Journal of Clinical Epigenetics Open Access

Keywords

Parkinson’s disease; Epigenetics; Manganese; Environmental health; Neurotoxicity; Neurodegenerative disorders

Manganese, Exposure and Health Effects

Manganese as a part of normal physiology: source in diet, role as a cofactor, deficiencies

Manganese (Mn) is a naturally occurring component in the environment and an essential trace element in for normal growth and development through maintaining of proper cellular functions and biochemical processes [1]. Manganese deficienciency would lead to some diseases and it may play a significant role in coronary spasm via reduction in SOD activity, resulting in increased superoxide levels which in turn inactivate NO, leading to coronary spasm [2]. In recent years it has been noted that there is more Mn content in infant formula than in human milk [3]. Although required by multiple physiological processes of the human body, elevated Mn in the body due to over exposure would elicit toxicological effects, particularly on the central nervous system (CNS).

Sources of toxicity and human exposure

Human exposure to Mn can occur through ingestions and inhalation. The general population can be exposed to Mn through the consumption of Mn-contaminated drinking water and food stuff [4]. However, Mn-induced toxicity mainly occurs in certain occupational settings through inhalation of Mn-containing dust [5]. With continuing improvement in production technology and prevention strategies, serious and acute poisoning of Mn have occurred rarely, but long-term and low dose exposure to Mn still exists which may impose health risks to factory workers and nearby residents. Besides, Mn is one of the constituents (24.4%) of in methylcyclopentadienyl manganese tricarbonyl (MMT) which has been used in leaded gasoline, unleaded gasoline, diesel fuel, fuel oil to improve combustion. The combustion of MMT in the combustion chamber leads to increased airborne Mn, including inorganic Mn particles such as manganese phosphate and manganese sulfate [6,7]. Additionally, Mn is also found in some fungicides and pesticides, which could gradually lead to potential contamination of soil and waterways [8].

Manganese absorption, transport and excretion

Through inhalation and ingestion, Mn can be absorbed [9] into blood where it is mainly (>99%) in the 2+ oxidation state (Mn⁺²) and bound to β-globulin and albumin with a small fraction with transferrin [10]. Facilitated by the divalent metal ion transporter 1 (DMT1), N-methyl-d-aspartate (NMDA) receptor channel and Zip8 [11], the Mn+2 transport across the blood–brain barrier and blood-cerebrospinal fluid barrier and accumulate in in center nerve system including the basal ganglia structures, specifically in the striatum, cerebellum and globus pallidus [12-16] where Mn cause pathologies of extrapyramidal system, referred to as manganism, or Parkinsin’s disease (PD)-like syndrome [17]. Besides, Mn has also been contributed to the etiology of other neurodegenerative diseases, such as Huntington’s disease and Alzheimer’s disease [18,19].

Absorption by astrocytes, levels in normal astrocytes vs. after exposure to Mn

Astrocytes the most abundant CNS cells (~ 50% by volume), can accumulate up to 50-fold higher Mn concentrations compared to neurons, thus serving as the main homeostatic and storage site for this metal [20]. At the subcellular level, the highest Mn concentration in astrocytes is noted within mitochondria [21]. In normal situation the intracellular concentration of Mn in astrocytes is 50-75 μM where it is an essential cofactor for the astrocyte-specific enzyme glutamine synthetase [22]. Astrocytes exposed to Mn (500 μM) had significantly reduced 3H-GABA uptake despite no change in membrane or cytosolic GAT3 protein levels. Mn accumulation in the membrane fraction of astrocytes was enhanced with fatty acid administration, and was negatively correlated with 3H-GABA uptake [23].

