The biochemical basis of microsatellite instability and abnormal immunohistochemistry and clinical behavior in Lynch syndrome: from bench to bedside

[Lynch syndrome: genetics and surgery]

Hereditary nonpolyposis colorectal cancer or Lynch Syndrome, caused by germinal mutations in mismatch deoxyribonucleic acid (DNA) repair genes, is the most common form of hereditary colorectal cancer. The identification of these individuals is not easy and is based on clinical and molecular criteria. A review is presented on the genetics and diagnosis in Lynch Syndrome, as well as on its surgical management and prevention.

Low frequency of Lynch syndrome among young patients with non-familial colorectal cancer

BACKGROUND & AIMS

Colorectal cancer (CRC) is uncommon in individuals <50 years old. Lynch syndrome is caused by germline mutations in DNA mismatch repair (MMR) genes and associated with early-onset CRC, but little is known about the proportion of young patients with apparently sporadic CRC who actually have Lynch syndrome. We examined patterns of microsatellite instability (MSI) and MMR genes among patients <50 years old with non-familial CRC (patients with not more than 1 family member with CRC).

METHODS

Tissue specimens were collected from 75 CRC patients <50 years old (mean age, 34.5 years) and analyzed using immunohistochemical analyses of MLH1, MSH2, MSH6, and PMS2. MSI and mutations in BRAF and KRAS were also analyzed.

RESULTS

Most cancers (72%) arose in the distal colon. MSI was detected in 21% of the samples, and loss of 1 or more MMR proteins was observed in 21%. Interestingly, only 38% of the MMR-deficient CRCs lost either MLH1 or MSH2, whereas 63% of the MMR-deficient CRC samples lost either PMS2 or MSH6. All 11 CRC samples that had lost MSH2, MLH1, or PMS2 had MSI, but only 2 of the 5 tumors that lost only MSH6 had MSI. There were no BRAF mutations in any tumor.

CONCLUSIONS

In young patients with apparently sporadic CRC, most tumors arise in the distal colon; only 21% have features of Lynch syndrome. Loss of MSH6 or PMS2 occurred in 13.3% of these tumors. Most tumors that lose MSH6 will not be detected in screens for MSI; CRC screening might be modified to identify more patients with Lynch syndrome.

Microsatellite alterations at selected tetranucleotide repeats are associated with morphologies of colorectal neoplasias

BACKGROUND & AIMS

Elevated microsatellite alterations at selected tetranucleotide repeats (EMAST) occurs during microsatellite instability (MSI) that is not associated with major defects in DNA mismatch repair (MMR) but rather the reduced (heterogenous) expression of the MMR protein hMSH3; it occurs in sporadic colorectal tumors. We examined the timing of development of EMAST during progression of colorectal neoplasias and looked for correlations between EMAST and clinical and pathology features of tumors.

METHODS

We evaluated tumor samples from a cohort of patients that had 24 adenomas and 84 colorectal cancers. EMAST were analyzed after DNA microdissection of matched normal and tumor samples using the polymorphic tetranucleotide microsatellite markers MYCL1, D9S242, D20S85, D8S321, and D20S82; data were compared with clinical and pathology findings. Traditional MSI analysis was performed and hMSH3 expression was measured.

RESULTS

Moderately differentiated adenocarcinomas and poorly differentiated adenocarcinomas had higher frequencies of EMAST (56.9% and 40.0%, respectively) than well-differentiated adenocarcinomas (12.5%) or adenomas (33.3%) (P = .040). In endoscopic analysis, ulcerated tumors had a higher frequency of EMAST (52.3%) than flat (44.0%) or protruded tumors (20.0%) (P = .049). In quantification, all tumors with >3 tetranucleotide defects lost MSH3 (>75% of cells); nuclear heterogeneity of hMSH3 occurred more frequently in EMAST-positive (40.0%) than in EMAST-negative tumors (13.2%) (P = .010).

CONCLUSIONS

EMAST is acquired during progression of adenoma and well-differentiated carcinomas to moderately and poorly differentiated carcinomas; it correlates with nuclear heterogeneity for hMSH3. Loss of hMSH3 corresponds with multiple tetranucleotide frameshifts. The association between EMAST and ulcerated tumors might result from increased inflammation.

