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Colorectal cancer (CRC) is a leading cause of cancer death in the United States. Standard treatment for advanced stage CRC for decades has included 5-fluorouracil based chemotherapy. More recently, targeted therapies for metastatic CRC are being used based on the individual cancer's molecular profile. In the past few years, several different molecular subtype schemes for human CRC have been developed. The molecular subtypes can be distinguished by gene expression signatures and have the potential to be used to guide treatment decisions. However, many subtyping classification methods were developed using mRNA expression levels of hundreds to thousands of genes, making them impractical for clinical use. In this study, we assessed whether an immunohistochemical approach could be used for molecular subtyping of colorectal cancers. We validated two previously published, independent sets of immunohistochemistry classifiers and modified the published methods to improve the accuracy of the scoring methods. In addition, we evaluated whether protein and genetic signatures identified originally in the mouse were linked to clinical outcomes of patients with CRC. We found that low DDAH1 or low GAL3ST2 protein levels in human CRCs correlate with poor patient outcome. The results of this study have the potential to impact methods for determining the prognosis and therapy selection for CRC patients.

Background Data are steadily accruing that demonstrate that intestinal tumors are frequently derived from multiple founding cells, resulting in tumors comprised of distinct ancestral clones that might cooperate or alternatively compete, thereby potentially impacting different phases of the disease process. Aim We sought to determine whether tumors with a multi-ancestral architecture involving at least two distinct clones show increased tumor number, growth, progression, or resistance to drug intervention. Methods Mice carrying the Min allele of Apc were generated that were mosaic with only a subset of cells in the intestinal epithelium expressing an activated form of PI3K, a key regulatory kinase affecting several important cellular processes. These cells were identifiable as they fluoresced green, whereas all other cells fluoresced red. Results Cell lineage tracing revealed that many intestinal tumors from our mouse model were derived from at least two founding cells, those expressing the activated PI3K (green) and those which did not (red). Heterotypic tumors with a multi-ancestral architecture as evidenced by a mixture of green and red cells exhibited increased tumor growth and invasiveness. Clonal architecture also had an impact on tumor response to low-dose aspirin. Aspirin treatment resulted in a greater reduction of heterotypic tumors derived from multiple founding cells as compared to tumors derived from a single founding cell. Conclusion These data indicate that genetically distinct tumor-founding cells can contribute to early intratumoral heterogeneity. The coevolution of the founding cells and their progeny enhances colon tumor progression and impacts the response to aspirin. These findings are important to a more complete understanding of tumorigenesis with consequences for several distinct models of tumor evolution. They also have practical implications to the clinic. Mouse models with heterogenous tumors are likely better for predicting drug efficacy as compared to models in which the tumors are highly homogeneous. Moreover, understanding how interactions among different populations in a single heterotypic tumor with a multi-ancestral architecture impact response to a single agent and combination therapies are necessary to fully develop personalized medicine.

Representative models are needed to screen new therapies for patients with cancer. Cancer organoids are a leap forward as a culture model that faithfully represents the disease. Mouse-derived cancer organoids (MDCOs) are becoming increasingly popular, however there has yet to be a standardized method to assess therapeutic response and identify subpopulation heterogeneity. There are multiple factors unique to organoid culture that could affect how therapeutic response and MDCO heterogeneity are assessed. Here we describe an analysis of nearly 3500 individual MDCOs where individual organoid morphologic tracking was performed. Change in MDCO diameter was assessed in the presence of control media or targeted therapies. Individual organoid tracking was identified to be more sensitive to treatment response than well-level assessment. The impact of different generations of mice of the same genotype, different regions of the colon, and organoid specific characteristics including baseline size, passage number, plating density, and location within the matrix were examined. Only the starting size of the MDCO altered the subsequent growth. These results were corroborated using ~ 1700 patient-derived cancer organoids (PDCOs) isolated from 19 patients. Here we establish organoid culture parameters for individual organoid morphologic tracking to determine therapeutic response and growth/response heterogeneity for translational studies.

The normal colon epithelium is transformed into its neoplastic counterpart through a series of genetic alterations in driver genes including activating mutations in PIK3CA. Treatment often involves surgery followed by 5-fluorouracil (5-FU) based therapy, which has limited efficiency and serious side effects. We sought to determine whether fisetin, a dietary flavonoid, alone or in combination with 5-FU affected tumorigenesis in the mammalian intestine. We first determined the effect of fisetin, 5-FU or their combination on PIK3CA-mutant and PIK3CA wild-type colon cancer cells by assessing cell viability, colony formation, apoptosis and effects on PI3K/AKT/mTOR signaling. Treatment of PIK3CA-mutant cells with fisetin and 5-FU reduced the expression of PI3K, phosphorylation of AKT, mTOR, its target proteins, constituents of mTOR signaling complex and this treatment increased the phosphorylation of AMPKα. We then determined whether fisetin and 5-FU together or singly affected tumorigenesis in Apc(Min/+) mice that also express constitutively active PI3K in the distal small intestine and colon. Tumor incidence was markedly lower in fisetin-treated FC(1)3K(1)Apc(Min/+) mice that also express constitutively active PI3K in distal small intestine and colon, as compared to control animals, indicating that fisetin is a strong preventive agent. In addition, the combination of fisetin and 5-FU also reduced the total number of intestinal tumors. Fisetin could be used as a preventive agent plus an adjuvant with 5-FU for the treatment of PIK3CA-mutant colorectal cancer.

Significance The premalignant colonic adenoma presents a long-lived target to prevent colon cancer. The levels of peptide biomarkers for four serum proteins—F5, ITIH4, LRG1, and VTN—are each elevated in sera in two adenoma-bearing animal models and in patients. The elevated levels of these serum proteins are correlated with total colonic adenoma number in the rat model. Longitudinal analysis of patients demonstrated their association with the subset of adenomas that continue to grow, but not with the total volume of the adenoma burden. This pattern of correlation with the number and growth, but not total adenoma volume, indicates that this class of marker can complement an emergent class of volume-dependent markers to detect the growing early colonic adenoma, minimizing overdiagnosis., A major challenge for the reduction of colon cancer is to detect patients carrying high-risk premalignant adenomas with minimally invasive testing. As one step, we have addressed the feasibility of detecting protein signals in the serum of patients carrying an adenoma as small as 6–9 mm in maximum linear dimension. Serum protein biomarkers, discovered in two animal models of early colonic adenomagenesis, were studied in patients using quantitative mass-spectrometric assays. One cohort included patients bearing adenomas known to be growing on the basis of longitudinal computed tomographic colonography. The other cohort, screened by optical colonoscopy, included both patients free of adenomas and patients bearing adenomas whose risk status was judged by histopathology. The markers F5, ITIH4, LRG1, and VTN were each elevated both in this patient study and in the studies of the Pirc rat model. The quantitative study in the Pirc rat model had demonstrated that the elevated level of each of these markers is correlated with the number of colonic adenomas. However, the levels of these markers in patients were not significantly correlated with the total adenoma volume. Postpolypectomy blood samples demonstrated that the elevated levels of these four conserved markers persisted after polypectomy. Two additional serum markers rapidly renormalized after polypectomy: growth-associated CRP levels were enhanced only with high-risk adenomas, while PI16 levels, not associated with growth, were reduced regardless of risk status. We discuss biological hypotheses to account for these observations, and ways for these signals to contribute to the prevention of colon cancer.

