Parkinson’s is a neurogenerative disorder that increases with severity over time. Parkinson’s disease the result of nuronal death in the brain causing a decreased concentration of the neurotransmitter dopamine which is used to signal muscle movement. Parkinson’s leads to tremors, cognitive dysfunction and many other psychological ailments. The etiological source of Parkinson’s is not yet fully known. So far the consensus between academics is that it is a combination of genetic and environmental factors. Chronic exposure to environmental toxicants such as lead has shown to be a risk factor. This has been hypothesised as lead can bioaccumulate in the Substantia Nigra and cause oxidative stress. Assessment of lead concentrations is examined in the blood for acute exposure while lead concentrations in bones have a higher residence time which can assess lifetime exposure. As exposure to lead can cause varying severities of Parkinson’s disease this review details the different approaches taken by current and past studies while discussing its exposure, mode of action and genetic effect. Lead is a dangerous toxicant because it can pass through the blood-brain barrier. It can achieve this because it can obstruct the regulatory mechanism of calcium ions within a cell, which can have severe effects on intracellular biological activities. When lead enters the brain, many neurological disorders can develop such as brain/nerve damage, mental retardation, Parkinson’s and dementia. Lead also has been shown to have to a toxic effect on geans which are associated with the brain, bone marrow, liver and lung cells.
- How is lead initially implicated with Parkinson’s? Cross-Sectional and ecological studies.
The investigation of lead for its possible ecological link to Parkinson’s was inspired by the discovery in 1920 that l-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) showed a chemically induced cause of Parkinson’s disease. This discovery showed that exogenous neurotoxins could cause Parkinson’s disease. Building on this discovery Seidler et al., 1996. Devised an ecological, case-controlled study that analysed several factors from living in rural areas, to show if and by how much they could change the risk of developing Parkinson’s disease. A rough 2:1 ratio was kept between controls and subjects who had Parkinson’s disease. A Total of 380 diagnosed subjects were recruited from hospitals, and 755 control subjects were recruited from local neighbourhoods and regional areas in Germany. The diagnosed subjects were interviewed about their possible environmental exposure to neurotoxic substances before developing the disease, and controls were interview for previous exposure within the past year. These groups were separated into never being exposed to occupational lead and ever being exposed. A statistical analysis of their results showed some significance for the link between lead exposure to Parkinson’s disease. This study was not conclusive as only 10% of the control and patients claimed they were exposed to heavy metals and lead exposure was self-reported exposure and which reduces the results scientific validity.
Winkel et al. conducted one of the first ecological studies implicating lead exposure to Parkinson’s., in 1995. This study analysed three post office workers who were in contact with lead sulphate batteries throughout thirty-six years. The three post office workers all showed symptoms of parkinsonism such as tremors, peripheral neuropathy, diminished mental processing (bradyphrenia), loss of memory, depression, muscular rigidity and Parkinson’s disease. The conclusion of this report highlighted that lead or lead compounds caused the prevalence of Parkinson’s between subjects. Kuhn et al. conducted a similar study., in 1998 which focused on nine postal workers who were exposed to Lead Sulphate battery powered wagons throughout thirty years. These batteries had positive and negatively charged lead rods that were contained in a solution of sulfuric acid. To prevent short-circuiting workers had to remove the lead precipitate out of the battery daily. As there was a lack of personal protective equipment, the workers would frequently be exposed to sulphur when changing the acid and lead when replacing the rods and removing the contaminated precipitate. This study labelled each a number from 1 to 9. Cases 1 to 3 and 6 to 7 was expertly assessed for the link between their occupation and their development of Parkinson’s. Case 8 was tested for neurological damage and cases 4 and five died before the study. Out of the nine postal workers, seven were studied, three had signs of cortical atrophy, and one had periventricular legions in the brain. The workers also had other indicative signs of idiopathic Parkinson’s disease such as diminished motor function, resting tremors and impairment of voluntary movement (akinesia). Through the high occurrence of these extrapyramidal symptoms, it was hypothesised that the chronic exposure to lead or the combination of the lead with sulphate present in the batteries was a factor in the development of Parkinson’s disease. This hypothesis does not explain the underlying pathological mode of action for the lead. As four of the subjects had evidence of axonal neuropathy showed that Parkinson’s disease could originate from a toxin.
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An ecological study conducted by Santurtún et al., 2016 analysed the geographic distribution of mortality as a result of Parkinson’s for 14 years. This data was collected, and cross-referenced with lead pollution concentrations in 50 different Spanish counties. Results showed that out of 36180 patients analysed most of the deaths accounted for patients over 65 and who lived in northern Spain. This co-incited with data connecting the northern states with increased air lead concentrations. The causal link between atmospheric lead and Parkinson’s disease could not be made, but from the available evidence, there was a significant link between Lead and Parkinson’s.