Immunohistochemistry for PMS2 and MSH6 alone can replace a four antibody panel for mismatch repair deficiency screening in colorectal adenocarcinoma

AIMS

Currently, testing for mismatch repair deficiency in colorectal cancers is initiated by performing immunohistochemistry with four antibodies (MLH1, PMS2, MSH2 and MSH6). If any one of these stains is negative the tumour is considered microsatellite unstable and, if clinical circumstances warrant it, the patient is offered genetic testing for Lynch's syndrome. Due to the binding properties of the mismatch repair heterodimer complexes, gene mutation and loss of MLH1 and MSH2 invariably result in the degradation of PMS2 and MSH6, respectively, but the converse is not true. We propose that staining for PMS2 and MSH6 alone will be sufficient to detect all cases of mismatch repair deficiency and should replace routine screening with all four antibodies.

METHODS

The electronic database of the department of Anatomical Pathology, Royal North Shore Hospital, Sydney, Australia, was searched for all colorectal carcinomas on which a four panel immunohistochemical microsatellite instability screen was performed. An audit of the slides for concordant loss of MLH1-PMS2 and MSH2-MSH6 was then undertaken. Unusual or discordant cases were reviewed and, in some cases, re-stained to confirm the staining pattern.

RESULTS

Of 344 cases of colorectal cancer which underwent four antibody immunohistochemistry, 104 displayed loss of at least one mismatch repair protein. Of these, 100 showed concordant mismatch repair loss (i.e., loss of MLH1 and PMS2 or loss of MSH2 and MSH6). The four discordant cases comprised two single negative cases (1 MSH6 negative/MSH2 positive case, 1 PMS2 negative/MLH1 positive) and two triple negative (both MLH1/PMS2/MSH6 negative). The microsatellite instability (MSI) group showed a relatively high median age (69.3 years) due to the departmental policy of testing all cases with possible MSI morphology regardless of age.

CONCLUSIONS

The sensitivity and specificity of a two panel test comprised of PMS2 and MSH6, compared to a four panel test, is 100%. No false negatives or positives were identified. We conclude that the two panel test should replace a four panel protocol for immunohistochemical screening for mismatch repair deficiency.

Evidence for an hMSH3 defect in familial hamartomatous polyps

BACKGROUND

Patients with hamartomatous polyposis syndromes have increased risk for colorectal cancer (CRC). Although progression of polyps to carcinoma is observed, pathogenic mechanisms remain unknown. The authors examined whether familial hamartomatous polyps harbor defects in DNA mismatch repair (MMR), and assayed for somatic mutation of PTEN, a gene inactivated in the germline of some hamartomatous polyposis syndrome patients.

METHODS

Ten hamartomatous polyposis syndrome patients were genotyped for germline mutations. Epithelial and nonepithelial polyp DNA were assayed for microsatellite instability (MSI) and PTEN frameshift mutation. DNA MMR and PTEN protein expression were assessed in all polyps by immunohistochemistry. In addition, 99 MSI-high sporadic CRCs and 50 each of hMLH1(-/-) and hMSH3(-/-) cell clones were examined for PTEN frameshifts.

RESULTS

Twenty-five (58%) of 43 hamartomatous polyposis syndrome polyps demonstrated dinucleotide or greater MSI in polyp epithelium, consistent with hMSH3 deficiency. MSI domains lost hMSH3 expression, and PTEN expression was lost in polyps from germline PTEN patients; sporadic hamartomatous polyps did not show any of these findings. PTEN analysis revealed wild type exon 7 and 8 sequences suggestive of nonexistent or rare events for PTEN frameshifts; however, MSI-high sporadic CRC showed 11 (11%) of 99 frameshifts within PTEN, with 4 tumors having complete loss of PTEN expression. Subcloning hMLH1(-/-) and hMSH3(-/-) cells revealed somatic PTEN frameshifts in 4% and 12% of clones, respectively.

CONCLUSIONS

Nondysplastic epithelium from hamartomatous polyposis syndrome polyps harbors hMSH3 defects, which may prime neoplastic transformation. Polyps from PTEN(+/-) patients lose PTEN expression, but loss is not a universal early feature of all hamartomatous polyposis syndrome. However, PTEN frameshifts can occur in hMSH3-deficient cells, suggesting that hMSH3 deficiency could drive hamartomatous polyposis syndrome tumorigenesis.

