Aneuploidy, in which cells carry an abnormal number of one or more chromosomes, is highly detrimental during mammalian development, since amplification of most human chromosomes is inviable during embryogenesis. Imbalanced and especially elevated expression from altered chromosomes is thought to tax cellular proteostasis, both by producing too much protein and when stoichiometric imbalance of interacting proteins disrupts cooperative folding (Donnelly and Storchová, 2015; Oromendia and Amon, 2014; Pavelka and Rancati, 2013). Yet ≥90% of tumors are aneuploid with little detriment and even benefits to cells, and the degree of aneuploidy is associated with poorer patient prognosis (Holland and Cleveland, 2012; Targa and Rancati, 2018). Aneuploidy is also common in several fungal species including fungal pathogens. In fact, chromosome amplification represents a major route to drug resistance in pathogenic infections, when amplification of drug transporters and defense mechanisms promotes drug evasion (Wertheimer et al., 2016; Ni et al., 2013; Bennett et al., 2014). Why aneuploidy is benign or beneficial in some cells but highly deleterious in others is not understood.

The yeast Saccharomyces cerevisiae has been a formidable model to understand why chromosome amplification is toxic in the first place. Several studies characterized suites of aneuploid laboratory strains to understand the mechanisms of aneuploidy toxicity and the effects of chromosomal amplification. In a well-studied laboratory strain, chromosome amplification leads to reduced cell growth, metabolic alterations, altered cell-cycle progression in part through aberrant cyclin regulation, activation of a common transcriptome program regardless of the amplified chromosome, and signatures of protein aggregation and defects clearing misfolded peptides, referred to as proteostasis stress (Torres et al., 2007; Torres et al., 2010; Oromendia et al., 2012; Sheltzer et al., 2012; Thorburn et al., 2013; Dephoure et al., 2014; Dodgson et al., 2016; Brennan et al., 2019). Despite the deleterious effects reported in lab strains, chromosome amplification is beneficial in the right environment and provides a rapid route to phenotypic evolution (Rancati et al., 2008; Pavelka et al., 2010; Yona et al., 2012; Filteau et al., 2015; Fontanillas et al., 2010). This is consistent with the prevalence of chromosome amplification in fungal pathogens emerging after drug-treatment regimens (Ni et al., 2013; Selmecki et al., 2009; Selmecki, 2006).

Studies in laboratory strains have clearly generated important information on the causes and consequences of aneuploidy. However, we previously reported a striking difference among wild isolates: a substantial number of wild strains are naturally aneuploid, in some cases carrying extra copies of multiple chromosomes (Gasch et al., 2016; Hose et al., 2015). Recent large-scale sequencing efforts provide confirmatory evidence, reporting over 20% of sequenced strains as aneuploid, with each of the 16 yeast chromosomes represented across affected strains (Peter et al., 2018). In contrast to well-studied laboratory strain W303, naturally aneuploid yeast show only subtle growth defects, no detectable metabolic differences, and lack evidence of the canonical stress response (Gasch et al., 2016; Hose et al., 2015). The relative tolerance is not a result of adaptation: we showed that naturally euploid strains selected for chromosome amplification also showed relatively mild growth defects, and euploid derivatives of aneuploid isolates grew similarly to the aneuploid parent. Although some strains show variable karyotypes over time, picking up or losing chromosomes during division, chromosome amplification in other strains is generally stable (Gasch et al., 2016). Thus, many wild yeast strains tolerate chromosome amplification whereas W303 cannot.

Here, we mapped the genetic basis for this phenotypic difference, by crossing a naturally aneuploid strain isolated from oak soil, YPS1009 with extra copies of Chromosome XII (Chr12), to laboratory strain W303 carrying an extra copy of Chr12. Mapping and confirmatory genetics reveal that the basis for the difference in aneuploidy tolerance lies in SSD1, encoding an RNA binding protein known to be hypomorphic in W303. Our results point to combinatorial dysfunction in mitochondrial physiology and cytosolic protein homeostasis as the basis for aneuploidy toxicity in ssd1- strains, and in wild-type aneuploids with drug-induced defects. Integrating our results with past yeast and mammalian studies presents a holistic view of eukaryotic responses to chromosome amplification.

