Oncogenic Adenomatous polyposis coli mutants impair the mitotic checkpoint through direct interaction with Mad2

Cohesin Rad21 mediates loss of heterozygosity and is upregulated via Wnt promoting transcriptional dysregulation in gastrointestinal tumors

Loss of heterozygosity (LOH) of the adenomatous polyposis coli (APC) gene triggers a series of molecular events leading to intestinal adenomagenesis. Haploinsufficiency of the cohesin Rad21 influences multiple initiating events in colorectal cancer (CRC). We identify Rad21 as a gatekeeper of LOH and a β-catenin target gene and provide evidence that Wnt pathway activation drives RAD21 expression in human CRC. Genome-wide analyses identified Rad21 as a key transcriptional regulator of critical CRC genes and long interspersed element (LINE-1 or L1) retrotransposons. Elevated RAD21 expression tracks with reactivation of L1 expression in human sporadic CRC, implicating cohesin-mediated L1 expression in global genomic instability and gene dysregulation in cancer.

Contribution of soft substrates to malignancy and tumor suppression during colon cancer cell division

In colon cancer, a highly aggressive disease, progression through the malignant sequence is accompanied by increasingly numerous chromosomal rearrangements. To colonize target organs, invasive cells cross several tissues of various elastic moduli. Whether soft tissue increases malignancy or in contrast limits invasive colon cell spreading remains an open question. Using polyelectrolyte multilayer films mimicking microenvironments of various elastic moduli, we revealed that human SW480 colon cancer cells displayed increasing frequency in chromosomal segregation abnormalities when cultured on substrates with decreasing stiffness. Our results show that, although decreasing stiffness correlates with increased cell lethality, a significant proportion of SW480 cancer cells did escape from the very soft substrates, even when bearing abnormal chromosome segregation, achieve mitosis and undergo a new cycle of replication in contrast to human colonic HCoEpiC cells which died on soft substrates. This observation opens the possibility that the ability of cancer cells to overcome defects in chromosome segregation on very soft substrates could contribute to increasing chromosomal rearrangements and tumor cell aggressiveness.

The Mad1-Mad2 balancing act--a damaged spindle checkpoint in chromosome instability and cancer

Cancer cells are commonly aneuploid. The spindle checkpoint ensures accurate chromosome segregation by controlling cell cycle progression in response to aberrant microtubule-kinetochore attachment. Damage to the checkpoint, which is a partial loss or gain of checkpoint function, leads to aneuploidy during tumorigenesis. One form of damage is a change in levels of the checkpoint proteins mitotic arrest deficient 1 and 2 (Mad1 and Mad2), or in the Mad1:Mad2 ratio. Changes in Mad1 and Mad2 levels occur in human cancers, where their expression is regulated by the tumor suppressors p53 and retinoblastoma 1 (RB1). By employing a standard assay, namely the addition of a mitotic poison at mitotic entry, it has been shown that checkpoint function is normal in many cancer cells. However, in several experimental systems, it has been observed that this standard assay does not always reveal checkpoint aberrations induced by changes in Mad1 or Mad2, where excess Mad1 relative to Mad2 can lead to premature anaphase entry, and excess Mad2 can lead to a delay in entering anaphase. This Commentary highlights how changes in the levels of Mad1 and Mad2 result in a damaged spindle checkpoint, and explores how these changes cause chromosome instability that can lead to aneuploidy during tumorigenesis.

A screen for selective killing of cells with chromosomal instability induced by a spindle checkpoint defect

BACKGROUND

The spindle assembly checkpoint is crucial for the maintenance of a stable chromosome number. Defects in the checkpoint lead to Chromosomal INstability (CIN), which is linked to the progression of tumors with poor clinical outcomes such as drug resistance and metastasis. As CIN is not found in normal cells, it offers a cancer-specific target for therapy, which may be particularly valuable because CIN is common in advanced tumours that are resistant to conventional therapy.