Purpose. Estimating the cancer risk associated with interplanetary travel is complicated by the lack of data for human exposure to high atomic number, high-energy (HZE) radiation encountered in space. Therefore, human exposure data from low linear energy transfer (low-LET) γ-ray radiation is used in cancer risk models with the critical assumption that HZE and γ-ray radiation have comparable biological effects, including the induction of the same spectrum of tumors with similar lethality. Recently this assumption has been challenged by several reports indicating that HZE radiation might produce more aggressive tumors. The goal of this research is to test the hypothesis that high-LET HZE radiation induced tumors are more aggressive than those that arise spontaneously or from exposure to low-LET γ-rays. Materials & methods. The FVB/N-Tg(MMTV-PyMT) and (DBAxFVB)F1-Tg(MMTV-PyMT) murine models of mammary cancer and the C3H-Tg(Afp-mCherry) and C3B6F1-Tg(Afp-mCherry) liver cancer models were used to determine the impact of exposure to 0.2 Gy of HZE 300 MeV/n silicon ions in comparison to 3 Gy of γ-rays and no radiation. Various measures of tumor aggressiveness including latency, size, pathological grade, prognostic biomarkers, metastasis, and survival were assessed. Results. For both mammary cancer models, there was no significant change in the latency of mammary tumors or in the number or size of pulmonary metastasis in silicon-irradiated mice compared to their sham-irradiated controls. For the C3H-Tg and C3B6-Tg liver cancer models, we did observe an increase in the incidence of liver cancer in irradiated mice, but no differences in measures of aggressiveness, including tumor size, grade, metastasis, or overall survival. Conclusion. The principle finding of our study is that tumors in the HZE-irradiated mice were not more aggressive than those arising from exposure to low-LET γ-rays or spontaneously. Thus, we conclude that enhanced aggressiveness does not appear to be a uniform characteristic of all tumors in HZE-irradiated animals.

Though many advancements in personalized medicine have been made, better methods are still needed to predict treatment benefit for patients with colorectal cancer. Patient-derived cancer organoids (PDCOs) are a major advance towards true personalization of treatment strategies. A growing body of literature is demonstrating the feasibility of PDCOs as an accurate and high-throughput preclinical tool for patient treatment selection. Many studies demonstrate that these cultures are readily generated and represent the tumors they were derived from phenotypically and based on their mutation profile. This includes maintenance of the driver muatations giving the cancer cells a selective growth advantage, and also heterogeneity, including molecular and metabolic heterogeneity. Additionally, PDCOs are now being utilized to develop patient biospecimen repositories, perform high to moderate-throughput drug screening, and to potentially predict treatment response for individual patients that are undergoing anti-cancer treatments. In order to develop PDCOs as a true clinical tool, further studies are required to determine the reproducibility and accuracy of these models to predict patient response.

Colorectal cancer originates within immunologically complex microenvironments. To date, the benefits of immunotherapy have been modest, except in neoantigen-laden mismatch repair–deficient tumors. Approaches to enhance tumor-infiltrating lymphocytes in the tumor bed may substantially augment clinical immunotherapy responses. In this article, we report that proteolysis of the tolerogenic matrix proteoglycan versican (VCAN) strongly correlated with CD8+ T cell infiltration in colorectal cancer, regardless of mismatch repair status. Tumors displaying active VCAN proteolysis and low total VCAN were associated with robust (10-fold) CD8+ T cell infiltration. Tumor-intrinsic WNT pathway activation was associated with CD8+ T cell exclusion and VCAN accumulation. In addition to regulating VCAN levels at the tumor site, VCAN proteolysis results in the generation of bioactive fragments with novel functions (VCAN-derived matrikines). Versikine, a VCAN-derived matrikine, enhanced the generation of CD103+CD11chiMHCIIhi conventional dendritic cells (cDCs) from Flt3L-mobilized primary bone marrow–derived progenitors, suggesting that VCAN proteolysis may promote differentiation of tumor-seeding DC precursors toward IRF8- and BATF3-expressing cDCs. Intratumoral BATF3-dependent DCs are critical determinants for T cell antitumor immunity, effector T cell trafficking to the tumor site, and response to immunotherapies. Our findings provide a rationale for testing VCAN proteolysis as a predictive and/or prognostic immune biomarker and VCAN-derived matrikines as novel immunotherapy agents.

Advances in DNA sequencing have created new opportunities to better understand the biology of cancers. Attention is currently focused on precision medicine: does a cancer carry a mutation that is targetable with already available drugs? But, the timing at which multiple, targetable mutations arise during the adenoma to carcinoma sequence remains unresolved. Borras and colleagues identified mutations and allelic imbalance in at-risk mucosa and early polyps in the human colon. Their analyses indicate that mutations in key genes can arise quite early during tumorigenesis and that polyps are often multiclonal with at least two clones. These results are consistent with the “Big Bang” model of tumorigenesis, which postulates that intratumoral heterogeneity is a consequence of a mutational burst in the first few cell divisions following initiation that drives divergence from a single founder with unique but related clones coevolving. Emerging questions center around the ancestry of the tumor and impact of early intratumoral heterogeneity on tumor establishment, growth, progression, and most importantly, response to therapeutic intervention. Additional sequencing studies in which samples, especially at-risk tissue and premalignant neoplasms, are analyzed from animal models and humans will further our understanding of tumorigenesis and lead to more effective strategies for prevention and treatment. Cancer Prev Res; 9(8); 638–41. ©2016 AACR. See related article by Borras, et al., Cancer Prev Res 2016;9(6):417–427

Human colorectal cancers often possess multiple mutations, including three to six driver mutations per tumor. The timing of when these mutations occur during tumor development and progression continues to be debated. More advanced lesions carry a greater number of driver mutations, indicating that colon tumors might progress from adenomas to carcinomas through the stepwise accumulation of mutations following tumor initiation. However, mutations that have been implicated in tumor progression have been identified in normal-appearing epithelial cells of the colon, leaving the possibility that these mutations might be present before the initiation of tumorigenesis. We utilized mouse models of colon cancer to investigate whether tumorigenesis still occurs through the adenoma-to-carcinoma sequence when multiple mutations are present at the time of tumor initiation. To create a model in which tumors could concomitantly possess mutations in Apc, Kras, and Pik3ca, we developed a novel minimally invasive technique to administer an adenovirus expressing Cre recombinase to a focal region of the colon. Here, we demonstrate that the presence of these additional driver mutations at the time of tumor initiation results in increased tumor multiplicity and an increased rate of progression to invasive adenocarcinomas. These cancers can even metastasize to retroperitoneal lymph nodes or the liver. However, despite having as many as three concomitant driver mutations at the time of initiation, these tumors still proceed through the adenoma-to-carcinoma sequence. Cancer Prev Res; 8(10); 952–61. ©2015 AACR.

