Two modes of microsatellite instability in human cancer: differential connection of defective DNA mismatch repair to dinucleotide repeat instability

HTLV-1 bZIP factor impairs DNA mismatch repair system

Adult T-cell leukemia (ATL) is a peripheral T-cell malignancy caused by human T-cell leukemia virus type 1 (HTLV-1). Microsatellite instability (MSI) has been observed in ATL cells. Although MSI results from impaired mismatch repair (MMR) pathway, no null mutations in the genes encoding MMR factors are detectable in ATL cells. Thus, it is unclear whether or not impairment of MMR causes the MSI in ATL cells. HTLV-1 bZIP factor (HBZ) protein interacts with numerous host transcription factors and significantly contributes to disease pathogenesis and progression. Here we investigated the effect of HBZ on MMR in normal cells. The ectopic expression of HBZ in MMR-proficient cells induced MSI, and also suppressed the expression of several MMR factors. We then hypothesized that the HBZ compromises MMR by interfering with a transcription factor, nuclear respiratory factor 1 (NRF-1), and identified the consensus NRF-1 binding site at the promoter of the gene encoding MutS homologue 2 (MSH2), an essential MMR factor. The luciferase reporter assay revealed that NRF-1 overexpression enhanced MSH2 promoter activity, while co-expression of HBZ reversed this enhancement. These results supported the idea that HBZ suppresses the transcription of MSH2 by inhibiting NRF-1. Our data demonstrate that HBZ causes impaired MMR, and may imply a novel oncogenesis driven by HTLV-1.

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.

MutL homolog 1 methylation and microsatellite instability in sporadic colorectal tumors among Filipinos

BACKGROUND

Colorectal cancer (CRC) ranks third in terms of incidence and second in mortality worldwide. In CRC, the silencing of mismatch repair genes, including the mutL homolog 1 (hMLH1) has been linked to microsatellite instability (MSI), the lengthening or shortening of microsatellite repeats. Very limited data have been presented so far on the link of hMLH1 methylation and MSI in Southeast Asia populations with sporadic CRC, and on its clinical significance.

AIM

To investigate the significance of the MSI status and hMLH1 methylation in CRC Filipino patients.

METHODS

Fifty-four sporadic CRC patients with complete clinical data were included in this study. Genomic DNA from CRC tumor biopsies and their normal tissue counterparts were profiled for MSI by high resolution melting (HRM) analysis using the Bethesda Panel of Markers (BAT25, BAT26, D2S123, D5S346, and D17S250). hMLH1 methylation screening was performed using bisulfite conversion and methylation specific polymerase chain reaction. Statistical analysis was conducted to calculate their associations to clinicopathological characteristics and survival relevance (Kaplan-Meier curves and the log-rank test).

RESULTS

hMLH1 methylation was observed in 9% and 35% of CRC and normal samples, respectively. Higher incidence of consistently methylated hMLH1 found in both normal and CRC was noticed for relation to location of tumor (P < 0.05). As for MSI status, D2S123 the most common unstable microsatellite and MSI-high (MSI-H) was the most common MSI profile, counted for 46% and 50% of normal and CRC tissues, respectively. The presence of MSI-low (MSI-L) and microsatellite stable (MSS) was 43% and 11% for normal, and 31% and 19% for CRC samples. The mean month of patients’ survival was shorter in patients whose normal and tumor tissues had methylated compared to those with unmethylated hMLH1 and with MSI-H compared to those with MSI-L/MSS (P < 0.05). This was supported by significant difference in Kaplan-Meier with log-rank analysis. This data indicated that hMLH1 methylation and high MSI status have prognostic value.

CONCLUSION

This study showed the clinical significance of hMLH1 methylation and MSI status in sporadic CRC Filipino patients, especially in the normal part of the tumor.

DNA polymerase delta Exo domain stabilizes mononucleotide microsatellites in human cells

In prokaryotes and yeasts, DNA polymerase proofreading (PPR) and DNA mismatch repair (MMR) cooperatively counteracts replication errors leading to repeat sequence destabilization (i.e. insertions/deletions of repeat units). However, PPR has not thus far been regarded as a mechanism stabilizing repeat sequences in higher eukaryotic cells. In a human cancer cell line, DLD-1, which carries mutations in both MSH6 and the Exo domain of POLD1, we previously observed that mononucleotide microsatellites were markedly destabilized whereas being stable in the simple MMR-defective backgrounds. In this study, we introduced the Exo domain mutation found in DLD-1 cells into MSH2-null HeLa cell clones, using CRISPR/Cas9 system. In the established Exo-/MMR-mutated HeLa clones, mononucleotide repeat sequences were remarkably destabilized as in DLD-1 cells. In contrast, dinucleotide microsatellites were readily destabilized in the parental MMR-deficient backgrounds, and the instability was not notably increased in the genome-edited HeLa clones. Here, we show an involvement of the Exo domain functions of DNA polymerase delta in mononucleotide repeat stabilization in human cells, which also suggests a possible role division between DNA polymerase and MMR in repeat maintenance in the human genome.

Precision length determination and in silico simulation in PCR of microsatellite repeat sequences

Despite being commonplace, polymerase chain reactions (PCRs) still contain many unknown aspects. One example is microsatellite PCR, which is now widely used for various purposes from ecology to cancer medicine. Since this category of repetitive DNA sequences induces polymerase slippage not only in vivo but also in vitro, microsatellite PCR products comprise a complex combination of DNA fragments with various lengths and have, therefore, been empirically interpreted. The primary obstacle for understanding microsatellite PCR was the intrinsic inaccuracy in sizing of DNA fragments in capillary electrophoresis (CE), which, however, has been overcome by elucidating intrinsic sizing errors in each fragment length range. Secondly, the slippage properties of the thermostable polymerases were first clarified in detail using primer extension assays. Furthermore, using the obtained slippage parameters and our original program, we have first reconstructed microsatellite PCR in silico. The entire processes of complex microsatellite PCR have, thus, been more clearly understood.

Germline Risk Contribution to Genomic Instability in Multiple Myeloma

Genomic instability, a well-established hallmark of human cancer, is also a driving force in the natural history of multiple myeloma (MM) - a difficult to treat and in most cases fatal neoplasm of immunoglobulin producing plasma cells that reside in the hematopoietic bone marrow. Long recognized manifestations of genomic instability in myeloma at the cytogenetic level include abnormal chromosome numbers (aneuploidy) caused by trisomy of odd-numbered chromosomes; recurrent oncogene-activating chromosomal translocations that involve immunoglobulin loci; and large-scale amplifications, inversions, and insertions/deletions (indels) of genetic material. Catastrophic genetic rearrangements that either shatter and illegitimately reassemble a single chromosome (chromotripsis) or lead to disordered segmental rearrangements of multiple chromosomes (chromoplexy) also occur. Genomic instability at the nucleotide level results in base substitution mutations and small indels that affect both the coding and non-coding genome. Sometimes this generates a distinctive signature of somatic mutations that can be attributed to defects in DNA repair pathways, the DNA damage response (DDR) or aberrant activity of mutator genes including members of the APOBEC family. In addition to myeloma development and progression, genomic instability promotes acquisition of drug resistance in patients with myeloma. Here we review recent findings on the genetic predisposition to myeloma, including newly identified candidate genes suggesting linkage of germline risk and compromised genomic stability control. The role of ethnic and familial risk factors for myeloma is highlighted. We address current research gaps that concern the lack of studies on the mechanism by which germline risk alleles promote genomic instability in myeloma, including the open question whether genetic modifiers of myeloma development act in tumor cells, the tumor microenvironment (TME), or in both. We conclude with a brief proposition for future research directions, which concentrate on the biological function of myeloma risk and genetic instability alleles, the potential links between the germline genome and somatic changes in myeloma, and the need to elucidate genetic modifiers in the TME.

