Effect of passenger mutations
Is cancer an evolutionary process whose course is determined by a delicate balance between driver and passenger mutations?
- Date : 13 Feb 2013
- Topic : Translational research
A typical cancer cell has thousands of mutations scattered throughout its genome and hundreds of mutated genes. However, only a handful of those genes, known as drivers, are responsible for uncontrolled growth of cancers. Cancer biologists have largely ignored the other mutations, believing they had little or no impact on cancer progression, but a new study from MIT, Harvard University, the Broad Institute and Brigham and Women’s Hospital reveals, shows for the first time, when enough of these so-called passenger mutations accumulate, they can slow or even halt tumour growth.
The findings, reported in the Proceedings of the National Academy of Sciences, suggest that cancer should be viewed as an evolutionary process whose course is determined by a delicate balance between driver and passenger mutations, according to Leonid Mirny, an associate professor of physics and health sciences and technology at MIT and senior author of the paper.
Furthermore, drugs that tip the balance in favour of the passenger mutations could offer a new way to treat cancer, beating it with its own weapon — mutations. Although the influence of a single passenger mutation is minuscule, collectively they can have a profound effect, according to the researchers. Lead author of the paper is Christopher McFarland, a graduate student at Harvard.
A simulation model
Cancer can take years or even decades to develop, as cells gradually accumulate the necessary driver mutations. Those mutations usually stimulate oncogenes such as Ras, which promotes cell growth, or turn off tumour-suppressing genes such as p53, which normally restrains growth.
Passenger mutations that arise randomly alongside drivers were believed to be fairly benign. However, Mirny and his colleagues suspected that the evolutionary process in cancer can proceed differently, allowing mutations with only a slightly harmful effect to accumulate.
To test this theory, the researchers created a computer model that simulates cancer growth as an evolutionary process during which a cell acquires random mutations. These simulations followed millions of cells: every cell division, mutation and cell death. They found that during the long periods between acquisitions of driver mutations, many passenger mutations arose. When one of the cancerous cells gains a new driver mutation, that cell and its progeny take over the entire population, bringing along all of the original cell’s baggage of passenger mutations.
This process repeats five to 10 times during cancer development; each time, a new wave of damaging passengers is accumulated. If enough deleterious passengers are present, their cumulative effects can slow tumour growth, the simulations found. Tumours may become dormant, or even regress, but growth can start up again if new driver mutations are acquired. This matches the cancer growth patterns often seen in human patients.
Cancer may not be a sequence of inevitable accumulation of driver events, but may be actually a delicate balance between drivers and passengers, according to Mirny and colleagues. Spontaneous remissions or remissions triggered by drugs may actually be mediated by the load of deleterious passenger mutations.
When they analysed passenger mutations found in genomic data taken from cancer patients, the researchers found the same pattern predicted by their model — accumulation of large quantities of slightly deleterious mutations.
Tipping the balance
In computer simulations, the researchers tested the possibility of treating tumours by boosting the impact of deleterious mutations. In their original simulation, each deleterious passenger mutation reduced the cell’s fitness by about 0.1%. When that was increased to 0.3%, tumours shrank under the load of their own mutations.
The same effect could be achieved in real tumours with drugs that interfere with proteins known as chaperones. After proteins are synthesised, they need to be folded into the correct shape, and chaperones help with that process. In cancerous cells, chaperones help proteins fold into the correct shape even when they are mutated, helping to suppress the effects of deleterious mutations.
Several potential drugs that inhibit chaperone proteins are now in clinical trials to treat cancer, although researchers had believed that they acted by suppressing the effects of driver mutations, not by enhancing the effects of passengers.
In current studies, the researchers are comparing cancer cell lines that have identical driver mutations but a different load of passenger mutations, to see which grow faster. They are also injecting the cancer cell lines into mice to see which are likeliest to metastasize.
The research was funded by the USA National Institutes of Health/National Cancer Institute Physical Sciences Oncology Center at MIT.
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