Double frameshift mutations in APC and MSH2 in the same individual

Germline mutations of the adenomatous polyposis coli (APC) gene in Algerian familial adenomatous polyposis cohort: first report

BACKGROUND

Familial adenomatous polyposis (known also as classical or severe FAP) is a rare autosomal dominant colorectal cancer predisposition syndrome, characterized by the presence of hundreds to thousands of adenomatous polyps in the colon and rectum from an early age. In the absence of prophylactic surgery, colorectal cancer (CRC) is the inevitable consequence of FAP. The vast majority of FAP is caused by germline mutations in the adenomatous polyposis coli (APC) tumor suppressor gene (5q21). To date, most of the germline mutations in classical FAP result in truncation of the APC protein and 60% are mainly located within exon 15.

MATERIAL AND METHODS

In this first nationwide study, we investigated the clinical and genetic features of 52 unrelated Algerian FAP families. We screened by PCR-direct sequencing the entire exon 15 of APC gene in 50 families and two families have been analyzed by NGS using a cancer panel of 30 hereditary cancer genes.

RESULTS

Among 52 FAP index cases, 36 had 100 or more than 100 polyps, 37 had strong family history of FAP, 5 developed desmoids tumors, 15 had extra colonic manifestations and 21 had colorectal cancer. We detected 13 distinct germline mutations in 17 FAP families. Interestingly, 4 novel APC germline pathogenic variants never described before have been identified in our study.

CONCLUSIONS

The accumulating knowledge about the prevalence and nature of APC variants in Algerian population will contribute in the near future to the implementation of genetic testing and counseling for FAP patients.

A novel mouse model of PMS2 founder mutation that causes mismatch repair defect due to aberrant splicing

Hereditary non-polyposis colorectal cancer, now known as Lynch syndrome (LS) is one of the most common cancer predisposition syndromes and is caused by germline pathogenic variants (GPVs) in DNA mismatch repair (MMR) genes. A common founder GPV in PMS2 in the Canadian Inuit population, NM_000535.5: c.2002A>G, leads to a benign missense (p.I668V) but also acts as a de novo splice site that creates a 5 bp deletion resulting in a truncated protein (p.I668*). Individuals homozygous for this GPV are predisposed to atypical constitutional MMR deficiency with a delayed onset of first primary malignancy. We have generated mice with an equivalent germline mutation (Pms2c.1993A>G) and demonstrate that it results in a splicing defect similar to those observed in humans. Homozygous mutant mice are viable like the Pms2 null mice. However, unlike the Pms2 null mice, these mutant mice are fertile, like humans homozygous for this variant. Furthermore, these mice exhibit a significant increase in microsatellite instability and intestinal adenomas on an Apc mutant background. Rectification of the splicing defect in human and murine fibroblasts using antisense morpholinos suggests that this novel mouse model can be valuable in evaluating the efficacy aimed at targeting the splicing defect in PMS2 that is highly prevalent among the Canadian Inuits.

Distinctive DNA mismatch repair and APC rare variants in African Americans with colorectal neoplasia

PURPOSE

African Americans have a higher incidence and mortality from colorectal cancer. This disparity might be due, in part, to the type of mutations in driver genes. In this study, we examined alterations specific to APC, MSH3, and MSH6 genes using targeted exome sequencing to determine distinctive variants in the course of neoplastic transformation.

EXPERIMENTAL DESIGN

A total of 140 African American colon samples (30 normal, 21 adenomas, 33 advanced adenomas and 56 cancers) were used as our discovery set on an Ion Torrent platform. A 36 samples subset was resequenced on an Illumina platform for variants' validation. Bioinformatics analyses were performed and novel validated variants are reported.

RESULTS

Two novel MSH6 variants were validated and mapped to the MutS-V region near the MSH2 binding site. For MSH3, 4 known variants were validated and were located in exon 10 (3 non-synonymous) and exon 18 (1 synonymous). As for APC, 20 variants were validated with 4 novel variants: 3 stopgain and 1 non-synonymous. These variants mapped prior to and on the Armadillo repeats region, to the 15-amino acid repeat region, and to the 20-amino acid repeats region, respectively.

CONCLUSION

We defined novel variants that target DNA mismatch repair and APC genes in African Americans with colorectal lesions. A greater frequency of variants in genes encoding DNA mismatch repair functions and APC likely plays major roles in colorectal cancer initiation and higher incidence of the disease in African Americans.