Differential genomic destabilisation in human cells with pathogenic MSH2 mutations introduced by genome editing

Repeat destabilisation is variously associated with human disease. In neoplastic diseases, microsatellite instability (MSI) has been regarded as simply reflecting DNA mismatch repair (MMR) deficiency. However, several discrepancies have been pointed out. Firstly, the MSI⁺ phenotype is not uniform in human neoplasms. Established classification utilises the frequency of microsatellite changes, i.e. MSI-H (high) and -L (low), the former regarded as an authentic MMR-defective phenotype. In addition, we have observed the qualitatively distinct modes of MSI, i.e. Type A and Type B. One discrepancy we previously pointed out is that tumours occurring in MMR gene knockout mice exhibited not drastic microsatellite changes typical in MSI-H tumours (i.e. Type B mode) but minor and more subtle alterations (i.e. Type A mode). In the present study, MSH2 mutations reported in Lynch syndrome (LS) kindred have been introduced into HeLa cells using the CRISPR/Cas9 system. The established mutant clones clearly exhibited MMR-defective phenotypes with alkylating agent-tolerance and elevated mutation frequencies. Nevertheless, microsatellites were not markedly destabilised as in MSI-H tumours occurring in LS patients, and all the observed alterations were uniformly Type A, which confirms the results in mice. Our findings suggest added complexities to the molecular mechanisms underlying repeat destabilisation in human genome.

Fluorescence detection of DNA mismatch repair in human cells

Mismatched base pairs, produced by nucleotide misincorporation by DNA polymerase, are repaired by the mismatch repair (MMR) pathway to maintain genetic integrity. We have developed a method for the fluorescence detection of the cellular MMR ability. A mismatch, which would generate a stop codon in the mRNA transcript unless it was repaired, was introduced into the gene encoding the enhanced green fluorescent protein (EGFP) in an expression plasmid. When MMR-proficient HeLa cells were transformed with this plasmid, the production of active EGFP was observed by fluorescence microscopy. It was assumed that the nick required to initiate the MMR pathway was produced non-specifically in the cells. In contrast, fluorescence was not detected for three types of MMR-deficient cells, LoVo, HCT116, and DLD-1, transformed with the same plasmid. In addition, the expression of a red fluorescent protein gene was utilized to avoid false-negative results. This simple fluorescence method may improve the detection of repair defects, as a biomarker for cancer diagnosis and therapy.

A specific mode of microsatellite instability is a crucial biomarker in adult T-cell leukaemia/lymphoma patients

Purpose

Microsatellite instability (MSI) has been a long-standing biomarker candidate for drug resistance in tumour cells. Despite numerous clinical studies, the data in the literature are not conclusive. The complexity of the MSI phenomenon in some malignancies may, at least partly, account for the discrepancy. In addition, methodological problems are also pointed out in the assay techniques. We previously established a unique fluorescent technique in which the major methodological problems in conventional assays are overcome. Application of this technique has revealed two distinct modes of microsatellite alterations, i.e. Type A and Type B. More importantly, we demonstrated that Type A MSI is the direct consequence of defective DNA mismatch repair (MMR) that causes cellular resistance against antineoplastic agents.

Method

We first applied this technique to adult T-cell leukaemia/lymphoma (ATLL).

Results

The MSI phenomenon was indeed observed in ATLLs (4/20, 20%). Intriguingly, the observed microsatellite alterations were invariably Type A, which implies that the tumours were MMR-defective. Indeed, clinical outcomes of patients with these MSI⁺ tumours were significantly worse. Furthermore, multivariate analysis revealed that Type A MSI is an independent prognostic factor.

Conclusion

These observations strongly suggest the possibility of Type A MSI as a prognostic and potentially predictive biomarker in ATLL.

Modal variety of microsatellite instability in human endometrial carcinomas

PURPOSE

Microsatellite instability (MSI) in human endometrial cancer (EC) was analysed using a unique fluorescent technique. MSI is associated with various human neoplasms. However, the reported frequency of MSI differs widely in each malignancy. Methodological difficulties have in fact been pointed out in its assay techniques.

METHODS

We previously established a sensitive fluorescent technique in which the major methodological problems are overcome. Application of this technique has revealed two distinct modes of microsatellite alterations, i.e. Type A and Type B. In the present study, we have applied this technique to 94 ECs.

RESULTS

Significant microsatellite alterations were observed in 38 (40.4%) tumours of the panel. The two modes, Type A and Type B, were indeed observed in this malignancy. More importantly, we found that the modes more closely correlated with the molecular and clinicopathological backgrounds of the tumours than the established and widely used MSI grades, MSI-H and MSI-L. Type B MSI widely correlated with family history of hereditary non-polyposis colorectal cancer-associated cancers, whereas MSI-H only did with that of colorectal cancer. Furthermore, mutation in the KRAS oncogene, which has been regarded as generally infrequent in microsatellite-unstable tumours, was clearly associated with Type A MSI.

CONCLUSIONS

Our observations may suggest a biological relevance and a potential utility of the modal classification of MSI and, furthermore, added complexities to genomic instability underlying tumourigenesis in human endometrium.

Microsatellite instability affecting the T17 repeats in intron 8 of HSP110, as well as five mononucleotide repeats in patients with colorectal carcinoma

AIM

To investigate mononucleotide markers: BAT-25, BAT-26, NR-21, NR-22 and NR-24 in patients with colorectal cancer (CRC), and the status of HSP110T17, KRAS, BRAF and the MLH1 promoter mutations in microsatellite unstable CRC.

METHODS

Genetic assessments were performed on samples obtained following resection of CRC in 200 patients.

RESULTS

Allelic variations of HSP110T17 were found in all 18 patients with microsatellite instabilities (MSIs) in at least three markers (high-frequency MSI). By contrast, mutations of HSP110T17 were absent in all 20 patients with no MSI frequency. Eight out of 182 patients with low (instability in one marker) or no frequency MSI had allelic shifts due to polymorphisms of BAT-25 (1.5%), NR-21 (1.75%) and NR-24 (1.5%). BRAF mutations were associated with >5 bp shortening of HSP110T17.

CONCLUSION

Patients with high-frequency MSI CRC had allelic variations of HSP110T17. BRAF mutations occur along with greater shortening in HSP110T17 during oncogenesis via the MSI pathway.

