Early cell lines had a limited lifetime because cells develop senescence after a finite number of cell divisions (the Hayflick limit). Immortalized (permanent) cell lines have the huge advantage of allowing cells to be cultured indefinitely. As well as allowing applications that last over long timescales, and avoiding having to devote significant effort into just keeping the cells alive, there is the convenience of being able to access the desired cells from major cell line repositories.
Cells can become immortal when mechanisms regulating cell division are subverted, allowing cells to keep on dividing. That happens naturally in cancers: normal cell division controls are dysregulated following mutations in certain cancer- susceptibility genes, or after transformation of cells by cancer-causing viruses. Some popular permanent cell lines were originally obtained from naturally occur ring tumors (such as the HeLa human cell line that originated from Henrietta Lacks, a patient with cervical cancer). And after being identified as key components in cellular transformation, oncogenes could be artificially expressed in cultured cells to induce transformation, resulting in immortalized cell lines. For example, many immortalized cell lines have been made by transfecting and expressing an oncogene from simian virus 40 (SV40). The gene product, the SV40 large T antigen, binds to and inhibits p53 and the pRb retinoblastoma protein, proteins that normally act as brakes on cell division.
Immortalized cell lines derived from tumors or artificially transformed cells are dis advantaged by genome instability, and aneuploidy is common. Molecular pathways in transformed cell lines, therefore, may not always be representative of those in the original untransformed cells. But for some purposes that is not a major concern. The major utility of immortal human lymphoblastoid cell lines, for example, is to provide an inexhaustible source of DNA from individuals of interest (Box 8.1).
Immortalized euploid cell lines
To make immortalized euploid cell lines that are more representative of the original cells, interest has focused on a final step in cell transformation: activation of TERT, telomerase reverse transcriptase. Recall from Section 2.4 that the DNA of telomeres consists of tandem repeats of the hexanucleotide TTAGGG, and that telomerase extends telomeric DNA by using its RNA component, TERC, to provide an RNA template for the TERT enzyme to make new TTAGGG repeats. Only a very few of our cells normally express telomerase because although the TERC RNA is expressed in all cells, the TERT enzyme is normally restricted to unspecialized cells in early development and to immortal stem cells that are needed to replenish body cells. Normal body cells can undergo just a limited number of cell divisions because DNA is lost at the telomeres at each round of DNA synthesis (the chromosome end-replication problem), causing progressive shortening of telomeres and eventually inducing cell senescence.
Transformed cells do not undergo senescence. But after several rounds of cell division, past the stage where senescence develops in the normal cells, trans formed cells undergo a proliferation crisis: the very few surviving cells are aneuploid cells that have reactivated telomerase to become immortal. The long route that transformed cells take to achieve immortality can be bypassed by just recreating the last step: by artificially expressing a TERT transgene in cultured euploid cells, it is possible to create euploid cell lines with telomerase activity that are effectively immortal.
