Figure 1 shows the overall setup of this study. All experiments were carried out on the hESC lines VUB03 and VUB1926,27. In vitro culture led to three independent events of gain in chromosome 1. VUB19 and one subline of VUB03 acquired a gain of the entire q arm of chromosome 1 (termed VUB191q21.1qter and VUB031q21.1qter), another subline of VUB03 gained a smaller region spanning 3.3 Mb in 1q32.1 (VUB031q32.1).
hESC 1q mis-specify to placode and non-neural ectoderm during neuroectoderm differentiation and differentiate to immature hepatoblasts and cardiac progenitors than hESCwt
We investigated the impact of the gain of 1q on trilineage differentiation by subjecting hESCwt and hESC1q to neuroectoderm (NE), hepatoblast (HEP) and cardiac progenitor (CP) differentiation (VUB031q21.1qter, VUB191q21.1qter, VUB031q32.1, VUB03wt and VUB19wt, all lines differentiated at least in triplicate, Fig. 1). We measured the expression of six lineage-specific markers and of NANOG and POUF51 to evaluate differentiation efficiency and to test for residual undifferentiated cells (Fig. 2A and 2C and Figure S1). All differentiated cells had almost undetectable levels of POU5F1 and NANOG mRNA, and no POU5F1-positive cells appeared in the immunostainings (Fig. 2A), indicating that all cells exited the undifferentiated state (Fig. 2C and Figure S1).
NE1q showed significantly lower mRNA levels of PAX6 and SOX1 compared to the levels in NEwt (Fig. 2C pPAX6=0.0323 and pSOX1=0.0002, unpaired t-test, N = 15), indicating a decreased neuroectodermal differentiation efficiency. In line with this, immunostaining showed a lower percentage of PAX6-positive cells in differentiated hESC1q than in their isogenic hESCwt counterparts, but without reaching statistical significance (Fig. 2B, NE1q = 32.15%, NEwt=61.24%, p = 0.0974, unpaired t-test, N = 3). For the HEP differentiation, we found no significant difference between HEPwt and HEP1q in the mRNA expression levels of HNF4A while AFP had a significantly lower expression in HEP1q (Fig. 2C, pHNF4A=0.2247, pAFP=0.0142, unpaired t-test, N = 9) and immunostaining for HNF4A showed similar percentages of HNF4A positive cells in both groups (Fig. 2B, HEP1q = 41.60%, HEPwt=29.79%, p = 0.4514, unpaired t-test, N = 3). Similarly, the mRNA levels of the CP marker GATA4 was not significantly different between CPwt and CP1q while NKX2-5 was significantly lower in CP1q (Fig. 2C, pGATA4=0.1537, pNKX2.5=0.0398, unpaired t-test, N = 18), which was confirmed by the immunostaining for GATA4 (Fig. 2B, CP1q = 29.56%, CPwt=22.74%, p = 0.4390, unpaired t-test).
Taken together, these results show that while hESC1q show a decreased differentiation efficiency into neuroectoderm, they do not remain undifferentiated, suggesting that part of the cells miss-specify to alternative cell fates. For the mesendoderm lineages, hESC1q appear to commit equally efficiently to this germ layer than their genetically balanced isogenic counterpart, as reflected by the similar expression for the early markers of endoderm and mesoderm induction, while showing a decreased expression of the markers for further cell-type commitment.
To further characterize the differentiated cells, we carried out bulk mRNA sequencing of 43 samples: 15 samples of neuroectoderm (NEwt=6, NE1q = 9, 14 of cardiac progenitors (CPwt=6, CP1q = 8) and 14 of hepatoblasts (HEPwt=6, HEP1q = 8). Differential gene expression analysis showed that NE1q differentially expressed 1603 genes as compared to NEwt, while HEP1q and CP1q differentially expressed 189 and 241 genes respectively as compared to HEPwt and CPwt (Figure S2, |log2fold-change|>1.0 and false discovery rate (FDR) < 0.05). We surmised that this significant difference in the number of differentially expressed genes was caused by miss-specification of hESC1q to neuroectoderm and were rather yielding a mixed cell population, which did not occur in the mesendoderm lineages.
