Familial polyposis: recent advances

Shifting the Focus of Signaling Abnormalities in Colon Cancer

Colon cancer tumorigenesis occurs incrementally. The process involves the acquisition of mutations which typically follow an established pattern: activation of WNT signaling, activation of RAS signaling, and inhibition of TGF-β signaling. This arrangement recapitulates, to some degree, the stem cell niche of the intestinal epithelium, which maintains WNT and EGF activity while suppressing TGF-β. The resemblance between the intestinal stem cell environment and colon cancer suggests that the concerted activity of these pathways generates and maintains a potent growth-inducing stimulus. However, each pathway has a myriad of downstream targets, making it difficult to identify which aspects of these pathways are drivers. To address this, we utilize the cell cycle, the ultimate regulator of cell proliferation, as a foundation for cross-pathway integration. We attempt to generate an overview of colon cancer signaling patterns by integrating the major colon cancer signaling pathways in the context of cell replication, specifically, the entrance from G1 into S-phase.

Mitotic recombination: a genotoxic effect of the antidepressant citalopram in Aspergillus nidulans

This report evaluates the potential of the antidepressant drug citalopram to induce homozygotization of genes previously present in a heterozygous condition, by homologous recombination. In order to address this question, a heterozygous diploid strain of the filamentous fungus Aspergillus nidulans and the homozygotization assay were utilized. Non-cytotoxic concentrations of citalopram (50, 75 and 100 μmol/L) showed a strong recombinogenic effect in A. nidulans, inducing homozygosis of the diploid strain's nutritional markers. The genetic markers exhibited homozygotization index (HI) rates higher than 2.0 and significantly different from HI control ones. Since citalopram has been previously characterized as a DNA synthesis inhibitor, the recombinogenic potential of this antidepressant in A. nidulans may be associated with the recombinational repair of citalopram-induced DNA strand breaks.

A mitotic recombination map proximal to the APC locus on chromosome 5q and assessment of influences on colorectal cancer risk

Background

Mitotic recombination is important for inactivating tumour suppressor genes by copy-neutral loss of heterozygosity (LOH). Although meiotic recombination maps are plentiful, little is known about mitotic recombination. The APC gene (chr5q21) is mutated in most colorectal tumours and its usual mode of LOH is mitotic recombination.

Methods

We mapped mitotic recombination boundaries ("breakpoints") between the centromere (~50 Mb) and APC (~112 Mb) in early colorectal tumours.

Results

Breakpoints were non-random, with the highest frequency between 65 Mb and 75 Mb, close to a low copy number repeat region (68–71 Mb). There were, surprisingly, few breakpoints close to APC, contrary to expectations were there constraints on tumorigenesis caused by uncovering recessive lethal alleles or if mitotic recombination were mechanistically favoured by a longer residual chromosome arm. The locations of mitotic and meiotic recombination breakpoints were correlated, suggesting that the two types of recombination are influenced by similar processes, whether mutational or selective in origin. Breakpoints were also associated with higher local G+C content. The recombination and gain/deletion breakpoint maps on 5q were not, however, associated, perhaps owing to selective constraints on APC dosage in early colorectal tumours. Since polymorphisms within the region of frequent mitotic recombination on 5q might influence the frequency of LOH, we tested the 68–71 Mb low copy number repeat and nearby tagSNPs, but no associations with colorectal cancer risk were found.

Conclusion

LOH on 5q is non-random, but local factors do not greatly influence the rate of LOH at APC or explain inter differential susceptibility to colorectal tumours.

Prevalence of mutations in APC, CTNNB1, and BRAF in Tunisian patients with sporadic colorectal cancer

Sporadic colorectal tumorigenesis is caused by alterations in the Wnt (APC, CTNNB1) and Ras pathways. Our objective was to analyze the occurrence of these genetic alterations in relation to tumor and patient characteristics. The prevalence of somatic alteration in the hot-spot regions of the APC, BRAF, and CTNNB1 genes was investigated in 48 unselected and unrelated Tunisian patients with sporadic colorectal cancer, and the association between the molecular features at these genes in relation to tumor and patient characteristics (age at diagnosis, sex, tumor localization, stage, and differentiation) was analyzed. Loss of heterozygosity was observed at the APC locus in 52% of the analyzed tumors. 6 novel mutations were detected by polymerase chain reaction sequencing in the mutation cluster region of the APC gene. No mutations were observed in the CTNNB1 gene in any tumor, but 8% of tumors harbored mutation in the BRAF gene. Clinicopathological analyses showed an association between APC point mutations and the earliest occurrence of sporadic colorectal cancer. The findings confirm the heterogeneity of APC gene alteration and also reveal a particular profile of this pathology among Tunisian patients that confirms the epidemiological data for this country.

Microsatellites: evolution and mutational processes

Microsatellites (simple sequence repeats) are ubiquitous in eukaryotic genomes, and they are highly polymorphic. They are currently the primary tools for most genetic mapping and for studies comparing the differentiation of human and other mammalian populations. More and more inherited human diseases are now recognized as resulting from mutations in particular microsatellites, and such microsatellite mutations can serve as markers for some cancers. The majority of microsatellite mutational changes probably consist of insertion or deletion of one or a few repeat units through replication slippage, whereas larger (much rarer) changes are important in producing observed allele distributions. Comparisons of microsatellite allele frequencies between humans and chimpanzees suggest that there are constraints on the overall length of microsatellites. Sequence analyses of microsatellites in diverse human and non-human populations indicate that the structure of many repeats may not be as simple as previously believed, in that alleles differ in base composition as well as in repeat length. Single base changes that result in long uninterrupted repeats may lead to increased mutation rates, including the extreme trinucleotide repeat instability responsible for several inherited diseases.

Difference in the Role of Loss of Heterozygosity at 10p15 (KLF6 Locus) in Colorectal Carcinogenesis between Sporadic and Familial Adenomatous Polyposis and Hereditary Nonpolyposis Colorectal Cancer Patients

OBJECTIVES

To clarify the role of the KLF6 (Kruppel-like factor 6) locus in multistep colorectal carcinogenesis, we analyzed loss of heterozygosity (LOH) at 10p15 (KLF6 locus) and mutations of the KLF6gene in 298 colorectal tumors at various pathological stages of sporadic and familial adenomatous polyposis (FAP) and hereditary nonpolyposis colorectal cancer (HNPCC) patients.

METHODS

10p15 LOH was analyzed using KLF6M1 and KLF6M2, and KLF6 gene mutation was analyzed using PCR-SSCP and sequencing.