Manganese environmental epigenetics and neurotoxicity

Mn causes oxidative stress in primary cultures of astrocytes, leading to the mitochondrial dysfunction and energy insufficiency [24]. The main phenotypic characteristic of Mn intoxication is motor impairment due to the accumulation of Mn in the basal ganglia. The symptoms of manganism include rigidity, rapid postural tremor, bradykinesis, gait disturbance, memory and cognitive deficit, and mood disorder [25,26]. While mechanisms of these extrapyramidal effects of Mn are unclear, results from in vitro and animal studies suggested that multiple pathways are involved. One of the main mechanistic pathways underlying the Mn-induced neurotoxicity is the effects on dopaminergic transmission and monoamine oxidase (MAO) [13,27-30]. MAO is a flavo-enzyme involved in the oxidative deamination of amine neurotransmitters, including serotonin, dopamine and noradrenaline [31]. MAO can be oxidized into aldehyde amine, enough to degrade biogenic amines, including neurotransmitters such as norepinephrine, dopamine and serotonin (5–HT). It is the key enzyme of dopamine degradation, with detoxification function. In addition, Mn has been reported to disturb the dopamine metabolism via direct oxidation of dopamine, inhibition of its synthesis, and inhibition of monoamine oxidase activity in brain mitochondria [32]. In addition, MAO-A was found in catecholamine neurons with the highest expression [33]. Further, Mn can cause oxidative stress in mitochondrial [34-36] where MnSOD is the primary antioxidant [37]. Human and animal studies suggested the susceptibility to Mn-induced neurotoxicity [38] involves the Mn metabolism, distribution and ROS generation. Mn redox ability depends on the state of charge. Mn2+ in the body is more toxic when it is oxidized to Mn4+ and Mn3+ [39,40] because the more Mn accumulation in the cells when Mn is oxidized [41]. In addition, Mn (III) was found to inhibit total cellular aconitase activity, reduce cellular serotonin more effectively and induced more oxidative stress compared to Mn (II) [42-44].

Glutamine can enhance the heat shock protein 70 (HSP70) [45], a protein playing an important role in preventing oxidative damage and protecting against neurodegeneration [46-49]. Mn are also important cofactors for various mitochondrial enzymes, as a result the high Mn levels in this organelle can directly interfere with oxidative phosphorylation leading to mitochondrial dysfunction [50-53]. Besides, Mn has been shown to trigger apoptosis in dopaminergic neurons in a caspase-3-dependent manner by activation of protein kinase C delta (PKC-δ) [54]. Futher more Mn can also induce oxidative stress [35,55,56]. Studies have shown that individual susceptibility exists and plays a role in metabolism and subsequent neurotoxic effects of Mn [38,57]. The most of above effects are involved in the area of the basal ganglia and the dopaminergic system. Recent studies found that Mn can also interfere on cortical structures and cognitive functions involving in the cerebral cortex [58,59] in which is chemic lesions are also found in pre-motor stages of Parkinson’s disease (PD) [60,61].

Parkinson’s Disease (PD) and Etiology

Parkinson’s disease (PD), also known as idiopathic or primary parkinsonism, is a chronic, progressive neurological disorder of CNS and one of the most common neurodegenerative disorders and the second most prevalent after Alzheimer’s disease comprising 1-2% of the population over 65 years of age [62]. The disease is more commonly found in people over 50 year old but it can also happen in younger patients. The exact mechanisms underlying PD has been unclear but it is believed that multiple factors are involved in case of sporadic PD which are the majority of PD cases [63]. About 15% of PD patients genetically inherited with gene mutations of SNCA, LRRK2, Parkin, PINK1, DJ-1 and ATP13A2 [64]. Clinically, PD is also characterized by a kind of active immune response [65]. The typical symptoms are movement-related including shaking, rigidity, slowness of movement and difficulty with walking and gait accompanied by thinking, sensory, sleep and emotional problems. Its pathogenesis is characterized by the loss of dopamine signaling due to the progressive degeneration or death of dopamine-generating neuron cells in the region of midbrain and accumulation of a protein termed as Lewy bodies in neurons. The main affected brain areas in PD include substantia nigra, basal ganglia and cerebral cortex [61,66]. In addition to the aging and heritage, exposure to environmental factors such as pesticides has been identified to be a risk factor of PD [67,68].

