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Wed March 27, 2013
Catalog Of Gene Markers For Some Cancers Doubles In Size
Originally published on Wed March 27, 2013 11:41 am
The largest gene-probing study ever done has fished out dozens of new genetic markers that flag a person's susceptibility to breast, ovarian and prostate cancer.
The 74 newly discovered genetic variants double the previously known number for these malignancies, all of which are driven by sex hormones.
Underscoring the sheer magnitude of the findings, they're contained in 15 scientific papers published simultaneously by five different journals. The Nature group of journals has collected them all here.
But while the discovery is a landmark in cancer genetics, knowing these susceptibility markers won't translate into much for patients for now.
The nearest hope is that the growing catalog of markers will allow researchers to fine-tune screening tests for these cancers. They might be able to identify which people to screen using existing tools such as mammography, PSA blood tests or ultrasound. And a patient's genetic profile could help determine what to do when a screening test comes back positive.
For instance, one paper suggests that if a woman's genes contain one of four newly discovered variants, she's at risk for a more aggressive type of breast cancer called ER-negative, which accounts for about one in four cases. So she might be advised to have earlier or more frequent mammograms.
But Douglas Easton of Cambridge University, an author of one new paper, cautions that there are "many hurdles" before these genetic signals can be incorporated into breast cancer screening. "If you're 40-years-old, I think it would be in your lifetime," Easton told reporters on a conference call. "It will take some time."
The same is true for men with prostate cancer, who are currently bedeviled by the ambiguities of a screening test called PSA. Right now, a test that finds an elevated PSA doesn't indicate if a man actually has prostate cancer, or whether it's a slow-growing or aggressive type.
Encouragingly, researchers are reporting 23 new genetic markers for prostate cancer. Sixteen of them are associated with more aggressive disease.
Rosalind Eeles of Britain's Institute of Cancer Research says men with many of these susceptibility markers in their genes have a nearly 50 percent chance of developing prostate cancer, versus about 10 percent for the average man.
So someday, if a man with one of these markers has an elevated PSA, his doctor might feel more urgency about doing a biopsy. And if the biopsy shows cancer, that might argue for treatment rather than "watchful waiting."
"If your risk is over 50 percent of getting prostate cancer in your lifetime, we don't know if we should just PSA-screen you, whether we should do a biopsy, whether we should do an MRI scan," Eeles says. "So it's very early days."
But Eeles says studies are beginning to reveal whether a man's genetic profile should trigger more aggressive screening or diagnostic testing.
The picture for ovarian cancer is murkier. Paul Pharoah of Cambridge University says the new research identifies three more markers that put women at higher risk, bringing the total number for ovarian cancer to 12. Among women with many of these markers, three out of 100 will develop ovarian cancer at some point — almost double the average risk.
Pharoah guesses it might take five to 10 years before genetic profiling will be commonly used to identify women with a higher risk of ovarian cancer.
The hurdles are more than technological. A commentary in Nature Genetics notes ethical, legal and public health issues. A big one is how to ensure that poor and uninsured people have an equal chance of benefiting from a cancer screening program that requires genetic profiling.
Translating the genetic risk markers into more effective treatment is even further down the road. It will require a deep understanding of how the genetic variations give rise to cancer, and what might be done to block their effects.
The cornucopia of new cancer markers comes from a mega-consortium of 160 research centers around the world called COGS. Researchers drew on data from more than 200,000 individuals, most of them from Europe, North America and Australia. Some had cancer, others were used for comparison.
Scientists first identified genetic variations in these individuals called SNPs, sometimes called "spelling errors" in the genetic code. This technology is called genome-wide association studies, or GWAS.
The researchers sorted out the SNPs they thought would shed the most light on the risk of the three hormone-driven cancers. They put more than 200,000 of these tiny variations on a so-called DNA chip or micro-array.
When DNA from a specific individual is washed over this plate, pieces that match will latch onto the SNPs. Researchers can read these signals and correlate them with the individual's disease – or lack of disease.
The 74 new markers are those that correlate most strongly with cancer, and sometimes with more aggressive subtypes.
As impressive as the new list of markers is, they explain only about a third of all breast, prostate or ovarian cancers. (It varies a lot. The markers account for about 30 percent of breast and prostate cancers, but only 4 percent of ovarian cancers.)
Easton, the Cambridge University scientist, says part of the reason is that there are probably thousands more markers yet to be discovered. Some of them may be associated with only rare cases of cancer. He says finding them all will require even bigger studies.