Microsatellite instability in tumors as a model to study the process of microsatellite mutations

Microsatellite Instability: From the Implementation of the Detection to a Prognostic and Predictive Role in Cancers

Microsatellite instability (MSI) has been identified in several tumors arising from either germline or somatic aberration. The presence of MSI in cancer predicts the sensitivity to immune checkpoint inhibitors (ICIs), particularly PD1/PD-L1 inhibitors. To date, the predictive role of MSI is currently used in the selection of colorectal cancer patients for immunotherapy; moreover, the expansion of clinical trials into other cancer types may elucidate the predictive value of MSI for non-colorectal tumors. In clinical practice, several assays are used for MSI testing, including immunohistochemistry (IHC), polymerase chain reaction (PCR) and next-generation sequencing (NGS). In this review, we provide an overview of MSI in various cancer types, highlighting its potential predictive/prognostic role and the clinical trials performed. Finally, we focus on the comparison data between the different assays used to detect MSI in clinical practice.

Genetic and molecular bases of esophageal Cancer among Iranians: an update

Abstract

Background

Esophageal cancer is one of the leading causes of cancer related deaths among the Iranians. There is still a high ratio of mortality and low 5 years survival which are related to the late onset and diagnosis. Majority of patients refer for the treatment in advanced stages of tumor progression.

Main body

It is required to define an efficient local panel of diagnostic and prognostic markers for the Iranians. Indeed such efficient specific panel of markers will pave the way to decrease the mortality rate and increase the 5 years survival among the Iranian patients via the early diagnosis and targeted therapy.

Conclusion

in present review we have reported all of the molecular markers in different signaling pathways and cellular processes which have been assessed among the Iranian esophageal cancer patients until now.

Genetic instability and increased mutational load: which diagnostic tool best direct patients with cancer to immunotherapy?

The occurrence of high rates of somatic mutations in cancer is believed to correspond to increased frequency of neo-epitope formation and tumor immunogenicity. Thus, classification of patients with cancer according to degree a somatic hyper-mutational status could be proposed as a predictive biomarker of responsiveness to immunotherapy with immune checkpoint inhibitors. Here, we discuss the suitable and reliable tests easily adoptable in clinical practice to assess somatic mutational status in patients with advanced cancer.

Asymmetrical male-mediated gene flow between harbor seal (Phoca vitulina) populations in Alaska

Harbor seals ( Phoca vitulina richardii (Gray, 1864)) in Alaska are currently treated as three distinct management stocks. Previous genetic analyses using mitochondrial DNA suggested that these stocks are differentiated genetically. We studied populations in Glacier Bay (GB; Southeast Alaska Stock), where harbor seals are declining, and Prince William Sound (PWS; Gulf of Alaska Stock), where the population has recently stabilized. Using six pairs of hypervariable microsatellite primers, we determined that these populations are a single panmictic unit with estimated migration rates of 22 animals/generation (PWS to GB) and 63 animals/generation (GB to PWS). The asymmetrical gene flow between GB and PWS is likely driven in part by a recent increase in competitors and predators of seals in GB. In contrast with males, emigration of females from PWS to GB (8.3 seals/generation) is higher than emigration of females from GB to PWS (3.3 seals/generation), likely because females use glacial ice as pupping habitat. Despite the high gene flow, the number of migrants per year (0.02% of the Gulf of Alaska population) is likely too low to influence the demographics of harbor seals in PWS, and the two populations may best be managed as separate stocks.

Sequence-dependent effect of interruptions on microsatellite mutation rate in mismatch repair-deficient human cells

