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Issues in Genotyping and Biomonitoring
But is it ready for prime time?
Hot, New Technology Shows Promise for Determining Breast-Cancer
Risk
Although they have received a great deal of attention in the press,
mutations in the BRCA1 and 2 genes are responsible for only a
small percentage of breast cancer cases. Since breast cancer is
a genetic disease, science needs to look for variations in other
genes to explain what causes the disease in the majority of the
women and men who get it. A recent advance in genetics may help
solve the riddle by looking for tiny differences in a person's
overall genetic makeup, or genome, said Jessica Everett, M.S.,
who is a genetic counselor at Cincinnati Children's Hospital Medical
Center.
“The big hot thing that people are talking about are common
changes in the genome that are called single nucleotide polymorphisms,
or SNPs (pronounced snips).” A person's genetic makeup is
derived from his or her DNA, a long chain of chemical compounds,
known as nucleotides or base pairs. These nucleotides come in just
four varieties, designated by the letters A, T, G and C. Everett
explained, “A SNP is just a change in the DNA sequence of
one letter. When we say they are ‘common,' we mean that if
we looked at 100 people, probably 99 of them would not have the
change, but one would.”
Scientists currently know of at least 10 million different kinds
of SNPs that can occur in humans, she said. “That accounts
for the reason we look and behave differently from one another.”
It also affects a person's susceptibility to diseases like breast
cancer. She explained, “There is a lot of buzz around the
idea that some these little variations may individually have a small
impact on a person's breast cancer risk, but when you add those
particular SNPs together, that may be enough to have a big influence
on the individual's risk for breast cancer.”
Although it seems straightforward, the determination of an individual
SNP's impact on breast cancer risk is anything but, Everett said.
“The way this kind of study works is that researchers gather
a group of women who have breast cancer and a group of women who
do not, and then look at a particular SNP location.” Perhaps
they find that the SNP is present more often in the women who
have breast cancer by a ratio of 9 percent to 5 percent. “The
idea is there may be some type of connection there. Maybe they're
finding that genotype (form of the gene) in women with breast
cancer, because it is somehow related to why they got breast
cancer.” However,
she cautioned, “This doesn't necessarily tell us there's
a direct cause and effect. It only tells us that it seems there
might be something going on here.”
The relationship between a certain SNP and cancer risk becomes
thornier because different SNPs can act against each other or can
cooperate with one another. “We know that individual genes
certainly don't act independently of each other, so looking at variation
in one SNP is not even going to come close to getting at the complexity
of the situation.”
She pointed to a study of SNP interactions as an indication of
the difficulties in determining the importance of certain SNPs to
breast cancer. The researchers in the study considered only 10 of
the different SNPs that might be related to breast-cancer risk.
When they looked at the number of genotypes that could arise from
combinations of two or three SNPs among those 10, they counted more
than 16,000 different genotypes, she said. “And that doesn't
even get close to understanding how all 10 of them interact together.”
The task is daunting, but studies of SNPs are making important
inroads, she said. “We're now at the stage where we can genotype
a person at probably 90 different SNPs and have the result in a
couple of days. We can do this quickly, we can do it easily, and
it's relatively inexpensive.” This is also presenting a problem,
she noted. “Because we can do it quickly, easily and inexpensively,
and because we're dealing with consumers who want to know their
risk and how they can do something to change it, we're starting
to run into situations where companies are coming to market with
risk-prediction tests based on SNP associations. What we need to
think about as scientists, researchers, health care providers and
health care consumers is: Is this really ready for prime time?”
One currently available test is an example. It claims to determine
an individual's five-year and lifetime breast cancer risks by
screening for 90 SNPs in 78 genes, she said. “They send
you back a piece of paper that says your five-year risk for breast
cancer is (for example) 12 percent and your lifetime risk for
breast cancer is 30 percent, so good luck,” Everett recounted.
Unfortunately, she said, clinicians don't really have good,
clear recommendations to give women based on these kinds of
risk numbers. “The first
thing that comes to my mind when I find these kinds of tests for
sale on the internet is whether it's OK for us to offer tests
to patients outside of a research setting when we don't know
if they are clinically relevant. My argument would be that
that is irresponsible, but I'm certainly not the only one having
a voice on the issue.”
Ethical questions aside, the research is continuing at a fast
pace. Everett commented, “Cancer is a genetic disease. Cancer
happens because things go wrong with genes, and it's this kind of
SNP testing that really shows a lot of the promise.” She added,
“Coming at this from a clinical perspective, my big question
is always about what we are going to do differently. How is this
going to be clinically relevant, and at that point, are we ready
to talk to patients about doing this particular kind of testing?”
© 2006 BCERC. All Rights Reserved BCERC Coordinating Center,
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