The Absence of Msh2 Alters Abelson Virus Pre-B-Cell Transformation by Influencing p53 Mutation

Abstract

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.

BCR/ABL Inhibits Mismatch Repair to Protect from Apoptosis and Induce Point Mutations

“…In addition, N-ethyl-N-nitrosourea, which induced mutagenesis in MMR-deficient cells, generated imatinibresistant BCR/ABL mutants in experimental conditions (21). Moreover, Msh2 À/À bone marrow cells infected by Abelson murine leukemia virus acquire transforming phenotype and p53-inactivating point mutations with much higher frequency than Msh2 +/+ counterparts (22). It could be also postulated that impaired MMR could facilitate point mutations acquired during unfaithful nucleotide excision repair and homologous recombination repair in BCR/ABL-positive leukemias (5,23).…”

Absence of p53 Complements Defects in Abelson Murine Leukemia Virus Signaling

“…Uninfected cultures contained no colonies. a Primary transformants derived from p53 wild-type or null mice following infection with P120 or P120/R273K were plated in liquid medium, and growth and viability were monitored until the cultures were fully established as judged by regular growth and low levels of apoptosis (12,18,27). The numbers in parentheses are the number of primary transformants that established per number examined.…”

“…The second stage of Ab-MLV transformation involves a period when primary transformants display erratic growth and high levels of apoptosis and many primary transformants die (12,27); cells from p53 null mice bypass the crisis phase of the transformation process (27). To determine if primary transformants from p53 null mice initiated with P120/R273K display a different profile than those expressing P120, cells were explanted from agar and expanded.…”

“…To monitor sensitivity to rapid apoptosis following irradiation, cells were treated with approximately 1,000 rad of ␥-irradiation by using a cesium 137 Gammacell-100 source. The cells were incubated, and the frequency of apoptotic cells was evaluated 12 to 16 h later (12,26).…”

Active Akt and Functional p53 Modulate Apoptosis in Abelson Virus-Transformed Pre-B Cells

“…Ab-MLVinfected pre-B cells must overcome the effects of this pathway in order to become fully transformed cell lines with malignant potential (32,47,48), and the transformation defects of some Ab-MLV mutants are reduced when p53-null cells are used as a target population (49,55). Many transformants acquire p53 mutations as they become established (21,32,47), while others down-modulate the expression of the p19Arf protein, a molecule that can activate p53 through effects on Mdm2 (42). This latter type of transformant retains functional p53 protein and responds to gamma irradiation by undergoing rapid apoptosis and upregulating the expression of the p53-responsive gene p21Cip1 (47).…”