Constitutive deficiency in DNA mismatch repair: is it time for Lynch III?

Strong Hereditary Predispositions to Colorectal Cancer

Cancer is one of the most common causes of death worldwide. A strong predisposition to cancer is generally only observed in colorectal cancer (5% of cases) and breast cancer (2% of cases). Colorectal cancer is the most common cancer with a strong genetic predisposition, but it includes dozens of various syndromes. This group includes familial adenomatous polyposis, attenuated familial adenomatous polyposis, MUTYH-associated polyposis, NTHL1-associated polyposis, Peutz-Jeghers syndrome, juvenile polyposis syndrome, Cowden syndrome, Lynch syndrome, and Muir-Torre syndrome. The common symptom of all these diseases is a very high risk of colorectal cancer, but depending on the condition, their course is different in terms of age and range of cancer occurrence. The rate of cancer development is determined by its conditioning genes, too. Hereditary predispositions to cancer of the intestine are a group of symptoms of heterogeneous diseases, and their proper diagnosis is crucial for the appropriate management of patients and their successful treatment. Mutations of specific genes cause strong colorectal cancer predispositions. Identifying mutations of predisposing genes will support proper diagnosis and application of appropriate screening programs to avoid malignant neoplasm.

Statistical methods for Mendelian models with multiple genes and cancers

Risk evaluation to identify individuals who are at greater risk of cancer as a result of heritable pathogenic variants is a valuable component of individualized clinical management. Using principles of Mendelian genetics, Bayesian probability theory, and variant-specific knowledge, Mendelian models derive the probability of carrying a pathogenic variant and developing cancer in the future, based on family history. Existing Mendelian models are widely employed, but are generally limited to specific genes and syndromes. However, the upsurge of multigene panel germline testing has spurred the discovery of many new gene-cancer associations that are not presently accounted for in these models. We have developed PanelPRO, a flexible, efficient Mendelian risk prediction framework that can incorporate an arbitrary number of genes and cancers, overcoming the computational challenges that arise because of the increased model complexity. We implement an 11-gene, 11-cancer model, the largest Mendelian model created thus far, based on this framework. Using simulations and a clinical cohort with germline panel testing data, we evaluate model performance, validate the reverse-compatibility of our approach with existing Mendelian models, and illustrate its usage. Our implementation is freely available for research use in the PanelPRO R package.

Multi-syndrome, multi-gene risk modeling for individuals with a family history of cancer with the novel R package PanelPRO

Identifying individuals who are at high risk of cancer due to inherited germline mutations is critical for effective implementation of personalized prevention strategies. Most existing models focus on a few specific syndromes; however, recent evidence from multi-gene panel testing shows that many syndromes are overlapping, motivating the development of models that incorporate family history on several cancers and predict mutations for a comprehensive panel of genes.

We present PanelPRO, a new, open-source R package providing a fast, flexible back-end for multi-gene, multi-cancer risk modeling with pedigree data. It includes a customizable database with default parameter values estimated from published studies and allows users to select any combinations of genes and cancers for their models, including well-established single syndrome BayesMendel models (BRCAPRO and MMRPRO). This leads to more accurate risk predictions and ultimately has a high impact on prevention strategies for cancer and clinical decision making. The package is available for download for research purposes at https://projects.iq.harvard.edu/bayesmendel/panelpro.

Genetic testing for inherited colorectal cancer and polyposis, 2021 revision: a technical standard of the American College of Medical Genetics and Genomics (ACMG)