Association between excessive Mn exposure and PD

Resemblance in extrapyramidal symptoms between MN-induced neurotoxicity or manganese and Parkinson's disease has led to extensive study the possible relationship between these two syndromes. Eepidemiological studies suggested that pathologic and clinical difference between that chronic manganese (Mn) intoxication and PD. In addition, animal studies have shown that the therapy compound for PD is not effective for Mninduced motor and non-motor deficits [59]. However, further evidences are required before the final conclusion can be made since genetically correlations between them have been keeping reported. Several genes have recently been identified to be genetic etiological factors of Parkinson's disease including alpha-synuclein (α-Syn), leucine-rich repeat kinase 2 (LRRK2) [69]. Functional or structural abnormality of α-Syn in the brain is a hallmark pathological feature of several neurodegenerative disorders [70]. In Parkinson’s disease the accumulation of intraneuronal Lewy bodies/Lewy neurites containing misfolded fibrillar α-Syn are found [71]. A recent study using Cynomolgus macaques indicated that Mn exposure promotes α-Syn accumulation in neuronal and glial cells in the frontal cortex grey and white matter [72]. The induction of α-Syn aggregation by Mn may be due to defence mechanisms since an in vitro study showed that transgenic dopaminergic neuronal cells stably expressing human wild-type α-Syn hampered the Mn-induced toxicity during the early stages of exposure [73,74]. The variation of LRRK2 gene was found to be a risk factor for both familial and sporadic PD [69]. Down-regulation of this gene led in increased Mn-induced toxicity [75] suggesting the protective role of LRRK2 gene in Mn toxicity. Additionally, the proteins encoded by parkin and ATP13A2 can protect cell from Mn-induced toxicity. [76,77]. Over expression of parkin in cell by transient transfection has been found to attenuate the toxicity of Mn. Similarly, cells harboring wild type ATP13A2 showed more cell viability when compared with the cells with mutant ATP13A2 after exposure to Mn. These findings suggest that Mn-induced Parkinsonism and PD disease share at least partial common way of genetic initiation in disease onset.

Epigenetic involvement in of parkinson’s disease and Mn-induced neurotoxicity

The normal physiological functions of cells are controlled by not only genetic mechanisms but also balanced epigenetic pattern. The epigenetic machinery plays an important role in the control of many cellular functions of the body. The epigenetic modifications include DNA methylation, histone modifications and non-coding RNAs (ncRNA) expression. Methylation of DNA, a process involving the addition of methyl groups to DNA typically at CpG dinucleotide context, can cause the conformational change of DNA structure and consequent alteration in gene expression [78,79]. DNA methylation is import regulation mechanism for mammalian development [80]. However, abnormal DNA methylation patterns, hypermethylation or hypomethylation can lead to various pathogenesis or oncogenesis. Hypermethylations are generally associated with gene silencing or down regulation, whereas hypomethylation or unmethylated promoters are mostly linked to gene activation [81]. Epigenetic regulation gene expression can also be through modification of histone through post-translational modifications such as acetylation, phosphorylation, methylation and ubiquitination [82]. Histone modifications are important in genetic process including transcriptional regulation, DNA repair, DNA replication, alternative splicing and chromosome condensation [82-84]. Another important epigenetic modifier is ncRNA including microRNAs (miRs) and long non-coding RNAs (LncRNAs) [85].

In recent decades, the role of epigenetic changes in the development of diseases has drawn great attentions. Epigenetic changes are reversible and heritable modifications in phenotype without alteration of the primary nucleotide sequence [86]. Evidences have suggested that epigenetic mechanisms, including DNA methylation, histone modification and ncRNA DNA may regulate the expression of PD-related genes and provoke PD. It has been shown that methylation of the α-Syn, may in involved in PD through abnormal expression and accumulation of the protein [87,88]. Hypomethylation of α-Syn was found in patients with sporadic Parkinson's disease which could lead to over-expression of α-Syn resulting in disease development [89]. Interestingly L-Dopa which has been used for years for treating Parkinson's disease can increase methylation of α-Syn [90]. Furthermore, α-Syn has been shown to sequester DNMT1 and consequently leading to epigenetic alterations of Lewy body [91]. Other epigenetic regulated genes involving in PD includes LRRK2, Parkin, PARK16/1q32, and GPNMB [92]. Histone modifications also play roles in PD disease and inhibitors for of histone acetyltransferases (HATs) and histone deacetylases (HDACs) showed effective in both in vitro and in vivo PD models [93]. Recent studies have shown that miRNAs are involved in PD [94-96].