RESULTS

It was found that the frequencies of LOH (sum of M1 and M2) were 4% in adenomas, 0% in intramucosal carcinomas, 35% in invasive carcinomas, and 33% in liver metastases in sporadic cases. Invasive carcinomas from FAP patients showed only 6% LOH, and invasive carcinomas from HNPCC patients exhibited 0% LOH. Mutation analysis of the KLF6 gene in 298 colorectal tumors detected no somatic mutations.

CONCLUSIONS

The present data suggest that LOH of the KLF6 locus at chromosome 10p15 contributes to the invasion step from an intramucosal carcinoma to an invasive carcinoma specifically in sporadic colorectal carcinogenesis, but is rarely involved in the carcinogenesis of FAP and HNPCC cases. Moreover, the absence of somatic mutations suggests the uncertainty of the KLF6 gene as a classical tumor suppressor gene at the lost 10p15 region in colorectal carcinogenesis.

The modifier of Min 2 (Mom2) locus: embryonic lethality of a mutation in the Atp5a1 gene suggests a novel mechanism of polyp suppression

Inactivation of the APC gene is considered the initiating event in human colorectal cancer. Modifier genes that influence the penetrance of mutations in tumor-suppressor genes hold great potential for preventing the development of cancer. The mechanism by which modifier genes alter adenoma incidence can be readily studied in mice that inherit mutations in the Apc gene. We identified a new modifier locus of ApcMin-induced intestinal tumorigenesis called Modifier of Min 2 (Mom2). The polyp-resistant Mom2R phenotype resulted from a spontaneous mutation and linkage analysis localized Mom2 to distal chromosome 18. To obtain recombinant chromosomes for use in refining the Mom2 interval, we generated congenic DBA.B6 ApcMin/+, Mom2R/+ mice. An intercross revealed that Mom2R encodes a recessive embryonic lethal mutation. We devised an exclusion strategy for mapping the Mom2 locus using embryonic lethality as a method of selection. Expression and sequence analyses of candidate genes identified a duplication of four nucleotides within exon 3 of the alpha subunit of the ATP synthase (Atp5a1) gene. Tumor analyses revealed a novel mechanism of polyp suppression by Mom2R in Min mice. Furthermore, we show that more adenomas progress to carcinomas in Min mice that carry the Mom2R mutation. The absence of loss of heterozygosity (LOH) at the Apc locus, combined with the tendency of adenomas to progress to carcinomas, indicates that the sequence of events leading to tumors in ApcMin/+ Mom2R/+ mice is consistent with the features of human tumor initiation and progression.

Allele-specific loss of heterozygosity in multiple colorectal adenomas: toward an integrated molecular cytogenetic map II

Colorectal cancer (CRC) remains a significant public health challenge despite our increased understanding of the genetic defects underlying the pathogenesis of this common disease. It has been thought that multiple mechanisms lead to the malignant phenotype, with familial predisposition syndromes accounting for only a small proportion of all CRC cases. To identify additional loci likely involved in CRC and to test the hypothesis of allele-specific loss of heterozygosity (LOH) for the localization of CRC susceptibility genes, we initially conducted a genome-wide allelotyping analysis of 48 adenomas from a patient with familial adenomatous polyposis coli (FAP) and 63 adenomas from 7 patients with sporadic CRC using 79 fluorescently tagged oligonucleotide primers amplifying microsatellite loci covering the human genome. Frequent allelic losses were identified at D17S802 (41%), D7S518 (40%), D18S53 (38%), D10S249 (32%), D2S391 (29%), D16S419 (27%), D15S1005 and D15S120 (24%), D9S274 and D11S1318 (23%), D14S65 (20%), D14S274 and D17S953 (19%), D19S424 (18%), D5S346 and D1S397 (15%), and D6S468 (13%) in multiple FAP adenomas. Common LOH was also detected at D4S1584 (42%), D11S968 (31%), D17S953 (28%), D5S394, D9S286 and D10S249 (24%), D8S511 (23%), D13S158 (21%), D7S669 (20%), D18S58 (19%), D2S162 and D16S432 (16%), D2S206 (15%), D7S496 and D17S946 (14%), D6S292 (13%), D4S1586 and D8S283 (11%), and D1S2766 (10%) in multiple CRC adenomas. In addition, allele-specific LOH at D5S346, D15S1005, and D15S120 was observed in multiple FAP adenomas (P < 0.01) and at D2S206 and D16S423 in multiple CRC (P < 0.05). To compare our data to previous reports, we determined the band-specific frequency of chromosomal imbalances in CRC karyotypes reported in the Mitelman database, and from the CGH results of cases accessible through the PROGENETIX website. Furthermore, published genome-wide allelotyping analysis of CRC and other allele-specific LOH studies were compiled and collated with our LOH data. The combined results not only provide a comprehensive view of genetic losses in CRC, indicating the comparability of these different techniques, but they also reveal different novel loci in multiple adenomas from FAP and sporadic CRC patients, suggesting that they represent a distinct subtype of CRC in terms of allelic losses. Allele-specific LOH is an alternative approach for cancer gene mapping.

HCCR-1-interacting molecule "deleted in polyposis 1" plays a tumor-suppressor role in colon carcinogenesis

BACKGROUND & AIMS

Human cervical cancer oncogene (HCCR-1) has appeared to act as a negative regulator of p53 and contributes to tumorigenesis of various organs including the colon. We identified the HCCR-1 binding protein deleted in polyposis 1 (DP1) and accessed the role of HCCR-1 and DP1 in colon tumorigenesis.

METHODS

Yeast 2-hybrid was used to identify HCCR-1 interacting proteins. Various molecular biological approaches were used to examine the expression profile of HCCR-1 and DP1, subcellular localization, epitope mapping, the biological role of DP1, and the serum HCCR-1 level. Loss of heterozygosity frequency around DP1 also was examined.

RESULTS

We identified that HCCR-1 interacted with DP1. These 2 proteins colocalized in mitochondria but the expression of HCCR-1 showed negative correlation with that of DP1 in colorectal cancer (CRC). DP1 played a tumor-suppressor role in colon tumorigenesis (ie, DP1-transfected RKO cells showed growth inhibition, apoptosis, decreased telomerase activity, and up-regulation of p53). These phenomena were reversed when HCCR-1 was overexpressed. Loss of heterozygosity around the DP1 gene was observed frequently (50%) in CRCs. We examined the use of serum HCCR-1 in CRC patients. The sensitivity of HCCR-1 (76.0%) for detecting CRC was proven to be much higher than that of CA19-9 (32.0%).