The Genetics of Colorectal Cancer

The hereditary colorectal cancer syndromes comprise a heterogeneous group of conditions with varying cancer risks, gastrointestinal polyp types, nonmalignant findings, and inheritance patterns. Although each one is unique in its own right, these syndromes often have overlapping features, making diagnoses difficult in select cases. Obtaining accurate polyp history (histologic type, number, location, and age of onset), cancer history (location, type, and age of onset), and other nonmalignant features is imperative in determining the likely disease diagnosis and thereby the appropriate genetic tests for precise diagnosis in a timely fashion. This process often necessitates collaboration among surgical oncology team members and genetic counselors.

Polyposis and early cancer in a patient with low penetrant mutations in MSH6 and APC: hereditary colorectal cancer as a polygenic trait

Hereditary non-polyposis colorectal cancer and familial adenomatus polyposis are autosomal dominant diseases accounting for 5-7% of all colorectal cancer cases. Inheritance of mutations associated with both syndromes in the same individual has, so far, only been observed in a few cases. This report outlines the findings in a proband of a HNPCC family, who presented with colorectal cancer and with multiple adenomas at the age of 18. He was shown to be compound heterozygous for MSH6 mutations: a nonsense mutation in exon 4 (c.1836 C>A, p.S612X); and a missense mutation in exon 5 (c.3226 C>T, p.R1076C). In addition, an APC missense mutation was revealed (c.7504 G>A, p.G2502S). Immunohisto-chemical analysis showed lack of expression of MSH6 in tumour tissue, as well as accumulation of betacatenin in the nuclei of the tumour cells. We suggest that the presence of mutations in both alleles of one gene and mutations in different genes, may influence the phenotype in hereditary colorectal cancer. Biallelic and/or polygenic mutations should be suspected when facing unusual severe variants of "classic monogenic phenotypes", such as HNPCC.

The natural history of a combined defect in MSH6 and MUTYH in a HNPCC family

In the inherited syndromes, MUTYH-associated polyposis (MAP) and hereditary nonpolyposis colorectal cancer (HNPCC), somatic mutations occur due to loss of the caretaker function that base-repair (BER) and mismatch repair (MMR) genes have, respectively. Recently, we identified a large branch from a MSH6 HNPCC family in which 19 family members are heterozygous or compound heterozygous for MUTYH germ line mutations. MSH6/MUTYH heterozygote mutation carriers display a predominant HNPCC molecular tumour phenotype, with microsatellite instability and underrepresentation of G>T transversions. A single unique patient is carrier of the MSH6 germline mutation and is compound heterozygote for MUTYH. Unexpectedly, this patient has an extremely mild clinical phenotype with sofar only few adenomas at age 56. Four out of five adenomas show characteristic G>T transversions in APC and/or KRAS2, as seen in MUTYH associated polyposis. No second hit of MSH6 is apparent in any of the adenomas, due to retained MSH6 nuclear expression and a lack of microsatellite instability. Although this concerns only one case, we argue that the chance to find an additional one is extremely small and currently a mouse model with this genotype combination is not available. Moreover, the patients brother who is also compound heterozygous for MUTYH but lacks the MSH6 germline mutation presented with a full blown polyposis coli. In conclusion, these data would support the notion that abrogation of both MSH6 DNA mismatch repair and base repair might be mutually exclusive in humans.

MSH6 germline mutations in early-onset colorectal cancer patients without family history of the disease

Germline MLH1 and MSH2 mutations are scarce in young colorectal cancer patients with negative family history of the disease. To evaluate the contribution of germline MSH6 mutations to early-onset colorectal cancer, we have analysed peripheral blood of 38 patients diagnosed with this disease before 45 years of age and who presented no family history of hereditary nonpolyposis colorectal cancer-related cancers. Blood samples from 108 healthy volunteers were analysed for those genetic alterations suspected to affect the function of MSH6. Of the seven (18.4%) MSH6 alterations found, we have identified three novel germline mutations, one 8 bp deletion leading to a truncated protein and two missense mutations resulting in the substitution of amino acids belonging to different polarity groups. High-frequency microsatellite instability was found in the patient with the MSH6 deletion, but not in the other 27 carcinomas analysed. No MLH1 promoter methylation was detected in tumour tissue. Our findings suggest that germline MSH6 mutations contribute to a subset of early-onset colorectal cancer. Further studies are warranted to understand the genetic and environmental factors responsible for the variable penetration of MSH6 germline mutations, as well as to identify other causes of early-onset colorectal cancer.