Loss of heterozygosity at the Ink4a/Arf locus facilitates Abelson virus transformation of pre-B cells

The biologic properties of leukemias arising from BCR/ABL-mediated transformation vary as a function of developmental origin and activity of the p19ARF gene

Recent reports have shown that upon expression of appropriate oncogenes, both stem cells and more differentiated progenitor populations can serve as leukemia-initiating cells. These studies suggest that oncogenic mutations subvert normal development and induce reacquisition of stem-like features. However, no study has described how specific mutations influence the ability of differentiating cell subsets to serve as leukemia-initiating cells and if varying such cellular origins confers a functional difference. We have examined the role of the tumor suppressor gene p19(ARF) in a murine model of acute lymphoblastic leukemia and found that loss of p19(ARF) changes the spectrum of cells capable of tumor initiation. With intact p19(ARF), only hematopoietic stem cells (HSCs) can be directly transformed by BCR/ABL expression. In a p19(ARF)-null genetic background expression of the BCR/ABL fusion protein renders functionally defined HSCs, common lymphoid progenitors (CLP), and precursor B-lymphocytes competent to generate leukemia stem cells. Furthermore, we show that leukemias arising from p19(ARF)-null HSC versus pro-B cells differ biologically, including relative response to drug insult. Our observations elucidate a unique mechanism by which heterogeneity arises in tumor populations harboring identical genetic lesions and show that activity of p19(ARF) profoundly influences the nature of tumor-initiating cells during BCR/ABL-mediated leukemogenesis.

Growth, differentiation, and malignant transformation of pre-B cells mediated by inducible activation of v-Abl oncogene

The nonreceptor tyrosine kinase, encoded by the v-Abl oncogene of Abelson murine leukemia virus induces transformation of progenitor B cells. The v-Abl oncogene promotes cell cycle progression and inhibits pre-B cell differentiation. The temperature-sensitive form of Abelson murine leukemia virus offers a reversible model to study the role of v-Abl in regulating growth and differentiation. Inactivation of v-Abl elevates p27 and Foxo3a levels and activates NF-kappaB/Rel, which leads to G1 arrest and induction of Ig L chain gene rearrangement, respectively. In turn, v-Abl reactivation reduces p27 and Foxo3a levels, thus permitting G1-arrested cells to reenter the cell cycle. However, the cell lines derived from SCID mice that are defective in the catalytic subunit of DNA-dependent protein kinase retain elevated levels of p27 and Foxo3a proteins despite reactivation of v-Abl. Consequently, these cells are locked in the G1 phase for an extended period of time. The few cells that manage to bypass the G1 arrest become tumorigenic and fail to undergo pre-B cell differentiation induced by v-Abl inactivation. Deregulation of p27, Foxo3a, c-myc, and NF-kappaB/Rel was found to be associated with the malignant transformation of SCID temperature-sensitive form of Abelson murine leukemia virus pre-B cells.

Rapid, stabilizing palindrome rearrangements in somatic cells by the center-break mechanism

DNA palindromes are associated with rearrangement in a variety of organisms. A unique opportunity to examine the impact of a long palindrome in mammals is afforded by the Line 78 strain of mice. Previously it was found that the transgene in Line 78 is likely to be palindromic and that the symmetry of the transgene was responsible for a high level of germ line instability. Here we prove that Line 78 mice harbor a true 15.4-kb palindrome, and through the establishment of cell lines from Line 78 mice we have shown that the palindrome rearranges at the impressive rate of about 0.5% per population doubling. The rearrangements observed to arise from rapid palindrome modification are consistent with a center-break mechanism where double-strand breaks, created through hairpin nicking of an extruded cruciform, are imprecisely rejoined, thus introducing deletions at the palindrome center. Significantly, palindrome rearrangements in somatic tissue culture cells almost completely mirrored the structures generated in vivo in the mouse germ line. The close correspondence between germ line and somatic events indicates the possibility that center-break modification of palindromes is an important mechanism for preventing mutation in both contexts. Permanent cell lines carrying a verified palindrome provide an essential tool for future mechanistic analyses into the consequences of palindromy in the mammalian genome.

The absence of Msh2 alters abelson virus pre-B-cell transformation by influencing p53 mutation

Defects in DNA mismatch repair predispose cells to the development of several types of malignant disease. The absence of Msh2 or Mlh1, two key molecules that mediate mismatch repair in eukaryotic cells, increases the frequency of mutation and also alters the response of some cells to apoptosis and cell cycle arrest. To understand the way these changes contribute to cancer predisposition, we examined the effects of defective mismatch repair on the multistep process of pre-B-cell transformation by Abelson murine leukemia virus. In this model, primary transformants undergo a prolonged apoptotic crisis followed by the emergence of fully transformed cell lines. The latter event is correlated to a loss of function of the p53 tumor suppressor protein and down-modulation of the p53 regulatory protein p19Arf. Analyses of primary transformants from Msh2 null mice and their wild-type littermates revealed that both types of cells undergo crisis. However, primary transformants from Msh2 null animals recover with accelerated kinetics, a phenomenon that is strongly correlated to the appearance of cells that have lost p53 function. Analysis of the kinetics with which p53 function is lost revealed that this change provides the dominant stimulus for emergence from crisis. Therefore, the absence of mismatch repair alters the molecular mechanisms involved in transformation by affecting a gene that controls apoptosis and cell cycle progression, rather than by affecting these processes directly.