Conservation over three mammalian genera—the mouse, rat, and human—has been found for a subset of the transcripts whose level differs between the adenoma and normal epithelium of the colon. Pde4b is one of the triply conserved transcripts whose level is enhanced both in the colonic adenoma and in the normal colonic epithelium, especially adjacent to adenomas. It encodes the phosphodiesterase PDE4B, specific for cAMP. Loss of PDE4B function in the ApcMin/+ mouse leads to a significant increase in the number of colonic adenomas. Similarly, Pde4b-deficient ApcMin/+ mice are hypersensitive to treatment by the inflammatory agent DSS, becoming moribund soon after treatment. These observations imply that the PDE4B function protects against ApcMin-induced adenomagenesis and inflammatory lethality. The paradoxical enhancement of the Pde4b transcript in the adenoma versus this inferred protective function of PDE4B can be rationalized by a feedback model in which PDE4B is first activated by early oncogenic stress involving cAMP and then, as reported for frank human colon cancer, inactivated by epigenetic silencing., Author summary We have used the extensive genetic variation between genera within the mammalian order–mouse versus rat versus human–to discover genes whose broadly conserved transcripts differ in level in early colonic tumors compared to the normal colonic epithelium. Then, we developed a quantitative functional analysis using a targeted inactivating mutation in a genetically homogeneous strain, the congenic C57BL/6J ApcMin/+ mouse. This physiological genetic analysis of cancer involves parsing gene action into positive versus negative effects on the cancer of interest. Combining our studies with reports in the literature, we deduced that, by catabolizing cyclic AMP, the phosphodiesterase encoded by the gene of interest, Pde4b, protects against the early stages of colon cancer in the mouse. In advanced colon cancer in human patients, this gene is silenced, losing its inferred protective role. Together, this study illustrates the power of combining discovery by conservation among diverse mammalian genera with functional analysis within a single experimental species to understand a novel facet of human cancer.

A central goal in the analysis of complex traits is to identify genes that modify a phenotype. Modifiers of a cancer phenotype may act either intrinsically or extrinsically on the salient cell lineage. Germline point mutagenesis by ethylnitrosourea can provide alleles for a gene of interest that include loss-, gain-, or alteration-of-function. Unlike strain polymorphisms, point mutations with heterozygous quantitative phenotypes are detectable in both essential and nonessential genes and are unlinked from other variants that might confound their identification and analysis. This report analyzes strategies seeking quantitative mutational modifiers of ApcMin in the mouse. To identify a quantitative modifier of a phenotype of interest, a cluster of test progeny is needed. The cluster size can be increased as necessary for statistical significance if the founder is a male whose sperm is cryopreserved. A second critical element in this identification is a mapping panel free of polymorphic modifiers of the phenotype, to enable low-resolution mapping followed by targeted resequencing to identify the causative mutation. Here, we describe the development of a panel of six “isogenic mapping partner lines” for C57BL/6J, carrying single-nucleotide markers introduced by mutagenesis. One such derivative, B6.SNVg, shown to be phenotypically neutral in combination with ApcMin, is an appropriate mapping partner to locate induced mutant modifiers of the ApcMin phenotype. The evolved strategy can complement four current major initiatives in the genetic analysis of complex systems: the Genome-wide Association Study; the Collaborative Cross; the Knockout Mouse Project; and The Cancer Genome Atlas.

Colorectal cancer often arises from adenomatous colonic polyps. Polyps can grow and progress to cancer, but may also remain static in size, regress, or resolve. Predicting which polyps progress and which remain benign is difficult. We developed a novel long-lived murine model of colorectal cancer with tumors that can be followed by colonoscopy. Our aim was to assess whether these tumors have similar growth patterns and histologic fates to human colorectal polyps to identify features to aid in risk stratification of colonic tumors. Long-lived ApcMin/+ mice were treated with dextran sodium sulfate to promote colonic tumorigenesis. Tumor growth patterns were characterized by serial colonoscopy with biopsies obtained for immunohistochemistry and gene expression profiling. Tumors grew, remained static, regressed, or resolved over time with different relative frequencies. Newly developed tumors demonstrated higher rates of growth and resolution than more established tumors that tended to remain static in size. Colonic tumors were hyperplastic lesions (3%), adenomas (73%), intramucosal carcinomas (20%), or adenocarcinomas (3%). Interestingly, the level of β-catenin was higher in adenomas that became intratumoral carcinomas than those that failed to progress. In addition, differentially expressed genes between adenomas and intramucosal carcinomas were identified. This novel murine model of intestinal tumorigenesis develops colonic tumors that can be monitored by serial colonoscopy, mirror growth patterns seen in human colorectal polyps, and progress to colorectal cancer. Further characterization of cellular and molecular features is needed to determine which features can be used to risk-stratify polyps for progression to colorectal cancer and potentially guide prevention strategies. Cancer Prev Res; 7(1); 105–13. ©2013 AACR.

Epidemiological studies indicate that sunlight exposure and vitamin D are each associated with a lower risk of colon cancer. The few controlled supplementation trials testing vitamin D in humans reported to date show conflicting results. We have used two genetic models of familial colon cancer, the Apc(Pirc/+) (Pirc) rat and the Apc(Min/+) (Min) mouse, to investigate the effect of 25-hydroxyvitamin D(3) [25(OH)D(3)] and two analogs of vitamin D hormone on colonic tumors. Longitudinal endoscopic monitoring allowed us to test the efficacy of these compounds in preventing newly arising colonic tumors and in affecting established colonic tumors. 25(OH)D(3) and two analogs of vitamin D hormone each failed to reduce tumor multiplicities or alter the growth patterns of colonic tumors in the Pirc rat or the Min mouse.

