Role of inherited defects ofMYH in the development of sporadic colorectal cancer

DARVIC: Dihedral angle-reliant variant impact classifier for functional prediction of missense VUS

BACKGROUND

Of the large number of genetic variants identified, the functional impact for most of them remains unknown. Mutations in DNA damage repair genes such as MUTYH, which is involved in repairing A:8-oxoG mismatches caused by reactive oxygen species, can cause a higher risk of cancer. Mutations happening in other key genes such as TP53 also pose huge health threats and risk of cancer. The interpretation of genetic variants' functional impact is a forefront issue that needs to be addressed. Many different in silico methods based on different principles have been developed and applied in interpreting genetic variants. However, a current challenge is that many existing methods tend to overpredict the pathogenicity of benign variants. A new approach is needed to tackle this issue to improve genetic variant interpretation through the use of in silico methods.

METHODS

In this study, we developed another protein structural-based approach called Dihedral angle-reliant variant impact classifier (DARVIC) to predict the deleterious impact of the coding-changing missense variants. DARVIC uses Ramachandran's principle of protein stereochemistry as the theoretical foundation and uses molecular dynamics simulations coupled with a supervised machine learning algorithm XGBoost to determine the functional impact of missense variants on protein structural stability.

RESULTS

We characterized the features of dihedral angles in dynamic protein structures. We also tested the performance of DARVIC in MUTYH and TP53 missense variants and achieved satisfactory results in reflecting the functional impacts of the variants on protein structure. The method achieved a balanced accuracy of 84% in a functionally validated MUTYH dataset containing both benign and pathogenic missense variants, higher than other existing in silico methods. Along with that, DARVIC was able to predict 119 (47%) deleterious variants from a dataset of 254 MUTYH VUS. Further application of DARVIC to a functionally validated TP53 dataset had a balanced accuracy of 94%, topping other methods, demonstrating DARVIC's robustness.

CONCLUSION

DARVIC provides a valuable tool to predict the functional impacts of missense variants based on their effects on protein structural stability and motion. At its current state, DARVIC performed well in predicting the functional impact of the missense variants both in MUTYH and TP53. We expect its high potential to predict functional impact for the missense variants in other genes.

Incident Cancer Risk and Signatures Among Older MUTYH Carriers: Analysis of Population-Based and Genomic Cohorts

MUTYH carriers have an increased colorectal cancer risk in case-control studies, with loss of heterozygosity (LOH) as the presumed mechanism. We evaluated cancer risk among carriers in a prospective, population-based cohort of older adults. In addition, we assessed if cancers from carriers demonstrated mutational signatures (G:C>T:A transversions) associated with early LOH. We calculated incident risk of cancer and colorectal cancer among 13,131 sequenced study participants of the ASPirin in Reducing Events in the Elderly cohort, stratified by sex and adjusting for age, smoking, alcohol use, BMI, polyp history, history of cancer, and aspirin use. MUTYH carriers were identified among 13,033 participants in The Cancer Genome Atlas and International Cancer Genome Consortium, and somatic signatures of cancers were analyzed. Male MUTYH carriers demonstrated an increased risk for overall cancer incidence [multivariable HR, 1.66; 95% confidence interval (CI), 1.03-2.68; P = 0.038] driven by increased colorectal cancer incidence (multivariable HR, 3.55; 95% CI, 1.42-8.78; P = 0.007), as opposed to extracolonic cancer incidence (multivariable HR, 1.40; 95% CI, 0.81-2.44; P = 0.229). Female carriers did not demonstrate increased risk of cancer, colorectal cancer, or extracolonic cancers. Analysis of mutation signatures from cancers of MUTYH carriers revealed no significant contribution toward early mutagenesis from widespread G:C>T:A transversions among gastrointestinal epithelial cancers. Among cancers from carriers, somatic transversions associated with base-excision repair deficiency are uncommon, suggestive of diverse mechanisms of carcinogenesis in carriers compared with those who inherit biallelic MUTYH mutations.

PREVENTION RELEVANCE

Despite absence of loss of heterozygosity in colorectal cancers, elderly male MUTYH carriers appeared to be at increased of colorectal cancer.

When you're strange: Unusual features of the MUTYH glycosylase and implications in cancer

MUTYH is a base-excision repair glycosylase that removes adenine opposite 8-oxoguanine (OG). Variants of MUTYH defective in functional activity lead to MUTYH-associated polyposis (MAP), which progresses to cancer with very high penetrance. Whole genome and whole exome sequencing studies have found MUTYH deficiencies in an increasing number of cancer types. While the canonical OG:A repair activity of MUTYH is well characterized and similar to bacterial MutY, here we review more recent evidence that MUTYH has activities independent of OG:A repair and appear centered on the interdomain connector (IDC) region of MUTYH. We summarize evidence that MUTYH is involved in rapid DNA damage response (DDR) signaling, including PARP activation, 9-1-1 and ATR signaling, and SIRT6 activity. MUTYH alters survival and DDR to a wide variety of DNA damaging agents in a time course that is not consistent with the formation of OG:A mispairs. Studies that suggest MUTYH inhibits the repair of alkyl-DNA damage and cyclopyrimidine dimers (CPDs) is reviewed, and evidence of a synthetic lethal interaction with mismatch repair (MMR) is summarized. Based on these studies we suggest that MUTYH has evolved from an OG:A mispair glycosylase to a multifunctional scaffold for DNA damage response signaling.

Association of monoallelic MUTYH mutation among Egyptian patients with colorectal cancer

Colorectal cancer (CRC) is a heterogeneous triat that involves both environmental and genetic factors. Genetic mutations of MUTYH (p.Y179C and p.G396D) have been reported to be associated with increased risk of CRC among several ethnic populations. The aim of this work is to assess the association of the monoallelic MUTYH mutations (p.Y179C and p.G396D) with increased risk of CRC among Egyptian patients. This study included 120 unrelated CRC Egyptian patients who were compared with 100 healthy controls from the same locality. For all individuals, DNA was genotyped for MUTYH p.Y179C and MUTYH p.G396D mutations using the T-ARMS-PCR technique. The frequencies of monoallelic MUTYH mutations showed a strong association with the increased risk of CRC among Egyptian patients compared with controls (12.5 vs. 4.0 %, OR = 3.49, 95 % CI = 1.12-10.90, P = 0.03). Moreover, the frequency of MUTYH p.Y179C mutation was noted to be significantly higher among CRC patients compared to controls rather than MUTYH p.G396D mutation. Interestingly, CRC patients with tumors in the right side colon showed an evidence for association with the MUTYH p.Y179C mutation compared with tumors in the left side colon (p = 0.01). MUTYH p.Y179C mutation was associated with an increased risk of CRC among Egyptian patients rather than MUTYH p.G396D mutation.