Mismatch repair protein expression and colorectal cancer in Hispanics from Puerto Rico

Colorectal cancer (CRC) is a leading cause of morbidity and mortality and alterations in mismatch repair (MMR) genes, leading to absent protein (negative) expression, are responsible for approximately 20% of CRC cases. Immunohistochemistry is a tool for prescreening of MMR protein expression in CRC but the literature on its use on Hispanics is scarce. However, Hispanics represent the second leading ethnicity in the United States (US) and CRC is a public health burden in this group. Our objectives were to determine the frequency of MMR protein-negative CRC and to evaluate its association with clinical and pathological characteristics among Hispanics from Puerto Rico, for the first time to our knowledge. A retrospective observational study of unselected CRC patients from the Puerto Rico Medical Center from 2001 to 2005 was done. MLH1 and MSH2, the most commonly altered MMR genes, protein expression was evaluated using immunohistochemistry, with microsatellite instability (MSI) and BRAF gene analyses in the absence of MLH1 protein expression. One-hundred sixty-four CRC patients were evaluated: the overall MMR protein-negative frequency was 4.3%, with 0.6% frequency of co-occurrence of MLH1-protein negative expression, MSI-high, and normal BRAF gene. MMR protein-negative expression was associated with proximal colon location (P = 0.02) and poor histological tumor differentiation (P = 0.001), but not with other characteristics. The frequency of MMR protein-negative CRC in Hispanics from Puerto Rico was lower than reported in other populations. This finding may explain the lower CRC incidence rate among US Hispanics as compared to US non-Hispanic whites and blacks.

Therapeutic targeting of the DNA mismatch repair pathway

The mismatch repair (MMR) pathway is involved in the removal of DNA base mismatches that arise either during DNA replication or are caused by DNA damage. Mutations in four genes involved in MMR, MSH2, MLH1, PMS2 and MSH6, predispose to a range of tumorigenic conditions, including hereditary nonpolyposis colon cancer, also known as Lynch syndrome. Here we discuss the canonical MMR pathway and the burgeoning evidence for noncanonical roles for the MMR genes, and highlight the therapeutic implications of MMR. In particular, we discuss how the DNA repair defect in MMR-deficient cancers could be exploited by the development of novel therapeutic strategies based on synthetic lethal approaches.

Interobserver variability in the evaluation of mismatch repair protein immunostaining

Immunohistochemical staining for mismatch repair proteins has during recent years been established as a routine analysis in many pathology laboratories with the aim to identify tumors linked to the hereditary nonpolyposis colorectal cancer syndrome. Despite widespread application, data on reliability are lacking. We therefore evaluated interobserver variability among 6 pathologists, 3 experienced gastrointestinal pathologists and 3 residents. In total, 225 immunohistochemically stained colorectal cancers were evaluated as having normal, weak, loss of, or nonevaluable mismatch repair protein staining. Full consensus was achieved in 51% of the stainings for MLH1, 61% for PMS2, 83% for MSH2, and 45% for MSH6. Weak stainings were the main cause of reduced consensus, whereas contradictory evaluations with normal as well as loss of staining were reported in 2% to 6% of the tumors. Interobserver variability was considerable, though experienced pathologists and residents reached the same level of consensus. Because results from immunohistochemical mismatch repair protein stainings are used for decisions on mutation analysis and as an aid in the interpretation of gene variants of unknown significance in hereditary nonpolyposis colorectal cancer, the interobserver variability identified highlights the need for quality assessment programs, including guidelines for classification of different expression patterns.

Clinical relevance of microsatellite instability in colorectal cancer

Microsatellite instability (MSI) is a clonal change in the number of repeated DNA nucleotide units in microsatellites. It arises in tumors with deficient mismatch repair due to the inactivation of one of the four mismatch repair genes: MSH2, MLH1, MSH6, and PMS2. In order to determine the MSI status of a tumor, microdissection and polymerase chain reaction-based detection strategies are required. For practical purposes, MSI is equivalent to the loss of staining by immunohistochemistry (IHC) of one of the mismatch repair genes since both signify an abnormality in mismatch repair. Of all colorectal cancers (CRCs), 15% to 20% display MSI or abnormal IHC (often referred to as microsatellite instability [MIN] pathway). The remaining 80% to 85% of CRCs are microsatellite stable but most are characterized by chromosomal instability (CIN pathway). Almost all Lynch syndrome tumors have MSI or abnormal IHC and they account for up to one third of all MIN CRCs (3% to 5% of all CRCs). The remaining MIN tumors are sporadic as a result of somatic inactivation of the MLH1 gene caused by methylation of its promoter. Thus, the presence of a MSI/IHC abnormality prompts further investigations to diagnose Lynch syndrome, whereas its absence excludes Lynch syndrome. We recommend screening all CRC tumors for IHC or MSI. MIN tumors have a more favorable outcome than CIN tumors, and fluorouracil-based adjuvant chemotherapy does not improve the outcome of stage II or stage III MIN tumors. More data are needed to determine how best to treat patients with stage II and stage III MIN CRCs.

