Epigenetic silencing of the intronic microRNA hsa-miR-342 and its host gene EVL in colorectal cancer.

MicroRNAs are small, non-coding RNAs that influence gene regulatory networks by post-transcriptional regulation of specific messenger RNA targets. MicroRNA expression is dysregulated in human malignancies, frequently leading to loss of expression of certain microRNAs. We report that expression of hsa-miR-342, a microRNA encoded in an intron of the gene EVL, is commonly suppressed in human colorectal cancer. The expression of hsa-miR-342 is coordinated with that of EVL and our results indicate that the mechanism of silencing is CpG island methylation upstream of EVL. We found methylation at the EVL/hsa-miR-342 locus in 86% of colorectal adenocarcinomas and in 67% of adenomas, indicating that it is an early event in colorectal carcinogenesis. In addition, we observed a higher frequency of methylation (56%) in histologically normal colorectal mucosa from individuals with concurrent cancer compared to mucosa from individuals without colorectal cancer (12%), suggesting the existence of a 'field defect' involving methylated EVL/hsa-miR-342. Furthermore, reconstitution of hsa-miR-342 in the colorectal cancer cell line HT-29 induced apoptosis, suggesting that this microRNA could function as a proapoptotic tumor suppressor. In aggregate, these results support a novel mechanism for silencing intronic microRNAs in cancer by epigenetic alterations of cognate host genes.

Sequential tumor biopsies in early phase clinical trials of anticancer agents for pharmacodynamic evaluation.

PURPOSE: In the setting of target-based anticancer drug development, it is critical to establish that the observed preclinical activity can be attributed to modulation of the intended target in early phase trials in human subjects. This paradigm of target modulation allows us to determine a Phase II or III dose (optimal biochemical/biological modulatory dose) that may not necessarily be the maximum tolerated dose. A major obstacle to target-based (often cytostatic) drug development has been obtaining relevant tumor tissue during clinical trials of these novel agents for laboratory analysis of the putative marker of drug effect. EXPERIMENTAL DESIGN: From 1989 to present, we have completed seven clinical trials in which the end point was a biochemical or biological modulatory dose in human tumor tissues (not surrogate tissue). Eligibility enrollment required that patients have a biopsiable lesion either with computerized tomography (CT) guidance or direct visualization and consent to sequential (pre and posttreatment) biopsies. RESULTS: A total of 192 biopsies were performed in 107 patients. All but 8 patients had sequential pre and posttreatment biopsies. Seventy-eight (73%) of the 107 patients had liver lesion biopsies. In eight patients, either one or both biopsies contained insufficient viable tumor tissue or no tumor tissue at all for analysis. Of a total of 99 patients in whom we attempted to obtain paired biopsies, a total of 87 (88%) were successful. Reasons for failure included patient refusal for a second biopsy (n = 2), vasovagal reaction with first biopsy precluding a second biopsy (n = 1), subcapsular hepatic bleeding (n = 1), and most commonly obtaining necrotic tumor, fibrous, or normal tissue in one of the two sequential biopsies (n = 8). CONCLUSIONS: This is the first and largest reported series demonstrating that with adequate precautions and experience, sequential tumor biopsies are feasible and safe during early phase clinical trials.

Differential degradation rates of inactivated alkyltransferase in blood mononuclear cells and tumors of patients after treatment with O(6)-benzylguanine.

