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Genetic susceptibility to non-polyposis colorectal cancer.

Familial colorectal cancer (CRC) is a major public health problem by virtue of its relatively high frequency. Some 15-20% of all CRCs are familial. Among these, familial adenomatous polyposis (FAP), caused by germline mutations in the APC gene, accounts for less than 1%. Hereditary non-polyposis colorectal cancer (HNPCC), also called Lynch syndrome, accounts for approximately 5-8% of all CRC patients. Among these, some 3% are mutation positive, that is, caused by germline mutations in the DNA mismatch repair genes that have so far been implicated (MLH1, MSH2, MSH6, PMS1, and PMS2). Most of the remaining patients belonging to HNPCC or HNPCC-like families are still molecularly unexplained. Among the remaining familial CRCs, a large proportion is probably caused by gene mutations and polymorphisms of low penetrance, of which the I1307K polymorphism in the APC gene is a prime example. Molecular genetic findings have enabled hereditary CRC to be divided into two groups: (1) tumours that show microsatellite instability (MSI), occur more frequently in the right colon, have diploid DNA, harbour characteristic mutations such as transforming growth factor beta type II receptor and BAX, and behave indolently, of which HNPCC is an example; and (2) tumours with chromosomal instability (CIN), which tend to be left sided, show aneuploid DNA, harbour characteristic mutations such as K-ras, APC, and p53, and behave aggressively, of which FAP is an example. This review focuses most heavily on the clinical features, pathology, molecular genetics, surveillance, and management including prophylactic surgery in HNPCC. Because of the difficulty in diagnosing HNPCC, a detailed differential diagnosis of the several hereditary CRC variants is provided. The extant genetic and phenotypic heterogeneity in CRC leads to the conclusion that it is no longer appropriate to discuss the genetics of CRC without defining the specific hereditary CRC syndrome of concern. Therefore, it is important to ascertain cancer of all anatomical sites, as well as non-cancer phenotypic stigmata (such as the perioral and mucosal pigmentations in Peutz-Jeghers syndrome), when taking a family cancer history.

Colorectal Neoplasms, Hereditary Nonpolyposis↗

The mouse Ms6-hm hypervariable microsatellite forms a hairpin and two unusual tetraplexes.

The mouse Ms6-hm microsatellite consists of a tandem array of the pentamer d(CAGGG)n. This microsatellite is extremely hypervariable, showing a germ line mutation rate of 2.5%/gamete. The mechanism responsible for this instability is not known. The ability to form intrastrand structures is a conserved feature of many hypervariable sequences, and it has been suggested that the formation of such structures might account for instability by affecting DNA replication, repair, or recombination. Here we show that this microsatellite is able to form intrastrand structures as well. Under physiological conditions, the Ms6-hm microsatellite forms a hairpin as well as two different unusual intrastrand tetraplexes. The hairpin forms in the absence of monovalent cation and contains G.A, G.C, and G.G base pairs in a 1:1:1 ratio. In the presence of K+, a tetraplex is formed in which the adenines are unpaired and extrahelical, and the cytosines are involved in C.C pairs. In Na+, a tetraplex forms that contains C.C+ pairs, with the adenines being intrahelical and hydrogen-bonded to guanines. Tetraplex formation in the presence of Na+ requires both cytosines and adenines and might reflect the altered internal dimensions of this tetraplex, perhaps resulting from the ability of the C.C+ pairs to become intercalated in this sequence context. Our demonstration of the stabilization of tetraplexes by hydrogen bonding between adenines and guanines expands the hydrogen-bonding possibilities for tetraplexes and suggests that the category of sequences with tetraplex-forming potential may be larger than previously appreciated.

Animals↗

Detection of circulating tumour DNA in the blood (plasma/serum) of cancer patients.