Microsatellite instability in colorectal cancer

Microsatellite instability (MSI) is a hypermutable phenotype caused by the loss of DNA mismatch repair activity. MSI is detected in about 15% of all colorectal cancers; 3% are of these are associated with Lynch syndrome and the other 12% are caused by sporadic, acquired hypermethylation of the promoter of the MLH1 gene, which occurs in tumors with the CpG island methylator phenotype. Colorectal tumors with MSI have distinctive features, including a tendency to arise in the proximal colon, lymphocytic infiltrate, and a poorly differentiated, mucinous or signet ring appearance. They have a slightly better prognosis than colorectal tumors without MSI and do not have the same response to chemotherapeutics. Discovery of MSI in colorectal tumors has increased awareness of the diversity of colorectal cancers and implications for specialized management of patients.

MSH6 and PMS2 mutation positive Australian Lynch syndrome families: novel mutations, cancer risk and age of diagnosis of colorectal cancer

BACKGROUND

Approximately 10% of Lynch syndrome families have a mutation in MSH6 and fewer families have a mutation in PMS2. It is assumed that the cancer incidence is the same in families with mutations in MSH6 as in families with mutations in MLH1/MSH2 but that the disease tends to occur later in life, little is known about families with PMS2 mutations. This study reports on our findings on mutation type, cancer risk and age of diagnosis in MSH6 and PMS2 families.

METHODS

A total of 78 participants (from 29 families) with a mutation in MSH6 and 7 participants (from 6 families) with a mutation in PMS2 were included in the current study. A database of de-identified patient information was analysed to extract all relevant information such as mutation type, cancer incidence, age of diagnosis and cancer type in this Lynch syndrome cohort. Cumulative lifetime risk was calculated utilising Kaplan-Meier survival analysis.

RESULTS

MSH6 and PMS2 mutations represent 10.3% and 1.9%, respectively, of the pathogenic mutations in our Australian Lynch syndrome families. We identified 26 different MSH6 and 4 different PMS2 mutations in the 35 families studied. We report 15 novel MSH6 and 1 novel PMS2 mutations. The estimated cumulative risk of CRC at age 70 years was 61% (similar in males and females) and 65% for endometrial cancer in MSH6 mutation carriers. The risk of developing CRC is different between males and females at age 50 years, which is 34% for males and 21% for females.

CONCLUSION

Novel MSH6 and PMS2 mutations are being reported and submitted to the current databases for identified Lynch syndrome mutations. Our data provides additional information to add to the genotype-phenotype spectrum for both MSH6 and PMS2 mutations.

Frameshift mutagenesis and microsatellite instability induced by human alkyladenine DNA glycosylase

Human alkyladenine DNA glycosylase (hAAG) excises alkylated purines, hypoxanthine, and etheno bases from DNA to form abasic (AP) sites. Surprisingly, elevated expression of hAAG increases spontaneous frameshift mutagenesis. By random mutagenesis of eight active site residues, we isolated hAAG-Y127I/H136L double mutant that induces even higher rates of frameshift mutation than does the wild-type hAAG; the Y127I mutation accounts for the majority of the hAAG-Y127I/H136L-induced mutator phenotype. The hAAG-Y127I/H136L and hAAG-Y127I mutants increased the rate of spontaneous frameshifts by up to 120-fold in S. cerevisiae and also induced high rates of microsatellite instability (MSI) in human cells. hAAG and its mutants bind DNA containing one and two base-pair loops with significant affinity, thus shielding them from mismatch repair; the strength of such binding correlates with their ability to induce the mutator phenotype. This study provides important insights into the mechanism of hAAG-induced genomic instability.

Secondary Prevention of Colorectal Cancer: Is There an Optimal Follow-up for Patients with Colorectal Cancer?