O(6)-Alkylguanine-DNA alkyltransferase (AGT) repairs O(6)-alkylating DNA adducts generated by alkylating therapeutic agents. Therefore, AGT activity may be an important marker of tumor and normal tissue sensitivity to chemotherapeutic agents and a predictor for the success of chemotherapeutic regimens. It is rapidly inactivated by O(6)-benzylguanine (BG) that mimics its substrates, O(6)-methylguanine and O(6)-chloroethylguanine DNA adducts. In a Phase I clinical trial, BG was given in increasing doses (from 10 to 120 mg/m(2)) by 1-h infusion. We previously reported depletion of AGT activity, and in this report, we demonstrate the relationship between degradation of BG-inactivated AGT protein and the depletion of AGT activity in peripheral blood mononuclear cells (PBMCs) and tumor samples obtained by computed tomography-guided cutting needle biopsy from patients prior to BG and either 2 or 18 h after BG. In PBMCs, BG inactivated AGT activity by over 95-100% at the end of a 1-h infusion, and depletion was maintained for 18 h. In contrast, AGT protein remained almost unchanged for up to 18 h after BG, suggesting that inactivated AGT proteins remain immunoreactive and are not rapidly degraded in PBMCs. In patient tumor biopsies, AGT activity was depleted approximately 90% 2 h after BG. Tumor AGT protein levels were reduced to approximately 40% of pretreatment values when detected by either Western blot or immunohistochemistry staining. In tumor samples obtained 18 h after BG, >95% inactivation of tumor AGT activity was observed at BG doses of 36-80 mg/m(2), and complete depletion of tumor AGT activity occurred at 120 mg/m(2) BG. However, residual AGT protein (5-10% of baseline) was detectable in all tumor samples. Therefore, the degradation of BG-inactivated AGT protein appeared to be much more rapid in tumors than that in PBMCs, which may impact on AGT regeneration rates as well. Because degradation of BG-inactivated AGT takes place slowly, antibody-based measurements of AGT protein correlate poorly with depletion of AGT activity immediately after BG. Thus, biochemical activity measurements remain the appropriate monitor of AGT during therapeutic modulation. These data provide the first and conclusive evidence of differential degradation rates of inactivated AGT in PBMCs and tumors of patients after treatment with BG and suggest that immunoreactive AGT measurements in PBMCs are a poor surrogate for AGT activity in tumor tissue.

Temozolomide: the effect of once- and twice-a-day dosing on tumor tissue levels of the DNA repair protein O(6)-alkylguanine-DNA-alkyltransferase.

Temozolomide (TMZ) is a methylating agent of the imidotetrazine class, whose cytotoxic product is O(6)-methylguanine DNA adducts, which initiate a futile recycling of the mismatch repair pathway causing DNA strand breaks and apoptotic cell death in mismatch repair proficient cells. The DNA repair protein O(6)-alkylguanine DNA alkyltransferase (AGT) repairs these adducts in a suicide manner and reduces the cytotoxic action of TMZ. An antitumor threshold is reached when sufficient adducts are formed by TMZ to inactivate AGT. In this study, we evaluated the relation between TMZ dosing and AGT depletion in patients with deep visceral tumors and in peripheral blood mononuclear cells (PBMCs) to determine whether the dose of TMZ was sufficient to inactivate AGT and lead to therapeutic efficacy. To do so, we compared single dose therapy with a novel twice daily regimen in a laboratory correlate-driven Phase I dose escalation study. p.o. bolus dose TMZ 200 mg/m(2) daily times five was compared with the same bolus on day 1 followed by nine doses at 12-h intervals of 50, 75, 90, or 100 mg/m(2). Dose-limiting toxicity in the bid regimen (grade IV thrombocytopenia and neutropenia) was seen at 100 mg/m(2), cumulative dose 1100 mg/m(2), and the maximum tolerated dose was 1010 mg/m(2). The degree of tumor tissue AGT activity depletion measured in biopsies before and on day 5 of therapy varied widely, between 0 (in 3 patients) and 99% (in 1), with the majority of patients (10 of 15) having 52-84% tumor AGT depletion. In contrast, AGT activity in PBMCs fell rapidly during TMZ administration to undetectable levels in all dosage groups on day 5 but did not correlate with tumor AGT depletion. TMZ pharmacokinetics were dose proportional; no accumulation occurred >5-day period in the bid regimen. Two partial responses were seen, lasting 3 and 4 months. Five additional patients achieved prolonged stabilization of disease for 4-6 monthly cycles. This is the first study to document that at maximum tolerated doses, TMZ depletes PBMC AGT but only partially and variably depletes visceral tumor AGT in most patients, even during twice daily dosing. Drug combinations or schedules designed to maximally deplete tumor AGT might improve TMZ efficacy.