PRINCIPAL FINDINGS

Here we identify genes that are required for the viability of cells with a CIN phenotype. We have used RNAi knockdown of the spindle assembly checkpoint to induce CIN in Drosophila and then screened the set of kinase and phosphatase genes by RNAi knockdown to identify those that induce apoptosis only in the CIN cells. Genes identified include those involved in JNK signaling pathways and mitotic cytoskeletal regulation.

CONCLUSIONS/SIGNIFICANCE

The screen demonstrates that it is feasible to selectively kill cells with CIN induced by spindle checkpoint defects. It has identified candidates that are currently being pursued as cancer therapy targets (e.g. Nek2: NIMA related kinase 2), confirming that the screen is able to identify promising drug targets of clinical significance. In addition, several other candidates were identified that have no previous connection with mitosis or apoptosis. Further screening and detailed characterization of the candidates could potentially lead to the therapies that specifically target advanced cancers that exhibit CIN.

Chromosome instability and deregulated proliferation: an unavoidable duo

The concept that aneuploidy is a characteristic of malignant cells has long been known; however, the idea that aneuploidy is an active contributor to tumorigenesis, as opposed to being an associated phenotype, is more recent in its evolution. At the same time, we are seeing the emergence of novel roles for tumor suppressor genes and oncogenes in genome stability. These include the adenomatous polyposis coli gene (APC), p53, the retinoblastoma susceptibility gene (RB1), and Ras. Originally, many of these genes were thought to be tumor suppressive or oncogenic solely because of their role in proliferative control. Because of the frequency with which they are disrupted in cancer, chromosome instability caused by their dysfunction may be more central to tumorigenesis than previously thought. Therefore, this review will highlight how the proper function of cell cycle regulatory genes contributes to the maintenance of genome stability, and how their mutation in cancer obligatorily connects proliferation and chromosome instability.

Chemoprevention of familial adenomatous polyposis by bromo-noscapine (EM011) in the Apc(Min/+) mouse model

Germline mutation of the tumor suppressor gene, adenomatous polyposis coli (APC), is responsible for familial adenomatous polyposis (FAP) with nearly 100% risk for colon cancer at an early age. Although FAP is involved in only 1% of all colon cancer cases, over 80% of sporadic cancers harbor somatic mutations of APC. We show here that bromo-noscapine (EM011), a rationally designed synthetic derivative of a natural nontoxic tubulin-binding alkaloid-noscapine, that reduces the dynamics of microtubules, causes a reversible G(2) /M arrest in wild type murine embryonic fibroblasts (MEFs), but an aberrant exit from a brief mitotic block, followed by apoptosis in MEFs after APC deletion with small interfering RNA. Furthermore, both β-catenin levels and activity fell to half the original levels with a concomitant reduction of cell proliferation-inducing cyclin D1, c-Myc, and induction of cytostatic protein p21 before caspase-3 activation. Additionally, we show a statistically significant reduction in the number of newly emerging intestinal polyps (to 35% compared with untreated mice) as well as the mean size of polyps (to 42% compared with untreated mice) in EM011-treated Apc(Min/+) mice as compared to their sham-treated control littermates. The remaining polyps in the EM011 treated group of Apc(Min/+) mice showed evidence of elevated apoptosis as revealed by immunohistochemistry. We failed to detect any evidence of histopathological and hematological toxicities following EM011 treatment. Taken together, our data are persuasive that a clinical trial of EM011 is possible for the prevention/amelioration of polyposis in FAP patients.

