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

Impaired Mitochondrial ATP Production Downregulates Wnt Signaling via ER Stress Induction

Wnt signaling affects fundamental development pathways and, if aberrantly activated, promotes the development of cancers. Wnt signaling is modulated by different factors, but whether the mitochondrial energetic state affects Wnt signaling is unknown. Here, we show that sublethal concentrations of different compounds that decrease mitochondrial ATP production specifically downregulate Wnt/β-catenin signaling in vitro in colon cancer cells and in vivo in zebrafish reporter lines. Accordingly, fibroblasts from a GRACILE syndrome patient and a generated zebrafish model lead to reduced Wnt signaling. We identify a mitochondria-Wnt signaling axis whereby a decrease in mitochondrial ATP reduces calcium uptake into the endoplasmic reticulum (ER), leading to endoplasmic reticulum stress and to impaired Wnt signaling. In turn, the recovery of the ATP level or the inhibition of endoplasmic reticulum stress restores Wnt activity. These findings reveal a mechanism that links mitochondrial energetic metabolism to the control of the Wnt pathway that may be beneficial against several pathologies.

DNA methylation changes and somatic mutations as tumorigenic events in Lynch syndrome-associated adenomas retaining mismatch repair protein expression

Background

DNA mismatch repair (MMR) defects are a major factor in colorectal tumorigenesis in Lynch syndrome (LS) and 15% of sporadic cases. Some adenomas from carriers of inherited MMR gene mutations have intact MMR protein expression implying other mechanisms accelerating tumorigenesis. We determined roles of DNA methylation changes and somatic mutations in cancer-associated genes as tumorigenic events in LS-associated colorectal adenomas with intact MMR.

Methods

We investigated 122 archival colorectal specimens of normal mucosae, adenomas and carcinomas from 57 LS patients. MMR-deficient (MMR-D, n = 49) and MMR-proficient (MMR-P, n = 18) adenomas were of particular interest and were interrogated by methylation-specific multiplex ligation-dependent probe amplification and Ion Torrent sequencing.

Findings

Promoter methylation of CpG island methylator phenotype (CIMP)-associated marker genes and selected colorectal cancer (CRC)-associated tumor suppressor genes (TSGs) increased and LINE-1 methylation decreased from normal mucosa to MMR-P adenomas to MMR-D adenomas. Methylation differences were statistically significant when either adenoma group was compared with normal mucosa, but not between MMR-P and MMR-D adenomas. Significantly increased methylation was found in multiple CIMP marker genes (IGF2, NEUROG1, CRABP1, and CDKN2A) and TSGs (SFRP1 and SFRP2) in MMR-P adenomas already. Furthermore, certain CRC-associated somatic mutations, such as KRAS, were prevalent in MMR-P adenomas.

Interpretation

We conclude that DNA methylation changes and somatic mutations of cancer-associated genes might serve as an alternative pathway accelerating LS-associated tumorigenesis in the presence of proficient MMR.

Fund

Jane and Aatos Erkko Foundation, Academy of Finland, Cancer Foundation Finland, Sigrid Juselius Foundation, and HiLIFE.

DNA hypermethylation appears early and shows increased frequency with dysplasia in Lynch syndrome-associated colorectal adenomas and carcinomas

BACKGROUND

Lynch syndrome (LS) is associated with germline mutations in DNA mismatch repair (MMR) genes. The first "hit" to inactivate one allele of the predisposing MMR gene is present in every cell, contributing to accelerated tumorigenesis. Less information is available of the nature, timing, and order of other molecular "hits" required for tumor development. To this end, MMR protein expression and coordinated promoter methylation were examined in colorectal specimens prospectively collected from LS mutation carriers (n = 55) during colonoscopy surveillance (10/2011-5/2013), supplemented with retrospective specimens.

RESULTS

Loss of MMR protein corresponding to the gene mutated in the germline increased with dysplasia, with frequency of 0 % in normal mucosa, 50-68 % in low-grade dysplasia adenomas, and 100 % in high-grade dysplasia adenomas and carcinomas. Promoter methylation as a putative "second hit" occurred in 1/56 (2 %) of tumors with silenced MMR protein. A general hypermethylation tendency was evaluated by two gene sets, eight CpG island methylator phenotype (CIMP) genes, and seven candidate tumor suppressor genes linked to colorectal carcinoma (CRC). Hypermethylation followed the same trend as MMR protein loss and was present in some low-grade dysplasia adenomas that still expressed MMR protein suggesting the absence of a "second hit." To assess prospectively collected normal mucosa for carcinogenic "fields," the specimen donors were stratified according to age at biopsy (50 years or below vs. above 50 years) and further according to the absence vs. presence of a (previous or concurrent) diagnosis of CRC. In mutation carriers over 50 years old, two markers from the candidate gene panel (SFRP1 and SLC5A8) revealed a significantly elevated average degree of methylation in individuals with CRC diagnosis vs. those without.

CONCLUSIONS

Our findings emphasize the importance and early appearance of epigenetic alterations in LS-associated tumorigenesis. The results serve early detection and assessment of progression of CRC.

DUOX2 and DUOXA2 form the predominant enzyme system capable of producing the reactive oxygen species H2O2 in active ulcerative colitis and are modulated by 5-aminosalicylic acid

BACKGROUND

NADPH oxidase-derived reactive oxygen species, such as H2O2, are part of the intestinal innate immune system but may drive carcinogenesis through DNA damage. We sought to identify the predominant enzyme system capable of producing H2O2 in active ulcerative colitis and assess whether it is affected by 5-aminosalicylic acid (5-ASA).

METHODS

We studied human mucosal biopsies by expression arrays, quantitative real-time polymerase chain reaction for NADPH oxidase family members, in situ hybridization (DUOX2 and DUOXA2) and immunofluorescence for DUOX, 8-OHdG (DNA damage), and γH2AX (DNA damage response) and sought effects of 5-ASA on ex vivo cultured biopsies and cultured rectal cancer cells.

RESULTS

DUOX2 with maturation partner DUOXA2 forms the predominant system for H2O2 production in human colon and is upregulated in active colitis. DUOX2 in situ is exclusively epithelial, varies between and within individual crypts, and increases near inflammation. 8-OHdG and γH2AX were observed in damaged crypt epithelium. 5-ASA upregulated DUOX2 and DUOXA2 levels in the setting of active versus quiescent disease and altered DUOX2 expression in cultured biopsies. Ingenuity pathway analysis confirmed that inflammation status and 5-ASA increase expression of DUOX2 and DUOXA2. An epithelial cell model confirmed that cultured cancer cells expressed DUOX protein and produced H2O2 in response to hypoxia and 5-ASA exposure.

