People with Parkinson's disease do not make enough of a brain chemical called dopamine, leading to muscle tremors, slowed movement and muscle rigidity. Parkinson's disease can be difficult to diagnose because other diseases show similar symptoms. Its similarity to other diseases has also made it difficult to discover its true causes, although we know that both genetics and environmental factors play a role, and some of those factors have been identified.
Early Onset Genes
People usually do not develop Parkinson's disease before the age of 50. However, a rare form called early-onset Parkinson's disease can begin in people as young as 18 (although an age range of 28 to 35 years is more common). Early-onset Parkinson's is a recessive disease, meaning that you must inherit two faulty copies of the causative gene, one from each of your parents, in order to develop the disease.
Most people with early-onset Parkinson's disease carry two mutant copies of the gene parkin. The product of the gene parkin breaks down proteins in nerve cells.
The second most common cause of early-onset Parkinson's is having two mutant copies of the gene PINK1, encoding a protein in the energy production centers of your cells (mitochondria) that protects your body from cellular stress.
Some people with only one mutant copy of parkin or PINK1 develop Parkinson's disease later in life. The normal copy of the gene is able to do the work of both copies until another factor disrupts the cell and triggers the onset of Parkinson's disease.
Mutations of the genes DJ1 and FBXO7 are associated with early-onset Parkinson's disease, but they are quite rare.
Other Genes
Gene mutations causing Parkinson's disease in people over 50 are generally dominant mutations, meaning that you only need to inherit one mutant copy of the gene to eventually develop the disease.
There are four known autosomal dominant gene mutations associated with Parkinson's disease. A mutation in the gene alpha-synuclein causes plaques to form in the brain; a mutation in UCHL1 disrupts the function of nerve cells; a mutation in HTRA2 makes nerve cells more susceptible to the effects of stress; and a mutation in LRRK2 disrupts an enzyme that interacts with parkin.
Environment
Very often, identical twins are discordant for Parkinson's disease: even though they are genetically identical, one twin will have the disease while the other does not. This and other evidence leads some scientists to believe that in most cases, non-genetic ("environmental") factors act on the aging brain to cause Parkinson's disease. Despite this fairly widespread belief, there are very few identified environmental factors associated with Parkinson's disease. Prolonged exposure to some pesticides or herbicides can increase your risk, while caffeine might protect you from developing Parkinson's disease.