Cause-specific mortality coding. methods in the collaborative ocular melanoma study coms report no. 14.

Ascertainment of cause of death is often sought in clinical trials in which mortality is an outcome of interest. Standardized methods of coding all-cause and disease-specific mortality were developed and evaluated in the Collaborative Ocular Melanoma Study randomized trial of pre-enucleation radiation of large choroidal melanoma. All available clinical and pathologic materials documenting events prior to each reported death were reviewed systematically by a Mortality Coding Committee (MCC) to determine whether melanoma metastasis or local recurrence was present at the time of death. A level of certainty was assigned based on availability of local or central review of pathology materials. The outcome of the mortality coding protocol was evaluated both by assessing agreement between the judgment of the MCC and the presumed cause of death reported by the clinical center and, for a subset of patients, by assessing agreement between the MCC classification and the cause of death reported on the death certificate. As of July 31, 1997 (the cutoff date for the initial mortality report), 435 (95%) of 457 deceased patient files had been reviewed. The MCC classified 269 patients (62%) as dead with melanoma metastasis, 22 (5%) as dead with another malignant tumor, and 92 (21%) as dead with a malignant tumor of uncertain origin. Thirty-eight patients (9%) died with no evidence of malignancy; in 14 cases (3%), the presence or absence of malignancy could not be established due to lack of clinical information. Fair agreement (kappa = 0.34) was observed between the determinations of the MCC based on detailed review of materials and the cause of death reported on the death certificate, but death certificates alone underestimated the proportion of deaths due to metastatic choroidal melanoma. Detailed mortality coding identified difficulties associated with accurate reporting of cause-specific mortality in patients with choroidal melanoma.

Short mononucleotide repeat sequence variability in mismatch repair-deficient cancers.

Mismatch repair-deficient cancers are characterized by widespread insertions and deletions in microsatellite sequences, including those comprised of mononucleotide repeats. Such alterations have been observed in relatively short mononucleotide tracts in several genes and often are interpreted to indicate that the affected genes normally act as tumor suppressors. To aid in the interpretation of such changes, we have systematically assessed their frequency within transcribed regions of the genome that are unlikely to play a tumorigenic role. The advent of the complete human genomic sequences of chromosome 22 allowed us to select 29 genes for this analysis, spaced at approximately 1-Mb intervals. Each of the selected genes had an (A)(8) or a (G)(8) tract deep within intronic sequences that was not included in the processed transcript. Surprisingly, we found that there was substantial variation in the prevalence of mutations among these tracts. Some tracts were altered in < 5% of the mismatch repair-deficient cancers studied, whereas other tracts were altered in nearly half of the cancers. In particular, (G)(8) tracts were considerably more prone to mutation than (A)(8) tracts, and the sequences or chromatin structures surrounding the mononucleotide tracts seemed to affect their mutability significantly.

Transforming growth factor-beta-induced growth inhibition in a Smad4 mutant colon adenoma cell line.

Transforming growth factor-beta (TGF-beta) inhibits growth and induces apoptosis of colon epithelial cells. Binding of TGF-beta to its receptor induces phosphorylation of the Smad proteins Smad2 and Smad3, which then form heteromeric complexes with Smad4, translocate to the nucleus, and activate gene transcription. Smad4 function has been considered an obligate requirement for TGF-beta signaling, and Smad4 mutations present in some cancers have been considered sufficient to inactivate TGF-beta signaling. In this work, we describe studies with a nontransformed human colon epithelial cell line that is mutant for Smad4 but remains growth-inhibited by TGF-beta. The colon cell line VACO-235 has lost one of its Smad4 alleles via a chromosome 18q deletion. The remaining allele bears two missense point mutations located in regions important for Smad4 trimer formation, which is thought necessary for Smad4 function. As expected, pSBE4-BV/Luc, a Smad4-activated transcriptional reporter, was inactive in VACO-235. Nonetheless, VACO-235 demonstrated 80% growth inhibition in response to TGF-beta, as well as retention of some TGF-beta-mediated activation of the p3TP-Lux transcriptional reporter. Transient transfection of the VACO-235 Smad4 mutant allele into a Smad4-null cell line confirmed that this allele is functionally inactive as assayed by both the pSBE4-BV and p3TP-Lux reporters. The simplest explanation of these results is that there is a non-Smad4-dependent pathway for TGF-beta-mediated signaling and growth inhibition in VACO-235 cells.