CONCLUSIONS

Both DUOX2 and DUOXA2 expression are involved specifically in inflammation and are regulated on a crypt-by-crypt basis in ulcerative colitis tissues. Synergy between inflammation, hypoxia, and 5-ASA to increase H2O2 production could explain how 5-ASA supports innate defense, although potentially increasing the burden of DNA damage.

Colorectal cancers show distinct mutation spectra in members of the canonical WNT signaling pathway according to their anatomical location and type of genetic instability

It is unclear whether the mutation spectra in WNT genes vary among distinct types of colorectal tumors. We have analyzed mutations in specific WNT genes in a cohort of 52 colorectal tumors and performed a meta-analysis of previous studies. Notably, significant differences were found among the mutation spectra. We have previously shown that in familial adenomatous polyposis, APC somatic mutations are selected to provide the "just-right" level of WNT signaling for tumor formation. Here, we found that APC mutations encompassing at least two beta-catenin down-regulating motifs (20 a.a. repeats) are significantly more frequent in microsatellite unstable (MSI-H) than in microsatellite stable (MSS) tumors where truncations retaining less than two repeats are more frequent (P = 0.0009). Moreover, in cases where both APC hits are detected, selection for mutations retaining a cumulative number of two 20 a.a. repeats became apparent in MSI-H tumors (P = 0.001). This type of mutations were also more frequent in proximal versus distal colonic tumors, regardless of MSI status (P = 0.0008). Among MSI-H tumors, CTNNB1 mutations were significantly more frequent in HNPCC than in sporadic lesions (28% versus 6%, P < 10-6) and were preferentially detected in the proximal colon, independently of MSI status (P = 0.017). In conclusion, the observed spectra of WNT gene mutations in colorectal tumors are likely the result from selection of specific levels of beta-catenin signaling, optimal for tumor formation in the context of specific anatomical locations and forms of genetic instability. We suggest that this may underlie the preferential location of MMR deficient tumors in the proximal colon.

CD4+ lymphocytes modulate prostate cancer progression in mice

Chronic inflammation contributes to the development of prostate cancer in humans. Here, we show that male Apc(Min/+) mice also develop prostate carcinoma with increasing age, mimicking that seen in humans in their 5th or 6th decade of life. Proinflammatory cytokines were significantly linked with cancer and increasing age in our mouse model; however, prostate and bowel tissues lacked evidence of inflammatory cell infiltrates other than mast cells. Lymphocytes protected against cancer, and protection from prostate cancer resided in antiinflammatory CD4(+)CD25(+) regulatory (T(REG)) cells that downregulated inflammatory cytokines. Supplementation with syngeneic T(REG) cells collected from wild-type mice reduced the levels of interleukin (IL)-6 (p < 0.05) and IL-9 (p < 0.001) and lowered prostate cancer risk (p < 0.05). Depletion of CD25(+) cells in 2-month-old animals increased the expression of IL-6 (p < 0.005) within prostate and increased the frequency of high-grade prostatic intraepithelial neoplasia (p < 0.05) and microinvasive prostatic carcinoma (p < 0.05) in dorsolateral prostate. Depletion of CD25(+) cells in young animals also increased the frequency of intestinal cancer in Min mice. Taken together, chronically elevated proinflammatory cytokines promoted carcinoma in Apc(Min/+) mice. T(REG) lymphocytes downregulated inflammation-associated carcinogenic processes and contributed to immune and epithelial homeostasis.

APC gene mutations causing familial adenomatous polyposis in Polish patients

Familial adenomatous polyposis (FAP) is a well-known hereditary condition characterised by alimentary system tumours. Tens to thousands of polyps occur in the colon and rectum of the patients. There is a high heterogeneity with regard to the number and time of the occurrence of polyps. The occurrence of FAP is associated with mutations in the APC tumour suppressor gene, which was described in 1991. Since then, many studies have been done to analyse the distribution of mutations in individual populations and to determine the function of the gene and a diagnostic approach to FAP. Here the APC gene was studied with respect to the occurrence of small mutations and large rearrangements in 300 unrelated Polish FAP families. Ninety-seven mutations were identified in 164 families. Out of these mutations, 80 were small mutations, including 58 small mutations that were first identified in the Polish population (42 novel and 16 described previously). An increased frequency of mutation c.3927_3931delAAAGA was observed in 10% of the Polish group. Seventeen large rearrangements were found in 29 families. Out of those rearrangements, 8 repeat rearrangements occurred in 20 families. A problem in fast molecular diagnostics of FAP is a high heterogeneity of mutations in the APC gene. It seems that a multiplex ligation-dependent probe amplification test and searching for small mutations by the use of screening methods at the 5' end of exon 15 and exons 14, 9, 11, 13, 5, and 3, help to improve the molecular diagnostics of FAP in Polish patients.

Recent advances in colorectal cancer genetics and diagnostics

Colorectal cancer (CRC) is one of the most prevalent cancers and leading cause of cancer mortality worldwide. It is also one of the most curable cancers if detected early. This review classifies the diverse disease subtypes using various parameters including phenotypes of the polyps and describes how recent advances in genetics have impacted on disease diagnostics. For familial syndromes, the discovery of initiating mutations in the germline made personalized medicine a reality. A model linking the main tumorigenesis (Wnt/TGF-beta-BMP/LKB-1/PI3K-AKT) pathways and a strategy for gene testing are proposed. For sporadic CRC, high throughput technology has enabled the discovery of susceptibility loci that increased CRC risk. The ramifications of screening the population for susceptibility loci are discussed.

Truncation mutations abolish chromatin-associated activities of adenomatous polyposis coli

The adenomatous polyposis coli (APC) is a tumor suppressor whose loss of function leads to colon cancer. APC shuttles between the nucleus and cytoplasm, however its role in the nucleus remains elusive. We have found that nuclear APC specifically associates with transcriptionally active chromatin through structural elements located downstream to the region of frequent truncation mutations found in colorectal tumors. We show that a recombinant APC fragment comprising such elements associates in vivo with euchromatin and preferentially binds in vitro to acetylated histone H3. Induction of DNA double-strand breaks (DSB) stimulates accumulation of APC at the damaged DNA chromatin marked by histone H2AX and S139-phosphorylated histone H2AX. A nuclear complex containing the DNA-dependent protein kinase catalytic subunit (DNAPKcs) and APC associates with chromatin in response to DNA DSB. APC knockdown with siRNA decreased the rate of DNA DSB-induced S139 histone H2AX phosphorylation in cells expressing endogenous full-length APC, but not in colon cancer cells with its truncation mutants, whereas ectopic APC expression stimulated the H2AX phosphorylation regardless of the type of endogenous APC. Our data suggest that APC involves in the DSB DNA repair and that truncation mutations impair chromatin-associated functions of APC.