Environmental factors, biological and chemical, have long-lasting phenotypic effects without apparent underlying genetic change through above epigenetic modifications. In another words, environmental factors may change the gene expression directly or indirectly through epigenetic alterations such as DNA methylation or histone modifications. Heavy metalloid(s) such as arsenic, cadmium, chromium, lead, Mercury, coppers, nickel have been found to cause adverse effects through aberration of epigenetic patterns, which has been well reviewed [97-99]. These epigenetic changes in the developmental stages due to prenatal exposure to the environmental factors including Mn may contribute the abnormal phenotype including neurodegeneration. It has been reported that epigenetic gene regulation may contribute to Mn-induced neurogenesis in mouse offspring after maternal exposure to MN. Sustained promoter hypermethylation of Mid1, Atp1a3, and Nr2f1 and transient hypermethylation in Pvalb and consequent down regulation of these genes were found in mouse offspring after maternal exposure to Mn [100]. Epidemiological studies reported the epigenetic changes including DNA methylation, histone modifications and microRNA in subjects exposed to metal-rich air particles containing Mn [101-104]. Although Mn is one of the inhalable metal components in these studies, no significant association was found between Mn exposure and epigenetic changes.

Conclusion

While the role of Mn in the pathogenesis of PD remains controversial, the role of Mn as a modifier of PD warrants future study. Mn has been shown to induce mitochondrial dysfunction and oxidative stress, α-Syn aggregation and dopaminergic neurons which are pathological changes in PD. Recent work has shown the important role of epigenetic regulation as mediator between environmental factors and gene in PD onset and development. So far epigenetic studies on Mn-induced neurotoxicity have been sparsely reported. More and more epigenetic roles in PD have been reported while these genes with epigenetic changes in PD have been not reported in Mn-induced toxicity. To understand the epigenetic effects of Mn on these genes would be helpful to differentiate these two syndromes. This kind of work can improve our understanding of the role of Mn on early events as well as late life abnormalities of the nervous system. Although conclusion about the association between PD and Mn exposure need more consolidated studies, studying the genetic and epigenetic mechanisms of the effect of Mn will be helpful to understand the etiology of PD which is essential for therapeutic strategies of this disease. These studies will also helpful in finding suitable biomarkers not only for health risk assessment of Mn and other related environmental factors but also diagnosis and treatment of PD.