Small amounts of free DNA circulate in both healthy and diseased human plasma/serum, and increased concentrations of DNA are present in the plasma of cancer patients. Characteristics of tumour DNA have been found in genetic material extracted from the plasma of cancer patients. These features include decreased strand stability and the presence of specific oncogene, tumour suppressor gene and microsatellite alterations. Point mutations of the ras genes have been detected in the plasma DNA of patients suffering from haematopoetic malignancies, colorectal and pancreatic cancer, sometimes prior to clinical diagnosis. Rearranged immunoglobulin heavy chain DNA has been found in the plasma of patients with non-Hodgkins lymphoma and acute B cell leukaemia. Microsatellite instability, expressed either as a new allele or a loss of one allele (LOH) occurs in the plasma and serum DNA of patients suffering from head and neck, lung and renal cell cancer. The results obtained in many different cancers have opened a new research area indicating that plasma DNA might eventually be a suitable target for the development of non-invasive diagnostic, prognostic and follow-up tests for cancer.

Animals↗

Regional reproducibility of microsatellite instability in sporadic colorectal cancer.

The reproducibility of microsatellite instability from different regions of the same sporadic colon cancer has not been addressed. We therefore microdissected and extracted DNA from three to nine separate regions of 13 highly unstable sporadic colon cancers. Each region was then evaluated by polymerase chain reaction amplification of 17 microsatellites: 10 tetranucleotide repeats, 2 noncoding mononucleotide repeats (BAT-26 and BAT-40), and 5 coding mononucleotide repeats (TGFBRII, BAX, MSH3, MSH6, IGFIIR). Microsatellite instability showed 100% regional reproducibility with respect to either the panel of 10 tetranucleotide repeats or BAT-26, and nearly 100% reproducibility with BAT-40, although regional variation in the percent instability and the size of unstable alleles was present. TGFBRII was more frequently mutated than any other coding mononucleotide repeat; frame shifts in this gene were identified in nearly every region of every tumor. Each of the five coding repeats showed regional variability in at least one tumor, and 10 of the 13 tumors showed variability with at least one coding repeat. This variability took the form of different mutant alleles (TGFBRII, BAX, MSH3) or mutations present in some but not all regions of a tumor (MSH6, IGFIIR, BAX, MSH3). We conclude that the regional reproducibility of generalized microsatellite instability as measured by noncoding repeats indicates that sampling is not a problem in these highly unstable tumors, and that the mismatch repair deficiency phenotype is acquired in the very late adenoma stage or early cancer stage of sporadic colonic tumorigenesis. The high frequency of TGFBRII mutations is consistent with acquisition of these mutations at a similar stage of tumorigenesis. The regional variability with respect to the presence or absence of a mutation in the other four coding mononucleotide repeats could lead to sampling error and is consistent with a somewhat later time of acquisition of these mutations. Genes Chromosomes Cancer 26:106-114, 1999.

Adult↗

Distal colorectal cancers with microsatellite instability (MSI) display distinct gene expression profiles that are different from proximal MSI cancers.

Promoter methylation of the mismatch repair gene plays a key role in sporadic microsatellite instability (MSI) colorectal cancers. However, promoter methylation often occurs in proximal colon cancers, and molecular phenotypes underlying MSI cancers in distal colon have not been fully clarified. Our goal was to clarify the difference between MSI and microsatellite stability (MSS) cancers and, furthermore, to determine distinct characteristics of proximal and distal MSI cancers. By DNA microarray analysis of 84 cancers (33 MSI and 51 MSS), we identified discriminating genes (177 probe sets), which predicted MSI status with a high accuracy rate (97.6%). These genes were related to phenotypic characteristics of MSI cancers. Next, we identified 24 probe sets that were differentially expressed in proximal and distal MSI cancers. These genes included promoter methylation-mediated genes, whose expression was significantly down-regulated in proximal MSI cancers. Among discriminating genes between MSI and MSS, nine methylation-mediated genes showed down-regulation in MSI cancers. Of these, 7 (77.8%) showed down-regulation in proximal MSI cancers. Furthermore, methylation-specific PCR confirmed that frequency of hMLH1 promoter methylation was significantly higher in proximal MSI cancers (P = 0.0317). These results suggested that there is a difference between proximal and distal MSI cancers in methylation-mediated influence on gene silencing. In conclusion, using DNA microarray, we could distinguish MSI and MSS cancers. We also showed distinct characteristics of proximal and distal MSI cancers. The inactivation form of hMLH, per se, differed between proximal and distal MSI cancers. These results suggested that distal MSI cancers constitute a distinct subgroup of sporadic MSI cancers.