MSH3 polymorphisms and protein levels affect CAG repeat instability in Huntington's disease mice

Expansions of trinucleotide CAG/CTG repeats in somatic tissues are thought to contribute to ongoing disease progression through an affected individual's life with Huntington's disease or myotonic dystrophy. Broad ranges of repeat instability arise between individuals with expanded repeats, suggesting the existence of modifiers of repeat instability. Mice with expanded CAG/CTG repeats show variable levels of instability depending upon mouse strain. However, to date the genetic modifiers underlying these differences have not been identified. We show that in liver and striatum the R6/1 Huntington's disease (HD) (CAG)∼100 transgene, when present in a congenic C57BL/6J (B6) background, incurred expansion-biased repeat mutations, whereas the repeat was stable in a congenic BALB/cByJ (CBy) background. Reciprocal congenic mice revealed the Msh3 gene as the determinant for the differences in repeat instability. Expansion bias was observed in congenic mice homozygous for the B6 Msh3 gene on a CBy background, while the CAG tract was stabilized in congenics homozygous for the CBy Msh3 gene on a B6 background. The CAG stabilization was as dramatic as genetic deficiency of Msh2. The B6 and CBy Msh3 genes had identical promoters but differed in coding regions and showed strikingly different protein levels. B6 MSH3 variant protein is highly expressed and associated with CAG expansions, while the CBy MSH3 variant protein is expressed at barely detectable levels, associating with CAG stability. The DHFR protein, which is divergently transcribed from a promoter shared by the Msh3 gene, did not show varied levels between mouse strains. Thus, naturally occurring MSH3 protein polymorphisms are modifiers of CAG repeat instability, likely through variable MSH3 protein stability. Since evidence supports that somatic CAG instability is a modifier and predictor of disease, our data are consistent with the hypothesis that variable levels of CAG instability associated with polymorphisms of DNA repair genes may have prognostic implications for various repeat-associated diseases.

The genetic instability of repetitive DNA sequences in particular genes can lead to numerous neurodegenerative, neurological, and neuromuscular diseases. These diseases show progressively increasing severity of symptoms through the life of the affected individual, a phenomenon that is linked with increasing instability of the repeated sequences as the person ages. There is variability in the levels of this instability between individuals—the source of this variability is unknown. We have shown in a mouse model of repeat instability that small differences in a certain DNA repair gene, MSH3, whose protein is known to fix broken DNA, can lead to variable levels of repeat instability. These DNA repair variants lead to different repair protein levels, where lower levels lead to reduced repeat instability. Our findings reveal that such naturally occurring variations in DNA repair genes in affected humans may serve as a predictor of disease progression. Moreover, our findings support the concept that pharmacological reduction of MSH3 protein should reduce repeat instability and disease progression.

Implication of microsatellite instability in human gastric cancers

Microsatellite instability, one of the phenomena implicated in gastric cancer, is mainly associated with the expansion or contraction of microsatellite sequences due to replication errors caused most frequently by mutations in the mismatch repair (MMR) and tumour suppressor genes. Tumours exhibiting microsatellite instability are proven to have truncated products resulting from frequent mutations in mononucleotide or dinucleotide runs in coding and non-coding regions of the targeted genes. Epigenetic changes like hypermethylation of the promoter region of MMR genes as well as gene silencing are also responsible for the microsatellite instability phenotypes. Assessing microsatellite instability in tumours has proved to be an efficient tool for the prognosis of various cancers including colorectal and gastric cancers. Such tumours are characterized by distinct clinicopathological profiles. Biotic agents like Epstein Barr Virus and H. pylori along with other factors like family history, diet and geographical location also play an important role in the onset of gastric carcinogenesis. Instability of mitochondrial DNA has also been investigated and claimed to be involved in the occurrence of gastric cancers in humans. Development of simplified but robust and reproducible microsatellite instability based molecular tools promises efficient prognostic assessment of gastric tumours.

FANCJ expression predicts the response to 5-fluorouracil-based chemotherapy in MLH1-proficient colorectal cancer

PURPOSE

Fanconi anemia protein, FANCJ, directly interacts with MLH1, a key protein involved in DNA mismatch repair. Deficient mismatch repair, or microsatellite instability, is a potent marker for the ineffectiveness of 5-fluorouracil (5-FU) in colorectal cancer (CRC). We investigated the significance of FANCJ expression in CRC, focusing on the effects of 5-FU-based adjuvant chemotherapy.

METHODS

Clinicopathologic features and immunohistochemical expression of FANCJ and MLH1 were studied in 219 patients with CRC. We also analyzed 5-FU sensitivity in CRC cell lines with varying levels of FANCJ expression.

RESULTS

FANCJ expression was elevated in tumor tissues compared with normal epithelial tissue. High expression of FANCJ was significantly associated with 5-FU resistance measured by the SDI test (P < 0.05) and poor recurrence-free survival (RFS) (P < 0.05). Among patients with stage II/III tumors who received 5-FU, patients with tumors exhibiting high FANCJ expression had significantly worse RFS than did patients with tumors exhibiting low FANCJ expression (P < 0.01). Among patients who did not receive adjuvant chemotherapy, FANCJ expression was not correlated with RFS (P = 0.76). High FANCJ expression was correlated with 5-FU resistance in tumors with normal MLH1 expression (P < 0.05) but not in tumors not expressing MLH1 (P = 0.9). In vitro, FANCJ overexpression was correlated with 5-FU resistance in MLH1-proficient HCT116 3-6 cells but not in MLH1-deficient HCT116 cells.

CONCLUSIONS

FANCJ could be a useful biomarker to predict the response to 5-FU and prognosis of CRC, particularly in tumors with normal MLH1 expression.

Isolating microsatellite loci: looking back, looking ahead

Microsatellite DNA loci are tandemly repeated simple sequence repeats (SSRs) that are ubiquitous in eukaryotic genomes. When flanked by unique sequences, length variation (driven by high rates of strand slippage during DNA replication) at a given repeat locus can be assayed by PCR and electrophoretic separation of the resulting DNA fragments (representing alleles defined by fragment size or repeat number at that locus). In nonmodel organisms that do not have sequence information at SSR loci (or at SSRs in a closely related taxon), microsatellites must be isolated and sequenced de novo. Traditionally, this has been accomplished with cloning of genomic DNA fragments enriched for SSRs, a protocol described in detail here. PCR primers flanking microsatellite repeats can be used to assay repeat length variation among individuals (typically through fluorescent labeling of one strand and capillary electrophoresis), useful for questions related to population variation, individual assignment, mating studies, selection scans, mapping, and phenotypic traits. High-throughput next-generation sequencing will likely supplant traditional cloning methods for the discovery of microsatellite loci.

Advances in understanding the relationship between cyclin E and human colorectal cancer

Cell cycle deregulation is one of important mechanisms leading to human colorectal cancer. It has been revealed that cyclin E is the most important regulatory factor for cell cycle control and plays an important role in the occurrence and development of human colorectal cancer. Detection of cyclin E expression can be used to assess the prognosis of colorectal cancer.