CONCLUSIONS

DP1 plays a tumor-suppressor role in CRC. DP1 and HCCR-1 are supposed to regulate each other negatively by interaction, but further study is required to get better insight into the biological significance of the interaction.

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) induces genetic changes in murine intestinal tumours and cells with ApcMin mutation

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is one of the mutagenic heterocyclic amines derived from cooked meat. In previous animal studies, spontaneous tumour formation in B6(Min/+) mice was associated with somatic loss of the wild-type Apc+ allele by loss of the entire chromosome 18 or by recombination. The objective of this study was to examine genetic changes caused by PhIP-exposure in a mouse intestinal cell line and in tumours from hybrid mice by keeping track of the chromosomes carrying the two Apc alleles. We transformed the SV40 T-immortalised intestinal epithelial cell line IMCE, derived from the B6(Min/+) mice by exposure to N-OH-PhIP, and studied the effect on Apc status and chromosome 18. Eighteen transformed cultures were obtained and all of them had retained the Apc+ allele. Five of seven transformed cultures were tumorigenic after implantation in nude mice. Chromosomal analysis of these five cultures and the parent IMCE cell line showed that the IMCE cells were near-tetraploid with an average of 77 chromosomes/cell, while the tumorigenic cell cultures were all triploid to hyper-triploid with a range of 61-69 chromosomes/cell. The number of copies of chromosome 18 was about four in the IMCE line and this copy number was retained in the transformed lines derived from IMCE. Changes in chromosome 18 and Apc during tumour development in vivo were examined in spontaneously formed and PhIP-induced intestinal tumours from two hybrid mice strains, i.e. B6(Min/+) - a murine FAP model - crossed with either AKR/J or A/J. We evaluated the allelic status of Apc, and the heterogenic microsatellite markers D18Mit19 and D18Mit4, located at the upper and lower ends of chromosome 18, respectively. In tumours from untreated animals, instability in the D18Mit19 and Apc was observed. Upon PhIP exposure, the B6(Min/A+) hybrid mouse tumours differed distinctly in genetic profile from those obtained from untreated animals and we detected three genetically different tumour groups, all of which had apparently retained Apc+. One group had allelic balance between the Apc(Min) and Apc+, the second had allelic imbalance between the Apc and D18Mit4 alleles, indicative of chromosomal stability in the first group and instability in the lower end of chromosome 18 in the second group, respectively. The third group showed variable allelic status of the three markers. A similar change in genetic profile was also seen in intestinal tumours of PhIP-exposed B6(Min/AKR+) hybrid mice, but it was less pronounced. Chromosomal breaks and/or recombinational events could be alternative explanations for the observed allelic imbalances in chromosome 18 markers in intestinal tumours from PhIP-exposed mice.

Genetic basis of variation in adenoma multiplicity in ApcMin/+ Mom1S mice

Apc(Min) mice have provided an example of a locus (Modifier of Min1; Mom1) modifying adenoma numbers in the intestines of inbred strains. Linkage analysis located Mom1 on chromosome 4, and further investigation identified secretory phospholipase A2 (Pla2g2a) as a candidate gene. Because of unknown variation introduced by a single founding male mouse, our Min stock, although Pla2g2a(Mom1-s), was not on a pure C57BL/6J background and exhibited several polymorphic loci, including a region on chromosome 18 distal to Apc. Through selective breeding for homozygosity for distal chromosome 18 markers, six recombinant lines that presented with limited intraline variation in adenoma numbers were established. One line (V) showed a particularly severe phenotype (mean adenoma number +/- SEM, 370 +/- 21) compared with the other lines that recorded significantly lower means (3- to 5-fold; P < 10(-3), t test). Intercrosses between lines I and V showed suppression of the severe phenotype in the N1 generation. In N2 (and subsequent) backcrosses, tumor multiplicity depended on the origins of the WT and Min Apc alleles. Mice carrying both alleles from line V had a severe phenotype; others had mild disease very similar to line I (likelihood ratio statistic > 49.0; likelihood of odds > 10; P < 10(-5)). Frequency of allele loss at Apc was increased significantly in adenomas of mice with more severe disease. We propose that a modifier gene close to Apc or structural variation on chromosome 18 modifies polyp numbers in our mice, possibly by altering the frequency of WT Apc allele loss.

APC gene loss of heterozygosity, mutations, E1317Q, and I1307K germ-line variants in sporadic colon cancer in Croatia

Colorectal carcinomas are characterized by multiple genetic aberrations that occur during tumorigenesis. Several tumor suppressor genes associated with colorectal carcinoma have been identified: MCC, APC, p53, nm23-H1, DCC, DPC4. We examined 73 cases of sporadic human colon cancer and corresponding normal tissue samples to evaluate the loss of heterozygosity (LOH) at the APC gene loci. The purpose of this study was also to evaluate whether the LOH at the APC gene is associated with clinicopathological characteristics in sporadic colon cancer. We also investigated presence and the frequency of the most common APC gene mutations and APC E1317Q and I1307K germ-line variants in Croatian colorectal cancer patients. Five markers in all patients were found to be heterozygous and informative for LOH analysis. LOH at the APC locus was detected in 30.1% of tumors were examined. The majority of APC gene LOH was observed in Dukes' B (55.6%) and in the moderately differentiated tumors (42.9%). Only 1309 APC gene mutation was detected in our samples. In one tumor sample, a new sporadic mutation of the APC gene in codon 1374 was detected. APC E1317Q and I1307K germ-line variants were not detected in our population. But APC E1317Q sporadic mutation was found in one tumor sample.

Analysis of adenomatous polyposis coli gene expression, APC locus-microsatellite instability and APC promoter methylation in the progression of melanocytic tumours

Adenomatous polyposis coli gene (APC) defects have been demonstrated for the first time in familial adenomatous polyposis. Recent reports indicate that the APC gene is an intermediary between cell adhesion molecules and the cytoskeleton and that it may function as a gatekeeper of colonic epithelial proliferation. The objective of this study was to analyse APC's presence in lentigos, primary melanomas and melanoma metastases. By immunohistochemistry, APC was demonstrated in all lentigos, in 75 out of 88 primary melanomas and in 16 out of 28 melanoma lymphatic metastases. The percentage of immunolabelled tumour cells (APC index) in lentigos ranged between 5 and 69%, in primary melanomas between 0 and 98% and in melanoma metastases between 0 and 52%. Statistically significant differences between lentigos and primary melanomas and between lentigos and metastases in APC expression were found. In a multivariate analysis, APC showed an independent prognostic impact. Analysis of microsatellite instability in the APC locus was performed on 29 melanomas. Microsatellite instability was found in 5/29 melanomas and loss of heterozygosity in 1/29 melanomas. Promoter methylation of APC was found in 6/10 APC-negative primary melanomas and in 9/10 APC-negative melanoma lymphatic metastases investigated. We conclude about important role of APC alterations for melanoma progression.