References

1. Takumi Ohishia,LiyunWanga, HirotoshiAkanea, AyakoShirakia, Ken Gotob et al. (2012) Reversible aberration of neurogenesis affecting late-stage differentiation in the hippocampal dentate gyrus of rat offspring after maternal exposure to manganese chloride. Reproductive Toxicology 34: 408-419.
2. Hiraoka Y, et al. (2012) Manganese Deficiency May Play a Role in Coronary Spasm, and Selenium Deficiency May Induce Cardiac Dysfunction through the Microcirculatory Disturbance. Circulation 126: A8892.
3. ChtourouY et al. (2011) Manganese induces oxidative stress, redox state unbalance and disrupts membrane bound ATPases on murine neuroblastoma cells in vitro: protective role of silymarin.Neurochem Res 36: 1546-1557.
4. Williams M et al. (2012) in Toxicological Profile for Manganese. Atlanta (GA).
5. Dobson AW, KM Erikson, and MAschner (2004) Manganese neurotoxicity. Annals of the New York Academy of Sciences 1012: 115-128.
6. Ávila DS et al. (2014) Manganese Neurotoxicity.  843-864.
7. Su C et al.(2015)Chronic exposure to manganese sulfate leads to adverse dose-dependent effects on the neurobehavioral ability of rats. Environ Toxicol.
8. Gunier RB (2013) Exposure to Manganese from Agricultural Pesticide Use and Neurodevelopment in Young Children. Robert Bruce Gunier 1-5.
9. PratapKarki Manganese Neurotoxicity: a Focus on Glutamate.
10. Transporters (2013) Annals of Occupational and Environmental Medicine. 25: 1-5.
11. O'Neal SL and WZheng (2015) Manganese Toxicity Upon Overexposure: a Decade in Review. Curr Environ Health Rep 2: 315-328.
12. BowmanAB, et al. (2011) Role of manganese in neurodegenerative diseases. Journal of Trace Elements in Medicine and Biology 25: 191-203.
13. Farina Met al.(2013) Metals, oxidative stress and neurodegeneration: a focus on iron, manganese and mercury. NeurochemInt 62: 575-594.
14. ShinotohH et al. (1997) Presynaptic and postsynaptic striatal dopaminergic function in patients with manganese intoxication: a positron emission tomography study. Neurology 48: 1053-1056.
15. Guilarte TR (2011) Manganese and Parkinson's disease: a critical review and new findings. Ciencia&saudecoletiva 16: 4549-4566.
16. Finkelstein Y et al.(2008) Differential deposition of manganese in the rat brain following subchronic exposure to manganese: a T1-weighted magnetic resonance imaging study. The Israel Medical Association journal: IMAJ 10: 793-798.
17. Bowman AB et al.(2011) Role of manganese in neurodegenerative diseases. Journal of Trace Elements in Medicine and Biology 25: 191-203.
18. Aschner M et al.(2007) Manganese: recent advances in understanding its transport and neurotoxicity. Toxicology and applied pharmacology 221: 131-147.
19. Bowman AB et al.(2011) Role of manganese in neurodegenerative diseases. J Trace Elem Med Biol 25: 191-203.
20. Farina M et al.(2013) Metals, oxidative stress and neurodegeneration: a focus on iron, manganese and mercury. Neurochemistry international 62: 575-594.
21. PratapKarki (2013) Manganese Neurotoxicity: a Focus on Glutamate Transporters. Annals of Occupational and Environmental Medicine 25:1-5.
22. Morello M, et al.(2008) Sub-cellular localization of manganese in the basal ganglia of normal and manganese-treated rats: an electron spectroscopy imaging and electron energy-loss spectroscopy study. Neurotoxicology 29: 60-72.
23. Tholey G et al.(1988) Concentrations of physiologically important metal ions in glial cells cultured from chick cerebral cortex. Neurochemical research 13: 45-50.
24. Fordahl SC and KM Erikson (2014) Manganese accumulation in membrane fractions of primary astrocytes is associated with decreased γ-aminobutyric acid (GABA) uptake, and is exacerbated by oleic acid and palmitate. Environmental Toxicology and Pharmacology 37: 1148-1156.
25. Chen CJ and SLLiao (2002) Oxidative stress involves in astrocytic alterations induced by manganese. Experimental neurology 175: 216-225.
26. Josephs KA et al.(2005) Neurologic manifestations in welders with pallidal MRI T1 hyperintensity. Neurology 64: 2033-2039.