Mechanisms underlying losses of heterozygosity in human colorectal cancers.

Losses of heterozygosity are the most common molecular genetic alteration observed in human cancers. However, there have been few systematic studies to understand the mechanism(s) responsible for losses of heterozygosity in such tumors. Here we report a detailed investigation of the five chromosomes lost most frequently in human colorectal cancers. A total of 10,216 determinations were made with 88 microsatellite markers, revealing 245 chromosomal loss events. The mechanisms of loss were remarkably chromosome-specific. Some chromosomes displayed complete loss such as that predicted to result from mitotic nondisjunction. However, more than half of the losses were associated with losses of only part of a chromosome rather than a whole chromosome. Surprisingly, these losses were due largely to structural alterations rather than to mitotic recombination, break-induced replication, or gene conversion, suggesting novel mechanisms for the generation of much of the aneuploidy in this common tumor type.

Contextual effects of transforming growth factor beta on the tumorigenicity of human colon carcinoma cells.

Transforming growth factor betas (TGF-betas) are a growth factor family with negative autocrine growth functions for most epithelial cells including colon carcinoma cell lines. Both type I (RI) and type II (RII) transmembrane TGF-beta receptors have been shown to be indispensable for TGF-beta-mediated cell growth regulation. Previous studies using different model systems have shown that both overexpression of TGF-beta1 and transfection of antisense TGF-beta1 to reduce TGF-beta1 expression could lead to increased tumorigenicity. These results are seemingly contradictory and suggest that effects of TGF-beta modulation on malignant properties of cancer cells may be contextual. This study addresses this issue using human colon carcinoma cells (CBS and FET) to determine the effects of modulation of the various components of the TGF-beta system on in vitro and in vivo growth properties in two independent isogenic models of colon carcinoma. Cells were stably transfected with a tetracycline-repressible RII expression vector (CBS4-RII), a tetracycline-repressible expression vector containing a truncated RII cDNA lacking the serine/threonine kinase domain (CBS4-deltaRII and FET6-deltaRII), or with a vector containing the TGF-beta1 cDNA (CBS4-beta1S and FET-beta1S). Expression of the truncated RII reduced TGF-beta sensitivity, whereas overexpression of RII increased TGF-beta sensitivity. TGF-beta overexpression did not affect TGF-beta response. In vivo tumorigenicity assays revealed that CBS4-RII cells had lower tumorigenicity than control cells, whereas CBS4-deltaRII and CBS4-beta1S had higher tumorigenicity than controls. The CBS4 cells are poorly tumorigenic in athymic mice, and the wild-type FET6 cells are nontumorigenic. FET6-deltaRII cells formed rapidly growing tumors, and FET-beta1S cells also formed tumors. These data illustrate the paradoxical tumor-promoting and -suppressing effects of TGF-beta signaling activity in two isogenic model systems from human colon carcinomas, thus demonstrating that the effects of modulation of TGF-beta expression or TGF-beta signaling capability affects malignancy in a contextual manner.

O6-benzylguanine: a clinical trial establishing the biochemical modulatory dose in tumor tissue for alkyltransferase-directed DNA repair.

