The studies, of a malignant melanoma and a lung cancer, reveal for the first time almost all of the mutations in the genomes of two cancers. All cancers are caused by mutations in the DNA of cancer cells which are acquired during a person’s lifetime.
Lung cancer causes around one million deaths worldwide each year, and almost all are associated with smoking. The number of mutations found- almost 23 000- suggest that a typical smoker would acquire one mutation for every 15 cigarettes smoked.
Although malignant melanoma comprises only 3 per cent of skin cancer cases, it is the cause of three out of four skin cancer deaths. The melanoma genome contained more than 30 000 mutations that carried a record of how and when they occurred during the patient’s life.
“These are the two main cancers in the developed world for which we know the primary exposure,” explains Professor Mike Stratton, from the Cancer Genome Project at the Wellcome Trust Sanger Institute. “For lung cancer, it is cigarette smoke and for malignant melanoma it is exposure to sunlight. With these genome sequences, we have been able to explore deep into the past of each tumour, uncovering with remarkable clarity the imprints of these environmental mutagens on DNA, which occurred years before the tumour became apparent.
“We can also see the desperate attempts of our genome to defend itself against the damage wreaked by the chemicals in cigarette smoke or the damage from ultraviolet radiation. Our cells fight back furiously to repair the damage, but frequently lose that fight.”
The studies, funded by the Wellcome Trust, used powerful new DNA sequencing technologies to decode completely the genome of both tumour tissue and normal tissue from a lung cancer and a malignant melanoma patient. By comparing the genome sequence from the cancer to the genome from healthy tissue they could pick up the changes specific to the cancer. The studies are the first to produce comprehensive genome-wide descriptions of all classes of mutation, producing rich accounts of the genetic changes in the development of the two cancers.
“In the melanoma sample, we can see sunlight’s signature writ large in the genome,” says Dr Andy Futreal, from the Wellcome Trust Sanger Institute. “However, with both samples, because we have produced essentially complete catalogues, we can see other, more mysterious processes acting on the DNA. Indeed, somewhere among the mutations we have found lurk those that drive the cells to become cancerous. Tracking them down will be our major challenge for the next few years.”
Identifying the causative mutations among the large number of mutations found poses a challenge, but the complete genome sequences mean, that for the first time, that challenge can be met.
“Nearly 10 years on, we are still reaping the benefit from the first human genome sequence and we have much still to do to get to grips with these new disrupted landscapes of cancer genomes,” explains Dr Peter Campbell from the Wellcome Trust Sanger Institute. “But the knowledge we extract over the next few years will have major implications for treatment. By identifying all the cancer genes we will be able to develop new drugs that target the specific mutated genes and work out which patients will benefit from these novel treatments.”
A complete genome catalogue for each patient would be expected to help select between treatments and to direct treatment in the most efficient and cost-effective way. The Sanger Institute is already working with researchers at Massachusetts General Hospital on a large scale project to tie genetic changes in cancers to their responses to anticancer treatments.
“We want to drive healthcare through better understanding of the biology of disease,” says Sir Mark Walport, Director of the Wellcome Trust. “Previous outcomes from our Cancer Genome Project are already being fed into clinical trials, and these remarkable new studies further emphasise the extraordinary scientific insights and benefits for patients that accrue from studying the genome of cancer cells.
“This is the first glimpse of the future of cancer medicine, not only in the laboratory, but eventually in the clinic. The findings from today will feed into knowledge, methods and practice in patient care.”
The human genome is large. Moreover, there are more than 100 different types of cancer and sequencing genomes is expensive. To ensure that thousands of cancers ultimately are sequenced in the same way as these two, the International Cancer Genome Consortium has been established, on the model of the Human Genome project itself to coordinate cancer genome sequencing across the globe.
These catalogues of mutations across the broad diversity of cancer types will provide powerful insights into the biology of cancer and will be the foundation for understanding cancer causation and improving prevention, detection and treatment.
Russian roulette
According to BBC The scientists found the DNA code for a skin cancer called melanoma contained more than 30,000 errors almost entirely caused by too much sun exposure.
The lung cancer DNA code had more than 23,000 errors largely triggered by cigarette smoke exposure.
From this, the experts estimate a typical smoker acquires one new mutation for every 15 cigarettes they smoke.
Although many of these mutations will be harmless, some will trigger cancer.
Wellcome Trust researcher Dr Peter Campbell, who conducted this research, published in the journal Nature, said: “It’s like playing Russian roulette.
“Most of the time the mutations will land in innocent parts of the genome, but some will hit the right targets for cancer.”
By quitting smoking, people could reduce their cancer risk back down to “normal” with time, he said.
The suspicion is lung cells containing mutations are eventually replaced with new ones free of genetic errors.
By studying the cancer catalogues in detail, the scientists say it should be possible to find exactly which lifestyle and environmental factors trigger different tumours.
Treatment and prevention
Tom Haswell, who was successfully treated 15 years ago for lung cancer, believes the research will benefit the next generation:
“For future patients I think it’s tremendous news because hopefully treatments can be targeted to their particular genome mutations, hopefully… reducing some of the side effects we get”.
Cancer experts have applauded the work.
The Institute of Cancer Research said: “This is the first time that a complete cancer genome has been sequenced and similar insights into other cancer genomes are likely to follow.
“As more cancer genomes are revealed by this technique, we will gain a greater understanding of how cancer is caused and develops, improving our ability to prevent, treat and cure cancer.”
Professor Carlos Caldas, from Cancer Research UK’s Cambridge Research Institute called the research “groundbreaking”.
“Like molecular archaeologists, these researchers have dug through layers of genetic information to uncover the history of these patients’ disease.
“What is so new in this study is the researchers have been able to link particular mutations to their cause.
“The hope and excitement for the future is that we will eventually have detailed picture of how different cancers develop, and ultimately how better to treat and prevent them.”