To identify the genetic basis of differential aneuploidy tolerance, we crossed a haploid derivative of oak-soil strain YPS1009 disomic for chromosome 12 (YPS1009_Chr12) to W303 disomic for the same chromosome (W303_Chr12, Figure 1A). Haploid F2 segregants all harbor two copies of Chr12 but display quantitatively different growth rates (Figure 1—figure supplement 1). To score aneuploidy sensitivity, we focused on W303_Chr12 phenotypes, namely small colony size, slow growth, and/or propensity of the culture to lose the amplified chromosome during passaging. We realized during tetrad dissection that W303-inherited auxotrophies, especially adenine auxotrophy, influenced aneuploidy tolerance (Figure 1—figure supplement 2A-B). We therefore selected an F2 segregant prototrophic for influential markers (called ‘sp100’), backcrossed it to the tolerant YPS1009_Chr12 parent, and scored aneuploidy sensitivity as above (Figure 1A) to generate pools of aneuploidy-sensitive and aneuploidy-tolerant segregants (Figure 1—figure supplement 2C-D). To control for other genetic influences on growth rate and/or colony size, we also performed a control cross of the euploid parents, generating pools of euploid segregants with small versus large colony sizes (see Materials and methods).

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Bulk analysis of aneuploidy-sensitive and -tolerant backcrossed segregants revealed a major-effect locus on Chr4 that was nearly fixed for W303 alleles in the aneuploidy-sensitive pool (Figure 1B and Figure 1—figure supplements 3, 4) but not small colonies from a euploid-control cross (Figure 1C, see Materials and methods). The locus spanned SSD1, encoding an RNA-binding protein. This locus harbors a premature stop codon in W303 that deletes 44% of the Ssd1 protein including conserved RNA binding domains (Uesono et al., 1997; Sutton et al., 1991). Ssd1 is best characterized for regulating localization and translation of cell-wall destined mRNAs, delivered by Ssd1 to the growing bud during active growth but to P-bodies for translational silencing following stress or mitotic defects (Jansen et al., 2009; Kurischko et al., 2011a). Ssd1 has also been implicated in a large number of suppressor screens and has a role in aging and quiescence (Miles et al., 2019; Li et al., 2013; Hu et al., 2018). W303 carries a premature stop codon that ablates RNA binding domains, which underlies several phenotypic differences reported between W303 and other strains (Kaeberlein et al., 2004; Moriya and Isono, 1999; Ohyama et al., 2010; Uesono et al., 1994).

Genetic analysis confirmed that SSD1 underlies the difference in aneuploidy tolerance. SSD1 deletion had little effect on the growth of euploid YPS1009 but significantly retarded YPS1009_Chr12 proliferation, comparable to W303_Chr12 (Figure 1D). The phenotype was true in both haploid and diploid versions of the strain (Figure 1—figure supplement 5). Growth rate was restored if cells lost the extra chromosome during passaging (Figure 1D, star) or if SSD1^YPS1009 was reintroduced (Figure 1E). To test if Ssd1’s role was exclusive to this genetic background or chromosome amplification, we deleted SSD1 in naturally aneuploid, diploid West African strain tetrasomic for Chr 8 (NCYC110_Chr8) and in a derived aneuploid vineyard strain, KCY40_Chr8 (Hose et al., 2015). SSD1 deletion sensitized cells to chromosome amplification, showing that the effect is independent of genetic background and duplicated chromosome (and is thus also independent of the rDNA locus on Chr 12) (Figure 1D). Reintroducing the YPS1009 allele of SSD1 complemented the aneuploidy sensitivity of multiple strain backgrounds (Figure 1E), whereas the W303 allele provided no complementation in the NCYC110_Chr8 ssd1Δ strain and partial complementation in YPS1009_Chr12 ssd1Δ (Figure 1D). Importantly, expressing the YPS1009 allele in W303_Chr12 largely corrected its sensitivity (with a remaining contribution likely from the adenine auxotrophy, see Figure 1—figure supplement 2B), demonstrating that the ssd1^W303 allele is responsible for aneuploidy sensitivity in W303. Thus, Ssd1 plays a generalizable role in tolerating chromosome amplification across multiple strains and chromosome duplications.

