Immunohistochemistry and microsatellite instability testing for selecting MLH1, MSH2 and MSH6 mutation carriers in hereditary non-polyposis colorectal cancer

Hereditary non-polyposis colorectal cancer (HNPCC) represents 1-3% of all colorectal cancers. HNPCC is caused by a constitutional defect in a mismatch repair (MMR) gene, most commonly affecting the genes MLH1, MSH2 and MSH6. The MMR defect results in an increased cancer risk, with the greatest lifetime risk for colorectal cancer and other cancers associated to HNPCC. The HNPCC-associated tumor phenotype is generally characterized by microsatellite instability (MSI) and immunohistochemical loss of expression of the affected MMR protein. The aim of this study was to determine the sensitivity of IHC for MLH1, MSH2 and MSH6, and MSI analysis in tumors from known MMR gene mutation carriers. Fifty-eight paired normal and tumor samples from HNPCC families enrolled in our high-risk colorectal cancer registry were studied for the presence of germline mutations in MLH1, MSH2 and MSH6 by DGGE and direct sequencing. MSI analysis and immunostaining for MLH1, MSH2 and MSH6 were evaluated. Of the 28 patients with a real pathogenic mutation, loss of immunohistochemical expression for at least 1 of these MMR proteins was found, and all except 1 have MSI-H. Sensitivity by MSI analysis was 96%. IHC analysis had a sensitivity of 100% in detecting MMR deficiency in carriers of a pathogenic MMR mutation, and can be used to predict which gene is expected to harbor the mutation for MLH1, MSH2 and MSH6. This study suggests that both analyses are useful for selecting high-risk patients because most MLH1, MSH2 and MSH6 gene carriers will be detected by this 2-step approach. This practical method should have immediate application in the clinical work of patients with inherited colorectal cancer syndromes.     

Characterization of MLH1 and MSH2 DNA mismatch repair proteins in cell lines of the NCI anticancer drug screen

Purpose and methods: The lack of a functional DNA mismatch repair (MMR) pathway has been recognized as a common characteristic of several different types of human cancers due to mutation affecting one of the MMR genes or due to promoter methylation gene silencing. These MMR-deficient cancers are frequently resistant to alkylating agent chemotherapy such as DNA-methylating or platinum-containing compounds. To correlate drug resistance with MMR status in a large panel of human tumor cell lines, we evaluated by Western blot the cellular levels of the two MMR proteins most commonly mutated in human cancers, MLH1 and MSH2, in the NCI human tumor cell line panel. This panel consists of 60 cell lines distributed among nine different neoplastic diseases. Results: We found that in most of these cell lines both MLH1 and MSH2 were expressed, although at variable levels. Five cell lines (leukemia CCRF-CEM, colon HCT 116 and KM12 and ovarian cancers SK-OV-3 and IGROV-1) showed complete deficiency in MLH1 protein. MSH2 protein was detected in all 57 cell lines studied. Absence of MLH1 protein was always linked to resistance to the methylating chemotherapeutic agent temozolomide. This resistance was independent of cellular levels of O⁶-alkylguanine DNA alkyltransferase. Based on data available for review in the NCI COMPARE database, cellular levels of MLH1 and MSH2 did not correlate significantly with sensitivity to any standard anticancer drug or with any characterized molecular target already tested against the same panel of cell lines. Conclusion: Based on evaluation of 60 tumor cell lines in the NCI anticancer drug screen, MLH1 deficiency was more common than MSH2 deficiency and was always associated with a high degree of temozolomide resistance. These data will enable correlations with other drug sensitivities and molecular targets in the COMPARE database to evaluate linked processes in tumor drug resistance. Received: 13 December 1999 / Accepted: 27 July 2000     

Loss of the mismatch repair protein MSH6 in human glioblastomas is associated with tumor progression during temozolomide treatment.