Exploratory plasma proteomic analysis in a randomized crossover trial of aspirin among healthy men and women

Long-term use of aspirin is associated with lower risk of colorectal cancer and other cancers; however, the mechanism of chemopreventive effect of aspirin is not fully understood. Animal studies suggest that COX-2, NFκB signaling and Wnt/β-catenin pathways may play a role, but no clinical trials have systematically evaluated the biological response to aspirin in healthy humans. Using a high-density antibody array, we assessed the difference in plasma protein levels after 60 days of regular dose aspirin (325 mg/day) compared to placebo in a randomized double-blinded crossover trial of 44 healthy non-smoking men and women, aged 21-45 years. The plasma proteome was analyzed on an antibody microarray with ~3,300 full-length antibodies, printed in triplicate. Moderated paired t-tests were performed on individual antibodies, and gene-set analyses were performed based on KEGG and GO pathways. Among the 3,000 antibodies analyzed, statistically significant differences in plasma protein levels were observed for nine antibodies after adjusting for false discoveries (FDR adjusted p-value<0.1). The most significant protein was succinate dehydrogenase subunit C (SDHC), a key enzyme complex of the mitochondrial tricarboxylic acid (TCA) cycle. The other statistically significant proteins (NR2F1, MSI1, MYH1, FOXO1, KHDRBS3, NFKBIE, LYZ and IKZF1) are involved in multiple pathways, including DNA base-pair repair, inflammation and oncogenic pathways. None of the 258 KEGG and 1,139 GO pathways was found to be statistically significant after FDR adjustment. This study suggests several chemopreventive mechanisms of aspirin in humans, which have previously been reported to play a role in anti- or pro-carcinogenesis in cell systems; however, larger, confirmatory studies are needed.

Whole-genome landscape of pancreatic neuroendocrine tumours

The diagnosis of pancreatic neuroendocrine tumours (PanNETs) is increasing owing to more sensitive detection methods, and this increase is creating challenges for clinical management. We performed whole-genome sequencing of 102 primary PanNETs and defined the genomic events that characterize their pathogenesis. Here we describe the mutational signatures they harbour, including a deficiency in G:C > T:A base excision repair due to inactivation of MUTYH, which encodes a DNA glycosylase. Clinically sporadic PanNETs contain a larger-than-expected proportion of germline mutations, including previously unreported mutations in the DNA repair genes MUTYH, CHEK2 and BRCA2. Together with mutations in MEN1 and VHL, these mutations occur in 17% of patients. Somatic mutations, including point mutations and gene fusions, were commonly found in genes involved in four main pathways: chromatin remodelling, DNA damage repair, activation of mTOR signalling (including previously undescribed EWSR1 gene fusions), and telomere maintenance. In addition, our gene expression analyses identified a subgroup of tumours associated with hypoxia and HIF signalling.

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.

Role of MUTYH in human cancer

MUTYH, a human ortholog of MutY, is a post-replicative DNA glycosylase, highly conserved throughout evolution, involved in the correction of mismatches resulting from a faulty replication of the oxidized base 8-hydroxyguanine (8-oxodG). In particular removal of adenine from A:8-oxodG mispairs by MUTYH activity is followed by error-free base excision repair (BER) events, leading to the formation of C:8-oxodG base pairs. These are the substrate of another BER enzyme, the OGG1 DNA glycosylase, which then removes 8-oxodG from DNA. Thus the combined action of OGG1 and MUTYH prevents oxidative damage-induced mutations, i.e. GC->TA transversions. Germline mutations in MUTYH are associated with a recessively heritable colorectal polyposis, now referred to as MUTYH-associated polyposis (MAP). Here we will review the phenotype(s) associated with MUTYH inactivation from bacteria to mammals, the structure of the MUTYH protein, the molecular mechanisms of its enzymatic activity and the functional characterization of MUTYH variants. The relevance of these results will be discussed to define the role of specific human mutations in colorectal cancer risk together with the possible role of MUTYH inactivation in sporadic cancer.

MUTYH Associated Polyposis (MAP)

MUTYH Associated Polyposis (MAP), a Polyposis predisposition caused by biallelic mutations in the Base Excision Repair (BER) gene MUTYH, confers a marked risk of colorectal cancer (CRC). The MAP phenotype is difficult to distinguish from other hereditary CRC syndromes. Especially from Familial Adenomatous Polyposis (FAP) and to a lesser extend Lynch Syndrome, which are caused by germline mutations in the APC and Mismatch Repair (MMR) genes, respectively.

Here we review research findings regarding MUTYH interactions, genotypic and phenotypic characteristics of MAP, as well as surveillance and treatment of the disease. The applied papers, published between 1/1 2002- 1/2 2008, were found through PubMed.

The exact role of MUTYH in CRC tumorgenesis is still uncertain, although MAP tumors show distinct molecular features, including somatic G:C>T:A transversions in the APC gene. Furthermore, cooperation between the BER and the MMR systems exists, as MUTYH interacts with MMR gene-products. Possibly, monoallelic defects in both pathways are of significance to CRC development.

Specific MUTYH variants are found to be characteristic in distinct ethnic populations, which could facilitate future genetic screening. Knowledge concerning functional consequences of many MUTYH germline mutations remains sparse. Most thoroughly investigated are the two most common MUTYH variants, Y179C and G396D, both generating dysfunctional gene products.

Phenotypic features of MAP include: development of 10-100 colorectal adenomas, debuting at 46-47 years, often CRC at time of clinical diagnosis, and in some, development of extracolonic manifestations.

Frequency of the common germline MUTYH mutations p.G396D and p.Y179C in patients diagnosed with colorectal cancer in Southern Brazil

INTRODUCTION

MUTYH-associated polyposis (MAP) is an autosomal recessive cancer predisposition syndrome associated with the development of colorectal tumors and colonic polyps at an early age. MAP syndrome is associated to germline biallelic mutations in the MUTYH gene which lead to deficient DNA repair through the base-excision repair system and accumulation of G:C→T:A transversions. Occurrence of such mutations in oncogenes and tumor suppressor genes drives colorectal carcinogenesis and is associated with the development of colonic polyps. Two common mutations, p.Y179C and p.G396D, are present in approximately 70-80% of MAP in European families with identified MUTYH germline mutations. The aim of this study was to assess the frequency of the germline MUTYH mutations p.Y179C and p.G396D in Brazilian patients with MAP and other hereditary colorectal cancer (CRC) phenotypes, as well as in sporadic CRC cases.

MATERIALS AND METHODS

A total of 75 patients were included. Samples were screened for the MUTYH germline mutations p.Y179C and p.G396D by allelic discrimination assays using allele-specific TaqMan® probes. In all mutation-positive cases, results were confirmed by sequencing.

