Cytogenetic Instability of Stem Cells

DIVIDING CELLS ARE subject to errors during cell division that can result in abnormal chromosome patterns. Cytogenetics, which involves the study of abnormal chromosomes, has shown that human and mouse embryonic stem (ES) cells from laboratories throughout the world tend to show the same chromosome aberrations. The most frequent change in human ES cells involves gain of chromosomes 12 or 17, both of which are associated with cancer, whereas mouse embryonic stem cells tend to acquire extra copies of chromosomes 8 or 11. There is no way to distinguish embryonic stem cells with abnormal chromosomes from normal stem cells without genetic testing, as both express the same proteins and typical stem cell markers, and the presence of chromosome changes does not affect the ability of these cells to give rise to different cell lineages.

Although accidents in division leading to extra or missing chromosomes occur in dividing cells of all tissues and species, most of these lead to cell death. However, when a specific chromosome change results in the affected cells having a growth advantage, cells with this change tend to increase and completely replace the normal cells in about 10 passages. Therefore, it is not surprising that the most common chromosome changes seen in human and mouse ES cells involve acquisition of those extra chromosomes that are associated with cancer when they occur in the body. For example, the presence of extra copies of chromosome 12, and often of chromosome 17, is characteristic of spontaneously developing germ cell tumors of the testis in human males.

In both cultured ES cells as well as germ cell tumors, the presence of extra chromosomes 12 and 17 tends to increase proliferation and mitotic instability, leading to acquisition of other chromosome changes. Because the most common secondary changes involve acquisition of extra copies of chromosome 20 and the X, these must also confer some growth advantage, as is shown by the fact that in rare cases human ES cell cultures have acquired an extra X or chromosome 20 as the sole abnormality. In mouse ES cells, loss of Y is a recurrent change, though not so frequent as acquisition of an extra chromosome 8.

Mouse ES cells tend to be more unstable than their human counterparts, often acquiring chromosome aberrations at early passages. The advantage of research using mouse ES cells is that these cells can undergo targeted mutations and, when injected into mouse embryos, can create chimeric offspring with specific genetic constitutions useful for research. However, if the ES cell has an extra chromosome 8, such cells are unlikely to enter the germ line. In fact, in mouse ES cells, there appears to be an inverse correlation between the efficiency of targeted mutations entering the germline and the growth rate of the ES cells in culture, perhaps because cells from the more rapidly proliferating cultures are likely to have an extra chromosome 8.

Although some researchers claim normal chromosomes in human ES cells after extended time in culture (more than 100 passages), others have reported recurrent aberrations involving chromosomes 12 and 17 occurring between passages 25 and 45. Despite optimal culture techniques, guaranteeing the genetic integrity of ES cells is difficult because of the stresses of tissue culture and the selective pressures exerted on the cells after cultures have been frozen and thawed. Because cultures of cryo—preserved ES cells tend to grow poorly after thawing, a few cells with a growth advantage resulting from an extra chromosome 12 or 17 can increase in number and eventually overgrow the normal cells.

...