Previous studies have shown that intestinal tumors from ApcMin/+ (Min) mice and familial adenomatous polyposis (FAP) patients are often polyclonal. We sought to determine whether polyclonality is unique to tumors arising from hereditary predispositions or, instead, is a common feature of intestinal tumorigenesis in other pathways to tumorigenesis. Ethylnitrosourea-induced intestinal tumors from mice wild type at the Apc locus and chimeric for the Rosa26 lineage marker were analyzed. Many were overtly polyclonal, being composed of a mixture of Rosa26+ and Rosa26− neoplastic cells. Statistical analyses revealed that polyclonality could be explained by interactions between two initiated clones separated by a very short distance. The frequency of overtly polyclonal tumors and the range of interactions estimated in this model are similar to those observed when analyzing familial tumors from Min mice. Thus, polyclonality does not depend on the familial pathway to tumorigenesis. Interactions between two initiated clones might provide a selective advantage during the early stages of intestinal tumorigenesis. Cancer Prev Res; 4(6); 916–23. ©2011 AACR.

C57BL/6J mice carrying the Min allele of Adenomatous polyposis coli (Apc) develop numerous adenomas along the entire length of the intestine and consequently die at an early age. This short lifespan would prevent the accumulation of somatic genetic mutations or epigenetic alterations necessary for tumor progression. To overcome this limitation, we generated F1 ApcMin/+ hybrids by crossing C57BR/cdcJ and SWR/J females to C57BL/6J ApcMin/+ males. These hybrids developed few intestinal tumors and often lived longer than 1 year. Many of the tumors (24-87%) were invasive adenocarcinomas, in which neoplastic tissue penetrated through the muscle wall into the mesentery. In a few cases (3%), lesions metastasized by extension to regional lymph nodes. The development of these familial cancers does not require chromosomal gains or losses, a high level of microsatellite instability, or the presence of Helicobacter. To test whether genetic instability might accelerate tumor progression, we generated ApcMin/+ mice homozygous for the hypomorphic allele of the Nijmegen breakage syndrome gene (Nbs1ΔB) and also treated ApcMin/+ mice with a strong somatic mutagen. These imposed genetic instabilities did not reduce the time required for cancers to form nor increase the percentage of cancers nor drive progression to the point of distant metastasis. In summary, we have found that the ApcMin/+ mouse model for familial intestinal cancer can develop frequent invasive cancers in the absence of overt genomic instability. Possible factors that promote invasion include age-dependent epigenetic changes, conservative somatic recombination, or direct effects of alleles in the F1 hybrid genetic background. [Cancer Res 2009;69(14):5768–75]

Cytochrome P450 1B1 (Cyp1b1) metabolism contributes to physiologic functions during embryogenesis but also to carcinogenic activation of polycyclic aromatic hydrocarbons (PAH). We generated Cyp1b1-deficient mice carrying the Min allele of the adenomatous polyposis coli gene. These Cyp1b1-deficient Min mice developed twice as many tumors as Min controls, which, however, remained similar in size and histology. Tumors from older (130 days) Cyp1b1-deficient Min mice selectively exhibited focal areas of nuclear atypia associated with less organized epithelia. The metabolism of endogenous substrates by Cyp1b1, therefore, suppresses tumor initiation but also affects progression. Treatment of Min mice with 7,12-dimethylbenzanthracene (DMBA) doubled both tumor multiplicity and size within 20 days but not when mice lacked Cyp1b1. This was paralleled by an abnormal staining of crypts with β-catenin, phospho-IκB kinase, and RelA, which may represent an early stage of tumorigenesis similar to aberrant crypt formation. Cyp1b1 deletion did not affect circulating DMBA and metabolites. Cyp1b1 expression was higher in the tumors compared with normal small intestines. Increased tumorigenesis may, therefore, arise from generation of DMBA metabolites by Cyp1b1 in the developing tumors. Benzo(a)pyrene (BP), which is similarly activated by Cyp1b1 in vitro, did not affect tumorigenesis in Min mice. By contrast, BP and DMBA each suppressed tumor multiplicity in the absence of Cyp1b1. Cyp1b1 metabolism of DMBA and endogenous oxygenation products may each affect a tumor-promoting nuclear factor-κB activation, whereas Ah receptor activation by PAH affects suppression. Tumorigenesis may, therefore, depend on activation of PAH by Cyp1b1 and on offsetting suppression by Cyp1b1 of endogenous tumor-enhancing substrates. [Cancer Res 2008;68(18):7394–402]

The phosphoinositide 3-kinase (PI3K) signaling pathway is critical for multiple important cellular functions, and is one of the most commonly altered pathways in human cancers. We previously developed a mouse model in which colon cancers were initiated by a dominant active PI3K p110-p85 fusion protein. In that model, well-differentiated mucinous adenocarcinomas developed within the colon and initiated through a non-canonical mechanism that is not dependent on WNT signaling. To assess the potential relevance of PI3K mutations in human cancers, we sought to determine if one of the common mutations in the human disease could also initiate similar colon cancers. Mice were generated expressing the Pik3caH1047R mutation, the analog of one of three human hotspot mutations in this gene. Mice expressing a constitutively active PI3K, as a result of this mutation, develop invasive adenocarcinomas strikingly similar to invasive adenocarcinomas found in human colon cancers. These tumors form without a polypoid intermediary and also lack nuclear CTNNB1 (β-catenin), indicating a non-canonical mechanism of tumor initiation mediated by the PI3K pathway. These cancers are sensitive to dual PI3K/mTOR inhibition indicating dependence on the PI3K pathway. The tumor tissue remaining after treatment demonstrated reduction in cellular proliferation and inhibition of PI3K signaling.

