CANCER AND UNLIMITED REPLICATIVE CAPACITY
If you remove a piece of tissue from an animal, break it up into single cells and put those cells into a suitable medium they will grow and duplicate themselves quite happily. They’ll keep reproducing for somewhere between 20 and 60 doublings of the cell population – and then stop. They have then entered the state of senescence or replicative senescence – cellular old age. The cells are not dead, although they will eventually die, they can exist for a long time in culture in this state of suspended animation: they have barely detectable metabolic activity and require almost nothing in the way of nutrients.
One reason for cells becoming senescent is that they have lost DNA from the ends of their chromosomes – recall that our DNA is split up into 23 pairs of chromosomes, so that means there are 92 (23 × 2 × 2) free ends of double-stranded DNA within the nucleus. These are a problem for the machinery that replicates most of our DNA because it can’t deal with the ends. Every time DNA is duplicated, therefore, which has to happen whenever a cell reproduces itself, the chromosomes are shortened. This might be serious at a very early stage if the DNA lost were important (e.g. encoded a protein). Presumably, for this reason, chromosomes are ‘capped’ at each end by repeated sequences of DNA – telomeres. Because telomeres are made up of a repetitive sequence that doesn’t code for protein, loss of these sequences does not affect the viability of individual cells. Nevertheless, telomere loss would be incompatible with the survival of the species if it happened in all cells and for that reason germ line cells and some stem cells express an enzyme, telomerase that can replicate the ends of chromosomes (Fig. 1).
Telomerase has its own RNA template, which, in effect, allows chromosomes to be temporarily extended to solve the problem. In all other types of cell, telomerase is almost undetectable.
In about 90% of human primary tumours examined, however, there is substantial telomerase expression (in benign tumours the figure is ~25%). This is a consequence of the mutations that make a tumour cell and it enables cancer cells to maintain the length of their telomeres and thus escape mechanisms that restrict the doublings of normal cells to a finite number. They can grow indefinitely. Tumour cells that do not express significant telomerase use another mechanism to get around the problem – alternative telomere lengthening (ALT) – a process of recombination between the ends (telomeres) of different chromosomes. It will be evident, however, that in the early stages of cancer development telomerase activity may be inadequate to compensate for chromosome erosion. Cells with critically shortened telomeres will then enter a growth ‘crisis’ that generally results in death. Cells that survive this crisis and emerge as a proliferating clone have usually lost p53, the key monitor of genomic integrity. The unprotected ends of chromosomes have undergone end-to-end fusions with the production of dicentric chromosomes and the initiation of the chromosomal breakage–fusion– bridge cycle. This results in unequal chromosomal segregation during mitosis that can give rise to gene amplification and deletion. Thus, for these cells, inadequate telomerase activity may promote malignant progression.
Despite the fact that telomerase is largely suppressed in somatic cells low levels of activity can be detected in most cells. This suggests that, yet again, nature works by balancing forces – in this case, chromosome length. On the one hand, you might wish your chromosomes to stay the same length so that you don’t grow old but, on the other, we know that very active telomerase is a major contributor to cancer development. Amazingly, we may be able to adjust our telomerase activity, and hence the rate at which our chromosomes disappear, by changing our lifestyle. In other words, telomerase may turn out to be another biochemical marker for stress. Individuals who endure stress over prolonged periods (e.g. people who care for patients with severe disabilities or are HIV positive but without overt symptoms) have lower telomerase activity and shorter telomeres than their less stressed brethren.
In addition to its action in telomere maintenance, other functions of the telomerase catalytic subunit are emerging including effects on chromatin structure, mitochondrial RNA processing and modulation of the WNT signalling pathway through its capacity to interact with β-catenin. The convergence of the reverse transcriptase activity (TERT) of telomerase and the WNT pathway is required for stem cell proliferation, a finding that has implications for tumour development as yet unresolved. In some human tumour cell lines at least TERT has a second mitochondrial activity in being able to influence cellular redox status, thereby of reactive oxygen species (ROS). ROS generation can occur, for example, as part of the p53-mediated DNA damage response leading to BAX translocation to mitochondria and the release of pro-apoptotic proteins. This effect on ROS endows TERT with yet another tumourigenic activity – the capacity to confer survival on cells that would otherwise be eliminated.