Microsatellite instability in ovarian and other pelvic carcinomas

Genomic instability in ovarian cancer: Through the lens of single nucleotide polymorphisms

Ovarian cancer (OC) is the deadliest gynecological malignancy among all female reproductive cancers. It is characterized by high mortality rate and poor prognosis. Genomic instability caused by mutations, single nucleotide polymorphisms (SNPs), copy number variations (CNVs), microsatellite instability (MSI), and chromosomal instability (CIN) are associated with OC predisposition. SNPs, which are highly prevalent in the general population, show a greater relative risk contribution, particularly in sporadic cancers. Understanding OC etiology in terms of genetic basis can increase the use of molecular diagnostics and provide promising approaches for designing novel treatment modalities. This will help deliver personalized medicine to OC patients, which may soon be within reach. Given the pivotal impact of SNPs in cancers, the primary emphasis of this review is to shed light on their prevalence in key caretaker genes that closely monitor genomic integrity, viz., DNA damage response, repair, cell cycle checkpoints, telomerase maintenance, and apoptosis and their clinical implications in OC. We highlight the current challenges faced in different SNP-based studies. Various computational methods and bioinformatic tools employed to predict the functional impact of SNPs have also been comprehensively reviewed concerning OC research. Overall, this review identifies that variants in the DDR and HRR pathways are the most studied, implying their critical role in the disease. Conversely, variants in other pathways, such as NHEJ, MMR, cell cycle, apoptosis, telomere maintenance, and PARP genes, have been explored the least.

Population analysis of microsatellite genotypes reveals a signature associated with ovarian cancer

Ovarian cancer (OV) ranks fifth in cancer deaths among women, yet there remain few informative biomarkers for this disease. Microsatellites are repetitive genomic regions which we hypothesize could be a source of novel biomarkers for OV and have traditionally been under-appreciated relative to Single Nucleotide Polymorphisms (SNPs). In this study, we explore microsatellite variation as a potential novel source of genomic variation associated with OV. Exomes from 305 OV patient germline samples and 54 tumors, sequenced as part of The Cancer Genome Atlas, were analyzed for microsatellite variation and compared to healthy females sequenced as part of the 1,000 Genomes Project. We identified a subset of 60 microsatellite loci with genotypes that varied significantly between the OV and healthy female populations. Using these loci as a signature set, we classified germline genomes as ‘at risk’ for OV with a sensitivity of 90.1% and a specificity of 87.6%. Cross-analysis with a similar set of breast cancer associated loci identified individuals ‘at risk’ for both diseases. This study revealed a genotype-based microsatellite signature present in the germlines of individuals diagnosed with OV, and provides the basis for a potential novel risk assessment diagnostic for OV and new personal genomics targets in tumors.

Frequency of mismatch repair deficiency in ovarian cancer: a systematic review This article is a US Government work and, as such, is in the public domain of the United States of America

Loss of mismatch repair (MMR) capacity may represent an important tumor initiating mechanism in ovarian cancer. We conducted a systematic review to analyze the frequency of microsatellite instability (MSI), immunohistochemical (IHC) staining for MMR proteins, and hypermethylation of the MLH1 promoter region in ovarian cancers. Studies examining MSI, loss of MMR gene expression by IHC staining and MLH1 promoter hypermethylation in ovarian cancer were identified by a systematic literature search of the PubMed electronic database through August 31, 2009. Pertinent data was extracted from eligible studies and estimates for pooled proportions were computed using random effects models. The pooled proportion of MSI detection was 0.10 (95% CI, 0.06-0.14) among 1,234 cases in 22 studies. Dinonucleotide markers had a higher frequency of instability than mononucleotide markers. The pooled proportion of MLH1 or MSH2 staining loss was 0.06 (95% CI, 0.01-0.17) among 474 cases in three studies, with a higher frequency of loss in MLH1. The pooled proportion of MLH1 methylation was 0.10 (95% CI, 0.06-0.15) among 672 cases in seven studies. Data reporting MSI and loss of MMR staining in the same cases was limited. Although MMR deficiency was found in all histologic subtypes, endometrioid cancers had the highest proportion. Approximately 10% of unselected ovarian cancers are related to MMR deficiency. While MMR deficiency is associated with improved survival in other MMR-deficiency related cancer sites, epidemiological and clinical factors related to the MMR-deficient phenotype have not been adequately studied in ovarian cancer to date.