Differences between familial and sporadic forms of colorectal cancer with DNA microsatellite instability

Microsatellite instability (MSI) is observed in approximately 13% of colorectal cancers. Genes containing a mononucleotide microsatellite in the coding sequence are particularly prone to inactivation in MSI tumourigenesis, and much work has been conducted to identify genes with high repetitive tract mutation rates in these tumours. MSI caused by deficient DNA mismatch-repair functions is a hallmark of cancers associated with the hereditary non-polyposis colorectal cancer syndrome but is also found in about 15% of all sporadic tumours.

APC mutations and other genetic and epigenetic changes in colon cancer

Relationships between adenomatous polyposis coli (APC) mutations, BRAF V600E mutations, and the CpG island methylator phenotype (CIMP) in colon cancer have not been explored. In addition, controversies exist about the proportion of tumors with APC mutations in the mutation cluster region (MCR); how commonly APC, Ki-ras, and p53 mutations occur in the same tumor; and whether APC mutations occur in sporadic microsatellite-unstable tumors. The APC gene was therefore sequenced in 90 colonic adenocarcinomas previously evaluated for CIMP, microsatellite instability, BRAF, Ki-ras, and p53. APC mutations were inversely related to BRAF mutations (P = 0.0003) and CIMP (P = 0.02) and directly related to p53 and Ki-ras mutations (P = 0.04). Slightly more than half of APC mutations occurred outside of the MCR, and frameshift mutations were more likely than nonsense mutations to occur in the MCR (21 of 28 versus 12 of 40, P = 0.0003). APC mutations were found in sporadic microsatellite-unstable tumors and were more likely to be frameshifts in short nucleotide repeats (P = 0.007). The occurrence of APC, Ki-ras, and p53 mutations together in the same tumor was uncommon (11.1%). In conclusion, an analysis restricted to the MCR will miss more than half of APC mutations as well as mischaracterize their mutational spectrum. The conventional wisdom that most colon cancers contain APC, Ki-ras, and p53 mutations is incorrect. Microsatellite instability may precede acquisition of APC mutations in sporadic microsatellite-unstable tumors. The relationships of APC mutations to other genetic and epigenetic alterations add to the already impressive genetic heterogeneity of colon cancer.

Chromosomal instability by beta-catenin/TCF transcription in APC or beta-catenin mutant cells

Adenomatous polyposis coli (APC/Apc) gene encodes a key tumor suppressor whose mutations activate beta-catenin/T-cell factor (TCF)-mediated transcription (canonical Wnt signaling). Here, we show that Wnt signaling can cause chromosomal instability (CIN). As an indicator of CIN, we scored anaphase bridge index (ABI) in mouse polyps and ES cells where Wnt signaling was activated by Apc or beta-catenin mutations. We found three to nine times higher ABI than in wild-type controls. Furthermore, karyotype analysis confirmed that the Wnt signal-activated ES cells produced new chromosomal aberrations at higher rates; hence CIN. Consistently, expression of dominant-negative TCFs in these cells reduced their ABI. We also found that Wnt signal activation increased phosphorylation of Cdc2 (Cdk1) that inhibited its activity, and suppressed apoptosis upon exposure of the cells to nocodazole or colcemid. The data suggest that Wnt signaling stimulates the cells to escape from mitotic arrest and apoptosis, resulting in CIN. In human gastric cancer tissues with nuclear beta-catenin, ABI was significantly higher than in those without. These results collectively indicate that beta-catenin/TCF-mediated transcription itself increases CIN through dysregulation of G2/M progression.

Crypt dynamics and colorectal cancer: advances in mathematical modelling

Mathematical modelling forms a key component of systems biology, offering insights that complement and stimulate experimental studies. In this review, we illustrate the role of theoretical models in elucidating the mechanisms involved in normal intestinal crypt dynamics and colorectal cancer. We discuss a range of modelling approaches, including models that describe cell proliferation, migration, differentiation, crypt fission, genetic instability, APC inactivation and tumour heterogeneity. We focus on the model assumptions, limitations and applications, rather than on the technical details. We also present a new stochastic model for stem-cell dynamics, which predicts that, on average, APC inactivation occurs more quickly in the stem-cell pool in the absence of symmetric cell division. This suggests that natural niche succession may protect stem cells against malignant transformation in the gut. Finally, we explain how we aim to gain further understanding of the crypt system and of colorectal carcinogenesis with the aid of multiscale models that cover all levels of organization from the molecular to the whole organ.

Cancer and Evolution

Cancer mostly is a disease of old age. Evolutionary pressures have pushed the somatic "error rate", especially the mutation rate, down to a level where for most organisms cancer is no longer of any selective significance. This appears to be a by-product of the selection that gives rise to senescence, following the arguments of Medawar, Holliday, and Kirkwood.

The development of a cancer is discussed from an evolutionary viewpoint, emphasising the role of selection versus mutation and the fact that each cancer is an independent evolutionary process. The nature of the selective advantages associated with the somatic genetic changes during a cancer's evolution can sometimes be inferred by reference to the known types of mutations found in cancers. Examples are given using colorectal cancer as a model. The major selective forces involve the balance between selection for increased growth rate and against apoptosis. There are strong arguments against the much discussed role of genomic instability as a requirement for cancer. Current evidence suggests that instability is a byproduct of selection against apoptosis. There is an important contrast between germ line and somatic changes, the former being the basis for inherited susceptibilities to cancer, while the latter are the fundamental changes that turn a normal cell into a cancer cell.

Tissue stem cells, as in the colonic crypt, provide the source, through division and differentiation, of the differentiated cells in a crypt. Mathematical models can provide an explanation for how the balance in a crypt between stem cells, intermediate proliferating cells, and non-proliferating differentiated cells is maintained. Perturbations of the renewal parameters in the model can explain the evolution of benign tumors, namely polyps or adenomas, and the eventually exponential growth of cells resulting in a fully developed carcinoma. It seems probable that the origin of most carcinomas is in the intermediate proliferating cells, and that these are therefore the likely source of cancer stem cells.

