Cellular Life Span Is Determined by DNA Sequences at the Ends of Chromosomes
Ihe final major dysfunction of growth control found within cancer cells is the most recently discovered, but also seems to be the most common single molecular lesion in cancers: the expression of a reverse transcriptase called telomerase.
(A reverse transcriptase is any enzyme that synthesizes DNA from an RNA template.) Telomerase is responsible for replicating telomeres, the specialized, noncoding regions of DNA found at the end of chromosomes. However, telomerase is normally expressed only in embryonic cells and in adult stum cells. (Stem cells are specialized normal ceils that do have limitless replicative potential, such as gamete-generating cells and the blood-forming cells of the bone marrow, as discussed later.) The vast majority of normal somatic cells do not express telomerase, but it is expressed in 85% to 90% of all cancers and is the major determinant of the “immortality” of cancer cells.Telomeres are segments of highly repetitive DNA, representing hundreds of repeats of the simple nucleotide sequence TTAGGG (in vertebrates), found at the ends of chromosomes. Telomeres serve as caps at chromosomal ends, protecting them against end-to-end joining of chromosomes. Telomeres also prevent the ends of chromosomes from being recognized as sites of DNA damage (double-strand DNA breaks). Most relevant for cancer, telomeres protect against the loss of coding DNA from each chromosomal end with every round of DNA replication; this is needed because normal DNA polymerases have a serious limitation: they cannot fully replicate the end of a double-strand DNA molecule. As a result, the ends of chromosomes become shorter with each round of DNA replication. (Bacteria solve this problem by having circular DNA chromosomes.)
Telomeres are expendable DNA, at the ends of chromosomes, whose progressive shortening does not compromise the coding function of the genome.
Although no coding sequence is lost, the shortening of telomeres nevertheless plays an important role in the cell. The shortening of telomeres serves as a kind of clock, measuring the number of times a cell has divided, and the length of the telomere reflects the age of the cell. Through poorly understood mechanisms, cells can detect the length of their telomeres, and when they reach a critically short length, the cell ceases to divide and is said to undergo senescence (Latin for “growing old”). As noted earlier, normal cells have a finite life span, such that a cell taken from a middle-aged human will divide 20 to 40 limes in culture before senescence. When placed into culture, the number of subsequent cell divisions before senescence reflects the original length of the telomeres. Further, various degenerative diseases, including cirrhosis of the liver, have been shown to accelerate telomere shortening. In principle, senescence is a powerful block to cancer because the original damaged cell (see Figure 2-1) would be unable to divide for a sufficient number of generations to accumulate the necessary multiple mutations required to produce a tumor. Telomerase expression (and other, less common means of elongating telomeres) effectively eliminates this block to cancer development by causing the cells to become immortal.Telomerase has both protein and RNA components. The protein provides the catalytic reverse transcriptase, allowing the enzyme to elongate the telomere sequence based on the RNA template it carries. That is, the RNA component of telomerase is complementary to the telomere DNA sequence and is used as the template for telomere DNA replication. Telomerase is not expressed in normal adult somatic cells except for stem cells, mentioned earlier. However, immortal tissue culture cells do express telomerase, as do cancer cells. Experimental expression of telomerase in human cells dramatically increases the replicative life span of the cells. Thus the observed expression of telomerase in the vast majority of human cancers permits these cells to divide indefinitely, providing yet another selective advantage for these cells to accumulate other damage over time.
In the last sections of this chapter, we turn our attention to the cancer cell in the context of a tumor, which is a population of cancer cells interacting with one another and with surrounding normal tissue. We end our discussion of the intrinsic growth controls of normal and cancer cells with an experimental result that seems to confirm the importance of the types of damage discussed thus far. This experiment showed that four genetic changes were sufficient to transform normal human kidney cells into cancer cells able to form tumors when transplanted into a mouse host (with no immune system). The four genetic changes were to “engineer” into the cells an activating mutation for the ras oncogene, inactivation of both the RB and p53 proteins, and expression of the catalytic subunit of telomerase. Thus, damage to the genes or expression of these molecules, emphasized here, reflects the minimum requirements for a normal cell to grow as a cancer.