Molecular pathogenesis of endometrial and ovarian cancer

Pregnancy, breastfeeding, and oral contraceptive pill use interrupt menstrual cycles and reduce endometrial and ovarian cancer risk. This suggests the importance of turnover within Mullerian tissues, where the accumulation of mutations in p53 and PTEN has been correlated with number of cycles. The most common type of endometrial cancer (Type I) is endometrioid and molecular abnormalities include mutations in PTEN, KRAS and β-catenin. The Type I precursor is Endometrial Intraepithelial Neoplasia which displays PTEN defects. Type II endometrial cancer (whose precursors are less clear) includes serous and clear cell tumors and the most common alteration is p53 mutation. For ovarian cancer, histopathologic types parallel endometrial cancer and include serous, mucinous, endometrioid, and clear cell; some molecular features are also shared. The most frequent type of ovarian cancer is high grade serous that often displays p53 mutation and its precursor lesions may originate from normal-appearing fallopian tube epithelium that contains a p53 "signature". Mutations in KRAS, BRAF and PTEN are described in mucinous, endometrioid and low grade serous cancers and these may originate from ovarian cortical inclusion cysts. A consideration of molecular and other pathogenetic features, like epidemiology and histopathology, may provide a better understanding of endometrial and ovarian cancer.

Systematic review and meta-analysis of ovarian cancers: estimation of microsatellite-high frequency and characterization of mismatch repair deficient tumor histology

PURPOSE

A meta-analytic approach was used to estimate the frequency of: (a) microsatellite instability-high (MSI-H) phenotype in unselected ovarian cancers and (b) various histologic subtypes of mismatch repair (MMR)-deficient epithelial ovarian cancers.

METHODS

A systematic search of the Medline electronic database was conducted to identify articles published between January 1, 1966, and December 31, 2007, that examined MMR deficiency in ovarian cancers. Data were extracted on the study population, sample size, MSI-H frequency, and histology of MMR-deficient ovarian tumors.

RESULTS

The pooled proportion of MSI-H ovarian cancers was 0.12 [95% confidence interval (CI), 0.08-0.17] from 18 studies with 977 cases. The proportion of histologic subtypes in the pooled analysis from 15 studies with 159 cases was serous at 0.32 (95% CI, 0.20-0.44), mucinous at 0.19 (95% CI, 0.12-0.27), endometrioid at 0.29 (95% CI, 0.22-0.36), clear cell at 0.18 (95% CI, 0.09-0.28), and mixed at 0.24 (95% CI, 0.07-0.47). There was significant heterogeneity between studies.

CONCLUSIONS

The frequency of the MSI-H phenotype in unselected ovarian cancers approximates 12%. MMR-deficient ovarian cancers also seem to be characterized by an overrepresentation of nonserous histologic subtypes. Knowledge of histologic subtype may aid clinicians in identifying the relatively large proportion of ovarian cancers due to MMR defects; such knowledge has potential implications for medical management.