RESULTS AND CONCLUSIONS

Biallelic germline MUTYH mutations were identified in 4 of 60 (6.6%) patients with a phenotype of hereditary colorectal cancer. Germline MUTYH mutation screening should be considered in the differential diagnosis of hereditary colorectal syndromes, and not only in MAP, but also in familial adenomatous polyposis and Bethesda criteria-positive families. Additional mutation screening studies of the MUTYH gene in a larger number of Brazilian patients will be necessary to confirm these results and determine the validity and applicability of MUTYH mutation screening in our population.

Association between monoallelic MUTYH mutation and colorectal cancer risk: a meta-regression analysis

Whether people who inherit a mutation in MUTYH from only one parent (monoallelic mutation) are at increased risk of colorectal cancer (CRC) remains controversial. Most previous studies and meta-analyses have not found statistically significant associations but, given carriers are relatively rare, may be underpowered to detect small increased risks. We have conducted a systematic review and meta-regression analysis of previously published case-control studies to estimate the strength of association for monoallelic MUTYH mutation and CRC risk. Potential sources of heterogeneity were evaluated. We have compared the carrier frequency in cases with a family history of CRC to that of controls, as a novel and powerful design, to measure statistical evidence of an association but not the strength of association. The magnitude of the genotype-disease association, estimated from a pooled odds ratio comparing cases unselected for family history with controls, was 1.15 (95% CI = 0.98-1.36) and not substantially altered by adjustment for potential sources of heterogeneity. Monoallelic mutation carrier frequency was greater for cases ascertained due to a family history (3.3%; SE 0.9%) than for controls (1.4%; SE 0.3%) (P = 0.02). Monoallelic MUTYH mutation carriers are at increased risk of CRC but the average increase is small.

Cancer risks for monoallelic MUTYH mutation carriers with a family history of colorectal cancer

Cancer risks for a person who has inherited a MUTYH mutation from only one parent (monoallelic mutation carrier) are uncertain. Using the Colon Cancer Family Registry and Newfoundland Familial Colon Cancer Registry, we identified 2,179 first- and second-degree relatives of 144 incident colorectal cancer (CRC) cases who were monoallelic or biallelic mutation carriers ascertained by sampling population complete cancer registries in the United States, Canada and Australia. Using Cox regression weighted to adjust for sampling on family history, we estimated that the country-, age- and sex-specific standardized incidence ratios (SIRs) for monoallelic mutation carriers, compared to the general population, were: 2.04 (95% confidence interval, CI 1.56-2.70; p < 0.001) for CRC, 3.24 (95%CI 2.18-4.98; p < 0.001) for gastric cancer, 3.09 (95%CI 1.07-12.25; p = 0.07) for liver cancer and 2.33 (95%CI 1.18-5.08; p = 0.02) for endometrial cancer. Age-specific cumulative risks to age 70 years, estimated using the SIRs and US population incidences, were: for CRC, 6% (95%CI 5-8%) for men and 4% (95%CI 3-6%) for women; for gastric cancer, 2% (95%CI 1-3%) for men and 0.7% (95%CI 0.5-1%) for women; for liver cancer, 1% (95%CI 0.3-3%) for men and 0.3% (95%CI 0.1-1%) for women and for endometrial cancer, 4% (95%CI 2-8%). There was no evidence of increased risks for cancers of the brain, pancreas, kidney, lung, breast or prostate. Monoallelic MUTYH mutation carriers with a family history of CRC, such as those identified from screening multiple-case CRC families, are at increased risk of colorectal, gastric, endometrial and possibly liver cancers.

A large-scale meta-analysis to refine colorectal cancer risk estimates associated with MUTYH variants

Background:

Defective DNA repair has a causal role in hereditary colorectal cancer (CRC). Defects in the base excision repair gene MUTYH are responsible for MUTYH-associated polyposis and CRC predisposition as an autosomal recessive trait. Numerous reports have suggested MUTYH mono-allelic variants to be low penetrance risk alleles. We report a large collaborative meta-analysis to assess and refine CRC risk estimates associated with bi-allelic and mono-allelic MUTYH variants and investigate age and sex influence on risk.

Methods:

MUTYH genotype data were included from 20 565 cases and 15 524 controls. Three logistic regression models were tested: a crude model; adjusted for age and sex; adjusted for age, sex and study.

Results:

All three models produced very similar results. MUTYH bi-allelic carriers demonstrated a 28-fold increase in risk (95% confidence interval (CI): 6.95–115). Significant bi-allelic effects were also observed for G396D and Y179C/G396D compound heterozygotes and a marginal mono-allelic effect for variant Y179C (odds ratio (OR)=1.34; 95% CI: 1.00–1.80). A pooled meta-analysis of all published and unpublished datasets submitted showed bi-allelic effects for MUTYH, G396D and Y179C (OR=10.8, 95% CI: 5.02–23.2; OR=6.47, 95% CI: 2.33–18.0; OR=3.35, 95% CI: 1.14–9.89) and marginal mono-allelic effect for variants MUTYH (OR=1.16, 95% CI: 1.00–1.34) and Y179C alone (OR=1.34, 95% CI: 1.01–1.77).

Conclusions:

Overall, this large study refines estimates of disease risk associated with mono-allelic and bi-allelic MUTYH carriers.

Tgf-beta signaling alterations and colon cancer

Colorectal cancer is the second most common cause of cancer-related death in the United States. Twin studies suggest that 35% of all colorectal cancer cases are inherited. High-penetrance tumor susceptibility genes account for at most 3-6% of all colorectal cancer cases and the remainder of the unexplained risk is likely due to a combination of low to moderate penetrance genes. Recent genome-wide association studies have identified several SNPs near genes belonging to the transforming growth factor beta (TGF-beta) superfamily such as GREM1 and SMAD7. Together with the recent discovery that constitutively decreased TGFBR1 expression is a potent modifier of colorectal cancer risk, these findings strongly suggest that germline variants of the TGF-beta superfamily may account for a sizeable proportion of colorectal cancer cases. The TGF-beta superfamily signaling pathways mediate many different biological processes during embryonic development, and in adult organisms they play a role in tissue homeostasis. TGF-beta has a central role in inhibiting cell proliferation and also modulates processes such as cell invasion, immune regulation, and microenvironment modification. Mutations in the TGF-beta type II receptor (TGFBR2) are estimated to occur in approximately 30% of colorectal carcinomas. Mutations in SMAD4 and BMPR1A are found in patients with familial juvenile polyposis, an autosomal dominant condition associated with an increased risk of colorectal cancer. This chapter provides an overview of the genetic basis of colorectal cancer and discusses recent discoveries related to alterations in the TGF-beta pathways and their role in the development of colorectal cancer.

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.