Loss of Ssd1 recapitulates multiple signatures of aneuploid W303

W303 studies reported a transcriptomic signature of aneuploidy independent of amplified chromosome identity, but this is not seen in wild aneuploid strains (Torres et al., 2007; Hose et al., 2015). To test dependence on Ssd1, we followed transcriptomes of natural aneuploids and their ssd1Δ derivatives, with or without extra chromosomes. We identified 861 genes with altered expression in both YPS1009_Chr12 ssd1Δ and NCYC110_Chr8 ssd1Δ mutants compared to their isogenic wild-type aneuploids (false discovery rate, FDR < 0.05, Figure 2A). Induced genes showed little change in euploid YPS1009 ssd1Δ but were up-regulated when SSD1 was deleted in the context of Chr12 amplification. NCYC110 showed similar trends, except that in this strain we observed a weak expression signature in euploid ssd1Δ cells that was exacerbated when Chr8 was amplified. Repressed genes included rRNA and tRNA processing and cytokinesis factors, whereas induced genes encompassed the environmental stress response (ESR Gasch et al., 2000), oxidoreductases, carbohydrate and energy metabolism, and genes involved in mitochondrial degradation (p<1e-4, hypergeometric test). This response effectively recapitulates the expression signature seen in W303 aneuploids (Torres et al., 2007, Figure 2A). The response was exacerbated with increasing DNA content in W303 carrying multiple extra chromosomes (Figure 2A). Thus, the previously reported aneuploidy transcriptome signature results from defective Ssd1 function, independent of affected chromosome, exacerbated with additional DNA content, and with some strain-specific nuances.

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We wondered if other aneuploidy signatures seen in W303 could be explained by defective Ssd1. In addition to growth delay, aneuploid W303 strains reportedly exhibit larger cell size, altered cyclin Cln2 abundance, delayed G1/S progression, metabolic defects, and signatures of proteotoxicity including protein aggregation and a defect degrading misfolded protein (Torres et al., 2007; Oromendia et al., 2012; Thorburn et al., 2013). While these seminal studies generated important information on aneuploidy toxicity in a sensitized strain, many phenotypes likely result from defective Ssd1. YPS1009_Chr12 ssd1Δ grows slower (Figure 1), produces ~33% higher optical versus cell density indicating larger size, and displays metabolic defects encompassing defective respiratory growth (see Figure 4). We confirmed that Ssd1 binds many cell-cycle transcripts including CLN2 (Supplementary file 2), which is translationally regulated by Ssd1 and explains altered G1/S progression (Ohyama et al., 2010).

Loss of SSD1 also explains proteotoxicity observed for aneuploid strains. First, we followed accumulation of human Von Hippel Lindau (VHL) protein upon over-expression. Because VHL cannot fold in the absence of its interacting proteins in yeast, accumulation of VHL-GFP foci reflects misfolded protein that has yet to be cleared by the proteasome (McClellan et al., 2005; Kaganovich et al., 2008). We found that significantly more YPS1009_Chr12 ssd1Δ cells accumulated VHL-GFP foci compared to wild type (Figure 2B). We also followed the protein disaggregase Hsp104, which binds misfolded and aggregated proteins in discrete protein quality-control centers (Kaganovich et al., 2008; Chernova et al., 2017). Wild-type aneuploids showed no obvious difference in the number of Hsp104-GFP foci compared to euploids (Figure 2C and Figure 2—figure supplement 1), indicating that gross protein aggregation is not a universal feature of aneuploid yeast. However, mutant aneuploids lacking SSD1 showed a higher proportion of cells with Hsp104-GFP foci, and more foci within those cells, compared to wild-type aneuploids (Figure 2C). Ssd1 was previously implicated in protein homeostasis after heat shock (Mir et al., 2009), but our results demonstrate that chromosome amplification in the absence of other stresses is enough to provoke misfolding in ssd1Δ cells. Together, these results show that myriad signatures of W303 aneuploidy can occur due to defective Ssd1.

Ssd1 mutants could be sensitive to specific genes on the amplified chromosomes, or they could have a generalized sensitivity to the burden of extra DNA/protein. To distinguish these models, we transformed YPS1009 strains with a barcoded, high-copy gene over-expression library and measured relative fitness costs after 5 generations of growth (see Materials and methods). Both the euploid and aneuploid ssd1Δ mutants were highly sensitive to the library (Figure 2D): genes that were deleterious in wild type were toxic in the mutant, while many genes with neutral effect in parental strains were deleterious in the absence of SSD1. We cannot exclude a defect maintaining the high-copy 2-micron plasmid (indeed, the mutant cells do not grow well with the empty vector, and Ssd1 is already implicated in plasmid maintenance Uesono et al., 1994). Nonetheless, both the euploid and aneuploid ssd1Δ mutants are highly sensitive to the 2-micron overexpression library.

Ssd1 binds RNAs and alters aneuploid proteomes

We focused on YPS1009 strains to elucidate Ssd1 function in aneuploidy tolerance. Revisiting the YPS1009_Chr12 ssd1Δ transcriptome revealed broader induction of genes, including mRNAs whose proteins localize to subcellular compartments such as mitochondria, ER, vacuole, peroxisome, plasma membrane, and the cell wall (p<1e-4, hypergeometric test). Many of these organelles functionally and physical interact (Scorrano et al., 2019), raising the possibility of broader inter-organelle issues. Consistent with this notion, the ssd1Δ aneuploid also showed transcriptional signatures of the unfolded ER-protein response (Travers et al., 2000) and mitochondrial protein import stress (Weidberg and Amon, 2018) (see Materials and methods).