Colorectal cancer (CRC) is the fourth most frequently diagnosed cancer and 30% of all cases of CRC are believed to have a familial component and up to one-third of these (10%) are hereditary. Pathogenic germline variants in multiple genes have been associated with predisposition to hereditary CRC or polyposis. Lynch syndrome (LS) is the most common hereditary CRC syndrome, caused by variants in the mismatch repair (MMR) genes MLH1, MSH2, MSH6, and PMS2 and is inherited in a dominant manner. Heritable conditions associated with colonic polyposis include familial adenomatous polyposis (FAP) associated with APC pathogenic variants, MUTYH-associated polyposis (MAP) caused by biallelic MUTYH pathogenic variants, and polymerase proofreading-associated polyposis (PPAP) caused by POLE or POLD1 pathogenic variants. Given the overlapping phenotypes of the cancer syndromes along with the limited sensitivity of using clinical criteria alone, a multigene panel testing approach to diagnose these conditions using next-generation sequencing (NGS) is effective and efficient. This technical standard is not recommended for use in the clinic for patient evaluation. Please refer to National Comprehensive Cancer Network (NCCN) clinical practice guidelines to determine an appropriate testing strategy and guide medical screening and management. This 2021 edition of the American College of Medical Genetics and Genomics (ACMG) technical standard supersedes the 2013 edition on this topic.

Hereditary Colorectal Cancer Syndromes

Awareness of hereditary colorectal cancer syndromes is important to facilitate their identification because affected patients are at increased risk for early onset, synchronous, and metachronous colorectal malignancies, and certain extracolonic malignancies depending on the syndrome. Identification of an affected individual allows for screening and early interventions for patients and their at-risk kindred. Genetic counseling and testing is important to the care of these patients. As knowledge of the genetic basis of these syndromes grows, unique genotype-phenotype profiles allow clinicians to tailor surveillance and treatment strategies based on individual risk.

The current value of determining the mismatch repair status of colorectal cancer: A rationale for routine testing

Colorectal Cancer (CRC) is the third most prevalent cancer in men and women. Up to 15% of CRCs display microsatellite instability (MSI). MSI is reflective of a deficient mismatch repair (MMR) system and is most commonly caused by hypermethylation of the MLH1 promoter. However, it may also be due to autosomal dominant constitutional mutations in DNA MMR, termed Lynch Syndrome. MSI may be diagnosed via polymerase chain reaction (PCR) or alternatively, immunohistochemistry (IHC) can identify MMR deficiency (dMMR). Many institutions now advocate universal tumor screening of CRC via either PCR for MSI or IHC for dMMR to guide Lynch Syndrome testing. The association of sporadic MSI with methylation of the MLH1 promoter and an activating BRAF mutation may offer further exclusion criteria for genetic testing. Aside from screening for Lynch syndrome, MMR testing is important because of its prognostic and therapeutic implications. Several studies have shown MSI CRCs exhibit different clinicopathological features and prognosis compared to microsatellite-stable (MSS) CRCs. For example, response to conventional chemotherapy has been reported to be less in MSI tumours. More recently, MSI tumours have been shown to be responsive to immune-checkpoint inhibition providing a novel therapeutic strategy. This provides a rationale for routine testing for MSI or dMMR in CRC.

Familial colorectal cancer

Identifying individuals with a genetic predisposition to developing familial colorectal cancer (CRC) is crucial to the management of the affected individual and their family. In order to do so, the physician requires an understanding of the different gene mutations and clinical manifestations of familial CRC. This review summarises the genetics, clinical manifestations and management of the known familial CRC syndromes, specifically Lynch syndrome, familial adenomatous polyposis, MUTYH-associated neoplasia, juvenile polyposis syndrome and Peutz-Jeghers syndrome. An individual suspected of having a familial CRC with an underlying genetic predisposition should be referred to a familial cancer centre to enable pre-test counselling and appropriate follow up.

Glioblastoma in a Patient with a Hereditary Cancer Syndrome

A 68-year-old Caucasian male presented to emergency with a spontaneous sudden onset headache localized behind the left eye. Pain was severe and associated with vomiting, agitation, anxiety, memory loss, and word-finding difficulty. There was no preceding trauma and no meningeal symptoms. Further history was significant for mitral valve repair, atrial fibrillation treated with coumadin, colorectal cancer treated with right-sided hemicolectomy 17 years prior, and multiple skin tumors, which included squamous cell and basal cell carcinomas of his face and ears. Family history was significant for colon cancer in his father (at age 50), a sister (at age 40), and a niece (at age 38), as well as primary brain cancer in his paternal aunt.