PURPOSE: Glioblastomas are treated by surgical resection followed by radiotherapy [X-ray therapy (XRT)] and the alkylating chemotherapeutic agent temozolomide. Recently, inactivating mutations in the mismatch repair gene MSH6 were identified in two glioblastomas recurrent post-temozolomide. Because mismatch repair pathway inactivation is a known mediator of alkylator resistance in vitro, these findings suggested that MSH6 inactivation was causally linked to these two recurrences. However, the extent of involvement of MSH6 in glioblastoma is unknown. We sought to determine the overall frequency and clinical relevance of MSH6 alterations in glioblastomas. EXPERIMENTAL DESIGN: The MSH6 gene was sequenced in 54 glioblastomas. MSH6 and O(6)-methylguanine methyltransferase (MGMT) immunohistochemistry was systematically scored in a panel of 46 clinically well-characterized glioblastomas, and the corresponding patient response to treatment evaluated. RESULTS: MSH6 mutation was not observed in any pretreatment glioblastoma (0 of 40), whereas 3 of 14 recurrent cases had somatic mutations (P = 0.015). MSH6 protein expression was detected in all pretreatment (17 of 17) cases examined but, notably, expression was lost in 7 of 17 (41%) recurrences from matched post-XRT + temozolomide cases (P = 0.016). Loss of MSH6 was not associated with O(6)-methylguanine methyltransferase status. Measurements of in vivo tumor growth using three-dimensional reconstructed magnetic resonance imaging showed that MSH6-negative glioblastomas had a markedly increased rate of growth while under temozolomide treatment (3.17 versus 0.04 cc/mo for MSH6-positive tumors; P = 0.020). CONCLUSIONS: Loss of MSH6 occurs in a subset of post-XRT + temozolomide glioblastoma recurrences and is associated with tumor progression during temozolomide treatment, mirroring the alkylator resistance conferred by MSH6 inactivation in vitro. MSH6 deficiency may therefore contribute to the emergence of recurrent glioblastomas during temozolomide treatment.     

Use of molecular tumor characteristics to prioritize mismatch repair gene testing in early-onset colorectal cancer.

PURPOSE: The relationships between mismatch repair (MMR) protein expression, microsatellite instability (MSI), family history, and germline MMR gene mutation status have not been studied on a population basis. METHODS: We studied 131 unselected patients with colorectal cancer diagnosed younger than age 45 years. For the 105 available tumors, MLH1, MSH2, MSH6, and PMS2 protein expression using immunohistochemistry (IHC) and MSI were measured. Germline DNA was screened for hMLH1, hMSH2, hMSH6, and hPMS2 mutations for the following patients: all from families fulfilling the Amsterdam Criteria for hereditary nonpolyposis colorectal cancer (HNPCC); all with tumors that were high MSI, low MSI, or that lacked expression of any MMR protein; and a random sample of 23 with MS-stable tumors expressing all MMR proteins. RESULTS: Germline mutations were found in 18 patients (nine hMLH1, four hMSH2, four hMSH6, and one hPMS2); all tumors exhibited loss of MMR protein expression, all but one were high MSI or low MSI, and nine were from a family fulfilling Amsterdam Criteria. Sensitivities of IHC testing, MSI (high or low), and Amsterdam Criteria for MMR gene mutation were 100%, 94%, and 50%, respectively. Corresponding positive predictive values were 69%, 50%, and 75%. CONCLUSIONS: Tumor IHC analysis of four MMR proteins and MSI testing provide a highly sensitive strategy for identifying MMR gene mutation-carrying, early-onset colorectal cancer patients, half of whom would have been missed using Amsterdam Criteria alone. Tumor-based approaches for triaging early-onset colorectal cancer patients for MMR gene mutation testing, irrespective of family history, appear to be an efficient screening strategy for HNPCC.     

Mismatch repair gene expression and genetic instability in testicular germ cell tumor

Human mismatch repair (MMR) genes encode highly conserved interacting proteins that correct replication errors predisposing to hereditary gastrointestinal and genitourinary malignancies. A subset of sporadic genitourinary tumors also exhibits MMR deficiency and can be identified by measuring the frequency of microsatellite instability (MSI) in cancer cell DNA. We investigated expression of the two most commonly mutated MMR genes, MSH2 and MLH1, in sporadic testicular germ cell tumor (GCT) in order to: (1) determine the expression pattern of MSH2 and MLH1 proteins in normal seminiferous tubules and histologically distinct GCT subtypes, (2) correlate MMR gene expression with genetic instability in GCT and (3) develop a panel of molecular markers that can identify genetically distinct subsets of GCT for prognostic assessment. MSH2 and MLH1 had differential staining patterns in normal seminiferous tubules and malignant tissues. MSH2 was expressed in all stages of spermatogenesis up to but excluding mature sperm whereas MLH1 was predominantly expressed in premeiotic germ cells. All histological GCT subtypes showed differential immunostaining for MSH2 and MLH1 however pure seminoma had statistically significant fewer low MSH2 staining tumors than other subtypes (p = 0.046). Twenty-five percent of GCT exhibited increased frequency of MSI (MSI+ tumors) with 73, 70 and 43% of MSI+ tumors exhibiting low MSH2, low MLH1 or low MSH2 and low MLH1 staining respectively. Fifteen percent of testicular GCT exhibited loss of heterozygosity (LOH) but no MSI (LOH only tumors). Only 28, 17 or 6% of LOH only tumors exhibited low MSH2, low MLH1 or low MSH2 and low MLH1 staining respectively.     