MUTYH-associated polyposis and colorectal cancer

This article reviews the role of defective base excision repair, and MUTYH specifically, in colorectal cancer etiology and discusses the consequences of MUTYH gene defects, with particular emphasis on clinical relevance to colorectal polyposis, colorectal cancer risk, and appraising the risk of extra-colonic malignancy. Evidence guiding clinical practice, in terms of surveillance recommendations and options for surgical and other prophylactic interventions, is reviewed.

Altered expression of MUTYH and an increase in 8-hydroxydeoxyguanosine are early events in ulcerative colitis-associated carcinogenesis

8-Hydroxy-guanine (8-OH-G) mismatches readily with adenine residues, leading to a G : C to T : A transversion mutation. The human mutY homologue (MUTYH) excises adenine misincorporated opposite 8-OH-G during replication and suppresses mutations caused by reactive oxygen species. We defined the expression of 8-hydroxydeoxyguanosine (8-OHdG) and MUTYH in ulcerative colitis (UC)-associated neoplasia by immunohistochemistry and compared this with expression in UC patients without neoplasia and patients unaffected by UC. We also performed mutation analyses for MUTYH and K-ras. 8-OHdG was expressed more intensely in the mucosa of UC-associated neoplasia and UC without neoplasm than in the mucosa unaffected by UC. Immunohistochemistry with two different types of MUTYH antibody showed that UC-associated neoplasia and UC without neoplasia exhibited strong cytoplasmic expression and attenuated nuclear expression of MUTYH when compared with patients unaffected by UC. No pathological MUTYH mutations were detected in any of the UC-associated neoplasia cases. However, K-ras mutation was detected in two cases, one of which showed G : C to T : A transversion mutation and attenuated nuclear staining of MUTYH. In conclusion, inflamed mucosa of UC is exposed to oxidative damage. An increase in cytoplasmic MUTYH, rather than its mutation, may contribute to the promotion of carcinogenesis in UC.

Increased colorectal cancer incidence in obligate carriers of heterozygous mutations in MUTYH

BACKGROUND & AIMS

MUTYH-associated polyposis (MAP) is an autosomal recessive disorder caused by mutations in the MUTYH gene. Patients with MAP are at extremely high risk of colorectal cancer, but the risks of colorectal and other cancers in heterozygous carriers of a single MUTYH mutation are uncertain. We performed a retrospective study of cancer incidence and causes of death among obligate MUTYH heterozygote individuals.

METHODS

MAP index cases were identified from polyposis registers in Germany, The Netherlands, and the United Kingdom. Cancer incidence, cancer mortality, and all-cause mortality data were collected from 347 parents of unrelated MAP index cases and the spouses of 3 index cases who were also found to be heterozygous for single MUTYH mutations. These data were compared with appropriate national sex-, age-, and period-specific population data to obtain standardized mortality ratios (SMR) and standardized incidence ratios (SIR).

RESULTS

There was a 2-fold increase in the incidence of colorectal cancer among parents of MAP cases, compared with the general population (SIR, 2.12; 95% confidence interval [CI]: 1.30-3.28). Their colorectal cancer mortality was not increased significantly (SMR, 1.02; 95% CI: 0.41-2.10) nor was overall cancer risk (SIR, 0.92; 95% CI: 0.70-1.18), cancer mortality (SMR, 1.12; 95% CI: 0.83-1.48), or overall mortality (SMR, 0.94; 95% CI: 0.80-1.08).

CONCLUSIONS

The risk of colorectal cancer in heterozygous carriers of single MUTYH mutations who are relatives of patients with MAP is comparable with that of first-degree relatives of patients with sporadic colorectal cancer. Screening measures should be based on this modest increase in risk.

Clinical implications of the colorectal cancer risk associated with MUTYH mutation

PURPOSE

Biallelic mutations in the base excision DNA repair gene MUTYH predispose to colorectal cancer (CRC). Evidence that monoallelic mutations also confer an elevated CRC risk is controversial. Precise quantification of the CRC risk and the phenotype associated with MUTYH mutations is relevant to the counseling, surveillance, and clinical management of at-risk individuals.

METHODS

We analyzed a population-based series of 9,268 patients with CRC and 5,064 controls for the Y179C and G396D MUTYH mutations. We related genotypes to phenotype and calculated genotype-specific CRC risks.

RESULTS

Overall, biallelic mutation status conferred a 28-fold increase in CRC risk (95% CI,17.66 to 44.06); this accounted for 0.3% of CRCs in the cohort. Genotype relative risks of CRC were strongly age dependent, but penetrance was incomplete at age 60 years. CRC that developed in the context of biallelic mutations were microsatellite stable. Biallelic mutation carriers were more likely to have proximal CRC (P = 4.0 x 10(-4)) and synchronous polyps (P = 5.7 x 10(-9)) than noncarriers. The performance characteristics of clinicopathologic criteria for the identification of biallelic mutations are poor. Monoallelic mutation was not associated with an increased CRC risk (odds ratio, 1.07; 95% CI, 0.87 to 1.31).

CONCLUSION

The high risk and the propensity for proximal disease associated with biallielic MUTYH mutation justify colonoscopic surveillance. Although mutation screening should be directed to patients with APC-negative polyposis and early-onset proximal MSS CRC in whom detection rates will be highest, the expanded phenotype associated with MUTYH mutation needs to be recognized. There is no evidence than monoallelic mutation status per se is clinically important.

Germline MutY human homologue mutations and colorectal cancer: a multisite case-control study

BACKGROUND & AIMS

The MutY human homologue (MYH) gene is a member of the base-excision repair pathway involved in the repair of oxidative DNA damage. The objective of this study was to determine colorectal cancer (CRC) risk associated with mutations in the MYH gene.

METHODS

A total of 3811 CRC cases and 2802 controls collected from a multisite CRC registry were screened for 9 germline MYH mutations; subjects with any mutation underwent screening of the entire MYH gene. Logistic regression was used to estimate age- and sex-adjusted odds ratios (AOR). Clinicopathologic and epidemiologic data were reviewed to describe the phenotype associated with MYH mutation status and assess for potential confounding and effect modification.

RESULTS

Twenty-seven cases and 1 control subject carried homozygous or compound heterozygous MYH mutations (AOR, 18.1; 95% confidence interval, 2.5-132.7). CRC cases with homozygous/compound heterozygous mutations were younger at diagnosis (P=.01), had a higher proportion of right-sided (P=.01), synchronous cancers (P<.01), and personal history of adenomatous polyps (P=.003). Heterozygous MYH mutations were identified in 87 CRC cases and 43 controls; carriers were at increased risk of CRC (AOR, 1.48; 95% confidence interval, 1.02-2.16). There was a higher prevalence of low-frequency microsatellite instability (MSI) in tumors from heterozygous and homozygous/compound heterozygous MYH mutation carriers (P=.02); MSI status modified the CRC risk associated with heterozygous MYH mutations (P interaction<.001).