Secondary prevention of colorectal cancer, as opposed to primary prevention, indicates that a person has already had the disease and there are steps being taken to prevent cancer recurrence, usually as metachronous tumors. This generally involves annual surveillance with colonoscopy after surgical removal of the initial cancer if some aspect of the colon remains. However, some familial cases may involve other modalities, such as cyclooxygenase inhibitors, as an adjunct after the initial operation. Genetic testing in suspected familial cases may identify candidates for secondary prevention. The timing for secondary prevention is critical to prevent recurrent advanced disease, which is detrimental to patient survival. Recommendations are often empiric, but some cases are based on the biological behavior of the tumor. Close follow-up with a competent health care provider, such as a gastroenterologist, is necessary to help prevent recurrence.

Performance of clinical guidelines compared with molecular tumour screening methods in identifying possible Lynch syndrome among colorectal cancer patients: a Norwegian population-based study

Background:

The aim of this study was to assess the performance of the Revised Bethesda Guidelines (RBG) and the accuracy of the Amsterdam II criteria (AM II) in identifying possible Lynch syndrome (LS) compared with the results of molecular tumour testing.

Methods:

Tumours from 336 unselected colorectal cancer patients were analysed by three molecular tests (namely microsatellite instability (MSI), BRAF mutation and methylation of mismatch-repair genes), and patients were classified according to the RBG and AM II criteria.

Results:

A total of 87 (25.9%) patients fulfilled the RBG for molecular tumour analyses (MSI and/or immunohistochemistry), and the AM II identified 8 (2.4%) patients as having possible LS. Molecular tests identified 12 tumours (3.6%) as probable LS. The RBG identified 6 of the 12 patients (sensitivity 50%), whereas 5 of the 8 patients who fulfilled the AM II criteria were not likely to be LS, based on molecular tests (predictive value of positive test, 38%).

Interpretation:

Assuming a fairly high accuracy of molecular testing, the performance of the RBG in identifying patients with possible LS was poor, and the AM II criteria falsely identified a large proportion as having possible LS. This favours the use of molecular testing in the diagnosis of possible LS.

Genetic testing for hereditary colorectal cancer

This article focuses on genetic testing for hereditary colorectal cancer syndromes. Genetic testing is now available in North America for all of the known hereditary colorectal cancer genes. In addition, most of these tests have improved significantly in the past few years with the inclusion of techniques to detect large rearrangements. As a result, clinicians are in a better position than ever to help families with these syndromes to identify the underlying genetic cause. This identification will ensure that they receive appropriate management, and will enable their relatives to determine their precise risks and to tailor their cancer surveillance.

Immunohistochemistry as first-line screening for detecting colorectal cancer patients at risk for hereditary nonpolyposis colorectal cancer syndrome: a 2-antibody panel may be as predictive as a 4-antibody panel

The utility of immunohistochemical detection of DNA mismatch repair proteins in screening colorectal cancer for hereditary nonpolyposis colorectal cancer (HNPCC) is being widely investigated. Currently, in both research and clinical settings, a 4-antibody panel that includes the 4 most commonly affected proteins (MLH1, MSH2, MSH6, and PMS2) is being used generally. On the basis of the biochemical properties of these proteins, we hypothesized that a 2-antibody panel, comprising MSH6 and PMS2, would be sufficient to detect abnormalities in all 4 proteins. We tested this hypothesis on a series of 232 colorectal carcinoma samples derived from 2 patient cohorts: (1) a prospectively accrued series of patients who were judged to carry a higher-than-average risk for HNPCC based on the revised Bethesda guidelines (n=190); and (2) a retrospective series of patients who were 40 years of age or younger (n=42). Immunohistochemical stains were regarded as negative (protein lost), when there was no nuclear labeling in tumor cells (with positive internal control). Overall, 70 of the 232 tumors demonstrated loss of at least one protein. The most common abnormality was concurrent loss of MLH1 and PMS2 (observed in 17% of the cases), followed by concurrent loss of MSH2 and MSH6 (6%). All MLH1 and MSH2-abnormal cases were also abnormal for PMS2 and MSH6, respectively, whereas 9 of 50 (18%) PMS2 and 6 of 20 (30%) MSH6-abnormal cases showed only isolated loss of PMS2 or MSH6 (with normal staining for MLH1 and MSH2). As such, our findings provide evidence that a 2-antibody panel (PMS2 and MSH6) is as effective as the current 4-antibody panel in detecting DNA mismatch repair protein abnormalities. Such a cost-effective approach carries significant implication, as immunohistochemistry is being widely used as first-line screening for HNPCC.