Adaptor Proteins, Signal Transducing↗

3-aminobenzamide and/or O6-benzylguanine evaluated as an adjuvant to temozolomide or BCNU treatment in cell lines of variable mismatch repair status and O6-alkylguanine-DNA alkyltransferase activity.

O6-benzylguanine (O6-BG) and 3-aminobenzamide (3-AB) inhibit the DNA repair proteins O6-alkylguanine-DNA alkyltransferase (AGT) and poly(ADP-ribose) polymerase (PARP) respectively. The effect of O6-BG and/or 3-AB on temozolomide and 1,3-bis(2-chloroethyl)-nitrosourea (BCNU) cytotoxicity, was assessed in seven human tumour cell lines: six with an AGT activity of > 80 fmol mg-1 protein (Mer+) and one with an AGT activity of < 3 fmol mg-1 protein (Mer-). Three of the Mer+ cell lines (LS174T, DLD1 and HCT116) were considered to exhibit resistance to methylation by a mismatch repair deficiency (MMR-), each being known to exhibit microsatellite instability, and DLD1 and HCT116 having well-characterised defects in DNA mismatch binding. Potentiation was defined as the ratio between an IC50 achieved without and with a particular inhibitor treatment. Temozolomide or BCNU cytotoxicity was not potentiated by either inhibitor in the Mer- cell line. Preincubation with O6-BG (100 microM for 1 h) was found to potentiate the cytotoxicity of temozolomide by 1.35- to 1.57-old in Mer+/MMR+ cells, but had no significant effect in Mer+/MMR- cells. In comparison, O6-BG pretreatment enhanced BCNU cytotoxicity by 1.94- to 2.57-fold in all Mer+ cell lines. Post-incubation with 3-AB (2 mM, 48 h) potentiated temozolomide by 1.35- to 1.59-fold in Mer+/MMR+ cells, and when combined with O6-BG pretreatment produced an effect which was at least additive, enhancing cytotoxicity by 1.97- to 2.16-fold. 3-AB treatment also produced marked potentiation (2.20- to 3.12-fold) of temozolomide cytotoxicity in Mer+/MMR- cells. In contrast, 3-AB produced marginal potentiation of BCNU cytotoxicity in only three cell lines (1.19- to 1.35-fold), and did not enhance the cytotoxicity of BCNU with O6-BG treatment in any cell line. These data suggest that the combination of an AGT and PARP inhibitor may have a therapeutic role in potentiating temozolomide activity, but that the inhibition of poly(ADP-ribosyl)ation has little effect on the cytotoxicity of BCNU.

Antineoplastic Agents, Alkylating↗

Instability of the D4Mit12 microsatellite marker in C57BL/6J x BALB/cJF1 hybrid mice is independent of the tumor phenotype.

We examined the stability of different types of DNA microsatellite markers in gamma-radiation-induced thymic lymphomas as well as normal, nontumorous tissues of F1 hybrid mice derived from the parental strains C57BL/6J, RF/J, and BALB/cJ. Surprisingly, the D4Mit12 microsatellite showed genomic instability in a significant fraction of DNAs of the (C57BL/6J x BALB/cJ) F1 hybrids with independence of the tumor phenotype. This instability was not found in the reciprocal F1 DNAs, in DNAs of the parental strains, or in DNAs of (C57BL/6J x RF/J) F1 hybrid mice. These results suggest that the instability of the D4Mit 12 microsatellite is dependent on its repeat length, the genetic background of F1 hybrids, and the maternal origin of the C57BL/6J allele.

Animals↗

Microsatellite instability in colorectal adenocarcinoma cell lines that have full-length adenomatous polyposis coli protein.

Almost 20% of colon cancers are characterized by genomic instability at simple repeated sequences. This instability is the result of a deficient DNA mismatch repair system. Sporadic, as well as hereditary carcinomas of the proximal colon display this effect. In this study, we examined colorectal adenocarcinoma cell lines, with or without wild-type adenomatous polyposis coli (APC) protein, for the presence of microsatellite instability. The three cell lines that maintained full-length APC protein also displayed the highest level of instability, suggesting a negative correlation between APC mutations and microsatellite instability. This data, in addition to other studies that show a negative correlation between microsatellite instability and mutations in p53 and K-ras, support the idea of a second pathway for colorectal cancer development.