Cancer genetics: colorectal cancer as a model

Cancer is essentially a somatic evolutionary process and is, therefore, effectively defined by the genetic and epigenetic changes underlying this process. An understanding of the function of these changes is fundamental to devising new approaches to prevention and treatment. Colorectal cancer (CRC), apart from its obvious importance as one of the most frequent cancers, provides an excellent model for such studies because of the availability of precursor adenoma lesions and the existence of several clear-cut familial inherited susceptibilities. These include familial adenomatous polyposis (FAP), which led to the identification of the APC gene and the importance of the Wnt pathway, and hereditary non-polyposis CRC (HNPCC), which identified the role of the mismatch repair genes in colorectal and other cancers. The presently known range of genetic and epigenetic changes in CRCs and adenomas is reviewed in this paper and the evidence against a requirement for genomic instability presented, together with a discussion of patterns of gene methylation, including especially our work on the homeobox gene, CDX1. Clearly, familial cancers, such as FAP and HNPCC, cannot account for more than perhaps 5% of the incidence of CRC. There is, however, evidence that approximately a further 25-30% have some inherited susceptibility. Based on the association of APC missense variants with multiple adenomas, we proposed that much of this may be due to the cumulative effects of low frequency, low penetrance variants, and the "rare variant hypothesis". The evidence for this from our work on multiple adenoma cases, and certain other examples, is discussed.

Association between cigarette smoking, APC mutations and the risk of developing sporadic colorectal adenomas and carcinomas

Background

The association between colorectal cancer (CRC) and smoking has not been consistent. Incomplete smoking history and association to a specific subset of CRC tumors have been proposed as explanations. The adenomatous polyposis coli (APC) gene has been reported to have a "gatekeeper" function in the colonic mucosa.

Methods

To evaluate the hypothesis that cigarette smoking is associated with adenoma and carcinoma development and further to investigate whether this association is due to mutations in the APC gene, we used a study population consisting of 133 cases (45 adenomas and 88 carcinomas) and 334 controls. All tumors were sequenced in the mutation cluster region (MCR) of the APC gene. Cases and controls were drawn from a homogeneous cohort of Norwegian origin.

Results

The mutational spectra of the APC gene revealed no difference in frequencies of mutations in cases based on ever and never smoking status. An overall case-control association was detected for adenomas and "ever smoking" OR = 1.73 (95% CI 0.83–3.58). For CRC cases several smoking parameters for dose and duration were used. We detected an association for all smoking parameters and "duration of smoking > 30 years", yielded a statistically significant OR = 2.86 (1.06–7.7). When cases were divided based on APC truncation mutation status, an association was detected in adenomas without APC mutation in relation to "ever smoking", with an OR = 3.97 (1.26–12.51). For CRC cases without APC mutation "duration of smoking > 30 years", yielded a statistically significant OR = 4.06 (1.20–13.7). The smoking parameter "starting smoking ≥ 40 years ago" was only associated with CRC cases with APC mutations, OR = 2.0 (0.34–11.95). A case-case comparison revealed similar findings for this parameter, OR = 2.24 (0.73–6.86).

Conclusion

Our data suggest an association between smoking and adenoma and CRC development. This association was strongest for cases without APC truncation mutation. This may implicate other factors in development of these tumors. The association detected between smoking and CRC cases with APC mutation was in relationship to the smoking parameter "starting smoking ≥ 40 years ago", a time period long enough to proceed CRC initiation.

Mutational Targets in Colorectal Cancer Cells with Microsatellite Instability

Cancers arise from the sequential acquisition of genetic alterations in specific genes. The high number of mutations in cancer cells led to the hypothesis that an early step in tumor progression is the generation of a genetic instability. The potent role of genetic instability in initiation and progression of colorectal cancers has been well defined in hereditary nonpolyposis colon cancer (HNPCC) syndrome. HNPCC is a common hereditary disorder caused by germline mutations of DNA mismatch repair (MMR) genes. Somatic loss of the normal allele of the predisposition gene leads to a strong "mutator phenotype", characterized by a high rate of mutations in repetitive sequences. Nevertheless, the observation of frequent alterations of key growth regulatory genes in MMR-deficient cells such as NF1, APC, p53, K-Ras, with no significant excess of frameshift mutations and changes at short coding repeats, suggest that even in the presence of an inherited tendency to genomic instability, tumor progression is mainly driven by a process of natural selection.

Immunoassay for wild-type protein in lymphocytes predicts germline mutations in patients at risk for hereditary colorectal cancer

Using colorectal cancer (CRC) as an example, we present the hypothesis that quantitative immunoassays for wild-type (full-length) proteins can be used to identify carriers of traits for hereditary diseases. In the case of hereditary CRC, this involves identifying individuals with germline mutations in a mismatch-repair (MMR) gene (mainly hMSH2 or hMLH1) or in the adenomatous polyposis coli (APC) gene. Because expression of wild-type protein should reflect wild-type gene dosage, we predicted that individuals harboring a germline mutation will have a reduction of approximately 50% in expression in lymphocytes of the corresponding full-length protein. In this pilot study, we tested lymphoblastoid cell lines that had been established from controls and individuals with, or at high risk for, hereditary CRC: 9 lines from healthy, unaffected individuals; 4 from affected members in familial adenomatous polyposis families (with known germ-line APC mutation); 42 from CRC patients in our Familial CRC Registry (increased risk of hereditary nonpolyposis colon cancer as assessed by family history, age at adenoma or carcinoma diagnosis, and other clinical criteria). For MSH2 and MLH1 we used western blots; for APC we used immunoprecipitation. All familial adenomatous polyposis lines had about 50% less immunoprecipitable full-length APC protein. Some cell lines (7 of 42) from Familial CRC Registry patients showed on western blots a reduction (mean 46%) in either MSH2 or MLH1 (relative to the other protein). All 7 subsequently were proved to contain a germline MMR mutation. We conclude that (1) because most of the expected CRC-causing germ line mutations are truncation-causing, immunoassays for wild-type protein should be able to identify most individuals with hereditary CRC-causing traits; (2) these assays, which are more practical and inexpensive than current mutation-detecting tests for hereditary CRC traits, have the potential for commercial development into broad-based population screens of high-risk patients and their families and the potential to save both lives and health-care dollars; (3) this strategy may be useful for other hereditary cancers and even other hereditary diseases; (4) our approach has the potential to greatly benefit public-health programs for cancer control.

