Parkinson’s disease (PD) Risk Factors
For Parkinson’s disease both genetic factors and environmental/lifestyle factors may responsible for the disease.
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Gender
PD occurs more frequently in men than in women. The male-to-female ratio is about 1.3 to 2.0, meaning men are approximately 1.3–2 times more likely to develop the disease.
However, this difference may partly be due to lifestyle and hormonal factors such as cigarette smoking habits, use of hormones after menopause, and caffeine consumption.
Age
Age is the most important risk factors for PD, chances of PD increase with age. The average age at which people develop Parkinson’s disease is around 60 years.
As people age, several biological problems develop inside cells, especially nerve cells, which may lead to degeneration of neurons. These age-related problems include:
1. Telomere dysfunction – damage or shortening of protective chromosome ends.
2. Genomic instability – increased DNA damage or mutations.
3. Epigenetic changes – alterations in gene activity without changing DNA sequence.
4. Ubiquitin-proteasome and autophagy-lysosomal system defects – failure of the cell’s waste-removal and protein-cleaning systems.
5. Mitochondrial defects – problems in energy-producing structures of cells.
Together, these abnormalities may cause neuronal death or death of nerve cells, which contributes to Parkinson’s disease progression.
Researchers have divided Parkinson’s Disease into different subtypes based on symptoms. These subtypes are:
1. Tremor-dominant subtypes
2. Postural-Instability-Gait-Disorder (PIGD)
In Tremor-dominant subtype, trembling/shaking is the main symptom and progress more slowly.
In Postural-Instability-Gait-Disorder (PIGD) subtype, patients face difficulty in balancing, posture is unstable, and have walking problems. PIGD shows more severe symptoms, progress faster and have poor prognosis.
Both subtypes, are not permanent, As Parkinson’s disease progresses and patients receive treatment, symptoms can change. A person who initially has tremor-dominant PD may later develop PIGD features.
So, researchers now think this motor subtypes may be less stable than previously believed, and may not always represent completely separate forms of the disease.
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Environmental risk factors
Researchers have suggested many factors that may increase Parkinson’s disease risk, including:
1. Pesticide exposure,
2. Heavy metal exposure,
3. Living in rural areas,
4. Farming/agricultural work,
5. Head injury,
6. History of melanoma (skin cancer),
7. High dairy consumption,
8. Type 2 Diabetes, and others.
Although these risk factors seem biologically possible, many findings are not consistently reproduced in different studies.
Lifestyle and other factors
Researchers found that cigarette smoking and caffeine consumption (coffee/tea).
are the most consistently observed factors linked with lower risk of PD.
Other possible protective factors include higher levels of urate (uric acid) in blood,
use of ibuprofen, and regular exercise.
Why smoking and Parkinson’s disease are interesting
Studies repeatedly show an inverse (negative) association between smoking and PD. People who smoke appear less likely to develop PD compared with non-smokers.
However, this does NOT mean smoking is healthy or recommended, because smoking causes many serious diseases including cancer, heart disease, and lung disease.
Researchers proposed several theories:
1. Personality hypothesis
Some scientists think people who are naturally prone to PD may have a more cautious or avoidance-type personality. Because of this personality trait they may be less likely to start smoking, or more likely to quit smoking. So, the lower smoking rate may reflect personality differences rather than smoking itself being protective.
2. Nicotine neuroprotection hypothesis
Another theory is that nicotine may protect dopamine-producing neurons. Nicotine has been shown experimentally to stimulate dopamine release in the striatum (brain region involved in movement), help preserve dopaminergic neuron function in laboratory models.
3. Other unknown substances in smoke
Researchers also suggest cigarette smoke may contain other unidentified compounds that could have neuroprotective effects.
Caffeine consumption
Studies show that people who drink caffeine have a lower risk of developing Parkinson’s disease. The relative risk reduction is about 0.5–0.8, meaning caffeine drinkers may have approximately 20–50% lower risk compared with non-drinkers.
Researchers found a dose-dependent effect, meaning the more caffeine consumed (within studied ranges), the stronger the protective association appeared.
Caffeine blocks a brain receptor called the adenosine A2A receptor. Because caffeine acts as an A2A receptor antagonist (blocker), scientists think it may:
- protect dopamine-producing neurons,
- reduce neurodegeneration,
- help preserve brain function.
This possible protective action is called a neuroprotective role.