Adenocarcinoma↗

Microsatellite instability, mismatch repair deficiency, and genetic defects in human cancer cell lines.

The instability of short repetitive sequences in tumor DNA can result from defective repair of replication errors due to mutations in any of several genes required for mismatch repair. Understanding this repair pathway and how defects lead to cancer is being facilitated by genetic and biochemical studies of tumor cell lines. In the present study, we describe the mismatch repair status of extracts of 22 tumor cell lines derived from several tissue types. Ten were found to be defective in strand-specific mismatch repair, including cell lines from tumors of the colon, ovary, endometrium, and prostate. The repair defects were independent of whether the signal for strand specificity, a nick, was 5' or 3' to the mismatch. All 10 defective cell lines exhibited microsatellite instability. Repair activity was restored to 9 of these 10 extracts by adding a second defective extract made from cell lines having known mutations in either the hMSH2 or hMLH1 genes. Subsequent analyses revealed mutations in hMSH2 (4 lines) and hMLH1 (5 lines) that could explain the observed microsatellite instability and repair defects. Overall, this study strengthens the correlation between microsatellite instability and defective mismatch repair and the suggestion that diminuition in mismatch repair activity is a step in carcinogenesis common to several types of cancer. It also provides an extensive panel of repair-proficient and repair-deficient cell lines for future studies of mismatch repair.

Adaptor Proteins, Signal Transducing↗

Microsatellite instability in hereditary and sporadic breast cancers.

Sporadic cancers and familial breast cancers are characterized by an increase in genetic instability. Little is known about whether mismatch repair defects accompany this genetic instability. We investigated invasive and/or in situ breast cancers from 30 women with deleterious BRCA1/2 mutations and unclassified variant BRCA1/2 alterations. Forty cases of sporadic breast cancers were also investigated, including 7 medullary carcinomas. Malignant and benign lesions were examined from all cases to better understand tumor progression. Automated immunohistochemistry, with antibodies directed against hMLH1 and hMSH2, was used to screen cases for possible mismatch repair defects. When loss of expression was noted, DNA ploidy was performed by cytomorphometry. DNA, after laser microdissection, was extracted from a majority of familial cases and their corresponding controls, and microsatellite instability analysis was performed. None of the familial or sporadic cases had loss of hMSH2 expression. All but one lesion, a DCIS arising in a deleterious BRCA2 mutation carrier, had loss of hMLH1 expression and a tetraploid profile by image cytomorphometry. There was no MSI in any explored lesions (n = 34), as determined by molecular analysis, including the DCIS with loss of hMLH1 expression. We conclude that DNA mismatch repair defects involving hMLH1 and hMSH2 underexpression are extremely rare events in sporadic and familial breast cancer. Mismatch repair gene mutations may be secondary random events in breast cancer progression.

Adaptor Proteins, Signal Transducing↗

Microsatellite instability occurs in distinct subtypes of pediatric but not adult central nervous system tumors.

Length alterations in microsatellite repeats, termed microsatellite instability (MSI), are found in 10-15% of sporadic colon, endometrial, and gastric cancers harboring defects in DNA mismatch repair (MMR) genes We used the microsatellite markers Big Adenine Tract (BAT) 26 and BAT-25 from the reference panel of five markers recommended by the National Cancer Institute to evaluate the incidence of MSI in 206 central nervous system tumors. We screened 102 pediatric and 104 adult cases representing 165 astrocytic and 41 nonastrocytic tumors. The overall incidence of MSI was 8% (16 of 206). All 16 tumors with MSI were found in pediatric rather than adult patients. MSI was associated with two distinct subtypes of pediatric tumors occurring in 27% (12 of 45) of WHO grade III and grade IV astrocytomas and 24% (4 of 17) of gangliogliomas We evaluated the difference in clinicopathological and genetic features among 45 high-grade pediatric astrocytomas by MSI status. The median survival for pediatric patients with MSI (n = 12) was 8 months compared with 15 months for those patients without MSI (n = 33; P = 0.18). The frequency of p53 gene mutations was 13% for pediatric patients with MSI (n = 8) compared with 47% for those patients without MSI (n = 19; P = 0.19). These results revealed a trend between MSI status and prog nosis and MSI status and frequency of p53 gene mutations. Our data suggest that pediatric high-grade astrocytomas can be attributed to two different genetic pathways: a MMR-deficient pathway and a MMR proficient pathway.

