Down Syndrome Patients Live Much Longer Now, Study Finds
The life expectancy for people born with Down syndrome has doubled over the last two decades, according to an analysis of U.S. death certificates.
In addition, for unknown reasons, rates of most cancers are far lower for people with this genetic condition than they are for the population at large, the data reveal.
The findings, by researchers from the U.S. Centers for Disease Control and Prevention in Atlanta and the University of British Columbia in Vancouver, suggest that the health of people with Down syndrome, a genetic abnormality causing an array of traits including mental retardation, has improved markedly over the years. The results are published in today’s edition of the medical journal The Lancet.
Several causes probably contribute to the change, including the demise of institutionalization, access to better care and advances in surgery to treat the congenital heart defects that accompany the syndrome in many cases.
Surgery for life-threatening heart defects, for example, is performed more often these days for people with Down syndrome--and earlier, before dangerous complications can develop, said Dr. Kevin M. Shannon, associate professor of pediatric cardiology at Mattel Children’s Hospital at UCLA.
There also is greater recognition that Down syndrome babies are entitled to aggressive medical interventions, doctors said.
The low rates of cancers, meanwhile, could teach scientists something about the very biology of malignancy. People with Down syndrome have an extra copy of chromosome 21 in all their cells--and thus extra copies of several hundred genes. Researchers think one or more of those genes, when present in an extra dose, may render cells more resistant to cancer.
Finding May Be Key to Lessening Tumor Risk
The finding is “extremely important,” said Dr. Julie R. Korenberg, vice chairwoman of pediatrics at Cedars Sinai Medical Center and a professor at UCLA. “The implications are that if we could understand the gene or genes responsible for the decrease, we would have a way of decreasing this tumor risk in the rest of us who don’t have three copies of chromosome 21.”
Dr. Charles Epstein, chief of the division of medical genetics at UC San Francisco, said the result could indicate that chromosome 21 houses so-called tumor suppressor genes, which put the brakes on abnormal, out-of-control cell division. If a cell had three copies of a tumor suppressor gene instead of just two, it would be harder for a cell to mutate to form cancer, Epstein said.
Chromosome 21 is known to carry some genes that might theoretically help lower cancer rates. One, called superoxide dismutase, helps destroy tissue-damaging chemicals known as free radicals. Korenberg, meanwhile, said her lab has discovered that a gene responsible for the production of another potential tumor-fighting chemical, known as endostatin, resides on chromosome 21.
The study, although large, has its limitations, the authors and other experts noted. It relies upon death certificate data to reach its conclusions--and such data often are incomplete and may be more likely to report some diseases than others. But it is unlikely that those shortcomings in the data could account for the new findings, the authors said.
Racial Disparities Also Found
The findings also highlight strong racial disparities in the life spans of people with Down syndrome. Although the disparities have narrowed in recent years, African Americans in particular had shorter life spans than did whites. The cause for this difference is unclear, the authors said.
Down syndrome is a genetic condition that usually stems from a mistake made during the creation of eggs or sperm. As a result, a child ends up with three copies of one of the human chromosomes instead of the normal two. The resulting imbalance in gene activity causes developmental abnormalities, including mental retardation, heart defects and unusual facial features.
In former decades, Down syndrome children often were institutionalized. Today, most live with their families or in smaller, group homes.
To track the changing welfare of people with Down syndrome, epidemiologist Quanhe Yang at the CDC and colleagues analyzed U.S. death certificate data for 1983 through 1997. They compared the age of death of nearly 18,000 people who were identified on those certificates as having Down syndrome to about 32 million who were not so identified.
In 1983, the authors found, the median age of death for people with the syndrome was 25. Over the 15 years, that increased to 49 years old in 1997. The median age of death for the general population was 73 in 1983, increasing to 76 in 1997.
The scientists also examined the death certificate data for an array of major medical conditions. (Such conditions often are recorded on death certificates, even if they are not the actual cause of death.)
They found that some diseases were noted much more frequently on the death certificates of people with Down syndrome. These included heart defects, dementia, thyroid disorders and diseases and infections of the lungs. All are known to be particular problems for people with Down syndrome.
The scientists also found that many cancers were strikingly less common in people with Down syndrome, cropping up on their death certificates at less than one-tenth the rate of that for the general population. The exceptions were leukemias and testicular cancers, both of which are elevated in people with Down syndrome.
“Breast, lung, colon, etc.--all of the common adult cancers that you hear of people you know dying from--all were much lower,” said Dr. Sonja A. Rasmussen, a coauthor of the study and a clinical geneticist at the CDC.
This disparity can’t be explained by the earlier age of death for people with Down syndrome, Rasmussen said, as age was statistically controlled for in the study. Moreover, she said, the low cancer finding fits with two earlier, smaller studies--as well as doctors’ general impression from their clinical experience with Down syndrome patients. “We think,” Rasmussen said, “the finding is real.”