A review of the clinical relevance of mismatch-repair deficiency in ovarian cancer

Ovarian cancer ranks fifth in both cancer incidence and mortality among women in the United States. Defects in the mismatch-repair (MMR) pathway that arise through genetic and/or epigenetic mechanisms may be important etiologically in a reasonable proportion of ovarian cancers. Genetic mechanisms of MMR dysfunction include germline and somatic mutations in the MMR proteins. Germline mutations cause hereditary nonpolyposis colorectal cancer (HNPCC), which is the third most common cause of inherited ovarian cancer after BRCA1 and BRCA2 mutations. An epigenetic mechanism known to cause inactivation of the MMR system is promoter hypermethylation of 1 of the MMR genes, mutL homolog 1 (MLH1). Various laboratory methods, in addition to clinical and histopathologic criteria, can be used to identify MMR-deficient ovarian cancers. Such methods include microsatellite instability analysis, immunohistochemistry, MLH1 promoter hypermethylation testing, and germline mutation analysis. In this review, the authors describe the existing literature regarding the molecular, clinical, and histologic characteristics of MMR-deficient ovarian cancers along with the possible effect on survival and treatment response. By further defining the profile of MMR-deficient ovarian cancers and their associated etiologic mechanisms, there may be a greater potential to distinguish between those of hereditary and sporadic etiology. The ability to make such distinctions may be of diagnostic, prognostic, and therapeutic utility.

Mismatch repair and treatment resistance in ovarian cancer

BACKGROUND

The treatment of ovarian cancer is hindered by intrinsic or acquired resistance to platinum-based chemotherapy. The aim of this study is to determine the frequency of mismatch repair (MMR) inactivation in ovarian cancer and its association with resistance to platinum-based chemotherapy.

METHODS

We determined, microsatellite instability (MSI) as a marker for MMR inactivation (analysis of BAT25 and BAT26), MLH1 promoter methylation status (methylation specific PCR on bisulfite treated DNA) and mRNA expression of MLH1, MSH2, MSH3, MSH6 and PMS2 (quantitative RT-PCR) in 75 ovarian carcinomas and eight ovarian cancer cell lines

RESULTS

MSI was detected in three of the eight cell lines i.e. A2780 (no MLH1 mRNA expression due to promoter methylation), SKOV3 (no MLH1 mRNA expression) and 2774 (no altered expression of MMR genes). Overall, there was no association between cisplatin response and MMR status in these eight cell lines. Seven of the 75 ovarian carcinomas showed MLH1 promoter methylation, however, none of these showed MSI. Forty-six of these patients received platinum-based chemotherapy (11 non-responders, 34 responders, one unknown response). The resistance seen in the eleven non-responders was not related to MSI and therefore also not to MMR inactivation.

CONCLUSION

No MMR inactivation was detected in 75 ovarian carcinoma specimens and no association was seen between MMR inactivation and resistance in the ovarian cancer cell lines as well as the ovarian carcinomas. In the discussion, the results were compared to that of twenty similar studies in the literature including in total 1315 ovarian cancer patients. Although no association between response and MMR status was seen in the primary tumor the possible role of MMR inactivation in acquired resistance deserves further investigation.

Molecular pathogenesis of ovarian borderline tumors: new insights and old challenges

Ovarian borderline (low malignant potential) tumors are a puzzling group of neoplasms that do not fall neatly into benign or malignant categories. Their behavior is enigmatic, their pathogenesis unclear, and their clinical management controversial, especially for serous borderline tumors (SBT), the most common type of ovarian borderline tumor. Clarifying the nature of borderline tumors and their relationship to invasive carcinoma has puzzled investigators since the category was created over 30 years ago. Much of the confusion and controversy concerning these tumors is due to a lack of understanding of their pathogenesis and an absence of a model for the development of ovarian carcinoma. This review summarizes recent molecular studies of ovarian borderline tumors with special emphasis on the role of SBT in tumor progression and its relationship to ovarian serous carcinoma.

The contribution of genetic and epigenetic changes in granulosa cell tumors of ovarian origin

PURPOSE

Granulosa cell tumors (GCTs) are relatively rare and are subtypes of the sex-cord stromal neoplasms. A better understanding of the molecular genetics underlying various steps in malignant transformation is critical to success in the battle against this disease. Changes in the status of methylation, known as epigenetic alterations, are one of the most common molecular alterations in human cancers, including GCTs. Chromosomal instability and microsatellite instability (MSI) are common in these GCTs. We tested the hypothesis that C-->T transition polymorphism in the promoter region of cytosine DNA-methyltransferase-3B (DNMT3B) and its altered expression are also associated with hypermethylation of the genes. We also attempted to determine the relationship between MSI of ovarian carcinoma and hMLH1 hypermethylation in these tumors.

