the Seeds of Evil
he war on cancer has resulted in the generation of an immense body of knowledge concerning the biology, diagnosis and therapy of various malignancies. Although this information has resulted in new and improved ways to detect and treat cancer, many patients initially respond to therapy, but eventually relapse. Until recently, the primary cause of cancer relapse was thought to be due to the development of drug resistance in the tumor cell population. An emerging concept in cancer biology is that a rare population of cancer stem cells (CSCs) exists among the heterogeneous cell mass that constitutes the tumor. These CSCs are like normal stem cells and have unique biological characteristics, including resistance to many of the therapies that are used in cancer treatment.
Mantle Cell Lymphoma
Mantle cell lymphoma (MCL) is a distinct subtype of non-Hodgkin’s lymphoma and accounts for 5% to 8% of adult non-Hodgkin’s lymphoma (NHL) in the U.S. and Europe. The disease is aggressive with a median patient survival of 2 to 4 years. Although standard chemotherapeutic regimens generally induce disease response, the responses are often partial and of short duration. Even when a complete remission is obtained, the median duration is short with no plateau in failure free survival. Improved outcomes occur with more intensive regimens +/- autologous or allogeneic hematopoietic stem cell rescue/transplantation. However, many patients still relapse.
The cancer stem cell hypothesis
There is increasing evidence that many cancers contain a small subset of cells with stem cell-like properties, often referred to as cancer stem cells (CSCs). They can self-renew to generate additional CSCs and also differentiate into phenotypically diverse cancer cells with limited proliferative potential as found in the bulk tumor. Since current cancer therapeutics have been developed based on killing differentiated cancer cells, CSCs may explain why there are often recurrences after chemotherapy (i.e. CSCs may survive the treatment and act as “seeds” for future relapse).
The cancer stem cell hypothesis states that tumors are composed of a heterogeneous population of cells, within which resides a small population of CSCs that are responsible for the maintenance and propagation of the tumors and are the likely cause of disease relapse and metastasis. These cells possess many of the same stem cell properties found in normal stem cells including: (i) the ability for self-renewal; (ii) the ability to differentiate; (iii) increased drug transport pumps (i.e. ABCG2, MDR-1); (iv) activation of anti-apoptotic pathways; (v) increased telomerase activity; (vi) activation of transcription factors required for self-renewal (i.e. BMI-1, Nannog ); and (vii) the ability to migrate.
The first conclusive evidence for the existence of CSCs was reported for acute myeloid leukemia (AML ) by researcher John Dick’s group. In these studies, a subpopulation of leukemia cells was identified that expressed the HSC markers CD34+/CD38-, similar to their normal counter-parts (Figure 1). More importantly, these cells had a frequency of <1/10,000 and were capable of initiating tumors in NOD/SCID mice. When mice were injected with several logs higher of cells devoid of the CD34+/CD38- cells, no engraftment was observed. Furthermore, the leukemia produced in the mice recapitulated the histological phenotype of the original leukemia, not the features of the CSC. For example, when NOD/SCID mice were injected with AML M2, CSCs that were phenotypically distinct from the bulk of the AML cells, the tumors produced were histologically identical to the patient’s AML M2 phenotype.
Since these seminal reports, CSCs have been identified in multiple myeloma, chronic myeloid leukemia, and in several solid tumors including breast, brain, prostate, colon and pancreatic cancer. It should be noted that CSCs may originate from normal stem cells, progenitor cells or mature cells which, through mutations or epigenetic mechanisms, may have reactivated self-renewal pathways.
Identification of MCL Stem Cells
While studying MCL-human mesenchymal stem cell (hMSC) interactions, we made the unexpected observation of the presence of clusters of small lymphoid-like cells under the mesenchymal stem cell layer during long-term co-culture of these cell populations. These clusters are reminiscent of cobblestone area forming cells (CAFCs) seen when bone marrow (BM) stromal cells are co-cultured with normal hematopoietic stem cells (Figure 2). It has been observed that the quiescent and more primitive hematopoietic cells preferentially locate within the adherent stromal layer, whereas the cells that migrate to the surface of the layer show increased proliferation and maturity and are then shed into the medium. Preliminary analysis of this MCL cell population demonstrates that these clusters contain self-renewing cells with the chromosomal translocation t(11; 14)(q13; q32) characteristic of MCL, yet have a unique immunophenotype, including the HSC marker CD133. The expression of CD133 and capacity for self-renewal are present only in the very small subset (approximately one in a million) of MCL cells that form CAFCs. In addition, our studies demonstrate that these cells have the ability to engraft NOD/SCID mice and produce MCL similar to samples originally obtained from patients. Thus, these rare CD133+ cells have many of the features of CSCs.
Our research has three potential areas of impact. First, it will provide a powerful model to identify how MCL-SC-stromal interactions affect MCL pathogenesis, a process still poorly understood for this disease. Secondly, understanding the pathways involved in maintenance of MCL-SC and their interaction with the microenvironment has significant translational potential for diagnosis, prognosis and therapeutic development. Thirdly, the in vitro and in vivo systems being developed will serve as “second generation” pre-clinical screens for therapeutic agents. A model such as this, based on patient derived samples in conjunction with a system to study drug effects on MCL-SC, more closely resembles the human disease and may yield more accurate and effective therapeutic information that will be immediately translated into clinical trials at CINJ.
Figure 1. Development of Normal and Cancer Hematopoietic Stem Cells. Human hematopoietic cells are organized in a hierarchy that is sustained by a small population of self-renewing hematopoietic stem cells (HSCs). HSCs give rise to progressively more lineage-restricted, differentiated progenitors with reduced self-renewal capacity, which in turn produce functionally mature blood cells. Disruption of pathways regulating self-renewal and differentiation through the acquisition of transforming mutations generates cancer stem cells capable of sustaining growth of the leukemia/lymphoma clone.
Figure 2. MCL Cells in Long-Term Culture. MCL stem cells (round dark cells) cluster in cobblestone areas (CAFC), and MCL cells (round bright cells) on human stromal cells (flat dark cells) (hMSC)
Daniel J. Medina is an assistant professor of medicine at UMDNJ-Robert Wood Johnson Medical School (RWJMS) and The Cancer Institute of New Jersey (CINJ). He received his PhD from the University of Rhode Island, followed by postdoctoral training at Yale School of Medicine. He joined the RWJMS faculty in 1995. His research is supported by a grant from the New Jersey Commission on Science and Technology - Stem Cell Research Grants and by the Century for the Cure Bike Ride. He works with a research team that includes Roger Strair, MD, PhD, professor of medicine at RWJMS and director of hematologic malignancies at CINJ; Tulin Budak-Alpdogan, MD, associate professor of medicine at RWJMS and CINJ; Lauri Goodel, MD, RWJMS associate professor of pathology and laboratory medicine; and Hana Aviv, PhD, RWJMS associate professor of pathology and laboratory medicine.