What is the current status of predictive
gene testing for cancer?
Tests for a few rare cancers are
already in clinical use. Predictive gene tests for more common types of
cancer are still primarily a research tool, difficult to execute and
available only through research programs to small numbers of people who
have a strong family history of disease. But the field of gene testing
is evolving rapidly, with new genes being discovered almost daily and
innovations in testing arriving almost as quickly. For example:
Predictive tests already are being
used routinely in selected families with retinoblastoma and Wilms'
tumor.
A gene test is available for
persons in the rare cancer-prone Li-Fraumeni families. However, it is
available only to participants in a research study, and experts caution
that it must be offered with great care, weighing benefits against
risks.
A test is in place for the gene
that triggers familial adenomatous polyposis, a tendency to form
hundreds of colon polyps, some of which, if not removed, will go on to
become cancerous. (But the condition can also be diagnosed without the
gene test.)
A set of genes that predispose a
person to a much more common type of colon cancer (hereditary
nonpolyposis colon cancer, or HNPCC) has been identified in high-risk
families. These genetic alterations are thought to be carried by as many
as 1 million Americans, and to cause about 90 percent of all inherited
colon cancers, or about 15 percent of the 160,000 colon cancers
diagnosed in the United States each year. The genes have also been
linked to cancers of the uterus, stomach, ovary, small intestine, gall
bladder, kidney, and ureter. Very high-risk families (three or more
affected members, at least one before age 50, over two or more
generations) are being tested at a few research centers. A blood test is
expected in a year or two.
The BRCA1 gene mutation
predisposes a person to hereditary breast cancer and ovarian cancer. A
mutant BRCA1 gene on chromosome 17 is probably responsible for about 5
percent of the 182,000 cases of breast cancer predicted for a single
year, and as many as a quarter of the cases occurring in women ages 45
and younger. A mutant BRCA1 gene is found in nearly half of the families
with a high incidence of breast cancer and in at least 80 percent of the
families with a history of both early onset breast and ovarian cancer.
With the isolation of the gene, a blood test is expected, but before it
becomes available, research studies must address important questions
about optimum management of BRCA1 mutation carriers. (On chromosome 13,
scientists have also found evidence of a second breast cancer gene
called BRCA2.)
Genes have been reported for
melanoma, leukemia, thyroid and renal cell cancer, and scientists are
closing in on genes for several other cancers.