High frequency of microsatellite instability and loss of mismatch-repair protein expression in patients with double primary tumors of the endometrium and colorectum

BACKGROUND: Patients with the familial syndrome hereditary nonpolyposis colorectal carcinoma (HNPCC) exhibit an increased risk for several tumor types, of which the greatest lifetime risk is for colorectal and endometrial carcinoma. HNPCC is caused by a germline mutation in one of several identified mismatch repair (MMR) genes and typically presents with microsatellite instability (MSI) and frequent loss of MMR protein expression in the tumor tissue. The objective of this study was to estimate the proportion of double primary tumors of the endometrium and colorectum that displays tumor characteristics suggestive of MMR deficiency. METHODS: The authors used the southern Sweden regional population-based Cancer Registry to identify women who developed double primary tumors of the endometrium and colorectum. Of the 256 women who were diagnosed with carcinoma at both of these sites during the period 1958-1998, 39 women had developed their first tumor before age 50 years. The authors successfully retrieved 67 tumors from 36 of these patients and analyzed them for MSI and immunohistochemical expression of the MMR genes, MLH1, MSH2, and MSH6. RESULTS: The MSI status of the 67 tumors was high MSI in 37 tumors, low MSI in 13 tumors, and microsatellite stable (MSS) in 17 tumors. Immunohistochemical loss of MMR protein expression was correlated with MSI status and was demonstrated in 29 high MSI tumors, in 1 low MSI tumor, and in 1 MSS tumor. A concordant loss of the same MMR protein in both tumors was found in 12 of 27 patients. CONCLUSIONS : The authors demonstrated a high frequency of MSI (75%) in tumors from women with endometrial and colorectal carcinoma who had their first tumor diagnosed before age 50 years and observed concordant immunohistochemical loss of MMR protein expression, suggestive of a possible underlying germline mutation, in 12 of 27 patients (44%). They concluded that double primary malignancies of the colorectum and endometrium at a young age should make the clinician suspect HNPCC.     

Microsatellite instability markers for identifying early-onset colorectal cancers caused by germ-line mutations in DNA mismatch repair genes.

PURPOSE: Microsatellite instability (MSI) testing of colorectal cancer tumors is used as a screening tool to identify patients most likely to be mismatch repair (MMR) gene mutation carriers. We wanted to examine which microsatellite markers currently used to detect MSI best predict early-onset colorectal cancer caused by germ-line mutations in MMR genes. EXPERIMENTAL DESIGN: Invasive primary tumors from a population-based sample of 107 cases of colorectal cancer diagnosed before age 45 years and tested for germ-line mutations in MLH1, MSH2, MSH6, and PMS2 and MMR protein expression were screened for MSI using the National Cancer Institute panel and an expanded 10-microsatellite marker panel. RESULTS: The National Cancer Institute five-marker panel system scored 31 (29%) as (NCI)MSI-High, 13 (12%) as (NCI)MSI-Low, and 63 (59%) as (NCI)MS-Stable. The 10-marker panel classified 18 (17%) as (10)MSI-High, 17 (16%) as (10)MSI-Low, and 72 (67%) as (10)MS-Stable. Of the 26 cancers that lacked the expression of at least one MMR gene, 24 (92%) were positive for some level of MSI (using either microsatellite panel). The mononucleotide repeats Bat26, Bat40, and Myb were unstable in all (10)MSI-High cancers and all MLH1 and MSH2 mutation carriers (100% sensitive). Bat40 and Bat25 were unstable in all tumors of MSH6 mutation carriers (100% sensitive). Bat40 was unstable in all MMR gene mutation carriers (100% sensitive). By incorporating seven mononucleotide repeats markers into the 10-marker panel, we were able to distinguish the carriers of MSH6 mutations (all scored (10)MSI-Low) from the MLH1 and MSH2 mutation carriers (all scored (10)MSI-High). CONCLUSIONS: In early-onset colorectal cancer, a microsatellite panel containing a high proportion of mononuclear repeats can distinguish between tumors caused by MLH1 and MSH2 mutations from those caused by MSH6 mutations.     