Colonic polyposis caused by mTOR-mediated chromosomal instability in Apc+/Delta716 Cdx2+/- compound mutant mice

The mammalian homeobox transcription factor CDX2 has key roles in intestinal development and differentiation. Heterozygous Cdx2 mice develop one or two benign hamartomas in the proximal colon, whereas heterozygous Apc(Delta716) mice develop numerous adenomatous polyps, mostly in the small intestine. Here we show that the colonic polyp number is about six times higher in Apc+/Delta716 Cdx2+/- compound mutant mice. Levels of both APC and CDX2 were significantly lower in the distal colon, which caused high anaphase bridge index (ABI) associated with a higher frequency of loss of heterozygosity (LOH) at Apc. In cultured rat intestinal epithelial and human colon cancer cell lines, suppression of CDX2 by antisense RNA caused marked increases in ABI and chromosomal aberrations. This was mediated by stimulation of the mTOR pathway, causing translational deregulation and G1-S acceleration, associated with low levels of p27 and activation of cyclin E-Cdk2. We obtained similar results in the colonic mucosa of Apc+/Delta716) Cdx2+/- compound mutant mice. Forced activation of mTOR through upstream regulator Akt also increased ABI in colon cancer cells. High ABI in all cell lines was suppressed by mTOR inhibitors LY294002 and rapamycin. These results suggest that reduced expression of CDX2 is important in colon tumorigenesis through mTOR-mediated chromosomal instability.

Genomic instability--the engine of tumorigenesis?

Human cancers harbour numerous mutations and it has been proposed that these result from some form of inherent genomic instability. Some cancers have proven genomic instability or features that are indicative of this. Inherited cancer syndromes exist that are caused by deficient DNA repair or chromosomal integrity. By contrast, theoretical analysis and experimental data from sporadic colorectal tumours provide little general evidence of genomic instability in early lesions. These apparently conflicting data raise the question of whether or not genomic instability is necessary for driving tumour growth, and whether or not it is the usual initiating event in tumorigenesis.

Diallyl disulfide induces ERK phosphorylation and alters gene expression profiles in human colon tumor cells

Diallyl disulfide (DADS), a compound found in processed garlic, has been shown to arrest unsynchronized human colon tumor cells (HCT-15) in the G(2)/M phase of the cell cycle. The present studies were designed to examine whether this cell cycle block related to alterations in protein kinase C (PKC), Ca(2+)/calmodulin-dependent protein kinase II (CAMK II) or extracellular signal-regulated kinase (ERK) activity. Exposing double thymidine synchronized HCT-15 cells to DADS (25, 50 and 100 micromol/L) for 4 h increased the G(2)/M population by 30, 31 and 63%, respectively, compared with controls (P < 0.05). PKC and CAM KII activities were not influenced by increasing DADS exposure and thus did not correlate with the block of cells in the G(2)/M phase. Although ERK activity increased by 44 and 60% after treatment with 100 and 500 micromol/L DADS (P < 0.05), it was not influenced by exposure to 25 or 50 micromol/L DADS. Western blot analysis revealed that although DADS (25, 50, 100 and 500 micromol/L) did not influence the quantity of ERK protein expressed, it did increase its phosphorylation by 39, 52, 73 and 61%, respectively, compared with controls (P < 0.05). These studies provide evidence that early alterations in ERK pathway signaling may contribute to the G(2)/M arrest observed after DADS exposure. Preliminary data generated using the Clonetech Atlas Human Cancer cDNA Expression Array suggest that alterations in cell cycle, DNA repair and cellular adhesion factors accompany DADS exposure and may also be involved in mediating the block in G(2)/M progression.

Analysis of the human APC mutation spectrum in a saccharomyces cerevisiae strain with a mismatch repair defect

Somatic APC mutations in colorectal tumors with an RER phenotype reflect excessive frameshift mutations, especially in simple repetition tracts within the coding sequence. Because this type of mutation is characteristic of cells with a deficient DNA MMR system, the APC mutation signature of RER tumors may be attributable to a defect in the MMR system. However, there is little experimental evidence to prove that the spectrum of mutations and the APC gene distribution are directly influenced by MMR system defects. We therefore examined the mutation spectrum of the MCR of the APC gene after transfection into both MMR-proficient and MMR-deficient yeast strains and compared it with a previously reported human APC mutation database. Small insertions or deletions in mono- or dinucleotide repeats were more common in the MMR-deficient than in the MMR-proficient strain (91.2% vs. 38.1%, Fisher's exact test p < 0.0001). Furthermore, the 2 mutation hot spots, 4385-4394(AG)(5) and 4661-4666(A)(6), found in the yeast system corresponded with those in human tumors. Combining our data with those from human tumors, there appears to be hypermutable mutations in specific simple repetitive sequences within the MCR, which are more prevalent in MMR-deficient cells and RER tumors than in MMR-proficient cells and non-RER tumors. We therefore consider that the differences in the spectra of RER and non-RER tumors are attributable at least in part to the MMR system of the host cells.

Intestinal adenomas can develop with a stable karyotype and stable microsatellites

Loss of function of the adenomatous polyposis coli (APC)/Apc tumor suppressor gene occurs early in the etiology of intestinal cancer in mammals. In human colonic tumors, genomic instability is proposed to be associated with tumor initiation by inducing loss of APC function. We have used a mouse model of inherited intestinal cancer (Apc(Min)/+, Min/+) to analyze the earliest stages of tumorigenesis in this organ. We find that tumors from C57BL/6 Min/+ mice have a stable karyotype and stable microsatellites. In contrast to previous claims, we find that homozygosity for the Min allele of Apc in tumors can proceed by homologous somatic recombination. Further, our analysis of early, benign human colorectal adenomas failed to reveal any evidence for generalized chromosomal or microsatellite instability. These results cast doubt on the hypothesis that either of these forms of genomic instability is necessary for the initial development of colorectal adenomas. We contrast our analysis of autochthonous primary tumors to other studies involving xenografts or cultured cells.

Distinction between familial and sporadic forms of colorectal cancer showing DNA microsatellite instability

Attempts to classify colorectal cancer into subtypes based upon molecular characterisation are overshadowed by the classical stepwise model in which the adenoma-carcinoma sequence serves as the morphological counterpart. Clarity is achieved when cancers showing DNA microsatellite instability (MSI) are distinguished as sporadic MSI-low (MSI-L), sporadic MSI-high (MSI-H) and hereditary non-polyposis colorectal cancer (HNPCC). Divergence of the 'methylator' pathway into MSI-L and MSI-H is at least partly determined by the respective silencing of MGMT and hMLH1. Multiple differences can be demonstrated between sporadic and familial (HNPCC) MSI-H colorectal cancer with respect to early mechanisms, evolution, molecular characterisation, demographics and morphology. By acknowledging the existence of multiple pathways, rapid advances in the fields of basic and translational research will occur and this will lead to improved strategies for the prevention, early detection and treatment of colorectal cancer.