Development of gastric carcinoma from intestinal metaplasia in Cdx2-transgenic mice

In the progression of chronic gastritis, gastric mucosal cells deviate from the normal pathway of gastric differentiation to an intestinal phenotype. Many epidemiologic studies have found an association between the formation of intestinal metaplasia and the development of gastric carcinoma. However, there is no direct evidence that shows intestinal metaplasia is a precursor lesion of gastric carcinoma, to date. We periodically examined the intestinal metaplastic mucosa of Cdx2-transgenic mice we have previously generated. Gastric polyps developed from intestinal metaplastic mucosa in all stomachs of Cdx2-transgenic mice examined. These gastric polyps consisted of intestinal-type adenocarcinoma that invaded the submucosa and muscularis propria and occasionally spread into the subserosa. p53 and APC gene mutations were recognized in the adenocarcinomas. The participation of APC and p53 gene mutations in gastric carcinogenesis from the intestinal metaplasia was verified by the Cdx2-transgenic mice, carrying Apc(Min) mutation or p53 deficiency, that developed gastric polyps much earlier than Cdx2 alone. We successfully showed that long-term intestinal metaplasia induces invasive gastric carcinoma. These results indicate that intestinal metaplasia itself plays a significant role in the genesis and progression of gastric carcinoma.

Adenomatous polyposis coli truncation mutations in 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced intestinal tumours of multiple intestinal neoplasia mice

The heterocyclic amine 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) induces intestinal tumours in C57BL/6J-multiple intestinal neoplasia (Min)/+ mice. The main mechanism for PhIP-induced tumour induction in Min/+ mice is loss of the wild-type adenomatous polyposis coli (Apc) allele, i.e. loss of heterozygosity (LOH). In this study, single injections of either 10, 17.5 or 25 mg/kg PhIP on days 3-6 after birth all increased the mean number of small intestinal tumours two to three-fold, from 37.7 in controls to 124.8 in the PhIP-treated Min/+ mice. In total, we analysed 292 small intestinal tumours and 253 of these had LOH. The frequency of LOH in the Apc gene was 88, 93, 83 and 84% in tumours of 0, 10, 17.5 and 25 mg/kg PhIP-treated mice, respectively. Therefore, these lower doses of PhIP did not reduce the frequency of LOH, as found in our previous study with a single injection of 50 mg/kg PhIP (Mutat. Res. 1-2 (2002) 157). In the second part of this study, we wanted to characterise Apc truncation mutations from tumour samples apparently retaining the Apc wild-type allele from this and two previous experiments with PhIP-exposed Min/+ mice. In the first half of exon 15 in Apc, we verified 25 mutations from 804 tumour samples of PhIP-treated mice. Of these were 60% G-->T transversions, and 16% G deletions, indicating that these are the predominant types of PhIP-induced truncation mutations in the Apc gene in Min/+ mice. Most of the mutations were located between codon 989 and 1156 corresponding to the first part of the beta-catenin binding region. We also identified two Apc truncation mutations from 606 spontaneously formed intestinal tumours from untreated Min/+ mice, one C-->T transition and one T insertion, which were different from those induced by PhIP.

Role of Smad4 (DPC4) inactivation in human cancer

The tumor suppressor gene Smad4 (DPC4) at chromosome 18q21.1 belongs to the Smad family, which mediates the TGFbeta signaling pathway suppressing epithelial cell growth. This review summarizes the mutational events of the Smad4 gene in human cancer. The Smad4 gene is genetically responsible for familial juvenile polyposis, an autosomal dominant disease characterized by predisposition to gastrointestinal polyps and cancer. In this syndrome, polyps are formed by inactivation of the Smad4 gene through germline mutation and loss of the unaffected wild-type allele. In pancreatic and colorectal cancer, inactivation of the Smad4 gene through homozygous deletion or intragenic mutation occurs frequently in association with malignant progression. However, mutation of this gene is seen only occasionally in the rest of human cancers. The majority of Smad4 gene mutations in human cancer are missense, nonsense, and frameshift mutations at the mad homology 2 region (MH2), which interfere with the homo-oligomer formation of Smad4 protein and the hetero-oligomer formation between Smad4 and Smad2 proteins, resulting in disruption of TGFbeta signaling. Supporting evidence for the above observation was provided by genetically manipulated mice carrying either a heterozygote of the Smad4 gene or a compound heterozygote of the Smad4 and APC genes, which develop either gastrointestinal polyps/cancer mimicking familial juvenile polyposis or progressed colorectal cancer, respectively.

Differentiation-dependent induction of CYP1A1 in cultured rat small intestinal epithelial cells, colonocytes, and human colon carcinoma cells: basement membrane-mediated apoptosis

Rat small intestinal epithelial cells and human colon adenocarcinoma cells cultured on Matrigel expressed the differentiation specific enzyme, sucrase-isomaltase, as determined by indirect immunofluorescence. Rat small intestinal epithelial cells, rat colonocytes, and human colon adenocarcinoma cells developed an altered morphology when cultured on Matrigel and became apoptotic within 24-48 h. Benzo[a]pyrene and 2,3,7,8-tetrachlorodibenzo-p-dioxin caused a 2- and 5-fold induction, respectively, of ethoxyresorufin-o-deethylase activity in rat small intestinal epithelial cells cultured on Matrigel. Benzo[a]pyrene- or 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced ethoxyresorufin-o-deethylase activity in rat small intestinal epithelial cells cultured on plastic was not detected. 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment caused a 14-fold induction of transfected, rat CYP1A1-promoter-luciferase activity in rat small intestinal epithelial cells cultured on Matrigel. Benzo[a]pyrene and 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment induced ethoxyresorufin-o-deethylase activity by 6- and 1.6-fold, respectively in rat colonocytes cultured on Matrigel. Induction of ethoxyresorufin-o-deethylase activity was not observed in rat colonocytes cultured on plastic. CYP1A1-promoter-luciferase activity was induced 3-fold by 2,3,7,8-tetrachlorodibenzo-p-dioxin in rat colonocytes cultured on Matrigel. Induction of CYP1A1-promoter-luciferase activity in rat small intestinal epithelial cells or rat colonocytes cultured on plastic was not observed. Ethoxyresorufin-o-deethylase activity in human colon adenocarcinoma cells, cultured on either plastic or Matrigel, was induced 7-fold by benzo[a]pyrene. 2,3,7,8-Tetrachlorodibenzo-p-dioxin-induced ethoxyresorufin-o-deethylase activity was 2-fold greater in human colon adenocarcinoma cells cultured on Matrigel compared to cells cultured on plastic. Extracellular matrix-mediated differentiation and apoptosis of intestinal cells provide in vitro systems for study of the regulation of CYP1A1 expression, carcinogen activation in the gut and mechanism(s) of apoptosis of colon cancer cells.

