Origin of multiple mutations in human cancers

Mms22p protects Saccharomyces cerevisiae from DNA damage induced by topoisomerase II

The cleavage reaction of topoisomerase II, which creates double-stranded DNA breaks, plays a central role in both the cure and initiation of cancer. Therefore, it is important to understand the cellular processes that repair topoisomerase II-generated DNA damage. Using a genome-wide approach with Saccharomyces cerevisiae, we found that Δmre11, Δxrs2, Δrad50, Δrad51, Δrad52, Δrad54, Δrad55, Δrad57 and Δmms22 strains were hypersensitive to etoposide, a drug that specifically increases levels of topoisomerase II-mediated DNA breaks. These results confirm that the single-strand invasion pathway of homologous recombination is the major pathway that repairs topoisomerase II-induced DNA damage in yeast and also indicate an important role for Mms22p. Although Δmms22 strains are sensitive to several DNA-damaging agents, little is known about the function of Mms22p. Δmms22 cultures accumulate in G₂/M, and display an abnormal cell cycle response to topoisomerase II-mediated DNA damage. MMS22 appears to function outside of the single-strand invasion pathway, but levels of etoposide-induced homologous recombination in Δmms22 cells are lower than wild-type. MMS22 is epistatic with RTT101 and RTT107, genes that encode its protein binding partners. Finally, consistent with a role in DNA processes, Mms22p localizes to discrete nuclear foci, even in the absence of etoposide or its binding partners.

BCR/ABL oncogenic kinase promotes unfaithful repair of the reactive oxygen species-dependent DNA double-strand breaks

The oncogenic BCR/ABL tyrosine kinase induces constitutive DNA damage in Philadelphia chromosome (Ph)-positive leukemia cells. We find that BCR/ABL-induced reactive oxygen species (ROSs) cause chronic oxidative DNA damage resulting in double-strand breaks (DSBs) in S and G(2)/M cell cycle phases. These lesions are repaired by BCR/ABL-stimulated homologous recombination repair (HRR) and nonhomologous end-joining (NHEJ) mechanisms. A high mutation rate is detected in HRR products in BCR/ABL-positive cells, but not in the normal counterparts. In addition, large deletions are found in NHEJ products exclusively in BCR/ABL cells. We propose that the following series of events may contribute to genomic instability of Ph-positive leukemias: BCR/ABL --> ROSs --> oxidative DNA damage --> DSBs in proliferating cells --> unfaithful HRR and NHEJ repair.

Thymic lymphomas arising in Msh2 deficient mice display a large increase in mutation frequency and an altered mutational spectrum

Mismatch repair (MMR) genes, such as Msh2, are classified as "mutator" genes, responsible for the microsatellite instability identified in many tumors. In the current study, the mutation frequency and mutational spectrum in thymic lymphoma arising in Msh2 deficient mice are investigated. Thymic lymphoma developed in Msh2-/- background displayed an eight to nine-fold increase in mutation frequency compared to the normal thymi in Msh2 deficient animals. Sequencing demonstrated significantly different mutational spectra between normal thymus tissue and thymic lymphomas in Msh2-/- mice (P=0.02). The tumor mutational spectrum is characterized by an increase in base substitutions occurring at A:T sites, and multiple mutations, as well as a minor increase in -1 frameshifts. We analyzed mutations in different parts of the tumors, and different regional hotspots could be identified. Several hotspot mutations that are a rare event in normal tissues were identified in the tumor tissues. We conclude that thymic lymphomas arising in Msh2 deficient genetic background are hypermutable and the altered mutational spectrum might be an indication of infidelity of DNA replication during tumorigenesis.

DNA polymerase beta imbalance increases apoptosis and mutagenesis induced by oxidative stress

Oxidative stress has been proposed to be one of the major causes leading to the accumulation of mutation that is associated with the initiation and progression of cancers. Elevated expression of DNA polymerase beta, an event found in many human tumors, has been shown to generate a mutator phenotype. Here, we demonstrated that overexpression of DNA polymerase beta strengthens the mutagenicity of oxidative damages, concomitantly with a higher cellular sensitivity and increased apoptosis. Deregulated expression of DNA polymerase beta could represent a predisposition factor for mutagenic effects of oxidative stress and thus have implication in the generation and/or evolution of cancer.

Mechanisms of dinucleotide repeat instability in Escherichia coli

The high level of polymorphism of microsatellites has been used for a variety of purposes such as positional cloning of genes associated with diseases, forensic medicine, and phylogenetic studies. The discovery that microsatellites are associated with human diseases, not only as markers of risk but also directly in disease pathogenesis, has triggered a renewed interest in understanding the mechanism of their instability. In this work we have investigated the role of DNA replication, long patch mismatch repair, and transcription on the genetic instability of all possible combinations of dinucleotide repeats in Escherichia coli. We show that the (GpC) and (ApT) self-complementary sequence repeats are the most unstable and that the mode of replication plays an important role in their instability. We also found that long patch mismatch repair is involved in avoiding both short deletion and expansion events and also in instabilities resulting from the processing of bulges of 6 to 8 bp for the (GpT/ApC)- and (ApG/CpT)- containing repeats. For each dinucleotide sequence repeat, we propose models for instability that involve the possible participation of unusual secondary structures.

Mismatch repair and drug responses in cancer

Defects in mismatch repair contribute to development of approximately 15% of colon cancers and to origination of endometrial, gastric and other cancers. Tumors with defects in mismatch repair exhibit marked resistance to alkylators and a variety of anticancer agents that modify DNA to create substrates for the mismatch repair system. These altered drug responses appear to derive from requirements for mismatch repair proteins in signalling apoptosis, altered cell cycle checkpoint behaviour and/or loss of mismatch repair dependent toxicity arising from futile repair cycling. Altered repair mechanisms for mismatched substrates in mismatch repair defective tumors provide both challenges for development of tumor-phenotype-screening methodologies to assure appropriate therapy is administered for these cancers and foci for development of new therapy approaches that capitalize on modified drug responses in mismatch repair- defective cells. Copyright 1999 Harcourt Publishers Ltd.

The Tyr-265-to-Cys mutator mutant of DNA polymerase beta induces a mutator phenotype in mouse LN12 cells

DNA polymerase beta functions in both base excision repair and meiosis. Errors committed by polymerase beta during these processes could result in mutations. Using a complementation system, in which rat DNA polymerase beta substitutes for DNA polymerase I of Escherichia coli, we previously isolated a DNA polymerase beta mutant in which Tyr-265 was altered to Cys (Y265C). The Y265C mutant is dominant to wild-type DNA polymerase beta and possesses an intrinsic mutator activity. We now have expressed the wild-type DNA polymerase and the Y265C mutator mutant in mouse LN12 cells, which have endogenous DNA polymerase beta activity. We demonstrate that expression of the Y265C mutator mutant in the LN12 cells results in an 8-fold increase in the spontaneous mutation frequency of lambdacII mutants compared with expression of the wild-type protein. Expression of Y265C results in at least a 40-fold increase in the frequency of deletions of three bases or more and a 7-fold increase in point mutations. Our results suggest that the mutations we observe in vivo result directly from the action of the mutator polymerase. To our knowledge, this is the first demonstration of a mutator phenotype resulting from expression of a DNA polymerase mutator mutant in mammalian cells. This work raises the possibility that variant polymerases may act in a dominant fashion in human cells, leading to genetic instability and carcinogenesis.