RESUMO
Colorectal cancer (CRC) is the second most common cause of cancer-related deaths in the Western world and interactions between genetic and environmental factors, including diet, are suggested to play a critical role in its etiology. We conducted a long-term feeding experiment in the mouse to address gene expression and methylation changes arising in histologically normal colonic mucosa as putative cancer-predisposing events available for early detection. The expression of 94 growth-regulatory genes previously linked to human CRC was studied at two time points (5 weeks and 12 months of age) in the heterozygote Mlh1(+/-) mice, an animal model for human Lynch syndrome (LS), and wild type Mlh1(+/+) littermates, fed by either Western-style (WD) or AIN-93G control diet. In mice fed with WD, proximal colon mucosa, the predominant site of cancer formation in LS, exhibited a significant expression decrease in tumor suppressor genes, Dkk1, Hoxd1, Slc5a8, and Socs1, the latter two only in the Mlh1(+/-) mice. Reduced mRNA expression was accompanied by increased promoter methylation of the respective genes. The strongest expression decrease (7.3 fold) together with a significant increase in its promoter methylation was seen in Dkk1, an antagonist of the canonical Wnt signaling pathway. Furthermore, the inactivation of Dkk1 seems to predispose to neoplasias in the proximal colon. This and the fact that Mlh1 which showed only modest methylation was still expressed in both Mlh1(+/-) and Mlh1(+/+) mice indicate that the expression decreases and the inactivation of Dkk1 in particular is a prominent early marker for colon oncogenesis.
Assuntos
Proteínas Adaptadoras de Transdução de Sinal/genética , Colo/metabolismo , Neoplasias do Colo/genética , Dieta Hiperlipídica , Regulação Neoplásica da Expressão Gênica , Mucosa/metabolismo , Proteínas Nucleares/genética , Proteínas Adaptadoras de Transdução de Sinal/deficiência , Animais , Proteínas de Transporte de Cátions/genética , Colo/patologia , Neoplasias do Colo/diagnóstico , Neoplasias Colorretais Hereditárias sem Polipose/diagnóstico , Neoplasias Colorretais Hereditárias sem Polipose/genética , Metilação de DNA , Modelos Animais de Doenças , Proteínas de Homeodomínio/genética , Humanos , Peptídeos e Proteínas de Sinalização Intercelular/genética , Camundongos , Camundongos da Linhagem 129 , Camundongos Knockout , Transportadores de Ácidos Monocarboxílicos , Mucosa/patologia , Proteína 1 Homóloga a MutL , Proteínas Nucleares/deficiência , Reação em Cadeia da Polimerase Via Transcriptase Reversa , Proteína 1 Supressora da Sinalização de Citocina , Proteínas Supressoras da Sinalização de Citocina/genética , Fatores de TempoRESUMO
The human mismatch repair (MMR) gene MSH2 is the second most frequently mutated hereditary nonpolyposis colorectal cancer (HNPCC) susceptibility locus. Given that missense mutations account for 17% of all identified alterations in this gene, the study of their pathogenicity is of increasing importance. Previously, we showed that pathogenic MSH2 missense mutations typically impaired the repair activity of the protein. In this study, we took advantage of its crystal structure and attempted to correlate the mismatch binding and ATP-catalyzed mismatch release activities with the location of 18 nontruncating MSH2 mutations. We observed that the MMR-deficient mutations situated in the amino-terminal connector and lever domains of MSH2 (V161D, G162R, G164R, L173P, L187P, C333Y, and D603N) affected protein stability, whereas mutations in the ATPase domain (A636P, G674A, C697F, I745_I746del, and E749 K) mainly caused defects in mismatch binding or release. Of the MMR-proficient variants, four (T33P, A272 V, G322D, and V923E) showed slightly reduced mismatch binding and/or release efficiencies compared to wild-type (WT) protein, while two variants (N127S and A834 T) showed no defects in the assays. Similar to our biochemical data, the mutations that affected protein stability were associated with an absence of the protein in tumors in immunohistochemical (IHC) analyses. In contrast, the protein with the mutation E749 K, which abrogates MMR but not protein stability, is well expressed in tumors. In conclusion, pathogenic missense mutations in MSH2 may interfere with different mechanisms that tend to cluster in separate protein domains with varying effects on protein stability, which could be taken into account when interpreting IHC data.
Assuntos
Neoplasias Colorretais Hereditárias sem Polipose/genética , Reparo de Erro de Pareamento de DNA/genética , Proteína 2 Homóloga a MutS/genética , Mutação de Sentido Incorreto , Células Cultivadas , Proteínas de Ligação a DNA/genética , Predisposição Genética para Doença , Variação Genética , Humanos , Domínios e Motivos de Interação entre Proteínas , Estabilidade Proteica , Proteínas Recombinantes/genéticaRESUMO
Hereditary non-polyposis colorectal cancer (HNPCC) is associated with germline mutations in mismatch repair (MMR) genes. Inherited missense mutations, however, complicate the diagnostics because they do not always cause unambiguous predisposition to cancer. This leads to variable and contradictory interpretations of their pathogenicity. Here, we establish evidence for the functionality of the 2 frequently reported variations, MSH2 N127S and G322D, which have been described both as pathogenic and non-pathogenic in literature and databases. We report the results of 3 different functional analyses characterizing the biochemical properties of these protein variants in vitro. We applied an immunoprecipitation assay to assess the MSH2-MSH6 interaction, a bandshift assay to study mismatch recognition and binding, and a MMR assay for repair efficiency. None of the experiments provided evidence on reduced functionality of these proteins as compared to wild-type MSH2. Our data demonstrate that MSH2 N127S and G322D per se are not sufficient to trigger MMR deficiency. This together with variable clinical phenotypes in the mutation carriers suggest no or only low cancer risk in vivo.