Epitope-positive truncating MLH1 mutation and loss of PMS2: implications for IHC-directed genetic testing for Lynch syndrome

We assessed mismatch repair by immunohistochemistry (IHC) and microsatellite instability (MSI) analysis in an early onset endometrial cancer and a sister’s colon cancer. We demonstrated high-level MSI and normal expression for MLH1, MSH2 and MSH6. PMS2 failed to stain in both tumors, strongly implicating a PMS2 defect. This family did not meet clinical criteria for Lynch syndrome. However, early onset endometrial cancers in the proband and her sister, a metachronous colorectal cancer in the sister as well as MSI in endometrial and colonic tumors suggested a heritable mismatch repair defect. PCR-based direct exonic sequencing and multiplex ligation-dependent probe amplification (MLPA) were undertaken to search for PMS2 mutations in the germline DNA from the proband and her sister. No mutation was identified in the PMS2 gene. However, PMS2 exons 3, 4, 13, 14, 15 were not evaluated by MLPA and as such, rearrangements involving those exons cannot be excluded. Clinical testing for MLH1 and MSH2 mutation revealed a germline deletion of MLH1 exons 14 and 15. This MLH1 germline deletion leads to an immunodetectable stable C-terminal truncated MLH1 protein which based on the IHC staining must abrogate PMS2 stabilization. To the best of our knowledge, loss of PMS2 in MLH1 truncating mutation carriers that express MLH1 in their tumors has not been previously reported. This family points to a potential limitation of IHC-directed gene testing for suspected Lynch syndrome and the need to consider comprehensive MLH1 testing for individuals whose tumors lack PMS2 but for whom PMS2 mutations are not identified.

From bacteria to plants: a compendium of mismatch repair assays

Mismatch repair (MMR) system maintains genome integrity by correcting mispaired or unpaired bases which have escaped the proofreading activity of DNA polymerases. The basic features of the pathway have been highly conserved throughout evolution, although the nature and number of the proteins involved in the mechanism vary from prokaryotes to eukaryotes and even between humans and plants. Cells deficient in MMR genes have been observed to display a mutator phenotype characterized by an increased rate in spontaneous mutation, instability of microsatellite sequences and illegitimate recombination between diverged DNA sequences. Studies of the mutator phenotype have demonstrated a critical role for the MMR system in mutation avoidance and genetic stability. Here, we briefly review our current knowledge of the MMR mechanism and then focus on the in vivo biochemical and genetic assays used to investigate the function of the MMR proteins in processing DNA mismatches generated during replication and mitotic recombination in Escherichia coli, Saccharomyces cerevisiae, Homo sapiens and Arabidopsis thaliana. An overview of the biochemical assays developed to study mismatch correction in vitro is also provided.

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.

Hereditary ovarian carcinoma: heterogeneity, molecular genetics, pathology, and management

Hereditary ovarian cancer accounts for at least 5% of the estimated 22,000 new cases of this disease during 2009. During this same time, over 15,000 will die from malignancy ascribed to ovarian origin. The bulk of these hereditary cases fits the hereditary breast-ovarian cancer syndrome, while virtually all of the remainder will be consonant with the Lynch syndrome, disorders which are autosomal dominantly inherited. Advances in molecular genetics have led to the identification of BRCA1 and BRCA2 gene mutations which predispose to the hereditary breast-ovarian cancer syndrome, and mutations in mismatch repair genes, the most common of which are MSH2 and MLH1, which predispose to Lynch syndrome. These discoveries enable relatively certain diagnosis, limited only by their variable penetrance, so that identification of mutation carriers through a comprehensive cancer family history might be possible. This paper reviews the subject of hereditary ovarian cancer, with particular attention to its molecular genetic basis, its pathology, and its phenotypic/genotypic heterogeneity.

