Challenging cases encountered in colorectal cancer screening for Lynch syndrome reveal novel findings: nucleolar MSH6 staining and impact of prior chemoradiation therapy

Mismatch repair deficiency: how reliable is the two-antibody approach? A national real-life study

AIMS

Traditionally, mismatch repair (MMR) status is determined by a panel of four antibodies (MLH1, PMS2, MSH2, MSH6). If all proteins are retained, cases are MMR proficient (pMMR), while loss of one or more proteins is indicative of MMR deficiency (dMMR). This approach has been challenged in favour of a two-antibody approach, using PMS2 and MSH6 as a first screening. Their retainment is deemed sufficient to declare cases pMMR. In this study we aim to verify the validity of the two-antibody approach.

METHODS AND RESULTS

We performed a nationwide study in colorectal cancer (CRC) and endometrial cancer (EC) diagnosed between 2016 and 2023, including 47,657 patients to evaluate the two-antibody approach. In 0.17% and 0.4% of cases of CRC and EC, respectively, dMMR cases would be missed with the two-antibody approach. Subgroup analyses pointed towards slightly increased miss rates in younger patients (under the age of 50 years) in both groups and identified special subtypes (signet ring cell carcinoma, medullary carcinoma, and mucinous carcinoma in CRC and clear cell carcinoma in EC) with increased miss rates. For these specific subgroups, a low threshold should be used for further testing. In case of ambiguous or heterogeneous staining patterns, four antibodies should be used.

CONCLUSION

In general, the application of a two-antibody MMR testing strategy does not lead to considerable failure of dMMR identification and saves costs.

Mismatch repair protein status of non-neoplastic uterine and intestinal mucosa in patients with Lynch syndrome and double somatic mismatch repair protein mutations

Mismatch repair (MMR) protein-deficient non-neoplastic colonic crypts and endometrial glands (dMMR crypts and glands) have been reported as a unique marker of underlying Lynch syndrome (LS). However, no large studies have directly compared the frequency of detection in cases with double somatic (DS) MMR mutations. We retrospectively analyzed 42 colonic resection specimens (24 LS and 18 DS) and 20 endometrial specimens (9 LS and 11 DS), including 19 hysterectomies and 1 biopsy for dMMR crypts and glands. All specimens were from patients with known primary cancers, including colonic adenocarcinomas and endometrial endometrioid carcinomas (including 2 mixed carcinomas). Four blocks of normal mucosa away from the tumor were selected from most cases, as available. MMR immunohistochemistry specific to the primary tumor mutations was analyzed. dMMR crypts were found in 65% of LS and 0% of DS MMR-mutated colonic adenocarcinomas (P < .001). Most dMMR crypts were detected in the colon (12 of 15) compared to the ileum (3 of 15). dMMR crypts showed single and grouped losses of MMR immunohistochemical expression. dMMR glands were found in 67% of LS and 9% (1 of 11) of DS endometrial cases (P = .017). Most dMMR glands were found in the uterine wall, with 1 LS and 1 DS case exhibiting dMMR glands in the lower uterine segment. Most cases exhibited multifocal and grouped dMMR glands. No morphologic atypia was identified in dMMR crypts or glands. Overall, we demonstrate that dMMR crypts and glands are highly associated with underlying LS, while being rarer in those with DS MMR mutations.

Patterns of DNA mismatch repair protein expression for primary and recurrent colorectal cancer at an advanced surgical unit: A retrospective audit

AIM

Lynch syndrome is an inherited cancer syndrome associated with an increased lifetime risk of colorectal cancer (CRC) and characterized by germline mutations to one of four DNA mismatch repair (MMR) genes. Immunohistochemical (IHC) testing is used to screen for Lynch syndrome; however, despite routine completion following resection of primary CRC, it is only variably completed following resection of recurrent disease. This may be significant, as MMR protein expression can change from primary to recurrent CRC. The primary aim of this study is to investigate how MMR profiles change from primary to recurrent CRC; the secondary aim is to assess rates of MMR testing of primary and recurrent disease.

METHOD

We conducted a retrospective analysis of patients undergoing surgery for recurrent CRC from 2018-19 at a high-volume institution. MMR profiles were obtained following both primary and recurrent resection of CRC, and MMR protein expression was evaluated from both time points.

RESULTS

A total of 107 patients met the inclusion criteria and IHC results were obtained for both primary and recurrent resections in 85 cases. MMR profiles changed in nine patients (10.6%), with a loss of staining from primary to recurrent disease in six (7.1%) and a gain of staining in three (3.5%). IHC testing was completed following 88.7% of primary and 39.3% of recurrent resections.

CONCLUSION

MMR profiles can change from primary to recurrent CRC and repeat MMR testing for recurrent CRC is completed in only a minority of cases.

Challenges and Therapeutic Opportunities in the dMMR/MSI-H Colorectal Cancer Landscape

About 5 to 15% of all colorectal cancers harbor mismatch repair deficient/microsatellite instability-high status (dMMR/MSI-H) that associates with high tumor mutation burden and increased immunogenicity. As a result, and in contrast to other colorectal cancer phenotypes, a significant subset of dMMR/MSI-H cancer patients strongly benefit from immunotherapy. Yet, a large proportion of these tumors remain unresponsive to any immuno-modulating treatment. For this reason, current efforts are focused on the characterization of resistance mechanisms and the identification of predictive biomarkers to guide therapeutic decision-making. Here, we provide an overview on the new advances related to the diagnosis and definition of dMMR/MSI-H status and focus on the distinct clinical, functional, and molecular cues that associate with dMMR/MSI-H colorectal cancer. We review the development of novel predictive factors of response or resistance to immunotherapy and their potential application in the clinical setting. Finally, we discuss current and emerging strategies applied to the treatment of localized and metastatic dMMR/MSI-H colorectal tumors in the neoadjuvant and adjuvant setting.

