Parkinson's disease affects about 1 percent of individuals age 60 years or older and 4 to 5 percent of those age 85 years or older, according to background information in the article. The neurodegenerative disorder is characterized by tremors, rigid movements, difficulty walking and a mask-like facial appearance. Though much about the development of Parkinson's disease remains unknown, family history has been found to significantly increase risk. Researchers have located several genes that may be implicated, including the parkin gene. About 50 percent of individuals with "juvenile" parkinsonism (in which features of Parkinson's disease develop before age 21 years) and 10 to 25 percent of those with early-onset (before age 50) Parkinson's disease have mutated parkin genes.
Mei Sun, M.D., Ph.D., Massachusetts General Hospital and Harvard Medical School, Boston, and colleagues studied 183 families in which two or more members had Parkinson's disease. The families also met one of two other criteria: either one individual in the family developed Parkinson's disease before age 54 years, or two affected siblings had identical versions of the parkin gene. Because siblings inherit one chromosome from their mother and one chromosome from their father, they can share either zero, one or two versions of each gene. At least one member of each family underwent comprehensive genetic screening to determine the number and nature of any mutations to the parkin gene.
Twenty-three (12.6 percent) of the families screened contained at least one member with a mutated parkin gene. Among those families, 10 (43 percent) had at least one member with two different mutations; three (13 percent) had at least one member with two copies of the same mutation; and 10 (43 percent) had at least one member with one mutated parkin gene and one normal parkin gene. Individuals with one mutated gene and one normal gene are known as heterozygotes; researchers had not previously determined whether heterozygotes were at increased risk for Parkinson's disease. Patients with parkin mutations developed Parkinson's disease at an average age of 42.9 years. Among the families who were selected because two siblings shared the same version of the parkin gene, heterozygotes developed Parkinson's disease an average of 11.7 years earlier than those with no mutations, and those with two or more mutations developed the condition 13.2 years earlier than heterozygotes.
"In conclusion, parkin mutations are not rare in this selected subset of familial Parkinson's disease samples (12.6 percent)," the authors write. "Single parkin mutations may increase susceptibility to the disease and decrease the onset age of Parkinson's disease (mean onset age, 49.6 years)." The fact that even heterozygotes developed Parkinson's disease at an earlier age than those with two normal copies indicates that the gene's effect may be stronger than previously recognized, and that screening for mutations in the gene may help identify those at risk of developing the condition.
(Arch Neurol. 2006;63:826-832. Available pre-embargo to the media at www.jamamedia.org.)
Editor's Note: This study was supported by the Bumpus Foundation and grants from the U.S. Public Health Service. The DNA samples contributed by the Parkinson Institute, Istituti Clinici di Perfezionamento, were from the Human Genetic Bank of Patients Affected by Parkinson Disease and Parkinsonisms, supported by a grant from the Italian Telethon Foundation.
Editorial: Genetic Studies Help Explain Parkinson's Disease Development
Understanding how mutations in parkin and other genes contribute to Parkinson's disease offers valuable clues to the still-mysterious pathways of its development, write Roger N. Rosenberg, M.D., editor, Archives of Neurology, and Uwe Beffert, Ph.D., both of University of Texas, Southwestern Medical Center, Dallas, in an accompanying editorial.
The editorialists note that if the results of this study are confirmed, they write, they "will require a reexamination of the genetic data associated with Parkinson's disease to take into account the possibility that heterozygous mutations in autosomal recessive genes such as parkin and PINK1 affect the susceptibility and age of onset of Parkinson's disease. These findings are provocative and add a new genetic dimension to the pathogenesis of Parkinson's disease."
(Arch Neurol. 2006;63:807-808. Available pre-embargo to the media at www.jamamedia.org.)
To contact editorialist Roger N. Rosenberg, M.D., call Aline McKenzie at 214-648-3404.