Adolescent↗

DNA mismatch repair deficiency accelerates endometrial tumorigenesis in Pten heterozygous mice.

PTEN mutation and microsatellite instability are two of the most common genetic alterations in uterine endometrioid carcinoma. Furthermore, previous studies have suggested an association between the two alterations, however the basis and consequence of the association is not understood. Recently it has been shown that 100% of female Pten(+/-) mice develop complex atypical hyperplasia by 32 weeks of age that progresses to endometrial carcinoma in approximately 20 to 25% of mice at 40 weeks. In an attempt to expand this mouse model of endometrial tumorigenesis and to further our understanding of the association betweenPten mutations and DNA mismatch repair deficiency, we generated Ptenheterozygous, Mlh1-null (mismatch repair deficient) mice. Significantly, the majority ofPten(+/-)/Mlh1(-/-)mice developed polypoid lesions in the endometrium at 6 to 9 weeks of age. By 14 to 18 weeks, all of the double-mutant mice had lesions histologically similar to those seen inPten(+/-) mice, and two of them exhibited invasive disease. Moreover, the frequency of loss of the wild-type Pten allele in the double-mutant mice at 14 to 18 weeks was similar to that seen in lesions from 40-week-old Pten(+/-) mice. Taken together, our results indicate that DNA mismatch repair deficiency can accelerate endometrial tumorigenesis inPten heterozygous mice and suggests that loss of the wild-type Pten allele is involved in the development/progression of tumors in this setting.

Adaptor Proteins, Signal Transducing↗

p53 Gene mutation and genetic instability in superficial multifocal esophageal squamous cell carcinoma.

Tumor multicentricity is occasionally observed in esophageal squamous cell carcinoma (SCC). We studied five surgically resected superficial multifocal esophageal SCCs for p53 gene mutation and genetic instability, using DNA extracted from microdissected areas. A total of 38 target areas (TAs) were analyzed in SCC, dysplasia, basal cell hyperplasia (BCH) and normal squamous epithelium. Analysis of the replication error (RER) at 10 microsatellite loci showed microsatellite instability in all TAs, as well as in normal squamous epithelium. p53 gene mutation was identified in 28.9% (11/38 TAs). All cases showed a common missense mutation in exon 8 at codon 273 (CGT-->CAT, Arg-->His), which was DNA contact mutation in the S10 beta strand. In association with microsatellite alterations, 7 of 9 TAs with p53 mutation in exon 8 at codon 273 also showed loss of heterozygosity (LOH) of p53 gene. LOH of p53 gene was detected in 83.8% (31/37 TAs). LOH at D2S123 on 2p16 near MSH2 gene and at D3S1611 on 3p22 near MLH1 gene was detected in 65.4% (17/26) and 71.4% (10/14) TAs, respectively. Frequencies of LOH at p53 and D2S123 were similar in non-cancerous areas and SCCs. LOH of p53 and D2S123 were found in 50% (5/10 TAs) of non-cancerous areas and 60% (9/15 TAs) of SCCs. Our results suggest that genetic instability induces esophageal tumor multicentricity, and that p53 gene contact mutation together with LOH are early events of the multistage carcinogenesis of multifocal primary esophageal SCC.

Carcinoma, Squamous Cell↗

CDX2, a human homologue of Drosophila caudal, is mutated in both alleles in a replication error positive colorectal cancer.

The Cdx2 gene is one of three murine homologues of the Drosophila homeobox gene caudal. Mice heterozygous for a null mutation in Cdx2 exhibit a variable phenotype including tail abnormalities, stunted growth and a homeotic shift of vertebrae. Most strikingly, however, 90% of heterozygous mice were reported to develop multiple intestinal adenomatous polyps, most notably in the proximal colon (Chawengsaksophak et al., 1997). These observations led us to propose that mutation of CDX2 may be involved in the genesis of some human colorectal tumours. A survey of DNA from 85 colorectal tumours revealed that one with extensive microsatellite instability (RER+ phenotype) has mutations in both alleles of CDX2. Both mutations occur in coding regions which contain repetitive elements and are consistent with those found in RER + tumours.