The molecular and genetic basis of colon cancer

Remarkable progress has been accomplished in understanding the molecular basis of genetic colon cancer syndromes including FAP and HNPCC, and their variants; of sporadic colon cancer; and of the rare hamartomatous polyp syndromes. This molecular progress now has to be translated into clinical progress in molecular diagnosis, and in pharmacologic therapy for colonic polyps and cancers. It is hoped that such progress will impact on the frequency and mortality of this very common and frequently fatal cancer.

De novo mutations in familial adenomatous polyposis (FAP)

Familial Adenomatous Polyposis (FAP) results from a germline mutation in the APC gene. A new mutation rate of 4-9 x 10(-6) mutations/gametes/generation has been reported. In other familial cancer syndromes a bias for paternal origin of new mutations has been described. This bias is probably due to a larger number of cell divisions during spermatogenesis compared to oogenesis; giving a larger susceptibility for mutagenesis. We report here a molecular genetic analysis of 26 FAP patients with putative de novo APC mutations. In 15 families the novel origin of the mutations was confirmed by haplotyping and sequencing. Analysis of 10 of these mutations, in which the parental origin could be established, gave a 6 : 4 distribution in favour of maternal origin. This is in agreement with a 1 : 1 distribution and does not indicate an expected paternal bias. Moreover, no parental age effect was identified. We propose that APC germline mutations are not premeiotic events but more likely arise during the meiosis. This would give an equal susceptibility for mutagenesis during spermatogenesis and oogenesis, respectively. The model is in concordance with the previously established difference between APC somatic mutations, as being a mitotic event and APC germline mutations, as being a meiotic event. The confirmation of 15 de novo mutations by a molecular genetic approach is in fine agreement with previous results based on clinical records.

The food mutagen 2-amino-9H-pyrido[2,3-b]indole (AalphaC) but not its methylated form (MeAalphaC) increases intestinal tumorigenesis in neonatally exposed multiple intestinal neoplasia mice

The heterocyclic amines 2-amino-9H-pyrido[2,3-b]indole (AalphaC) and 2-amino-3-methyl-9H-pyrido[2,3-b]indole (MeAalphaC) are carcinogenic in several organs in rodents, but not in the intestinal tract. However, AalphaC induces DNA adducts, mutations and preneoplastic aberrant crypt foci (ACF) in rodent colons. The purpose of this study was to examine whether AalphaC and MeAalphaC could affect intestinal tumorigenesis in C57BL/6J-Min/+ (multiple intestinal neoplasia) mice. These mice are heterozygous for a germline nonsense mutation in codon 850 of the tumor suppressor gene adenomatous polyposis coli (Apc), producing a truncated non-functional Apc protein. They develop multiple intestinal adenomas, and are particularly susceptible to intestinal carcinogens that affect the Apc gene, especially when exposed neonatally. Whole litters consisting of Min/+ and +/+ (wild-type) mice of both sexes were given a single s.c. injection of 0.22 mmol/kg AalphaC (40.3 mg/kg) or MeAalphaC (43.4 mg/kg) or the vehicle 1:1 dimethylsulfoxide:0.9% NaCl on days 3-6 after birth, and were terminated at 11 weeks. AalphaC increased the number and diameter of small intestinal tumors, but not the number of colonic tumors or dysplastic ACF, in female and male Min/+ mice separately. In pooled data from females and males, colonic tumors and ACF found after AalphaC exposure appeared to be smaller than the spontaneous lesions, indicating later induction, slower growth or both. In contrast to AalphaC, MeAalphaC did not affect intestinal tumorigenesis in Min/+ mice. No effects were found by any of the amino-alpha-carbolines in the +/+ mice. AalphaC was less potent than the heterocyclic amine 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine.

One dose of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) or 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) induces tumours in Min/+ mice by truncation mutations or LOH in the Apc gene

The C57BL/6J-Min/+ (multiple intestinal neoplasia) mouse has a heterozygous nonsense Apc(Min) (adenomatous polyposis coli) mutation, and numerous adenomas spontaneously develop in the intestine. Neonatal exposure of Min/+ mice to the food carcinogens 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) or 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) (one injection of 50mg/kg) increased the number of small intestinal tumours about three- and two-fold, respectively. The number of colonic tumours was only increased in males. We examined whether the wild-type Apc allele was affected in intestinal tumours induced by either PhIP or IQ. In spontaneously formed and in IQ-induced small intestinal and colonic tumours from these mice, the main mechanism for tumour induction was loss of wild-type Apc allele, i.e. loss of heterozygosity (LOH). In contrast to the IQ-induced (84% LOH) and spontaneously (88% LOH) formed tumours, only 55% of the PhIP-induced small intestinal tumours from males showed LOH. Tumours that apparently had retained the wild-type Apc allele were further analysed for the presence of truncated Apc proteins by the in vitro synthesised protein (IVSP) assay. Truncated Apc proteins, indicating truncation mutations in exon 15 of the Apc gene, were detected in two of the 12 PhIP-induced tumours in segment 2 (codons 686-1217), and two of five IQ-induced tumours, one in segment 2 and the other in segment 3 (codons 1099-1693). Three of these four mutations, all in segment 2 of the Apc gene, were confirmed by sequencing. The PhIP-induced mutations were detected at codon 1125 (C deletion) and 1130 (G-T transversion), and the IQ-induced mutation was at codon 956 (C-T transition). Importantly, no truncated proteins were detected in tumours from unexposed mice with apparently retained wild-type Apc allele. These results show that one injection of either PhIP or IQ induces intestinal tumours in the Min/+ mice by inactivation of the wild-type Apc allele either by causing LOH or truncation mutations.