EXPERIMENTAL DESIGN

We studied chromosome instability in 25 GCTs by detecting gross chromosome rearrangements in cultured peripheral blood lymphocytes. MSI was assessed using six microsatellite markers (BAT25, BAT26, D2S123, D5S346, D11S1318, and D17S250). Using sensitive methylation-specific PCR, we searched for aberrant promoter hypermethylation in a panel of genes including p16, BRCA1, RASSF1A, ER-alpha, TMS1, TIMP3, Twist, GSTP1, AR, and hMLH1. Polymorphism in the DNMT3B gene was assessed by the PCR-RFLP method, and DNMT3B expression was studied by reverse transcription-PCR assay.

RESULTS

Chromosome instability was indicated by significantly higher frequencies of chromosome aberrations (6.24%; P < 0.001) compared with controls (2.12%). The most frequently observed changes include trisomy 14 and monosomy 22. MSI has been found in 19 of 25 tumors, and loss of heterozygosity has been found in 9 of 25 tumors. Frequencies of methylation in GCTs were 40% for p16 and ER-alpha; 36% for BRCA1 and RASSF1A; 28% for hMLH1; 24% for TIMP3, Twist, and GSTP1; and 20% in TMS1 and AR. TT genotype was found only in two cases; the remainder were either CC or CT type. There was no significant alteration in the expression of DNMT3B in these patients.

CONCLUSIONS

Coexistence of chromosome instability, MSI, and hypermethylation suggests that both genetic and epigenetic mechanisms may act in concert to inactivate the above-mentioned genes in these GCTs. These mechanisms can be an early event in the pathogenesis of these tumors, and it can be a critical step in the tumorigenic process. All these events might play an important role in early clinical diagnosis and in chemotherapeutic management and treatment of the disease. Larger studies may lend further understanding to the etiology and clinical behavior of these tumors.

Molecular genetic alterations in endometrioid carcinomas of the ovary: similar frequency of beta-catenin abnormalities but lower rate of microsatellite instability and PTEN alterations than in uterine endometrioid carcinomas

Endometrioid carcinomas of the ovary closely resemble their uterine counterparts. It has been suggested that the former tumors have the same molecular alterations (microsatellite instability [MSI], PTEN, and beta-catenin) described in endometrioid carcinomas of the uterus. We analyzed 55 ovarian carcinomas, including 22 endometrioid, 18 clear cell, and 15 mixed types. MSI was detected in 5 of 39 cases (13%). MLH1 promoter hypermethylation was identified in 2 of the 5 MSI-positive tumors. PTEN was mutated in 5 of 54 cases (9%); of these, 3 had MSI and exhibited frameshift mutations in short-coding mononucleotide repeats. Beta-catenin nuclear expression was detected in 11 of 54 cases (20%) by immunostaining; of these, 7 exhibited CTNNB1 gene mutations. These alterations were found more frequently in endometrioid carcinomas than in tumors of the other 2 groups. Among the former tumors, MSI was detected in 3 of 17 cases (17.5%); PTEN mutations, in 3 of 21 (14%); and beta-catenin, in 8 of 21 (38%). The molecular alterations were found more often in tumors associated with endometriosis than in tumors without endometriosis. Six endometrioid tumors demonstrating matrix metalloproteinase-7 (MMP-7) immunoreactivity with nuclear accumulation of beta-catenin had good outcomes, in contrast to poor outcomes in 7 of 9 predominantly nonendometrioid tumors demonstrating expression of MMP-7 only. We found a similar frequency of beta-catenin abnormalities but lower rates of MSI and PTEN alterations than in uterine endometrioid carcinomas. Alterations in beta-catenin and PTEN genes, as well as MSI, are frequent in low-stage ovarian carcinomas of endometrioid type that have a favorable prognosis.