Although microsatellite mutation rates generally increase with increasing length of the repeat tract, interruptions in a microsatellite may stabilize it. We have performed a direct analysis of the effect of microsatellite interruptions on mutation rate and spectrum in cultured mammalian cells. Two mononucleotide sequences (G(17) and A(17)) and a dinucleotide [(CA)(17)] were compared with interrupted repeats of the same size and with sequences of 8 repeat units. MMR-deficient (MMR(-)) cells were used for these studies to eliminate effects of this repair process. Mutation rates were determined by fluctuation analysis on cells containing a microsatellite sequence at the 5' end of an antibiotic-resistance gene; the vector carrying this sequence was integrated in the genome of the cells. In general, interrupted sequences had lower mutation rates than perfect ones of the same size, but the magnitude of the difference was dependent upon the sequence of the interrupting base(s). Some interrupted repeats had mutation rates that were lower than those of perfect sequences of the same length but similar to those of half the length. This suggests that interrupting bases effectively divide microsatellites into smaller repeat runs with mutational characteristics different from those of the corresponding full-length microsatellite. We conclude that interruptions decrease microsatellite mutation rate and influence the spectrum of frameshift mutations. The sequence of the interrupting base(s) determines the magnitude of the effect on mutation rate.

The rise, fall and renaissance of microsatellites in eukaryotic genomes

Microsatellites are among the most versatile of genetic markers, being used in an impressive number of biological applications. However, the evolutionary dynamics of these markers remain a source of contention. Almost 20 years after the discovery of these ubiquitous simple sequences, new genomic data are clarifying our understanding of the structure, distribution and variability of microsatellites in genomes, especially for the eukaryotes. While these new data provide a great deal of descriptive information about the nature and abundance of microsatellite sequences within eukaryotic genomes, there have been few attempts to synthesise this information to develop a global concept of evolution. This review provides an up-to-date account of the mutational processes, biases and constraints believed to be involved in the evolution of microsatellites, particularly with respect to the creation and degeneration of microsatellites, which we assert may be broadly viewed as a life cycle. In addition, we identify areas of contention that require further research and propose some possible directions for future investigation.

Evolution of hypervariable microsatellites in apomictic polyploid lineages of Ranunculus carpaticola: directional bias at dinucleotide loci

Microsatellites are widely used in genetic and evolutionary analyses, but their own evolution is far from simple. The mechanisms maintaining the mutational patterns of simple repeats and the typical stable allele-frequency distributions are still poorly understood. Asexual lineages may provide particularly informative models for the indirect study of microsatellite evolution, because their genomes act as complete linkage groups, with mutations being the only source of genetic variation. Here, we study the direction of accumulated dinucleotide microsatellite mutations in wild asexual lineages of hexaploid Ranunculus carpaticola. Whereas the overall number of contractions is not significantly different from that of expansions, the within-locus frequency of contractions, but not of expansions, significantly increases with allele length. Moreover, within-locus polymorphism is positively correlated with allele length, but this relationship is due solely to the influence of contraction mutations. Such asymmetries may explain length constraints generally observed with microsatellites and are consistent with stable, bell-shaped allele-frequency distributions. Although apomictic and allohexaploid, the R. carpaticola lineages show mutational patterns resembling the trends observed in a broad range of organisms, including sexuals and diploids, suggesting that, even if not of germline origin, the mutations in these apomicts may be the consequence of similar mechanisms.

Microsatellite instability and methylation of the DNA mismatch repair genes in head and neck cancer

BACKGROUND

Methylation in the promoter region of the DNA mismatch repair genes hMLH1 and hMSH2 and microsatellite instability at three loci were analyzed in the tumor tissue from patients with head and neck cancer.

METHODS

Microsatellite instability and promoter methylation were investigated by PCR, denaturing-polyacrylamide gel electrophoresis and digestion with methylation-specific restriction enzymes.

RESULTS

Microsatellite instability was observed in 41% of the patients. hMLH1 and hMSH2 genes were methylated in 47% and 30% of the patients, respectively. BAT25 and BAT26 instability were associated with age and histopathology, respectively. Methylation frequency of the hMLH1 gene promoter was significantly higher in patients displaying a high level of microsatellite instability. Instability at the BAT 26 and D2S123 loci were associated with the MSI-high status.

CONCLUSIONS

Our results indicate that microsatellite instability and modifications in the hMLH1 and hMSH2 genes are implicated in a significant proportion of the patients with head and neck cancer.

