Small mutations accumulating in DNA are unlikely to be fully responsible for the ageing process, finds a new study carried out in collaboration with the University of Birmingham.
The research into the theories of ageing found that human cells and tissues can accumulate many more mutations than are normally present, without the body showing the features associated with ageing.
The new study, published today in Nature Genetics, compared DNA taken from individuals with inherited mutations in genes involved in DNA replication with DNA from individuals who have normal versions of these genes.
The research team, including experts at the University of Birmingham, Wellcome Sanger Institute, and the University of Edinburgh, aimed to understand the impact of defective DNA replication on cancer risk and features associated with ageing. The results suggest that build-up of mutations in normal cells is unlikely to be the only factor in the development of age-related disease, adding to the ongoing debate about the causes of ageing.
One of the current models of ageing suggests that accumulation of mutations in the DNA of healthy cells results in the changes that we see as the body grows older. The model is based on the observation that mutations accumulate in normal cells throughout life, and theorises that the greater number of mutations in older people compared to younger people impairs the function of genes and disturbs cell function, ultimately leading to diseases of old-age and the visible features typically associated with ageing.
However, this new research shows that human cells and tissues can function apparently normally with many more mutations than are usually present, suggesting that ageing may not be due to build-up of these types of mutations alone.
DNA replication is required to duplicate the DNA in a cell ready for cell division. It involves creating an entire error-free copy of the human genome from the existing strand, and is undertaken with very high accuracy in normal healthy cells by proteins called DNA polymerases. When the DNA polymerases have a mutation, causing them to be faulty, it leads to more DNA errors, or small mutations, accumulating with each and every cell replication.
In this study, researchers including scientists from the Cancer Grand Challenges Mutographs team, applied recently developed cutting-edge techniques to sequence the DNA of normal cells and tissues from patients who have inherited mutated versions of the DNA polymerase genes, POLE and POLD1.
By comparing tissue samples with unaffected individuals, they found that normal tissues from those who had a faulty DNA polymerase had elevated mutation rates. These study participants did not, however, show features of early onset ageing or age-related diseases despite having accumulated numbers of mutations that would have made them hundreds of years old in terms of their ‘mutational age’. Therefore, other than an increased risk of certain cancers, the research shows that cells can accumulate many mutations and not show features associated with ageing, challenging the current model. Further research is therefore needed to understand the biological processes underlying ageing.
Dr Phil Robinson, co-first author and Wellcome Clinical PhD Fellow at the Wellcome Sanger Institute, said: “By focusing on people who have a known increased risk of cancer, we discovered that most or potentially all of the cells in these individuals carry an increased burden of mutations. We were amazed to see that normal and seemingly healthy cells could tolerate so many mutations. This research has given us an insight into the potential reasons for their increased risk of cancer and also offers an immensely valuable window into the process of ageing. Our research shows that a higher mutational burden does not appear to result in early onset signs and features that we typically associate with ageing. While other types of mutations could potentially play a role, it suggests that there is a more complex process behind ageing than the accumulation of mutations alone.”
Dr Claire Palles, co-first author and Birmingham Fellow at the Institute of Cancer and Genomic Sciences at the University of Birmingham, said: “Knowing the impact of different polymerase mutations on the genetic material in cells is crucial if we are to fully understand the risk of patients with these mutations developing certain cancers and age-related disease. By showing that healthy cells can contain as many mutations as some cancer cells do, it emphasises that there are more factors that go into making a cell cancerous than mutational burden. Further research is now necessary to understand how our findings fit into what we already know about inherited syndromes that make a person more susceptible to cancer, and if there is any way to help keep their risk of developing disease as low as possible.”
Dr David Scott, director of Cancer Grand Challenges at Cancer Research UK, said: “The Cancer Grand Challenges Mutographs team is making incredible progress in helping us better understand the roles mutations play in tumour development. But as this study shows, the scope of the team’s work goes beyond cancer and is helping us to understand more about mutations in normal tissue and even the processes of ageing.”
Professor Sir Mike Stratton, senior author and Director of the Wellcome Sanger Institute, said: “Understanding why our cells age and the mechanisms behind ageing may help us find new ways to protect against age-related disease. This research indicates that accumulation of mutations during the course of a lifetime is unlikely, on its own, to account for the constellation of features that we term ageing. Further studies are therefore required to understand what changes occurring in cells during life cause the behaviours associated with ageing.”