Early phase evaluation of anticancer drugs has traditionally used toxicity (usually hematological) rather than efficacy end points to establish appropriate dosing schedules. To establish a biochemical efficacy end point for overcoming alkylguanine DNA alkyltransferase (AGT)-mediated tumor cell resistance to 1,3-bis(2-chloroethyl)-1-nitrosourea, we performed a novel dose escalation clinical trial for the AGT-depleting agent O6-benzylguanine (BG). The dose of BG required to deplete AGT to undetectable levels (BMD(T)) in sequential computed tomography-guided tumor tissue biopsies before BG and 18 h after BG was determined. Thirty patients received doses of BG ranging from 10 to 120 mg/m2. In tumor tissue, AGT depletion >86% of baseline was demonstrated at all doses tested. Residual tumor AGT activity, present 18 h after BG doses of 10-80 mg/m2, was eliminated at the 120 mg/m2 dose and is thus the BMD(T) of BG. BG pharmacokinetics are characterized by the rapid, dose-independent clearance of BG from plasma Metabolism of BG to its biologically active metabolite, 8-oxo-benzylguanine (8-oxo-BG), was found. The t(1/2) of 8-oxo-BG is longer than BG. Plasma concentrations of 8-oxo-BG well above 200 ng/ml 18 h after the end of the BG infusion were observed at the highest dose levels tested and appeared to correlate with depletion of AGT activity to undetectable levels in tumor tissue. AGT activity in peripheral blood mononuclear cells at baseline did not correlate with tumor tissue AGT activity. Depletion of AGT activity to undetectable levels in peripheral blood mononuclear cells occurred at lower doses and was not a reliable predictor for tumor tissue depletion. No serious side effects were observed with administration of BG alone or in combination with 13 mg/m2 1,3-bis(2-chloroethyl)-1-nitrosourea. This is the first clinical study in which biochemical analyses from pre- and posttreatment tumor biopsies have been used as an efficacy end point for the clinical development of an anticancer agent. From our tumor tissue biopsy data, we have established that a BG dose of 120 mg/m2 infused over 1 h should be used in Phase II clinical trials.

Control of type II transforming growth factor-beta receptor expression by integrin ligation.

Ectopic expression of the alpha5 integrin subunit in cancer cells with little or no endogenous expression of this integrin often results in reduced proliferation as well as reduced malignancy. We now show that inhibition resulting from ectopic expression of alpha5 integrin is due to induction of autocrine negative transforming growth factor-beta (TGF-beta) activity. MCF-7 breast cancer cells do not express either alpha5 integrin or type II TGF-beta receptor and hence are unable to generate TGF-beta signal transduction. Ectopic expression of alpha5integrin expression enhanced cell adhesion to fibronectin, reduced proliferation, and increased the expression of type II TGF-beta receptor mRNA and cell surface protein. Receptor expression was increased to a higher level in alpha5 transfectants by growth on fibronectin-coated plates. Induction of type II TGF-beta receptor expression also resulted in the generation of autocrine negative TGF-beta activity because colony formation was increased after TGF-beta neutralizing antibody treatment. Transient transfection with a TGF-beta promoter response element in tandem with a luciferase cDNA into cells stably transfected with alpha5 integrin resulted in basal promoter activities 5-10-fold higher than those of control cells. Moreover, when alpha5 transfectants were treated with a neutralizing antibody to either TGF-beta or integrin alpha5, this increased basal promoter activity was blocked. Autocrine TGF-beta activity also induced 3-fold higher endogenous fibronectin expression in alpha5 transfectants relative to that of control cells. Re-expression of type II receptor by alpha5 transfection also restored the ability of the cells to respond to exogenous TGF-beta and led to reduced tumor growth in athymic nude mice. Taken together, these results show for the first time that TGF-beta type II receptor expression can be controlled by alpha5beta1 ligation and integrin signal transduction. Moreover, TGF-beta and integrin signal transduction appear to cooperate in their tumor-suppressive functions.

Detection of poly(ADP-ribose) polymerase cleavage in response to treatment with topoisomerase I inhibitors: a potential surrogate end point to assess treatment effectiveness.