Identification of predictive factors for the occurrence of predisposing MLH1 and MSH2 germline mutations among Sardinian patients with colorectal carcinoma

Factors predictive of carrying MLH1 and MSH2 germline mutations in patients with colorectal cancer (CRC) are as yet unknown. The aim of this population-based study, was to further define the role of MLH1/MSH2 mutations through an evaluation clinic program with 362 consecutive Sardinian CRC patients. Eight MLH1/MSH2 germline mutations were detected in 21 (6%) patients. Examining family cancer history, MLH1/MSH2 mutations were found in 14/48 (29.2%) probands from CRC families and, among them, in 10/13 (76.9%) families fulfilling the Amsterdam criteria. The patients with low familial recurrence (two CRCs in the family) presented a much lower frequency of MLH1/MSH2 mutations (2/55; 3.6%). Significantly higher rates of MLH1/MSH2 mutations were found in patients with age of onset 45 years (P=0.012) or with 3 affected family members (P=0.009). While no significant predictive value was found for the presence of endometrial cancer within the family, earlier age of diagnosis and/or familial CRC recurrence should be considered as strong predictors for the occurrence of MLH1/MSH2 mutations, and therefore useful in recommending CRC patients for genetic testing.

Correlation between the allelic distribution of STRs in a Finnish population and phenotypically different gastrointestinal tumours: a study using four X-chromosomal markers (DXS7423, DXS8377, ARA, DXS101)

Microsatellite instability in tumours has been suggested as a model to study the process of short tandem repeat (STR) mutations. In the present study we have determined the allelic variation of four X-STRs (DXS7423, DXS8377, DXS101 and ARA) in a Finnish population of 103 individuals, and assessed whether a comparable allelic distribution could be found in a series of gastrointestinal cancers differing by the level of microsatellite instability. Fifty-seven gastric and colorectal cancers were stratified by autosomal STRs, and the mononucleotide marker BAT-26 into stable, low-level unstable and high-level unstable microsatellite (MSI-H) cancers, of which the last produced the majority of X-STR alleles. For the four markers analysed, a significant correlation of allele distribution between our Finnish population sample and MSI-H tumours was noted. Together, the eight MSI-H tumours found represented 80%, 66-80% and 100% of the DXS101 alleles in the Finnish, and in previously described Caucasian and Korean population samples, respectively. Of the ARA, DXS7423 and DXS8377 alleles in the Finnish population, 42%, 75% and 79% were found in the MSI-H cancers, respectively. The results suggest that analysis of STR variation in a relatively small number of MSI-H cancers may aid in pre-evaluation of their allelic distribution in a population.

Hereditary nonpolyposis colorectal cancer (Lynch syndrome): criteria for identification and management

Hereditary nonpolyposis colorectal carcinoma (HNPCC), or Lynch syndrome, is an autosomal dominant syndrome accounting for 5 to 10% of the total colorectal cancer population. Patients with this syndrome develop colorectal carcinoma at an early age, but disease onset can happen in all age groups. Usually the carcinomas are synchronous or metachronous, and most of them arise proximal to the splenic flexure. The prognosis is better than for the sporadic form of cancer, and there is increased risk for cancer development in certain extracolonic sites, such as the endometrium, ovary, stomach, small bowel, hepatobiliary tract, ureter, and renal pelvis. Most patients with HNPCC have a mutation in one of two DNA mismatch repair genes, hMSH2 or hMLH 1. More than 90% of colorectal carcinoma patients with hMSH2 or hMLH1 demonstrate high-frequency microsatellite instability (MSI-H). If a patient is suspected to belong to an HNPCC family, the first screening test should be immunohistochemistry for the detection of hMLH1 and hMSH2 proteins, and if it is indicative, it should be followed by genomic sequencing for the identification of mutations in the mismatch repair genes. Genetic counseling and surveillance for high risk HNPCC family members should begin at age 25. Surveillance includes annual colonoscopy of the entire large bowel, with fecal occult blood testing performed twice a year. Systematic surveillance and individually designed treatment of affected patients may help to detect cancers at an earlier stage and subsequently improve the prognosis of the disease further.

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.