To test if Ssd1 binds a broader set of mRNAs, we sequenced RNAs recovered from Ssd1 immunoprecipitation (see Materials and methods). The 286 associated mRNAs (FDR < 0.05, Supplementary file 2) were heavily enriched for transcripts encoding cell-wall proteins as expected, but the group was also enriched for cell-cycle regulated mRNAs (including cyclins CLN2 and CLB2/4/5) and those involved in budding, RNA metabolism, and sterol transport (p<1e-4). Myriad other functions were also represented, such as proteins in chromatin regulation, transcription, lipid biogenesis, endocytosis, protein homeostasis, and mitochondrial function, some previously noted (Jansen et al., 2009; Hogan et al., 2008). Interestingly, Ssd1 also bound mRNAs encoding osmotic-response regulators (SLN1, SHO1, MSB2, HOT1), notable since osmotic stress was recently implicated in aneuploidy responses of a different lab strain (Tsai et al., 2019). Ssd1-bound mRNAs were not enriched for those encoded on the amplified chromosome, nor transcripts disproportionately expressed compared to DNA content (Hose et al., 2015), suggesting that the mechanism of aneuploidy tolerance is not through modulation of amplified-gene expression. Most (60%) bound transcripts were not differentially expressed in the aneuploid mutant, downplaying a generalizable role in regulating mRNA abundance. In turn, most mRNAs differentially expressed in the aneuploid mutant are not Ssd1-bound, suggesting widespread secondary responses to the primary defect(s).

Since Ssd1 regulates translation via direct RNA binding (Jansen et al., 2009; Wanless et al., 2014; Kurischko et al., 2011b), we next used quantitative proteomics to measure effects on the cellular proteome. 301 of 3906 measured proteins were more abundant in the aneuploid mutant versus wild type (FDR < 0.05, Figure 3A). Many emerge from induced transcripts; however, a large fraction of proteins was elevated beyond mRNA differences, including cell-wall proteins, nuclear-encoded mitochondrial proteins (p=6e-4, hypergeometric test), and others. Many of these proteins were also elevated in the euploid mutant without significant mRNA changes (Figure 3A). Although there was no enrichment for proteins encoded by Ssd1-bound transcripts, several elevated proteins emerge from Ssd1 targets, including cell-wall transcripts and several nuclear transcripts encoding mitochondrial proteins. For example, UTH1 encoding a mitochondrial protein linked to aging is bound by Ssd1 in our and other studies and is known to be translationally regulated by direct Ssd1 binding at specific mRNA locations (Wanless et al., 2014; Camougrand et al., 2004; Camougrand et al., 2000). Although protein induction was evident in euploid ssd1Δ cells, the defect was exacerbated by Chr12 amplification, with ~4X more Uth1 protein despite little difference in mRNA (Figure 3B). Other mitochondrial proteins emanating from Ssd1-bound mRNAs were also significantly elevated in the ssd1Δ aneuploid, including several mitochondrial ribosomal proteins (Figure 3B). Thus, Ssd1 affects the proteome of aneuploid cells, including from bound transcripts.

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Ssd1 is important for mitochondrial function and inheritance

Our past work revealed that wild aneuploid strains down-regulate many nuclear encoded mitochondrial transcripts, a response also seen in Down syndrome models (Helguera et al., 2013; Liu et al., 2017), hinting that mitochondrial regulation is important for tolerating chromosome amplification (Hose et al., 2015). In the current work, multiple lines implicated mitochondrial effects in ssd1Δ aneuploids. To explore this, we tested mitochondrial function in YPS1009 strains. We found a synergistic defect between SSD1 deletion and aneuploidy when cells experienced mitochondrial stress. YPS1009_Chr12 ssd1Δ, NCYC110_Chr8 ssd1Δ (Figure 4A), and aneuploid W303 (Hose et al., 2015) were all sensitive to non-fermentable acetate, beyond what is expected from the additive effects of aneuploidy and reduced respiratory growth rate. This demonstrates Ssd1-dependent respiratory dysfunction autonomous of strain background or affected chromosome. YPS1009_Chr12 ssd1Δ cells were also susceptible to sub-lethal doses of carbonyl-cyanide 3-chlorophenylhydrazone (CCCP), which uncouples mitochondrial membrane potential (Figure 4B). Notably, aneuploid ssd1Δ cells were no more sensitive than expected to cell wall or ER stress (Figure 4—figure supplement 1), indicating a specific interaction with mitochondrial challenge. In the process of this work, we discovered that ssd1Δ aneuploid cells also showed a striking difference in mitochondrial morphology. Wild-type YPS1009_Chr12 grew well but harbored many globular mitochondria compared to the euploid tubular shape (Figure 4C). Although the impact of this morphology is not clear, YPS1009_Chr12 ssd1Δ displayed significant differences, including more tubular forms and increased mitochondria fragmentation (Figure 4D). The West African aneuploid did not display globular mitochondria but did display dysfunction (Figure 4A). Thus, ssd1Δ cells show numerous signs of mitochondrial dysfunction compared to wild-type aneuploid cells.