Concurrent loss of MLH1, PMS2 and MSH6 immunoexpression in digestive system cancers indicating a widespread dysregulation in DNA repair processes

Immunohistochemical analysis of mismatch repair (MMR) protein expression is widely used to identify tumors with a deficient MMR (dMMR). MMR proteins (MLH1/PMS2 and MSH2/MSH6) work as functional heterodimers, which usually leads to the loss of expression in only one functional MMR heterodimer. Recently, there have been studies showing the simultaneous loss of immunoexpression in proteins of both heterodimers. Yet, this phenomenon has been rarely investigated. In this study, we retrospectively considered cases of different digestive system cancers (gastric cancer, ampullary cancer, small bowel cancer, colorectal cancer), which were immunohistochemically tested for dMMR within a 4-year period at our university hospital (n=352). Of the 103 cases showing dMMR, 5 cases (1.4% of all, 5.1% of dMMR cases) showed a concurrent loss of MLH1, PMS2 and MSH6 immunoexpression, whereas in the other 98 dMMR cases only one MMR heterodimer was affected. MLH1^-/PMS2^-/MSH6^- cancer cases almost arose throughout the entire digestive tract: from the gastric antrum to the left colic flexur. To provide a comprehensive molecular characterization of this MLH1^-/PMS2^-/MSH6^- immunophenotype, tumors were analyzed for microsatellite instability, MLH1 promotor hypermethylation and BRAF exon 15 status. Furthermore, we performed next-generation sequencing focusing on genes related to DNA repair. Here, we could detect pathogenic germline variants as well as multiple sporadic mutations in different genes involved in MMR and homologous recombination repair (HRR) respectively. The affected MMR/HRR-related genes were: ATM, BARD1, BRCA1, CDK12, CHEK1, CHEK2, FANCA, MLH1, MSH6, PALB2, TP53. Considering the biologic function of HRR/MMR proteins as potential drug targets and the low frequency of most of these mutations in digestive system cancers in general, their common occurrence in our MLH1^-/PMS2^-/MSH6^- cases seems to be even more noteworthy, highlighting the need for recognition, awareness and further investigation of this unusual IHC staining pattern.

Immunohistochemistry in gynecologic carcinomas: Practical update with diagnostic and clinical considerations based on the 2020 WHO classification of tumors

This review provides an update on immunohistochemistry applications-diagnostic, prognostic, and predictive-in the pathology evaluation of gynecologic carcinomas. The 5th edition of the WHO Classification of Female Genital Tumors introduced important changes in the diagnostic classification of lower genital tract, endometrial, and ovarian carcinomas, with major influence on the routine pathology practice. Lower genital tract carcinomas and their precursor lesions are now classified based on their human papillomavirus (HPV)-associated and HPV-independent pathogenesis, reflecting the clinically significant prognostic differences and impacting the therapeutic decision-making. Immunohistochemical markers have an increasing role in the pathology evaluation of endometrial carcinomas: in addition to their traditional use in the differential diagnosis and histologic subtyping, they have also been recently advocated for prognostic classification as surrogates for the TCGA (The Cancer Genome Atlas) molecular groups. New entities - mesonephric-like adenocarcinoma and gastric (gastrointestinal)-type mucinous adenocarcinoma of the endometrium - have also been added and often require immunostains for diagnostic confirmation. Ovarian carcinomas frequently show overlapping morphologic patterns and heterogeneous appearance within the same tumor, necessitating immunohistochemical work-up. Beyond diagnostic applications, there is increasing clinical demand for screening of inherited cancer syndromes, prediction of prognosis and guiding targeted therapy. Practical issues and pitfalls related to mismatch repair protein immunohistochemistry, HER2, and PD-L1 testing are also discussed.

Identifying mismatch repair-deficient colon cancer: near-perfect concordance between immunohistochemistry and microsatellite instability testing in a large, population-based series

AIMS

Establishing the mismatch repair (MMR) status of colorectal cancers is important to enable the detection of underlying Lynch syndrome and inform prognosis and therapy. Current testing typically involves either polymerase chain reaction (PCR)-based microsatellite instability (MSI) testing or MMR protein immunohistochemistry (IHC). The aim of this study was to compare these two approaches in a large, population-based cohort of stage 2 and 3 colon cancer cases in Northern Ireland.

METHODS AND RESULTS

The study used the Promega pentaplex assay to determine MSI status and a four-antibody MMR IHC panel. IHC was applied to tumour tissue microarrays with triplicate tumour sampling, and assessed manually. Of 593 cases with available MSI and MMR IHC results, 136 (22.9%) were MSI-high (MSI-H) and 135 (22.8%) showed abnormal MMR IHC. Concordance was extremely high, with 97.1% of MSI-H cases showing abnormal MMR IHC, and 97.8% of cases with abnormal IHC showing MSI-H status. Under-representation of tumour epithelial cells in samples from heavily inflamed tumours resulted in misclassification of several cases with abnormal MMR IHC as microsatellite-stable. MMR IHC revealed rare cases with unusual patterns of MMR protein expression, unusual combinations of expression loss, or secondary clonal loss of expression, as further illustrated by repeat immunostaining on whole tissue sections.

CONCLUSIONS

MSI PCR testing and MMR IHC can be considered to be equally proficient tests for establishing MMR/MSI status, when there is awareness of the potential pitfalls of either method. The choice of methodology may depend on available services and expertise.

Variable Expression of MSH6 in Endometrial Carcinomas With Intact Mismatch Repair and With MLH1 Loss Due to MLH1 Methylation

Immunohistochemistry for mismatch repair proteins MLH1, MSH2, MSH6, and PMS2 is an effective screen to detect individuals at risk for Lynch syndrome. College of American Pathologists guidelines stipulate that protein expression should be reported as present versus absent, as most patients with germline mutations in a mismatch repair gene have complete loss of protein expression in tumor cells. A similar approach is employed to screen for cancer patients eligible for immune checkpoint blockade. This "all or none" interpretive approach ignores substantial evidence that mismatch repair may be more finely regulated by other mechanisms. We have observed clinically that MSH6 expression is variable, even in carcinomas that are overall considered positive for MSH6 expression. A proof-of-principle study was therefore designed to more rigorously quantify the protein expression of MSH6 and its binding partner, MSH2, using image analysis applied to age-matched endometrioid grade 2 subsets that were either mismatch repair intact or MLH1-deficient due to MLH1 gene methylation. In both endometrioid groups, MSH6 expression was significantly lower than MSH2 expression. MSH6 expression increased in higher grade, mismatch repair intact serous carcinomas, but it was still significantly lower than that for MSH2. MSH2 expression was consistently high across the 3 different tumor groups. These results suggest that MSH6 expression is subject to wide fluctuations in expression, even when overall its expression is considered intact. While such fluctuations are likely not relevant for Lynch syndrome screening, they may be more impactful when considering patients eligible for immune checkpoint blockade.

Unexpected expression of mismatch repair protein is more commonly seen with pathogenic missense than with other mutations in Lynch syndrome

It has been observed that some patients with colorectal cancer due to germline or double somatic pathogenic variants in the mismatch repair (MMR) genes may have intact protein expression in their tumors as assessed by immunohistochemistry (IHC). This has been speculated to occur more frequently in Lynch syndrome (LS) cases due to pathogenic missense mutations, leading to expression of a full-length but nonfunctional protein with retained antigenicity. Our goals were to study the frequency of unexpected MMR expression in colorectal cancers among LS cases with missense mutations, LS cases with truncating mutations, as well as cases with double somatic MMR mutations and evaluate if the unexpected MMR expression is more common in certain categories. IHC slides were available for 82 patients with MMR deficiency without methylation, which included 56 LS cases and 26 double somatic MMR mutation cases. Sixteen of 82 MMR-defective cases showed unexpected MMR expression, with 10 cases showing tumor staining weaker than the control and 6 cases (7%) showing intact staining. Unexpected MMR expression was most commonly seen with LS cases with missense mutations (4 of 9, 44%), followed by MMR double somatic mutation cases (7 of 26, 27%), and finally by LS cases with truncating mutations (5 of 47, 11%). Cautious interpretation of MMR IHC is advised when dealing with tumor staining that is weaker than the control regardless of the percentage of tumor staining as these cases may harbor pathogenic MMR gene mutations. Missense mutations may account for some LS cases that may be missed by IHC alone. Strict adherence to proper interpretation of IHC with attention to staining intensity and the status of heterodimer partner protein will prevent many potential misses.

