Why are imprints unstable in pluripotent stem cells?

Pluripotent stem cells (PSCs) possess the remarkable ability to self-renew and differentiate into nearly any cell type, making them invaluable for both research and therapeutic applications. Despite these powerful attributes, PSCs are vulnerable to genetic and epigenetic instabilities that can undermine their reliability and safety. While genetic abnormalities can be routinely monitored with established guidelines, epigenetic instabilities often go unchecked. Among the most recurrent epigenetic defects in PSCs are errors in genomic imprinting - a process that governs parent-of-origin-specific monoallelic expression of certain genes through differential marking of the two parental alleles by DNA methylation. When disrupted, it becomes a source of a dozen developmental conditions known as imprinting diseases. In PSCs, once imprinting errors arise, they remain throughout cellular differentiation, casting uncertainty over the use of PSC-derived cells for disease modelling and regenerative medicine. In this review, we provide an overview of imprinting defects in both mouse and human PSCs, delving into their origins and consequences. We also discuss potential correction strategies that aim to enhance imprinting stability, ultimately paving the way for safer, more reliable PSC use in research and clinical applications.