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We wondered if Ssd1 plays a direct role in mitochondrial function, perhaps by localizing nuclear-encoded mitochondrial mRNAs as it does cell-wall mRNAs. We first scored Ssd1 localization by centrifugation-based cellular fractionation. Ssd1 was reproducibly recovered in the organelle-enriched fraction that was depleted of cytosolic actin but enriched for markers of mitochondria (Cox2), ER (Dpm1), and vacuole (Vph1, Figure 5A), which themselves interact. Attempts to separate the components by immunoprecipitation of Ssd1 from the fractions were not successful. We next followed cellular localization of MMR1 transcript, encoding a bud-mitochondria localized protein involved in mitochondrial inheritance (Itoh et al., 2004), by single-molecule RNA FISH (smFISH, Figure 5B). In most wild-type cells, mother-encoded MMR1 was directed to the nascent bud, before the nucleus migrated. Although scoring precise differences in mRNA patterns was challenging, YPS1009_Chr12 ssd1Δ cells displayed twice as many buds lacking MMR1 as wild-type aneuploids (Figure 5C, p<0.02, Fisher’s exact test). In addition, the mutant showed double the cells lacking mitochondria as indicated by Rhodamine staining (Figure 5D), consistent with a defect in mitochondrial inheritance. It is possible that the mutant suffers from delayed dynamics, rather than fully aberrant localization in individual cells. Other nuclear-encoded mitochondrial mRNAs bound by Ssd1 were too abundant to follow by smFISH and will require further delineation. Nonetheless, together our work shows that Ssd1 associates with organelle fractions that include mitochondria (Figure 5A), binds several nuclear-encoded mitochondrial transcripts (Supplementary file 2), and can influence abundance of mitochondrial proteins (Figure 3B) or localization patterns of bound transcript (Figure 5B–C), consistent with the requirement of SSD1 for proper mitochondrial function in aneuploid cells (Figure 4).

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Combinatorial mitochondrial dysfunction and proteostasis stress underlie aneuploidy sensitivity in ssd1Δ cells

A remaining question is why Ssd1 dependence and mitochondrial dysfunction are more severe in aneuploids. We reasoned that underlying ssd1Δ defects are exacerbated by effects of chromosome amplification. One candidate is proteome stress that may emerge from over-production of proteins from the amplified chromosome in the absence of Ssd1-dependent translational silencing, which could tax the proteostatic buffering capacity specifically in the mutant (Donnelly and Storchová, 2015; Oromendia and Amon, 2014; Oromendia et al., 2012). Many recent studies have revealed a connection between mitochondrial function and cytosolic proteome stress: defects in mitochondrial protein import induce cytosolic proteostatic defense mechanisms, and misfolded cytosolic proteins interact with and can even be cleared by mitochondria (Qureshi et al., 2017; Ruan et al., 2017; Wrobel et al., 2015). Furthermore, mitochondrial defects and cytosolic proteostasis stress co-emerge in neurological syndromes, aging, and aneuploidy (Oromendia and Amon, 2014; Helguera et al., 2013; Ruan et al., 2017; Franco-Iborra et al., 2018; Kauppila et al., 2017).

To test the model that synergistic dysfunction underlies aneuploidy sensitivity in ssd1Δ aneuploid yeast, we applied nourseothricin (NTC), among the aminoglycoside drugs that induce mistranslation and protein misfolding (Ling et al., 2012). We confirmed that NTC treatment increased the number of Hsp104-GFP foci in the aneuploid wild type, and discovered that CCCP produced an even stronger effect even though wild-type aneuploids grew well in the drug (Figures 6 and 4B). Wild-type aneuploids were slightly sensitive to NTC, but the mutant was significantly more sensitive, beyond the expected additivity of aneuploidy and NTC response, revealing a synergistic defect induced by the drug in combination with aneuploidy (Figure 6A). The NTC sensitivity suggests that wild-type aneuploids with full length SSD1 can largely buffer proteostasis upon chromosome amplification but may exist near capacity. Mitochondrial defect, protein over-abundance, and mislocalized transcripts/proteins resulting from SSD1 deletion may simply push cells over the edge (see Discussion).