Cleavage of poly(ADP-ribose) polymerase (PARP) by caspases is a prominent characteristic of apoptosis or programmed cell death shown to be induced by topoisomerase (Topo) inhibitors. Because Topo I inhibitors have been shown to be effective in the treatment of some patients with colon cancer, we considered the possibility of using PARP cleavage as an early predictor of responsiveness to this class of agents. We show cleavage of PARP in response to treatment with Topo I inhibitors in colon cancer both in vitro and in vivo: (a) in vitro in SW480, HCT116, VACO5, VACO6, VACO8, VACO411, VACO425, and VACO451 human colon cancer cell lines treated with topotecan (TPT) or CPT-11; (b) in vivo in SW480, VACO451, and VRC5 colon cancer xenografts grown in athymic mice treated with TPT or CPT-11; and (c) in vivo in colon cancer samples from patients undergoing a Phase II clinical trial with CPT-11. Our results show a strong correlation between percentage of PARP cleavage and percentage of acridine orange-positive cells in colon cancer cell lines treated with 0.1 microM TPT for 24 and 48 h, confirming that PARP cleavage is a useful marker for programmed cell death in colon cancer cell lines. Results from experiments performed on colon cancer xenografts also show an association between PARP cleavage and response to treatment with TPT or CPT-11. The increase of PARP cleavage in xenografts and in clinical samples corresponding to treatment with Topo I inhibitors suggests that this procedure may have early predictive value to assess effectiveness of treatment. These results provide the basis for determining the validity of using PARP cleavage as an early marker of chemotherapeutic effectiveness in human samples.

Mutational inactivation of transforming growth factor beta receptor type II in microsatellite stable colon cancers.

We previously demonstrated that mutational inactivation of transforming growth factor beta type II receptors (RIIs) is very common among the 13% of human colon cancers with microsatellite instability. These mutations principally cluster in the BAT-RII polyadenine sequence repeat. Among microsatellite stable (MSS) colon cancers, we now find that non-BAT-RII point mutations inactivate RII in another 15% of cases, thus doubling the known number of colon cancers in which RII mutations are pathogenetic. Functional analysis confirms that these mutations inactivate RII signaling. Moreover, another 55% of MSS colon cancers demonstrate a transforming growth factor beta signaling blockade distal to RII. The transforming growth factor beta pathway and RII in particular are major targets for inactivation in MSS colon cancers as well as in colon cancers with microsatellite instability.

Somatic mutations of the mitochondrial genome in human colorectal tumours.

Alterations of oxidative phosphorylation in tumour cells were originally believed to have a causative role in cancerous growth. More recently, mitochondria have again received attention with regards to neoplasia, largely because of their role in apoptosis and other aspects of tumour biology. The mitochondrial genome is particularly susceptible to mutations because of the high level of reactive oxygen species (ROS) generation in this organelle, coupled with a low level of DNA repair. However, no detailed analysis of mitochondrial DNA in human tumours has yet been reported. In this study, we analysed the complete mtDNA genome of ten human colorectal cancer cell lines by sequencing and found mutations in seven (70%). The majority of mutations were transitions at purines, consistent with an ROS-related derivation. The mutations were somatic, and those evaluated occurred in the primary tumour from which the cell line was derived. Most of the mutations were homoplasmic, indicating that the mutant genome was dominant at the intracellular and intercellular levels. We showed that mitochondria can rapidly become homogeneous in colorectal cancer cells using cell fusions. These findings provide the first examples of homoplasmic mutations in the mtDNA of tumour cells and have potential implications for the abnormal metabolic and apoptotic processes in cancer.

Autocrine transforming growth factor alpha provides a growth advantage to malignant cells by facilitating re-entry into the cell cycle from suboptimal growth states.

