Adenoma development in a patient with MUTYH-associated polyposis (MAP): new insights into the natural course of polyp development

Biallelic MUTYH mutations can mimic Lynch syndrome

The hallmarks of Lynch syndrome (LS) include a positive family history of colorectal cancer (CRC), germline mutations in the DNA mismatch repair (MMR) genes, tumours with high microsatellite instability (MSI-H) and loss of MMR protein expression. However, in ∼10-15% of clinically suspected LS cases, MMR mutation analyses cannot explain MSI-H and abnormal immunohistochemistry (IHC) results. The highly variable phenotype of MUTYH-associated polyposis (MAP) can overlap with the LS phenotype, but is inherited recessively. We analysed the MUTYH gene in 85 'unresolved' patients with tumours showing IHC MMR-deficiency without detectable germline mutation. Biallelic p.(Tyr179Cys) MUTYH germline mutations were found in one patient (frequency 1.18%) with CRC, urothelial carcinoma and a sebaceous gland carcinoma. LS was suspected due to a positive family history of CRC and because of MSI-H and MSH2-MSH6 deficiency on IHC in the sebaceous gland carcinoma. Sequencing of this tumour revealed two somatic MSH2 mutations, thus explaining MSI-H and IHC results, and mimicking LS-like histopathology. This is the first report of two somatic MSH2 mutations leading to an MSI-H tumour lacking MSH2-MSH6 protein expression in a patient with MAP. In addition to typical transversion mutations in KRAS and APC, MAP can also induce tumourigenesis via the MSI-pathway.

MUTYH hotspot mutations in unselected colonoscopy patients

AIM

Biallelic MutY human homologue (MUTYH) germline mutations predispose to recessively inherited adenomatous polyposis, designated MUTYH-associated polyposis (MAP), and colorectal cancer (CRC). The hotspot mutations p.Y179C and p.G396D account for the majority of pathogenic variants of MUTYH in Caucasians. Our aim was to evaluate the prevalence of MUTYH mutations in a prospective cohort of unselected patients with different colorectal diseases.

METHOD

The hotspot mutations p.Y179C and p.G396D were genotyped in 352 consecutive patients undergoing colonoscopy at our tertiary referral centre. Exons 2-14 were sequenced in hotspot mutation carriers to exclude additional variants.

RESULTS

Overall, we identified five heterozygous p.Y179C mutations and three heterozygous p.G396D mutations in seven hotspot mutation carriers (risk allele frequencies 0.7% and 0.4%, respectively). Two of these hotspot mutation carriers harboured a heterozygous p.Q338H variant, which is of uncertain clinical significance, on the other allele. Three individuals were biallelic MUTYH variant carriers (p.Y179C/p.G382D: typical MAP; p.Y179C/p.Q338H: atypical MAP with late onset and lower polyp burden; p.G382D/p.Q338H: inflammatory bowel disease), and four subjects were monoallelic mutation carriers.

CONCLUSION

MUTYH-associated disease, and hence genetic counselling and MUTYH genetic testing, should be considered in the clinical routine of an endoscopy unit, but the wide range of phenotypes represents a challenge for patient identification. The clinical significance of p.Q338H should be evaluated in future case-control studies because compound heterozygotes for pathogenic mutations and p.Q338H may be at increased risk for mild polyposis or CRC. In addition, MUTYH should be assessed as a potential susceptibility gene for the development of colitis-associated CRC in future.

MUTYH-associated polyposis - variability of the clinical phenotype in patients with biallelic and monoallelic MUTYH mutations and report on novel mutations

To further characterize 215 APC mutation-negative patients with colorectal neoplasias classified in classical, attenuated, or atypical familial adenomatous polyposis (FAP) coli we performed mutation screening in the Mut Y homologue (MUTYH) gene. The incidence was 15% for biallelic and 3.7% for monoallelic MUTYH mutations. We describe six novel MUTYH mutations in biallelic constellation and two novel monoallelic missense mutations. Of 33 MUTYH-associated polyposis coli (MAP) patients 57% were attenuated familial adenomatous polyposis (AFAP) patients, 10% display early-onset classical FAP and 18% had only few adenomas at higher age. Biallelic cases had a high incidence of extracolonic polyposis in 32% and colorectal cancer (CRC) in 33% of the cases. The clinical picture of MAP ranged from classical FAP or synchronous CRC at age 30 years to few adenomas at age 54 years without evidence of CRC, initially suspected for hereditary non-polyposis colorectal cancer (HNPCC). The mean age of onset was 43 years, with 11 (33%) patients being younger than 40 years of age, indicating that the clinical manifestation can be earlier than so far reported. Monoallelic MUTYH mutation carriers had a positive family history in seven of eight cases allowing the hypothesis of a disease-causing synergism of MUTYH mutations with other genes.

MUTYH-associated polyposis (MAP)

The human mutY homologue (MUTYH) gene is responsible for inheritable polyposis and colorectal cancer. This review discusses the molecular genetic aspects of the MUTYH gene and protein, the clinical impact of mono- and biallelic MUTYH mutations and histological aspects of the MUTYH tumors. Furthermore, the relationship between MUTYH and the mismatch repair genes in colorectal cancer (CRC) families is examined. Finally, the role of other base excision repair genes in polyposis and CRC patients is discussed.