Role of heterozygous APC mutation in niche succession and initiation of colorectal cancer--a computational study

Mutations in the adenomatous polyposis coli (APC) gene are found in most colorectal cancers. They cause constitutive activation of proliferative pathways when both alleles of the gene are mutated. However studies on individuals with familial adenomatous polyposis (FAP) have shown that a single mutated APC allele can also create changes in the precancerous colon crypt, like increased number of stem cells, increased crypt fission, greater variability of DNA methylation patterns, and higher somatic mutation rates. In this paper, using a computational model of colon crypt dynamics, we evolve and investigate a hypothesis on the effect of heterozygous APC mutation that explains these different observations. Based on previous reports and the results from the computational model we propose the hypothesis that heterozygous APC mutation has the effect of increasing the chances for a stem cell to divide symmetrically, producing two stem cell daughters. We incorporate this hypothesis into the model and perform simulation experiments to investigate the consequences of the hypothesis. Simulations show that this hypothesis links together the changes in FAP crypts observed in previous studies. The simulations also show that an APC(+/-) stem cell gets selective advantages for dominating the crypt and progressing to cancer. This explains why most colon cancers are initiated by APC mutation. The results could have implications for preventing or retarding the onset of colon cancer in people with inherited or acquired mutation of one APC allele. Experimental validation of the hypothesis as well as investigation into the molecular mechanisms of this effect may therefore be worth undertaking.

Translating tumor biology into personalized treatment planning: analytical performance characteristics of the Oncotype DX Colon Cancer Assay

BACKGROUND

The Oncotype DX Colon Cancer Assay is a new diagnostic test for determining the likelihood of recurrence in stage II colon cancer patients after surgical resection using fixed paraffin embedded (FPE) primary colon tumor tissue. Like the Oncotype DX Breast Cancer Assay, this is a high complexity, multi-analyte, reverse transcription (RT) polymerase chain reaction (PCR) assay that measures the expression levels of specific cancer-related genes. By capturing the biology underlying each patient's tumor, the Oncotype DX Colon Cancer Assay provides a Recurrence Score (RS) that reflects an individualized risk of disease recurrence. Here we describe its analytical performance using pre-determined performance criteria, which is a critical component of molecular diagnostic test validation.

RESULTS

All analytical measurements met pre-specified performance criteria. PCR amplification efficiency for all 12 assays was high, ranging from 96% to 107%, while linearity was demonstrated over an 11 log2 concentration range for all assays. Based on estimated components of variance for FPE RNA pools, analytical reproducibility and precision demonstrated low SDs for individual genes (0.16 to 0.32 CTs), gene groups (≤ 0.05 normalized/aggregate CTs) and RS (≤ 1.38 RS units).

CONCLUSIONS

Analytical performance characteristics shown here for both individual genes and gene groups in the Oncotype DX Colon Cancer Assay demonstrate consistent translation of specific biology of individual tumors into clinically useful diagnostic information. The results of these studies illustrate how the analytical capability of the Oncotype DX Colon Cancer Assay has enabled clinical validation of a test to determine individualized recurrence risk after colon cancer surgery.

Regulation of Lrp6 phosphorylation

The Wnt/beta-catenin signaling pathway plays important roles in embryonic development and tissue homeostasis, and is implicated in human disease. Wnts transduce signals via transmembrane receptors of the Frizzled (Fzd/Fz) family and the low density lipoprotein receptor-related protein 5/6 (Lrp5/6). A key mechanism in their signal transduction is that Wnts induce Lrp6 signalosomes, which become phosphorylated at multiple conserved sites, notably at PPSPXS motifs. Lrp6 phosphorylation is crucial to beta-catenin stabilization and pathway activation by promoting Axin and Gsk3 recruitment to phosphorylated sites. Here, we summarize how proline-directed kinases (Gsk3, PKA, Pftk1, Grk5/6) and non-proline-directed kinases (CK1 family) act upon Lrp6, how the phosphorylation is regulated by ligand binding and mitosis, and how Lrp6 phosphorylation leads to beta-catenin stabilization.