27. KlosKJ, et al.(2006), Neuropsychological profiles of manganese neurotoxicity. European journal of neurology: the official journal of the European Federation of Neurological Societies 13:1139-1141.
28. Sriram K et al. (2010) Dopaminergic neurotoxicity following pulmonary exposure to manganese-containing welding fumes. Archives of toxicology 84: 521-540.
29. Roth JA et al.(2013)The effect of manganese on dopamine toxicity and dopamine transporter (DAT) in control and DAT transfected HEK cells. Neurotoxicology 35: 121-128.
30. Zhang SZZhou and J Fu (2003)  Effect of manganese chloride exposure on liver and brain mitochondria function in rats. Environmental research 93:149-157.
31. Leung TK, L Lim and JC Lai (1993) Brain regional distributions of monoamine oxidase activities in postnatal development in normal and chronically manganese-treated rats. Metabolic brain disease 8: 137-149.
32. Shih JC, Cloningafter cloningknock-out mice and physiological functions of MAO A and B. Neurotoxicology 25: 21-30.
33. Subhash MN and TS Padmashree (1991) Effect of manganese on biogenic amine metabolism in regions of the rat brain. Food and chemical toxicology: an international journal published for the British Industrial Biological Research Association 29: 579-582.
34. Weyler W, YP Hsu and XO (1990) Breakefield Biochemistry and genetics of monoamine oxidase. Pharmacology & therapeutics 47:391-417.
35. Taylor MD, et al.(2006) Effects of inhaled manganese on biomarkers of oxidative stress in the rat brain. Neurotoxicology 27: 788-797.
36. Milatovic D et al.(2009) Oxidative damage and neurodegeneration in manganese-induced neurotoxicity. Toxicology and applied pharmacology 240: 219-225.
37. Martinez-Finley, EJ et al.(2013) Manganese neurotoxicity and the role of reactive oxygen species. Free radical biology & medicine 62: 65-75.
38. MelovS.(2004) Modeling mitochondrial function in aging neurons. Trends in neurosciences 27: 601-606.
39. Curran CP et al.(2009) Incorporating genetics and genomics in risk assessment for inhaled manganese: from data to policy. Neurotoxicology 30: 754-760.
40. Reaney SH and DR Smith (2005) Manganese oxidation state mediates toxicity in PC12 cells. Toxicology and Applied Pharmacology 205: 271-281.
41. Reaney SH, CL Kwik-Uribe and DR Smith (2002) Manganese oxidation state and its implications for toxicity. Chemical Research in Toxicology 15:1119-1126.
42. Reaney SH, GBench and DR Smith (2006) Brain accumulation and toxicity of Mn(II) and Mn(III) exposures. Toxicological Sciences 93: 114-124.
43. Reaney SH, Kwik-Uribe CL, Smith DR (2002) Manganese oxidation state and its implications for toxicity. Chem Res Toxicol 15:1119-1126.
44. Sidoryk-Wegrzynowicz, M and M Aschner(2013) Manganese toxicity in the central nervous system: the glutamine/glutamate-gamma-aminobutyric acid cycle. Journal of internal medicine 273: 466-477.
45. Chen JY, TsaoGC, Zhao Q et al. (2001) Differential cytotoxicity of Mn(II) and Mn(III): special reference to mitochondrial [Fe-S] containing enzymes. ToxicolApplPharmacol  175:160-168.
46. HamielCR et al. (2009) Glutamine enhances heat shock protein 70 expression via increased hexosamine biosynthetic pathway activity. American journal of physiology Cell physiology 297: 1509-1519.
47. Turturici G, GSconzo and F Geraci(2011) Hsp70 and its molecular role in nervous system diseases. Biochemistry research international 2011: 618127.
48. Gifondorwa DJ, et al. (2007)Exogenous delivery of heat shock protein 70 increases lifespan in a mouse model of amyotrophic lateral sclerosis. The Journal of neuroscience: the official journal of the Society for Neuroscience 27: 13173-13180.
49. Wang AM et al. (2013) Activation of Hsp70 reduces neurotoxicity by promoting polyglutamine protein degradation. Nature chemical biology 9: 112-118.
50. Paul S and SMahanta (2014) Association of heat-shock proteins in various neurodegenerative disorders: is it a master key to open the therapeutic door? Molecular and cellular biochemistry 386: 45-61.
51. Pan Chen, SCTanara, Aaron Bowmanc and M Aschner (2015) Manganese-induced neurotoxicity: from C. elegans to humans. Toxicol Res 4: 191-201.