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Clustal Omega alignment of YPS1009 and seven other strains with truncated alleles.

Fasta files were recapitulated by mapping SNPs from the published vcf file⁸ onto the S288c sequence and performing a multiple alignment using Clustal Omega (https://www.ebi.ac.uk/Tools/msa/clustalo/). Allelic differences are highlighted in yellow below. Only regions with polymorphisms are shown.

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This raised an important prediction: if synergistic defects in mitochondrial function and proteostasis sensitize ssd1Δ cells to chromosome amplification, then combinatorial drug treatment to mimic these defects should selectively target wild-type aneuploids. In fact, this was the case: wild-type euploid and aneuploid cells tolerated short-term CCCP and NTC individually, but when combined aneuploid growth was significantly delayed beyond the euploid strain and the expectation of additive effects (Figure 6C). Longer-term combinatorial drug treatment limited growth of euploid YPS1009 but selectively blocked proliferation in YPS1009_Chr12 (Figure 6D). The effect was persistent across strains and chromosome amplifications: combinatorial treatment halted over-night growth of W303 with duplications of Chr8, Chr10, and Chr16 and NCYC110 carrying extra Chr 8 (although in this strain CCCP was actually protective against NTC toxicity in the euploid cells at the doses used)(Figure 6D).

Our work has several major implications for understanding the consequences of aneuploidy and how to modulate them. First, we resolve the discrepancy in the literature between wild and laboratory-strain responses to aneuploidy, by showing that mutation of a single gene explains the phenotypic difference among strains studied here. Many of the yeast phenotypes previously reported as signatures of aneuploidy, including proteostasis defects, metabolic defects, cell-cycle defects, and transcriptome response, can be caused by SSD1 deletion or mutation as seen in the commonly used W303 lab strain. This result explains why wild yeast from our studies, other studied S. cerevisiae strains, and pathogenic fungi do not show major defects upon chromosome amplification – these fungi have mechanisms to tolerate extra chromosomes. Our results underscore the importance of studying multiple strain backgrounds to understand model organism biology. At the same time, although W303 is clearly a sensitized strain we highlight that many important insights have come from its dissection. We propose that integrating our results with past yeast and mammalian studies reconciles to a holistic view of eukaryotic aneuploidy physiology.

Our model posits that Ssd1’s function in translational silencing and mitochondrial physiology enable aneuploidy tolerance in wild yeast. The wild aneuploid strains studied here do not show signs of metabolic or proteostatic stress under standard growth conditions. But mimicking ssd1Δ defects through combinatorial drug treatment sensitizes cells to extra chromosomes, showing that it is indeed combined dysfunction in mitochondria and proteome management that is responsible for aneuploidy sensitivity. We propose that, under normal conditions, wild aneuploids handle the extra chromosome by buffering the effects of gene/protein amplification – yet cells may exist close to their proteostatic buffering capacity. Additional stress on the proteostasis system, due to drugs or SSD1 deletion, pushes cells beyond capacity, thereby limiting fitness.

How does Ssd1 fulfill this function? Ssd1 has a clear role in translational regulation: it localizes to P-bodies during times of stress, suppresses encoded protein abundance via direct RNA binding, and is linked to the reduction of polysomes in aged cells (Jansen et al., 2009; Kurischko et al., 2011a; Hu et al., 2018; Wanless et al., 2014; Kurischko et al., 2011b). Ssd1’s role in mitigating proteostatic stress likely emerges via RNA binding, since the ssd1^W303 allele lacks the carboxyl-terminal RNA binding domain (Uesono et al., 1997). A remaining question is if mRNAs bound by Ssd1 are especially relevant to proteome homeostasis. Brennan et al. (2019) recently identified aggregation-prone proteins in aneuploid W303, including proteins encoded on and off the amplified chromosomes. The hypothesis was raised that aggregation may be a beneficial mode of protein-dosage compensation (Brennan et al., 2019). However, that aggregation is a hallmark of ssd1 deficiency, which itself causes aneuploidy sensitivity, argues against a beneficial function of aggregates. Instead, it points to a protective role for Ssd1 in handling aggregation-prone proteins. Consistent with this notion, the set of 22 proteins most prone to aggregation across W303 aneuploids is enriched for proteins encoded by Ssd1-bound transcripts identified in our study (p=0.005, hypergeometric test). Furthermore, as a group, proteins encoded by Ssd1 targets are predicted to display substantially higher fractions of disordered regions (based both on median IUPred score compared to all proteins and the fraction of residues with scores > 0.5, Mann Whitney p<2e-16); the trends remain significant even after disordered cell-wall proteins are removed from consideration. Although details of Ssd1’s function remain to be worked out, these results are consistent with a role for Ssd1 in regulating where and when mRNAs are translated to minimize aggregation and misfolding, and to enable normal cells to handle extra chromosomes.