Frequent loss of mutation-specific mismatch repair protein expression in nonneoplastic endometrium of Lynch syndrome patients

Lynch syndrome is most often caused by a germline mutation in one of four DNA mismatch repair (MMR) genes (MLH1, PMS2, MSH2, or MSH6) or EPCAM and is associated with a significantly increased risk of endometrial cancer in affected women. Although universal screening of endometrial cancer for Lynch syndrome is becoming increasingly common by various algorithms using MMR immunohistochemistry and/or microsatellite instability testing by PCR, establishing the diagnosis of Lynch syndrome can be still challenging. MMR-deficient nonneoplastic colonic crypts have been recently described in Lynch syndrome patients with colorectal carcinoma, and have been proposed to be a novel indicator of Lynch syndrome. Presence of MMR-deficient nonneoplastic endometrial glands have not yet been systematically evaluated in Lynch syndrome patients. We performed MMR protein immunohistochemistry in prophylactic hysterectomies and endometrial curettings/biopsies from 27 patients with known Lynch syndrome confirmed by germline mutation analysis. A total of 56 control benign endometrial tissues were also analyzed, and included benign endometrium adjacent to MMR-deficient sporadic (MLH1 promoter hypermethylated) endometrial carcinoma (n = 9), adjacent to MMR-intact sporadic endometrial carcinoma (n = 27), and normal endometrium from hysterectomies performed for benign disease (n = 20). MMR protein deficient nonneoplastic endometrial glands were identified in 70% (19 of 27) of Lynch syndrome patients. In all 19 cases the MMR protein loss was specific for the patients' known germline mutation. None of the control cases showed loss of MMR protein expression in nonneoplastic endometrium. Our findings suggest that MMR-deficient nonneoplastic endometrial glands may be a unique, specific marker of Lynch syndrome, and may provide an important insight into the pathogenesis of Lynch syndrome-associated endometrial cancer. Evaluation of MMR protein expression of benign background endometrium in endometrial cancer patients may be further explored as a possible useful addition to the Lynch syndrome screening algorithm.

Mismatch Repair Protein Deficiency/Microsatellite Instability Is Rare in Cholangiocarcinomas and Associated With Distinctive Morphologies

OBJECTIVES

Although germline mutations of mismatch repair (MMR) genes (Lynch syndrome) are not typically associated with cholangiocarcinomas, the US Food and Drug Administration recently approved the use of pembrolizumab in patients with advanced solid tumors at all sites that show MMR deficiency or associated high microsatellite instability.

METHODS

We analyzed 96 cases of intra- and extrahepatic cholangiocarcinomas for morphology using H&E and for MMR status using immunohistochemical staining. We submitted any results with MMR loss for microsatellite instability testing.

RESULTS

We found that 6% of samples showed MMR deficiency. The best predictive factor was a nontypical infiltrating pattern of invasion (P < .0001). No patients with MMR deficiency had a history of a cancer typically associated with Lynch syndrome.

CONCLUSIONS

Solid, mucinous, or signet-ring appearance of a cholangiocarcinoma should prompt MMR testing for immunotherapy options but should not necessarily raise concern about Lynch syndrome.

Molecular pathology of Lynch syndrome

Lynch syndrome (LS) is characterised by predisposition to colorectal, endometrial, and other cancers and is caused by inherited pathogenic variants affecting the DNA mismatch repair (MMR) genes MLH1, MSH2, MSH6, and PMS2. It is probably the most common predisposition to cancer, having an estimated prevalence of between 1/100 and 1/180. Resources such as the International Society for Gastrointestinal Hereditary Cancer's MMR gene variant database, the Prospective Lynch Syndrome Database (PLSD), and the Colon Cancer Family Register (CCFR), as well as pathological and immunological studies, are enabling advances in the understanding of LS. These include defined criteria by which to interpret gene variants, the function of MMR in the normal control of apoptosis, definition of the risks of the various cancers, and the mechanisms and pathways by which the colorectal and endometrial tumours develop, including the critical role of the immune system. Colorectal cancers in LS can develop along three pathways, including flat intramucosal lesions, which depend on the underlying affected MMR gene. This gives insights into the limitations of colonoscopic surveillance and highlights the need for other forms of anti-cancer prophylaxis in LS. Finally, it shows that the processes of autoimmunisation and immunoediting fundamentally constrain the development of tumours in LS and explain the efficacy of immune checkpoint blockade therapy in MMR-deficient tumours. © 2020 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Microsatellite Stable Colorectal Cancer With an Immunogenic Phenotype: Challenges in Diagnosis and Treatment

BACKGROUND

Patients with deficient microsatellite mismatch repair (dMMR) colorectal cancer (CRC) may respond to immune checkpoint inhibition (ICI), whereas patients with microsatellite-stable (MSS) CRC have not demonstrated response. However, a proportion of MSS tumors display histomorphologic features characteristic of dMMR tumors consistent with an increased antigenicity. Therefore, a subset of patients with CRC not currently receiving ICI treatment may derive benefit from ICI therapy. We review tumors in which the histologic features suggestive of dMMR were in disagreement with the DNA mismatch repair proteins obtained by immunohistochemistry (IHC). Possible causes of such disagreement are discussed.

MATERIALS AND METHODS

Three patients with CRC suggestive of histomorphologic immunogenicity underwent evaluation by IHC staining for mismatch repair (MMR) status, next-generation sequencing assays, and/or polymerase chain reaction.

RESULTS

Findings compatible with an immunogenic response were similarly observed in all patients. Case 1 highlighted the limiting factors inherent to IHC staining for MMR status: a biopsy initially interpreted as MSS was subsequently interpreted as being dMMR. Case 2 examined the challenges in reconciling histologic characteristics traditionally associated with dMMR CRCs but ultimately determined to be MSS. Case 3 examined the microsatellite instability of CRC resulting from MLH1-methylation and/or MSH6 mutation.

CONCLUSIONS

We demonstrated the challenges in establishing MMR status when confronted with conflicting results from histology, IHC, polymerase chain reaction, and next-generation sequencing. Given that dMMR status has been shown to be a biomarker for ICI responsiveness, the importance of accurate identification is critical.

