About a decade ago, the apolipoprotein E (apo E) e4 allele was linked with an increased risk of developing Alzheimer’s disease. The most important implication of the new finding is that it suggests a tie between these two neurodegenerative diseases, Dr. Jeffrey M. Vance said at the annual meeting of the American Society of Human Genetics. But the impact of the e4 allele on Parkinson’s disease seems weaker than its impact on Alzheimer’s disease.

Other neurodegenerative diseases also have recently been linked to the e4 allele. People who carry at least one of these alleles and develop multiple sclerosis have a more severe form of the disease. And people with the e4 allele who develop amyotrophic lateral sclerosis have an earlier age of onset.

The neurological process that ties the e4 allele to these four diseases remains unknown. “It’s probably some basic biologic process that involves the human body’s reaction to stress,” said Dr. Vance, co director of the Center for Human Genetics at Duke University in Durham, N.C.

He and his associates analyzed the apolipoprotein genes carried by 1,051 patients with Parkinson’s disease; 944 close relatives served as controls. All of the patients came from families with two or more affected members. Most of the unaffected relatives were siblings of the patients.

The analysis showed that people who carried one or two apo E e4 alleles were more likely to have a severe form of Parkinson’s disease and developed the disease an average of about 4 years earlier in life than patients who lacked an e4 allele. The differences in both disease severity and age of onset were statistically significant.

Two other alleles can potentially fill the apo E locus: e2 and 3. People who carried one or two e2 alleles seemed to be protected against having Parkinson’s disease. The e3 allele seemed to have no effect on the severity or age of onset of Parkinson’s disease. A similar pattern of effects also exists for Alzheimer’s disease.