Predictive gene tests look for disorders that "run in families" as the
result of a faulty gene that is inherited.
When a mutated gene is inherited because it was carried in the
reproductive cells (egg or sperm), the mutation will be present in cells
throughout the body. This means that the mutation can be detected in white
blood cells in a blood sample, for instance.
Predictive gene tests are presently available for diseases such as
Tay-Sachs disease and cystic fibrosis, and tests are being developed for
many more conditions, including a predisposition to ALS, or
amyotrophic lateral sclerosis, the fatal nerve degeneration known as
Lou Gehrig's disease;
Huntington's disease, a devastating disorder of middle age that causes
dementia and ends in death; some forms of Alzheimer's disease; and
catastrophically high cholesterol.
Genes have also been found for several types of cancer that can run in
families.
Several of these are rare conditions that affect only a few people: a
childhood eye cancer known as
retinoblastoma;
Wilms' tumor, a kidney cancer that usually appears before age 5; and
the
Li-Fraumeni syndrome, in which children and young adults of the family
develop an assortment of cancers, including
sarcomas in the bones and soft tissues of the arms and legs, brain
tumors, acute leukemia, and breast cancer. In 1993, scientists identified
the gene that causes familial adenomatous polyposis, an inherited
predisposition to form
precancerous polyps. This condition is believed to be responsible for
about 1 percent of colon cancers.
More recently, scientists have identified gene mutations that are
linked to inherited tendencies toward common cancers, including colon
cancer and breast cancer. Families who carry these altered genes may also
have an increased risk of other cancers. Women with an altered copy of the
BRCA1 breast cancer susceptibility gene, in particular, are susceptible to
ovarian cancer as well. People who inherit cancer genes are more likely to
develop cancer at a young age, because the predisposing gene damage is
present throughout their lives, ready to set cancer's uncontrolled growth
in motion should the normal allele be lost or inactivated.
Such inherited, or familial, forms of cancer represent perhaps about 5
to 10 percent of all cancers. The great majority of people who get breast
cancer or colon cancer have not inherited such highly active altered
genes. This is true even for many families that have several members with
cancer; certain cancers are so common that some clusters are bound to
happen purely by chance. Cases that are diagnosed at older ages, in
particular, are more likely to be caused by acquired mutations.
Nevertheless, because breast and colon cancer are so widespread, even a
small fraction of the total equals a very large number. It is estimated
that as many as 1 in 300 women may carry inherited mutations of breast
cancer susceptibility genes, and approximately the same proportion of
Americans carry mutations that make them susceptible to colon cancer.