MSH6 immunohistochemical heterogeneity in colorectal cancer: comparative sequencing from different tumor areas

Mismatch repair protein (MMR) immunohistochemistry is an important tool in screening for Lynch syndrome in colorectal cancer patients. Unusual staining patterns such as heterogeneous MSH6 staining have been reported in colorectal and endometrial cancers. We aim to better understand MSH6 staining heterogeneity in colorectal cancer by comparative sequencing of different tumor areas for MMR and DNA polymerase mutations. Whole-section slides of 1754 colorectal cancers were reviewed for heterogeneous MSH6 staining, defined as discrete tumor areas with abrupt loss of staining juxtaposed to tumor areas with retained staining. Nine cases (0.05%) demonstrated heterogeneous MSH6 staining; none received neoadjuvant therapy prior to the specimen collection. The area of tumor with loss of MSH6 expression ranged from 5% to 60% (average 22%). Four cases had enough tissue remaining in both retained and lost MSH6 areas to perform tumor sequencing on both areas. All 9 cases were negative for MSH6 germline mutation; MSH6 heterogeneous staining was seen in tumors with MLH1 or PMS2 abnormalities (6 cases of MLH1 methylation, 2 PMS2 germline mutation, 1 MLH1 germline mutation). In addition, case 1 also had a somatic POLD1 exonuclease domain mutation (p.Y405C) in the MSH6 loss area but not in the intact area. We recommend reporting MSH6 heterogeneous pattern as MSH6 staining is present with a comment stating that the heterogeneous pattern typically does not indicate germline mutation in MSH6 but is commonly associated with abnormality in another MMR gene such as MLH1 or PMS2, or even other DNA repair genes such as DNA polymerase.

Pure Discrete Punctate Nuclear Staining Pattern for MLH1 Protein Does Not Represent Intact Nuclear Expression

Immunohistochemical staining for DNA mismatch repair (MMR) proteins is commonly used to screen for Lynch syndrome. Several laboratories have noticed a discrete punctate nuclear staining pattern for MLH1 that caused confusion in interpretation. This study was designed to investigate whether this particular staining pattern represents intact nuclear expression of MLH1. MMR proteins immunostaining and follow-up testing in 161 consecutive colorectal adenocarcinoma cases (86 biopsies, 75 resections) were retrospectively reviewed. Both discrete punctate nuclear staining and diffuse nuclear staining patterns for MLH1 were observed in internal control cells in 76 biopsies and 27 resections. Only diffuse nuclear staining was seen in the remaining 10 biopsies and 48 resections (P < .0001). Pure discrete punctate nuclear staining pattern for MLH1 was observed in 11 tumors (9 biopsies, 2 resections), and completely negative staining was seen in 13 tumors (2 biopsies, 11 resections; P = .003). Those 24 tumors (21 patients) invariably showed loss of PMS2. Three patients whose biopsies showed pure punctate staining for MLH1 underwent repeat testing on resections: 1 retained the punctate staining and 2 showed complete loss of MLH1. Nine patients who showed loss of PMS2 and pure punctate MLH1 staining underwent molecular testing: 4 had BRAF V600E mutations and 1 had MLH1 gene mutation. Our data showed that discrete punctate nuclear staining for MLH1 is more commonly seen in biopsy specimens. Pure discrete punctate staining pattern is paired with loss of PMS2 expression and may be associated with BRAF or MLH1 gene mutation, thus it should not be interpreted as intact nuclear expression.

Role of intravoxel incoherent motion MRI in preoperative evaluation of DNA mismatch repair status in rectal cancers

AIM

To explore the role of intravoxel incoherent motion (IVIM) magnetic resonance imaging (MRI) in evaluating DNA mismatch repair (MMR) status of rectal cancers preoperatively.

MATERIALS AND METHODS

Seventy-six patients with a diagnosis of rectal cancer confirmed at endoscopic biopsy were enrolled prospectively and underwent IVIM MRI before surgery.

RESULTS

The perfusion fraction (f) values of MMR proteins (MMRP) positive rectal cancers were significantly higher than negative cancers. The f values could differentiate MMRP positive rectal cancers from negative cancers with an area under the curve (AUC) of 0.695. The vascular endothelial growth factor (VEGF) and vascular endothelial growth factor receptor 2 (VEGFR2) expression rates of positive MMRP rectal cancers were significantly higher than negative cancers.

CONCLUSION

This pilot study indicated that the f value derived from IVIM MRI differed significantly between rectal cancers with different MMRP expression levels, which might be involved with different VEGF and VEGFR2 expression rates.

Granular dot-like staining with MLH1 immunohistochemistry is a clone-dependent artefact

Immunohistochemistry (IHC) for DNA mismatch repair proteins MLH1, PMS2, MSH2, and MSH6 is used for microsatellite instability (MSI) screening in colorectal carcinoma (CRC) and endometrial carcinoma (EC). Loss of PMS2, with retained MLH1 staining occurs in germline mutations of PMS2 gene, and is an indication for genetic testing. We report a pitfall of immunohistochemical interpretation in an EC, initially regarded as MLH1-positive and PMS2-negative. Review of the MLH1-IHC (M1-clone) revealed a granular, dot-like, nuclear staining. On repeating the MLH1-IHC with a different clone (ES05-clone), complete negativity was noted, and on molecular testing, MLH1 promotor methylation was detected. The dot-like pattern was therefore adjudged a clone-dependent artefact. On reviewing the archived MLH1-IHC slides, we observed the same dot-like pattern in two CRCs; in both cases the M1-clone had been used. Awareness of this artefact may prevent reporting errors, and unnecessary referrals for germline mutation testing.

Mismatch Repair System Deficiency Is Associated With Response to Neoadjuvant Chemoradiation in Locally Advanced Rectal Cancer

PURPOSE

Defective mismatch repair system (dMMR) has been shown to have a favorable impact on outcome in patients with colorectal cancer treated with surgery or immunotherapy, with adjuvant chemotherapy being discouraged unless there is nodal involvement. Its impact on radiosensitivity is unknown in patients with colorectal cancer.

METHODS AND MATERIALS

Patients treated for locally advanced rectal cancer between 2000 and 2016 were studied. Reported points included age, sex, clinical and radiologic tumor stages at diagnosis, modalities of neoadjuvant treatment, posttreatment pathologic staging, tumor regression score, and local, distant relapse-free, and overall survival. An inverse probability of treatment weighting propensity score analysis was performed to evaluate the association of mismatch repair proficiency with surgical and clinical outcomes.

RESULTS

Among the 296 patients included, 23 (7.8%) had dMMR. Median follow-up was 43.0 months (interquartile range, 27.9-66.7). Patients with dMMR were significantly younger than the others. After inverse probability of treatment weighting propensity score matching, dMMR patients had higher pathologic downstaging rate (P < .0001), higher tumor regression grade (P = .024), and a longer recurrence-free survival (P < .0001).