Y-chromosomal microsatellite mutation rates: differences in mutation rate between and within loci

Precise estimates of mutation rates at Y-chromosomal microsatellite STR (short tandem repeat) loci make an important basis for paternity diagnostics and dating of Y chromosome lineage origins. There are indications of considerable locus mutation rate variability between (inter-) and within (intra-) loci. We have studied nine Y-STR loci-DYS19, DYS389I/II, DYS390, DYS391, DYS392, DYS393, DYS385, and DYS388-in 1,766 father-son pairs of confirmed paternity (a total of 15,894 meioses). Five biallelic markers were also analyzed in the fathers-Tat, YAP, 12f2, SRY1532, and 92R7-defining haplogroups 1, 2, 3, 4, 9, and 16, respectively. A total of 36 fragment length mutations were observed: 24 gains (22 single-step, two double-step) and 12 single-step losses. Thus, there was a significant surplus of gains (p=0.045). Overall, the mutation rate was positively correlated to STR repeat length and there was a significant relative excess of losses in long alleles and gains in short alleles (p=0.043). In contrast to the situation in autosomal STR loci and in MSY-1, no noteworthy correlation between mutation rate and the father's age at the child's birth was observed. We observed significant interlocus differences in Y-STR mutation rates (p<0.01). The number of observed mutations ranged from zero in DYS392 to eight in DYS391 and DYS390. We have also demonstrated obvious differences in mutation rates between the haplogroups studied (p=0.024), a phenomenon that is a reflection of the dependence of mutation rate on allele size. Our study has thus demonstrated the necessity of not only locus-specific, but even allele-specific, mutation rate estimates for forensic and population genetic purposes, and provides a considerable basis for such estimates.

High somatic instability of a microsatellite locus in a clonal tree, Robinia pseudoacacia

Robinia pseudoacacia L. is a clonal tree species. To investigate a mutation within eight microsatellite loci of R. pseudoacacia, we analyzed DNA samples obtained from different leaf samples within each ramet, leaves from ramets within the genet, and seeds. Of the eight loci, locus Rops15 (AG motif) displayed hypermutability. The mutation rates of Rops15 within each ramet, among ramets within the genet, and offspring were 6.27% (ranging from 0 to 31.1%), 6.11% (from 0 to 25.0%) and 3.78% (from 0 to 10.9%), respectively. The mutation rate increased with allele size (13-71 repeat units). The mutation patterns observed in Rops15 were distinctive in two ways. First, there was a significant bias toward additions over deletions, and both addition and deletion of single repeats were dominant at alleles with lengths less than 232 bp (63 repeats). Second, for the longest allele of 248 bp (71 repeats), the number of losses was higher than the number of gains. These observations suggest that the mutation patterns of microsatellites in R. pseudoacacia may follow a generalized stepwise mutation model, and that the tendency of long alleles to mutate to shorter lengths acts to prevent infinite growth. Finally, the observation of somatic hypermutability at locus Rops15 highlights the need for caution when using highly polymorphic microsatellites for population genetic structure and paternity analysis in tree species.

Evaluation of gastrointestinal cancer tissues as a source of genetic information for forensic investigations by using STRs

Malignant tissue samples may sometimes be the only source of biological material for forensic investigations, including identification of individuals or paternity testing. However, in use of such samples, uncertainties due to microsatellite instability (MSI) and loss of heterozygosity (LOH) often associated with neoplasias may be encountered. In this study, we have analysed the applicability of autosomal tetranucleotide short tandem repeat (STR) markers, which are routinely used in forensic analysis, to gain genetic information. MSI and LOH were analysed in 41 surgically removed gastrointestinal cancer specimens and the adjascent non-cancerous tissue marginals. The cancer specimens showed great variability in their genetic phenotypes due to MSI or LOH, with only 32% being microsatellite-stable. Of the 15 autosomal STR loci analysed, only TH01 had no MSI-type alteration in these samples. The loci most frequently affected by MSI were D8S1179, D21S11, D18S51 and D19S433 (MSI in 15-17% of cases). LOH-type alterations were observed at all of the loci, including the amelogenin locus used for sex determination. The highest LOH frequency was found at locus D18S51 (27%). The genetic alterations at the marker loci may indicate false homozygosity or heterozygosity, and false gender may result from erroneous deduction of DNA profiles. Therefore, typing of autosomal STRs from malignant tissues in forensic settings warrants careful interpretation of MSI and LOH results together with microscopic analysis of a tissue specimen. Results by two commercially available and widely used forensic DNA profiling kits used here were comparable.