ACMG technical standards and guidelines for genetic testing for inherited colorectal cancer (Lynch syndrome, familial adenomatous polyposis, and MYH-associated polyposis)

Lynch syndrome, familial adenomatous polyposis, and Mut Y homolog (MYH)-associated polyposis are three major known types of inherited colorectal cancer, which accounts for up to 5% of all colon cancer cases. Lynch syndrome is most frequently caused by mutations in the mismatch repair genes MLH1, MSH2, MSH6, and PMS2 and is inherited in an autosomal dominant manner. Familial adenomatous polyposis is manifested as colonic polyposis caused by mutations in the APC gene and is also inherited in an autosomal dominant manner. Finally, MYH-associated polyposis is caused by mutations in the MUTYH gene and is inherited in an autosomal recessive manner but may or may not be associated with polyps. There are variants of both familial adenomatous polyposis (Gardner syndrome--with extracolonic features--and Turcot syndrome, which features medulloblastoma) and Lynch syndrome (Muir-Torre syndrome features sebaceous skin carcinomas, and Turcot syndrome features glioblastomas). Although a clinical diagnosis of familial adenomatous polyposis can be made using colonoscopy, genetic testing is needed to inform at-risk relatives. Because of the overlapping phenotypes between attenuated familial adenomatous polyposis, MYH-associated polyposis, and Lynch syndrome, genetic testing is needed to distinguish among these conditions. This distinction is important, especially for women with Lynch syndrome, who are at increased risk for gynecological cancers. Clinical testing for these genes has progressed rapidly in the past few years with advances in technologies and the lower cost of reagents, especially for sequencing. To assist clinical laboratories in developing and validating testing for this group of inherited colorectal cancers, the American College of Medical Genetics and Genomics has developed the following technical standards and guidelines. An algorithm for testing is also proposed.

Simultaneous colonic adenocarcinoma and medulloblastoma in a 12-year-old with biallelic deletions in PMS2

We describe a 12-year-old girl, simultaneously presenting with colonic adenocarcinoma and medulloblastoma from bialleic deletions in the mismatch repair gene PMS2. Her distinctive physical and clinical findings are characteristic of constitutional mismatch repair deficiency syndrome. Earlier recognition of such findings may permit better screening and more effective treatment.

Multiple pilomatricomas with somatic CTNNB1 mutations in children with constitutive mismatch repair deficiency

Constitutional mismatch repair deficiency (CMMR-D) due to biallelic germline mutations in one of four mismatch repair genes causes a childhood cancer syndrome characterized by a broad tumor spectrum including hematological malignancies, and brain and Lynch syndrome-associated tumors. Herein, we report three children who had in addition to CMMR-D-associated malignancies multiple pilomatricomas. These are benign skin tumors of hair matrical differentiation frequently associated with somatic activating mutations in the ß-catenin gene CTNNB1. In two of the children, the diagnosis of CMMR-D was confirmed by the identification of biallelic germline PMS2 mutations. In the third individual, we only found a heterozygous germline PMS2 mutation. In all nine pilomatricomas with basophilic cells, we detected CTNNB1 mutations. Our findings indicate that CTNNB1 is a target for mutations when mismatch repair is impaired due to biallelic PMS2 mutations. An elevated number of activating CTNNB1 alterations in hair matrix cells may explain the development of multiple pilomatricomas in CMMR-D patients. Of note, two of the children presented with multiple pilomatricomas and other nonmalignant features of CMMR-D before they developed malignancies. To offer surveillance programs to CMMR-D patients, it may be justified to suspect CMMR-D syndrome in individuals fulfilling multiple nonmalignant features of CMMR-D (including multiple pilomatricomas) and offer molecular testing in combination with interdisciplinary counseling.