CONCLUSIONS

Homozygous/compound heterozygous MYH mutations account for less than 1% of CRC cases. Heterozygous carriers are at increased risk of CRC. Further studies are needed to understand the possible interaction between the base excision repair and low-frequency MSI pathways.

MUTYH-associated polyposis

MUTYH-associated polyposis (MAP) is an autosomal recessive disorder characterised by adenomatous polyps of the colorectum and a very high risk of colorectal cancer. It appears to be at least as prevalent as autosomal dominant familial adenomatous polyposis (that is caused by truncating mutations in the APC gene) with which it shares important gastroenterological features. It was first recognised as recently as 2002 and its full phenotype and natural history are still being characterised. Key extracolonic manifestations include a predisposition to duodenal adenomas and cancer and a modest increase in risk for several extraintestinal tumours. Testing for mutations in the MUTYH gene is indicated in patients who have multiple colorectal adenomas or a family history suggestive of autosomal recessive colorectal cancer and for the siblings and spouses of patients with MAP in order to inform surveillance and treatment for patients and their families.

Recent advances in colorectal cancer genetics and diagnostics

Colorectal cancer (CRC) is one of the most prevalent cancers and leading cause of cancer mortality worldwide. It is also one of the most curable cancers if detected early. This review classifies the diverse disease subtypes using various parameters including phenotypes of the polyps and describes how recent advances in genetics have impacted on disease diagnostics. For familial syndromes, the discovery of initiating mutations in the germline made personalized medicine a reality. A model linking the main tumorigenesis (Wnt/TGF-beta-BMP/LKB-1/PI3K-AKT) pathways and a strategy for gene testing are proposed. For sporadic CRC, high throughput technology has enabled the discovery of susceptibility loci that increased CRC risk. The ramifications of screening the population for susceptibility loci are discussed.

Pathological features of colorectal carcinomas in MYH-associated polyposis

AIMS

MYH is a DNA glycosylase in the base excision repair pathway. Germ-line biallelic mutations in the MYH gene are associated with the development of multiple colorectal adenomas and colorectal carcinoma (CRC). A slightly increased risk of CRC is suggested in monoallelic MYH mutation carriers. The aim was to characterize the histopathological features of carcinomas from biallelics and monoallelics.

METHODS AND RESULTS

Clinicopathological features of 57 colorectal carcinomas from 50 patients identified in familial CRC registries were recorded. These included 16 cancers from 14 MYH biallelics; 25 cancers from 22 MYH monoallelics; and 16 cancers from 14 controls. Carcinomas in biallelics demonstrated tubular, papillary or cribriform patterns as the predominant histological subtype, and main histological groups differed according to mutation status (P = 0.0053). All biallelic cancers were low grade, with high-grade tumours more common in monoallelics and controls (P = 0.002). Synchronous polyps were observed in 75% of biallelics, 33% of monoallelics and 43% of controls (P = 0.035). Serrated carcinoma was the predominant type in 12% (3/25) of the monoallelics but in none of the biallelics or controls. MYH immunohistochemistry failed to distinguish between groups.

CONCLUSIONS

Neither pathological features nor immunohistochemistry could predict the MYH mutation status of CRCs in this study.

Recently identified colon cancer predispositions: MYH and MSH6 mutations

Single-gene germline mutations conferring a high lifetime risk of colorectal cancer (CRC) account for up to 6% of all CRC cases. The most widely studied monogenic colorectal cancer syndromes include familial adenomatous polyposis (FAP) and Lynch syndrome. However, additional syndromes continue to be defined and new predisposition genes are continuing to be identified. Most recently, MYH-associated polyposis (MAP) and an "atypical Lynch syndrome" related to the presence of MSH6 mutations have been linked to an increased risk of CRC. In this review, we summarize basic information related to these newly recognized gene mutations, including the accumulating data on the prevalence and penetrance of deleterious mutations, as well as the management options for identified carriers and their families. Recognizing these heritable syndromes is essential and predictive genetic testing will continue to transform the field of cancer risk assessment by offering the opportunity to focus on more precise risk management and cancer prevention.

The hOGG1 Ser326Cys polymorphism is not associated with colorectal cancer risk

BACKGROUND

Little is known about the genetic risk factors associated with colorectal cancer. Although the Ser326Cys polymorphism of 8-oxoguanine DNA glycosylase (hOGG1) is consistently associated with a range of cancers, there is no consensus regarding this polymorphism and colorectal cancer risk.

METHODS

In the present study, conducted in a Korean population, we used the TaqMan assay to investigate whether the hOGG1 Ser326Cys polymorphism was associated with colorectal cancer in 439 colorectal cancer patients and 676 healthy normal controls. We also examined whether the hOGG1 Ser326Cys polymorphism is associated with tumor location, microsatellite instability (MSI) status and tumor-node-metastasis (TNM) stage in colorectal cancer.

RESULTS

We found no significant difference between the cancer and control populations in terms of genotype distribution (CC, CG and GG). In addition, we found no association between the hOGG1 Ser326Cys polymorphism and cancer risk, MSI status, TNM stage or tumor location in colorectal cancer patients.

CONCLUSIONS

These results suggest that unlike for other cancer types, the hOGG1 Ser326Cys polymorphism is not a major genetic risk factor for colorectal cancer.

Germline MYH mutations in a clinic-based series of Canadian multiple colorectal adenoma patients

OBJECTIVES

MYH is a member of the DNA base excision repair (BER) pathway and mutations of this gene predispose to the development of colorectal neoplasia in an autosomal recessive transmission pattern. Our objective was to determine the significance of MYH mutations in a series of Canadian patients with multiple adenomas.

METHODS

We screened for germline MYH mutations (by dHPLCO) in 20 clinic-based multiple adenoma patients who were adenomatous polyposis coli (APC) mutation-negative. Suspected mutations were confirmed by sequence analysis.

RESULTS

Six of 20 (30%) patients carried pathogenic biallelic MYH mutations, 1 Y165C homozygote and 5 compound heterozygotes of other sequence variants. We identified three novel variants, Q377X, 1314delA, and P281L, which are likely pathogenic. Twenty-nine relatives of the Y90X/1103delC compound heterozygous carrier were also screened for germline MYH mutations, and 1 homozygous and 14 heterozygous carriers were identified.

CONCLUSIONS

Among patients with multiple adenomas, biallelic MYH mutations account for approximately 30% of APC mutation negative cases and two thirds of these carry mutations other than the "common" Y165C and G382D variants. Clinical screening algorithms which focus only on the Y165C and G382D alleles are inadequate since additional pathogenic mutations may be identified by screening the entire gene.

