RESUMO
Mutations in the human DNA mismatch repair (MMR) gene MLH1 are associated with hereditary nonpolyposis colorectal cancer (Lynch syndrome, HNPCC) and a significant proportion of sporadic colorectal cancer. The inactivation of MLH1 results in the accumulation of somatic mutations in the genome of tumor cells and resistance to the genotoxic effects of a variety of DNA damaging agents. To study the effect of MLH1 missense mutations on cancer susceptibility, we generated a mouse line carrying the recurrent Mlh1(G67R) mutation that is located in one of the ATP-binding domains of Mlh1. Although the Mlh1(G67R) mutation resulted in DNA repair deficiency in homozygous mutant mice, it did not affect the MMR-mediated cellular response to DNA damage, including the apoptotic response of epithelial cells in the intestinal mucosa to cisplatin, which was defective in Mlh1(-/-) mice but remained normal in Mlh1(G67R/G67R) mice. Similar to Mlh1(-/-) mice, Mlh1(G67R/G67R) mutant mice displayed a strong cancer predisposition phenotype. However, in contrast to Mlh1(-/-) mice, Mlh1(G67R/G67R) mutant mice developed significantly fewer intestinal tumors, indicating that Mlh1 missense mutations can affect MMR tumor suppressor functions in a tissue-specific manner. In addition, Mlh1(G67R/G67R) mice were sterile because of the inability of the mutant Mlh1(G67R) protein to interact with meiotic chromosomes at pachynema, demonstrating that the ATPase activity of Mlh1 is essential for fertility in mammals.
Assuntos
Proteínas Adaptadoras de Transdução de Sinal/genética , Proteínas Adaptadoras de Transdução de Sinal/metabolismo , Reparo de Erro de Pareamento de DNA , Meiose/genética , Neoplasias/genética , Neoplasias/metabolismo , Proteínas Nucleares/genética , Proteínas Nucleares/metabolismo , Proteínas Adaptadoras de Transdução de Sinal/deficiência , Animais , Apoptose/efeitos dos fármacos , Linhagem Celular , Cromossomos/genética , Cisplatino/farmacologia , Dano ao DNA , Predisposição Genética para Doença/genética , Glicina/genética , Glicina/metabolismo , Masculino , Camundongos , Camundongos Knockout , Instabilidade de Microssatélites , Proteína 1 Homóloga a MutL , Mutação/genética , Neoplasias/patologia , Proteínas Nucleares/deficiência , Fenótipo , Taxa de SobrevidaRESUMO
PURPOSE: Telomerase activity appears to play a crucial role in the development of many tumors. More than 80% of all malignant human tumors show an increased telomerase activity. However, conflicting results have been reported about telomerase activity in sarcomas. The aim of the study was to obtain more information about telomerase activity in sarcomas based on a large number of cases. METHODS: Telomerase activity was measured in 69 different tumor samples (33 malignant bone tumors and 36 soft tissue sarcomas). Tumor samples were obtained intraoperatively and frozen immediately in liquid nitrogen. Telomerase activity was detected by the telomeric repeat amplification assay (TRAP-assay). RESULTS: Only 7% of the samples showed telomerase activity. No correlation between staging and telomerase activity could be observed. DISCUSSION: The fact that only five out of 69 examined tumor samples showed a telomerase activity provides experimental evidence that in sarcomas the reactivation of telomerase may play a subordinate role. Our results suggest that alternative mechanisms for cell immortalization, yet to be determined, seem to be involved in the development and/or maintenance of soft-tissue sarcomas and malignant bone tumors.