Discovery of EST-SSRs in lung cancer: tagged ESTs with SSRs lead to differential amino acid and protein expression patterns in cancerous tissues

Tandem repeats are found in both coding and non-coding sequences of higher organisms. These sequences can be used in cancer genetics and diagnosis to unravel the genetic basis of tumor formation and progression. In this study, a possible relationship between SSR distributions and lung cancer was studied by comparative analysis of EST-SSRs in normal and lung cancerous tissues. While the EST-SSR distribution was similar between tumorous tissues, this distribution was different between normal and tumorous tissues. Trinucleotides tandem repeats were highly different; the number of trinucleotides in ESTs of lung cancer was 3 times higher than normal tissue. Significant negative correlation between normal and cancerous tissue showed that cancerous tissue generates different types of trinucleotides. GGC and CGC were the more frequent expressed trinucleotides in cancerous tissue, but these SSRs were not expressed in normal tissue. Similar to the EST level, the expression pattern of EST-SSRs-derived amino acids was significantly different between normal and cancerous tissues. Arg, Pro, Ser, Gly, and Lys were the most abundant amino acids in cancerous tissues, and Leu, Cys, Phe, and His were significantly more abundant in normal tissues than in cancerous tissues. Next, the putative functions of triplet SSR-containing genes were analyzed. In cancerous tissue, EST-SSRs produce different types of proteins. Chromodomain helicase DNA binding proteins were one of the major protein products of EST-SSRs in the cancerous library, while these proteins were not produced from EST-SSRs in normal tissue. For the first time, the findings of this study confirmed that EST-SSRs in normal lung tissues are different than in unhealthy tissues, and tagged ESTs with SSRs cause remarkable differences in amino acid and protein expression patterns in cancerous tissue. We suggest that EST-SSRs and EST-SSRs differentially expressed in cancerous tissue may be suitable candidate markers for lung cancer diagnosis and prediction.

Microsatellite instability in pediatric high grade glioma is associated with genomic profile and differential target gene inactivation

High grade gliomas (HGG) are one of the leading causes of cancer-related deaths in children, and there is increasing evidence that pediatric HGG may harbor distinct molecular characteristics compared to adult tumors. We have sought to clarify the role of microsatellite instability (MSI) in pediatric versus adult HGG. MSI status was determined in 144 patients (71 pediatric and 73 adults) using a well established panel of five quasimonomorphic mononucleotide repeat markers. Expression of MLH1, MSH2, MSH6 and PMS2 was determined by immunohistochemistry, MLH1 was assessed for mutations by direct sequencing and promoter methylation using MS-PCR. DNA copy number profiles were derived using array CGH, and mutations in eighteen MSI target genes studied by multiplex PCR and genotyping. MSI was found in 14/71 (19.7%) pediatric cases, significantly more than observed in adults (5/73, 6.8%; p = 0.02, Chi-square test). MLH1 expression was downregulated in 10/13 cases, however no mutations or promoter methylation were found. MSH6 was absent in one pediatric MSI-High tumor, consistent with an inherited mismatch repair deficiency associated with germline MSH6 mutation. MSI was classed as Type A, and associated with a remarkably stable genomic profile. Of the eighteen classic MSI target genes, we identified mutations only in MSH6 and DNAPKcs and described a polymorphism in MRE11 without apparent functional consequences in DNA double strand break detection and repair. This study thus provides evidence for a potential novel molecular pathway in a proportion of gliomas associated with the presence of MSI.

Fusion tyrosine kinase NPM-ALK Deregulates MSH2 and suppresses DNA mismatch repair function novel insights into a potent oncoprotein

The fusion tyrosine kinase NPM-ALK is central to the pathogenesis of ALK-positive anaplastic large cell lymphoma (ALK(+)ALCL). We recently identified that MSH2, a key DNA mismatch repair (MMR) protein integral to the suppression of tumorigenesis, is an NPM-ALK-interacting protein. In this study, we found in vitro evidence that enforced expression of NPM-ALK in HEK293 cells suppressed MMR function. Correlating with these findings, six of nine ALK(+)ALCL tumors displayed evidence of microsatellite instability, as opposed to none of the eight normal DNA control samples (P = 0.007, Student's t-test). Using co-immunoprecipitation, we found that increasing levels of NPM-ALK expression in HEK293 cells resulted in decreased levels of MSH6 bound to MSH2, whereas MSH2·NPM-ALK binding was increased. The NPM-ALK·MSH2 interaction was dependent on the activation/autophosphorylation of NPM-ALK, and the Y191 residue of NPM-ALK was a crucial site for this interaction and NPM-ALK-mediated MMR suppression. MSH2 was found to be tyrosine phosphorylated in the presence of NPM-ALK. Finally, NPM-ALK impeded the expected DNA damage-induced translocation of MSH2 out of the cytoplasm. To conclude, our data support a model in which the suppression of MMR by NPM-ALK is attributed to its ability to interfere with normal MSH2 biochemistry and function.

Childhood brain tumours due to germline bi-allelic mismatch repair gene mutations

Childhood brain tumours may be due to germline bi-allelic mismatch repair (MMR) gene mutations in MLH1, MSH2, MSH6 or PMS2. These mutations can also lead to colorectal neoplasia and haematological malignancies. Here, we review this syndrome and present siblings with early-onset rectal adenoma and papillary glioneural brain tumour, respectively, due to novel germline bi-allelic PMS2 mutations. Identification of MMR protein defects can lead to early diagnosis of this condition. In addition, assays for these defects may help to classify brain tumours for research protocols aimed at targeted therapies.

Concurrent genetic alterations in DNA polymerase proofreading and mismatch repair in human colorectal cancer

Genomic sequences encoding the 3' exonuclease (proofreading) domains of both replicative DNA polymerases, pol delta and pol epsilon, were explored simultaneously in human colorectal carcinomas including six established cell lines. Three unequivocal sequence alterations, including one previously reported, were found, and all these were considered as dysfunctional mutations in light of the local amino-acid sequences. In particular, the F367S mutation found in the POLE gene encoding the pol epsilon catalytic subunit, which includes the proofreading domain, is the first found in human diseases. Surprisingly, the tumours carrying these proofreading domain mutations were all defective in DNA mismatch repair (MMR). In addition to the two cell lines with acknowledged MMR gene mutations, the third tumour was also demonstrated to harbour a distinct mutation in MLH1, and indeed exhibited a microsatellite-unstable phenotype. These findings suggest that, in concert with MMR deficiency, defective polymerase proofreading may also contribute to the mutator phenotype observed in human colorectal cancer. Our observations may suggest previously unrecognised complexities in the molecular abnormalities underlying the mutator phenotype in human neoplasms.

High incidence and frequency of LOH are associated with aggressive features of high-grade HER2 and triple-negative breast cancers

BACKGROUND

Basal-like and HER2-overexpressing breast carcinomas are histologically undifferentiated, high-grade tumors with a high proliferation rate and associated with a poor outcome. Most basal-like tumors lack the expression of ER, PR, and HER2 (triple-negative; TN). Loss of heterozygosity (LOH) is thought to reflect random chromosomal instability, and recent studies have shown that DNA-copy number alterations or LOH occur with a high frequency in basal-like and HER2-amplified tumors.

METHODS

The levels and patterns of LOH were analyzed by the microsatellite alteration analysis using fluorescence-labeled primers and an automated DNA sequencer at 5 randomly selected loci in 246 Japanese primary breast cancers. Associations between the level of LOH and breast cancer subtypes and tumor aggressiveness were investigated.

