Mlh1-dependent suppression of specific mutations induced in vivo by the food-borne carcinogen 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP)

Induction of mismatch repair deficiency, compromised DNA damage signaling and compound hypermutagenesis by a dietary mutagen in a cell-based model for Lynch Syndrome

The prevalent cancer predisposition Lynch syndrome (LS, OMIM #120435) is caused by an inherited heterozygous defect in any of the four core DNA mismatch repair (MMR) genes MSH2, MSH6, MLH1 or PMS2. MMR repairs errors by the replicative DNA polymerases in all proliferating tissues. Its deficiency, following somatic loss of the wild type copy, results in a spontaneous mutator phenotype that underlies the rapid development of, predominantly, colorectal cancer (CRC) in LS. Here we have addressed the hypothesis that aberrant responses of intestinal stem cells to diet-derived mutagens may be causally involved in the restricted cancer tropism of LS. To test this we have generated a panel of isogenic mouse embryonic stem (mES) cells with heterozygous or homozygous disruption of multiple MMR genes and investigated their responses to the common dietary mutagen and carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Our data reveal that PhIP can inactivate the wild type allele of heterozygous mES cells via the induction of either loss of heterozygosity (LOH) or intragenic mutations. Moreover, while protective DNA damage signaling (DDS) is compromised, PhIP induces more mutations in Msh2, Mlh1, Msh6 or Pms2-deficient mES cells than in wild type cells. Combined with their spontaneous mutator phenotypes, this results in a compound hypermutator phenotype. Together, these results indicate that dietary mutagens may promote CRC development in LS at multiple levels, providing a rationale for dietary modifications in the management of LS.

Excision of translesion synthesis errors orchestrates responses to helix-distorting DNA lesions

An Msh2/Msh6-dependent DNA repair mechanism mitigates the mutagenicity of photolesions and induces cell cycle responses by excising incorrect nucleotides incorporated by postreplicative translesion synthesis.

In addition to correcting mispaired nucleotides, DNA mismatch repair (MMR) proteins have been implicated in mutagenic, cell cycle, and apoptotic responses to agents that induce structurally aberrant nucleotide lesions. Here, we investigated the mechanistic basis for these responses by exposing cell lines with single or combined genetic defects in nucleotide excision repair (NER), postreplicative translesion synthesis (TLS), and MMR to low-dose ultraviolet light during S phase. Our data reveal that the MMR heterodimer Msh2/Msh6 mediates the excision of incorrect nucleotides that are incorporated by TLS opposite helix-distorting, noninstructive DNA photolesions. The resulting single-stranded DNA patches induce canonical Rpa–Atr–Chk1-mediated checkpoints and, in the next cell cycle, collapse to double-stranded DNA breaks that trigger apoptosis. In conclusion, a novel MMR-related DNA excision repair pathway controls TLS a posteriori, while initiating cellular responses to environmentally relevant densities of genotoxic lesions. These results may provide a rationale for the colorectal cancer tropism in Lynch syndrome, which is caused by inherited MMR gene defects.

Tumor characteristics as an analytic tool for classifying genetic variants of uncertain clinical significance

It is important to identify a germline mutation in a patient with an inherited cancer syndrome to allow mutation carriers to be included in cancer surveillance programs, which have been proven to save lives. Many of the mutations identified result in premature termination of translation, and thus in loss-of-function of the encoded mutated protein. However, the significance of a large proportion of the sequence changes reported is unknown. Some of these variants will be associated with a high risk of cancer and have direct clinical consequence. Many criteria can be used to classify variants with unknown significance; most criteria are based on the characteristics of the amino acid change, on segregation data and appearance of the variant, on the presence of the variant in controls, or on functional assays. In inherited cancers, tumor characteristics can also be used to classify variants. It is worthwhile to examine the clinical, morphological and molecular features of a patient, and his or her family, when assessing whether the role of a variant is likely to be neutral or pathogenic. Here we describe the advantages and disadvantages of using the tumor characteristics of patients carrying germline variants of uncertain significance (VUS) in BRCA1, BRCA2, or in one of the mismatch repair (MMR) genes, MLH1, MSH2, or MSH6, to infer pathogenicity.

Role of the mismatch repair gene, Msh6, in suppressing genome instability and radiation-induced mutations

Mismatch repair (MMR) is critical for preserving genomic integrity. Failure of this system can accelerate somatic mutation and increase the risk of developing cancer. MSH6, in complex with MSH2, is the MMR protein that mediates DNA repair through the recognition of 1- and 2-bp mismatches. To evaluate the effects of MSH6 deficiency on genomic stability we compared the frequency of in vivo loss of heterozygosity (LOH) between MSH6-proficient and deficient, 129S2xC57BL/6 F1 hybrid mice that were heterozygous for our reporter gene Aprt. We recovered mutant cells that had functionally lost APRT protein activity and categorized the spectrum of mutations responsible for the LOH events. We also measured the mutant frequency at the X-linked gene, Hprt, as a second reporter for point mutation. In Msh6-/-Aprt+/- mice, mutation frequency at Aprt was elevated in both T cells and fibroblasts by 2.5-fold and 5.7-fold, respectively, over Msh6+/+Aprt+/- littermate controls. While a modest increase in mitotic recombination (MR) was observed in MSH6-deficient fibroblasts compared to wild type controls, point mutation was the predominant mechanism leading to APRT deficiency in both cell types. Base substitution, consisting of multiple types of transitions, accounted for all of the point mutations identified within the Aprt coding region. We also assessed the role of MSH6 in preventing mutations caused by a common environmental mutagen, ionizing radiation (IR). In Msh6-/-Aprt+/- mice, 4Gy of X-irradiation induced a significant increase in point mutations at both Aprt and Hprt in T cells, but not in fibroblasts. These findings indicate that MutS alpha reduces spontaneous and IR-induced mutation in a cell type-dependant manner.

Induction of aberrant crypt foci in DNA mismatch repair-deficient mice by the food-borne carcinogen 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP)

Disruption of the DNA mismatch repair (MMR) pathway results in elevated mutation rates, inappropriate survival of cells bearing DNA damage, and increased cancer risk. Relatively little is known about the impact of environmentally relevant carcinogens on cancer risk in individuals with MMR-deficiency. We evaluated the effect of MMR status (Mlh1(+/+) versus Mlh1(-/-)) on the carcinogenic potential of the cooked-meat mutagen, 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP) in mice. PhIP exposure did not obviously increase lymphoma or small intestinal tumorigenesis in either Mlh1-deficient or -proficient mice. In contrast, the frequency of aberrant crypt foci (ACF), a preneoplastic biomarker for colon tumorigenesis, was increased by PhIP, and the increase due to PhIP was significantly greater in Mlh1(-/-) versus wild-type littermates. This apparent heightened susceptibility to induction of ACF parallels the previously reported hypermutability of Mlh1-deficient mice to PhIP and is consistent with the hypothesis that MMR-deficiency would increase the likelihood of PhIP-induced carcinogenic mutations. Further evaluation of the risk that consumption of heterocyclic amines may impart to MMR-deficient individuals therefore is warranted.