Frequent and multiple mutations at minisatellite loci in sporadic human colorectal and gastric cancers--possible mechanistic differences from microsatellite instability in cancer cells

Minisatellites (MNs), composed of 5 to 100 nucleotide repeat units, range from 0.5 to 30 kb in length, and have been reported to be mutated in various human malignancies. In this study, frequencies of MN mutations in sporadic human colorectal (34 cases) and gastric cancers (24 cases) at various clinicopathological stages were assessed by multilocus DNA fingerprint analysis with three MN probes, Pc-1, 33.6 and 33.15. MN mutations were observed in both colorectal and gastric cancers, but at a significantly higher frequency in the former (56%) than in the latter (25%). Multiplicities of MN mutations were 1.50 +/- 1.81 and 0.46 +/- 1.10 in colorectal and gastric cancers, respectively, and the difference was also significant. Neither the presence nor multiplicity of MN mutations in either colorectal or gastric cancer cases had any correlation with the pathological stage, histological grading or the presence of microsatellite instability (MSI). Although the biological relevance of MN mutations still remains to be clarified, a subset of colorectal and gastric cancers could feature a new type of genomic instability, distinct from MSI.

COL11A1 in FAP polyps and in sporadic colorectal tumors

BACKGROUND

We previously reported that the alpha-1 chain of type 11 collagen (COL11A1), not normally expressed in the colon, was up-regulated in stromal fibroblasts in most sporadic colorectal carcinomas. Patients with germline mutations in the APC gene show, besides colonic polyposis, symptoms of stromal fibroblast involvement, which could be related to COL11A1 expression. Most colorectal carcinomas are suggested to be a result of an activated Wnt- pathway, most often involving an inactivation of the APC gene or activation of beta-catenin.

METHODS

We used normal and polyp tissue samples from one FAP patient and a set of 37 sporadic colorectal carcinomas to find out if the up-regulation of COL11A1 was associated with an active APC/beta-catenin pathway.

RESULTS

In this study we found a statistically significant difference in COL11A1 expression between normal tissue and adenomas from one FAP patient, and all adenomas gave evidence for an active APC/beta-catenin pathway. An active Wnt pathway has been suggested to involve stromal expression of WISP-1. We found a strong correlation between WISP-1 and COL11A1 expression in sporadic carcinomas.

CONCLUSIONS

Our results suggest that expression of COL11A1 in colorectal tumors could be associated with the APC/beta-catenin pathway in FAP and sporadic colorectal cancer.

Screening for mutations of the APC gene in 66 Italian familial adenomatous polyposis patients: evidence for phenotypic differences in cases with and without identified mutation

Germline mutations in the APC gene are responsible for familial adenomatous polyposis (FAP), a dominantly inherited syndrome characterized by the development of hundreds to thousands of polyps in the colon and in the rectum of affected individuals and by variable extracolonic manifestations (gastric and duodenal polyps, osteomas, retinal lesions, and desmoid tumors). Through the combined use of single-strand conformation polymorphism (SSCP) analysis and the protein truncation test (PTT), we have screened 66 Italian FAP patients and found 29 different APC mutations in a total of 34 cases. Of the identified mutations, 15 were nonsense, 12 were 1- to 5-bp deletions or insertions and two were complex rearrangements, all leading to the formation of premature stop codons. Only 10 mutations had been already previously described at the germline level, confirming the high heterogeneity of the APC mutational spectrum. The mean age of diagnosis in mutation positive cases and their affected relatives was significantly lower than in cases without identified mutation (30.6 vs 39.1 years, respectively; p = 0.003). In addition, among patients without a family history of polyposis, all mutation-positive cases displayed at least one of the extracolonic manifestations usually associated with FAP, whereas in one-half of the cases without identified mutation, none of these phenotypes was observed. Although a fraction of apparently mutation-negative cases were likely to be due to limitations of the mutation screening strategy, our results suggest, in agreement with previous reports, that allelic and/or genetic heterogeneity might be responsible for the phenotypic variability observed in FAP patients.

Defects in mismatch repair occur after APC mutations in the pathogenesis of sporadic colorectal tumours

The roles of the intrinsic mutation rate and genomic instability in tumorigenesis are currently controversial. In most colorectal tumours, it is generally supposed that the first mutations occur at the adenomatous polyposis coli (APC) locus; APC mutations are thought to provide cells with a selective advantage but have no known effect on the mutation rate. It has also been suggested that genomic instability is the initiating event in colorectal tumorigenesis and, if this is true, mutations of DNA mismatch repair (MMR) genes (or at similar loci) are the most likely candidates. If defective MMR precedes APC mutations, the APC mutations of colon tumours with defective MMR and hence replication errors (RER+) should differ from those of RER- tumours, in at least three specific ways: (1) a higher frequency of allele loss at APC in RER- tumours; (2) more frameshift than nonsense mutations in RER+ tumours; and (3) APC mutations in simple repeat sequences [(N)n, (N1N2)n, or (N1N2N3)n] in RER+ tumours. We found no evidence that sporadic RER+ and RER- colon cancers (including cell lines) differ in any of these three ways. Although it remains possible that MMR is abnormal in tumours from HNPCC families before APC mutations occur, it is likely that in sporadic colon tumours, APC mutations, rather than genomic instability, are the initiating events in tumorigenesis.

APC mutations in sporadic colorectal tumors: A mutational "hotspot" and interdependence of the "two hits"

Although APC mutations occur at a high frequency in colorectal cancers, few studies have performed a comprehensive analysis by screening the whole gene for mutations and assessing allelic loss. APC seems to act as a tumor-suppressor gene in a "nonclassical" fashion: data from familial adenomatous polyposis (FAP) show that the site of the germ-line mutation determines the type of "second hit" in FAP tumors, and simple protein inactivation is selected weakly, if at all. In this study, we screened the entire coding region of APC for mutations and assessed allelic loss in a set of 41 colorectal cancer cell lines. Of 41 cancers, 32 (83%) showed evidence of APC mutation and/or allelic loss. We identified several APC mutations and found a "hotspot" for somatic mutation in sporadic colorectal tumors at codon 1,554. Our results suggest that APC mutations occur in the great majority of colorectal cancers, the exceptions almost all being RER+ tumors, which may substitute for altered APC function by mutations in beta-catenin and/or at other loci. When combined with previously published data, our results show that there is interdependence of the "two hits" at APC in sporadic colorectal tumors as well as in FAP. APC mutations in the "mutation cluster region," especially those close to codon 1,300, are associated with allelic loss, whereas tumors with mutations outside this region tend to harbor truncating mutations. The causes of this phenomenon are probably selection for retained N-terminal and lost C-terminal APC functions, effects on beta-catenin levels, and APC protein stability.