Background: Epidemiologic studies in humans have shown associations between greater sunlight exposure, higher serum 25-hydroxycholecalciferol [25(OH)D3] concentrations, and reduced colon cancer risk. However, results from a limited number of vitamin D supplementation trials in humans have not shown a protective effect. Objective: We sought to determine whether adding to the diet increasing amounts of either 25(OH)D3, the stable metabolite measured in serum and associated with cancer risk, or cholecalciferol (vitamin D3), the compound commonly used for supplementation in humans, could reduce emergent adenomas (chemoprevention) or decrease the growth of existing adenomas (treatment) in the colons of vitamin D–sufficient rats carrying a truncation mutation of adenomatous polyposis coli (Apc), a model of early intestinal cancer. Methods: Apc Pirc/+ rats were supplemented with either vitamin D3 over a range of 4 doses (6–1500mg/kg body weight per day) or with 25(OH)D3 over a range of 6 doses (60–4500 mg/kg body weight per day) beginning after weaning. Rats underwent colonoscopy every other week to assess effects on adenoma number and size. At termination (140 d of age), the number of tumors in the small intestine and colon and the size of tumors in the colon were determined, and serum calcium and 25(OH)D3 measurements were obtained. Results: At lower doses (those that did not affect body weight), neither of the vitamin D compounds reduced the number of existing or emergent colonic tumors (P-trend > 0.24). By contrast, supplementation at higher doses (those that cause a suppression in body weight gain) with either 25(OH)D3 or vitamin D3 caused a dose-dependent increase in colonic tumor number in both males and females (P-trend < 0.003). Conclusions: No evidence for protection against colon tumor development was seen with lower dose supplementation with either cholecalciferol or 25-hydroxycholecalciferol. Thus, the association between sunlight exposure and the incidence of colon cancer may involve factors other than vitamin D concentrations. Alternative hypotheses warrant investigation. Furthermore, this study provides preliminary evidence for the need for caution regarding vitamin D supplementation of humans at higher doses, especially in individuals with sufficient serum 25(OH)D3 concentrations. JN utrdoi: 10.3945/jn.114.204396.

In previous studies demonstrating the polyclonal structure of familial intestinal adenomas, high tumor multiplicity made it difficult to eliminate the possibility that polyclonality arose by the random collision of distinct initiated clones as opposed to some form of clonal interaction. We sought to test further the random collision hypothesis. Chimeric mice carrying the multiple intestinal neoplasia ( Min ) mutation of the adenomatous polyposis coli gene ( Apc ) and homozygous for the tumor resistance allele of the Mom1 locus were established. These chimeras also display a strong propensity for tumors of polyclonal structure, despite their markedly reduced tumor multiplicity. Considering tumor sizes and multiplicities, the observed fraction of overtly polyclonal heterotypic adenomas was significantly higher than predicted by the random collision hypothesis. This finding supports models of polyclonality involving interaction among multiple initiated clones. The extent of clonal interaction was assessed by statistical analyses that relate the observed frequency of overtly polyclonal heterotypic tumors to the geometry of the chimeric patches and the pattern of underlying crypts. These statistical calculations indicate that the familial adenomas of the Apc Min /+ mouse may commonly form through interactions between clones as close as 1–2 crypt diameters apart.

It recently has been recognized that men develop colonic adenomas and carcinomas at an earlier age and at a higher rate than women. In the Apc(Pirc/+) (Pirc) rat model of early colonic cancer, this sex susceptibility was recapitulated, with male Pirc rats developing twice as many adenomas as females. Analysis of large datasets revealed that the Apc(Min/+) mouse also shows enhanced male susceptibility to adenomagenesis, but only in the colon. In addition, WT mice treated with injections of the carcinogen azoxymethane (AOM) showed increased numbers of colonic adenomas in males. The mechanism underlying these observations was investigated by manipulation of hormonal status. The preponderance of colonic adenomas in the Pirc rat model allowed a statistically significant investigation in vivo of the mechanism of sex hormone action on the development of colonic adenomas. Females depleted of endogenous hormones by ovariectomy did not exhibit a change in prevalence of adenomas, nor was any effect observed with replacement of one or a combination of female hormones. In contrast, depletion of male hormones by orchidectomy (castration) markedly protected the Pirc rat from adenoma development, whereas supplementation with testosterone reversed that effect. These observations were recapitulated in the AOM mouse model. Androgen receptor was undetectable in the colon or adenomas, making it likely that testosterone acts indirectly on the tumor lineage. Our findings suggest that indirect tumor-promoting effects of testosterone likely explain the disparity between the sexes in the development of colonic adenomas.

Prior to the advent of genetic engineering in the mouse, the rat was the model of choice for investigating the etiology of cancer. Now, recent advances in the manipulation of the rat genome, combined with a growing recognition of the physiological differences between mice and rats, have reignited interest in the rat as a model of human cancer. Two recently developed rat models, the polyposis in the rat colon (Pirc) and Kyoto Apc Delta (KAD) strains, each carry mutations in the intestinal-cancer-associated adenomatous polyposis coli (Apc) gene. In contrast to mouse models carrying Apc mutations, in which cancers develop mainly in the small intestine rather than in the colon and there is no gender bias, these rat models exhibit colonic predisposition and gender-specific susceptibility, as seen in human colon cancer. The rat also provides other experimental resources as a model organism that are not provided by the mouse: the structure of its chromosomes facilitates the analysis of genomic events, the size of its colon permits longitudinal analysis of tumor growth, and the size of biological samples from the animal facilitates multiplexed molecular analyses of the tumor and its host. Thus, the underlying biology and experimental resources of these rat models provide important avenues for investigation. We anticipate that advances in disease modeling in the rat will synergize with resources that are being developed in the mouse to provide a deeper understanding of human colon cancer.

Previous studies of Min /+ (multiple intestinal neoplasia) mice on a sensitive genetic background, C57BL/6 (B6), showed that adenomas have lost heterozygosity for the germ-line Apc Min mutation in the Apc (adenomatous polyposis coli) gene. We now report that on a strongly resistant genetic background, AKR/J (AKR), Min -induced adenoma multiplicity is reduced by about two orders of magnitude compared with that observed on the B6 background. Somatic treatment with a strong mutagen increases tumor number in AKR Min /+ mice in an age-dependent manner, similar to results previously reported for B6 Min /+ mice. Immunohistochemical analyses indicate that Apc expression is suppressed in all intestinal tumors from both untreated and treated AKR Min /+ mice. However, the mechanism of Apc inactivation in AKR Min /+ mice often differs from that observed for B6 Min /+ mice. Although loss of heterozygosity is observed in some tumors, a significant percentage of tumors showed neither loss of heterozygosity nor Apc truncation mutations. These results extend our understanding of the effects of genetic background on Min -induced tumorigenesis in several ways. First, the AKR strain carries modifiers of Min in addition to Mom1 . This combination of AKR modifiers can almost completely suppress spontaneous intestinal tumorigenesis associated with the Min mutation. Second, even on such a highly resistant genetic background, tumor formation continues to involve an absence of Apc function. The means by which Apc function is inactivated is affected by genetic background. Possible scenarios are discussed.