Microsatellite analysis of the DCC gene in nephroblastomas: pathologic correlations and prognostic implications

Microsatellite instability has been reported in a wide variety of cancer types. Inactivation or loss of tumour suppressor genes has been shown to result in cell cycle deregulation and neoplastic growth. We conducted a microsatellite study using fluorescent-based DNA technology to determine whether mutations in the microsatellite sequences of the deleted in colorectal cancer (DCC) gene, a tumour suppressor at 18q21.1, have any pathologic correlation or prognostic significance in nephroblastomas. Normal and tumour DNA was isolated from 106 cases of nephroblastoma using the standard proteinase K digestion and phenol-chloroform extraction method from paraffin wax-embedded tissue. Polymerase chain reaction using three microsatellite markers; D18S21, D18S34 and D18S58, for the DCC gene were performed. The polymerase chain reaction products were analysed on the ALF Express Automated DNA sequencer. The results were correlated with age at diagnosis, preoperative chemotherapy, clinicopathological stage, histological classification and patient outcome using chi(2) test. Allelic imbalance/loss of heterozygosity appeared to be a more frequent genetic aberration than microsatellite instability with 20% of cases showing allelic imbalance/loss of heterozygosity and only 9% of cases showing microsatellite instability. Genetic aberrations were more frequent in unfavourable histology tumours compared to favourable histology tumours (P=0.012). All patients with genetic aberrations for more than one DCC marker died independent of histological classification and stage (P=0.016). There was no statistically significant difference when DCC aberrations were compared with age at diagnosis, preoperative chemotherapy and clinicopathological stage. In conclusion, this study has found that multiple aberrations involving the DCC locus may play a role in the progression of nephroblastomas, and hence confer a poorer prognosis.

The mutational spectrum of human autosomal tetranucleotide microsatellites

We studied by multiplex amplification and single-run electrophoretic analysis 10 microsatellite loci, composed of nine tetranucleotide-repeats (D1S1612, D3S2387, D4S2431, D5S2501, D10S1237, D15S657, D16S2622, D18S1270, and IFNAR-ALU) and one trinucleotide repeat (D2S1353). After elimination of proven null allele events involving D1S1612 and D5S2501 and of all data of D3S2387, in which we suspected but could not prove the occurrence of null alleles, we were left with nine loci, encompassing 24,224 meioses and 23 mutations. Twenty-two of the mutations (96%) were single-step events. Moreover, 18 of the mutations were paternal, four were maternal, and one was indeterminate. There was no significant difference between the number of additions and deletions in the mutants. Our findings are compatible with a simple model in which tetranucleotide microsatellites mutate primarily in paternal germinative cells by DNA slippage, such that the vast majority of mutations are equiprobable additions or deletions of a single-repeat unit. By combining the data from our tetranucleotide loci with literature information of highly and lowly mutable microsatellites, we observed a very highly significant correlation between mutation rate and the geometric mean of the length of the longest perfect repeat region (LRPR), compatible with a power or exponential relationship. The variation of the length of the LRPR explained as much as 80% of the variance of the mutation rate of autosomal tetranucleotide microsatellites.

Chromosomal abnormalities and microsatellite instability in sporadic endometrial cancer