Black Tea consumption
Protection may not come only from caffeine. For example, black tea contains antioxidants and other beneficial compounds. These substances may help protect brain cells independently of caffeine.
Drinking caffeine, especially coffee or tea, has been linked with a lower risk of PD. Caffeine may protect brain cells by blocking certain brain receptors, and antioxidants in tea may also contribute to this protective effect.
Several substances that may influence the risk or progression of PD, but it also emphasizes that the scientific evidence is still unclear in many cases.
Uric acid and Parkinson’s disease
Uric acid is produced during purine metabolism in the body. It acts as an antioxidant; it can neutralize harmful free radicals.
A meta-analysis of 13 studies found that people with PD tend to have lower serum uric acid levels than healthy controls, and patients with more advanced Parkinson’s disease often have even lower levels than those in early stages.
This suggests uric acid might have a protective role, but the relationship is uncertain.
Some large studies, found no direct relationship between uric acid and PD. This suggests low uric acid may be associated with PD, but it may not actually cause or prevent it.
Ibuprofen and Parkinson’s disease
Studies suggest that Ibuprofen may reduce PD risk. However other NSAIDs (non-steroidal anti-inflammatory drugs) have not shown consistent results. So, the protective effect is still uncertain.
Statins and lipid levels
Researchers have also studied statins (cholesterol-lowering drugs), and blood lipid levels, but results remain inconclusive because different types of statins behave differently such as hydrophilic statins are water-soluble, hydrophobic/lipophilic statins are fat-soluble, lipid interactions are complex, and studies use different methods. Therefore, scientists cannot yet reach any conclusions.
Genetics
Twin and family studies
Early studies on twins, and families in which PD appeared repeatedly, suggested that genes can contribute to the disease. Some families showed Mendelian inheritance patterns, meaning the disease was inherited in predictable genetic ways:
Dominant inheritance – one mutated gene copy can cause disease.
Recessive inheritance – two mutated copies are usually required.
Family and twin studies showed that Parkinson’s disease can be inherited genetically. This led to the discovery of important genes such as SNCA and PRKN, which helped scientists better understand the disease mechanisms.
Because of improved genetic technology and large population studies, scientists have discovered more than 20 monogenic forms of Parkinson’s disease, and more than 100 genetic loci (gene regions) associated with increased disease risk.
Mutations
PARK-SNCA (PARK1)
Although mutations in the SNCA gene are rare causes of Parkinson’s disease, scientists now know that the protein it produces α-synuclein which plays a major role in disease development.
What is α-synuclein?
α-synuclein is a small protein normally helps nerve cells function properly.
Evidence that α-synuclein can be harmful. Studies in genetically modified (transgenic) mice showed that excessive α-synuclein production causes movement problems, degeneration of the substantia nigra (dopamine-producing brain region).
How α-synuclein becomes toxic?
The protein becomes harmful when too much normal protein accumulates, gene multiplication causes overproduction, mutations alter the protein, or dopamine chemically modifies it.
These changes cause α-synuclein to form abnormal clumps (oligomers and aggregates), which damage neurons, especially through interactions with cell membranes/lipids.
Lewy bodies and Lewy neurites
Abnormal aggregated α-synuclein is the main component of Lewy bodies and Lewy neurites.
These are the characteristic pathological features of Parkinson’s disease.
Many studies suggest α-synuclein pathology may begin in the peripheral nervous system, or olfactory bulb (smell-related brain region), then gradually spread upward through the brain from lower brainstem (caudal) to upper brain regions (rostral).
The Braak hypothesis.
α-synuclein is a normal brain protein, but when it accumulates abnormally and forms toxic clumps, it damages dopamine-producing neurons. These abnormal protein aggregates form Lewy bodies, a key feature of PD, and may spread gradually through the nervous system.
Although mutations in the SNCA gene are uncommon, researchers discovered that some people have duplication, triplication, or quadruplication of the SNCA gene.
When extra gene copies are present, the body produces too much α-synuclein, excessive protein accumulates, toxic aggregates form, and neurons become damaged which leads to PD.
Gene dosage effect- A gene dosage effect means the greater the number of SNCA gene copies, the more severe the disease. For example, people with SNCA triplication usually develop Parkinson’s disease at a younger age, more severe symptoms, and more cognitive impairment, compared with those having only duplication.
PARK-PArkiN (PARK2)
PRKN (Parkin) is the most common gene linked to autosomal recessive Parkinson’s disease. People who inherit two abnormal PRKN gene copies account for about 50% of early-onset Parkinson’s disease cases.
