Mechanisms underlying abnormal growth of malignant stem and progenitor cells
Improved understanding of the molecular mechanisms responsible for human stem cell transformation is essential for a better understanding of leukemia pathogenesis and to guide rational development of additional mechanism-based therapies. Our goal is to develop improved understanding of differences in growth regulation between malignant and normal hematopoietic stem cells and the mechanisms underlying hematopoietic stem cell transformation in patients with leukemia.
We have extensively characterized abnormalities in cellular growth regulation in chronic myelogenous leukemia (CML), a lethal hematological malignancy resulting from transformation of a hematopoietic stem cell by the BCR/ABL gene. We are currently investigating molecular mechanisms crucial for stem cell transformation in CML. We have developed a novel human model of CML based on viral vector mediated delivery of the BCR/ABL gene into human progenitor cells to identify key signaling mechanisms underlying stem cell transformation in CML. This model has been well characterized in vitro and is being extended to in vivo studies. This approach will be extended to study the mechanisms of malignant transformation in other stem cell malignancies such as AML and MDS.
Since the mechanisms underlying stem cell transformation in primary MDS and AML are poorly understood, we are characterizing MDS and AML stem and progenitor cells in functional assays in vitro and in vivo as well as by gene expression profiling, and are investigating the functional significance of genes that are abnormally expressed in malignant stem cells. In this context, we have studied the Delta-like 1 (DLK1) gene that may play a role in maintaining stem cell self-renewal, and which is aberrantly expressed on MDS CD34+ cells, and shown that DLK expression inhibits hematopoietic cell differentiation and proliferation and may play a potential role in hematopoietic abnormalities observed in MDS.
Therapeutic targeting of leukemia stem cells
An ultimate goal of the above studies is to develop strategies for selective therapeutic targeting of leukemia stem cells based on improved understanding of the differences between normal and malignant stem cells. We have studied the effects of newly developed targeted therapies on leukemia stem cells. We have detected persistence of malignant stem cells in CML patients in complete cytogenetic remission following treatment with the BCR/ABL kinase inhibitor Imatinib (Gleevec) and elucidated key mechanisms underlying resistance of malignant stem cells to this agent, including:
(a) a dominant anti-proliferative rather than apoptosis-inducing effect on CML stem and progenitor cells with resistance of non-dividing CML CD34+ cells to apoptosis,
(b) preservation of viability of Imatinib-treated stem cells through growth factor mediated signals from the microenvironment, and
(c) presence of BCR/ABL kinase domain mutations in residual stem cells from CML patients in CCR.
On the basis of these observations, we have initiated and/or are in the process of developing clinical trials aimed at residual leukemia stem cells in imatinib-treated CML patients based on
(a) immune targeting using a BCR/ABL peptide vaccine;
(b) immune targeting using non-myeloablative allogeneic HCT and adoptive immunotherapy approaches;
(c) more potent kinase inhibitors that can target Imatinib-resistant cells;
(d) modulation of epigenetic regulation of gene expression in malignant stem cells; and
(e) activation of dormant leukemia stem cells using growth factors or other mechanisms.
We are also collaborating with investigators in the new drug development program to perform correlative laboratory studies to evaluate targeting malignant stem cells in patients enrolled in clinical trials of new therapeutic agents.
Hematopoietic stem cell transformation in therapy-related leukemia
Therapy-related myelodysplasia/acute myelogenous leukemia (t-MDS/AML) is a lethal complication of cancer therapy. To better understand the pathogenesis of t-MDS/AML and to identify predictors of risk, we have initiated a prospective, longitudinal evaluation of patients with HD or NHL undergoing autologous hematopoietic cell transplantation (HCT) who are at high risk for development of t-MDS/AML.
In addition to collection of relevant demographic, clinical, and therapeutic information, samples are being collected and stored pre-HCT, from the PBSC autograft, and various time points post-transplant to perform assessments of factors predicting for development of t-MDS/AML and to investigate pathogenetic mechanisms.
This project is aimed at understanding
(a) early events leading to stem cell transformation and development of leukemia;
(b) mechanisms of susceptibility- genetic, environmental influences and their interactions;
(c) evolution of cellular and molecular abnormalities in stem cells during myeloid leukemogenesis;
(d) identification of patients at risk for leukemia; and
(e) development of novel preventive/ therapeutic strategies.
It is hypothesized that acquired defects in DNA repair, DNA damage response, hematopoietic regulation and clonal genetic abnormalities contribute to the development of t-MDS/AML. The findings from this study are expected to have broader implications for the pathogenesis of de novo AML and MDS in general.
Ongoing studies investigating the sequential acquisition of abnormalities in DNA repair, DNA damage response, hematopoietic regulation, telomere function, and cytogenetic abnormalities in the pathogenesis of therapy-related leukemia are supported by the Lymphoma SPORE. In addition we are investigating genome-wide abnormalities in gene expression and of genetic lesions during the course of development of t-MDS/AML. The candidate genes identified by these studies will be investigated using models developed in the course of our studies in CML as described above. Successful completion of these studies is expected to provide insights into pathogenesis and lead to development of predictors for t-MDS/AML.