Microsatellite instability and expression of MLH1 and MSH2 in carcinomas of the small intestine

BACKGROUND: Carcinomas of the small intestine are rare, but the risk is greatly increased in patients with hereditary nonpolyposis colorectal cancer (HNPCC) due to an inherited mismatch repair (MMR) gene mutation, most commonly affecting the genes MLH1 or MSH2. Defective MMR is characterized by microsatellite instability (MSI) and loss of MMR protein expression in the tumor tissue. However, a subset of several sporadic tumor types, including about 15% of colon cancers, also evolve through defective MMR. METHODS: The authors have assessed the frequency of MSI and analyzed the immunohistochemical expression of MLH1 and MSH2 in a population-based series of 89 adenocarcinomas of the small intestine. To study the contribution of MSI and defective MMR protein expression in young patients, 43 cancers of the small intestine from patients below age 60 years (including 24 tumors from the population-based series and an additional 19 tumors from young individuals) were also analyzed. RESULTS: MSI was detected in 16/89 tumors (18%) in the population-based series, and immunohistochemistry revealed loss of expression for MLH1 in 7/16 MSI tumors and in 2/73 MSS tumors, whereas all tumors showed normal expression for MSH2. Among the young patients, the authors identified MSI in 10/43 tumors (23%), and 6 of these 10 MSI tumors showed immunohistochemical loss of MMR protein expression, which affected MLH1 in 3 cases and MSH2 in 3 cases. CONCLUSIONS: The frequency of MSI (18%) in adenocarcinomas of the small intestine equals that of colon cancer. However, silencing of MLH1 seems to explain the MSI status in only about half of the MSI tumors. Among patients with cancer of the small intestine before age 60 years, MSI is found in 23% of the cases, with MLH1 and MSH2 being affected at equal frequencies, indicating that HNPCC may underly a subset of such cases.     

结直肠癌组织MSI状态及其与临床病理参数之间的相关性

目的:分析结直肠癌组织微卫星不稳定（MSI）状态及其与临床病理参数之间的相关性。方法:利用免疫组化法检测MLH1、MSH2、MSH6、PMS2错配修复蛋白的表达,分析441例结直肠癌组织的MSI状态。结果:免疫组化检测发现,441例结直肠癌中,微卫星稳定（MSS）为375例,MLH1、MSH2、MSH6、PMS2错配修复蛋白任一表达缺失共66例,占14.97%（66/441）;其中MLH1、MSH2、MSH6、PMS2单一表达缺失率分别为1.4%（6/441）、0.2%（1/441）、0.7%（3/441）、2.3%（10/441）;MLH1和PMS2同时表达缺失率9.1%（40/441）,MSH2和MSH6同时表达缺失率1.1%（5/441）,MSH6和PMS2同时表达缺失率0.2%（1/441）。结直肠癌患者MSI与MSS在民族、肿瘤部位、分化程度、T分期、N分期、肿瘤大小等临床病理特征方面存在差异,而在性别、年龄、大体类型、病理类型、M分期、临床分期、神经和脉管侵犯方面均无明显差异。结论:新疆少数民族、右半结肠、低分化、T4、N0、肿瘤＞5 cm的结直肠癌患者更易发生MSI。     

Elucidation of genomic origin of synchronous endometrial and ovarian cancer (SEO) by genomic and microsatellite analysis