Genomic and epigenetic instability in colorectal cancer pathogenesis

Colorectal cancer arises as a consequence of the accumulation of genetic alterations (gene mutations, gene amplification, and so on) and epigenetic alterations (aberrant DNA methylation, chromatin modifications, and so on) that transform colonic epithelial cells into colon adenocarcinoma cells. The loss of genomic stability and resulting gene alterations are key molecular pathogenic steps that occur early in tumorigenesis; they permit the acquisition of a sufficient number of alterations in tumor suppressor genes and oncogenes that transform cells and promote tumor progression. Two predominant forms of genomic instability that have been identified in colon cancer are microsatellite instability and chromosome instability. Substantial progress has been made to identify causes of chromosomal instability in colorectal cells and to determine the effects of the different forms of genomic instability on the biological and clinical behavior of colon tumors. In addition to genomic instability, epigenetic instability results in the aberrant methylation of tumor suppressor genes. Determining the causes and roles of genomic and epigenomic instability in colon tumor formation has the potential to yield more effective prevention strategies and therapeutics for patients with colorectal cancer.

Hereditary colorectal cancer syndromes: familial adenomatous polyposis and lynch syndrome

Familial colorectal cancer (CRC) accounts for 10% to 20% of all cases of CRC. Two major autosomal dominant forms of heritable CRC are familial adenomatous polyposis (FAP) and Lynch syndrome (also known as hereditary nonpolyposis colorectal cancer). Along with the risk for CRC, both syndromes are associated with elevated risk for other tumors. Improved understanding of the genetic basis of these diseases has not only facilitated the identification and screening of at-risk individuals and the development of prophylactic or early-stage intervention strategies but also provided better insight into sporadic CRC. This article reviews the clinical and genetic characteristics of FAP and Lynch syndrome, recommended screening and surveillance practices, and appropriate surgical and nonsurgical interventions.

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.

Immunohistochemistry versus microsatellite instability testing for screening colorectal cancer patients at risk for hereditary nonpolyposis colorectal cancer syndrome. Part I. The utility of immunohistochemistry

The utility of immunohistochemical detection of DNA mismatch repair (MMR) protein in screening colorectal tumors for hereditary nonpolyposis colorectal cancer (HNPCC) syndrome has been the focus of much intensive research over the last 10 years. Particular attention has been given to the relative usefulness of immunohistochemistry (IHC) versus testing of tumor microsatellite instability (MSI). Earlier work that focused on mutL homolog 1 (MLH1) and mutS homolog 2 (MSH2) has created a false impression that IHC has a lower sensitivity than MSI testing in predicting germline mutation. More recent studies that included postmeiotic segregation increased 2 (PMS2) and MSH6, on the other hand, have demonstrated an IHC predictive value that is virtually equivalent to that of MSI testing. Such added value of PMS2 and MSH6 can be explained by the biological and biochemical properties of the MMR proteins. On the premise that IHC with PMS2 and MSH6 is as sensitive as MSI testing, given that IHC is easily available and generally inexpensive and, importantly, identifies the affected gene, it is reasonable to regard IHC as a more optimal first-line screening tool than MSI testing for identifying HNPCC. MSI testing can provide a fallback position in equivocal situations, while remaining an important research tool. However, for IHC to be used as a first-line screening test requires that both pathologists and clinicians be aware that IHC results may be construed as "genetic information," and that appropriate procedures should be established to ensure patient understanding and consent.

Hereditary colorectal cancer syndromes: molecular genetics, genetic counseling, diagnosis and management

Hereditary forms of colorectal cancer, as is the case with virtually all forms of hereditary cancer, show extensive phenotypic and genotypic heterogeneity, a phenomenon discussed throughout this special issue of Familial Cancer. Clearly, the family physician, oncology specialist, genetic counselor, and cancer geneticist must know fully the complexity of hereditary cancer syndromes, their differential diagnosis, in order to establish a diagnosis, direct highly-targeted surveillance and management, and then be able to communicate effectively with the molecular geneticist so that an at-risk patient's DNA can be tested in accord with the syndrome of concern. Thus, a family with features of the Lynch syndrome will merit microsatellite instability testing, consideration for immunohistochemistry evaluation, and mismatch repair gene testing, while, in contrast, a patient with FAP will require APC testing. However, other germline mutations, yet to be identified, may be important should testing for these mutations prove to be absent and, therein, unrewarding to the patient. Nevertheless, our position is that if the patient's family history is consistent with one of these syndromes, but a mutation is not found in the family, we still recommend the same surveillance and management strategies for patients from families with an established cancer-causing germline mutation. Our purpose in this paper is to provide a concise coverage of the major hereditary colorectal cancer syndromes, to discuss genetic counseling, molecular genetic evaluation, highly targeted surveillance and management, so that cancer control can be maximized for these high hereditary cancer risk patients.