CONCLUSIONS

dMRR was associated with significant tumor downstaging after neoadjuvant chemoradiation and with increased recurrence-free survival. dMMR patients may have more radiosensitive tumors.

Site-specific Differences in Colonic Adenocarcinoma: KRAS Mutations and High Tumor Budding Are More Frequent in Cecal Adenocarcinoma

Recent literature indicates that adenocarcinomas of the cecum differ with respect to molecular alterations compared with noncecal proximal colon adenocarcinomas and that cecal tumor site may be a prognostically relevant variable. We compared molecular alterations, histopathologic features, and disease-specific survival in a series of 328 colonic adenocarcinomas identified over a 2-year period and stratified by tumor location (cecum, right colon, and left colon). Overall, cecal adenocarcinomas demonstrated the highest frequency of molecular abnormalities with 74% harboring either a KRAS exon 2 or 3 mutation, a BRAF mutation, or DNA mismatch repair protein deficiency. KRAS mutations were more frequently seen in the cecum compared with all other tumor sites (P=0.03). KRAS mutations were identified in 46% of cecal adenocarcinomas compared with only 25% of adenocarcinomas of the right colon (P=0.004). Cecal adenocarcinomas more frequently displayed adverse histopathologic features, in particular high tumor budding (31%), compared with tumors of the right colon (18%; P=0.04) and tumors of the left colon (17%; P=0.02). Overall stage was the most important independent predictor of disease-specific survival in the multivariable analysis; however, cecal tumor site and high tumor budding were also predictive of poor survival, particularly in patients with stage III or IV tumors. In conclusion, cecal adenocarcinomas are characterized by a high frequency of KRAS mutations compared with noncecal right colon tumors, frequently display high tumor budding, and may be a prognostically relevant variable, particularly in patients with stage III or IV disease.

Comprehensive Paired Tumor/Germline Testing for Lynch Syndrome: Bringing Resolution to the Diagnostic Process

PURPOSE

The current diagnostic testing algorithm for Lynch syndrome (LS) is complex and often involves multiple follow-up germline and somatic tests. We aimed to describe the results of paired tumor/germline testing performed on a large cohort of patients with colorectal cancer (CRC) and endometrial cancer (EC) to better determine the utility of this novel testing methodology.

MATERIALS AND METHODS

We retrospectively reviewed a consecutive series of patients with CRC and EC undergoing paired tumor/germline analysis of the LS genes at a clinical diagnostic laboratory (N = 702). Microsatellite instability, MLH1 promoter hypermethylation, and germline testing of additional genes were performed if ordered. Patients were assigned to one of five groups on the basis of prior tumor screening and germline testing outcomes. Results for each group are described.

RESULTS

Overall results were informative regarding an LS diagnosis for 76.1% and 60.8% of patients with mismatch-repair-deficient (MMRd) CRC and EC without and with prior germline testing, respectively. LS germline mutations were identified in 24.8% of patients in the group without prior germline testing, and interestingly, in 9.5% of patients with previous germline testing; four of these were discordant with prior tumor screening. Upon excluding patients with MLH1 promoter hypermethylation and germline mutations, biallelic somatic inactivation was seen in approximately 50% of patients with MMRd tumors across groups.

CONCLUSION

Paired testing identified a cause for MMRd tumors in 76% and 61% of patients without and with prior LS germline testing, respectively. Findings support inclusion of tumor sequencing as well as comprehensive LS germline testing in the LS testing algorithm. Paired testing offers a complete, convenient evaluation for LS with high diagnostic resolution.

A practical guide to biomarkers for the evaluation of colorectal cancer

Evaluation of microsatellite instability (MSI) of every colorectal cancer (CRC) is important for prognostic and therapeutic purposes, while molecular testing helps identify actionable targeted therapy for patients with metastatic disease. This review will discuss the biomarkers commonly encountered in the clinical evaluation of CRC, and practical issues regarding MSI screening, reporting, interpretation, molecular test indication, and specimen requirements.

Two-stain immunohistochemical screening for Lynch syndrome in colorectal cancer may fail to detect mismatch repair deficiency

Universal screening for Lynch syndrome in colorectal cancer is recommended, and immunohistochemistry for the mismatch repair proteins is commonly used. To reduce cost, some screen using only MSH6 and PMS2, with reflex to the partner stain if either are absent (two-stain method). An expression pattern revealing absent MSH2 and intact MSH6 is not expected, but could result in failed Lynch syndrome detection. We analyzed tumors with absent MSH2 but any degree of MSH6 expression to determine if the two-stain method could miss MSH2 mutations. One-thousand seven-hundred thirty colorectal cancer patients from the Ohio Colorectal Cancer Prevention Initiative underwent tumor screening using microsatellite instability and immunohistochemistry. The two-stain method was used for 1235 cases; staining for all four proteins was completed for 495 cases. The proportion of positive cells and staining intensity were reviewed for MSH6, as well as MSH2 when available. Patients with mismatch repair deficiency underwent next-generation sequencing of germline DNA for mismatch repair genes. If negative, tumor next-generation sequencing was performed to assess for somatic mutations. Overall, thirty-three (1.9%, 33/1730) MSH2-absent cases were identified. Of those, fourteen had no MSH6 expression but eight (0.5%, 8/1730) had ambiguous and eleven (0.6%, 11/1730) had convincing MSH6 expression that could have been interpreted as intact. Germline next-generation sequencing identified MSH2 mutations in 11/14 cases with absence of both stains, 7/8 cases with ambiguous MSH6 expression, and 9/11 cases with convincing MSH6 expression. All remaining cases, except one, had double somatic mutations. The two-stain method fails to detect some patients with Lynch syndrome: (1) significant staining weaker than the control may be incorrectly interpreted as intact MSH6, or (2) Weak or focal/patchy MSH6 can be retained with the absence of MSH2. Accordingly, we recommend the four-stain method be used for optimal Lynch syndrome screening detection.