The role of selection in progressive neoplastic transformation

Mathematical modeling indicates that selective growth of cells with biallelic mutations in tumor suppressor genes is the driving force in the development of most human tumors, and that increased mutation rate is not required. Spontaneous neoplastic transformation of cells in culture offers the opportunity for quantitative analysis of all stages of neoplastic progression, the cellular variation that underlies it, and the selective conditions that promote it. Most of the early work on spontaneous transformation was done in primary cultures of mouse embryo cells, but established mouse cell lines have been used more in recent years. The main criteria for transformation have been tumorigenesis in mice, increase in saturation density, and production of discrete, multilayered foci in confluent cell cultures. Spontaneous transformation in NIH 3T3 mouse fibroblasts is efficiently evoked by progressive selection under prolonged contact inhibition at high population density or during multiplication at low population density in suboptimal concentrations or types of serum. In general, it is a multistep process with some stages of progression occurring before there is any visible sign of transformed foci. There is a high degree of heritable heterogeneity in the original NIH 3T3 cell population for susceptibility to transformation. Isolation and expansion of minority susceptible clones from a relatively refractory population exhibit transformation long before the polyclonal parental population does because of the increased proportion of susceptible cells in these clones. There are indications that the selective conditions induce selectable variants. Tumor development in animals and man shares important characteristics with spontaneous transformation in culture, including a major role for selection, but the selective conditions for clonal expansion probably vary with the dynamics of differentiation in each tissue. These considerations support a role for an altered microenvironment (as in the aging process) in selective growth of rogue clones.

The APC/beta-catenin pathway in ulcerative colitis-related colorectal carcinomas: a mutational analysis

BACKGROUND

Although the APC/beta-catenin pathway is known to play a crucial role in sporadic colorectal carcinogenesis, its influence on ulcerative colitis (UC)-related neoplastic progression is unknown. To elucidate the role of the APC-/beta-catenin pathway in UC-related carcinogenesis, the authors identified APC and beta-catenin mutations in a set of UC-related and sporadic colorectal carcinomas.

METHODS

The mutational cluster region of APC (codon 1267 to 1529) and exon 3 of the beta-catenin were directly sequenced.

RESULTS

Only 1 of 30 UC-related tumors (3%) showed an APC mutation whereas 11 of the 42 sporadic carcinomas (26%) had mutations within the mutational cluster region. Within the sporadic carcinoma group, only 8% of the right-sided carcinomas showed APC mutations whereas 50% of the left-sided carcinomas had mutations within the mutational cluster region. None of the tumors in either group showed a beta-catenin mutation.

CONCLUSIONS

Mutations of the APC and beta-catenin are rare in UC-related tumors. These genes may be altered because of mutations outside the regions studied, or by epigenetic silencing. Alternatively, other proteins involved in the APC/beta-catenin signaling cascade may be altered, or this pathway may be involved infrequently in UC-related carcinogenesis. The significant difference in frequency of APC mutations between right- and left-sided sporadic tumors suggests different molecular pathways in these two tumor sites.

Emerging pathways in colorectal-cancer development

The past decade has seen the emergence of new pathways in the development of colorectal cancer. There is now clear evidence that subsets of these tumours do not show chromosomal instability and do not follow the suppressor pathway. Instead, about 15% of colorectal cancers are characterised by microsatellite instability (MSI). This feature arises through defective DNA mismatch repair, which is related either to a germline mutation (as in hereditary non-polyposis colorectal carcinoma) or to failure to express a mismatch-repair gene. CpG-island methylation has been linked to sporadic cancers with a high frequency of MSI. This type of methylation leads to loss of gene expression when it occurs in the promoter region of a gene. Tumours may have high or low type C (cancer-related) CpG-island methylation. When methylation affects hMLH1 (mismatch repair gene), the resultant cancer has high MSI.

How carcinogens (or telomere dysfunction) induce genetic instability: associated-selection model

Carcinogens induce carcinogen-specific genetic instability (defects in DNA repair). According to the 'direct-selection' model, defects in DNA repair per se provide an immediate growth advantage. According to the 'associated-selection' model, carcinogens merely select for cells with adaptive mutations. Like any mutations, adaptive mutations occur predominantly in genetically unstable cells. The 'associated-selection' model predicts that carcinogen-driven selection minimizes cytotoxic but maximizes mutagenic effects of carcinogens. A purely mutagenic (neither cytotoxic, nor cytostatic) environment will favor effective DNA repair, whereas any growth-limiting conditions (telomerase deficiency, anticancer drugs) will select for genetically unstable cells. Genetic instability is a postmark of selective pressure rather than a hallmark of cancer per se. Once selected, genetic instability facilitates the development of resistance to any other growth-limiting conditions. As an example, a putative link between prior exposure to carcinogens and the ability to develop a telomerase-independent growth is discussed.

Aneuploid colon cancer cells have a robust spindle checkpoint

Colon cancer cells frequently display minisatellite instability (MIN) or chromosome instability (CIN). While MIN is caused by mismatch repair defects, the lesions responsible for CIN are unknown. The observation that CIN cells fail to undergo mitotic arrest following spindle damage suggested that mutations in spindle checkpoint genes may account for CIN. However, here we show that CIN cells do undergo mitotic arrest in response to spindle damage. Although the maximum mitotic index achieved by CIN lines is diminished relative to MIN lines, CIN cells clearly have a robust spindle checkpoint. Consistently, mutations in spindle checkpoint genes are rare in human tumours. In contrast, the adenomatous polyposis coli (APC) gene is frequently mutated in CIN cells. Significantly, we show here that expression of an APC mutant in MIN cells reduces the mitotic index following spindle damage to a level observed in CIN cells, suggesting that APC dysfunction may contribute to CIN.