ObjectiveElucidation of clonal origin of synchronous endometrial and ovarian cancers (SEOs).MethodsWe reviewed 852 patients who diagnosed endometrial and/or ovarian cancer. Forty-five (5.3%) patients were diagnosed as SEOs. We evaluated blood and tissue samples from 17 patients. We analyzed the clonal origins of 41 samples from 17 patients by gene sequencing, mismatch microsatellite instability (MSI) polymerase chain reaction assay and immunohistochemical (IHC) staining of 4 repair genes.ResultsSixteen of 17 patients had at least 2 or more trunk mutations shared between endometrial and ovarian cancer suggesting the identical clonal origins. The shared trunk mutation are frequently found in endometrial cancer of the uterus, suggesting the uterine primary. Four out of 17 (24%) SEOs had mismatch repair (MMR) protein deficiency and MSI-high (MSI-H) states. One case was an endometrial carcinoma with local loss of MSH6 protein expression by IHC staining, and the result of MSI analysis using the whole formalin-fixed, paraffin-embedded specimen was microsatellite stable. In contrast, ovarian tissue was deficient MMR and MSI-H in the whole specimen. This indicated that MMR protein deficiency could occur during the progression of disease.ConclusionMost SEOs are likely to be a single tumor with metastasis instead of double primaries, and their origin could be endometrium. In addition, SEOs have a high frequency of MMR gene abnormalities. These findings not only can support the notion of uterine primary, but also can help to expect the benefit for patients with SEOs by immuno-oncology treatment.     

Toward new strategies to select young endometrial cancer patients for mismatch repair gene mutation analysis.

PURPOSE: To determine the frequency of mismatch repair (MMR) gene germline mutations in endometrial cancer patients who were diagnosed at less than 50 years of age; to relate the presence of mutations to family history, histopathologic data, presence of tumor microsatellite instability (MSI), and immunostaining; and to formulate criteria for genetic testing in these patients. PATIENTS AND METHODS: Endometrial cancer patients (N = 58), who were diagnosed at less than 50 years of age, were included and questioned about their family history. Mutation analysis of the MLH1, MSH2, and MSH6 genes was performed (denaturing gradient gel electrophoresis and sequence analysis to detect small mutations and multiplex ligation-dependent probe amplification to detect large deletions or duplications). For MSI analysis, five consensus markers were used, and immunostaining of the three MMR proteins was performed. RESULTS: In five of 22 patients with a positive first-degree family history for hereditary nonpolyposis colorectal cancer (HNPCC)-related cancers, pathogenic germline mutations were found (one MLH1, three MSH2, and one MSH6). Four mutation carriers belonged to families fulfilling the revised Amsterdam criteria. No mutations were found in the 35 patients without such family history (P =.006). MSI was detected in 20 of 57 cancers, among which four were from mutation carriers. In 23 of 51 cancers, one or more MMR protein was absent; in all five mutation carriers, immunostaining indicated the involved MMR gene. CONCLUSION: In 23% of the young endometrial cancer patients with at least one first-degree relative with an HNPCC-related cancer, an MMR gene mutation was detected. Therefore, presence of an HNPCC-related cancer in a first-degree relative seems to be an important selection criterion for mutation analysis. Subsequent immunostaining of MMR proteins will point to the gene(s) that should be analyzed.     

Microsatellite instability in colorectal cancer: from molecular oncogenic mechanisms to clinical implications

Background

Microsatellite instability (MSI) constitutes an important oncogenic molecular pathway in colorectal cancer (CRC), representing approximately 15% of all colorectal malignant tumours. In roughly one third of the cases, the underlying DNA mismatch repair (MMR) defect is inherited through the transmission of a mutation in one of the genes involved in MMR, predominantly MSH2 and MLH1, or less frequently, MSH6 or PMS2. In the overwhelming number of sporadic cases, MSI results from epigenetic MLH1 silencing through hypermethylation of its promoter. MMR deficiency promotes colorectal oncogenesis through the accumulation of numerous mutations in crucial target genes harbouring mononucleotide repeats, notably in those involved in the control of cell proliferation and differentiation, as well as DNA damage signalling and repair.

Design

In this review, we describe the molecular aspects of the MMR system and the biological consequences of its defect on the oncogenic process, and we discuss the various experimental systems used to evaluate the efficacy of cytotoxic drugs on MSI colorectal cells lines. There is increasing evidence showing that MSI CRCs differ from all CRCs in terms of prognosis and response to the treatment. We report the clinical studies that have evaluated the prognostic and predictive value of MSI status on clinical outcome in patients treated with various chemotherapy regimens used in the adjuvant setting or for advanced CRCs.