The germline MLH1 K618A variant and susceptibility to Lynch syndrome-associated tumors

Missense variants discovered during sequencing of cancer susceptibility genes can be problematic for clinical interpretation. MLH1 K618A, which results from a 2-bp alteration (AAG→GCG) leading to a substitution of lysine to alanine in codon 618, has variously been interpreted as a pathogenic mutation, a variant of unknown significance, and a benign polymorphism. We evaluated the role of MLH1 K618A in predisposition to cancer by genotyping 1512 control subjects to assess its frequency in the general population. We also reviewed the literature concerning MLH1 K618A in families with colorectal cancer. The measured allele frequency of the K618A variant was 0.40%, which is remarkably close to the 0.44% summarized from 2491 control subjects in the literature. K618A was over-represented in families with suspected Lynch syndrome. In 1366 families, the allele frequency was 0.88% (OR = 2.1, 95% CI = 1.3 to 3.5; P = 0.006). In studies of sporadic cancers of the type associated with Lynch syndrome, K618A was over-represented in 1742 cases (allele frequency of 0.83) (OR = 2.0, 95% CI = 1.2 to 3.2; P = 0.008). We conclude that MLH1 K618A is not a fully penetrant Lynch syndrome mutation, although it is not without effect, appearing to increase the risk of Lynch syndrome-associated tumors approximately twofold. Our systematic assessment approach may be useful for variants in other genes.

Microsatellite instability and colorectal cancer

CONTEXT

About 15% of colorectal cancers are characterized by genomic microsatellite instability, and of these, about 1 in 5 (2%-4% overall) are due to Lynch syndrome, a dominantly inherited condition predisposing the patient to cancers of multiple organ systems, including the gastrointestinal tract. Identification of individuals with Lynch syndrome allows for increased surveillance of the affected individual and of potentially affected family members.

OBJECTIVE

To review the literature on microsatellite instability in colorectal cancer and current laboratory diagnostic testing strategies for the detection of Lynch syndrome.

DATA SOURCES

This review is based on peer-reviewed literature, published guidelines from professional organizations (Evaluation of Genomic Applications in Practice and Prevention Working Group, National Comprehensive Cancer Network), and information from clinical laboratories performing microsatellite instability testing.

CONCLUSIONS

Universal screening for Lynch syndrome in all individuals affected with colorectal cancer has been recommended by the Evaluation of Genomic Applications in Practice and Prevention Working Group. Preliminary screening tests can identify individuals unlikely to be affected by Lynch syndrome, thereby reducing the need for full gene analysis. Immunohistochemistry and polymerase chain reaction-based tests for microsatellite instability have similar clinical sensitivity and specificity, and each method has advantages and limitations. BRAF and MLH1 methylation testing are useful reflex tests for those with a defect in MLH1 identified by immunohistochemistry. Emerging technologies, such as high-throughput sequencing, may substantially affect diagnostic algorithms in the future.

The gastrointestinal phenotype of germline biallelic mismatch repair gene mutations

OBJECTIVES

A novel cancer syndrome associated with biallelic mismatch repair (MMR) mutations has been described recently. Patients presenting with childhood-onset gastrointestinal (GI) cancers may carry biallelic MMR mutations and have a distinct phenotype from classic Lynch syndrome. The aim of this study was to characterize patients with GI small bowel and/or colorectal cancers (CRCs) who have germline biallelic MMR mutations.

METHODS

A search of a Canadian GI cancer registry and literature review to identify patients with biallelic MMR was conducted.

RESULTS

The database identified 237 patients with intestinal cancer diagnosed before the age of 35 years. Five (2.1%) patients had biallelic MMR mutations. Overall, 32 individuals, from 29 families, with biallelic MMR gene mutations and GI cancers were identified by the registry and literature review. Among the 29 patients with CRCs, the mean age of first cancer diagnosis was 16.4 years (range: 5-28). More than one-third of patients had multiple colorectal adenomas (>10 polyps). Six individuals with biallelic MMR gene mutations have been reported with small bowel adenocarcinoma (mean age 20 years (range: 11-41)). Café-au-lait (CAL) macules were reported in 72% and, based on mutation analysis, consanguinity was suspected in 52% of kindred. Of the 29 kindred, 19 (66%) had PMS2 mutations, 6 (21%) had MSH6 mutations, 3 (10%) had MLH1 mutations, and 1 (3%) had MSH2 mutation.

CONCLUSIONS

Biallelic MMR mutations are an underrecognized cause of small bowel and colonic cancers in children and young adults. This distinct phenotype includes multiple adenomatous polyps and CAL skin lesions. It is important to identify such patients, so that families can be referred for genetic testing and counseling.

