What Causes Parkinson’s Disease?

Parkinson’s disease is a chronic and progressive neurological disorder. It causes trembling, slow or interrupted movements (termed “bradykinesia”), rigidity, and postural problems, among other symptoms. 

Two biological processes are seen with this condition. The first is the death of neurons (brain cells) in the substantia nigra of the basal ganglia region of the brain, which is the area that controls movement. The second is the accumulation of alpha-synuclein protein, forming deposits in the called Lewy bodies. In turn, these factors lead to the insufficient production of dopamine (a brain chemical that helps regulate movement).

Currently, Parkinson’s disease is thought to arise due to a combination of genetic and environmental factors. Though this condition isn’t inherited, it’s believed that having certain genes can increase your susceptibility. Alongside genetics, exposure to certain toxins and a number of lifestyle factors increase the risk of developing it.

Theories

More research is needed to determine the exact causes of the condition. However, several theories have been proposed, and the current belief is that the disease arises due to a combination of certain genetic, environmental, and behavioral factors.

Genetic Factors

Genetic mutations are thought to play a significant role in the development of Parkinson’s disease in about 5 to 10% of cases. These are linked to specific mutations of the synuclein alpha (SNCA) and leucine-rich repeat kinase 2 (LRRK2) genes, among others. 

These mutations allow synuclein alpha to build up and form Lewy bodies, impacting the natural release of a brain chemical called dopamine. Dopamine plays an important role in helping to control voluntary and involuntary movements.

Genetics alone is rarely the cause of Parkinson’s disease. However, it is believed they can increase the likelihood of someone developing the condition when combined with other factors.

Mitochondrial Dysfunction

Another hypothesis as to the cause of Parkinson’s disease is dysfunction of the mitochondria, which are the parts of a cell involved with respiration and energy production. This dysfunction in the brain cells of the substantia nigra leads to their degradation and death. It likely occurs alongside other factors in the disease, such as exposure to toxins in the environment and lifestyle risk factors.

Oxidative Stress

Reactive oxygen species (ROS) are naturally occurring molecules in the cell. They play a role in cellular communication, helping areas of the body communicate with each other. Elevated levels of ROS—usually occurring as a result of mitochondrial dysfunction—can cause oxidative stress. Oxidative stress can damage or even kill necessary cells.

Certain stressors, such as exposure to certain environmental toxins, metals, and others boost ROS, which can damage and degenerate cells, leading to Parkinson’s disease.

Exposure to Toxins

Though more research is needed, there’s evidence that Parkinson’s disease can arise due to exposure to environmental toxins, which can trigger oxidative stress. In particular, studies have found a connection between exposure to certain pesticides, such as paraquat and rotenone, and oxidative stress. 

Elevated levels of iron may have a similar effect. This is because iron plays a role in cellular metabolism. Significantly elevated levels have been found in the brains of those with Parkinson’s disease.

Is Parkinson’s Disease Hereditary?

Some rare types of Parkinson’s disease cases are hereditary (passed from parents to child). Mutations of several genes, including synuclein alpha (SNCA) and leucine-rich repeat kinase 2 (LRRK2), are directly inherited, causing what are called monogenic cases of the condition (cases that occur because of a mutation on a single gene). In these cases, the inheritance pattern has been found to vary.

Some forms of the disease are autosomal dominant, meaning only one parent with the mutation is needed to pass the disease along. Other types are autosomal recessive, in which the disease arises only when both parents have it.

That said, having the mutated gene may not always lead to Parkinson’s disease in these cases. A study of twins estimated the overall heritability—the extent to which genetic differences lead to the condition—to be 30%, indicating that other factors are more significant. 

Ultimately, about nine out of 10 cases of Parkinson’s disease aren’t directly linked to gene mutations. Further research is needed to fully understand the genetic component of this condition.

Who Gets Parkinson’s Disease?

Globally, it’s estimated 0.3% of adults over 40 develop Parkinson’s disease. Some populations are more likely to develop this condition.

• Age: Aging increases the chances of developing this condition. While seen in less than 1% of those aged 45 to 54, the prevalence rises to 4% of people assigned male at birth and 2% of those assigned female at birth who are age 85 or older.
• Sex: Across global populations, those assigned male at birth are twice as likely as those assigned female to develop Parkinson’s disease.
• Ethnicity: Certain ethnicities and races may be more prone to develop Parkinson’s disease. Ashkenazi Jews are susceptible to a genetic form of the condition. Also, studies have found elevated rates among Inuit, Alaska Native, and Native American populations.

Risk Factors

Further research is needed to fully grasp factors that increase the risk of developing Parkinson’s disease. Notably, It has been found that those that get regular physical activity and consume caffeine regularly may have the condition at lower rates. However, there’s evidence that this disease has several risk factors, including:

Head Trauma

A history of head trauma due to falls or contact sports has been associated with Parkinson’s disease risk. Studies have found a mild to moderate effect, with the chances of developing the condition increasing with the number of incidents.

Environmental Exposure

A number of studies have suggested that a history of being exposed to chemicals and toxins is a risk factor. Several links have been found:

• Pesticides and herbicides: Working with pesticides or herbicides, as in agricultural or landscaping work, has been found to increase the chances of developing Parkinson’s disease.
• Airborne toxins: Some studies have found living in urban or industrial areas are risk factors. Specifically, regular exposure to air or environments with high levels of pollutants, copper, manganese, and lead may increase your chances.
• Solvents: Working with or being regularly exposed to certain industrial solvents or paints, especially trichloroethylene (used to make refrigerants and as a metal degreaser), has been found to increase risk.

High Dairy Consumption  

Some studies have found higher rates of Parkinson’s disease among those who consume excess dairy. A 2014 meta-analysis of multiple studies in the European Journal of Epidemiology noted associations between higher rates of milk, cheese, and other product consumption and disease onset. This effect was more pronounced among those assigned male at birth.

Insufficient Vitamin D

Another risk factor for Parkinson’s disease is not getting enough vitamin D. Studies have linked not getting enough exposure to direct sunlight—a common source of this vitamin—to increased chances of developing Parkinson’s disease.

A Quick Review

Parkinson’s disease is a progressive neurodegenerative disorder characterized by tremors and motor difficulties. The exact causes of this condition are unknown, though a combination of genetic and environmental causes are thought to lead to it. Risk factors for Parkinson’s disease include older age, being assigned male at birth, and exposure to pesticides and airborne toxicants, among others.