DNA mismatch repair protein deficient non-neoplastic colonic crypts: a novel indicator of Lynch syndrome

Lynch syndrome is the most common form of hereditary colorectal carcinoma. However, establishing the diagnosis of Lynch syndrome is challenging, and ancillary studies that distinguish between sporadic DNA mismatch repair (MMR) protein deficiency and Lynch syndrome are needed, particularly when germline mutation studies are inconclusive. The aim of this study was to determine if MMR protein-deficient non-neoplastic intestinal crypts can help distinguish between patients with and without Lynch syndrome. We evaluated the expression of MMR proteins in non-neoplastic intestinal mucosa obtained from colorectal surgical resection specimens from patients with Lynch syndrome-associated colorectal carcinoma (n = 52) and patients with colorectal carcinoma without evidence of Lynch syndrome (n = 70), including sporadic MMR protein-deficient colorectal carcinoma (n = 30), MMR protein proficient colorectal carcinoma (n = 30), and "Lynch-like" syndrome (n = 10). MMR protein-deficient non-neoplastic colonic crypts were identified in 19 of 122 (16%) patients. MMR protein-deficient colonic crypts were identified in 18 of 52 (35%) patients with Lynch syndrome compared to only 1 of 70 (1%) patients without Lynch syndrome (p < 0.001). This one patient had "Lynch-like" syndrome and harbored two MSH2-deficient non-neoplastic colonic crypts. MMR protein-deficient non-neoplastic colonic crypts were not identified in patients with sporadic MMR protein-deficient or MMR protein proficient colorectal carcinoma. Our findings suggest that MMR protein-deficient colonic crypts are a novel indicator of Lynch syndrome, and evaluation for MMR protein-deficient crypts may be a helpful addition to Lynch syndrome diagnostics.

Rectal cancer and the pathologist

Examination of the rectum by pathologists is instrumental in the management of patients affected by rectal carcinoma. That role includes evaluation of multiple gross and microscopic features that convey prognostic implications. The analysis is based on the authors' experience handling rectal specimens along with review of the pertinent literature in these areas: margins of excision, quality of the mesorectum, diligence and techniques to sample lymph nodes, tumor budding, grading of residual amount of carcinoma after preoperative therapy, vascular/perineural invasion, and staging the tumor. Pathologists must communicate the findings in a clear manner. Evaluation of margins and completeness of mesorectum are markers of the quality of surgical excision. The number of lymph nodes obtained and examined is dependent in great part on the diligence of the pathologist finding them in the mesenteric adipose tissue. There are grades for budding and response to prior chemoradiation therapy. The location of vascular invasion (extramural vs. intramural) may predict aggressive behavior. Pathologists proactively are to choose sections of tumor for molecular testing. Meticulous macro- and microscopic evaluation of specimens for rectal carcinoma by pathologist is needed to determine an accurate assessment of staging and other prognostic factors. The modern pathologists play a pivotal part in the care and management of patients suffering from rectal adenocarcinoma. That role goes from the initial histological diagnosis to the gross and microscopic examination of the excised specimens. Based on that examination pathologists issue statements that not only evaluate the quality of the surgical procedure, but also through the application of molecular tests they give light on prognostic factors and information for therapeutic purposes.

Histologic Features Do Not Reliably Predict Mismatch Repair Protein Deficiency in Colorectal Carcinoma: The Results of a 5-Year Prospective Evaluation

Most major professional medical organizations advocate universal screening for Lynch syndrome in colorectal carcinoma; however, some allow for a selective screening approach based on clinicopathologic factors including assessment of histologic features of mismatch repair protein deficiency (MMRD). We performed a prospective evaluation for histopathologic features of MMRD in colorectal carcinomas that underwent universal screening for Lynch syndrome to evaluate the ability of histology to predict MMRD. In total, 947 resected colorectal carcinomas over a 5-year period were prospectively analyzed for histologic features of MMRD and for DNA mismatch repair protein abnormalities. Histologic features of MMRD were reported as present in 281 of 947 (30%) tumors with only 109 (39%) cases demonstrating MMRD by immunohistochemistry. Histologic features of MMRD had a sensitivity of 74% [95% confidence interval (CI), 66%-80%], specificity of 78% (95% CI, 75%-81%), positive predictive value of 39% (95% CI, 32%-44%), and negative predictive value of 94% (95% CI, 92%-96%). Histologic features of MMRD in left colon/rectal tumors had a significantly lower sensitivity of 56% (95% CI, 41%-77%) compared with right colon tumors (P=0.02). Histologic rereview identified that tumor-infiltrating lymphocytes (TILs) were most likely to be incorrectly reported as absent, and 72% of cases incorrectly assessed as lacking TILs demonstrated MMRD by immunohistochemistry. We demonstrate that histologic features of MMRD do not reliably predict the presence of MMRD by immunohistochemistry. Interpretative errors in the assessment of histologic features of MMRD occur, particularly for TILs and in tumors of the left colon/rectum.

Gynaecological neoplasms in common familial syndromes (Lynch and HBOC)

Recognising hereditary predisposition in a cancer patient has implications both for the patient and the patient's kindred. For the latter, cascade germline testing can reassure those not-at-risk family members while carriers can be enrolled in cancer screening and prevention programs that are medically effective and economically sustainable for health care systems. Furthermore, in many of these syndromes, ramifications of molecular phenotypes are increasing, and it is now emerging that, in addition, they convey prognostic and predictive information. Although cancer predisposition syndromes are rare, these molecular phenotypes also occur as somatic events in sporadic cancer settings. The information obtained from these molecular phenotypes, regardless of germline or somatic origin, is being incorporated into clinical management in view of their manifold significance. Thus, increasingly, bespoke management of cancer patients involves testing for both germline and somatic mutations in tumours. Lynch syndrome and BRCA-1 and BRCA-2-associated hereditary breast and ovarian cancer are hereditary cancer syndromes frequently involving the gynaecological tract but tumours associated with similar molecular alterations may also occur sporadically. Thus, the molecular phenotype of mismatch repair deficiency, microsatellite instability or hypermutator phenotype may be attributable to germline or somatic events. Similarly, homologous recombination deficiency or 'BRCAness' in ovarian cancers may be syndromic or sporadic. While hereditary syndromes are well recognised, the prognostic and predictive implications of these molecular phenotypes have only recently been elucidated and these aspects will finally ensure that molecular screening may become standard of care. Thus, nowadays pathologists are asked to designate the molecular phenotype of these cancers and then determine whether it is due to hereditary or sporadic causes.

Immunohistochemical Pitfalls: Common Mistakes in the Evaluation of Lynch Syndrome

At least 15% of colorectal cancers diagnosed in the United States are deficient in mismatch repair mechanisms. Most of these are sporadic, but approximately 3% of colorectal cancers result from germline alterations in mismatch repair genes and represent Lynch syndrome. It is critical to identify patients with Lynch syndrome to institute appropriate screening and surveillance for patients and their families. Exclusion of Lynch syndrome in sporadic cases is equally important because it reduces anxiety for patients and prevents excessive spending on unnecessary surveillance. Immunohistochemistry is one of the most widely used screening tools for identifying patients with Lynch syndrome.

Practical Immunohistochemistry in Neoplastic Pathology of the Gastrointestinal Tract, Liver, Biliary Tract, and Pancreas

CONTEXT

- Immunomarkers with diagnostic, therapeutic, or prognostic values have been increasingly used to maximize the benefits of clinical management of patients with neoplastic diseases of the gastrointestinal tract, liver, biliary tract, and pancreas.