Msh6 protects mature B cells from lymphoma by preserving genomic stability

Most human B-cell non-Hodgkin's lymphomas arise from germinal centers. Within these sites, the mismatch repair factor MSH6 participates in antibody diversification. Reminiscent of the neoplasms arising in patients with Lynch syndrome III, mice deficient in MSH6 die prematurely of lymphoma. In this study, we characterized the B-cell tumors in MSH6-deficient mice and describe their histological, immunohistochemical, and molecular features, which include moderate microsatellite instability. Based on histological markers and gene expression, the tumor cells seem to be at or beyond the germinal center stage. The simultaneous loss of MSH6 and of activation-induced cytidine deaminase did not appreciably affect the survival of these animals, suggesting that these germinal center-like tumors arose by an activation-induced cytidine deaminase-independent pathway. We conclude that MSH6 protects B cells from neoplastic transformation by preserving genomic stability.

Application of molecular diagnostics for the detection of Lynch syndrome

Lynch syndrome (LS), or hereditary nonpolyposis colorectal cancer, is the most common hereditary colorectal cancer (CRC) syndrome, accounting for approximately 2-5% of all newly diagnosed cases of CRC. Patients with LS have an increased lifetime risk of colorectal (52.2% in women and 68.7% in men) and endometrial cancer (15-70%), as well as certain extra-colonic cancers. Germline mutations in one of several DNA mismatch repair genes underlie LS. Molecular testing has emerged as an indispensable strategy for the diagnosis of LS. The diagnostic work-up of at-risk individuals includes a careful family history evaluation, microsatellite instability, immunohistochemistry and germline DNA analysis. A positive test result can guide clinicians in formulating the appropriate screening, surveillance and management strategies. However, because of the absence of an overt phenotype, such as a diffuse polyposis, it is not always straightforward to recognize LS clinically.

Constitutional mismatch repair deficiency and childhood leukemia/lymphoma--report on a novel biallelic MSH6 mutation

Biallelic mutations of mismatch repair genes cause constitutional mismatch repair deficiency associated with an increased risk for childhood leukemia/lymphoma. We report on a case with constitutional mismatch repair deficiency caused by a novel MSH6 mutation leading to a T-cell lymphoma and colonic adenocarcinoma at six and 13 years of age, respectively. A review of the literature on hematologic malignancies in constitutional mismatch repair deficiency showed that in almost half of the 47 known constitutional mismatch repair deficiency families, at least one individual is affected by a hematologic malignancy, predominantly T-cell lymphomas. However, diagnosing constitutional mismatch repair deficiency may be difficult when the first child is affected by leukemia/lymphoma, but identification of the causative germline mutation is of vital importance: (i) to identify relatives at risk and exclude an increased risk in non-mutation carriers; (ii) to prevent hematopoietic stem cell transplantation from sibling donors also carrying a biallelic germline mutation; and (iii) to implement effective surveillance programs for mutation carriers, that may reduce constitutional mismatch repair deficiency-associated mortality.

Sebaceous lesions and their associated syndromes: part II

Sebaceous lesions are associated with two syndromes with widespread multisystem disorders and tumors. Linear sebaceous nevus syndrome has been traditionally known as the triad of sebaceous nevus of Jadassohn, seizures, and mental retardation. This syndrome encompasses a much broader spectrum of multisystem disorders, which is explored below. Muir-Torre syndrome is described as the presence of sebaceous tumors or keratoacanthomas with an underlying visceral malignancy. It is caused by mutations in DNA mismatch repair genes. We discuss its relationship with Lynch syndrome and suggest a comprehensive algorithm on how to screen patients with sebaceous neoplasms for Muire-Torre syndrome. We also provide suggested intensive cancer screening guidelines based on recommendations for patients with Lynch syndrome that may also be of value for patients with Muir-Torre syndrome.