APC mutations are sufficient for the growth of early colorectal adenomas

It is not clear whether APC mutations are sufficient for early colorectal adenomas to grow or whether additional mutations at other loci are required. We previously have screened 210 early colorectal adenomas from familial adenomatous polyposis patients for mutations and allelic loss at APC. Here, we determined whether allelic loss at APC had any effect on the nearby alpha-catenin gene. However, loss on 5q in familial adenomatous polyposis adenomas rarely extended as far as alpha-catenin, and no differences in alpha-catenin protein expression were found in tumors that showed loss encompassing both APC and alpha-catenin. We then screened all 210 tumors for mutations at candidate loci other than APC (K-ras, beta-catenin, and allelic loss at 1p33-p35 and 1p36) and for microsatellite instability (MSI). Each of these loci has been implicated previously in early colorectal tumorigenesis. One tumor harbored a beta-catenin mutation and another MSI, but none showed K-ras mutation or allelic loss at 1p33-p35 or 1p36. These data support the following hypotheses derived from sporadic colorectal tumors: beta-catenin mutations are generally an alternative to mutations at APC, MSI is not usually an early phenomenon in colorectal tumorigenesis, and K-ras mutations are more typical of large- and moderate-sized adenomas. Contrary to some previous reports, chromosome 1p allelic loss is infrequent in very early adenomas. APC mutations are generally sufficient for colorectal tumors to grow to about 1-cm diameter, although chance mutations at other loci can provide these early colorectal adenomas with a selective advantage, and some colorectal tumors may develop along a pathway not involving APC.

Involvement of APC and K-ras mutation in non-polypoid colorectal tumorigenesis

The aim of this study was to clarify the role of APC and K-ras mutations in non-polypoid colorectal tumorigenesis. DNA from 63 adenomas (31 polypoid, 17 superficial elevated, 15 superficial depressed), 66 submucosally invasive carcinomas (47 polypoid, 19 non-polypoid) and 34 advanced carcinomas were examined for K-ras codon 12 point mutations and APC mutations in the mutation cluster region. K-ras mutation: the frequency in superficial depressed adenomas was lower than that in polypoid adenomas (0% vs 31%: P= 0.018). The frequency in non-polypoid carcinomas was lower than that in polypoid carcinomas (11% vs 56%: P = 0.0008), and was relatively low compared with that in polypoid adenomas (11% vs 31%). APC mutation: the frequency in superficial depressed adenomas was lower than that in polypoid adenomas (7% vs 43%: P = 0.016), and that in polypoid carcinomas was similar to that in non-polypoid carcinomas. Polypoid adenomas, polypoid carcinomas and advanced carcinomas had almost the same frequency. There may be some pathway other than the conventional adenoma-carcinoma sequence in development of non-polypoid carcinomas. The precursors of most non-polypoid carcinomas are considered to be de novo or superficial depressed adenomas. In this non-polypoid pathway, APC mutation seems to be requisite but K-ras mutation not. It is possible that new APC mutations are acquired after the development of superficial depressed adenomas.

The type of somatic mutation at APC in familial adenomatous polyposis is determined by the site of the germline mutation: a new facet to Knudson's 'two-hit' hypothesis

APC is often cited as a prime example of a tumor suppressor gene. Truncating germline and somatic mutations (or, infrequently, allelic loss) occur in tumors in FAP (familial adenomatous polyposis). Most sporadic colorectal cancers also have two APC mutations. Clues from attenuated polyposis, missense germline variants with mild disease and the somatic mutation cluster region (codons 1,250-1,450) indicate, however, that APC mutations might not result in simple loss of protein function. We have found that FAP patients with germline APC mutations within a small region (codons 1,194-1,392 at most) mainly show allelic loss in their colorectal adenomas, in contrast to other FAP patients, whose 'second hits' tend to occur by truncating mutations in the mutation cluster region. Our results indicate that different APC mutations provide cells with different selective advantages, with mutations close to codon 1,300 providing the greatest advantage. Allelic loss is selected strongly in cells with one mutation near codon 1,300. A different germline-somatic APC mutation association exists in FAP desmoids. APC is not, therefore, a classical tumor suppressor. Our findings also indicate a new mechanism for disease severity: if a broader spectrum of mutations is selected in tumors, the somatic mutation rate is effectively higher and more tumors grow.

Higher frequency of Smad4 gene mutation in human colorectal cancer with distant metastasis

We have previously detected an increased frequency of loss of heterozygosity (LOH) on chromosome 18q during progression of colorectal carcinomas. To clarify the target of 18qLOH, mutation of Smad4 and Smad2 genes was analysed in 176 colorectal tumors with different stages, including liver metastasis, from 111 sporadic, 52 familial adenomatous polyposis (FAP) and nine hereditary nonpolyposis colorectal cancer (HNPCC) patients. Mutation of other Smad gene families in the TGF-beta signaling pathway was also examined. Twenty-one Smad4 mutations and one Smad2 mutation were detected, whereas mutation of Smad3, 6 and 7 genes was not detected. Smad4 mutations included seven frameshift, one inframe deletion, four nonsense and nine missense mutations, 95% of which resulted in alteration of Smad4 protein regions included in homo-oligomer and hetero-oligomer formation. Frequencies of tumors with Smad4 mutation were 0/40 (0%) in adenoma, 4/39 (10%) in intramucosal carcinoma, 3/44 (7%) in primary invasive carcinoma without distant metastasis, 6/17 (35%) in primary invasive carcinoma with distant metastasis, and 11/36 (31%) in distant metastasis (metastatic/non-metastatic: P=0.006 approximately 0.01). Loss of the other allele was observed in 19 of 20 (95%) invasive and metastasized carcinomas with Smad4 mutations. In four cases both primary and metastasized carcinomas in the same patients showed the same mutations. The present results suggest that Smad4 gene is one of true targets of 18qLOH, and that its inactivation is involved in advanced stages, such as distant metastasis, in human colorectal carcinogenesis.

Inherited susceptibility to colorectal adenomas and carcinomas: evidence for a new predisposition gene on 15q14-q22

BACKGROUND & AIMS

The aim of this study was to evaluate the role of known colorectal adenoma and carcinoma susceptibility genes and to locate a novel susceptibility gene in an Ashkenazi family (SM1311) with dominantly inherited predisposition to colorectal adenomas and carcinomas.