MSI-low, a real phenomenon which varies in frequency among cancer types

This study assessed whether low-level microsatellite instability (MSI-L) is a phenomenon specific to colorectal cancers or is also present in other tumour types. Breast (grade III ductal and lobular), endometrial and ovarian carcinomas, as well as colorectal cancers, were analysed for MSI-L using eight microsatellite markers. The markers were selected from a panel that had previously been shown to be sensitive for the detection of MSI-L in colorectal cancers. It was found that MSI-L was present in 30 of 87 (35%) colorectal cancers, 2 of 59 (3%) grade III breast carcinomas, 1 of 35 (3%) lobular breast cancers, 16 of 50 (32%) endometrial cancers, and 9 of 34 (26%) ovarian cancers. These results suggest that MSI-L is a very rare occurrence in breast carcinomas, but does occur as a real phenomenon in colorectal, endometrial, and ovarian carcinomas, which are all part of the hereditary non-polyposis colon cancer (HNPCC) syndrome. PCR artefact was also found to masquerade as MSI-L; criteria for the assessment of MSI-L are suggested to eliminate this problem.

Microsatellite instability and expression of hMLH1 and hMSH2 proteins in ovarian endometrioid cancer

Microsatellite instability and loss of heterozygosity has been implicated in ovarian carcinogenesis. The reported frequency of microsatellite instability in human ovarian cancer varies significantly owing to the use of heterogeneous tumor histotypes and various microsatellite markers in different laboratories. In this study, we determined the frequency of microsatellite instability in 74 ovarian endometrioid carcinomas using four microsatellite markers (BAT25, BAT26, D5S346, D17S250), and examined hMLH1 and hMSH2 protein expression. In all, 20% of the tumors were microsatellite instability high (two or more markers showing instability) and 12% were microsatellite instability low (one marker showed instability). Loss of hMLH1 and/or hMSH2 expression was found in nine of 15 microsatellite instability-high tumors. The microsatellite instability-high phenotype tended to occur more frequently in low-grade tumors (P=0.053), but did not correlate with clinical stage. Totally, 38% of cases also displayed loss of heterozygosity at D17S250; this loss of heterozygosity was associated with high clinical stage (P=0.097). Our results indicate that both microsatellite and loss of heterozygosity at D17S250 are involved in the development of ovarian endometrioid carcinoma.

Mismatch repair gene expression defects contribute to microsatellite instability in ovarian carcinoma

BACKGROUND

hMLH1, the human MutL homologue, has been linked to microsatellite instability (MSI) in gastrointestinal tumors. However, to the authors' knowledge, the role of hMLH1, the other mismatch repair genes (MMR), and MSI in ovarian carcinoma has not been well defined. The purpose of the current study was to determine the relation between MSI of ovarian carcinoma and MMR gene expression, hMLH1 and hMSH2 hypermethylation, and hMLH1 and hMSH2 null mutations.

METHODS

hMLH1 mRNA was detected by reverse transcriptase-polymerase chain reaction (RT-PCR) and amplification of cDNA using a housekeeping gene (glycerol 3-phosphate dehydrogenase) as a control for mRNA quality and quantity. Methylation-specific PCR (MS-PCR) was used to correlate methylation of the hMLH1 and hMSH2 CpG islands with mRNA expression status. Similar techniques were used to evaluate the concomitant expression of five other MMR: hMSH2, hMSH3, hMSH6, PMS1, and PMS2. Microsatellite instability was studied using the National Cancer Institute consensus markers (D2S123, D5S346, D17S250, BAT25, and BAT26) and NM23 as described previously.