CBS human colon carcinoma cells are poorly tumorigenic in athymic nude mice, whereas FET colon carcinoma cells are non-tumorigenic. Both cell lines have well differentiated properties in tissue culture. Transforming growth factor alpha (TGF-alpha) was ectopically expressed by stable transfection of a TGF-alpha cDNA under repressible tetracycline control. The TGF-alpha-transfected cells showed enhanced clonal initiation and shortened lag phase growth in tissue culture without an alteration in doubling time in exponential phase relative to untransfected cells. Furthermore, the TGF-alpha transfectants showed increased independence from exogenous growth factors in clonal growth assays and induction of DNA synthesis after release from quiescence. Growth factor independence was associated with sustained epidermal growth factor receptor activation in quiescent TGF-alpha-transfected cells and the requirement of exogenous insulin for stimulation of quiescent cells to re-enter the cell cycle. Higher cloning, reduced lag time in tissue, and the acquisition of growth factor independence for DNA synthesis without a change in doubling time of TGF-alpha-transfected cells indicate that autocrine TGF-alpha functions by facilitating re-entry into the cell cycle from sub-optimal growth states rather than promoting or controlling the proliferation of actively cycling cells. The modulation of growth regulation by autocrine TGF-alpha was associated with increased malignant properties as TGF-alpha transfectants showed increased tumorigenicity in athymic nude mice. The administration of tetracycline reversed the effects of TGF-alpha expression in these cells both in vivo and in vitro, indicating that the alterations of the biological properties were due to the expression of TGF-alpha. Since these cells are continuously grown in a completely chemically defined medium without serum supplementation, it was possible to assign the mechanism underlying the generation of growth factor independence to the replacement of a requirement for exogenous insulin in parental cells by autocrine TGF-alpha.

Chromosome number and structure both are markedly stable in RER colorectal cancers and are not destabilized by mutation of p53.

Fourteen colorectal cancer cell lines, categorized according to the presence or absence of microsatellite instability, were further analysed for chromosomal stability by karyotyping. NonRER (microsatellite stable) cell lines typically displayed highly aberrant karyotypes with alterations not only of chromosome number but also of chromosome structure including chromosomal deletions, inversions, and translocations. RER (microsatellite unstable) cell lines, in contrast, displayed significantly fewer alterations of chromosome number. Moreover, RER cell lines also displayed significantly fewer cytogenetically evident alterations of chromosome structure. Compared to NonRER colon cancers, RER colon cancers are significantly less likely to have undergone chromosomal gain, loss, or breakage. Characterization of p53 gene status by gene sequencing was performed in an attempt to determine if p53 gene status correlated with the chromosomal stability of the RER cancers. Gene mutations in p53 were present in all of the NonRER colon cancers. However, p53 gene mutations were also found present in four of nine of the RER colon cancers. Unexpectedly, RER colon cancers bearing mutant p53 demonstrated the same stability of chromosome number, and the same stability of chromosome structure, as the RER colon cancers with wild-type p53. Therefore, in RER colon cancers specific p53 independent mechanisms actively maintain the stability of both chromosome number and structure.

Expression of transforming growth factor-beta receptor type II and tumorigenicity in human breast adenocarcinoma MCF-7 cells.

To analyze transforming growth factor-beta (TGF-beta) response during MCF-7 cell progression, early passage (MCF-7E, < 200 passage) and late passage (MCF-7L, > 500 passage) cells were compared. MCF-7E cells showed an IC50 of approximately 10 ng/ml of TGF-beta1, whereas MCF-7L cells were insensitive. MCF-7E cells contained approximately threefold higher levels of TGF-beta receptor type II (TbetaRII) mRNA than MCF-7L, but their TbetaRI levels were similar. MCF-7E parental cells showed higher TbetaRII promoter activity than MCF-7L cells, which could be attributed to changes in Sp1 nuclear protein levels. Receptor cross-linking studies indicated that the cell surface receptor levels parallel mRNA levels in both cell lines. Limiting dilution clones of MCF-7E cells were established to determine the heterogeneity of TbetaRII expression in this cell line, and they showed varying degrees of TbetaRII expression. Fibronectin was induced at higher levels in cells expressing higher TbetaRII levels. All three TGF-beta isoforms were detected in limiting dilution clones and parental cells, but TGF-beta1 was more abundant relative to TGF-beta2 or 3, and no correlation between TGF-beta isoform profile with TGF-beta sensitivity was found. MCF-7L cells were tumorigenic and formed xenografts rapidly and progressively, whereas MCF-7E parental and limiting dilution clonal cells showed transient tumor formation followed by regression. These results indicate that decreased TbetaRII transcription in breast cancer cells leads to a loss of TbetaRII expression, resulting in cellular resistance to TGF-beta which contributes to escape from negative growth regulation and tumor progression.