Conclusion

In view of this, the opportunity of a systematic MSI phenotyping in the clinical management of patients with CRC is further discussed.     

Promoter methylation and expression of MGMT and the DNA mismatch repair genes MLH1, MSH2, MSH6 and PMS2 in paired primary and recurrent glioblastomas

Epigenetic silencing of the O(6) -methylguanine-DNA methyltransferase (MGMT) gene promoter is associated with prolonged survival in glioblastoma patients treated with temozolomide (TMZ). We investigated whether glioblastoma recurrence is associated with changes in the promoter methylation status and the expression of MGMT and the DNA mismatch repair (MMR) genes MLH1, MSH2, MSH6 and PMS2 in pairs of primary and recurrent glioblastomas of 80 patients, including 64 patients treated with radiotherapy and TMZ after the first operation. Among the primary tumors, the MGMT promoter was methylated in 31 patients and unmethylated in 49 patients. In 71 patients (89%), the MGMT promoter methylation status of the primary tumor was retained at recurrence. MGMT promoter methylation, but not MGMT protein expression, was associated with longer progression-free survival, overall survival and postrecurrence survival (PRS). Moreover, PRS was increased under salvage chemotherapy. Investigation of primary and recurrent glioblastomas of 43 patients did not identify promoter methylation in any of the four MMR genes. However, recurrent glioblastomas demonstrated significantly lower MSH2, MSH6 and PMS2 protein expression as detected by immunohistochemistry. In conclusion, reduced expression of MMR proteins, but not changes in MGMT promoter methylation, is characteristic of glioblastomas recurring after the current standards of care.     

O6-Alkylguanine-DNA alkyltransferase activity of human malignant glioma and its clinical implications

Summary Activity of the DNA repair protein O⁶-alkylguanine-DNA alkyltransferase (AGT) is an important determinant of responsiveness of tumor cells to chloroethylnitrosoureas (CENUs), representative chemotherapeutic agents for primary malignant gliomas. In order to assess the real states of this repair protein in human malignant gliomas, we assayed AGT activity in surgically extirpated 42 malignant glioma samples and studied the distribution of the activity under certain clinical conditions. There were wide variations in AGT activity between individuals. No significant difference in AGT activity on average was seen either between glioblastoma and anaplastic astrocytoma, nor between primary and recurrent tumors. Among 42 malignant gliomas, 7 samples (16.7%) had low AGT activity less than 0.1 pmoles/mg protein. In the case of glioblastoma, tumors possessing higher AGT activity tended to be less responsive to post-operation remission-induction therapy including CENUs. The result of the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) chemosensitivity assay by using the corresponding surgical specimens suggested a close relationship between cellular resistance to CENUs and AGT activity. It was found to be unlikely that a short term administration of CENUs had a significant effect on AGT activity of brain tumors in human body. We could detect a bit of definite evidences of the relevance of AGT to resistance to CENUs and need to conduct further investigations for other resistance factors.     

Altered expression of MLH1, MSH2, and MSH6 in predisposition to hereditary nonpolyposis colorectal cancer.

PURPOSE: A considerable fraction (30% to 70%) of families with verified or putative hereditary nonpolyposis colorectal cancer fails to show mutations in DNA mismatch repair (MMR) genes. Our purpose was to address the genetic etiology of such families. Materials and METHODS: We scrutinized a population-based cohort of 26 families from Finland that had screened mutation-negative by previous techniques. Blood was tested for allelic messenger RNA (mRNA) expression of MLH1, MSH2, and MSH6 by single nucleotide primer extension (SNuPE), and tumor tissue for MMR protein expression by immunohistochemistry (IHC) as well as for microsatellite instability (MSI). Full-length cDNAs of genes implicated by SNuPE or IHC were cloned and sequenced. RESULTS: Unbalanced mRNA expression of MLH1 alleles was evident in two families. An inherited nonsense mutation was subsequently identified in one family, and complete silencing of the mutated allele was identified in the other family. Extinct protein expression by IHC implicated MLH1 in these two and in four other families, MSH2 in four families, and MSH6 in one family. Although no unequivocal genomic mutations were detected in the latter families, haplotype and other findings provided support for heritable defects. With one exception, all tumors with IHC alterations showed MSI, in contrast to the remaining families, which showed neither IHC changes nor MSI. CONCLUSION: Our expression-based strategy stratified the present "mutation-negative" cohort into two discrete categories: families linked to the major MMR genes MLH1, MSH2, and MSH6 (11 [42%] of 26) and those likely to be associated with other, as yet unknown susceptibility genes (15 [58%] of 26).     