METHODS

Clinicopathologic and family history data were collected. Genetic linkage and mutational analyses were used to investigate the genetic basis of the family's disease.

RESULTS

Affected members of SM1311 develop multiple tubular, villous, tubulovillous, and/or serrated colorectal adenomas throughout the large bowel, and some develop colon carcinoma. There are no extracolonic features clearly associated with disease in SM1311. We have shown that the family's phenotype does not result from APC mutations (including the I1307K variant) or from genetic changes in the other known genes that predispose to colon cancer. Using genetic linkage analysis, supplemented by allele loss in tumors, we have provided evidence for a new colorectal cancer susceptibility gene, CRAC1 (colorectal adenoma and carcinoma), mapping to chromosome 15q14-q22.

CONCLUSIONS

We provide evidence for a novel colorectal adenoma and carcinoma susceptibility gene on chromosome 15q14-q22. Further studies are needed to confirm this localization and to evaluate the contribution of CRAC1 to this disease.

Mutations of adenomatous polyposis coli (APC) gene are uncommon in sporadic desmoid tumours

Desmoids are locally aggressive, non-metastasizing soft-tissue tumours, whose aetiology is still unclear. In patients affected with familial adenomatous polyposis (FAP), the incidence of desmoids is much higher than in the general population. The APC gene, which is responsible for FAP, is involved in the development of desmoids associated with this syndrome. In this study 16 sporadic and four FAP-related desmoids were analysed in order to investigate the possible involvement of APC in non-syndromic cases also. The 5' end (exons 1-11) and the coding portion of exon 15 of APC were screened using the in vitro synthesized-protein assay (IVSP). Exons 5, 6, 8-14, and a region of exon 15 spanning codons 1036-1634 were investigated by single-strand conformation polymorphism (SSCP) analysis. APC germline mutations were identified in all FAP patients, but not in sporadic cases. Somatic mutations were found in three FAP-associated desmoids (75%) and two sporadic tumours (12.5%). In one of the latter cases, both alleles were affected. These findings indicate a limited role of the gene in the development of desmoid tumours outside FAP.

Phenotypic and genotypic characteristics of aberrant crypt foci in human colorectal mucosa

Aberrant crypt foci (ACF) in colorectal mucosa are proposed to be the earliest morphological lesion in the development of neoplasia, but their characteristics remain controversial. We therefore studied the epithelial phenotype and genotype of ACF from patients with familial adenomatous polyposis (FAP) and of sporadic ACF by evaluating glycoprotein markers associated with neoplasia (lectins Dolichus biflorus agglutinin and peanut agglutinin; monoclonal antibody CA 19-9 against sialyl Lewis-a blood group substance), expression of proliferating cell nuclear antigen, and ras proto-oncogene mutations. The utility of the markers was established by comparing adenomas and hyperplastic polyps. Most FAP ACF resembled adenomas and were found to differ from sporadic ACF in their high frequency of dysplasia, staining with Dolichus biflorus agglutinin, expression of sialyl Lewis-a, proliferation in the epithelium of upper crypts, and low frequency of ras gene mutations (P = .04 to < .0000001). By contrast, sporadic ACF and a subset of FAP ACF had phenotypic characteristics resembling hyperplastic polyps but usually had ras mutations, which were inversely related to dysplasia (P = .00009). Our findings suggest that "aberrant crypt focus" is a generic term analogous to "polyp" and requires further histopathologic, phenotypic, or genotypic classification into dysplastic and heteroplastic (hetero = other, plasia = form) types. Dysplastic ACF represent potential precursors to colorectal adenomas and adenocarcinomas, but heteroplastic ACF appear to be associated, rather than precursor, lesions.

[Evaluation of cancer risk through genetic analysis?]

BACKGROUND

The recent literature of familial cancer, specifically related to germline mutations of RB1, p53, NF1, ATM, BRCA1, Mismatch repair genes and APC is reviewed.

RESULTS AND CONCLUSIONS

Germline mutations do not relate to an increased tumor risk of any single tissue, but instead to spectra of neoplastic diseases. The genetic background plays a major role in modifying the cancer risk. Therefore, mass screening for mutations of single genes seems to be without practical value. Only in combination with an adequate and informative family history can molecular genetic analysis significantly support the care for the individual. Comparison of the data of patients inheriting germline mutations and the experience from the corresponding "knockout" mouse demonstrate that only the p53 and APC knockout mice are useful models of human disease.

Allele loss in colorectal cancer at the Cowden disease/juvenile polyposis locus on 10q

The genes that are mutated in inherited cancer syndromes are often involved in the pathogenesis of sporadic cancers of the types that characterize those syndromes. In colorectal cancer such loci include the familial adenomatous polyposis (APC) gene and the hereditary nonpolyposis colorectal cancer (DNA mismatch repair) genes. Juvenile hamartomatous polyposis syndromes, which include Juvenile Polyposis and Cowden disease, also predispose to colorectal cancer. The gene for Cowden disease has recently been localized to chromosome 10q22-q23, and a juvenile polyposis locus, JP1, has been reported as mapping to the same location. We have studied up to 70 cases of sporadic colorectal cancer for allele loss at markers predominantly on the long arm of chromosome 10, including loci flanking the putative Cowden Disease/JP1 locus. Frequencies of allele loss of about 35% were found close to this locus, whereas low frequencies of allele loss were found elsewhere on 10q. Mutations at the putative Cowden Disease/JP1 locus may therefore be important in sporadic colorectal cancer and fine mapping of allele loss on 10q in sporadic colon cancers may help to refine the position of this gene.

Lessons from the p53 mutant mouse

The use of the mouse as a model organism in cancer research has a long and productive history, from the earliest studies of chemical carcinogenesis to the recent advances in gene targeting. Many of the basic principles of tumorigenesis have been formed in whole or in part through the study of tumor development in the mouse. Over the past decade, the major experimental approach has been to generate cancer-prone strains, either through transgenic technologies or, more recently, gene targeting. Here, I will review the state of the field of gene targeting of tumor-suppressor genes and concentrate on the p53 mutant strains and the lessons learned from the p53 mutant mouse.

Mechanisms of carcinogenesis and the mutant mouse

Gene targeting in embryonic stem cells has allowed the creation of several new mouse strains carrying mutations in the murine homologs of human tumor suppressor genes. The phenotypes of these mutant strains are reviewed. In addition, recent experiments utilizing the ApcMin and p53 mutant animals to address specific mechanisms of carcinogenesis are discussed.