Genomic Stability of Induced Pluripotent Stem Cells During Repeated Cell Expansion

Abstract

Induced pluripotent stem cells offer a renewable source of patient-specific cells for disease modeling, drug development, and regenerative medicine. However, repeated in vitro expansion can impose selective pressure that alters genomic integrity, epigenetic state, cellular fitness, and differentiation potential. Clinically relevant abnormalities include chromosomal aneuploidy, copy-number changes, point mutations, structural variants, mitochondrial alterations, and clonal selection of cells with growth advantages. These changes may emerge gradually, remain below the detection threshold of routine karyotyping, or become enriched during prolonged culture and manufacturing. This review summarizes the biological mechanisms, risk factors, clinical implications, diagnostic approaches, management strategies, emerging technologies, and quality recommendations relevant to genomic stability during induced pluripotent stem cell expansion. A risk-based testing framework incorporating morphology, growth monitoring, cytogenetic analysis, genomic sequencing, identity testing, and functional assessment is essential before clinical translation.

Introduction

Induced pluripotent stem cells are generated by reprogramming differentiated somatic cells into a pluripotent state. Their ability to self-renew and differentiate into multiple lineages has created major opportunities in precision medicine, transplantation research, toxicology, and developmental biology. Nevertheless, pluripotency does not guarantee genomic normality. Reprogramming may reveal pre-existing somatic variants, while prolonged culture can favor subclones that proliferate more rapidly than genetically intact cells.

Repeated expansion is often necessary to obtain sufficient material for experiments or manufacturing, but every passage represents an opportunity for replication stress, DNA damage, mitotic error, oxidative injury, and selection. Genomic stability must therefore be considered a dynamic quality attribute rather than a one-time characteristic established at the beginning of a cell line's history. For healthcare professionals, the central concern is whether culture-associated abnormalities could compromise safety, alter therapeutic efficacy, or invalidate disease-modeling conclusions.

Epidemiology / Disease Burden

There is no single prevalence estimate for genomic abnormalities in expanded induced pluripotent stem cell cultures because reported frequencies vary according to donor characteristics, reprogramming method, passage number, culture conditions, assay sensitivity, and the definition of abnormality. Large-scale genomic analyses have consistently shown that apparently normal lines may contain low-frequency mutations or copy-number alterations that are missed by conventional cytogenetics. Recurrent abnormalities involving chromosomes or genomic regions associated with cellular growth have been reported across different laboratories and tissue sources.

The burden is primarily a translational and clinical safety burden rather than a conventional disease burden. Abnormal clones can distort phenotypic assays, create misleading disease models, affect differentiation efficiency, and potentially increase tumorigenic risk after transplantation. In research settings, unrecognized clonal evolution can reduce reproducibility between laboratories. In clinical manufacturing, it may affect product release, comparability, and regulatory acceptability.

Pathophysiology

Genomic instability during repeated expansion results from the interaction of intrinsic cellular stress and culture-driven selection. Reprogramming involves extensive changes in chromatin organization, DNA methylation, replication programs, and metabolic state. These transitions can expose replication vulnerabilities and permit survival of cells carrying pre-existing variants. Once pluripotent cells are established, high proliferative activity increases the number of DNA replication and mitotic events, thereby increasing opportunities for mutation and chromosome mis-segregation.

Culture conditions can intensify these processes. Excessive confluence, variable cell density, enzymatic dissociation, hypoxia, nutrient limitation, oxidative stress, and inappropriate substrate composition may impair DNA repair or increase apoptosis. Conversely, cells with alterations affecting apoptosis, cell-cycle control, or stress responses may gain a selective advantage. Copy-number changes involving regions that support self-renewal can therefore become enriched over time. Mitochondrial heteroplasmy may also shift during expansion, with consequences for bioenergetics and lineage specification. Importantly, a normal karyotype does not exclude single-nucleotide variants, small insertions or deletions, mosaic abnormalities, or epigenetic drift.

Risk Factors

Important risk factors include prolonged culture duration, high passage number, repeated thawing and refreezing, frequent single-cell dissociation, poor-quality starting material, and inadequate control of cell density. Donor age and the genetic background of the source tissue may influence the number of pre-existing somatic variants. Reprogramming technique, vector integration, incomplete reprogramming, and exposure to selective small molecules can also affect genomic behavior.

Laboratory-specific risks include inconsistent media preparation, fluctuating incubation conditions, contamination, operator-dependent handling, and the use of poorly characterized feeder cells or extracellular matrices. Expansion from a very small founder population may amplify a rare abnormal clone through a bottleneck effect. Repeated subcloning can further reduce genetic diversity and unintentionally select cells with unusual growth kinetics. These risks support the use of standardized master and working cell banks rather than indefinite serial passage.

Clinical Features

Genomic instability usually has no specific visible clinical or culture phenotype. Affected cultures may retain typical pluripotent morphology and express established markers while showing altered proliferation, differentiation bias, stress tolerance, or clonal dominance. Some abnormalities are associated with increased growth, resistance to apoptosis, or incomplete maturation. Others may remain functionally silent under routine laboratory conditions but become important after directed differentiation or transplantation.

