BACKGROUND:Cell therapies based on human pluripotent stem cells (hPSCs) are in clinical trials with the aim of restoring vision in people with age-related macular degeneration. The final cell therapy product consists of retinal pigment epithelium (RPE) cells differentiated from hPSCs. However, hPSCs recurrently acquire genetic abnormalities that give them an advantage in culture with unknown effects to the clinically-relevant cell progeny. One of the most common genetic abnormalities in hPSCs is the sub-karyotype 20q11.21 copy number variant, known to carry oncogenes. Understanding the impact of this variant on RPE differentiation and its potential for malignant transformation is crucial for the development of safe and effective cell therapies. METHODS:We monitored the RPE differentiation efficiency of hPSCs with or without the 20q11.21 variant. We then phenotyped the purified RPE cells for functionality, purity and tumorigenicity potential. RESULTS:We observed that 20q11.21 clones exhibited an enhanced differentiation capacity, developing pigmented foci at a higher rate and yield compared to normal clones. Gene expression analysis confirmed the upregulation of key RPE markers in 20q11.21 clones. The enhanced differentiation capacity of 20q11.21 clones was found to be dependent on the activity of BCL-XL, located within the amplicon. Furthermore, we demonstrated that 20q11.21-containing RPE cells displayed a mature phenotype, maintained long-term stability, and exhibited no malignant transformation capacity in vitro. CONCLUSION:We demonstrated that gain of 20q11.21 enhances the speed and yield of RPE differentiation without compromising the phenotype of the derivatives. Finally, we discovered that 20q11.21-localised BCL-XL is important for RPE differentiation with potential non-canonical roles in retinal biology.

Culture-acquired variants in human pluripotent stem cells (hPSCs) hinder their applications in research and clinic. However, the mechanisms that underpin selection of variants remain unclear. Here, through analysis of comprehensive karyotyping datasets from over 23,000 hPSC cultures of more than 1,500 lines, we explored how culture conditions shape variant selection. Strikingly, we identified an association of chromosome 1q gains with feeder-free cultures and noted a rise in its prevalence in recent years, coinciding with increased usage of feeder-free regimens. Competition experiments of multiple isogenic lines with and without a chromosome 1q gain confirmed that 1q variants have an advantage in feeder-free (E8/vitronectin), but not feeder-based, culture. Mechanistically, we show that overexpression of MDM4, located on chromosome 1q, drives variants' advantage in E8/vitronectin by alleviating genome damage-induced apoptosis, which is lower in feeder-based conditions. Our study explains condition-dependent patterns of hPSC aberrations and offers insights into the mechanisms of variant selection.

Human pluripotent stem cell (hPSC)-derived cardiomyocytes have emerged as powerful tools for disease modelling and cell therapy. The production of cardiomyocytes from hPSCs typically requires expanding large numbers of hPSCs and maintaining them in culture for extended periods of time. This in turn predisposes hPSCs to the acquisition of non-random genetic changes, including recurrent gains of chromosome 1q. Here, we show that gain of chromosome 1q in hPSCs affects both the efficiency of differentiation to cardiomyocytes and phenotype of the differentiated cells. Mechanistically, we show that aberrant activation of the Wnt signalling pathway underpins the skewed differentiation of variant 1q hPSCs. Collectively, our data demonstrates that the presence of genetically variant cells in cultures is a significant concern for production of hPSC-derived cardiomyocytes for research or clinical applications. Further, our results suggest new approaches for removing genetically variant cells for future clinical applications.### Competing Interest StatementThe authors have declared no competing interest.

Summary Since the first derivation of human pluripotent stem cells (hPSCs), the number of culture conditions has steadily increased, making hPSC culture more facile. Nonetheless, there remains the persistent issue of culture-acquired genetic changes, hampering the reproducibility of hPSC research and jeopardising their clinical use. Here, we utilised comprehensive karyotyping datasets from over 20,000 hPSC cultures sampled under different conditions to ascertain association of genetic changes with specific culture regimens. We found condition-dependent patterns of aberrations, with higher prevalence of chromosome 1q gains in recent years, associated with increased use of contemporary, feeder-free cultures. Mechanistically, we show the context-dependent selection of 1q variants is mainly driven by MDM4 , a gene amplified in many cancers, located on chromosome 1q. To facilitate reproducibility of hPSC research and their safe clinical utility, we provide a unique hPSC karyotype resource for informing the risk assessment of genetic aberrations and developing strategies to suppress their occurrence.

Chromosome 20 abnormalities are some of the most frequent genomic changes acquired by human pluripotent stem cell (hPSC) cultures worldwide. Yet their effects on differentiation remain largely unexplored. We investigated a recurrent abnormality also found on amniocentesis, the isochromosome 20q (iso20q), during a clinical retinal pigment epithelium differentiation. Here we show that the iso20q abnormality interrupts spontaneous embryonic lineage specification. Isogenic lines revealed that under conditions that promote the spontaneous differentiation of wild-type hPSCs, the iso20q variants fail to differentiate into primitive germ layers and to downregulate pluripotency networks, resulting in apoptosis. Instead, iso20q cells are highly biased for extra-embryonic/amnion differentiation following inhibition of DNMT3B methylation or BMP2 treatment. Finally, directed differentiation protocols can overcome the iso20q block. Our findings reveal in iso20q a chromosomal abnormality that impairs the developmental competency of hPSCs toward germ layers but not amnion, which models embryonic developmental bottlenecks in the presence of aberrations.

Human pluripotent stem cells (PSCs) are subject to the appearance of recurrent genetic variants on prolonged culture. We have now found that, compared with isogenic differentiated cells, PSCs exhibit evidence of considerably more DNA damage during the S phase of the cell cycle, apparently as a consequence of DNA replication stress marked by slower progression of DNA replication, activation of latent origins of replication, and collapse of replication forks. As in many cancers, which, like PSCs, exhibit a shortened G1 phase and DNA replication stress, the resulting DNA damage may underlie the higher incidence of abnormal and abortive mitoses in PSCs, resulting in chromosomal non-dysjunction or cell death. However, we have found that the extent of DNA replication stress, DNA damage, and consequent aberrant mitoses can be substantially reduced by culturing PSCs in the presence of exogenous nucleosides, resulting in improved survival, clonogenicity, and population growth.

Human pluripotent stem cells (hPSCs) are prone to acquiring genetic changes upon prolonged culture. Particularly common are copy number changes, including gains of chromosomes 1q, 12p, 17q, and 20q, and/or loss of chromosomes 10p and 18q. The variant cells harboring common genetic changes display altered behaviors compared to their diploid counterparts, thus potentially impacting upon the validity of experimental results and safety of hPSC-derived cellular therapies. Hence, a critical quality attribute in hPSC maintenance should include frequent monitoring for genetic changes arising in cultures. This in turn places large demands on the genotyping assays for detection of genetic changes. Traditional methods for screening cells entail specialized cytogenetic analyses, but their high costs and a lengthy turnaround time make them impractical for high-throughput analyses and routine laboratory use. Here, we detail a protocol for a rapid, accessible, and affordable PCR-based method for detection of frequently occurring copy number changes in hPSCs. © 2019 by John Wiley & Sons, Inc.