Intestinal tumors from mice and humans can have a polyclonal origin. Statistical analyses indicate that the best explanation for this source of intratumoral heterogeneity is the presence of interactions among multiple progenitors. We sought to better understand the nature of these interactions. An initial progenitor could recruit others by facilitating the transformation of one or more neighboring cells. Alternatively, two progenitors that are independently initiated could simply cooperate to form a single tumor. These possibilities were tested by analyzing tumors from aggregation chimeras that were generated by fusing together embryos with unequal predispositions to tumor development. Strikingly, numerous polyclonal tumors were observed even when one genetic component was highly, if not completely, resistant to spontaneous tumorigenesis in the intestine. Moreover, the observed number of polyclonal tumors could be explained by the facilitated transformation of a single neighbor within 144 μm of an initial progenitor. These findings strongly support recruitment instead of cooperation. Thus, it is conceivable that these interactions are necessary for tumors to thrive, so blocking them might be a highly effective method for preventing the formation of tumors in the intestine and other tissues.

Human colorectal cancers are known to possess multiple mutations, though how these mutations interact in tumor development and progression has not been fully investigated. We have previously described the FCPIK3ca* murine colon cancer model which expresses a constitutively activated phosphoinositide-3 kinase (PI3K) in the intestinal epithelium. The expression of this dominantly active form of PI3K results in hyperplasia and invasive mucinous adenocarcinomas. These cancers form via a non-canonical mechanism of tumor initiation that is mediated through activation of PI3K and not through aberrations in WNT signaling. Since the Adenomatous Polyposis Coli (APC) gene is mutated in the vast majority of human colon cancers and often occurs simultaneously with PIK3CA mutations, we sought to better understand the interaction between APC and PIK3CA mutations in the mammalian intestine. In this study, we have generated mice in which the expression of a constitutively active PI3K and the loss of APC occur simultaneously in the distal small intestine and colon. Here we demonstrate that expression of a dominant active PI3K synergizes with loss of APC activity resulting in a dramatic changes in tumor multiplicity, size, morphology, and invasiveness. Activation of the PI3K pathway is not able to directly activate WNT signaling through the nuclear localization of CTNNB1 (β-catenin) in the absence of aberrant WNT signaling. Alterations at the transcriptional level, including increased CCND1, may be the etiology of synergy between these activated pathways.

The phosphatidylinositide-3-kinase (PI3K) signaling pathway is critical for multiple cellular functions including metabolism, proliferation, angiogenesis, and apoptosis, and is the most commonly altered pathway in human cancers. Recently, we developed a novel mouse model of colon cancer in which tumors are initiated by a dominant active PI3K (FC PIK3ca*). The cancers in these mice are moderately differentiated invasive mucinous adenocarcinomas of the proximal colon that develop by 50 days of age. Interestingly, these cancers form without a benign intermediary or aberrant WNT signaling, indicating a non-canonical mechanism of tumorigenesis. Since these tumors are dependent upon the PI3K pathway, we investigated the potential for tumor response by the targeting of this pathway with rapamycin, an mTOR inhibitor. A cohort of FC PIK3ca* mice were treated with rapamycin at a dose of 6 mg/kg/day or placebo for 14 days. FDG dual hybrid PET/CT imaging demonstrated a dramatic tumor response in the rapamycin arm and this was confirmed on necropsy. The tumor tissue remaining after treatment with rapamycin demonstrated increased pERK1/2 or persistent phosphorylated ribosomal protein S6 (pS6), indicating potential resistance mechanisms. This unique model will further our understanding of human disease and facilitate the development of therapeutics through pharmacologic screening and biomarker identification.

Mutation of the APC (adenomatous polyposis coli) gene is an early event in colon tumor development in humans. Mice carrying Min (multiple intestinal neoplasia), a mutant allele of Apc, develop intestinal and mammary tumors as adults. To study the role of the Apc gene in development, we have investigated the phenotype of embryos homozygous for ApcMin (Min). Development of the primitive ectoderm fails prior to gastrulation in homozygous Min embryos. By midgestation, the presumed homozygotes consist of a mass of trophoblast giant cells with an additional cluster of much smaller embryonic cells. These results indicate that functional Apc is required for normal growth of inner cell mass derivatives.

A widely accepted paradigm in the field of cancer biology is that solid tumors are uni-ancestral being derived from a single founder and its descendants. However, data have been steadily accruing that indicate early tumors in mice and humans can have a multi-ancestral origin in which an initiated primogenitor facilitates the transformation of neighboring co-genitors. We developed a new mouse model that permits the determination of clonal architecture of intestinal tumors in vivo and ex vivo, have validated this model, and then used it to assess the clonal architecture of adenomas, intramucosal carcinomas, and invasive adenocarcinomas of the intestine. The percentage of multi-ancestral tumors did not significantly change as tumors progressed from adenomas with low-grade dysplasia [40/65 (62%)], to adenomas with high-grade dysplasia [21/37 (57%)], to intramucosal carcinomas [10/23 (43%]), to invasive adenocarcinomas [13/19 (68%)], indicating that the clone arising from the primogenitor continues to coexist with clones arising from co-genitors. Moreover, neoplastic cells from distinct clones within a multi-ancestral adenocarcinoma have even been observed to simultaneously invade into the underlying musculature [2/15 (13%)]. Thus, intratumoral heterogeneity arising early in tumor formation persists throughout tumorigenesis.

Aberrations in the phosphoinositide 3-kinase (PI3K) signaling pathway play a key role in the pathogenesis of numerous cancers by altering cellular growth, metabolism, proliferation, and apoptosis. Mutations in the catalytic domain of PI3K that generate a dominantly active kinase are commonly found in human colorectal cancers and have been thought to drive tumor progression but not initiation. However, the effects of constitutively activated PI3K upon the intestinal mucosa have not been previously studied in animal models. Here, we show that the expression of a dominantly active form of the PI3K protein in the mouse intestine results in hyperplasia and advanced neoplasia. Mice expressing constitutively active PI3K in the epithelial cells of the distal small bowel and colon rapidly developed invasive adenocarcinomas in the colon that spread into the mesentery and adjacent organs. The histologic characteristics of these tumors were strikingly similar to invasive mucinous colon cancers in humans. Interestingly, these tumors formed without a benign polypoid intermediary, consistent with the lack of aberrant WNT signaling observed. Together, our findings indicate a noncanonical mechanism of colon tumor initiation that is mediated through activation of PI3K. This unique model has the potential to further our understanding of human disease and facilitate the development of therapeutics through pharmacologic screening and biomarker identification. Cancer Res; 72(12); 2931–6. ©2012 AACR.