Incidence and functional consequences of hMLH1 promoter hypermethylation in colorectal carcinoma.

Inactivation of the genes involved in DNA mismatch repair is associated with microsatellite instability (MSI) in colorectal cancer. We report that hypermethylation of the 5' CpG island of hMLH1 is found in the majority of sporadic primary colorectal cancers with MSI, and that this methylation was often, but not invariably, associated with loss of hMLH1 protein expression. Such methylation also occurred, but was less common, in MSI- tumors, as well as in MSI+ tumors with known mutations of a mismatch repair gene (MMR). No hypermethylation of hMSH2 was found. Hypermethylation of colorectal cancer cell lines with MSI also was frequently observed, and in such cases, reversal of the methylation with 5-aza-2'-deoxycytidine not only resulted in reexpression of hMLH1 protein, but also in restoration of the MMR capacity in MMR-deficient cell lines. Our results suggest that microsatellite instability in sporadic colorectal cancer often results from epigenetic inactivation of hMLH1 in association with DNA methylation.

Aberrant regulation of transforming growth factor-alpha during the establishment of growth arrest and quiescence of growth factor independent cells.

Autocrine transforming growth factor alpha (TGFalpha) is an important positive growth effector in malignant cells and plays a significant role in generating the growth factor-independent phenotype associated with malignant progression. However, the molecular mechanisms by which TGFalpha confers a growth advantage in progression is poorly understood. The highly tumorigenic cell line HCT116 up-regulates TGFalpha mRNA expression during growth arrest, whereas the poorly tumorigenic growth factor-dependent FET cell line down-regulates TGFalpha mRNA expression as it becomes quiescent. We have identified a 25-bp sequence at -201 to -225 within the TGFalpha promoter which mediates the differential regulation of TGFalpha expression during quiescence establishment in these two cell lines. This same sequence confers TGFalpha promoter responsiveness to exogenous growth factor or autocrine TGFalpha. The abberant upregulation of TGFalpha mRNA in quiescent HCT116 cells may allow them to return to the dividing state under more stringent conditions (nutrient replenishment alone) then quiescent FET cells (requires nutrients and growth factors). Antisense TGFalpha approaches showed that the dysregulated TGFalpha expression in quiescent HCT116 cells is a function of the strong TGFalpha autocrine loop (not inhibited by blocking antibodies) in these cells.

Mutations of mitotic checkpoint genes in human cancers.

Genetic instability was one of the first characteristics to be postulated to underlie neoplasia. Such genetic instability occurs in two different forms. In a small fraction of colorectal and some other cancers, defective repair of mismatched bases results in an increased mutation rate at the nucleotide level and consequent widespread microsatellite instability. In most colorectal cancers, and probably in many other cancer types, a chromosomal instability (CIN) leading to an abnormal chromosome number (aneuploidy) is observed. The physiological and molecular bases of this pervasive abnormality are unknown. Here we show that CIN is consistently associated with the loss of function of a mitotic checkpoint. Moreover, in some cancers displaying CIN the loss of this checkpoint was associated with the mutational inactivation of a human homologue of the yeast BUB1 gene; BUB1 controls mitotic checkpoints and chromosome segregation in yeast. The normal mitotic checkpoints of cells displaying microsatellite instability become defective upon transfer of mutant hBUB1 alleles from either of two CIN cancers.