highly expressed in CALM/AF10 positive human
leukemia cells. Therefore, I sought to determine whether the
homeobox gene Meis1 collaborates with CALM/AF10 in inducing
leukemia. In order to achieve this goal, lethally irradiated
non-transgenic mice were transplanted with IgH-CALM/AF10 transgenic
bone marrow cells transduced with a Meis1 expressing retrovirus.
The transplanted mice developed an acute leukemia with a penetrance
of 100% and a median latency period of 187 days. The leukemia
showed predominantly myeloid features such as the presence of
myeloid marker positive cells. The myeloid blast cells infiltrated
in multiple hematopoietic as well as non-hematopoietic organs. The
leukemic cells were also positive for the B-cell marker B220. Cells
that were positive for both lymphoid and myeloid markers, a
characteristic feature of CALM/AF10-induced leukemia, were also
detected in all the mice. The leukemic cells had clonal DJH
rearrangements. Overall, these data suggest that the transformed
cell might be an early progenitor cell capable of lymphoid as well
as myeloid differentiation or that the leukemia was initiated by a
B220+ IgH DJ rearranged cell with blocked lymphoid differentiation,
which started a default myeloid differentiation program. By
performing serial secondary and tertiary transplantations the
leukemic nature of the disease could be confirmed. Colony forming
cell assays showed that CALM/AF10 in collaboration with Meis1
failed to induce the transformation of hematopoietic progenitors in
vitro. This could either be due to the lack of required growth
factors and conditions necessary for the proliferation of the
transformable cell or lack of additional events essential for
progression towards leukemia development. In conclusion, I have
demonstrated that Meis1 collaborates with CALM/AF10 in inducing
acute myeloid leukemia. Additional, detailed analyses of the
leukemia initiating cell in these models would help to better
understand the pathogenesis of CALM/AF10-induced leukemia.