RESULTS

The incidence and frequency of LOH was significantly higher in HER2 (56.3, 26.7%) and TN groups (44.4, 27.1%) than in luminal A (ER-positive and/or PR-positive and HER2-negative) groups (32.0, 12.2%). The incidence and frequency of LOH increased as nuclear grade was elevated. There were significantly more grade 3 tumors in the HER2 (80.0%) and TN (68.2%) subgroups (p < 0.0001). Even in HER2 and TN cases, the incidence and frequency of LOH was significantly higher in nuclear grade 3 cases than in grade 1 or 2 cases. Relapse-free survival of patients with LOH was significantly shorter than for those without LOH. In addition, the survival time was shorter as the frequency of LOH elevated. The incidence of LOH was an independent prognostic factor for relapse-free survival by multivariate analysis.

CONCLUSION

High incidence and frequency of LOH, which indicate increased genetic instability, were found to be associated with the aggressive features of high-grade HER2 and TN breast cancers.

The Mutyh base excision repair gene influences the inflammatory response in a mouse model of ulcerative colitis

BACKGROUND

The Mutyh DNA glycosylase is involved in the repair of oxidized DNA bases. Mutations in the human MUTYH gene are responsible for colorectal cancer in familial adenomatous polyposis. Since defective DNA repair genes might contribute to the increased cancer risk associated with inflammatory bowel diseases, we compared the inflammatory response of wild-type and Mutyh(-/-) mice to oxidative stress.

METHODOLOGY/PRINCIPAL FINDINGS

The severity of colitis, changes in expression of genes involved in DNA repair and inflammation, DNA 8-oxoguanine levels and microsatellite instability were analysed in colon of mice treated with dextran sulfate sodium (DSS). The Mutyh(-/-) phenotype was associated with a significant accumulation of 8-oxoguanine in colon DNA of treated mice. A single DSS cycle induced severe acute ulcerative colitis in wild-type mice, whereas lesions were modest in Mutyh(-/-) mice, and this was associated with moderate variations in the expression of several cytokines. Eight DSS cycles caused chronic colitis in both wild-type and Mutyh(-/-) mice. Lymphoid hyperplasia and a significant reduction in Foxp3(+) regulatory T cells were observed only in Mutyh(-/-) mice.

CONCLUSIONS

The findings indicate that, in this model of ulcerative colitis, Mutyh plays a major role in maintaining intestinal integrity by affecting the inflammatory response.

The distribution of microsatellites in the Nasonia parasitoid wasp genome

Microsatellites are important molecular markers used in numerous genetic contexts. Despite this widespread use, the evolutionary processes governing microsatellite distribution and diversity remain controversial. Here, we present results on the distribution of microsatellites of three species in the parasitic wasp genus Nasonia generated by an in silico data-mining approach. Our results show that the overall microsatellite density in Nasonia is comparable to that of the honey bee, but much higher than in eight non-Hymenopteran arthropods. Across the Nasonia vitripennis genome, microsatellite density varied both within and amongst chromosomes. In contrast to other taxa, dinucleotides are the most abundant repeat type in all four species of Hymenoptera studied. Whether the differences between the Hymenoptera and other taxa are of functional significance remains to be determined.

De novo hepatocellular carcinoma in a liver graft with sustained hepatitis C virus clearance after living donor liver transplantation

The occurrence of de novo hepatocellular carcinoma (HCC) after liver transplantation (LT) for advanced HCCs has been extremely limited. In this article, a case of de novo HCC in a liver graft with sustained hepatitis C virus clearance after living donor liver transplantation (LDLT) for multiple HCCs and hepatitis C cirrhosis is reported. The recipient was a 58-year-old female, and the left lobe living donor was the 30-year-old healthy daughter of the recipient. Three years after LDLT, the patient received 48 weeks of interferon treatment for recurrent hepatitis C with advanced fibrosis. The patient has shown successful viral clearance since then. However, an HCC was recognized in the liver graft during a follow-up computed tomography scan performed 6 years after LDLT, and it was surgically resected. To analyze its origin [either from the patient (metastatic) or from the living donor (de novo)], genotyping by microsatellite analysis of tissue and blood samples from the donor and recipient was performed, and it revealed that the HCC originated from the donor. To the best of our knowledge, this is the first report of de novo HCC in a liver graft with sustained hepatitis C virus clearance after LT for advanced HCCs and hepatitis C cirrhosis.

Chemosensitivity and survival in gastric cancer patients with microsatellite instability

INTRODUCTION

Conflicting data exist regarding the relevance of high-frequency microsatellite instability (MSI-H) for predicting the prognosis and benefits of 5-fluorouracil (5-FU)-based chemotherapy. This study investigated the usefulness of MSI as either a prognostic indicator or predictor of distinct clinical attributes regarding the use of adjuvant chemotherapy with 5-FU and its analogues in gastric cancer.

MATERIALS AND METHODS

Data and tumor specimens were collected from 240 gastric cancer patients from 1993 to 2002. Five microsatellite loci were analyzed using a high-intensity microsatellite analysis reported previously. A Cox proportional hazard model was used to compare the clinical data and survival as well as any associations between MSI and 5-FU treatment status of patients with MSI or microsatellite stability (MSS) gastric cancers. A 3-(4,5-dimethyl-2-thiazolyl) -2,5-diphenyl-2H-tetrazolium bromide (MTT) assay was conducted in 168 cases to investigate chemosensitivity to 5-FU.

RESULTS

This analysis identified 22 MSI-H (9.4%), 25 MSI-L (10.7%), and 193 MSS (79.9%) tumors. Gastric cancer with MSI-H tended to have increased likelihood to show higher age, antral location of the tumor, and lymph vessel involvement (P < 0.05). Univariate analyses failed to show any difference between the MSI-H and MSS/MSI-L groups with respect to overall survival. Furthermore, survival after the administration of 5-FU did not correlate with MSI status, and MSI was not associated with 5-FU sensitivity by MTT assay.

CONCLUSION

The results of this study indicate that MSI status has no clear influence on overall survival or response to 5-FU in gastric cancer.

Role of MUTYH and MSH2 in the Control of Oxidative DNA Damage, Genetic Instability, and Tumorigenesis

Mismatch repair is the major pathway controlling genetic stability by removing mispairs caused by faulty replication and/or mismatches containing oxidized bases. Thus, inactivation of the Msh2 mismatch repair gene is associated with a mutator phenotype and increased cancer susceptibility. The base excision repair gene Mutyh is also involved in the maintenance of genomic integrity by repairing premutagenic lesions induced by oxidative DNA damage. Because evidence in bacteria suggested that Msh2 and Mutyh repair factors might have some overlapping functions, we investigated the biological consequences of their single and double inactivation in vitro and in vivo. Msh2−/− mouse embryo fibroblasts (MEF) showed a strong mutator phenotype at the hprt gene, whereas Mutyh inactivation was associated with a milder phenotype (2.9 × 10−6 and 3.3 × 10−7 mutation/cell/generation, respectively). The value of 2.7 × 10−6 mutation/cell/generation in Msh2−/−Mutyh−/− MEFs did not differ significantly from Msh2−/− cells. When steady-state levels of DNA 8-oxo-7,8-dihydroguanine (8-oxoG) were measured in MEFs of different genotypes, single gene inactivation resulted in increases similar to those observed in doubly defective cells. In contrast, a synergistic accumulation of 8-oxoG was observed in several organs of Msh2−/−Mutyh−/− animals, suggesting that in vivo Msh2 and Mutyh provide separate repair functions and contribute independently to the control of oxidative DNA damage. Finally, a strong delay in lymphomagenesis was observed in Msh2−/−Mutyh−/− when compared with Msh2−/− animals. The immunophenotype of these tumors indicate that both genotypes develop B-cell lymphoblastic lymphomas displaying microsatellite instability. This suggests that a large fraction of the cancer-prone phenotype of Msh2−/− mice depends on Mutyh activity. [Cancer Res 2009;69(10):4372–9]