RESULTS

One hundred twenty-five primary tumors were analyzed. High-frequency MSI (MSI-H) was found in 21 tumors (16.8%). hMLH1 mRNA was absent in 10 of these 21 tumors (47.6%). In each case, coordinated hypermethylation of both regions A and C of the promoter was identified. Microsatellite stable and low-frequency MSI tumors all were found to express not only hMLH1 but the other MMR genes as well (P < 0.001). Absence of expression of hMSH2 and the four other MMRs occurred in tumors with absent hMLH1 mRNA expression because of CpG island hypermethylation. No absence of expression of hMSH2, hMSH3, hMSH6, PMS1, or PMS2 was found to occur in tumors expressing hMLH1. None of the 11 MSI-H tumors without promoter hypermethylation demonstrated a null mutation in hMLH1 or hMSH2.

CONCLUSIONS

A molecular mechanism to explain > 50% of the MSI-H phenotype in ovarian carcinoma cases was demonstrated. MSI-H may occur because of MMR defects, especially hMLH1 promoter hypermethylation. Additional mechanisms are required to explain the balance between the cases of MSI-H as well as the phenomenon of MSI-L tumors.

The clinical importance and prognostic implications of microsatellite instability in sporadic cancer

AIMS

The genetic abnormality known as microsatellite instability (MSI), first identified in colorectal cancer in 1993, has subsequently been recognised in other malignancies. These cancers are caused by a defect in the nuclear mismatch repair system, allowing mutations to accumulate with every cellular division. Hereditary Non Polyposis Colon Cancers (HNPCC) and associated malignancies demonstrating MSI have a unique histological appearance, improved prognosis and altered response to chemotherapy and radiotherapy. This review examines the incidence of MSI and its clinical significance in commonly occurring solid malignancies.

METHOD

A medline based literature search was performed using the key words 'Microsatellite Instability' and the name of the specific malignancy being investigated. Additional original papers were obtained from citations in those articles identified in the original medline search.

RESULTS

MSI has been detected in many solid malignancies although the definition of instability applied has been variable. It is most commonly found in sporadic malignancies that also occur in the HNPCC syndrome such as colorectal, stomach, endometrial and ovarian cancer. MSI may impart a favorable prognosis in colorectal, gastric, pancreatic and probably oesophageal cancers but a poor prognosis in non small cell lung cancer. In clinical studies colorectal cancers demonstrating MSI respond better to chemotherapy while in vitro studies using MSI positive cell lines show resistance to radiotherapy and chemotherapy.

CONCLUSION

MSI may be a useful genetic marker in prognosis and could be an influential factor in deciding treatment options. However, in many cancers its significance remains unclear and more evaluation is required.

Risk assessment & genetic testing

Ovarian cancer is the fifth most common cause of cancer death in women in Western countries and family history is one of the strongest known risk factors. Approximately 5 to 13% of all ovarian cancer cases are caused by the inheritance of cancer predisposing genes with an autosomal pattern of transmission. The inherited fraction of ovarian cancer may differ between populations. Based on analysis of familial ovarian cancer pedigrees and other epidemiological studies, three hereditary ovarian cancer syndromes have been defined. The identification of the genes responsible for most hereditary ovarian cancers has open a new area of early detection methods and preventive procedures specifically dedicated to women identified as carrying ovarian cancer predisposing genes. Predictive oncology is best performed by a dedicated unit with professionals aware of all the issues surrounding genetic testing.

Microsatellite instability, MLH-1 promoter hypermethylation, and frameshift mutations at coding mononucleotide repeat microsatellites in ovarian tumors

BACKGROUND

Microsatellite instability (MI) is frequent in endometrial carcinomas (ECs), but its occurrence in ovarian tumors is uncertain. Microsatellite instability positive ECs frequently are associated with frameshift mutations in coding mononucleotide tracts in IGFIIR, BAX, hMSH6, and hMSH3.

METHODS

DNA from 52 consecutive patients with ovarian tumors (10 benign, 7 borderline, and 35 malignant) was obtained from neoplastic and normal tissue. After preliminary results, the series was expanded by including 41 additional, previously selected, endometrioid and clear cell carcinomas. Microsatellite instability analysis was assessed by evaluating three (CA)n dinucleotide repeats (D2S123, D5S346, D17S250) and two mononucleotide tracts (BAT 25 and BAT 26). Frameshift mutations at coding mononucleotide repeats (IGFIIR, TGF beta II, BAX, hMSH6, and hMSH3) were investigated by single-strand conformation polymorphism analysis and DNA sequencing. MLH-1 methylation was assessed by methylation specific PCR.