Identification of MYH mutation carriers in colorectal cancer: a multicenter, case-control, population-based study

BACKGROUND & AIMS

Whereas it has conclusively been demonstrated that biallelic MutY human homolog (MYH) mutations confer a significant risk for colorectal cancer (CRC), the influence of monoallelic mutations remains controversial. Characterization of MYH-associated CRC is critical to identify individuals who might benefit from preventive strategies. This prospective, multicenter, case-control, population-based study was aimed at (1) establishing the CRC risk associated with specific germline MYH mutations and (2) devising a set of clinical criteria to identify MYH carriers among newly diagnosed CRC.

METHODS

Genotyping for Y165C and G382D was performed by TaqMan technology. Single-stranded conformation polymorphism analysis was performed in heterozygotes to screen for mutations in the entire gene. All individuals were re-screened for any additional pathogenic variant.

RESULTS

Biallelic and monoallelic MYH mutations were found in 8 (0.7%) and 19 (1.7%) of 1116 CRC patients, respectively. None of the 934 control subjects carried biallelic mutations, whereas 22 (2.3%) of them were monoallelic carriers. In a meta-analysis including all previous case-control studies, monoallelic MYH carriers were not at increased risk for CRC (odds ratio, 1.11; 95% confidence interval, 0.90-1.37), although a significant association was found with the Y165C mutation in either homozygotes or heterozygotes (odds ratio, 1.67; 95% confidence interval, 1.17-2.40). Furthermore, presence of more than 15 synchronous colorectal adenomas or CRC diagnosed before the age of 50 years was the most effective set of criteria for the identification of biallelic MYH mutation carriers.

CONCLUSIONS

This study proposes the first set of clinical criteria designed to identify CRC patients with biallelic MYH mutations, and it argues against an increased risk for monoallelic carriers.

Implication of MYH in colorectal polyposis

OBJECTIVE

The aim of this study was to determine the frequency of MYH mutations in one large population of polyposis patients without APC mutation identified.

SUMMARY BACKGROUND DATA

Familial adenomatous polyposis (FAP) is the most known inherited colorectal cancer syndrome. In 70% to 80% of polyposis patients, an APC mutation is found. Patients with polyposis but no APC mutation are considered as APC-muted patients and followed as their relatives accordingly. Biallelic mutation of MYH has been found to responsible of colorectal polyposis and cancer in an autosomal recessive pattern of inheritance.

METHODS

Between 1978 and 2004, 433 patients were operated for polyposis. A mutation on APC was identified in 322 patients. Among the remaining patients, 44 were identified as possible MYH-muted patients and contacted, and 31 signed informed consent. Clinical data were obtained from the patients' medical notes. Germline mutation of MYH was searched by sequencing the whole gene. To confirm the deleterious effects of biallelic MYH mutation, transversions on K-ras and APC were searched.

RESULTS

There were 9 women and 22 men with a mean age of 53.9 years (range, 22-68 years) at the time of diagnosis. The mean number of polyps was 62.8 (range, 11-266). Eighteen patients (58.1%) had a colorectal cancer. We found biallelic MYH mutation in 6 patients (19.3%; 95% confidence interval, 5.2%-33.5%) and 5 (83.3%) had transversions in K-ras and/or APC.

CONCLUSION

MYH is a new gene responsible for about 1.4% of all adenomatous polyposis and about 20% of adenomatous polyposis without APC mutation identified. Search for MYH biallelic mutation in these patients should be systematic as it changes their and relatives'surveillance.

MYH mutations are rare in prostate cancer

PURPOSE

Oxidative stress is considered a risk factor for prostate cancer development and is associated with the production of reactive oxygen species (ROS). The base excision repair gene MYH protects against ROS-mediated damage to DNA. Inherited MYH mutations predispose to colorectal adenomas and cancer. A compromised base-excision repair function due to defective MYH may contribute to prostate carcinogenesis. Here, we examine the genetic contribution of MYH to prostate cancer risk.

METHODS

Patients diagnosed with high-grade prostatic intraepithelial neoplasia (HGPIN) alone (n = 45), prostate cancer alone (n = 123) or both (n = 82) were screened for the two most common mutations in the MYH gene using PCR-based RFLP analysis. A single patient with an inherited MYH mutation as well as a subset of 26 patients presenting with a family history of colorectal cancer were screened for additional MYH mutations by direct sequencing of the entire coding region.

RESULTS

Biallelic germline mutations in MYH were not detected among prostate cancer patients. Only a single patient was a heterozygous carrier for the Y165C missense mutation. Allelic deletion or somatic mutation of the remaining MYH allele was not identified in this patient's tumor DNA. Two patients harbored V22M polymorphism and three patients were carriers of Q324H polymorphism.

CONCLUSIONS

MYH mutations are unlikely to contribute to prostate cancer risk.

Genetic alterations in sporadic and hereditary colorectal cancer: implementations for screening and follow-up

The genetics underlying an inherited predisposition to cancer are rapidly being uncovered. This fact may ultimately lead to the routine use of molecular tools to diagnose these disorders, and establish interventions to prevent the development of cancer. Among the multiple cancer family syndromes, several are known to be associated with the development of colon cancer. These disorders may be diagnosed during evaluation of the index patient or during screening of family members who are at risk. Although the effectiveness of screening and surveillance strategies is unproven in controlled clinical trials for any of these syndromes, the high cancer risk warrants screening, and reasonable recommendations can be made. Several other genetic syndromes are associated with gastrointestinal polyposis. The risk of colon cancer in these diseases is uncertain, and may not be increased and they are not mentioned in this review. Examples include Cowden disease, intestinal ganglioneuromatosis, Ruvalcaba-Myhre-Smith syndrome, Devon family syndrome, and Cronkite-Canada syndrome.

The genetics of FAP and FAP-like syndromes

The presence of multiple adenomatous polyps in the large bowel confers a high lifetime risk of colorectal cancer. Although many cases of classical familial adenomatous polyposis (> 100 polyps) can be accounted for by mutations in the adenomatous polyposis coli (APC) gene, a large group of patients remains with multiple (5-100) adenomas and in whom there is no detectable APC mutation. Recently two new genetic variants have been found to be associated with multiple colorectal adenomas and cancer, MYH/MUTYH on chromosome 1p and the HMPS/CRAC1 locus on chromosome 15q13-q14. New information also continues to emerge regarding the less common hamartomatous polyposis conditions, Peutz-Jeghers syndrome and Juvenile Polyposis syndrome. In approximately half to two thirds of these families, germline genetic variants can now be uncovered. In this review we draw together some of the most recent information pertinent to the molecular pathogenesis of colorectal polyposis.

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.