To determine the alternate cell fate acquired by hESC1q upon neuroectoderm differentiation, we carried out differential gene expression analysis of the NE1q and the NEwt relative to bulk RNA sequencing data of undifferentiated hESC (N = 38 samples, archived lab data). First, we tested the expression of neuroectoderm markers, which we found to be less induced in both NE1q than in NEwt (Fig. 2D). We then queried the data for the expression of different sets of markers for embryonic and extra-embryonic lineages that appear in early human development. Table S1 shows the lists of gene sets we curated from published data28–44. We found NE1q showed high levels of expression of markers of non-central nervous system (non-CNS) ectodermal lineages such as non-neural ectoderm and of cranial placode (Fig. 2E and Fig. 2F, Figure S3). To confirm this, we double-stained NEwt and NE1q for the non-CNS marker TFAP2A39 and the NE marker PAX6 and found that while hESCwt efficiently differentiate to a homogeneous neuroectoderm cell-population, hESC1q produce a mix of neuroectoderm and non-CNS cells (Fig. 2G).
Next, we used this same approach to study the cell types obtained from the HEP and CP samples. While hESC1q equally induce some of the early HEP and CP differentiation markers as hESCwt, 68.6% (24/35) and 63.2% (12/19) of HEP and CP lineage-specific markers have a lower expression in the mutant cells (Fig. 2H and Fig. 2K). HNF4 and GATA4 stainings (Fig. 1A, Figure S1) and RNA sequencing data (Figure S4) did not suggest miss-specification in hESC1q. While differentiating to HEP and CP, we analysed the expression of markers specific to different stages of the differentiation of these cell types.
Human ESC-derived HEP have a profile between hepatoblast stage 2 and fetal hepatocyte 2 (Fig. 2D), and cells with a gain of 1q show overall lower expression of 64% of markers (16/25). Gene-set enrichment analysis of the differential gene expression analysis between HEPwt and HEP1q showed that 145 of the 162 significantly enriched gene-sets in the canonical pathways gene set had negative enrichment scores and that they were frequently related to processes key to hepatocyte and liver function, including cholesterol metabolism, ferroptosis, transsulfuration, plasma lipoprotein remodelling, folate metabolism, selenium micronutrient network and metabolism of steroids (FDR < 0.05, Table S2). Gene ontology enrichment analysis showed that 1574 of the 1913 gene sets had negative enrichment scores, including amino acid metabolic process, fatty acid metabolic process and steroid metabolic process (Fig. 2J, Table S2).
In the case of CP, CP1q and CPwt equally express a profile between cardiac progenitors and cardiomyocytes, where CP1q have lower expression of genes marking the later stages of differentiation (Fig. 2L). Gene set enrichment analysis of the CP samples showed that CAR1q have negative enrichment scores for 157 of the 178 significantly enriched canonical pathway gene-sets, including sets related to dilated cardiomyopathy, folding of actin, striated muscle contraction and hypertrophic cardiomyopathy (Table S2). These genes are key to correct heart contraction functions. Gene ontology enrichment analysis identified negative enrichment scores for 1146 of the 1786 significantly enriched set, including terms, such as l band, Z disc, sarcomere, myofibril, contractile fiber, muscle system process and muscle development (Fig. 2M, Table S2).
Human ESC 1q have an MDM4-driven competitive advantage that is retained during differentiation
Human ESC1q have a well-established competitive advantage over their genetically balanced counterparts in the undifferentiated state9,11,14. We next aimed at determining whether the cells retain this selective advantage during differentiation, and which gene is driving their winning phenotype.
We first looked at the 53 commonly deregulated genes in our HEP, CP and NE samples, and found that MDM4, a regulator of p53 activity that is located in the common region of gain and which has been previously suggested as a key gene for the gain of 1q11, is consistently upregulated in all samples (Fig. 3A). Gene set enrichment analysis of the differentially expressed genes in NE, HEP and CP from 1q and wt cells for the Reactome pathways related to p53 signaling indeed shows that the transcriptional regulation by p53, the regulation of p53 activity, the G1/S damage checkpoint and the p53 dependent responses to DNA-damage in G1 and S are all significantly negatively enriched in cells carrying a gain of 1q (Fig. 3B, Table S2, Table S3). This led us to the hypothesis that the higher expression of MDM4 in cells with a gain of 1q results in the inhibition of p53-mediated transcriptional activation upon DNA damage, leading to a decreased induction of apoptosis upon DNA damage, thus providing a competitive advantage to the mutant cells (Fig. 3C).
