PROJECT SUMMARY/ABSTRACT
Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm diagnosed in about 5,000 Americans
each year, characterized by the presence of the t(9;22) Philadelphia chromosome and its protein product, the
BCR-ABL1 tyrosine kinase, in the leukemic cells. Current therapy for CML is centered on tyrosine kinase
inhibitors (TKIs) such as imatinib mesylate, which can induce cytogenetic and molecular remissions in most
patients, who then enjoy normal age-adjusted life expectancy. This clinical success will lead to an estimated
prevalence of ~200,000 CML patients in the U.S. by the year 2050, with attendant drug costs of >$10 billion that
can include burdensome out-of-pocket payments for patients. In addition, TKIs can have substantial side effects,
some of which are potentially life-threatening. Accordingly, recent efforts have been made to stop TKI therapy in
CML patients who have achieved molecular remission. About half of such patients experience progressive
molecular relapse following TKI stopping, but the other half either remain molecularly undetectable or experience
low-level recurrence that does not progress (collectively termed treatment-free remission or TFR), suggesting
that there are biological mechanisms that limit the ability of small numbers of leukemic stem cells to expand and
cause disease. These mechanisms are not understood, and represent a major unmet need in current CML
research, as we do not have validated approaches to increase the proportion of CML patients who are eligible
to stop their TKI nor to increase the proportion of patients who can maintain TFR.
This application seeks to address these unmet needs by exploiting a newly developed CML mouse model
of TFR. In this model, hematopoietic stem cells (HSCs) from an existing conditional double transgenic (dTg)
mouse model of CML are transplanted into recipient congenic B6 mice without the use of conditioning radiation.
The resulting bone marrow (BM) chimeras contain clones of dTg HSC in a background of normal BM,
representing a physiological model of early CML. Mice bearing large (>10%) clones of dTg HSC uniformly
develop CML-like leukemia when BCR-ABL1 expression is induced, but mice with smaller (<2%) dTg clones do
not develop CML, but instead exhibit fluctuating low levels of dTg granulocytes in peripheral blood that mimics
TFR. The proposed project will test whether two distinct processes, oncogene-induced replicative stress and the
BM microenvironment/niche, are involved in the ability of the host to control the malignancy. Aim 1 will test
whether oncogene-induced replicative stress is involved by a incorporating loss-of-function mutation in the p19Arf
gene in the dTg donor cells. Aim 2 will test whether elements of the BM niche, including immune cells and
cytokines, are required for disease control. Aim 3 will translate these findings to human CML BM samples using
the spatial mass cytometry to define the topography of the CML niche and correlate this with TFR outcomes.
These results will yield important new knowledge about the pathophysiology of TFR that should motivate novel
therapeutic approaches to increase the frequency and durability of TFR in CML patients.