The role of MYH and microsatellite instability in the development of sporadic colorectal cancer

Biallelic germline mutations in MYH are associated with colorectal neoplasms, which develop through a pathway involving somatic inactivation of APC. In this study, we investigated the incidence of the common MYH mutations in an Australian cohort of sporadic colorectal cancers, the clinicopathological features of MYH cancers, and determined whether inactivation of mismatch repair and base excision repair (BER) were mutually exclusive. The MYH gene was sequenced from lymphocyte DNA of 872 colorectal cancer patients and 478 controls. Two compound heterozygotes were identified in the cancer population and all three cancers from these individuals displayed a prominent infiltration of intraepithelial lymphocytes. In total, 11 heterozygotes were found in the cancer group and five in the control group. One tumour from an individual with biallelic germline mutation of MYH also demonstrated microsatellite instability (MSI) as a result of biallelic hypermethylation of the MLH1 promoter. Although MYH-associated cancers are rare in a sporadic colorectal population, this study shows that these tumours can develop through either a chromosomal or MSI pathway. Tumours arising in the setting of BER or mismatch repair deficiency may share a biological characteristic, which promotes lymphocytic infiltration.

Association of MUTYH and colorectal cancer

Mutations in the MUTYH gene have been reported to be associated with increased risk of developing colorectal cancer. In this study, we confirmed this association using original data on 928 colorectal cancer cases and 845 healthy controls from Scotland. We then conducted a meta-analysis from published data on the association between mutations at MUTYH and colorectal cancer risk. We show for the first time a small but significant mono-allelic effect with a genotype relative risk (GRR) of 1.27 (95% confidence interval (CI): 1.01-1.61), and confirm and give a more precise estimate of the strong bi-allelic effect with an estimated GRR of 117 (95% CI: 74-184). This study underscores the need for large sample sizes in order to identify small gene effects when the disease allele frequency is low.

Risk of colorectal cancer in monoallelic and biallelic carriers of MYH mutations: a population-based case-family study

Previous case-control studies have suggested that carriers of monoallelic germline mutations in the MYH gene may be at increased risk of colorectal cancer. We applied a kin-cohort design, using a modified segregation analysis, to estimate the colorectal cancer risk using 300 first-degree relatives of 39 colorectal cancer cases who were monoallelic or biallelic carriers of MYH mutations. We found that monoallelic carriers had a 3-fold increased risk of colorectal cancer (hazard ratio, 2.9; 95% confidence interval, 1.2-7.0; P = 0.02) and biallelic carriers a 50-fold increased risk (hazard ratio, 53; 95% confidence interval, 14-200; P < 0.0001). This analysis illustrates the potential of family analysis to estimate cancer risk for low-frequency mutations and, based on the proportion of relatives predicted to be carriers, we believe that this constitutes the largest study of monoallelic carriers to date.

Germline mutations in the MYH gene in Swedish familial and sporadic colorectal cancer

Biallelic germline mutations in the base excision repair gene MYH have been shown to predispose to a proportion of multiple colorectal adenomas and cancer. To evaluate the contribution of MYH mutations to non- FAP, non-HNPCC familial colorectal cancer, 84 unrelated Swedish individuals affected with colorectal cancer from such families were screened for germline mutations in the coding sequence of the gene. None of the cases was found to carry any pathogenic sequence change. We then determined the prevalence of the two most common pathogenic MYH mutations found in Caucasians, Y165C and G382D, in 450 Swedish sporadic colorectal cancer cases and 480 Swedish healthy controls. The frequency of both variants in Swedish cases and controls was similar to those previously reported. In addition, we found that previously unknown sequence variations at the position of amino acid 423 (R423Q, R423P, and R423R) appear to occur more frequently in cases than in controls (p = 0.02), a finding that warrants future studies.

MUTYH Mutations Do Not Cause HNPCC or Late Onset Familial Colorectal Cancer

Recently, carriers of biallelic mutations in the base excision repair gene MUTYH, have been demonstrated to have a predisposition for multiple adenomas and colorectal cancer. Still, many questions remain unanswered concerning MUTYH. We have addressed the following: Do biallelic MUTYH mutation carriers invariably demonstrate FAP, and may MUTYH be a gene causing HNPCC, HNPCC-like or dominantly inherited late onset colorectal cancer? We examined affecteds from our total series of HNPCC, HNPCC-like and dominantly inherited late onset colorectal cancer kindreds not demonstrated to have any MMR mutations. Bloodsamples from 96 patients were subjected to sequencing of exon 7 and exon 13 in the MUTYH gene. Two heterozygotes and one homozygote for the European founder mutations were found. The homozygous carrier did not meet criteria for FAP/AFAP. We conclude that MUTYH, when mutated, causes a rare recessively inherited disorder including colorectal- and duodenal cancers. It is not verified that heterozygous carriers of MUTYH mutations have an increased risk of cancer, and they do not explain the occurrence of familial colorectal cancer in the population.

DNA methylation patterns in adenomas from FAP, multiple adenoma and sporadic colorectal carcinoma patients

Colorectal adenomas have traditionally been regarded as homogeneous. The aim of our study was to identify molecular features that may differentiate sporadic adenomas from familial adenomas such as Familial Adenomatous Polyposis (FAP) and Multiple Adenoma patients. DNA methylation was tested at Methylated IN Tumor (MINT) loci (1,2,12,31) and the CpG promoter region of genes MLH1, HPP1, MGMT, p14ARF and p16INK4a in FAP-associated adenomas (33) from 5 patients with a known APC mutation (Group 1, FAP), adenomas (29) from 4 Multiple Adenoma patients (Group 2 Multiple), adenomas (14) from 3 patients with sporadic colorectal cancers showing high microsatellite instability (Group 3, MSI-H) and adenomas (16) from 7 patients, with sporadic colorectal cancers showing microsatellite stable or low level instability (Group 4, MSS/MSI-L). Aberrant Crypt Foci (ACFs), Hyperplastic Polyps (HPs) and cancers were also examined for methylation status as well as K-ras mutation. Multiple Adenoma patients were examined for germline polymorphisms in the base excision repair gene, MYH. The familial syndrome, FAP -associated adenomas showed a significantly low frequency of MINT methylation (15.5%,) compared to sporadic MSS/MSI-L-associated adenomas (35.5%). Group 3 (MSI-H) adenomas were different in that many showed serration and a high level of methylation (57.1%). Group 2, Multiple Adenoma cases, resembled sporadic MSS/MSI-L-associated adenomas. However the promoter regions of key genes, MGMT, p14ARF and p16INK4a were methylated to a greater extent than MINTs in both sporadic and familial adenomas. Genetic profiling of adenomas supports the concept that adenomas belonging to familial syndromes pursue a different pathway to tumorigenesis than their sporadic counterpar/ts from their earliest formation.