Apical membrane localization of the adenomatous polyposis coli tumor suppressor protein and subcellular distribution of the beta-catenin destruction complex in polarized epithelial cells

The adenomatous polyposis coli (APC) protein is implicated in the majority of hereditary and sporadic colon cancers. APC is known to function as a tumor suppressor through downregulation of beta-catenin as part of a high molecular weight complex known as the beta-catenin destruction complex. The molecular composition of the intact complex and its site of action in the cell are still not well understood. Reports on the subcellular localization of APC in various cell systems have differed significantly and have been consistent with an association with a cytosolic complex, with microtubules, with the nucleus, or with the cortical actin cytoskeleton. To better understand the role of APC and the destruction complex in colorectal cancer, we have begun to characterize and isolate these complexes from confluent polarized human colon epithelial cell monolayers and other epithelial cell types. Subcellular fractionation and immunofluorescence microscopy reveal that a predominant fraction of APC associates tightly with the apical plasma membrane in a variety of epithelial cell types. This apical membrane association is not dependent on the mutational status of either APC or beta-catenin. An additional pool of APC is cytosolic and fractionates into two distinct high molecular weight complexes, 20S and 60S in size. Only the 20S fraction contains an appreciable portion of the cellular axin and small but detectable amounts of glycogen synthase kinase 3beta and beta-catenin. Therefore, it is likely to correspond to the previously characterized beta-catenin destruction complex. Dishevelled is almost entirely cytosolic, but does not significantly cofractionate with the 20S complex. The disproportionate amount of APC in the apical membrane and the lack of other destruction complex components in the 60S fraction of APC raise questions about whether these pools of APC take part in the degradation of beta-catenin, or alternatively, whether they could be involved in other functions of the protein that still must be determined.

Frequent activation of the beta-catenin-Tcf signaling pathway in nonfamilial colorectal carcinomas with microsatellite instability

It has been reported that wild-type APC protein forms a complex with beta-Catenin and GSK3beta, inducing degradation of beta-Catenin in normal cells. Both beta-Catenin and APC gene mutations have recently been shown to activate the same signaling pathway. Frequent mutations of beta-Catenin in hereditary nonpolyposis colorectal carcinomas have also been reported. It was, however, controversial whether the mutation of the beta-Catenin gene was frequent in nonfamilial colorectal carcinomas with high-frequency microsatellite instability (MSI-H). We analyzed the mutations of the APC and beta-Catenin genes in 56 nonfamilial colorectal carcinomas stratified according to the presence or absence of microsatellite instability (MSI). APC mutations were identified in 11 of 22 (50%) cases of MSI-H and 14 of 34 (41%) cases of microsatellite-stable (MSS)/low-frequency microsatellite instability (MSI-L). In contrast, the frequency of beta-Catenin mutations was significantly higher in MSI-H (6/22; 27%) than in MSS/MSI-L (1/34; 3%) (P = 0.01). beta-Catenin mutations were not detected in carcinomas with APC mutation. APC mutation occurred irrespective of MSI status. beta-Catenin mutation, however, occurred frequently in MSI-H carcinomas. Our data suggest that activation of the beta-Catenin-Tcf signaling pathway, through either beta-Catenin or APC mutation, frequently contributes to MSI-H nonfamilial colorectal carcinomas (17/22; 77%).

Different mechanisms in the tumorigenesis of proximal and distal colon cancers

Environment or genetic constitutions can lead to an increase of genetic or epigenetic events and increase the risk for malignancy. Genomic instability is seen in most types of malignancies. Two forms of genetic instability have been described in colorectal cancer: chromosomal instability (CIN), and microsatellite instability (MIN). Almost all sporadic MIN tumors occur in the proximal colon, whereas most sporadic CIN tumors are distributed in the distal colon. The two familial syndromes, familial adenomatous polyposis and Lynch syndrome, constitute models for the different carcinogenic mechanisms in CIN and MIN tumors, respectively. This article reviews the principal differences between CIN and MIN tumors, evidence for a proximal and distal route in carcinogenesis, gender differences, and aspects of methylation in CIN and MIN colorectal tumorigenesis.

Pathogenesis of non-familial colorectal carcinomas with high microsatellite instability

AIMS

Microsatellite instability (MSI) was first observed in hereditary non-polyposis colorectal carcinoma (HNPCC) and was subsequently seen in non-familial colorectal carcinoma. The relation between MSI and cancer associated genes in non-familial colorectal carcinomas has yet to be evaluated. To clarify this matter, changes in cancer associated genes were examined in non-familial colorectal carcinomas.

METHODS

Alterations in the adenomatous polyposis coli (APC), p53, and Ki-ras genes were analysed in 24 MSI high (alterations in four to seven of seven loci), nine MSI low (alterations in one to three of seven loci), and 31 MSI negative non-familial carcinomas. The hMSH2 and hMLH1 genes were also analysed in 24 MSI high carcinomas.

RESULTS

Both the frequencies and types of alterations in the APC and p53 genes in MSI high carcinomas were the same as those in MSI low and MSI negative carcinomas; however, they were different from those seen in HNPCC. The frequency of Ki-ras mutation was significantly lower in the MSI high cases (two of 24; 8%) than in the others (15 of 38; 39%). Somatic mutation of hMSH2 or hMLH1 was detected in six of 24 (25%) of the MSI high cases.

CONCLUSIONS

These results suggest that APC and p53 alterations occur irrespective of microsatellite instability status in non-familial colorectal carcinomas, and that Ki-ras mutation is not involved in MSI high non-familial colorectal carcinoma. The pathogenesis of these carcinomas may differ from both the usual adenoma-carcinoma sequence and HNPCC carcinogenesis.

The role of hypermethylation of the hMLH1 promoter region in HNPCC versus MSI+ sporadic colorectal cancers

INTRODUCTION

Hypermethylation of the promoter region of the hMLH1 gene is associated with absent expression of MLH1 protein in sporadic colorectal cancers with microsatellite instability (MSI+), and it has been proposed that methylation may be a mechanism of inactivation in Knudson's hypothesis. The incidence of hypermethylation of the hMLH1 promoter in hereditary non-polyposis colorectal cancer (HNPCC) versus MSI+ sporadic colorectal cancer was investigated and compared.

METHODS

DNA was available from 10 HNPCC colorectal cancers (median age 58 years, range 39-67) with germline mutations in hMLH1 and 10 MSI+ sporadic colorectal cancers (mean age 79 years, range 41-85). MSI was determined by amplification of BAT26 and TGF-beta RII. The methylation status of the hMLH1 promoter was studied by the polymerase chain reaction (PCR) based HpaII restriction enzyme assay technique. Evidence of allelic loss at hMLH1 was searched for in the HNPCC colorectal cancers.

RESULTS

All cases were confirmed to be MSI+. The promoter region of hMLH1 was hypermethylated in seven of 10 MSI+ sporadic cancers versus 0 of 10 HNPCC cancers (p<0.002). Evidence of loss of heterozygosity at hMLH1 was observed in eight of the 10 HNPCC colorectal cancers.