Molecular mechanisms associated with chromosomal and microsatellite instability in sporadic glioblastoma multiforme

OBJECTIVE: Two chromosomal instability (CIN) pathways are described in glioblastoma multiforme (GBM), type 1 and type 2, which can be observed in up to 70% of the cases. Microsatellite instability (MSI) plays a pathogenic role in sporadic cancers such as colon, gastric and endometrial carcinomas with deficient mismatch repair (MMR). We aimed to perform a comprehensive analysis of the relationship between CIN and MSI mechanisms in sporadic glioblastomas. METHODS: 129 GBMs were examined (109 newly diagnosed and 20 relapses) investigating MSI, immunohistochemical expression of MMR proteins as well as sequencing and promoter methylation of hMLH1. We characterized the molecular changes frequently correlated with CIN in MSI+ GBMs and compared them with 26 microsatellite-stable tumors. RESULTS: Low-level MSI was observed in 11 of 129 (8.5%) cases and was higher in relapses than in primary GBMs (25 vs. 5.5%, p = 0.027). High-level MSI was not found in any case. A deficient expression of MLH1 and PMS2 without hMLH1 inactivation was observed only in one giant cell GBM. 55% of the MSI+ GBMs showed a profile which did not correspond to one of the known CIN pathways. An inverse association was observed between MSI and mutations of both p53 and PTEN. CONCLUSIONS: Our data suggest that CIN and MSI contribute to the genomic instability in GBMs via independent pathways. Since MSI was significantly more frequent in relapses, it might play a role in the malignant progression of GBM.     

Immunohistochemical expression pattern of MMR protein can specifically identify patients with colorectal cancer microsatellite instability

The microsatellite instability (MSI) pathway is found in most cases of hereditary nonpolyposis colorectal cancer (HNPCC) and in 12 % of sporadic colorectal cancer (CRC). It involves inactivation of deoxyribonucleic acid mismatch repair (MMR) genes MLH1, MSH2, PMS2, and MSH6. MMR germline mutation detections are an important supplement to HNPCC clinical diagnosis. It enables at-risk and mutation-positive relatives to be informed about their cancer risks and to benefit from intensive surveillance programs that have been proven to reduce the incidence of CRC. In this study, we analyzed for the first time in Tunisia the potential value of immunohistochemical assessment of MMR protein to identify microsatellite instability in CRC. We evaluate by immunohistochemistry MMR protein expression loss in tumoral tissue compared to positive expression in normal mucosa. Immunohistochemistry revealed loss of expression for MLH1, MSH2, MSH6, and PMS2 in 15, 21, 13, and 15 % of cases, respectively. Here, we report a more elevated frequency of MSI compared to data of the literature. In fact, by immunohistochemistry, 70 % of cases were shown to be MSS phenotype, whereas 30 % of cases, in our set, were instable. Moreover, according to molecular investigation, 71 % of cases were instable (MSI-H) and remaining cases were stable (29 %). Thus, we found a perfect association between MMR immunohistochemical analyses and MSI molecular investigation. Immunohistochemical analysis of MMR gene product expression may allow one to specifically identify MSI phenotype of patients with colorectal carcinomas.     

Microsatellite instability and mutation analysis among southern Italian patients with colorectal carcinoma: detection of different alterations accounting for MLH1 and MSH2 inactivation in familial cases.