Epigenetic repression of DNA mismatch repair by inflammation and hypoxia in inflammatory bowel disease-associated colorectal cancer

Sporadic human mismatch repair (MMR)-deficient colorectal cancers account for approximately 12.5% of all cases of colorectal cancer. MMR-deficient colorectal cancers are classically characterized by right-sided location, multifocality, mucinous histology, and lymphocytic infiltration. However, tumors in germ-line MMR-deficient mouse models lack these histopathologic features. Mice lacking the heterotrimeric G protein alpha subunit Gialpha2 develop chronic colitis and multifocal, right-sided cancers with mucinous histopathology, similar to human MMR-deficient colorectal cancer. Young Gialpha2-/- colonic epithelium has normal MMR expression but selectively loses MLH1 and consequently PMS2 expression following inflammation. Gialpha2-/- cancers have microsatellite instability. Mlh1 is epigenetically silenced not by promoter hypermethylation but by decreased histone acetylation. Chronically inflamed Gialpha2-/- colonic mucosa contains patchy hypoxia, with increased crypt expression of the hypoxia markers DEC-1 and BNIP3. Chromatin immunoprecipitation identified increased binding of the transcriptional repressor DEC-1 to the proximal Mlh1 promoter in hypoxic YAMC cells and colitic Gialpha2-/- crypts. Treating Gialpha2-/- mice with the histone deacetylase inhibitor suberoylanilide hydroxamic acid significantly decreased colitis activity and rescued MLH1 expression in crypt epithelial cells, which was associated with increased acetyl histone H3 levels and decreased DEC-1 binding at the proximal Mlh1 promoter, consistent with a histone deacetylase-dependent mechanism. These data link chronic hypoxic inflammation, epigenetic MMR protein down-regulation, development of MMR-deficient colorectal cancer, and the firstmouse model of somatically acquired MMR-deficient colorectal cancer.

Pediatric duodenal cancer and biallelic mismatch repair gene mutations

Gastrointestinal malignancies are extremely rare in the pediatric population, and duodenal cancers represent an even more unusual entity. Intestinal cancers in young adults and children have been observed to be associated with functional deficiencies of the mismatch repair (MMR) system causing a cancer-predisposition syndrome. We report the case of a 16-year-old female with duodenal adenocarcinoma and past history of medulloblastoma found to have a novel germline bialleleic truncating mutation (c.[949C>T]+[949C>T]) of the PMS2 gene.

Review of the Lynch syndrome: history, molecular genetics, screening, differential diagnosis, and medicolegal ramifications

More than one million patients will manifest colorectal cancer (CRC) this year of which, conservatively, approximately 3% (approximately 30,700 cases) will have Lynch syndrome (LS), the most common hereditary CRC predisposing syndrome. Each case belongs to a family with clinical needs that require genetic counseling, DNA testing for mismatch repair genes (most frequently MLH1 or MSH2) and screening for CRC. Colonoscopy is mandated, given CRC's proximal occurrence (70-80% proximal to the splenic flexure). Due to its early age of onset (average 45 years of age), colonoscopy needs to start by age 25, and because of its accelerated carcinogenesis, it should be repeated every 1 to 2 years through age 40 and then annually thereafter. Should CRC occur, subtotal colectomy may be necessary, given the marked frequency of synchronous and metachronous CRC. Because 40-60% of female patients will manifest endometrial cancer, tailored management is essential. Additional extracolonic cancers include ovary, stomach, small bowel, pancreas, hepatobiliary tract, upper uroepithelial tract, brain (Turcot variant) and sebaceous adenomas/carcinomas (Muir-Torre variant). LS explains only 10-25% of familial CRC.

