Genetic stability in human pluripotent stem cells

Pluripotent stem cells (SC) possess the capacity of self-renewal and can differentiate into different cell types of the adult body. These unique characteristics have made SCs not only an attractive potential therapeutic tool for numerous human diseases, but also a research tool for studying early human development and an interesting cellular model for human disorders, particularly for those for which there is no animal model available.

Pluripotent SCs are immortal cell lines and are often kept in culture for long periods of time without this apparently affecting their pluripotent capacity. Nevertheless, it is by now well known these cells present different forms of genetic instability, most probably as part of their adaptation to the culture conditions.

The best characterized have been human embryonic SC (hESC), in which many different laboratories have found gross chromosomal abnormalities to appear in culture. Several studies have now revealed smaller chromosomal aberrations, the size depending on the resolution of the method of analysis used. These aberrations are predominantly chromosomal amplifications, often involving known stemness genes, and seem to be rather the norm than the exception. Furthermore, it has been shown that these aberrations clearly affect the expression of the genes involved in the abnormality.

The genetic instability of hESC is not limited to chromosomal aberrations, but extends to epigenetic changes and mitochondrial mutations. HESC undergo hypomethylation of oncogene promotors, and hypermethylation of tumour suppressor genes, although imprinted genes seem to maintain their normal expression patterns. Another epigenetic trait that has been found to change is the X inactivation status. While some lines do not present any X inactivation in their undifferentiated state, and undergo inactivation upon differentiation, other lines show X inactivation from early passages onwards.

Resequencing of the mitochondrial genome in several hESC lines has shown that the lines acquired mutations that stably took over the culture. This type of mutation is challenging to explain, as the mitochondrial genome is present in many copies in each mitochondrion, of which each cell contains hundreds.

All these abnormalities share the fact that the cells carrying them are better adapted to the culture, and are able to take over the entire culture by starting from one mutated cell. This opens interesting lines of research on how these mutations occur and how we can prevent this occurrence, and how they change the biology of the cell to enable it to thrive much better than the non mutated cells.

Considering all the published findings, it is quite likely that hESC present other forms of instability that have not yet been detected. This raises the question of what is normal for a hESC, and where to set the threshold to consider these cells safe for therapeutic use, or as valid models in research.