BACKGROUND: Microsatellite instability (MSI) is due to defective DNA mismatch repair (MMR) and has been detected at various rates in colorectal carcinoma (CRC). The role of MSI in colorectal tumorigenesis was assessed further in this study by both microsatellite analysis of two CRC subsets [unselected patients (n = 215) and patients <50 years of age (n = 95)], and mutation screening of the two major MMR genes MLH1 and MSH2 among familial CRC cases. PATIENTS AND METHODS: PCR-based microsatellite analysis was performed on paraffin-embedded tissues. In CRC families, MLH1/MSH2 mutation analysis and MLH1/MSH2 immunostaining were performed on germline DNA and MSI+ tumour tissues, respectively. RESULTS: The MSI+ phenotype was detected in 75 (24%) patients, with higher incidence in early-onset or proximally located tumours. Among 220 patients investigated for family cancer history, MSI frequency was markedly higher in familial [18/27 (67%)] than in sporadic [32/193 (17%)] cases. Three MLH1 and six MSH2 germline mutations were identified in 14 out of 36 (39%) CRC families. Prevalence of MLH1/MSH2 mutations in CRC families was significantly increased by the presence of: (i) fulfilled Amsterdam criteria; (ii) four or more CRCs; or (iii) one or more endometrial cancer. While MSH2 was found mostly mutated, almost all [8/9 (89%)] familial MSI+ cases with loss of the MLH1 protein were negative for MLH1 germline mutations. CONCLUSIONS: Both genetic (for MSH2) and gene-silencing (for MLH1) alterations seem to be involved in CRC pathogenesis.     

Heterogeneous microsatellite instability observed within epithelium of ulcerative colitis

Microsatellite instability (MSI) has been associated with colitic cancer. However, reported frequency of MSI was varied and the association of MSI with mismatch repair (MMR) deficiency was unclear. In addition, the occurrence of genetic alterations in stromal cells within ulcerative colitis (UC) is still controversial. We therefore sampled 164 microareas in various pathological lesions of UC with or without colitic cancer and studied the MSI status in relation to the DNA repair protein expressions. A total of 129 microfoci from colorectal tissue of 5 colitic cancer patients and 35 microfoci of 7 UC patients (without neoplasm) were carefully sampled by laser-capture microdissection. MSI was analyzed in each microsamples. The protein expression of MMR genes (MLH1, MSH2, MSH6), O(6)-methylguanine-DNA methyltransferase and p53 were assessed by immunohistochemical analysis. Variety of di-nulcleotide microsatellite markers was altered in individual microfoci from different morphological epithelial lesions, in full range of nonneoplastic epithelium to colitic cancer. Interestingly, MSI was not observed in stromal cells at any sites, including those within colitic cancer/dysplasia lesions. Expression of the MMR proteins was not lost in any of the lesions examined. Microsatellite alterations rather seem to be related to the initiation than to the progression of colitic cancer.     

Relationship between MLH1, PMS2, MSH2 and MSH6 gene-specific alterations and tumor mutational burden in 1057 microsatellite instability-high solid tumors

Microsatellite instability-high (MSI-H) and tumor mutational burden (TMB) are predictive biomarkers for immune-checkpoint inhibitors (ICIs). Still, the relationship between the underlying cause(s) of MSI and TMB in tumors remains poorly defined. We investigated associations of TMB to mismatch repair (MMR) protein expression patterns by immunohistochemistry (IHC) and MMR mutations in a diverse sample of tumors. Hypothesized differences were identified by the protein/gene affected/mutated and the tumor histology/primary site. Overall, 1057 MSI-H tumors were identified from the 32 932 tested. MSI was examined by NGS using 7000+ target microsatellite loci. TMB was calculated using only nonsynonymous missense mutations sequenced with a 592-gene panel; a subset of MSI-H tumors also had MMR IHC performed. Analyses examined TMB by MMR protein heterodimer impacted (loss of MLH1/PMS2 vs. MSH2/MSH6 expression) and gene-specific mutations. The sample was 54.6% female; mean age was 63.5 years. Among IHC tested tumors, loss of co-expression of MLH1/PMS2 was more common (n = 544/705, 77.2%) than loss of MSH2/MSH6 (n = 81/705, 11.5%; P < .0001), and was associated with lower mean TMB (MLH1/PMS2: 25.03 mut/Mb vs MSH2/MSH6 46.83 mut/Mb; P < .0001). TMB also varied by tumor histology: colorectal cancers demonstrating MLH1/PMS2 loss had higher TMBs (33.14 mut/Mb) than endometrial cancers (20.60 mut/Mb) and other tumors (25.59 mut/Mb; P < .0001). MMR gene mutations were detected in 42.0% of tumors; among these, MSH6 mutations were most common (25.7%). MSH6 mutation patterns showed variability by tumor histology and TMB. TMB varies by underlying cause(s) of MSI and tumor histology; this heterogeneity may contribute to differences in response to ICI.