OBJECTIVES

- To review the characteristics of immunomarkers that are commonly used in surgical pathology practice for neoplasms of the gastrointestinal tract, liver, biliary tract, and pancreas, and to summarize the clinical usefulness of immunomarkers that have been discovered in recent years in these fields.

DATA SOURCES

- Data sources include literature review, authors' research data, and personal practice experience.

CONCLUSIONS

- Immunohistochemistry is an indispensable tool for the accurate diagnosis of neoplastic diseases of the gastrointestinal tract, liver, biliary tract, and pancreas. Useful immunomarkers are available to help distinguish malignant neoplasms from benign conditions, determine organ origins, and subclassify neoplasms that are morphologically and biologically heterogeneous. Specific immunomarkers are also available to help guide patient treatment and assess disease aggressiveness, which are keys to the success of personalized medicine. Pathologists will continue to play a critical role in the discovery, validation, and application of new biomarkers, which will ultimately improve patient care.

Targeted sequencing of 36 known or putative colorectal cancer susceptibility genes

BACKGROUND

Mutations in several genes predispose to colorectal cancer. Genetic testing for hereditary colorectal cancer syndromes was previously limited to single gene tests; thus, only a very limited number of genes were tested, and rarely those infrequently mutated in colorectal cancer. Next-generation sequencing technologies have made it possible to sequencing panels of genes known and suspected to influence colorectal cancer susceptibility.

METHODS

Targeted sequencing of 36 known or putative CRC susceptibility genes was conducted for 1231 CRC cases from five subsets: (1) Familial Colorectal Cancer Type X (n = 153); (2) CRC unselected by tumor immunohistochemical or microsatellite stability testing (n = 548); (3) young onset (age <50 years) (n = 333); (4) proficient mismatch repair (MMR) in cases diagnosed at ≥50 years (n = 68); and (5) deficient MMR CRCs with no germline mutations in MLH1, MSH2, MSH6, or PMS2 (n = 129). Ninety-three unaffected controls were also sequenced.

RESULTS

Overall, 29 nonsense, 43 frame-shift, 13 splice site, six initiator codon variants, one stop codon, 12 exonic deletions, 658 missense, and 17 indels were identified. Missense variants were reviewed by genetic counselors to determine pathogenicity; 13 were pathogenic, 61 were not pathogenic, and 584 were variants of uncertain significance. Overall, we identified 92 cases with pathogenic mutations in APC,MLH1,MSH2,MSH6, or multiple pathogenic MUTYH mutations (7.5%). Four cases with intact MMR protein expression by immunohistochemistry carried pathogenic MMR mutations.

CONCLUSIONS

Results across case subsets may help prioritize genes for inclusion in clinical gene panel tests and underscore the issue of variants of uncertain significance both in well-characterized genes and those for which limited experience has accumulated.

Molecular genetics of microsatellite-unstable colorectal cancer for pathologists

Background

Microsatellite-unstable colorectal cancers (CRC) that are due to deficient DNA mismatch repair (dMMR) represent approximately 15% of all CRCs in the United States. These microsatellite-unstable CRCs represent a heterogenous group of diseases with distinct oncogenesis pathways. There are overlapping clinicopathologic features between some of these groups, but many important differences are present. Therefore, determination of the etiology for the dMMR is vital for proper patient management and follow-up.

Main body

Epigenetic inactivation of MLH1 MMR gene (sporadic microsatellite-unstable CRC) and germline mutation in an MMR gene (Lynch syndrome, LS) are the two most common mechanisms in the pathogenesis of microsatellite instability in CRC. However, in a subset of dMMR CRC cases that are identified by screening tests, no known LS-associated genetic alterations are appreciated by current genetic analysis. When the etiology for dMMR is unclear, it leads to patient anxiety and creates challenges for clinical management.

Conclusion

It is critical to distinguish LS patients from other patients with tumors due to dMMR, so that the proper screening protocol can be employed for the patients and their families, with the goal to save lives while avoiding unnecessary anxiety and costs. This review summarizes the major pathogenesis pathways of dMMR CRCs, their clinicopathologic features, and practical screening suggestions. In addition, we include frequently asked questions for MMR immunohistochemistry interpretation.

Neoadjuvant therapy in microsatellite-stable colorectal carcinoma induces concomitant loss of MSH6 and Ki-67 expression

Universal screening using immunohistochemistry for DNA mismatch-repair proteins (MLH1, MSH2, MSH6, and PMS2) is advocated by major professional medical organizations to identify Lynch syndrome-associated colorectal carcinoma. Loss of MSH6 expression independent of MSH2 expression has been reported in microsatellite-stable (MSS) colorectal carcinoma after neoadjuvant therapy. The mechanism remains unclear. We studied the immunohistochemical expression of MSH2, MSH6, and Ki-67 in MSS colorectal carcinoma with (n=50) or without (n=64) preoperative neoadjuvant therapy and Lynch syndrome-associated colorectal carcinoma with confirmed MSH6 germline mutation (n=3). Twelve of 50 MSS colorectal carcinoma postneoadjuvant resections demonstrated reduced MSH6 expression, with loss of expression ranging from 20% to 100% of tumor cells. Eight of 64 MSS colorectal carcinomas without neoadjuvant therapy also exhibited reduced MSH6 expression but to a lesser degree (10%-50% of tumor cells with loss of expression). In both subgroups, concomitant loss of MSH6 and Ki-67 expressions was demonstrated in the same tumor areas in consecutive tissue sections. However, 3 cases of Lynch syndrome-associated colorectal carcinoma due to germline MSH6 mutation revealed complete loss of MSH6 expression with discordant positive Ki-67 staining in the tumor cells. The MSH2-independent, Ki-67-related expression of MSH6 in colorectal carcinoma helps to explain the heterogeneous MSH6 staining in MSS colorectal carcinoma with or without neoadjuvant therapy.

Select biomarkers for tumors of the gastrointestinal tract: present and future

CONTEXT

Advances in molecular biomarkers of the gastrointestinal tract have contributed to a decline in the incidence of and mortality from diseases of the gastrointestinal tract. The discovery and clinical validation of new biomarkers are important to personalized cancer therapy, and numerous clinical trials are currently ongoing to help identify individualized therapy affecting these biomarkers and molecular mechanisms they represent. Distinct molecular pathways leading to cancers of the colorectum, esophagus, stomach, small bowel, and pancreas have been identified. Using biomarkers in these pathways to direct patient care, including selection of proper molecular testing for identification of actionable mutations and reporting the results of these biomarkers to guide clinicians and genetic counselors, is paramount.

OBJECTIVE

To examine and review select clinically actionable biomarkers of the colon, esophagus, stomach, small bowel, and pancreas, including present and future biomarkers with relevant clinical trials.