Simulation-based analyses reveal stable microsatellite sequences in human pancreatic cancer

Genomic analysis using tissue samples is an essential approach in cancer genetics. However, technical and biological limits exist in this approach. Microsatellite instability (MSI) is frequently observed in human tumors. MSI assays are now prevalent and regarded as commonplace. However, several technical problems have been left unsolved in the conventional assay technique. Indeed, the reported frequencies of MSI differ widely in each malignancy. An example is pancreatic cancer. Using a unique fluorescent technique, we found that MSI is extremely infrequent in this malignancy, despite the relatively high frequencies in some reports. In a series of simulations, we have demonstrated that the extremely low frequency was derived neither from less sensitive assays nor from a scarcity of cancer cells in tissue samples. Furthermore, analyzing laser-capture microdissection (LCM)-processed cell populations of a microsatellite-unstable colorectal cancer cell line, HCT116, we have shown that MSI can be detected only when comparing two cell populations that have grown independently to a sufficiently large size. When MSI is not detected in analyses using tissue samples, LCM is not advisable. We therefore did not extend our study to LCM of tissue specimens. We conclude that microsatellite sequence alterations are not detectable in human pancreatic cancer.

Type A microsatellite instability in pediatric gliomas as an indicator of Turcot syndrome

Microsatellite instability (MSI) is present in hereditary conditions due to mismatch repair (MMR) gene mutations. Following MSI analysis, tumor samples are classified into MSS (stable), MSI-L (low instability), and MSI-H (high instability) based on the fraction of unstable loci. Another MSI-based classification takes into account the size difference between mutant alleles in tumor DNA compared to wild-type alleles; two types of MSI, A and B, are recognized using this approach, type A being characterized by smaller, more subtle allelic shifts compared to type B. Biallelic mutations of MMR genes are associated with pediatric cancers, including glial tumors, in Turcot syndrome type 1 (TS1). However, most TS1-associated gliomas so far analyzed did not display MSI. We investigated the frequency of MSI in a series of 34 pediatric gliomas of different grade using a panel of five mononucleotide quasimonomorphic markers. Subtle qualitative changes were observed for the majority of markers in two glioblastomas (5.9% of the total series and 33.3% of glioblastomas). In both cases, family histories were compatible with TS1, and mutations of the PMS2 and MLH1 genes were identified. In one family, the MSI patterns were compared between the glioblastoma and a colon cancer from an affected relative, showing a clear qualitative difference, with the former displaying type A and the latter type B instability, respectively. These results were confirmed using additional microsatellite markers, indicating that knowledge of the association between TS1-related glial tumors and subtle type A MSI is important for full ascertainment of TS1 patients and appropriate counselling.

The prevalence of microsatellite instability in head and neck squamous cell carcinoma

PURPOSE

The present study aimed to use the most definitive available techniques to resolve controversy in the literature as to the prevalence of microsatellite instability (MSI) in head and neck squamous cell carcinoma (HNSCC).

METHODS

Eighty patients with advanced HNSCC were enrolled in the study that examined 20 microsatellite markers with automatic fragment analysis. These markers included ones derived from the NCI reference panel and ones previously reported to detect MSI in HNSCC (HNSCC panel).

RESULTS

Only one of 80 tumors could be considered positive for MSI. For this case, both panels showed MSI-High (8/10 positive markers for the NCI reference panel and 6/10 positive markers for the HNSCC panel). Qualitatively, the observed MSI could be classified as Type B MSI.

CONCLUSIONS

The present results indicate that MSI has a low prevalence in HNSCC.

Exclusive KRAS mutation in microsatellite-unstable human colorectal carcinomas with sequence alterations in the DNA mismatch repair gene, MLH1

Microsatellite instability (MSI) is regarded as reflecting defective DNA mismatch repair (MMR). MMR defects lead to an increase in point mutations, as well as repeat instability, on the genome. However, despite the highly unstable microsatellites, base substitutions in representative oncogenes or tumor suppressors are extremely infrequent in MSI-positive tumors. Recently, the heterogeneity in MSI-positive colorectal tumors is pointed out, and the 'hereditary' and 'sporadic settings' are proposed. Particularly in the former, base substitution mutations in KRAS are regarded as relatively frequent. We sequenced the KRAS gene in a panel of 76 human colorectal carcinomas in which the MSI status has been determined. KRAS mutations were detected in 22 tumors (28.9%). Intriguingly, all of the KRAS-mutant MSI-H (high) tumors harbored sequence alterations in an essential MMR gene, MLH1, which implies that KRAS mutation more frequently and almost exclusively occurs in MMR gene-mutant MSI-H tumors. Furthermore, in contrast with the prevailing viewpoint, some of these tumors are derived from sporadic colorectal cancer patients. The tight connection between MMR gene mutation and KRAS mutation may suggest previously unrecognized complexities in the relationship between MSI and the mutator phenotype derived from defective MMR.

Carcinogenesis and microsatellite instability: the interrelationship between genetics and epigenetics

DNA mismatch repair (MMR) deficiency results in a strong mutator phenotype and high-frequency microsatellite instability (MSI-H), which are the hallmarks of tumors arising within Lynch syndrome. MSI-H is characterized by length alterations within simple repeated sequences, microsatellites. Lynch syndrome is primarily due to germline mutations in one of the DNA MMR genes; mainly hMLH1 or hMSH2 and less frequently hMSH6 and rarely hPMS2. Germline hemiallelic methylation of MLH1, termed epimutation, has been reported to be a new cause of Lynch syndrome. MSI-H is also observed in approximately 15% of colorectal, gastric and endometrial cancers and in lower frequencies in a minority of other tumors, where it is associated with the hypermethylation of the promoter region of hMLH1. MSI-H underlies a distinctive tumorigenic pathway because cancers with MSI-H exhibit many differences in genotype and phenotype relative to cancers without MSI-H, irrespective of their hereditary or sporadic origins. Genetic, epigenetic and transcriptomic differences exist between cancers with and those without the MSI-H. The BRAF V600E mutation is associated with sporadic MSI-H colorectal cancers (CRCs) harboring hMLH1 methylation but not Lynch syndrome-related CRCs. The differences in genotype and phenotype between cancers with and those without MSI-H are likely to be causally linked to their differences in biological and clinical features. Therefore, the diagnosis of MSI-H in cancers is thus considered to be of increasing relevance, because MSI-H is a useful screening marker for identifying patients with Lynch syndrome, a better prognostic factor and could affect the efficacy of chemotherapy. This review addresses recent advances in the field of microsatellite instability research.