What's new in hereditary colorectal cancer?

Precancerous polyposes other than classic familial adenomatous polyposis and the condition hereditary nonpolyposis colorectal cancer, or Lynch syndrome, continue to present major diagnostic challenges for the anatomic pathologist. This editorial highlights the practical significance of novel insights and clinical guidelines in the recent literature, as well as in 4 contributions to this edition of the Archives of Pathology & Laboratory Medicine. The first section will address attenuated familial adenomatous polyposis and a newly recognized type of autosomal-recessive adenomatous polyposis associated with the DNA repair gene MYH. The remainder of the editorial discusses the role of the revised Bethesda guidelines in the diagnosis of hereditary nonpolyposis colorectal cancer and concludes with the recently identified serrated pathway syndrome.

The dialectics of cancer: A theory of the initiation and development of cancer through errors in RNAi

The recent discoveries of the RNA-mediated interference system in cells could explain all of the known features of human carcinogenesis. A key, novel idea, proposed here, is that the cell has the ability to recognise a mutated protein and/or mRNA. Secondly, the cell can generate its own short interfering RNA (siRNA) using an RNA polymerase to destroy mutated mRNA, even when only a single base pair in the gene has mutated. The anti-sense strand of the short RNA molecule (called sicRNA), targets the mutated mRNA of an oncogene or a tumour suppressor. The resulting double stranded RNA, using the RNA-induced silencing complex in the cytoplasm dices the mutated mRNA. In cancer-prone tissues, during cell mitosis, the sicRNA complex can move into the nucleus to target the mutated gene. The sicRNA, possibly edited by dsRNA-specific adenosine deaminase, converting adenosines to inosines, can be retained in the nucleus, with enhanced destructive capability. The sicRNA triggers the assembly of protein complexes leading to epigenetic modification of the promoter site of the mutant gene, specifically methylation of cytosines. In some instances, instead of methylation, the homologous DNA is degraded, leading to loss of heterozygosity. The factors controlling these two actions are unknown but the result is gene silencing or physical destruction of the mutant gene. The cell survives dependent on the functioning of the single, wild-type allele. An error in RNAi defence occurs when the sicRNA enters the nucleus and targets the sense strand of the wrong DNA. The sicRNA, because of the similarity of its short sequence and relaxed stringency, can target other RNAs, which are being transcribed. This can result in the methylation of the wrong promoter site of a gene or LOH of that region. In the vast majority of these cases, the aberrant hybridisations will have no effect on cell function or apoptosis eliminates non-viable cells. On a rare occasion, a preneoplastic cell is initiated when aberrant hybridisations switches on/off a gene involved in apoptosis, as well as a gene involved in cell proliferation and DNA damage surveillance. Genetic instability results when the sicRNA competes for a repeat sequence in the centromere or telomere, leading to gross chromosomal rearrangements. A malignancy develops when the sicRNAs fortuitously targets a microRNA (miRNA) or activates a transcription factor, resulting in the translation of a large number of new genes, alien to that tissue. This leads to dedifferentiation of the tissue, a resculpting of the histone code, chromosomal rearrangements, along a number of specific pathways, the gain of immortality and the dissemination of a metastatic cancer.

MutYH (MYH) and colorectal cancer

MAP (MutYH-associated polyposis) is a recently described colorectal adenoma and carcinoma predisposition syndrome that is associated with biallelic-inherited mutations of the human MutY homologue gene, MutYH. MutYH is often also termed MYH. MAP tumours display a mutational signature of somatic guanine-to-thymine transversion mutations in the adenomatous polyposis coli and K-ras genes, reflecting the normal role of MutYH in the base excision repair of adenines misincorporated opposite 7,8-dihydro-8-oxoguanine, a prevalent and stable product of oxidative damage to DNA. However, the full genetic pathway of MAP tumorigenesis has not been elucidated.

MYH Y165C and G382D mutations in hepatocellular carcinoma and cholangiocarcinoma patients

PURPOSE

Production of reactive oxygen species (ROS) during chronic inflammation has been implicated in the progression of liver diseases and carcinogenesis. Subjects with inflammatory liver disease and one non-functional allele of the base excision repair gene, MYH, may be more susceptible to progression to cancer due to MYH haploinsufficiency in repairing oxidative damage caused by ROS. Here, we investigated the association of two common germline MYH mutations in patients with hepatocellular carcinoma (HCC) and cholangiocarcinoma.

METHODS

DNA from patients with HCC (n=48) or cholangiocarcinoma (n=84) compared to non-cancerous controls (n=308) were genotyped for the Y165C and G382D mutations in MYH.

RESULTS

There was no significant difference in MYH mutation carrier status between patients with HCC (1/48), cholangiocarcinoma (3/84), and non-cancerous controls (4/308).

CONCLUSIONS

Patients with HCC or cholangiocarcinoma do not have an increased incidence of monoallelic MYH mutations pre-disposing them to disease.

Inherited colorectal cancer syndromes

Colorectal cancer is one of the major causes of cancer deaths in both men and women. It is estimated that approximately 5% to 10% of patients with colorectal cancer have an inherited germline mutation that predisposes them to cancer. Clinically, hereditary colorectal cancer syndromes can be divided into those associated with colonic polyposis (familial adenomatous polyposis, attenuated familial adenomatous polyposis, and MYH-associated polyposis) and those not associated with colonic polyposis (hereditary nonpolyposis colon cancer). Treatment options for these patients include multiple aggressive screening regimens, chemopreventive medications, and prophylactic surgery. Selection of the appropriate management approach is best made using information obtained from the patient's clinical examination, the family medical history, and genetic evaluation. Compliance is improved when patients completely understand their disease and participate fully in the formulation of the treatment plan. Although not proved, it seems reasonable that this approach may prevent the poor outcomes so frequently associated with inherited cancer syndromes.

Frequency of the Common MYH Mutations (G382D and Y165C) in MMR Mutation Positive and Negative HNPCC Patients

Recently mutations in the MYH gene have been associated with a milder form of adenomatous polyposis which is characterized by a variable level of colonic polyps ranging from a few to several hundred. In the context of HNPCC it is not unusual to identify patients with a smattering of polyps. The MYH gene product is involved in DNA repair and indeed the hMSH2/hMSH6 complex (both genes being essential elements of the DNA mismatch repair pathway) is required to stimulate MYH activity. We reasoned that because of the clinical similarity of a subset of HNPCC patients to those described with MYH mutations and the role of the hMSH2/hMSH6 complex in the activation of MYH protein that MYH mutations may account for a small proportion of HNPCC patients. In a study of 442 HNPCC patients we identified MYH mutations at the same frequency as that expected in the general population. Nevertheless, two HNPCC families were identified harbouring biallelic changes in MYH.