Cumulative lifetime incidence of extracolonic cancers in Lynch syndrome: a report of 121 families with proven mutations

Lynch syndrome or hereditary non-polyposis colorectal cancer is caused by mutations of DNA mismatch repair (MMR) genes. The extracolonic tumour spectrum includes endometrial, ovarian, gastric, small bowel, pancreatic, hepatobiliary, brain, and urothelial neoplasms. Families were referred on the basis of clinical criteria. Tumour immunohistochemistry and microsatellite testing were performed. Appropriate patients underwent sequencing of relevant exons of the MMR genes. Proven and obligate mutation carriers and first-degree relatives (FDRs) with a Lynch syndrome spectrum cancer were considered mutation carriers, as were a proportion of untested, unaffected FDRs based on the proportion of unaffected relatives testing positive in each age group. Kaplan-Meier analysis of risk to 70 years was calculated. One hundred and eighty-four Lynch syndrome spectrum extracolonic cancers in 839 proven, obligate, or assumed mutation carriers were analysed. Cumulative risk for females of an extracolonic tumour is 47.4% (95% CI 43.9-50.8). The risk to males is 26.5% (95% CI 22.6-30.4). There was no reduction in gynaecological malignancies due to gynaecological screening (examination, transvaginal ultrasound scan, hysteroscopy and endometrial biopsy). Males have a higher risk of gastric cancer than females (p = 0.0003). Gastric cancer risk in those born after 1935 does not justify surveillance. These penetrance estimates have been corrected for ascertainment bias and are appropriate for those referred to a high-risk clinic.

Genetic epidemiology studies in hereditary non-polyposis colorectal cancer

Genetic epidemiology studies in hereditary non-polyposis colorectal cancer (HNPCC) have the potential to radically improve assessment of disease risk such that more individualised information can be provided to patients susceptible to developing disease. Studies of HNPCC initially focused on disease associations and the definition of the disease and its association with different cancers within the context of an inherited predisposition. With the identification of the genetic basis of HNPCC, new insights into the disease have been forthcoming and many advances in our understanding have been made. There have been many reports examining potential modifier genes in HNPCC, yet the results remain controversial as many findings have not been replicated and therefore no clear consensus as to the role of specific modifier genes has been reached. This review focuses on some of the factors associated with disease risk in HNPCC and where some of the difficulties lie in assessing the value of genetic epidemiology studies in this disorder.

Identifying Muir-Torre syndrome in a patient with glioblastoma multiforme

Patients with Muir-Torre syndrome, an autosomal-dominant familial tumor condition caused by germline mutation of the DNA mismatch repair genes, MSH2 or MLH1, present with tumors of the sebaceous gland and visceral malignancies characterized by microsatellite instability. Here we show development of glioblastoma multiforme in a patient with Muir-Torre syndrome. Immunohistochemical analysis of the brain tumor and colon cancer revealed loss of the DNA mismatch repair gene detected by the genetic test, suggesting a pathogenic link.

Compound heterozygosity for two MSH2 mutations suggests mild consequences of the initiation codon variant c.1A>G of MSH2

Mono-allelic germline mutations in mismatch repair (MMR) genes lead to Lynch syndrome, an autosomal dominant syndrome with an increased risk of predominantly colorectal and endometrial cancers. Bi-allelic germline mutations in MMR genes predispose to haematological malignancies, brain tumours, gastrointestinal tumours, polyposis and features of neurofibromatosis type 1 in early childhood.We report a brother and a sister with bi-allelic germline mutations in MSH2; a pathogenic deletion of the first 6 exons and a variant of the initiation codon (c.1A>G (p.Met1?)), whereas their phenotypes (four colorectal cancers, small bowel carcinoma and 15 adenomas at age 39 and 48, and colorectal cancer, endometrial cancer and four adenomas at age 33 and 44, respectively) are more suggestive of a mono-allelic pathogenic MMR gene mutation. The carcinomas showed microsatellite instability in the presence of MLH1, PMS2, MSH2 and MSH6 proteins, indicating that the variant c.1A>G leads to an alternative protein with reduced activity that is retained in the tumours.Our data suggest that the MSH2 variant c.1A>G (p.Met1?) should not be considered as a regular pathogenic mutation that leads to a strongly increased cancer risk, though it possibly contributes to a more severe phenotype when combined with a truncating mutation on the other allele.

Constitutional mismatch repair-deficiency syndrome: have we so far seen only the tip of an iceberg?