Tumor suppressor interactions with microtubules: keeping cell polarity and cell division on track

Tumor suppressor proteins protect cells and tissues from malignant transformation. Among their diverse actions, many of these proteins interact with the microtubule cytoskeleton. This review focuses on the interactions of several tumor suppressors with microtubules and speculates on how disruption of microtubule-dependent processes may contribute to cancer development and spread. We conclude that several tumor suppressors stabilize microtubules and organize microtubule arrays, functions that are likely to be important in preventing tumorigenesis. How tumor suppressors link microtubule stability with cell fate, and how their mutation affects the response of cancer cells to anti-microtubule chemotherapy drugs, remains unclear; these should prove fertile areas for future research.

Recruitment of Cdc20 to the kinetochore requires BubR1 but not Mad2 in Drosophila melanogaster

To prevent aneuploidy, cells require a mitotic surveillance mechanism, the spindle assembly checkpoint (SAC). The SAC prevents metaphase/anaphase transition by blocking the ubiquitylation and destruction of cyclin B and securin via the Cdc20-activated anaphase-promoting complex or cyclosome (APC/C)-mediated proteolysis pathway. This checkpoint involves the kinetochore proteins Mad2, BubR1, and Cdc20. Mad2 and BubR1 are inhibitors of the APC/C, but Cdc20 is an activator. Exactly how the SAC regulates Cdc20 via unattached kinetochores remains unclear; in vertebrates, most current models suggest that kinetochore-bound Mad2 is required for initial binding to Cdc20 to form a stable complex that includes BubR1. Here, we show that the Mad2 kinetochore dimerization recruitment mechanism is conserved and that the recruitment of Cdc20 to kinetochores in Drosophila requires BubR1 but not Mad2. BubR1 and Mad2 can bind to Cdc20 independently, and the interactions are enhanced after cells are arrested at mitosis by the depletion of Cdc27 using RNA interference (RNAi) in S2 cells or by MG132 treatment in syncytial embryos. These findings offer an explanation of why BubR1 is more important than Mad2 for SAC function in flies. These findings could lead to a better understanding of vertebrate SAC mechanisms.

Topoisomerase IIalpha binding domains of adenomatous polyposis coli influence cell cycle progression and aneuploidy

BACKGROUND

Truncating mutations in the tumor suppressor gene APC (Adenomatous Polyposis Coli) are thought to initiate the majority of colorectal cancers. The 15- and 20-amino acid repeat regions of APC bind beta-catenin and have been widely studied for their role in the negative regulation of canonical Wnt signaling. However, functions of APC in other important cellular processes, such as cell cycle control or aneuploidy, are only beginning to be studied. Our previous investigation implicated the 15-amino acid repeat region of APC (M2-APC) in the regulation of the G2/M cell cycle transition through interaction with topoisomerase IIalpha (topo IIalpha).

METHODOLOGY/PRINCIPAL FINDINGS

We now demonstrate that the 20-amino acid repeat region of APC (M3-APC) also interacts with topo IIalpha in colonic epithelial cells. Expression of M3-APC in cells with full-length endogenous APC causes cell accumulation in G2. However, cells with a mutated topo IIalpha isoform and lacking topo IIbeta did not arrest, suggesting that the cellular consequence of M2- or M3-APC expression depends on functional topoisomerase II. Both purified recombinant M2- and M3-APC significantly enhanced the activity of topo IIalpha. Of note, although M3-APC can bind beta-catenin, the G2 arrest did not correlate with beta-catenin expression or activity, similar to what was seen with M2-APC. More importantly, expression of either M2- or M3-APC also led to increased aneuploidy in cells with full-length endogenous APC but not in cells with truncated endogenous APC that includes the M2-APC region.

CONCLUSIONS/SIGNIFICANCE

Together, our data establish that the 20-amino acid repeat region of APC interacts with topo IIalpha to enhance its activity in vitro, and leads to G2 cell cycle accumulation and aneuploidy when expressed in cells containing full-length APC. These findings provide an additional explanation for the aneuploidy associated with many colon cancers that possess truncated APC.