Advanced Intestinal Cancers often Maintain a Multi-Ancestral Architecture.

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.

Improved Detection of Microsatellite Instability in Early Colorectal Lesions.

Microsatellite instability (MSI) occurs in over 90% of Lynch syndrome cancers and is considered a hallmark of the disease. MSI is an early event in colon tumor development, but screening polyps for MSI remains controversial because of reduced sensitivity compared to more advanced neoplasms. To increase sensitivity, we investigated the use of a novel type of marker consisting of long mononucleotide repeat (LMR) tracts. Adenomas from 160 patients, ranging in age from 29-55 years old, were screened for MSI using the new markers and compared with current marker panels and immunohistochemistry standards. Overall, 15 tumors were scored as MSI-High using the LMRs compared to 9 for the NCI panel and 8 for the MSI Analysis System (Promega). This difference represents at least a 1.7-fold increase in detection of MSI-High lesions over currently available markers. Moreover, the number of MSI-positive markers per sample and the size of allelic changes were significantly greater with the LMRs (p = 0.001), which increased confidence in MSI classification. The overall sensitivity and specificity of the LMR panel for detection of mismatch repair deficient lesions were 100% and 96%, respectively. In comparison, the sensitivity and specificity of the MSI Analysis System were 67% and 100%; and for the NCI panel, 75% and 97%. The difference in sensitivity between the LMR panel and the other panels was statistically significant (p<0.001). The increased sensitivity for detection of MSI-High phenotype in early colorectal lesions with the new LMR markers indicates that MSI screening for the early detection of Lynch syndrome might be feasible.

Transformation of epithelial cells through recruitment leads to polyclonal intestinal tumors.

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.

Regulated Expression of Chromobox Homolog 5 Revealed in Tumors of Apc(Min) (/+) ROSA11 Gene Trap Mice.

The gene-trap lacZ reporter insertion, ROSA11, in the Cbx5 mouse gene illuminates the regulatory complexity of this locus in Apc(Min) (/+) mice. The insertion site of the ß-Geo gene-trap element lies in the 24-kb intron proximal to the coding region of Cbx5. Transcript analysis indicates that two promoters for Cbx5 flank this insertion site. Heterozygotes for the insertion express lacZ widely in fetal tissues but show limited expression in adult tissues. In the intestine, strong expression is limited to proliferative zones of crypts and tumors. Homozygotes for ROSA11, found at a lower than Mendelian frequency, express reduced levels of the coding region transcript in normal tissues, using a downstream promoter. Analysis via real-time polymerase chain reaction indicates that the upstream promoter is the dominant promoter in normal epithelium and tumors. Bioinformatic analysis of the Cbx5 locus indicates that WNT and its target transcription factor MYC can establish a feedback loop that may play a role in regulating the self-renewal of the normal intestinal epithelium and its tumors.

Clonal structure of carcinogen-induced intestinal tumors in mice.

Previous studies have shown that intestinal tumors from Apc(Min)(/+) (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.

A statistical test of the hypothesis that polyclonal intestinal tumors arise by random collision of initiated clones.

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.

Polyclonality of familial murine adenomas: analyses of mouse chimeras with low tumor multiplicity suggest short-range interactions.

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.

Insertion of the beta Geo promoter trap into the Fem1c gene of ROSA3 mice.

ROSA3 mice were developed by retroviral insertion of the beta Geo gene trap vector. Adult ROSA3 mice exhibit widespread expression of the trap gene in epithelial cells found in most organs. In the central nervous system the highest expression of beta Geo is found in CA1 pyramidal cells of the hippocampus, Purkinje cells of the cerebellum, and ganglion cells of the retina. Characterization of the genomic insertion site for beta Geo in ROSA3 mice shows that the trap vector is located in the first intron of Fem1c, a gene homologous to the sex-determining gene fem-1 of Caenorhabditis elegans. Transcription of the Rosa3 allele (R3) yields a spliced message that includes the first exon of Fem1c and the beta Geo coding region. Although normal processing of the Fem1c transcript is disrupted in homozygous Rosa3 (Fem1c(R3/R3)) mice, some tissues show low levels of a partially processed transcript containing exons 2 and 3. Since the entire coding region of Fem1c is located in these two exons, Fem1c(R3/R3) mice may still be able to express a putative FEM1C protein. To this extent, Fem1c(R3/R3) mice show no adverse effects in their sexual development or fertility or in the attenuation of neuronal cell death, another function that has been attributed to both fem-1 and a second mouse homolog, Fem1b. Examination of beta Geo expression in ganglion cells after exposure to damaging stimuli indicates that protein levels are rapidly depleted prior to cell death, making the beta Geo reporter gene a potentially useful marker to study early molecular events in damaged neurons.