Chromosome instability and cancer

Chromosomal aneuploidy and cancer. The majority of tumor cells have abnormally high numbers of chromosomes, or aneuploidy. In some cases, such as colorectal cancer cells, aneuploidy is also accompanied by an increase in the rate of chromosome loss or gain. Despite the prevalence of these gross alterations in the genome, the defects underlying aneuploidy and the role of they play in cancer onset or progression remain controversial. 

In the early part of the 20th century, a scientist named Theodor Boveri observed cancer cells to have extra structures associated with the heritable material inside cells. He speculated that these extra structures might be the underlying cause of cancer. These structures turned out to be extra copies of the chromosomes and twice as many centrosomes as normal, a state associated with tetraploidy.  

adenomatous polyposis coli (APC) mutations act dominantly to inhibit microtubule dynamics. To understand the underlying causes for the high rates of chromosome instability observed in some colorectal tumor cells, I asked whether APC mutants affected mitosis. APC is part of a well-described regulatory complex that normally targets the cell polarity/transcription factor, b-catenin, for degradation. In cancer patients, loss of both alleles of APC causes an accumulation of b-catenin in the nucleus and transcriptional changes consistent with cancer progression. Thus, b-catinin stabilization is a recessive phenotype. APC also associates with microtubules and actin, suggesting it plays a key regulatory role in coordinating extra-cellular signals with cell polarity. Since nearly all APC mutants found in cancer patients are non-sense mutations that result in a truncated protein product, we asked if these were sufficient to act dominantly on microtubule function. Remarkably, we found that a single truncating mutation, APC1-1450, is sufficient to decrease microtubule dynamics by preventing the interaction of APC with the EB1 microtubule plus-end binding protein (Green and Kaplan, JCB 2003; Green et al, MCB 2005). Importantly, this microtubule defect is independent of changes in b-catenin levels and leads to dramatic changes in spindle organization.   

Mitotic defects caused by dominant APC mutants. The changes in microtubule dynamics cause defects in anaphase spindle elongation and ulimately mitotic catastrophe.  In vitro, this led to an increase in the number of binculeate and multi-nucleated cells. 

Using the Min mouse model for intestinal cancer (APCMin/+), we showed that spindles in the crypts of the small intestine were rotated relative to wild type crypts. We propose that the rotated spindles reflect the same spindle anchoring defects observed in cell culture. These defects were found in crypt tissue that had not lost the second copy of APC, consistent witht the dominant activity of APC truncations. Importantly, we also found an increase in the incidence of tetraploid cells in heterozygous APCMin/+ crypts with an accompanied increase in tetrapolar spindles.These results are consistent with the model that a single mutant allele of APC can result in mitotic defects in cells prior to loss of heterozygosity (LOH). It is unclear if these changes contribute directly to LOH or if other changes due to changes in microtubule dynamics are important for the transition of pre-cancerous cells to cancer cells. 

Tetraploids and cancer progression. We expect that tetraploid cells in the crypt will not be stable. They will either fail to proliferate or they will proliferate in a way to reduce the number of centrosomes and potentially the number of chromosomes. Our observations of dysplastic tissue in Min intestines are consistent with these possibilites. We observe a low but significant number of aneuploid cells with gains in chromosomes in these regions, even though the majority of cells appear near diploid. 

It is possible that the most important consequence of tetraploids, or changes in microtubule dynamics, are to alter the selective pressure experienced by crypt cells in the intestine. In this case, increases in chromosome number may be less important than changes to cell programming. Indeed, we obesrve that Min crypts have a heterogenous expression profile of the stem cell marker, Musashi. 

We are currently exploring the possibility that early changes in cellular programming caused by dominant APC mutations promote heterogeneity at both genetic and epigenetic levels. Our working model is that the APCMin/+ cells experience an elevated level of stress and this contributes to novel selective pressures that ultimately give rise to cells that escape their normal regulatory niche. © K.B. Kaplan 2012