Alterations in a cell's chromosomal content from normal is known as polyploidy or aneuploidy. Polyploidy is a change that is a multiple of the haploid chromosome content while aneuploidy is a change in the chromosome content that is not a multiple of the haploid number. There are many instances demonstrating that polyploidy is reasonably well tolerated at the organismal level, and whole genome duplications likely have served to promote the evolution of species (1). However, this is not the case for aneuploidy where the gain or loss of individual chromosomes has been demonstrated to result in lethality and the development of disease (1). Mitosis is a highly regulated process with various surveillance mechanisms in place to coordinate the proper segregation of genetic material between the daughter cells. Nevertheless, even in the presence of these checkpoints aneuploidy can occur, as chromosome mis-segregation has been estimated to happen at a rate of once in every 104 to 105 cell divisions in mammalian cells (2).
Aneuploidy has long been known to be a characteristic of cancer cells (3), and changes in chromosome number have been proposed to be a mechanism by which cancer cells acquire additional copies of oncogenes or lose the expression of tumor suppressor genes, thereby driving the tumorigenic process. Interestingly, individuals with Down syndrome (DS) are at an increased risk to develop leukemia, retinoblastoma, and germ cell tumors, but are less likely to develop other solid tumors (4, 5). As I develop my own research group, I am seeking to further define how the presence of an extra chromosome influences the fitness of mammalian cells, and how these differences might lend insight into the role of aneuploidy in cancer.
Generation and characterization of mouse aneuploid cells
In most species, the presence of an extra chromosome is associated with decreased fitness and reduced viability (1). However, how the presence of an extra chromosome reduces cellular fitness and viability at the molecular level is not well understood. We wished to determine the effect of an extra chromosome on mammalian cellular fitness. To accomplish this, we generated trisomic (Ts) mouse embryo fibroblasts (MEFs) utilizing a breeding scheme with mouse strains that harbor Robertsonian translocations for different chromosomes. Mice harboring different Robertsonian chromosomes can be bred so that the Robertsonian chromosomes are combined in different ways and non-disjunction events are predisposed to produce Ts embryos. We generated MEFs trisomic for Chromosomes 1, 13, 16, and 19. We chose to make MEFs for these trisomies because they span a large range of chromosome physical size and the known coding gene spectrum.
We observed a number of interesting phenotypes culturing these cells (6). First, transcriptional analysis from early passage cells indicated that the extra chromosome was being transcribed. All of the transcripts from the additional...