PRKN-related PD form often begins at a younger age and may show several characteristic symptoms such as:
Movement-related symptoms
- Dystonic gait- abnormal walking caused by involuntary muscle contractions.
- Leg tremor- shaking in the legs while resting, or while standing.
- Cervical dystonia- abnormal twisting or posture of the neck.
- Dopa-responsive dystonia- dystonia that improves strongly with levodopa treatment.
- Freezing- sudden inability to start or continue walking.
- Festination- rapid short shuffling steps.
- Retropulsion- tendency to fall backward.
Other neurological features
Marked sleep benefit symptoms improve temporarily after sleep.
Hyperreflexia- exaggerated reflexes.
Ataxia- poor coordination and balance.
Peripheral neuropathy- nerve damage outside the brain/spinal cord causing numbness or weakness.
Dysautonomia- autonomic nervous system dysfunction, affecting blood pressure, sweating, digestion, bladder function.
Pattern of symptoms- Usually symptoms affect both sides relatively equally (symmetrical). Rarely, it may appear as Hemiparkinsonism-hemiatrophy
Parkinson’s symptoms mainly on one side of the body, with wasting/shrinking (atrophy) of that side.
PARK-Lrrk2 (PARK8)
LRRK2 is the most common gene causing autosomal dominant Parkinson’s disease.
In autosomal dominant inheritance one abnormal copy of the gene can increase disease risk. G2019S mutation, is the most important LRRK2 mutation.
This mutation occurs in both familial PD, and sporadic (non-family) PD.
The mutation has age-dependent penetrance, not everyone carrying the mutation develops Parkinson’s disease immediately or at all, but the likelihood increases with age.
The G2019S mutation causes about 1–3% of sporadic PD, 3–4% of familial PD in most Caucasian populations. But it is much more common in North African Berbers, Iberian populations, and Ashkenazi Jewish populations, where it may account for up to 40% of cases.
The G2019S mutation is rare in Asians. Instead, Asians more commonly carry G2385R, and R1628P
Most people with LRRK2 mutations develop PD later in life, and resemble typical PD clinically. Often, they are difficult to distinguish from patients without the mutation.
Common symptoms include mainly PIGD phenotype: posture and gait problems, less REM sleep behavior disorder (RBD), relatively preserved sense of smell (olfaction). Some patients may also develop orthostatic hypotension, dementia, hallucinations, corticobasal syndrome and primary progressive aphasia
Patients may have Lewy bodies, or lack Lewy bodies entirely. Their pathology may overlap with Synucleinopathies (abnormal α-synuclein accumulation), Tauopathies (abnormal tau protein accumulation).
PARK-GBA
The PARK-GBA produces an enzyme called glucocerebrosidase. This enzyme works inside lysosomes.
When a person inherits two abnormal GBA gene copies, they can develop Gaucher Disease. This is the most common lysosomal storage disorder. Because enzyme activity becomes very low and glucocerebroside accumulates in spleen, liver, and bone marrow.
Gaucher’s disease patients have increased risk of Parkinson’s disease.
Mutations in GBA are now considered the single most important genetic risk factor for Parkinson’s disease in the general population.
GBA mutations are found in about 10% of sporadic PD, and more than 40% of familial PD in Ashkenazi Jewish populations.
Compared with typical Parkinson’s disease, GBA-associated PD often has younger age at onset, more cognitive impairment, more REM sleep behavior disorder (RBD).
Researchers believe that reduced glucocerebrosidase function impairs lysosomal activity. As a result, cellular waste removal becomes defective, α-synuclein accumulates, toxic protein aggregates form. This promotes Parkinson’s disease pathology.
Postmortem studies show classic Parkinson’s disease changes, plus widespread Lewy bodies in cortical brain areas.
Besides major genes like SNCA, PRKN, LRRK2, and GBA, researchers have identified many rarer genes linked to Parkinson’s disease.
Autosomal dominant genes- These include VPS35, EIF4G, DNAJC13, CHCHD2, Autosomal recessive genes- These include PINK1, DJ1, ATP13A2, GIGYF2, PLA2G6, FBXO7, DNAJC6, SYNJ1, and VPS13C.
These rare genetic forms often show atypical features meaning symptoms that differ from classic Parkinson’s disease.
Examples may include earlier onset, cognitive problems, dystonia, psychiatric symptoms, pyramidal signs, ataxia.
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