DATA SOURCES

Extensive literature review and practical and consultation experience of the authors.

CONCLUSIONS

Although numerous biomarkers have been identified and are currently guiding patient therapy, few have shown evidence of clinical utility in the management of patients with gastrointestinal cancers. Inconsistent results and discordant proposed algorithms for testing were identified throughout the literature; however, the potential for biomarkers to improve outcomes for patients with gastrointestinal cancer remains high. Continued advances through high-quality studies are needed.

Heterogenous mismatch-repair status in colorectal cancer

Abstract

Virtual Slides

The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/1771940323126788

Immunohistochemical detection of BRAF V600E mutant protein using the VE1 antibody in colorectal carcinoma is highly concordant with molecular testing but requires rigorous antibody optimization

The BRAF V600E mutation occurs in 15% of colorectal carcinomas (CRCs) and has important genetic, prognostic, and therapeutic implications. A monoclonal antibody (VE1) targeting the BRAF V600E mutant protein has become available with variable efficacy in literature reports. We investigated the utility of the VE1 antibody in detecting BRAF V600E mutant protein in two cohorts: (1) a retrospectively accrued series of 103 resected CRCs with (N = 57) and without (N = 46) known BRAF V600E mutation status by PCR and (2) a prospective series of 25 CRCs requiring BRAF analysis during routine screening for Lynch syndrome. All 74 cases with positive BRAF V600E mutation demonstrated cytoplasmic positivity with the VE1 antibody with most tumors (70/74, 95%) demonstrating moderate to strong staining. Of the 54 BRAF V600E-negative cases, 51/54 CRCs (94%) were negative with the VE1 antibody while 3 CRCs (6%) demonstrated weak cytoplasmic staining. The sensitivity and specificity of VE1 was 100% and 94%, respectively. Ten BRAF V600E-mutated CRCs had adjacent precursor lesions including 7 sessile serrated adenomas associated with CRCs with high-level microsatellite instability (MSI-H). All 10 precursor adenomas were positive for VE1 staining with the 7 sessile serrated adenomas maintaining preserved MLH1 expression. Our results indicate that VE1 immunohistochemistry is a useful surrogate for the detection of the BRAF V600E mutation in CRC, although weak staining must be evaluated by BRAF PCR analysis to exclude a false positive result. In addition, the BRAF V600E mutation appears to be an early event before the divergent development into MSS and MSI-H pathways.

Secondary mutation in a coding mononucleotide tract in MSH6 causes loss of immunoexpression of MSH6 in colorectal carcinomas with MLH1/PMS2 deficiency

Immunohistochemical staining for DNA mismatch repair proteins may be affected by various biological and technical factors. Staining variations that could potentially lead to erroneous interpretations have been recognized. A recently recognized staining variation is the significant reduction of staining for MSH6 in some colorectal carcinomas. The frequency and specific characteristics of this aberrant MSH6 staining pattern, however, have not been well analyzed. In this study of 420 colorectal carcinoma samples obtained from patients fulfilling the Revised Bethesda Guidelines, we detected 9 tumors (2%) showing extremely limited staining for MSH6 with positive staining present in <5% of the tumor cells. Our analyses showed that these tumors belonged to two distinct categories: (1) MLH1 and/or PMS2 protein-deficient carcinomas (n=5, including 1 with a pathogenic mutation in PMS2); and (2) MLH1, PMS2 and MSH2 normal but with chemotherapy or chemoradiation therapy before surgery (n=4). To test our hypothesis that somatic mutation in the coding region microsatellite of the MSH6 gene might be a potential underlying mechanism for such limited MSH6 staining, we evaluated frameshift mutation in a (C)(8) tract in exon 5 of the MSH6 gene in seven tumors that had sufficient DNA for analysis, and detected mutation in four; all four tumors belonged to the MLH1/PMS2-deficient group. In conclusion, our data outline the main scenarios where significant reduction of MSH6 staining is more likely to occur in colorectal carcinoma, and suggest that somatic mutations of the coding region microsatellites of the MSH6 gene is an underlying mechanism for this staining phenomenon in MLH1/PMS2-deficient carcinomas.

"Null pattern" of immunoreactivity in a Lynch syndrome-associated colon cancer due to germline MSH2 mutation and somatic MLH1 hypermethylation

Lynch syndrome accounts for approximately 3% of newly diagnosed colorectal cancers and is caused by germline defects in DNA mismatch repair genes. Screening of patients for Lynch syndrome can be done by immunohistochemical staining for a panel of mismatch repair proteins and/or DNA testing for microsatellite instability. We describe a unique "null" immunophenotype in a Lynch syndrome associated colon cancer in a 71-year-old woman who also had a personal history of ureteral cancer and a strong family history of various malignancies. Immunohistochemical stains for MLH1, MSH2, PMS2, and MSH6 were completely negative in the tumor cells, with positive staining in stromal and inflammatory cells. Mutation analysis using peripheral blood showed a germline G587R mutation in the MSH2 gene. Further testing revealed the tumor to be positive for MLH1 promoter hypermethylation. Normal colonic mucosa adjacent to the tumor was negative for MLH1 promoter methylation. The lack of immunostaining for any of the 4 DNA mismatch repair proteins in this extremely unusual case was, therefore, related to a germline MSH2 mutation and somatic MLH1 promoter hypermethylation.

Analysis of DNA mismatch repair proteins expression and BRAF V600E mutation in a subset of early- and late-onset colorectal carcinoma patients in Mexico

BACKGROUND AND AIMS

A third of colorectal carcinomas (CRC) affect patients <50 years of age. Fifteen percent of CRC cases with microsatellite instability are due to inherited germ-line mutations in DNA mismatch repair genes. The rest have an epigenetic hypermethylation of the MLH1 promoter in whom the BRAF V600E mutation is a common hallmark. Immunohistochemistry helps to classify colorectal cancers with 100% specificity and 92% sensitivity. We undertook this study to determine if age is a risk factor for defective MMR protein expression and BRAF mutations in our population and to compare these results with the histopathological tumor features.

METHODS

Immunohistochemistry for MLH1 and MSH2 and RT-PCR BRAF V600E mutation was performed on tissue specimens from 57 patients <50 years of age. Data on age, gender, tumor location, histology, depth of infiltration, and the presence of metastatic lymph nodes were collected. Forty eight patients >50 years of age were used as a control group. A statistical analysis using ANOVA, χ(2), and Spearman's rho test were performed.

RESULTS

Absent MMR protein expression was more prevalent in patients <50 years of age. No BRAF V600E mutations were detected in either group. Medullary and mucinous types were more prevalent among young patients, whereas intestinal type was more frequent in older patients (p = 0.0008). No differences were found regarding clinicopathological stages between groups.

CONCLUSIONS

We found an association between young age and defective MMR expression. No V600E BRAF mutations were detected in either group.