Telomere shortening is one of the suspected causes of ageing. So what are telomeres? They are sequences of DNA at the end of each chromosome. The DNA in each of our cells would be 2-3 metres long if stretched out. To squeeze this into a tiny cell nucleus, it is coiled tightly into structures called chromosomes. When a cell is getting ready to divide and make a new cell, chromosomes are duplicated so the old cell can keep a copy and the new cell can get a copy. However, because of the way new DNA strands are synthesised, the very ends of chromosomes can’t be copied.
To make sure that important information isn’t lost each time a cell divides, chromosomes have ‘disposable DNA’ sequences at each end. These are called telomeres and this DNA isn’t essential for normal cell function. Telomeres are essentially there to be lost, a little at a time, each time the cell divides. They also act as stoppers which prevent chromosomes from fusing with each other into one giant DNA mess.
Once all the telomeric sequence is gone, the cell can’t divide anymore and is now senescent. Once this happens in many cells, particularly in stem cells, there is no way for the body to make new cells to replace old damaged ones. This means that ageing tissues will start malfunctioning and disease will manifest.
There is a lot of individual variation in the length of telomeres and the rate of telomere shortening. However, on average older populations of people and animals have shorter telomeres because their cells have undergone more divisions. Environment also has an effect: both smoking and psychological stress can shorten telomeres. This is one way in which ageing and increased stress leads to disease.
Telomeres can be lengthened by an enzyme called telomerase. But this enzyme is not active in most adult cells. It is active in stem cells and in cancer cells. So we can’t just reactivate telomerase in all cells as an anti-ageing treatment because it would most likely cause cancer.
And to throw another a spoke in the wheels, a study published in 2009 showed that up to a third of people have stable or increasing telomere lengths. These people didn’t have a higher chance of getting cancer than those with shortening telomeres. There hasn’t been enough followup to know if the people with longer telomeres will live longer. So it still isn’t clear whether telomere shortening is a cause or a by-product of ageing?
The Scientific Papers:
Epel et al. Accelerated telomere shortening in response to life stress. PNAS, 2004
Nordfjäll et al.The individual blood cell telomere attrition rate is telomere length dependent. PLoS Genetics, 2009