The life expectancy for people born with Down's syndrome has doubled over the past 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 for the population at large, the data reveal.
The findings, by researchers from the U.S. Centers for Disease Control and Prevention and the University of British Columbia in Vancouver, suggest that the health of people with Down's syndrome, a genetic abnormality causing an array of traits including mental retardation, has improved markedly over the years. The results were published in Friday'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's _ and earlier, before dangerous complications can develop, said Dr. Kevin M. Shannon, associate professor of pediatric cardiology at Mattel Children's Hospital at the University of California, Los Angeles.
There is also greater recognition that babies with Down's are entitled to aggressive medical interventions, doctors said.
In former decades, children with Down's syndrome were often institutionalized. Today, most live with their families or in smaller group homes.
The low rates of cancers, meanwhile, could teach scientists something about the very biology of malignancy. People with Down's syndrome possess 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.
The finding is "extremely important," said Dr. Julie R. Korenberg, vice chair of pediatrics at Los Angeles' 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.
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 are often 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.
The findings also highlight strong racial disparities in life span of people with Down's syndrome. Although the disparities have narrowed in recent years, blacks in particular had shorter life spans than did whites. The cause for this difference is unclear, the authors said.
Epidemiologist Quenhe Yang at the CDC and colleagues analyzed U.S. death certificate data for the years 1983 through 1997. They compared the age of death of nearly 18,000 people who were identified on those certificates as having Down's syndrome to roughly 32-million who were not thus identified.
In 1983, the authors found, the median age of death for people with Down's was 25 years. That number increased, over the 15-year period, to 49 years 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 are often recorded on death certificates, even if they are not the cause of death.)
They found that some diseases were noted much more frequently on the death certificates of people with Down's syndrome. These included heart defects, dementias, thyroid disorders and diseases and infections of the lungs.
The scientists also found that many cancers were strikingly less common in people with Down's, cropping up on their death certificates at less than one-tenth the rate of that for the general population. The exception were leukemias and testicular cancers, both of which are elevated in people with Down's.