RESUMO
Hemizygosity for genes that are essential for DNA mismatch repair (MMR) was found to underlie cancer predisposition in hereditary nonpolypsis colorectal cancer (HNPCC). Loss of the wild-type allele generates a MMR-deficient cell compartment with a high propensity to oncogenic transformation. MMR deficiency not only accelerates spontaneous mutagenesis resulting from DNA replication errors, but also affects the cellular response to genotoxic agents. To study the consequences of MMR deficiency in vitro and to provide experimental access to HNPCC we have generated MMR-deficient cell lines and mice. The combination of MMR deficiency and exposure to genotoxic agents strongly accelerated lymphomagenesis.
Assuntos
Pareamento Incorreto de Bases/genética , Proteínas de Ligação a DNA , Linfoma/genética , Mutagênicos/toxicidade , Proteínas Proto-Oncogênicas/genética , Animais , Células Cultivadas , Reparo do DNA/genética , Predisposição Genética para Doença , Linfoma/induzido quimicamente , Camundongos , Proteína 2 Homóloga a MutSRESUMO
Cancer predisposition in hereditary non-polyposis colon cancer (HNPCC) is caused by defects in DNA mismatch repair (MMR). Mismatch recognition is attributed to two heterodimeric protein complexes: MutSalpha (refs 2, 3, 4, 5), a dimer of MutS homologues MSH2 and MSH6; and MutSbeta (refs 2,7), a dimer of MSH2 and MSH3. These complexes have specific and redundant mismatch recognition capacity. Whereas MSH2 deficiency ablates the activity of both dimers, causing strong cancer predisposition in mice and men, loss of MSH3 or MSH6 (also known as GTBP) function causes a partial MMR defect. This may explain the rarity of MSH6 and absence of MSH3 germline mutations in HNPCC families. To test this, we have inactivated the mouse genes Msh3 (formerly Rep3 ) and Msh6 (formerly Gtmbp). Msh6-deficient mice were prone to cancer; most animals developed lymphomas or epithelial tumours originating from the skin and uterus but only rarely from the intestine. Msh3 deficiency did not cause cancer predisposition, but in an Msh6 -deficient background, loss of Msh3 accelerated intestinal tumorigenesis. Lymphomagenesis was not affected. Furthermore, mismatch-directed anti-recombination and sensitivity to methylating agents required Msh2 and Msh6, but not Msh3. Thus, loss of MMR functions specific to Msh2/Msh6 is sufficient for lymphoma development in mice, whereas predisposition to intestinal cancer requires loss of function of both Msh2/Msh6 and Msh2/Msh3.
Assuntos
Neoplasias Colorretais Hereditárias sem Polipose/genética , Reparo do DNA/genética , Proteínas de Ligação a DNA/genética , Deleção de Genes , Predisposição Genética para Doença/genética , Proteínas Associadas à Resistência a Múltiplos Medicamentos , Alelos , Animais , Pareamento Incorreto de Bases/genética , Morte Celular/efeitos dos fármacos , Neoplasias Colorretais Hereditárias sem Polipose/metabolismo , Neoplasias Colorretais Hereditárias sem Polipose/mortalidade , Neoplasias Colorretais Hereditárias sem Polipose/patologia , Proteínas de Ligação a DNA/deficiência , Proteínas de Ligação a DNA/metabolismo , Proteínas de Ligação a DNA/fisiologia , Feminino , Incidência , Masculino , Metilnitronitrosoguanidina/toxicidade , Camundongos , Camundongos Transgênicos , Proteína 3 Homóloga a MutS , Mutagênese Insercional , Oligodesoxirribonucleotídeos/genética , Oligodesoxirribonucleotídeos/metabolismo , Células-Tronco/citologia , Células-Tronco/efeitos dos fármacos , Células-Tronco/metabolismo , Taxa de Sobrevida , Fatores de TempoRESUMO
The majority of tumors associated with the nonpolyposis form of familial colorectal cancer (HNPCC) shows a specific form of genetic instability which is manifested by length alterations of mono- or dinucleotide repeat sequences [e.g., (A)n or (CA)n]. This phenomenon was termed the RER+ (replication error-positive) phenotype, MSI or MIN (microsatellite instability), and found to result from defects in the cells' DNA mismatch repair system. This system recognizes and restores misincorporated bases or slippage errors which frequently occur during DNA replication. Loss of DNA mismatch repair therefore strongly accelerates the evolutionary process of mutagenesis and selection which underlies the development of cancer. In addition to mutation avoidance, DNA mismatch repair also plays a crucial role in the toxicity of a number of DNA-damaging drugs that are used in cancer chemotherapy. In experimental systems, mismatch-repair-deficient cells are highly tolerant to the methylating chemotherapeutic drugs streptozocin and temozolomide and, albeit to a lesser extent, to cisplatin and doxorubicin. These drugs are therefore expected to be less effective on mismatch-repair-deficient tumors in humans. MIN was also found in a substantial portion of sporadic (nonfamilial) human tumors. However, in many cases the extent of microsatellite instability was not as dramatic as found in HNPCC-related tumors and the underlying genetic defect is unclear. Therefore, while the mismatch repair status of tumors may become an important determinant in the choice of chemotherapeutic intervention, the significance of MIN in sporadic cancer remains elusive.