To determine if cells with a gain of 1q retained their ability to take over the culture during differentiation, we carried out competition assays during NE, HEP and CP induction. For this, 10% of hESC1q stably expressing a fluorescent protein was introduced into a unlabeled hESCwt culture. Differentiation was initiated the next day, and was controlled by immunostaining for PAX6, HNF4A and GATA4 (Fig. 3D). To measure culture-take over, the proportion of hESC1q was determined by flow cytometry at the onset and at the end of differentiation.
We found that in all three differentiations, the cells with a gain of 1q outcompeted wild type cells (Fig. 3E). During the NE induction, the proportion of 1q cells increased in average 33.9%±2.7% during the 8-day differentiation, from a mean of 10.9% at the onset to 44.9% at day 8 (p < 0.0001, unpaired t-test, N = 18–21). This was similar for the 8-day differentiation to HEP, where the 1q cells increased in average 49.3%±4.9% (mean at day 0 = 11.6%, mean at day 8 = 60.9%, p < 0.0001, unpaired t-test, N = 9). This increase was less pronounced during the 5-day CP differentiation, with an average 22.7%±3.2% increase (mean at day 0 = 10.9%, mean at day 5 = 33.5%, p < 0.0001, unpaired t-test, N = 18–21), which may be attributable to the shorter time span of the CP differentiation as compared to NE and HEP.
Next, we tested the role of MDM4 in the competitive advantage of cells with a 1q. For the undifferentiated cells, we downregulated MDM4 for 24h by siRNA in hESC1q on the day before to start of the competition assay. The cells were imaged daily, and counted at the start and end of differentiation (Fig. 3F). On average, hESC1q increased from 14.0 to 33.7% in three days in the untreated condition, while no increase was observed after siRNA treatment (N = 3, p1q = 0.0028, psi1q=0.7905, 2-way ANOVA, Fig. 3G). We also carried out the same experiment on a hESC line carrying the recurrent gain of 20q11.21, which provides the cells with a Bcl-xL-mediated decreased sensitivity to apoptosis16,17. Downregulation of MDM4 did not affect the growth advantage of the hESC20q11.21 cells (Fig. 3H and 3I), showing that the suppression of the competitive advantage by modulating the MDM4 expression is specific to cells with a gain of 1q and not due to a general decrease in cellular fitness.
For the competition assays during differentiation, the hESC1q were treated for 24h with the siRNA and mixed at a 1:9 ratio, and differentiation was initiated the next day. Differentiation was shortened to 4 days, as this was the time previous work in the lab showed that a single siRNA transfection could reliably sustain a gene’s downregulation22. Figure 3J shows the effect of the siRNA on the expression of MDM4 and of downstream targets of the p53 signaling pathway in the three hESC1q.In the untreated conditions of the competition assays, the fraction of cells with a gain of 1q became significantly larger in all three differentiations (Fig. 3K and 3L). The increase was most pronounced after NE induction, with an average increase of 30.0% (p1q = 0.0053, 2-way ANOVA). In HEP and CP, the mean increases were 11.3% (p1q = 0.0131, 2-way ANOVA) and 11.67 (p1q = 0.0149, 2-way ANOVA), respectively. Conversely, in the siRNA-treated competition assays, the fraction of 1q cells remained unchanged (Fig. 3K and 3L). Taken together, these results show that reducing the levels of MDM4 in the mutant cells abolishes their competitive advantage both in the undifferentiated state and during differentiation.
Higher MDM4 expression in hESC1q results in a decreased sensitivity to DNA-damage induced apoptosis
Next, we sought to elucidate by which mechanisms higher expression of MDM4 confers the competitive advantage to the cells. We first tested the hypothesis that the higher expression of MDM4 by cells carrying a gain of 1q leads to a decreased p53-mediated apoptosis in response to DNA damage. For this, we induced DNA damage in hESC1q and hESCwt using bleomycin and carried out a time-course measurement of apoptosis and cell death.