RESULTS

Microsatellite instability was identified in only 2 of the 52 (3.8%) tumors of the initial series (1 endometrioid and 1 clear cell carcinoma). After expanding the initial series of 15 endometrioid and clear cell carcinomas with 41 additional endometrioid and clear cell carcinomas, MI was found in 7 of the total series of 56 endometrioid and clear cell carcinomas (12.5%). Frameshift mutations in coding mononucleotide tracts were detected in BAX (6 of 7), IGFIIR (1 of 7), and MSH3 (2 of 7). MLH-1 promoter hypermethylation was identified in three of six MI positive tumors.

CONCLUSIONS

Microsatellite instability was infrequent in this series of ovarian tumors, and it was limited to endometrioid and clear cell carcinomas. Like EC, many ovarian carcinomas with MI follow the same process of MLH-1 promoter methylation and accumulation of mutations in coding mononucleotide tracts.

Microsatellite instability, MLH-1 promoter hypermethylation, and frameshift mutations at coding mononucleotide repeat microsatellites in ovarian tumors

BACKGROUND

Microsatellite instability (MI) is frequent in endometrial carcinomas (ECs), but its occurrence in ovarian tumors is uncertain. Microsatellite instability positive ECs frequently are associated with frameshift mutations in coding mononucleotide tracts in IGFIIR, BAX, hMSH6, and hMSH3.

METHODS

DNA from 52 consecutive patients with ovarian tumors (10 benign, 7 borderline, and 35 malignant) was obtained from neoplastic and normal tissue. After preliminary results, the series was expanded by including 41 additional, previously selected, endometrioid and clear cell carcinomas. Microsatellite instability analysis was assessed by evaluating three (CA)n dinucleotide repeats (D2S123, D5S346, D17S250) and two mononucleotide tracts (BAT 25 and BAT 26). Frameshift mutations at coding mononucleotide repeats (IGFIIR, TGF beta II, BAX, hMSH6, and hMSH3) were investigated by single-strand conformation polymorphism analysis and DNA sequencing. MLH-1 methylation was assessed by methylation specific PCR.

RESULTS

Microsatellite instability was identified in only 2 of the 52 (3.8%) tumors of the initial series (1 endometrioid and 1 clear cell carcinoma). After expanding the initial series of 15 endometrioid and clear cell carcinomas with 41 additional endometrioid and clear cell carcinomas, MI was found in 7 of the total series of 56 endometrioid and clear cell carcinomas (12.5%). Frameshift mutations in coding mononucleotide tracts were detected in BAX (6 of 7), IGFIIR (1 of 7), and MSH3 (2 of 7). MLH-1 promoter hypermethylation was identified in three of six MI positive tumors.

CONCLUSIONS

Microsatellite instability was infrequent in this series of ovarian tumors, and it was limited to endometrioid and clear cell carcinomas. Like EC, many ovarian carcinomas with MI follow the same process of MLH-1 promoter methylation and accumulation of mutations in coding mononucleotide tracts.

A change in microsatellite instability caused by cisplatin-based chemotherapy of ovarian cancer