- Leads mechanism of exposure.
As lead has such a wide variety of uses, it made it easier for humans to get exposed to organic and inorganic forms. In the European Union, the two primary sources of dietary lead exposure are from cereals, grains, vegetables and tap water. (EFSA 2012). When lead (Pb2+) enters the body, it gets stored in bones and blood. Some of the most common forms of exposure are inhalation, ingestion and physical contact. Inhalation of lead fumes or particles is an issue for occupational health as workers may unwittingly expose themselves as the lead is odourless. During metal processing and soldering, escaping lead fumes can be inhaled by workers. Lead particles can also originate from fragmentation or working with lead paint. Lead particles depositing on surfaces such as food, clothes and other items can then be ingested and enter the body. Some studies have also shown leads ability to absorb through the skin (CDC, 2018). In the European Union, the two primary sources of dietary lead exposure are from cereals, grains, vegetables and tap water. (EFSA 2012).
- How have cohort and case-control studies been used to implicate lead in Parkinson’s Disease?
A case-controlled study conducted by Gorell et al., 1997, investigated the etiological importance of chronic exposure to lead, lead-copper and lead-iron for a period over 20 years. This case-controlled study gave the first look a population-based exposure in a workplace. The sample population were required to pass a Mini-Mental state exam, so no test subject was cognitively impaired. Following the study by Seidler et al., 1996, case subjects had to have some form of bradykinesia, tremor, muscular rigidity and loss/decreased postural reflexes. The sample population composed of 144 cases and 464 controls recruited from the metropolitan Detroit area. These remaining participants completed a risk factor questionnaire on their past occupations lasting six months or more. An industrial hygienist assessed the risk of exposure to several heavy metals including Lead. Before this study, Lead was not sufficiently associated with Parkinson’s disease but was tested as the exposure was already measured in workplaces. Once again exposure was categorised into ever being exposed vs never being exposed to heavy metal. Out of 608 participants, there was a total of 15.1% who were exposed to lead, and 5.8% were exposed to lead over a period higher than 20 years. The odds ratio (OR) shown for the relation between lead and Parkinson’s disease was not statistically significant for short-term exposure (≤20 years) but long-term exposure (>20 years) the OR goes to 2.05 p=0.059 showing there is a borderline significance relating Parkinson’s to Lead
In 2006 Coon et al. set up a test to investigate the link between chronic occupational exposure to lead on the findings from several studies including Gorell et al. 1997. In this study, the left tibia and calcaneus bones were analysed using blood levels in combination with K-shell X-ray fluorescence to measure a patients acute and chronic lead exposure. They compiled their data to estimate the bodies clearance rate of bone lead and applied it to a pharmacokinetic model which tells when did exposure occur. In this study they started by carefully selecting their patients through a series of screening processes through the Henry Ford Health System database screened 121 Parkinson’s disease patients and 414 controls who were 50 years or older. Occupational data was taken from the subjects, and a historical guide was made since they were 18 to the time of recruitment. The results showed that concentrations of lead increases within bones over time. This accumulation of lead gets slowly released back into the body, causing chronic exposure. The subjects showed that those who had increased exposure to Lead had an increased risk of developing Parkinson’s disease. This result also supported the results from Gorell et al. (1997) which stated a two-fold increased the risk of developing Parkinson’s disease from lead exposure.
A case-control study conducted by Weisskopf et al. in 2010 tried to replicate the work conducted by Coon et al. but increasing the population size. This study aimed to link bone Lead concentrations and the occurrence of Parkinson disease due to cumulative exposure. This study took a sample of 330 people who had Parkinson disease and 308 controls who had no evidence of Parkinson. Lead has a half-life of 20-30 years when deposited in bones (ATSDR 2007), making it an acceptable biomarker to get a more accurate representation of cumulative exposure. Using K-shell x-ray fluorescence technique, lead concentrations were taken from the left tibia and patella bones in the patients. They concluded that since the tibia was cortical bone, it had a low turnover rate for lead and gave a half-life of around 20 years. The patella is a trabecular bone which has a higher turnover rate of a decade which gives a shorter reference time to exposure which may affect the visible occurrence of Parkinson. The patients were categorised by their age, race, education and smoking habits which yielded similar bone lead concentrations throughout all patients. This categorisation also made results independent of these factors thus creating less bias. From this study, they could not find an association with the patella bone lead concentrations and Parkinson disease, but they did find a link between bone lead concentrations in the tibia. This finding supports the previous evidence showing that chronic exposure throughout 20 years gives a higher risk of developing Parkinson.