Figure 4A shows the percentages of live and of apoptotic and dead cells for hESCwt (N = 3), hESC1q (N = 3) and hESC1q treated with siRNA against MDM4 (N = 3), at the start of bleomycin treatment and at the subsequent 2, 4 and 6-hour time-points. hESCwt start undergoing apoptosis at 2h after exposure to bleomycin, followed by a rapid decrease in the numbers of live cells. In contrast, apoptotic cells start appearing in hESC1q as from 4h of exposure and reach 41.9% of apoptotic cells at 6h, as compared to 78.6% in hESCwt (unpaired t-test, p = 0.0169). Treating hESC1q with siMDM4 significantly increases their sensitivity to DNA damage, with apoptosis initiating at 2h, and reaching 49.4% at 4h. siMDM4-treated hESC1q cells do not reach same levels of dead cells at 6h as in hESCwt cells, although the differences are not statistically significant (54.9% vs 78.6%, unpaired t-test, p = 0.1530, Fig. 4A). This may be explained by an incomplete transfection of the cells, or to overall insufficiently stable downregulation of MDM4 to the levels of hESCwt.
This raised the question of how a decrease of sensitivity to DNA-damage could provide a competitive advantage to hESC and differentiating cells. We observed during daily monitoring of the competition assays that hESC1q started outcompeting their genetically balanced counterparts once the cultures became confluent. Previous work has indicated that hPSC are prone to replication stress and DNA damage45,46, which can be mitigated by addition of nucleosides to the medium47 and exacerbated in higher cell density culture due to medium acidification48,49. We therefore hypothesized that higher culture cell density generates the conditions for a strong competitive advantage of 1q gains, by increasing the levels of DNA damage.
We studied the cell proliferation dynamics of hESCwt and hESC1q over time in culture in pure form and mixed at a 1:9 ratio (N = 3 for each condition). Daily cell numbers count showed that the numbers of hESCwt and hESC1q increased similarly until day 7, suggesting that they have similar cell doubling times. As from day 8, hESC1q continue proliferating even when they have reached a very high density (Fig. 4B). Daily analysis of the ratio between hESCwt and hESC1q showed no statistically significant changes until day 3, after which the proportion of hESC1q started steadily increasing (Fig. 4C, N = 3). This coincided with culture dishes at day 3 still showing empty areas, whereas at day 4 the cells had reached confluence, suggesting that this is a flipping point for 1q gains to start providing a competitive advantage (Fig. 4D). To investigate the relationship between cell culture density and DNA damage, we measured DNA damage by γH2AX staining in hESC1q and hESCwt, in low and high cell density cultures (mean of 7,547 cells/cm2 and 200,050cm/cm2 respectively, representative images are shown in Fig. 4E and cell counts shown in Fig. 4F), and found that cells grown at low density had significantly lower numbers of γH2AX foci than when grown in high density (Fig. 4G,wtlow:11.38, wthigh:17.95, 1qlow:13.71, 1qhigh:26.22, p < 0.0001, one-way ANOVA). While there were no differences in DNA-damage between in hESC1q and hESCwt grown at low density (Fig. 4G, 1.2-fold change, p = 0.2314, one-way ANOVA), at high density, hESC1q showed higher γH2AX foci counts than hESCwt (Fig. 4G, 1.5-fold change, p < 0.0001, one-way ANOVA). Lastly, we counted the numbers of cells in the cell cultures until day 5 and calculated the densities. The mean densities in days 1 and 2 are in the range of the ‘low density’ group in the DNA damage staining, with a mean of 33,711 cells/cm2 at day 1 and 87,854 cells/cm2 at day 2. From day 3, the densities are in the range and above of the ‘high density’ group, with 265,224cells/cm2 at day 3, 331,680 cells/cm2 at day 4 and 351,475 cells/cm2 at day 5. This indicates that at this point, the cells start undergoing DNA damage more frequently, reaching the condition when a decreased p53-mediated induction of apoptosis starts providing a selective advantage, so cells with a gain of 1q will start outcompeting their genetically balanced counterparts.