Defective DNA mismatch repair and nonfunctional mechanisms controlling the proper progression of the cell cycle have been proposed as being responsible for the genomic instability and accumulation of karyotypic alterations in endometrial cancer (EC). To assess whether numerical chromosomal anomalies (aneuploidy) and microsatellite instability (MSI) might be representative of distinctive tumour behaviour, paraffin-embedded tissue samples from 86 patients with sporadic EC were evaluated by both fluorescence in situ hybridisation (FISH) and microsatellite analysis, using free nuclei and genomic DNAs (respectively). Approximately one-third of the tumours analysed (24/74; 32%) exhibited MSI, whereas 38/86 (44%) of the EC samples displayed aneuploidy. The majority of the unstable cases (15/24; 63%) were from advanced-stage patients. Conversely, 23 (61%) out of the 38 tumours with aneuploidy were from early-stage patients. No apparent correlation was found between MSI and aneuploidy, whereas the immunohistochemical (IHC) analysis revealed that inactivation of the MLH1 mismatch repair gene may be involved in the majority of the MSI+ sporadic ECs. No genetic or cytogenetic alteration analysed here seems to add any significant predictive value to the stage of disease.

Prevalence and prognostic role of microsatellite instability in patients with rectal carcinoma

BACKGROUND

Association between microsatellite instability (MSI) and favorable postoperative survival in patients with colorectal cancer receiving adjuvant chemotherapy has been indicated. To evaluate whether an analogous positive prognostic role of MSI could be present in rectal carcinoma (RC; most RC patients receive adjuvant radiotherapy), PCR-based microsatellite analysis of archival RCs and statistical correlation with clinico-pathological parameters were performed.

PATIENTS AND METHODS

DNA from paraffin-embedded paired samples of tumors and corresponding normal tissue from 91 RC patients was analyzed for MSI using five microsatellite markers (tumors were classified as MSI(+) when two or more markers were unstable).

RESULTS

Seventeen (19%) RC patients exhibited a MSI(+) phenotype. Prevalence of instability was found in patients with earlier RC onset (28% in cases with diagnosis age < or =55 years versus 15% in cases >55 years), whereas similar MSI frequencies were observed in patients with different disease stage or receiving different adjuvant therapies. While MSI was detected in seven (64%) of 11 familial patients, a remarkably lower MSI incidence was observed in sporadic cases (10/80; 12.5%). A significant association with better disease-free survival (DFS) and overall survival (OS) was found for MSI(+) patients (median DFS/OS, 30/32 months) in comparison to MSI(-) ones (median DFS/OS, 18/21 months) (P <0.001).

CONCLUSIONS

MSI was demonstrated to be a strong molecular prognostic marker in rectal carcinoma, independent of the administered treatment (radiotherapy, chemotherapy or both).

Microsatellite instability and mutation analysis of candidate genes in unselected sardinian patients with endometrial carcinoma

BACKGROUND

Microsatellite instability (MSI) is due mostly to a defective DNA mismatch repair (MMR). Inactivation of the two principal MMR genes, hMLH1 and hMSH2, and the PTEN tumor suppressor gene seems to be involved in endometrial tumorigenesis. In this study, Sardinian patients with endometrial carcinoma (EC) were analyzed to assess the prevalence of both the mutator phenotype (as defined by the presence of MSI and abnormal MMR gene expression at the somatic level) and the hMLH1, hMSH2, and PTEN germline mutations among patients with MSI positive EC.

METHODS

Paraffin embedded tissue samples from 116 consecutive patients with EC were screened for MSI by polymerase chain reaction-based microsatellite analysis. Immunohistochemistry (IHC) with anti-hMLH1 and anti-hMSH2 antibodies was performed on MSI positive tumor tissue sections. Germline DNA was used for mutational screening by denaturing high-performance liquid chromatography analysis and automated sequencing.

RESULTS

Thirty-nine patients with EC (34%) exhibited MSI; among them, 25 tumor samples (64%) showed negative immunostaining for hMLH1/hMSH2 proteins (referred to as IHC negative). No disease-causing mutation within the coding sequences of the hMLH1/hMSH2 and PTEN genes was found in patients with EC who had the mutator phenotype (MSI positive and IHC negative), except for a newly described hMLH1 missense mutation, Ile655Val, that was observed in 1 of 27 patients (4%). Although MSI was more common among patients with advanced-stage EC and increased as the tumor grade increased, no significant correlation with disease free survival or overall survival was observed among the two groups (MSI positive or MSI negative) of patients with EC.