Adenocarcinoma↗

Telomere instability detected in sporadic colon cancers, some showing mutations in a mismatch repair gene.

Human telomeres are essential for genome stability and are composed of long simple tandem repeat arrays (STRs), comprising the consensus TTAGGG repeat interspersed, at the proximal end, with sequence-variant repeats. While the dynamics of telomere attrition through incomplete replication has been studied extensively, the effects on telomeres of error-prone DNA repair processes, known to affect other STRs, are poorly understood. We have compared the TTAGGG and sequence-variant interspersion patterns in the proximal 720 bp of telomeres in colon cancer and normal DNA samples. The frequency of telomere mutations was 5.8% per allele in a randomly collected panel of sporadic colon cancers, showing that telomere mutations occur in vivo. The mutation frequency rose to 18.6% per allele in sporadic tumours that exhibit instability at the polyA tract in the TGFbetaRII gene and to 35% per allele in tumours with somatic mutations in the hMSH2 gene. The majority of the characterized mutations resulted in the loss of one or a few repeats. If the mutation spectrum and frequency described here is reiterated in the rest of the array, there is the potential for extensive telomere destabilization especially in mismatch repair-defective cells. This may in turn lead to a greater requirement for telomere length maintenance earlier in tumourigenesis.

Base Pair Mismatch↗

Defective DNA mismatch repair in long-term (> or =3 years) survivors with pancreatic cancer.

BACKGROUND/AIMS: Defective DNA mismatch repair (MMR) in pancreatic cancer, reported in up to 13% of sporadic pancreatic cancers, may predict a good prognosis. To determine if long-term survival in pancreatic cancer could be attributed to defective DNA MMR, we ascertained its prevalence in 35 pancreatic cancer patients who survived > or =3 years after surgery. METHODS: We performed immunohistochemistry (IHC) for MMR proteins hMLH1, hMSH2, and hMSH6 in all 35 tumors and microsatellite instability (MSI) studies in 34/35 tumors using 10 microsatellite markers in paired normal and tumor DNA. Defective DNA MMR was defined as absence of protein expression on IHC and/or MSI in > or =30% of markers studied. RESULTS: On IHC, 3/35 (8.6%) tumors had defective DNA MMR. All 3 had absent expression of a DNA MMR protein (hMLH1 in 2 and hMSH2) and 2/3 also had MSI; the third could not be tested. Definitely 2, and probably all 3 patients had hereditary nonpolyposis colon cancer as determined by clinical and genetic profiles. CONCLUSION: Defective DNA MMR is uncommon in long-term survivors of pancreatic cancer and does not account for the survival benefit in those with sporadic pancreatic cancer.

Adaptor Proteins, Signal Transducing↗

Telomerase inhibition by an siRNA directed against hTERT leads to telomere attrition in HT29 cells.

Human telomerase is a ribonucleoprotein complex composed of at least the reverse catalytic transcriptase hTERT and RNA component hTR. The enzyme stabilizes telomere length and thereby contributes to unlimited cell proliferation, i.e. immortality. Reactivation of telomerase activity during carcinogenesis is a common hallmark in most human tumor types. Consequently, telomerase is an attractive molecular target toward which to direct cancer therapeutic agents. RNA interference (RNAi) has been shown to be an effective method for inhibiting the expression of a given gene in human cells by targeting with short duplex RNA (short-interfering RNA or siRNA). Thus, we evaluated the ability of siRNAs to inhibit telomerase activity in the HT29 immortal human colorectal adenocarcinoma cell line as a model for colorectal carcinogenesis. By transient expression of a specific siRNA directed against hTERT, we reduced telomerase activity in the transfected cells. Moreover, telomere lengths were reduced in cells stably expressing this particular RNA sequence, cloned as an shRNA into an eukaryotic expression vector. The cell clone that displayed a telomerase-negative phenotype showed dramatically reduced telomere lengths and stopped proliferation. We observed that the vector was integrated into the cell genome and, despite telomere shortening, cells retained their MSI phenotype. We conclude that we have developed a potent telomerase inhibitor leading to cell death.

Adenocarcinoma↗