DNA repair dysfunction in gastrointestinal tract cancers

The DNA repair system surveys the genome, which is always suffering from exposure to both exogenous as well as endogenous mutagens, to maintain the genetic information. The fact that the basis of this DNA repair system is highly conserved, from prokaryote to mammalian cells, suggests the importance of precise genome maintenance mechanisms for organisms. In the past 15 years, considerable progress has been made in understanding how repair processes interact and how disruptions of these mechanisms lead to the accumulation of mutations and carcinogenesis. In 1993, two groups reported that DNA mismatch repair could be associated with hereditary non-polyposis colorectal cancer, indicating a connection between faulty DNA repair function and cancer. More recently, an inherited disorder of DNA glycosylase, which removes mutagenic oxidized base from DNA, has been reported in individuals with a predisposition to multiple colorectal adenomas and carcinomas. This is the first report that directly indicates the role of the repair of oxidative DNA in human inherited cancer. Studies from gene knockout mice have elucidated the principal role of these repair systems in the process of carcinogenesis. Moreover, clinical samples derived from cancer patients have shown the direct involvement. This review focuses on the function of DNA mismatch repair and oxidative DNA/nucleotide repair among various DNA repair systems in cells, both of which are essentially involved in the carcinogenesis of gastrointestinal tract cancer.

The genome-wide determinants of human and chimpanzee microsatellite evolution

Mutation rates of microsatellites vary greatly among loci. The causes of this heterogeneity remain largely enigmatic yet are crucial for understanding numerous human neurological diseases and genetic instability in cancer. In this first genome-wide study, the relative contributions of intrinsic features and regional genomic factors to the variation in mutability among orthologous human-chimpanzee microsatellites are investigated with resampling and regression techniques. As a result, we uncover the intricacies of microsatellite mutagenesis as follows. First, intrinsic features (repeat number, length, and motif size), which all influence the probability and rate of slippage, are the strongest predictors of mutability. Second, mutability increases nonuniformly with length, suggesting that processes additional to slippage, such as faulty repair, contribute to mutations. Third, mutability varies among microsatellites with different motif composition likely due to dissimilarities in secondary DNA structure formed by their slippage intermediates. Fourth, mutability of mononucleotide microsatellites is impacted by their location on sex chromosomes vs. autosomes and inside vs. outside of Alu repeats, the former confirming the importance of replication and the latter suggesting a role for gene conversion. Fifth, transcription status and location in a particular isochore do not influence microsatellite mutability. Sixth, compared with intrinsic features, regional genomic factors have only minor effects. Finally, our regression models explain approximately 90% of variation in microsatellite mutability and can generate useful predictions for the studies of human diseases, forensics, and conservation genetics.

TGF-betaRII, BAX, IGFIIR, caspase-5, hMSH3 and hMSH6 alterations are not associated with microsatellite instability or p53 mutations in invasive urothelial carcinoma of the urinary bladder

AIM

The aim of this study was to determine the potential synchronous contribution of alterations in TGF-betaRII, BAX, IGFIIR, caspase-5, hMSH3 and hMSH6 genes to the development and clinical outcome of bladder cancer, in relation to p53 mutations, microsatellite status and hMLH1/hMSH2 expression.

METHODS

Molecular biology techniques as well as immunohistochemistry were applied in 69 samples from patients with urothelial carcinoma.

RESULTS

Microsatellite alterations were observed in TGF-betaRII(A)(10 )(16%) and BAX(G)(8 )(3%), irrespective of the presence of p53 mutations, but not in IGFIIR(G)(8), caspase-5(A)(10, ) hMSH3(A)(8) and hMSH6(C)(8). A statistically significant correlation could be found only between hMLH1 expression and the presence of microsatellite instability (Fisher's exact test, p = 0,013). Survival analysis indicated that apart from grade and T-category, hMLH1 expression was the only parameter significantly affecting overall survival (p = 0.021 in univariate and p = 0.015 in multivariate analysis) and recurrence-free survival (p = 0.0463 in univariate and p = 0.022 in multivariate analysis).

CONCLUSIONS

We conclude that alterations of the examined target genes of MSI are rare in urinary bladder carcinoma and they are not associated with microsatellite instability or the presence of p53 mutations.

The MRE11/RAD50/NBS1 complex destabilization in Lynch-syndrome patients

Lynch syndrome is an inherited disease leading to the development predominantly of colorectal cancer (CRC). The crucial cause is malfunction of DNA mismatch repair that is characterized by high level of microsatellite instability; however, new knowledge of two MSI modes (types A and B) suggests a more complex molecular basis of this syndrome. To investigate, whether the extensive alterations in individual MSI markers (type B) can indicate the potential deficiency of DNA double-strand break (DSB) repair in Lynch-syndrome-related tumours, we evaluated the MSI type and alterations in the MRE11 and RAD50 repeats that are associated with the reduced protein expression and functional impairment of the MRE11-RAD50-NBS1 (MRN) complex. Of 27 CRCs, 21 samples manifested type B in at least one MSI+ marker. From type B tumours, the genetic alterations were identified in 16 (76%) samples; seven, one and eight cases manifested mutations in MRE11, RAD50 and both genes, respectively. However, predominantly biallelic MRE11 alterations with simultaneously developed RAD50 mutations impaired the protein expressions with different intensity and location in tumour. Of six tumours presenting changes Collapse

Key Words Collapse

MESH Headings

• Acid Anhydride Hydrolases
• Adult
• Cell Cycle Proteins/genetics
• Colorectal Neoplasms, Hereditary Nonpolyposis/genetics
• DNA Breaks, Double-Stranded
• DNA Mismatch Repair
• DNA Repair Enzymes/genetics
• DNA, Neoplasm
• DNA-Binding Proteins/genetics
• Humans
• MRE11 Homologue Protein
• Microsatellite Instability
• Mutation
• Nuclear Proteins/genetics

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Grants Collapse

Affiliation(s)

Aster Alemayehu Laboratory of Cancer Genetics, Cancer Research Institute of Slovak Academy of Sciences, Bratislava, Slovakia
Ivana Fridrichova

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High-resolution fluorescent analysis of microsatellite instability in gastric cancer

BACKGROUND

Microsatellite instability (MSI) is associated with various human malignancies and regarded as reflecting cellular deficiency in DNA mismatch repair (MMR). Analysis of MSI has been prevalent in the field of oncology, and numerous data have accumulated in the literature. It has been reported that the MSI+ phenotype is relatively frequent in gastric cancer. The reported frequencies of MSI+ gastric tumors, however, are diverse.

AIM AND METHODS

To determine the frequencies of the MSI+ phenotype and defective MMR in gastric cancer, we examined tumors derived from 167 patients with sporadic gastric cancer, using our unique fluorescent technique, 'high-resolution fluorescent microsatellite analysis'.

RESULTS

High-resolution fluorescent microsatellite analysis allowed us the unequivocal designation of MSI. The frequencies of MSI-H and MSI-L were 11 and 9.6%, respectively. In addition to the distinction based on the frequency of microsatellite changes, MSI was classifiable into two distinct categories, type A and type B, according to the mode of length changes in the dinucleotide microsatellites. Type A and type B MSI were observed in 14 and 6.6%, respectively. The overall frequency of MSI was 21%. Intriguingly, MSI did not correlate with any of commonly used clinicopathological variables. In addition, neither MSI-H nor MSI-L correlated with family history of malignancies or patient history of multiple cancers. Instead, type B MSI was significantly more frequent in patients with family history of gastric cancer. Type A MSI appeared to occur more frequently in tumors of patients with a history of double cancer, which, however, was not statistically significant.

