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.
adult stem cells
Unlike ES cells, which can give rise to any lineage, adult stem cells can only generate cells of a specific lineage. Adult stem cells are present in specialized tissues throughout the body, such as bone marrow or skin, and are capable of unlimited production of differentiated cells. For adult stem cells to be able to divide continuously, they must have an active telomerase gene, which is characteristic of all ES cells and is needed for duplication of chromosome ends (called telomeres). The presence of the telom-erase gene enables stem cells to maintain the integrity of the chromosomes throughout many cell divisions, whereas differentiated cells of the body do not have this gene and therefore can undergo only a limited number of divisions. Because several mutations are required for a normal cell to become a cancer cell, including the ability to make telomerase, it is generally believed that cancer cells derive from mutated adult stem cells because differentiated cells have a limited life-span and therefore cannot accumulate the necessary mutations for malignant transformation.
Most mutations in adult stem cells that give rise to cancer or leukemia tend to be lineage specific because certain changes will promote growth in one tissue but not another. For example, chronic myeloid leukemia is caused by specific chromosome changes in a bone marrow stem cell. Because the normal stem cells in the bone marrow divide only when more blood cells are needed, they become quiescent as a result of continuous blood cell production by the mutated stem cells. This enables the abnormal stem cells populating the bone marrow to divide continuously, thereby producing the elevated blood cell count characteristic of this leukemia. The mutation in chronic myeloid leukemia involves a specific exchange between chromosomes 9 and 22, but this chromosome aberration has no effect in any other tissue because chromosome changes associated with cancer are lineage specific.
Although there are few reports of chromosome studies of cultured adult stem cells, it has been shown that cultured human mesenchymal stem cells have spontaneously acquired an extra chromosome 8, which is a common finding in cancers of mesenchymal origin. The fact that cultured adult stem cells can undergo tissue-specific chromosome changes associated with malignant diseases emphasizes the need to ensure that any adult stem cells used therapeutically, including repro-grammed cells, be monitored for genetic changes.