Studies of tumors from human familial adenomatous polyposis, sporadic colon cancer, and mouse and rat models of intestinal cancer indicate that the majority of early adenomas develop through loss of normal function of the Adenomatous polyposis coli ( APC ) gene. In murine models of familial adenomatous polyposis, specifically the multiple intestinal neoplasia mouse (Min) and the polyposis in the rat colon (Pirc) rat, most adenomas have lost their WT copy of the Apc gene through loss of heterozygosity by homologous somatic recombination. We report that large colonic adenomas in the Pirc rat have no detectable copy number losses or gains in genomic material and that most tumors lose heterozygosity only on the short arm of chromosome 18. Examination of early mouse and rat tumors indicates that a substantial subset of tumors shows maintenance of heterozygosity of Apc in genomic DNA, apparently violating Knudson's two-hit hypothesis. Sequencing of the Apc gene in a sampling of rat tumors failed to find secondary mutations in the majority of tumors that maintained heterozygosity of Apc in genomic DNA. Using quantitative allele-specific assays of Apc cDNA, we discovered two neoplastic pathways. One class of tumors maintains heterozygosity of Apc Min /+ or Apc Pirc/+ RNA expression and may involve haploinsufficiency for Apc function. Another class of tumors exhibits highly biased monoallelic expression of the mutant Apc allele, providing evidence for a stochastic or random process of monoallelic epigenetic silencing of the tumor suppressor gene Apc .

Familial adenomatous polyposis (FAP) is a human cancer syndrome characterized by the development of hundreds to thousands of colonic polyps and extracolonic lesions including desmoid fibromas, osteomas, epidermoid cysts, and congenital hypertrophy of the pigmented retinal epithelium. Afflicted individuals are heterozygous for mutations in the APC gene. Detailed investigations of mice heterozygous for mutations in the ortholog Apc have shown that other genetic factors strongly influence the phenotype. Here we report qualitative and quantitative modifications of the phenotype of Apc mutants as a function of three genetic variables: Apc allele, p53 allele, and genetic background. We have found major differences between the Apc alleles Min and 1638N in multiplicity and regionality of intestinal tumors, as well as in incidence of extracolonic lesions. By contrast, Min mice homozygous for either of two different knockout alleles of p53 show similar phenotypic effects. These studies illustrate the classic principle that functional genetics is enriched by assessing penetrance and expressivity with allelic series. The mouse permits study of an allelic gene series on multiple genetic backgrounds, thereby leading to a better understanding of gene action in a range of biological processes.

Using a mouse predisposed to neoplasia by a germ line mutation in Apc (Apc(Min)), we tested whether induced hyperplasia is sufficient to increase intestinal tumor multiplicity or size in the intestine. We found that hyperplasia in the jejunum correlated with a significant increase in tumor multiplicity. However, tumor multiplicity was unchanged in the hyperplastic colon. This result indicates that even an intestine predisposed to neoplasia can, in certain regions including the colon, accommodate net increased cell growth without developing more neoplasms. Where hyperplasia correlated with increased tumor multiplicity, it did not increase the size or net growth of established tumors. This result suggests that the event linking hyperplasia and neoplasia in the jejunum is tumor establishment. Two novel observations arose in our study: the multiple intestinal neoplasia (Min) mutation partially suppressed both mitosis and transforming growth factor alpha-induced hyperplasia throughout the intestine; and zinc treatment alone increased tumor multiplicity in the duodenum of Min mice.

Rationale and Objectives We sought to demonstrate the viability of microcomputed tomographic colonography (μCTC) as a tool for monitoring tumorigenesis in mouse models of human colorectal cancer during prospective longitudinal studies. The precision and accuracy of volumetric measurements were determined to assess whether changes in tumor volume over time were readily detectable. Materials and Methods All animal studies were conducted under the guidelines set forth by the Institutional Animal Care and Use Committee of the American Association for Assessment and Accreditation of Laboratory Animal Care. μCTC was performed on C57BL/6J (B6) mice carrying the Min allele of Apc , ultimately yielding 18 scans. Assessments of scan quality and tumor volume were both performed once per week over 8 weeks. Results Scans with a good quality rating had a mean standard deviation in tumor volume measurement of 8%. By contrast, scans with a poor quality rating had a mean standard deviation in tumor volume measurement of 35%. Variables affecting μCTC scan quality in living mice included bowel preparation, motion artifact, and tumor morphology. Tumor volume measurements were highly correlated with tumor weight ( r 2 = 0.87). Conclusions The reproducibility of tumor volume measurement at μCTC in living mice makes prospective longitudinal evaluation of colonic tumor response feasible. For μCTC scans of good quality, a 16% change in tumor volume can be detected at the 95% confidence level.

The ApcMin mouse model of colorectal cancer provides a discrete, quantitative measurement of tumor multiplicity, allowing for robust quantitative trait locus analysis. This advantage has previously been used to uncover polymorphic modifiers of the Min phenotype: Mom1, which is partly explained by Pla2g2a; Mom2, a spontaneous mutant modifier; and Mom3, which was discovered in an outbred cross. Here, we describe the localization of a novel modifier, Mom7, to the pericentromeric region of chromosome 18. Mom7 was mapped in crosses involving four inbred strains: C57BL/6J (B6), BTBR/Pas (BTBR), AKR/J (AKR), and A/J. There are at least two distinct alleles of Mom7: the recessive, enhancing BTBR, AKR, and A/J alleles and the dominant, suppressive B6 allele. Homozygosity for the enhancing alleles increases tumor number by approximately threefold in the small intestine on both inbred and F1 backgrounds. Congenic line analysis has narrowed the Mom7 region to within 7.4 Mb of the centromere, 28 Mb proximal to Apc. Analysis of SNP data from various genotyping projects suggests that the region could be as small as 4.4 Mb and that there may be five or more alleles of Mom7 segregating among the many strains of inbred mice. This has implications for experiments involving ApcMin and comparisons between different or mixed genetic backgrounds.

The random collision hypothesis is a mathematical idealization of intestinal tumor formation that can account for the polyclonal origin of tumors without requiring a mechanistic description of clonal interaction. Using data from recent polyclonality studies in mice, we develop a statistical procedure to test the random collision hypothesis. Elements from stochastic geometry and approximations due to Armitage (1949, Biometrika 36, 257-266) support a statistical model of tumor count data. Bayesian analysis yields the posterior distribution of the number of heterotypic tumors, from which p-values are computed to test random collision.

