Aneuploidy, an imbalanced karyotype, is a widely observed feature of cancer cells that has long been hypothesized to promote tumorigenesis. Here we evaluate the fitness of cells with constitutional trisomy or chromosomal instability (CIN) in vivo using hematopoietic reconstitution experiments. We did not observe cancer but instead found that aneuploid hematopoietic stem cells (HSCs) exhibit decreased fitness. This reduced fitness is due at least in part to the decreased proliferative potential of aneuploid hematopoietic cells. Analyses of mice with CIN caused by a hypomorphic mutation in the gene Bub1b further support the finding that aneuploidy impairs cell proliferation in vivo. Whereas nonregenerating adult tissues are highly aneuploid in these mice, HSCs and other regenerative adult tissues are largely euploid. These findings indicate that, in vivo, mechanisms exist to select against aneuploid cells.

SUMMARY Phenotypic variability is a hallmark of diseases involving chromosome gains and losses, such as Down Syndrome and cancer. Allelic variances have been thought to be the sole cause of this heterogeneity. Here, we systematically examine the consequences of gaining and losing single or multiple chromosomes to show that the aneuploid state causes non-genetic phenotypic variability. Yeast cell populations harboring the same defined aneuploidy exhibit heterogeneity in cell cycle progression and response to environmental perturbations. Variability increases with degree of aneuploidy and is partly due to gene copy number imbalances, suggesting subtle changes in gene expression impact the robustness of biological networks and cause alternate behaviors when they occur across many genes. As inbred trisomic mice also exhibit variable phenotypes, we further propose that non-genetic individuality is a universal characteristic of the aneuploid state that may contribute to variability in presentation and treatment responses of diseases caused by aneuploidy.

The molecular chaperone, Hsp104, is a hexameric protein belonging to the triple-A ATPase family (1). Along with its roles in protein folding and conferring thermotolerance, this chaperone is essential for propagation of prions in budding yeast. Prions are infectious proteins that occur in two conformations, a properly folded non-infectious conformation and a misfolded infectious amyloid conformation (2). Yeast has more than a dozen prions, which pass from mother to daughter cells as prion seeds. The steady-state number of prion seeds is maintained by Hsp104, which severs the seeds; this severing reaction is dependent on the ATPase activity of Hsp104 and involves the presentation of the prion to Hsp104 by the molecular chaperones, Sis1 and Ssa1/Ssa2, followed by threading of the prions through the Hsp104 channel (3). Yeast prions are cured by inhibiting the ATPase activity of Hsp104; this inhibition can be caused by either overexpressing a dominant-negative Hsp104 mutant or by inactivating Hsp104 with guanidine. Curing is then caused by absence of prion seed severing, in combination with dilution of the seeds by cell division (4 -6).Surprisingly A red/white colony assay is the standard method of detecting these two conformations. In this assay, yeast having nonsense mutations in the adenine synthesis pathway are plated on limiting adenine medium. Another method of detecting these two conformations is by fluorescence imaging using GFP-labeled Sup35. Using these methods, we previously showed that the curing of [PSI ϩ ] by overexpression of Saccharomyces cerevisiae Hsp104 (Sc-Hsp104) requires a new activity of Hsp104 called trimming (13). Like severing, trimming reduces the size of the prion seeds. However, unlike severing, trimming, which may occur through removal of Sup35 molecules from the ends of the prion fibers, is not accompanied by an increase in seed number. The dissolution of the prion seeds caused by overexpression of Sc-Hsp104 was shown to require the trimming activity of Hsp104. Mutants of Hsp104 that cannot trim also cannot cure [PSI ϩ ] prion by overexpression (14). For example, overexpression of the T160M point mutant of Sc-Hsp104 that lacks trimming activity but has severing activity comparable with wildtype Sc-Hsp104 does not cure [PSI ϩ ]. Consistent with our in vivo observations, in vitro evidence for dissolution of the prion seeds by Hsp104 has been obtained by Shorter and Lindquist (15). Other chaperones and cochaperones such as Hsp70, Sti1,