Our results also reveal that Ssd1 affects mitochondrial physiology in aneuploid cells. Defects in mitochondrial function and cytosolic proteome management have long been linked, in neurological syndromes, aging, and even aneuploidy. Disruption of mitochondrial protein folding, import, and localization induces cytosolic protein stress and triggers cytosolic proteostasis systems (Wrobel et al., 2015; Wang and Chen, 2015; Nargund et al., 2012), consistent with our observation that CCCP induces cytosolic Hsp104 foci (Figure 6B). Conversely, clearance of misfolded cytosolic proteins relies on mitochondria: in addition to providing sufficient ATP for chaperone function, mitochondria can retain and import misfolded cytosolic proteins for sequestration and degradation (Ruan et al., 2017; Zhou et al., 2014). Extreme cytosolic misfolding, for example aggregated Huntington protein, perhaps consequently causes mitochondrial dysfunction (Franco-Iborra et al., 2018; Ocampo et al., 2010). Defects in these processes also co-occur in aneuploid syndromes, which produce altered mitochondrial morphology and function and premature aging phenotypes (Oromendia and Amon, 2014; Bambrick and Fiskum, 2008; Chang and Min, 2005). It is possible that mitochondrial defects in ssd1Δ cells arise as a secondary consequence of Ssd1 dysfunction; however, that Ssd1 binds several nuclear-encoded mitochondrial mRNAs, controls protein abundance of several of them, and purifies with mitochondria-enriched fractions raises the possibility of a more direct function. Given its role in localizing cell-wall mRNAs to the bud neck during division, Ssd1 may play a broader role in localizing and/or handling mitochondrial mRNAs – we provide evidence for one, MMR1, which showed a defect in localization patterns consistent with a defect in mitochondrial inheritance.

Our model that normal wild strains can handle the stress of extra chromosomes but exist near their buffering capacity is compatible with results from other systems. Some, but notably not all, aneuploid mouse and human cell lines show indirect signs of proteome stress, including increased autophagy and sensitivity to 17-AAG that inhibits Hsp90 chaperone (which is also required for proper chromosome segregation, confounding interpretation Chen et al., 2012). However, not all aneuploid lines display these signatures (Santaguida et al., 2015; Stingele et al., 2013; Stingele et al., 2012; Donnelly et al., 2014; Tang et al., 2011) and observed phenotypes are reportedly weaker than seen in W303 aneuploids (Brennan et al., 2019), as predicted by our study. While some phenotypic differences may result from differences in the load and identity of the aneuploid chromosome, another possibility is that proteostasis stress is not a universal feature of aneuploid cells. Rather, it may reflect a variable response influenced by environmental, developmental, or genetic differences in mitochondrial/proteostatic buffering capacity across lines. It has long been known that trisomy 21 produces phenotypes of variable severity in Down syndrome (DS), implicating genetic modifiers that augment tolerance (Antonarakis, 2017). A recent proteomic study showed that proteomes of unrelated DS skin fibroblasts showed some commonalities, including down-regulation of nuclear-encoded mitochondrial proteins, while other responses (such as altered lysosome activity) were variable across unrelated individuals and may thus contribute to variable DS severity (Liu et al., 2017). It is possible that natural genetic variation in wild yeast strains also contributes to natural variation in aneuploidy sensitivity. Interestingly, a recent large-scale genome sequencing study reported at least five truncated SSD1 alleles segregating in yeast populations (Figure 6—source data 1) (Peter et al., 2018). The power of yeast genetics provides an opportunity to identify other modifiers of aneuploidy tolerance.