In a therapeutic context, potential consequences include reduced product consistency, unexpected differentiation, impaired tissue function, immune or inflammatory effects, and an increased theoretical risk of residual proliferative cells or tumor formation. The clinical significance depends on the abnormality, its variant allele fraction, the intended cell type, the route of administration, the extent of purification, and the duration of patient follow-up.

Diagnosis

Assessment should combine orthogonal methods. Morphology, growth curves, viability, sterility, mycoplasma testing, and pluripotency marker analysis provide essential baseline information but are insufficient for genomic risk assessment. G-banded karyotyping remains useful for detecting large chromosomal abnormalities, although its resolution is limited and it may miss mosaicism below approximately the laboratory-specific detection threshold.

Chromosomal microarray analysis and single-nucleotide polymorphism arrays improve detection of copy-number changes and loss of heterozygosity. Fluorescence in situ hybridization can target suspected abnormalities but is not a comprehensive screening method. Next-generation sequencing, including targeted panels, whole-exome sequencing, or whole-genome sequencing, can identify coding and noncoding variants, depending on assay design and validation. Low-frequency mosaic variants require adequate read depth, validated bioinformatic pipelines, and comparison with the original donor sample when available. Long-read sequencing and single-cell genomic approaches are emerging tools for resolving structural variants and clonal architecture. Testing should also include cell-line identity, donor matching, and functional differentiation assays.

Treatment & Management

Management begins with prevention and early detection. Laboratories should establish passage limits, define acceptable growth and morphology criteria, maintain detailed records, and minimize unnecessary expansion. Master cell banks should be created after comprehensive characterization, with working banks used for routine production. Cells should be expanded under controlled conditions that reduce confluence-associated stress, avoid excessive manipulation, and support consistent recovery after thawing.

When a genomic abnormality is detected, the result should be confirmed using an independent method and assessed for clonality, abundance, stability, and functional relevance. Discarding the affected culture is generally appropriate when the abnormality is recurrent, expanding, biologically plausible, or associated with a growth advantage. In selected research applications, a culture may be retained only after documented risk assessment and transparent reporting. Re-derivation from an earlier bank or a new donor sample is preferable to attempting to eliminate a dominant abnormal clone through unvalidated selection methods. Before clinical use, release decisions should integrate genomic, phenotypic, microbiological, identity, potency, and tumorigenicity-related data.

Recent Advances / Emerging Therapies

Technological advances are improving the detection and interpretation of culture-associated genomic change. Single-cell sequencing can distinguish multiple subclones that are obscured in bulk analysis, while longitudinal sampling can identify the rate at which variants expand. Automated imaging and machine-learning systems may assist in detecting subtle changes in morphology, growth, and differentiation, although such tools require rigorous validation. Multi-omics approaches combining genome, transcriptome, epigenome, and mitochondrial data may better define whether a variant has functional consequences.

Improved reprogramming systems, integration-free methods, optimized extracellular matrices, chemically defined media, and controlled oxygen conditions may reduce stress and variability. Genome editing can correct disease-causing variants, but editing itself introduces risks involving off-target changes, chromosomal abnormalities, and clonal selection. Consequently, edited and unedited lines require comparable or more extensive genomic surveillance. Future manufacturing platforms may use real-time release testing and computational models to predict clonal evolution before a product reaches the clinic.

Guideline Recommendations

Guideline-based quality programs should define genomic stability as a critical quality attribute and specify testing at reprogramming, banking, representative expansion points, and final product manufacture. No single assay is adequate for all applications. At minimum, programs should include authenticated cell identity, sterility and mycoplasma testing, pluripotency confirmation, cytogenetic evaluation, and an appropriately sensitive genomic assay. High-risk clinical applications warrant broader sequencing, validated thresholds for mosaic abnormalities, and documented interpretation by qualified specialists.

Testing frequency should be risk based and linked to passage number, culture duration, manufacturing scale, and intended clinical use. Laboratories should preserve early-passage reference material, use validated standard operating procedures, investigate unexpected growth or differentiation changes, and report deviations transparently. Regulatory submissions should describe donor history, reprogramming method, culture conditions, passage history, assay limitations, acceptance criteria, and the rationale for concluding that observed abnormalities do not compromise safety or efficacy.

Conclusion

Repeated expansion of induced pluripotent stem cells can promote genomic drift and selection of abnormal subclones, even when cultures retain normal morphology and pluripotency markers. The principal safeguards are controlled culture conditions, limited and documented passage history, early creation of master banks, longitudinal surveillance, and complementary genomic and functional testing. Karyotyping alone is inadequate for detecting the full spectrum of relevant abnormalities. A scientifically justified, risk-based quality strategy is essential to preserve reproducibility in research and to support the safety, consistency, and regulatory credibility of cell-based clinical products.