It may be possible to identify at what age someone might develop Parkinson's disease by studying their genes, a study suggests.
The age of onset of Parkinson's could be determined by genes
A study of 183 families affected by Parkinson's found a genetic mutation is linked to the age it affects someone.
People with one mutation develop the disease 12 years earlier than average, results in Archives of Neurology show.
Two mutated genes are linked with disease which starts 13 years earlier, they found.
Currently, genetic screening for Parkinson's is not available in the UK.
The prevalence of Parkinson's increases with age - appearing in 1% of people over 60 and 4-5% of those over 85 - but it can develop in much younger patients.
Research has already shown that inheriting two abnormal copies of the parkin gene is associated with the development of early-onset Parkinson's - which refers to disease which appears before the age of 50.
But a team led by Massachusetts General Hospital researchers has now found that even a single mutated copy of parkin reduces the age of onset of the disease.
The study was done as part of the international GenePD project, which is looking at siblings with Parkinson's disease.
Families were selected for the study if affected siblings had inherited identical versions of the chromosome 6 region, which contains the parkin gene, or at least one affected member developed symptoms before the age of 54.
Complete genetic screens were carried out on the patient with the earliest onset of symptoms and, if that patient was found to have parkin mutations, other affected family members were screened.
Among families with parkin mutations, 13 had at least one member with mutations on both copies of the gene, while in 10 families affected members had a single mutated copy of parkin.
In total, 18 different mutations were found, four of which had not previously been reported.
In people with parkin mutations, the average age when the disease became apparent was about 43.
In a subgroup of families who had identical chromosome markers, those with no mutations in the gene developed symptoms at an average age of about 61, those with a single abnormal copy of parkin at around age 50, and those who inherited two mutated copies had onset at around age 36.
"This is the first time anyone has shown that a single parkin mutation can lower the age of onset," said Dr James Gusella, study leader and director of the Massachusetts General Hospital Center for Human Genetic Research.
"We don't know if that would be sufficient to cause the disease.
"It's more likely that these mutations increase susceptibility to other factors underlying the development of Parkinson's," he added.
Kieran Breen, the director of research, at the Parkinson's Disease Society said: "There isn't currently a case for genetic testing to take place, even where a relative has the condition.
"Screening to identify the gene may only cause unnecessary alarm or worry in people who may never go on to develop Parkinson's.
"Research is ongoing on this subject in order to gain a greater understanding of the factors associated with Parkinson's."