Heterozygous mutations in one of the mismatch repair (MMR) genes MLH1, MSH2, MSH6 and PMS2 cause the dominant adult cancer syndrome termed Lynch syndrome or hereditary non-polyposis colorectal cancer. During the past 10 years, some 35 reports have delineated the phenotype of patients with biallelic inheritance of mutations in one of these MMR genes. The patients suffer from a condition that is characterised by the development of childhood cancers, mainly haematological malignancies and/or brain tumours, as well as early-onset colorectal cancers. Almost all patients also show signs reminiscent of neurofibromatosis type 1, mainly café au lait spots. Alluding to the underlying mechanism, this condition may be termed as "constitutional mismatch repair-deficiency (CMMR-D) syndrome". To give an overview of the current knowledge and its implications of this recessively inherited cancer syndrome we summarise here the genetic, clinical and pathological findings of the so far 78 reported patients of 46 families suffering from this syndrome.

The clinical phenotype of Lynch syndrome due to germ-line PMS2 mutations

BACKGROUND & AIMS

Although the clinical phenotype of Lynch syndrome (also known as hereditary nonpolyposis colorectal cancer) has been well described, little is known about disease in PMS2 mutation carriers. Now that mutation detection methods can discern mutations in PMS2 from mutations in its pseudogenes, more mutation carriers have been identified. Information about the clinical significance of PMS2 mutations is crucial for appropriate counseling. Here, we report the clinical characteristics of a large series of PMS2 mutation carriers.

METHODS

We performed PMS2 mutation analysis using long-range polymerase chain reaction and multiplex ligation-dependent probe amplification for 99 probands diagnosed with Lynch syndrome-associated tumors showing isolated loss of PMS2 by immunohistochemistry. Penetrance was calculated using a modified segregation analysis adjusting for ascertainment.

RESULTS

Germ-line PMS2 mutations were detected in 62% of probands (n = 55 monoallelic; 6 biallelic). Among families with monoallelic PMS2 mutations, 65.5% met revised Bethesda guidelines. Compared with the general population, in mutation carriers, the incidence of colorectal cancer was 5.2-fold higher, and the incidence of endometrial cancer was 7.5-fold higher. In North America, this translates to a cumulative cancer risk to age 70 years of 15%-20% for colorectal cancer, 15% for endometrial cancer, and 25%-32% for any Lynch syndrome-associated cancer. No elevated risk for non-Lynch syndrome-associated cancers was observed.

CONCLUSIONS

PMS2 mutations contribute significantly to Lynch syndrome, but the penetrance for monoallelic mutation carriers appears to be lower than that for the other mismatch repair genes. Modified counseling and cancer surveillance guidelines for PMS2 mutation carriers are proposed.

Café-au-lait macules and pediatric malignancy caused by biallelic mutations in the DNA mismatch repair (MMR) gene PMS2

A 14-year-old male presented with a T4 sigmoid adenocarcinoma, <10 colonic adenomas and multiple café-au-lait macules. Family history was not suggestive of a dominant hereditary form of colorectal cancer. Evaluation of the tumor revealed abnormal immunohistochemical staining of the PMS2 protein and high frequency microsatellite instability. Germline analysis identified biallelic PMS2 missense mutations. A new cancer syndrome caused by biallelic mutations in the mismatch repair genes, including PMS2, is now emerging and is characterized by café-au-lait macules, colonic polyps and a distinctive tumor spectrum.

Biallelic PMS2 mutations and a distinctive childhood cancer syndrome

Biallelic mutations in PMS2, a gene usually associated in heterozygous form with hereditary nonpolyposis colorectal cancer (HNPCC), results in a recently described childhood cancer syndrome. The tumor spectrum encompasses atypical brain cancers, hematologic malignancies, and colonic polyposis and cancer. Cutaneous stigmata resembling café-au-lait macules with more diffuse margins are frequently seen. Onset is as young as 2 years. The risk of second malignancy is high. Evidence exists for surveillance for bowel cancer, but surveillance for the wider tumor spectrum is of uncertain benefit. We report a consanguineous Australian-Lebanese family with multiple affected individuals shown to be homozygous for a PMS2 exon 7 deletion. We also review published cases of biallelic mutations in HNPCC-related genes. Early recognition of this familial cancer syndrome is critical, and should prompt investigation for familial HNPCC mutations.