TruncatingAPCmutations have dominant effects on proliferation, spindle checkpoint control, survival and chromosome stability

Abstract

The majority of human tumour cells are aneuploid owing to an underlying chromosome instability phenotype. While the genetic lesions that cause chromosome instability remain undefined, mouse ES cells harbouring homozygous adenomatous polyposis coli (APC) mutations are frequently tetraploid. In addition, colon cancer cells with APC mutations have weakened kinetochore-microtubule interactions. Furthermore, mitotic spindles assembled in APC-depleted Xenopus egg extracts are aberrant. Therefore, to determine whether APC mutations can initiate chromosome instability in human cells, we expressed N-terminal APC fragments in HCT-116 cells, a near diploid colon cancer cell line with two wild-type APC alleles. We show that cells expressing N-APC mutants exit mitosis prematurely in the presence of spindle toxins, consistent with a spindle checkpoint defect. In addition, N-APC cells show enhanced survival following prolonged spindle damage. In contrast to controls, the N-APC survivors frequently contain dicentric chromosomes and then go on to become highly aneuploid. These observations suggest that truncating APC mutations can exert dominant effects which in turn can initiate chromosome instability. As such, APC mutation not only compromises tumour suppressor function but may also have oncogenic properties. We suggest therefore that the initial APC mutation acts as a `double whammy', destabilising the genome and setting the stage for deregulated proliferation upon loss of the second APC allele.

Mechanical factors activateß‐catenin‐dependent oncogene expression in APC1638N/+mouse colon

“…In addition, it has been shown in vivo that mechanical irritation of the epithelium can lead to an increased mutation rate (Takahashi et al, 2000). This suggests that cells of the APC deficient colon, which can already experience chromosomal missegregation due to dominant negative effects of truncated APC protein or insufficient full length APC during mitosis (Fodde et al, 2001;Tighe et al, 2004;Aoki et al, 2007;Draviam et al, 2006), may be exposed to an alternative route to loss of heterozygosity when subjected to mechanical stress, in addition to the effects demonstrated here on tumor gene expression in the haploinsufficient tissue.…”

Bimodal activation of BubR1 by Bub3 sustains mitotic checkpoint signaling

“…Parental Flp-In TRex-HeLa or -DLD-1 parental cells that stably express mRFP-tagged histone H2B (H2B-mRFP) were as previously described (55,56). Stable, isogenic cell lines expressing MycGFP-BubR1 were generated using FRT/Flp-mediated recombination (57). Expression of MycGFP-BubR1 was induced with 1 μg/mL tetracycline.…”

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

“…In most colon cancer cells, the pathway is activated through mutations in the adenomatous polyposis coli (APC), AXIN1 or b-catenin (CTNNB1) gene. Recent reports have suggested that APC protein plays additional roles in chromosome segregation and CIN (Fodde et al, 2001;Kaplan et al, 2001;Dikovskaya et al, 2004;Green and Kaplan, 2004;Tighe et al, 2004). It has been found that truncation mutations in the APC gene cause structural abnormalities of chromosomes (Fodde et al, 2001), aneuploidy (Fodde et al, 2001;Kaplan et al, 2001) and spindle aberrations (Fodde et al, 2001).…”

“…These reports suggest that lack of APC is involved in cancer progression through induction of CIN. In addition, recent analyses have indicated that N-terminal APC fragments can suppress the mitotic spindle checkpoint and induce CIN (Tighe et al, 2004).…”