The treatment of localized colorectal cancer (CRC) depends on resection of the primary tumor with adequate margins and sufficient lymph node sampling. A novel imaging agent that accumulates in CRCs and the associated lymph nodes is needed. Cellectar Biosciences has developed a phospholipid ether analog platform that is both diagnostic and therapeutic. CLR1502 is a near-infrared fluorescent molecule, whereas 124/131 I-CLR1404 is under clinical investigation as a PET tracer/ therapeutic agent imaged by SPECT. We investigated the use of CLR1502 for the detection of intestinal cancers in a murine model and 131 I-CLR1404 in a patient with metastatic CRC. Mice that develop multiple intestinal tumors ranging from adenomas to locally advanced adenocarcinomas were utilized. After 96 hours post CLR1502 injection, the intestinal tumors were analyzed using a Spectrum IVIS (Perkin Elmer) and a Fluobeam (Fluoptics). The intensity of the fluorescent signal was correlated with the histological characteristics for each tumor. Colon adenocarcinomas demonstrated increased accumulation of CLR1502 compared to non-invasive lesions (total radiant efficiency: 1.76610 10 vs 3.27610 9 respectively, p=0.006). Metastatic mesenteric tumors and uninvolved lymph nodes were detected with CLR1502. In addition, SPECT imaging with 131 I-CLR1404 was performed as part of a clinical trial in patients with advanced solid tumors. 131 I-CLR1404 was shown to accumulate in metastatic tumors in a patient with colorectal adenocarcinoma. Together, these compounds might enhance our ability to properly resect CRCs through better localization of the primary tumor and improved lymph node identification as well as detect distant disease.

A tumor suppressor gene is identified by genetic changes that inactivate the function of the gene. The Adenomatous polyposis coli (APC) gene was first localized by the heterozygous Herrera deletion carried in a human familial polyposis kindred.1 Molecular analyses in the human and in animal models are summarized by the “gatekeeper” metaphor: loss of function of both alleles of the autosomal APC/Apc locus is necessary for colon cancer. In our recent quantitative study in murine models, published in The Proceedings of the National Academy of Sciences of the United States of America, both genetic and epigenetic changes have been identified, inactivating the Apc gatekeeper.2 A number of investigators are actively addressing issues of the stability, reversibility and environmental sensitivity of the genetic and epigenetic mechanisms affecting colonic and other cancers. We limit this commentary to identifying issues arising from our study. The Min mouse and Pirc rat genetic models for early familial colon cancer display greater genomic stability than many advanced human colon cancers.3 As in many human cases, the loss of function of the wild-type Apc allele from the mutant heterozygote involves irreversible genetic change: intragenic mutation in the wild-type Apc allele or conservative somatic recombination.4,5 By contrast, epigenetic changes involve a spectrum of mechanisms, each reversible in principle. Developmental “programmed” epigenesis is commonly implemented by the spatial and temporal elaboration of transcription factors that act in trans to control the activity of both alleles of target genes, including the synthesis and modification of specific chromatin proteins.6 “Stochastic” epigenetic change, such as the initiation of X inactivation in diploid eutherian females, acts in cis to control the activity of only one of the alleles of a target gene, chosen at random. For both programmed and stochastic epigenetic change, fundamental issues include mitotic transmission, reversibility and causative role in the hierarchy of biological change. These issues are illuminated by studies of gene silencing in S. cerevisiae, an organism deficient in cytosine methylaton.7 Single-cell monitoring demonstrates monoallelic silencing. Both monoallelic and biallelic silencing show measurable rates of reversion. The issues of mitotic transmission and reversibility come into sharp focus as the spectrum of molecular epigenetic mechanisms becomes defined. For example, X inactivation is observed over a series of molecular events: successive modification of the inactive X-chromosome by the non-coding RNA XIST, histone modification and symmetric methylation of CpG dyads.8 Female embryonic stem cells expressing XIST show random but unstable monoallelic X inactivation with finite reversion rates.9 We have used a quantitative Pyrosequencing assay to detect changes in the tumor genome. Analysis of allelic ratios in cDNAs from F1 hybrid genetic constructs expands the dynamic range to detect monoallelic silencing regardless of mechanism. Precise measurement of allelic ratios permits a quantitative assessment of the extent to which a tumor contains cells with a range of genetic or epigenetic states. Our study identified four ways in which the Apc gatekeeper function is affected in colonic tumors: the conservative pathway, with mitotic recombination usually limited to the chromosome carrying the Apc locus; a haploinsufficient pathway, in which the Apc locus is expressed at normal total levels but with half of the transcripts carrying the germline mutation in Apc; a stochastic silencing pathway, in which only the mutated Apc allele is expressed; and biallelic silencing of the Apc locus (Fig. 1). In principle, the haploinsufficient pathway must involve further events elsewhere in the genome or epigenome, since only a finite number of tumors form in the heterozygous animals. Use of F1 analysis of the tumor genome and epigenome complements that provided by mutation detection to create a comprehensive understanding of the above four ways by which the Apc gatekeeper function is affected in colon cancer. Figure 1 Familial colon cancer in humans and in the Pirc rat and Min mouse kindreds arises in heterozygotes for a mutation (red star) in the APC/Apc gatekeeper gene. Tumors arise when the remaining wild-type allele of APC/Apc loses its function. This can involve ... The cDNA assay for silencing can detect states that may be less stable than that involving the methylation of CpG dyads. It is interesting to ask, what level of mitotic stability of an epigenetic state is necessary to support the sub-exponential growth of a neoplasm? What environmental signals affect this stability? Does the estimated level of admixture in early tumors represent polyclonality,10 or does it represent a spectrum of epigenetic states in dynamic equilibrium?8 If stochastic silencing occurs over the entire genome, including genes for which biallelic silencing is detrimental to the tumor, can over-silencing lead to tumor stasis or regression? Recent quantitative studies of the growth profiles of early adenomas in Min mice show that a tumor can go through successive cycles of growth, stasis and partial regression.11 Does the longitudinal fate of a colonic tumor depend on the way in which the gate is opened? Overall, this report raises fundamental biological questions and provides a strategy to begin addressing these questions for the tumor, its genome and its epigenome.

Evidence is presented for the existence of teratocarcinoma transplantation antigens (Gt) encoded within the H-2 complex and present also on adult tissues. It has not been possible to separate these Gt loci from H-2 by recombination, and Gt factors map to each end of the H-2 complex. Previous reports indicating separation of all Gt loci from H-2 are reinterpreted. One class of such apparent recombinants has been shown to result from the outgrowth of tumor variants in mice of resistant genotype.

Evidence is presented for the existence of two strong murine teratocarcinoma transplantation antigens (Gt) on the cell line PCC3. It is shown that the loci governing expression of these antigens are linked to the H-2 complex. These loci have been further mapped with respect to the brachyury marker (T) and H-2: Gt-1 lies 5 +/- 2 crossover units proximal to H-2 and 12 +/- 2 crossover units distal to T; Gt-2 lies 21 +/- 4 crossover units distal to H-2. It is possible that these strong transplantation antigens provide an embryonic analogue to the adult major histocompatibility system.