Ssd1 is orthologous to human Dis3L2 (Heinicke et al., 2007), an RNA binding protein best characterized for its ability to degrade poly-uridylated RNAs targeted for decay by terminal-uridyl transferases (TUTases) (Astuti et al., 2012; Chang et al., 2013; Lubas et al., 2013; Malecki et al., 2013; Morris et al., 2013; Ustianenko et al., 2013). Ssd1 is thought to have lost its catalytic activity (perhaps concomitant with loss of TUTase enzymes from S. cerevisiae; Uesono et al., 1997; Viegas et al., 2015). Dis3L2 was first identified in the causal mapping of Perlman syndrome, characterized by cellular over-growth, and is also implicated in Wilms tumor (Astuti et al., 2012). Dis3L2 shares several features with Ssd1: both can localize to the cytosol and nucleus, both bind RNAs and interact with P-bodies, and ablation of both proteins produces protein inclusion bodies (Astuti et al., 2012; Malecki et al., 2013; Mori et al., 2018; Liu et al., 2018; Thomas et al., 2015). Dis3L2 is also implicated in apoptosis trigged by mitochondrial signals (Liu et al., 2018; Thomas et al., 2015). Remarkably, mutation of Dis3L2 is also linked to aneuploidy: knockdown of Dis3L2 actually increases chromosome instability, leading to chromosome loss and aneuploidy (Astuti et al., 2012). Dissecting its role in aneuploidy syndromes is an exciting avenue for future work.

Sequencing data for genetic mapping are available in the Short Read Archive (SRA) under access number PRJNA548343, and MULTIPOOL output files are available in Supplementary file 1. RNA and RNA Immunoprecipitation (RIP) sequencing data are available from the GEO database under accession number GSE132425, and processed data are also available in Supplementary file 2. Raw proteomic data are available in the PRIDE database (Project accession # PXD013847); processed data are available in Supplementary file 2, and normalized protein abundance data are available in Dataset 3.

1. 1. Evgenia Shishkova
   2. Joshua J Coon

   (2020) PRIDE

   ID PXD013847. Aneuploid yeast proteomes in wild-type and ssd1 strains.

   https://www.ebi.ac.uk/pride/archive/projects/PXD013847
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Author details

1. James Hose

   Center for Genomic Science Innovation, University of Wisconsin–Madison, Madison, United States

   Contribution

   Conceptualization, Formal analysis, Investigation, Methodology

   Competing interests

   No competing interests declared

2. Leah E Escalante

   1. Center for Genomic Science Innovation, University of Wisconsin–Madison, Madison, United States
   2. Laboratory of Genetics, University of Wisconsin-Madison, Madison, United States

   Contribution

   Investigation, Methodology

   Competing interests

   No competing interests declared

3. Katie J Clowers

   Laboratory of Genetics, University of Wisconsin-Madison, Madison, United States

   Present address

   Ginkgo Bioworks, Boston, United States

   Contribution

   Methodology

   Competing interests

   No competing interests declared

4. H Auguste Dutcher

   1. Center for Genomic Science Innovation, University of Wisconsin–Madison, Madison, United States
   2. Laboratory of Genetics, University of Wisconsin-Madison, Madison, United States

   Contribution

   Formal analysis, Investigation

   Competing interests

   No competing interests declared

5. DeElegant Robinson

   Center for Genomic Science Innovation, University of Wisconsin–Madison, Madison, United States

   Contribution

   Investigation, Methodology

   Competing interests

   No competing interests declared

6. Venera Bouriakov

   1. Center for Genomic Science Innovation, University of Wisconsin–Madison, Madison, United States
   2. Great Lakes Bioenergy Research Center, Madison, United States

   Contribution

   Investigation

   Competing interests

   No competing interests declared

7. Joshua J Coon

   1. Center for Genomic Science Innovation, University of Wisconsin–Madison, Madison, United States
   2. Great Lakes Bioenergy Research Center, Madison, United States
   3. Department of Biomolecular Chemistry, University of Wisconsin–Madison, Madison, United States
   4. Department of Chemistry, University of Wisconsin–Madison, Madison, United States
   5. Morgridge Institute for Research, Madison, United States

   Contribution

   Supervision

   Competing interests

   No competing interests declared

8. Evgenia Shishkova

   1. Center for Genomic Science Innovation, University of Wisconsin–Madison, Madison, United States
   2. Morgridge Institute for Research, Madison, United States

   Contribution

   Methodology

   Competing interests

   No competing interests declared

9. Audrey P Gasch

   1. Center for Genomic Science Innovation, University of Wisconsin–Madison, Madison, United States
   2. Laboratory of Genetics, University of Wisconsin-Madison, Madison, United States
   3. Great Lakes Bioenergy Research Center, Madison, United States

   Contribution

   Conceptualization, Formal analysis, Supervision, Funding acquisition, Project administration

   For correspondence

   agasch@wisc.edu

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-8182-257X

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