CONCLUSIONS

In patients with MSI positive EC, epigenetic inactivations rather than genetic mutations of the MMR genes seem to be involved in endometrial tumorigenesis. No prognostic value was demonstrated for MSI in patients with EC.

Germ cells microsatellite instability. The effect of different mutagens in a mismatch repair mutant of Drosophila (spel1)

Mismatch repair (MMR) process confers a type of genomic stability that maintains stable single repeated sequences, hence a failure of this process could deviate in cancer development. A characteristic phenotype of MMR-deficient cells is microsatellite instability (MSI) that could be modulated by mutagenic agents. The induction of MSI by the mutagens, bleomycin (BLM), hydrogen peroxide (H(2)O(2)), 2-acetylaminofluorene (2-AAF) and ethidium bromide (EB) was evaluated in vivo, by using a Drosophila melanogaster-null mutant of the msh2 mismatch repair gene (spel1). Whereas in the germ cells of the spel1 strain, we found microsatellite mutations in the five repeated sequences studied in untreated individuals, no alterations were found in the MMR-proficient strain. On the other hand, the data obtained from the treatment experiments show that BLM and 2-AAF induced a slight mutagenic effect in the MMR-deficient background but not in the normal one. These results indicate that the use of the Drosophila spel1 mutant (MMR-deficient) could be of relevant importance to identify environmental factors involved in carcinogenesis processes through genomic instability.

Hypermutability at a poly(A/T) tract in the human germline

Poly(A/T) tracts are abundant simple sequence repeats (SSRs) within the human genome. They constitute part of the coding sequence of a variety of genes, encoding polylysine stretches that are important for protein function. Assessment of poly(A/T) tract stability is also used to identify microsatellite unstable colorectal cancers, which are characteristic of tumours defective in DNA mismatch repair. Despite their importance, little is known about the stability of poly(A/T) SSRs in the human germline. We have determined the stability of a paradigm poly(A/T) tract, BAT-40, by study of population allele frequencies, mutation frequency in families and mutation frequency in sperm DNA. We show that the locus is polymorphic, with a level of heterozygosity of 59.7%. Germline mutation was observed in 13 of 187 germline transmissions (7.0%) in 10 families suggesting BAT-40 is unstable in the germline. Further evidence for germline instability at BAT-40 was provided by small pool PCR analysis of matched blood and sperm DNA templates, revealing a significantly elevated frequency of mutation in the germline (P < 0.001). These findings provide insight into poly(A/T) tract stability in the germline. They also have relevance to the study of gene expression and to determination of microsatellite instability in tumours.

Analysis of microsatellite instability and hypermutation of immunoglobulin variable genes in Werner syndrome

Werner syndrome (WS) is a human premature aging syndrome, which is associated with high frequencies of neoplasia and genetic instability. We have examined the occurrence of microsatellite instability, which may result from defective mismatch repair, in lymphoblastoid cell lines derived from nine WS patients. Instability was measured at the D2S123 locus by gel analysis of PCR products. Three WS cell lines had 4-13% altered alleles, compared with 0% in the other six lines. The increased frequency of microsatellite instability could not readily be associated with overt cancer or any other known clinical condition in the three patients. To examine whether the WS defect affected the humoral immune system, we measured the hypermutation of immunoglobulin variable genes in peripheral blood cells from the WS patient who donated the cell line with the highest frequency of microsatellite instability. The frequency and pattern of mutation was similar to that from normal individuals, suggesting that the Werner protein is not involved in generating hypermutation.

Microsatellite instability as a tool for the classification of gastric cancer

Microsatellite instability (MSI) is a common feature of gastric cancers that reflects underlying mismatch-repair deficiency in the tumor, caused most frequently by methylation of the hMLH1 promoter. Tumors with MSI have been found to inactivate certain target genes by permitting an increased frequency of mutations in mononucleotide runs in their coding regions. Gastric tumors with MSI have a distinct clinicopathological profile with a relatively good prognosis. Using the simple and robust methodologies available, MSI detection in gastrointestinal tumors promises to be one of the first widely used molecular prognostic tests for human cancer. Here, we review the molecular context of this exciting prospect with respect to one of the world's most prevalent cancers, that of the stomach.