To clarify the mechanism of acquired CDDP resistance in ovarian cancer, we compared the microsatellite instability (MSI) by the amplification of 10 microsatellite loci and immunohistochemical detection of hMSH2 and hMLH1 expression between the primary resected tumours and the secondary resected residual tumours after 5 or 6 courses of CDDP-based chemotherapy in the 24 cases of ovarian cancer. Of the 24 primary resected tumours, 9 (37.5%) showed MSI (7 cases of MSI-L, 2 cases of MSI-H), while 15 (72.5%) were microsatellite stable tumours (MSS). The primary tumours also had MSI in the residual tumours after CDDP-based chemotherapy. However, all of the cases with MSS in the primary resected tumours exhibited MSI (2 cases were MSI-L, and 13 cases were MSI-H) in the residual tumours after CDDP-based chemotherapy (P< 0.001). Furthermore, 11 (73.3%) of these cases which changed from MSS to MSI also had a change in the expression of hMLH1 from positive to undetectable (P< 0.001). Our data suggest that tumour MSI changes during CDDP-based chemotherapy, and that the loss of hMLH1 expression is one of the factors that has the greatest effect on this transformation.

Microsatellite instability and mutational analysis of transforming growth factor beta receptor type II gene (TGFBR2) in sporadic ovarian cancer

AIMS

To investigate the possible role of mutations in the transforming growth factor beta receptor type II gene (TGFBRII) in ovarian cancer and its relation to microsatellite instability (MSI), 43 sporadic ovarian tumours were analysed for mutations over the entire coding region of the TGFBRII gene.

METHODS

Mutational analysis was performed using the polymerase chain reaction (PCR), single strand conformation polymorphism (SSCP) gel analysis, and direct sequencing. MSI analysis included both mononucleotide and dinucleotide microsatellite markers used for radiolabelling and gene scanning.

RESULTS

No pathogenic mutations were detected, although sequencing of the polyadenine (poly A) tract in exon 3 using conventional techniques revealed a spurious frameshift mutation that was not present in the same samples analysed using a proofreading Taq polymerase. MSI analysis demonstrated an MSI negative phenotype in 40 of the 43 tumours. None of the three MSI positive tumours demonstrated MSI for mononucleotide markers only.

CONCLUSIONS

These findings suggest that: (1) MSI (both conventional and mononucleotide) is infrequent in ovarian cancer and (2) inactivation of the MSH2, MLH1, and MSH6 mismatch repair genes and TGFBR2 gene mutations do not play a major role in ovarian cancer tumorigenesis. The spurious TGFBR2 frameshift mutations detected by sequencing after conventional PCR underline the importance of confirming putative mutations in repetitive sequences by alternative methods.

Allelotype of papillary serous peritoneal carcinomas

OBJECTIVE

Papillary serous peritoneal carcinoma (PSPC) is histologically indistinguishable from papillary serous ovarian carcinoma (PSOC) with a similar clinical presentation, yet may differ in its carcinogenesis. The purpose of this study was to determine the incidence of allelic loss and the frequency of p53 mutation by p53 overexpression in PSPC compared to PSOC.

METHODS

An allelotype analysis of 26 patients with PSPC was performed using 39 microsatellite markers from 25 chromosomal arms. Thirty-seven previously studied patients with PSOC served as the comparison. P53 mutations were detected by immunohistochemical protein overexpression.

RESULTS

There was significantly less LOH in PSPC than PSOC. Both the number of chromosomes with LOH and the proportion of tumors with allelic loss were less frequent. Significant LOH, defined as >/=30% of informative tumors having loss at a chromosome locus, was seen on 4 chromosome arms in PSPC: 12p, 17p, 17q, and 18q, compared to 18 arms in PSOC: 4q, 5q, 6p, 6q, 9p, 9q, 12p, 12q, 13q, 15q, 16q, 17p, 17q, 18q, 19p, 19q, 22q, and Xq (P < 0.001). The median LOH frequency was higher in PSOC than PSPC, 43% versus 33%, respectively (P = 0.013), and more PSOC tumors had LOH than PSPC tumors, 91% versus 65% (P = 0.042). P53 overexpression was detected in 80% of PSPC tumors.

CONCLUSIONS

LOH occurs less frequently in PSPC compared to PSOC. Chromosomal regions with high frequencies of LOH common to PSPC and PSOC, such as 12p, 17p, 17q, and 18q, may harbor tumor suppressor genes important in the carcinogenesis of both malignancies and likely include p53.