Colon Cancer Patients With Genetic Abnormality Live Longer
Colorectal cancer is the second-leading cause of cancer deaths in the U.S.
(after lung cancer), and the third most common type of cancer among men and
women, according to the American Cancer Society.
Patients with colorectal cancer tumors that display microsatellite
instability appear to have a greater chance for long-term survival and to be at
lower risk for spread of their cancers to other organs than similar patients
with tumors that do not show the abnormality, according to Steven Gallinger,
MD, and colleagues from the University of Toronto, Mount Sinai Hospital, and
Cancer Care Ontario, all in Toronto.
In a study of 607 people aged 50 and under who were diagnosed with
colorectal cancer, 75% of patients with the genetic abnormality lived at least
five years after diagnosis, compared with 54% of those whose tumors did not
display the abnormality.
Intriguingly, patients whose tumors displayed microsatellite instability
were more likely to live longer and have a lower risk for cancer spread even
when they had multiple tumors, or tumors that extended deeply into the colon or
surrounding tissues, the researchers say.
"Now we can think of colon cancer as two different diseases from a
fundamental level," says co-researcher Mark Redston, MD. "I would guess
that in the future all cancers will be stratified into multiple diseases -- any
single cancer in five or 10 years will be stratified, by the types of genes,
into different diseases," Redston tells WebMD. Redston is assistant
professor of laboratory medicine and pathobiology and associate scientist at
the Centre for Cancer Genetics at the University of Toronto.
Microsatellite instability is a type of genetic error found in the
"letter" pairs that make up DNA, the basic set of instructions found in
every cell of the body. The term "microsatellite" refers to small
series of letter pairs that are repeated several times over in short stretches
of so-called "junk" or "nonsense" DNA that lie between genes.
When cells are damaged or divide during normal growth, the DNA in the cells
splits apart like two halves of a zipper and then reassembles, with each letter
matching up with its designated partner. Sometimes the zipper gets snagged and
has to be repaired by a special DNA mismatch repair system, but if the repair
system fails to work as it should, microsatellite stretches of DNA may hook up
with the wrong partner, resulting in the condition known as microsatellite
Microsatellite instability has been found in a high percentage of tumors
that occur in people with hereditary nonpolyposis colorectal cancer (HNPCC), an
inherited form of colon cancer in which multiple tumors occur at a young age.
The genetic abnormality is also found to a smaller degree in "sporadic"
colorectal cancers, which arise without any known cause.