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A cohort study conducted by Willis et al., in 2010 looked at 29 million people who availed of Medicare in 2003. This study gives community-based information about heavy metal exposure and the potential neurotoxic effect across the United States of America in comparison to smaller case-controlled studies of a select group. Only subjects who had stayed in the same residence for the past eight years were included in this study, reducing bias as it accounted for the movement in and out of states which could lead to bias results. Out of this population, 35,000 people with Parkinson’s disease were categorised by age, sex and race. These categories were then compared to counties that had high levels of Copper, Lead and Manganese air pollution and counties with lower levels of air pollution. The use of pollution rates in an area helped reduce recall bias from cases as they did not rely on self-reported cases of exposure seen in the report by Seidler et al., 1996. Although the primary data showed that areas that had higher levels of lead pollution had an increased occurrence of Parkinson’s disease, the study could not show a substantial correlation. The shorter analysis period of 8 years may be the cause in the lower correlation between lead and Parkinson’s. Previous studies conducted Coon et al.,. Gorell et al., and Weisskopf et al. showed that the onset of Parkinson’s disease correlates with a period of exposure for more than 20 years.
As lead requires chronic exposure for 20 years and its complex mechanistic pathway in biota, it is difficult to reach a consensus. A Case-controlled study in 2010 by Firestone et al., tried to resolve the inconsistencies within this field. An expert neurologist analysed candidates and selected cases with idiopathic Parkinson’s disease between 1992 and 2006. A total of 404 Parkinson disease cases and 526 unrelated controls were selected. Information was collected on their medical history, smoking habits, where they were demographically situated and information on past jobs which lasted over six months. Each job was assessed for exposure to heavy metals, solvents and pesticides. Candidates were separated by sex as occupations were generally dissimilar. These groups were further separated into never exposed if the exposure period was less than 20 years and ever exposed if exposure was greater than 20 years. The results showed that there was not a higher occurrence of Parkinson’s disease with higher exposure to lead, which contradicts previous studies conducted by [Gorell et al., 1999; Coon et al., 2006]. Variability may have come from the different study designs and a more significant impact of outliers due to their smaller study population.
- The likely mechanism(s) by which the toxicant exerts its effect.
Lead is a known potent neurotoxicant which has many mechanisms which it can exert its effect. When lead enters the body, it substitutes hydroxyapatite crystals in the bones. During bone remodelling, production and reabsorption, lead gets discharged back into the circulatory system. Lead then diffuses through the blood-brain barrier and attaches to sulfhydryl groups. This attachment has been associated with oxidative damage to intracellular neurons (Coon et al.,2006). The current understanding of the connection between lead and Parkinson’s disease is not yet understood but in-vitro experiments suggest that Lead can propagate fibrillation and attach to negatively charged carboxylates, this attachment restricts electrostatic repulsion and facilitating collapse to the partially folded conformation which promotes the aggregation of α-synuclein. (Fink, 2006, Uversky et al., 2001).
This accumulation leads to the presence of Lewy bodies present in the remaining living neurons which can act as an indicator for diagnosis of Parkinson’s. (Uversky et al., 2001).
Animal testing has also shown that lead can proliferate lipid peroxidation and damage cellular membranes which leads to neuronal death (Sandhir et al., 1994). It is thought that Leads ability to increase free radicals induces oxidative stress in the Basal Ganglia and Substantia Nigra, damage to this part of the brain associated with Parkinson’s disease. Oxidative stress can damage the mitochondria, kill neurons through overactivation (Excitotoxicity) and it can increase cytosolic free calcium which can kill neuronal cells (Uversky et al., 2001) Experiments on rats have also shown that lead exposure diminishes the release of dopamine by reducing depolarisation and it also decreases the sensitivity of the subsequent D1 receptor. Altering the body’s regulation of dopamine upsets any dopamine-dependent behaviours (Weisskopf et al., 2010).
- How interaction with other agents modifies the toxicant’s effects.
Case-control studies showed that the dual interaction between iron and lead significantly increases the chance in developing Parkinson’s disease OR = 2.84 p = 0.036 and more notably lead and copper OR = 5.25 p = 0.006 (Gorell et al., 1997, Gorell et al., 1999, Coon et al., 2006). The combination of these heavy metals is proposed to substantially increase levels of oxidative stress in comparison to lead by its self. Kuhn et al. (1998) have also proposed that the combination of sulphate and lead have a potential magnifying which could increase the risk of developing Parkinson’s disease. When exposed to several cations there is an observed increase in amyloid-beta, amyloid precursor protein, presenilin and β secretase complex in the brain suggests amyloidogenic processing. By processing amyloid- β precursor protein it can no longer repair or grow existing neurons causing neuronal death and the development of Parkinson’s disease, Alzheimer’s disease. (Ashok et al., 2015). Authors also observed (Ashok et al., 2015) increased levels of malondialdehyde (MDA), reduced activity of antioxidant enzymes, and the induction of 1L-1α and IL-1β in the frontal cortex and hippocampus of rats exposed to As + Pb + Cd mixture. #
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