Hematopoietic Stem Cells Are Not the Direct Target of Spontaneous Leukemic Transformation in p18INK4C-Null Reconstituted Mice

Abstract

Cell cycle inhibitors are important regulators in normal tissue regeneration and disruption of the regulators are involved in cancer development. Our recent study showed that the absence of the CDK inhibitor p18 INK4C (p18) enhances self-renewal of normal hematopoietic stem cell (HSC) in vivo, whereas previous studies by others showed an increased incidence of leukemogenesis in older p18-null mice. Here, we have examined potential leukemogenesis during experimentally induced regeneration of HSC in the absence of p18 in order to gauge the relation between these two processes. Reconstituted mice with p18-deficient HSCs under the condition of repetitive proliferative stress (serial transplantation) were followed for >3 years. T cell leukemia from the p18À/À origin was recapitulated 24 months after secondary transplantation. However, no myeloid leukemia was found in the recipients. The T cell leukemia-initiating cells (mainly in a CD3 lo cell subset) did not share the same immunophenotype with normal HSCs and, in fact, the function of HSCs was significantly compromised with decreased abundance in the leukemic mice. Furthermore, we found that the p15 or p16 gene promoters were frequently methylated in the leukemic cells but not in HSCs. Our present study argues against the possibility of overgrowth of p18-null HSCs leading to a leukemic phenotype. The data also support the notion that p18 has an independent role in T cell maintenance such that CD3 + CD8 + cells, unlike HSCs, are more accessible to leukemogenic transformation after the loss of p18. (Cancer Res 2006; 66(1): 343-51)

Gene therapy approaches for stem cell protection

“…One requirement of successful engraftment is the return of donor hematopoietic cells to a quiescent state. 8,9,17 Continued cycling of all engrafted cells would result in hematopoietic cell depletion, absence of serial self-renewal of stem cell and lack of stable engraftment. 5,6,19 The same requirement of primitive stem cell to return to the quiescent state is probably true for stem cell repopulation of intestine, lung, esophagus and skin.…”

“…24 The subset of hematopoietic stem cells that survive total body irradiation has been shown to be in quiescence. 8,9,18,19 5-Fluorouracil (5-FU)-resistant stem cells are radiation resistant (using 5-FU, a drug known to kill cycling cells). In contrast, busulfan is toxic for hematopoietic stem cells in quiescence.…”

“…[5][6][7] Mice deficient in p21 cip/wafl or p18 INCÀ4C demonstrate altered self-renewal capacity and altered competitive repopulation capacity compared to marrow stem cells from litter mates not lacking these tumor suppressor gene or cell cycle altering gene functions. 8,9 Antioxidant approaches toward enhancing stem cell protection include transfection of the transgenes for manganese superoxide dismutase (MnSOD), catalase, glutathione, peroxidase, metallothione and others. 10,11 While there is some protective effect of antioxidant transgene expression shown with respect to clonogenic survival of cells in culture, 12,13 no assays of stem cell competitive repopulation capacity have been reported.…”

“…11,15 The capacity of donor stem cells to home and engraft into the ionizing irradiated or toxin-damaged recipient microenvironment depends on intact DNA damage repair systems, and antioxidant stress handling mechanisms in the engrafting cells. 8,9 ATMÀ/À mouse hematopoietic stem cells have a deficient proliferative advantage compared to wild-type cells, engrafting into the same irradiated host which can be partially ameliorated by treating the host and/or deficient cells with the free radical scavenger N-acetyl-cysteine. 3,4 The success of hematopoietic cell engraftment in non-myeloablated recipients may be attributable to the reduced-irradiation dose-dependent decrease in toxicity to the microenvironment while still clearing of an adequate niche by the sublethal total body irradiation.…”

“…Resistance of a subset of hematopoietic stem cells to agents such as ionizing irradiation and alkalinizing agents which target cycling cells confirmed that quiescence or maintenance in the G 0 phase of the cell cycle is itself protective. [6][7][8][9] The subset of rodent hematopoietic stem cells which resists total body irradiation can be identified as those cells with a lower metabolic rate, low respiratory rate, as reflected by rhodamine dye exclusion from mitochondria. 35 Stem cell hierarchy is not the same for all organspecific, self-renewing populations.…”