CONCLUSION

In gastric cancer, contribution of defective MMR to the risk of multiple cancer or familial predisposition appears more limited than has been expected. The relationship between MSI and high risk of cancer may have been oversimplified, at least in gastric cancer.

The associated contributions of p53 and the DNA mismatch repair protein Msh6 to spontaneous tumorigenesis

DNA mismatch repair (MMR) is a highly conserved system that repairs DNA adducts acquired during replication, as well as some forms of exogenous/endogenous DNA damage. Additionally, MMR proteins bind to DNA adducts that are not removed by MMR and influence damage-response mechanisms other than repair. Hereditary non-polyposis colorectal cancer, as well as mouse models for MMR deficiency, illustrate that MMR proteins are required for maintenance of genetic stability and tumor suppression. In both humans and mice, the phenotype associated with Msh6-associated tumorigenesis is distinct from that of Msh2. In this study, we hypothesized that Msh6-/-;p53+/- mice would display earlier tumor onset than their Msh6-/- or p53+/- counterparts, indicating that concomitant loss of these two tumor suppressors contributes to tumorigenesis via mechanisms that are only partially interrelated. We generated a Msh6-/-;p53+/- mouse model which succumbed to malignant disease at an accelerated rate and with a tumor spectrum distinct from both Msh6-/- and p53+/- models. Alteration of tumor phenotype in the Msh6-/-;p53+/- mice included a marked increase in microsatellite instability that was associated with loss of heterozygosity of the remaining p53 allele. Also, genetic instability was inversely correlated with survival. This manuscript marks the first in vivo investigation into the association between Msh6 and p53, and their combined role in the suppression of spontaneous tumorigenesis, cell survival and genomic stability. Our results support the hypothesis that p53 and Msh6 are functionally interrelated and that, with concomitant mutation, these tumor suppressors act together to accelerate tumorigenesis.

The clinical relevance of microsatellite alterations in head and neck squamous cell carcinoma: a critical review

Triggered by the existing confusion in the field, the current paper aimed to review the current knowledge of both microsatellite instability (MSI) and loss of heterozygosity (LOH) detected by microsatellite markers in head and neck squamous cell carcinoma (HNSCC), and to provide the reader with an assessment of their prognostic and predictive value in this tumor type. For both MSI and LOH, various detection methods were included such as mono- and polynucleotidemarkers and gel- as well as automated analyses. Only studies based on PCR techniques with microsatellite markers were considered. Taking the methodological problems occurring in investigations with microsatellite markers into account, LOH seems to be more common than MSI in HNSCC. Although both types of microsatellite alterations have been correlated with clinicopathological features of this tumor type, only LOH seems to have a clear prognostic value. The predictive value of both MSI and LOH is debatable. More research has to be performed to clearly establish LOH detection as a translational application in the HNSCC field, aiming to predict response to treatments or outcome, and eventually to use as a therapeutic target.

Molecular testing for microsatellite instability and its value in tumor characterization

Molecular analysis of tumor tissue has become a rapidly expanding field in medical research, exploiting the advantages of new technologies adapted to high-throughput examination of genetic alterations, gene and protein expression patterns. Only exceptionally, these approaches have found their way into routine clinical diagnosis and therapy. Microsatellite instability testing has been established as a very powerful tool to identify patients with hereditary nonpolyposis colorectal cancer, one of the most common familial cancer syndromes. In addition, there is emerging evidence that microsatellite instability analysis may become increasingly important for the clinician, having considerable impact on patients' prognosis as well as therapeutic decisions, at least in colorectal cancer patients. A better understanding of the microsatellite instability phenotype, its pathogenesis and implications for the course of the disease will pave the way for novel diagnostic and therapeutic strategies specifically tailored to microsatellite-unstable tumors. This review summarizes the current significance of molecular testing for microsatellite instability in several tumor entities and provides prospects of future developments.

Genotyping of simple sequence repeats--factors implicated in shadow band generation revisited

PCR amplification of microsatellite sequences generates, besides the main product corresponding to allele size, also additional, undesired products usually shorter by multiples of the repeated unit. These extra products known as shadow bands or stutter products may complicate genotyping. The mechanism by which these artifacts are formed is not well understood and so no effective remedy has been found to cope with these spurious products. In this study, using the DNA templates containing the CAG/CTG repeats flanked by gene-specific sequences and universal priming sites, we analyzed the effects of many PCR variables on the shadow band generation. The most important result was that at the decreased temperature of the denaturation step during PCR cycling the shadow bands were either not formed or were strongly suppressed. Several possible sources of this effect are discussed.

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.

Involvement of mismatch repair in transcription-coupled nucleotide excision repair

Nucleotide excision repair (NER) is a versatile repair pathway to remove a variety of DNA distorting lesions. NER operate via two subpathways, that are global genome repair (GGR) and transcription coupled nucleotide excision repair (TCR). GGR removes DNA damage from the genome over all, whilst TCR is selectively directed to DNA lesions in the transcribed strand of expressed genes. The damage recognition step in GGR and TCR is also different. In GGR, the XPC-HR23B complex is an essential factor to recruit proteins for subsequent process. In TCR, a stalled RNA polymerase II is a presumed trigger to initiate TCR machinery in concert with Cockayne syndrome (CS) proteins. Mismatch repair (MMR) keeps fidelity of DNA replication through correcting replication errors. A distinctive feature of MMR pathway is that this repair is directed exclusively to the newly synthesized strand. This characteristic contributes to mediation of cytotoxity by methylating agents, and MMR deficient cells are more resistant to methylating agents than MMR proficient cells. The interaction between MMR and NER has been reported by several investigators. However, the most controversial problem is the role of MMR in TCR TCR in E. coli requires the participation of the MutS and MutL MMR proteins. On the contrary, TCR in yeast is independent of the yeast MutS and MutL homologues. To date, in mammalian cells, there are conflicting evidences for the association of MMR with TCR pathway. The aim of this article is to provide a brief overview of the recent literature on this subject.

Deregulation of Cyclin E induces chromosomal instability in human colon cancer

AIM: To study the influence of stably inducible expression of Cyclin E on the chromosomal instability in human colon cancer cell line DLD1.

METHODS: Tetracycline-responsive gene-inducible cell line DLD1tTA-cyclin E was generated. Western blot was used to examine the induction of Cyclin E expression upon removal of doxycycline. 4'-6-Diamidino-2-phenylindole (DAPI) staining was performed to detect the percentage of cells with chromosomal instability 1, 3, 5, 7 and 14 d after the induction of Cyclin E expression.

RESULTS: Western blot showed that the peak of Cyclin E expression appeared 96 h after induction. The percentage of cells with chromosomal instability in tet-off DLD1tTA-Cyclin E cells ranged from 0.97% to 1.22% (t = 3.81, P > 0.01). However, the percentages of cells with chromosomal instability were 2.41%, 3.63%, 3.92%, 6.17% and 8.34%, respectively, 1, 3, 5, 7 and 14 d after the induction of Cyclin E expression (t = 4.77, P < 0.01).

CONCLUSION: Deregulation of cyclin E can induce the formation of aneuploidy in human colon cancer cell line DLD1, and it also plays an important role in the pathway of chromosomal instability.