CONCLUSION

While mutations and allelic loss are responsible for the MSI+ phenotype in HNPCC cancers, the majority of MSI+ sporadic cancers are hypermethylated in the promoter region of hMLH1. These data further support our argument that tumours from HNPCC patients, which almost always acquire a raised mutation rate, mostly follow a different pathway from MSI+ sporadic tumours.

Different pathways of colorectal carcinogenesis and their clinical pictures

An entertaining debate in cancer genetics concerns the relative importance of selection for a growth advantage and genomic instability in tumorigenesis. Below, I present evidence that shows that selection is more important, particularly in the early stages of tumorigenesis, but that genomic instability is also an indisputable feature of many cancers. One attractive possibility is that some mutations may have pleiotropic effects both on cell replication or apoptosis and on genomic instability.

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.

Mismatch repair processing of carcinogen-DNA adducts triggers apoptosis

The DNA mismatch repair pathway is well known for its role in correcting biosynthetic errors of DNA replication. We report here a novel role for mismatch repair in signaling programmed cell death in response to DNA damage induced by chemical carcinogens. Cells proficient in mismatch repair were highly sensitive to the cytotoxic effects of chemical carcinogens, while cells defective in either human MutS or MutL homologs were relatively insensitive. Since wild-type cells but not mutant cells underwent apoptosis upon treatment with chemical carcinogens, the apoptotic response is dependent on a functional mismatch repair system. By analyzing p53 expression in several pairs of cell lines, we found that the mismatch repair-dependent apoptotic response was mediated through both p53-dependent and p53-independent pathways. In vitro biochemical studies demonstrated that the human mismatch recognition proteins hMutSalpha and hMutSbeta efficiently recognized DNA damage induced by chemical carcinogens, suggesting a direct participation of mismatch repair proteins in mediating the apoptotic response. Taken together, these studies further elucidate the mechanism by which mismatch repair deficiency predisposes to cancer, i.e., the deficiency not only causes a failure to repair mismatches generated during DNA metabolism but also fails to direct damaged and mutation-prone cells to commit suicide.

Genetic pathways in colorectal and other cancers

Cells from cancers show aberrant behaviour such as unrestrained growth, invasion into adjacent tissue and metastasis. All these features of cancer cell behaviour can be explained in terms of genetic changes and the functional impact of these changes. In this review, colorectal cancer (CRC) is examined as a classical example of multistep carcinogenesis. First there is an overview which shows that cancers develop by a process of somatic evolution. This gives rise to preferred genetic pathways of tumorigenesis. The factors which may influence the development and ultimate choice of genetic pathways are then examined. Next, CRC is studied as a specific disease and the putative genetic pathways are described. The mutations that comprise these pathways and the possible functional sequelae of these are explored. The review concludes with a look at those avenues which may further elucidate the natural history of CRC and lead to improved therapy.

Mechanisms of inactivation of mismatch repair genes in human colorectal cancer cell lines: the predominant role of hMLH1

Fifteen to twenty-five percent of sporadic colorectal carcinomas are replication error (RER) positive. Because the frequency of mutations in the mismatch repair genes (hMLH1 and hMSH2) is low in these tumors, we have investigated the role of mutational inactivation, methylation of the promoter region, and loss of heterozygosity (LOH) as a possible explanation for the mutator phenotype of RER+ colorectal cancer cell lines. Genomic DNA was extracted from a panel of 49 human colorectal cancer cell lines. The RER status was determined by amplification of BAT-26. All exons of hMLH1 and hMSH2 were amplified with the PCR and screened by using single-strand conformational polymorphism and direct sequencing. The methylation status was ascertained by methylation-specific PCR after bisulfite modification of DNA. Western blotting for hMLH1 was performed on methylated cell lines before and after the addition of the demethylating agent 5-azacytidine. LOH was sought by GENESCAN analysis of amplified CA repeat markers and indirectly by determining the number of homozygotes in the cell lines and human random controls. Twelve cell lines from ten tumors (24%) were RER+. Hypermethylation of the hMLH1 promoter occurred in five of ten (50%) RER+ tumors, whereas three of thirty-two (6%) RER tumors showed partial methylation. None of the fully methylated cell lines expressed hMLH1, although all reexpressed hMLH1 after treatment with 5-azacytidine. There was no LOH in the RER+ tumors in either hMLH1 or hMSH2. Our results suggest that mutations of hMLH1 together with hypermethylation of the promoter region, but not LOH, are the cause of the mutator phenotype in the majority (70%) of RER+ tumors.

Genetic pathways in colorectal and other cancers

Cells from cancers show aberrant behaviour such as unrestrained growth, invasion into adjacent tissue and metastasis. All these features of cancer cell behaviour can be explained in terms of genetic changes and the functional impact of these changes. In this review, colorectal cancer (CRC) is examined as a classical example of multistep carcinogenesis. First there is an overview which shows that cancers develop by a process of somatic evolution. This gives rise to preferred genetic pathways of tumorigenesis. The factors which may influence the development and ultimate choice of genetic pathways are then examined. Next, CRC is studied as a specific disease and the putative genetic pathways are described. The mutations that comprise these pathways and the possible functional sequelae of these are explored. The review concludes with a look at those avenues which may further elucidate the natural history of CRC and lead to improved therapy.

Genetic pathways of colorectal carcinogenesis rarely involve the PTEN and LKB1 genes outside the inherited hamartoma syndromes

Germline mutations of the PTEN/MMAC1/TEP and LKB1 genes cause hamartomas to develop in the gastrointestinal tracts of patients with Cowden syndrome and Peutz-Jeghers syndrome, respectively. PTEN mutations may also be responsible for some cases of juvenile polyposis. Histologically, hamartomas appear benign, but there is good evidence that in these syndromes, the hamartomas can progress to colorectal carcinoma. It remains unknown whether or not cancers that develop from hamartomas acquire a spectrum of mutations similar to those in sporadic colon cancers. PTEN and LKB1 are candidate genes for mutations in sporadic colon cancers, either as initiating events in tumorigenesis or providing a selective advantage during tumor growth. Using single-strand conformational polymorphism analysis, we have screened a set of sporadic colon cancers for somatic mutations in PTEN and LKB1. No variants predicted to alter protein function were detected in LKB1, but 1 of 72 cancers showed a somatic mutation in PTEN, together with allele loss. This cancer did not have a detectable APC mutation or allele loss at APC. It remains possible that PTEN and LKB1 are inactivated in other sporadic colon cancers by means such as deletion or promoter methylation. Like BRCA1 and BRCA2, however, it appears that PTEN and LKB1 mutations can cause cancers when present in the germline, but occur rarely in the soma.