52. Gavin CE, KK Gunterand TE Gunter (1992) Mn2+ sequestration by mitochondria and inhibition of oxidative phosphorylation. Toxicology and applied pharmacology 115: 1-5.
53. MaleckiEA (2001) Manganese toxicity is associated with mitochondrial dysfunction and DNA fragmentation in rat primary striatal neurons. Brain research bulletin 55:225-228.
54. Sriram K et al. (2010)Mitochondrial dysfunction and loss of Parkinson's disease-linked proteins contribute to neurotoxicity of manganese-containing welding fumes. FASEB journal: official publication of the Federation of American Societies for Experimental Biology 24: 4989-5002.
55. Tsuboi Y (2012) Environmental-Genetic Interactions in the Pathogenesis of Parkinson’s Disease. Experimental Neurobiology 21: 123-128.
56. HaMai D and SC Bondy(2004) Oxidative basis of manganese neurotoxicity. Annals of the New York Academy of Sciences 1012: 129-141.
57. Milatovic D et al.(2007) Manganese induces oxidative impairment in cultured rat astrocytes. Toxicological sciences : an official journal of the Society of Toxicology 98:198-205.
58. Rentschler G et al. (2012) ATP13A2 (PARK9) polymorphisms influence the neurotoxic effects of manganese. Neurotoxicology 33: 697-702.
59. Guilarte TR (2013) Manganese neurotoxicity: new perspectives from behavioral, neuroimaging, and neuropathological studies in humans and non-human primates. Frontiers in Aging Neuroscience 5.
60. Bouabid S, et al. (2014) Manganese-Induced Atypical Parkinsonism Is Associated with Altered Basal Ganglia Activity and Changes in Tissue Levels of Monoamines in the Rat. Plos One 9.
61. Ferrer I (2009) Early involvement of the cerebral cortex in Parkinson's disease: convergence of multiple metabolic defects. ProgNeurobiol 88: 89-103.
62. Ferrer I et al.(2011) Neuropathology of sporadic Parkinson disease before the appearance of parkinsonism: preclinical Parkinson disease. Journal of Neural Transmission 118: 821-839.
63. Gasser T (2009) Genomic and proteomic biomarkers for Parkinson disease. Neurology72: S27-S31.
64. Thomas B and MF Beal (2007) Parkinson's disease. Human Molecular Genetics 16: R183-R194.
65. Klein C and A Westenberger(2012) Genetics of Parkinson's Disease. Cold Spring Harbor Perspectives in Medicine 2.
66. Phani S, JD Loike and S Przedborski (2012)Neurodegeneration and inflammation in Parkinson's disease. Parkinsonism & related disorders 18: S207-S209.
67. Ferrer I et al.(2012) Neurochemistry and the non-motor aspects of PD. Neurobiology of Disease 46: 508-526.
68. Baltazar MT et al. (2014) Pesticides exposure as etiological factors of Parkinson's disease and other neurodegenerative diseases-A mechanistic approach. Toxicology Letters 230: 85-103.
69. Kamel F(2013) Epidemiology. Paths from pesticides to Parkinson's. Science 341: 722-723.
70. Lin MK and MJ Farrer (2014) Genetics and genomics of Parkinson's disease. Genome Med 6: 48.
71. Recasens A and B Dehay (2014) Alpha-synuclein spreading in Parkinson's disease. Front Neuroanat 8: 159.
72. Luk KC et al. (2012) Pathological α-synuclein transmission initiates Parkinson-like neurodegeneration in nontransgenic mice. Science 338: 949-953.
73. Verina T, JS Schneider and TR Guilarte (2013) Manganese exposure induces alpha-synuclein aggregation in the frontal cortex of non-human primates. ToxicolLett 217: 177-183.
74. HarischandraDS et al. (2015) alpha-Synuclein protects against manganese neurotoxic insult during the early stages of exposure in a dopaminergic cell model of Parkinson's disease. ToxicolSci 143: 454-468.
75. Pifl C et al. (2004) alpha-Synuclein selectively increases manganese-induced viability loss in SK-N-MC neuroblastoma cells expressing the human dopamine transporter. NeurosciLett 354: 34-37.
76. Lovitt B et al. (2010) Differential effects of divalent manganese and magnesium on the kinase activity of leucine-rich repeat kinase 2 (LRRK2). Biochemistry 49: 3092-3100.
77. Higashi Yet al. 2004Parkin attenuates manganese-induced dopaminergic cell death. J Neurochem 89:  1490-1497.
78. Tan J et al. (2011) Regulation of intracellular manganese homeostasis by Kufor-Rakeb syndrome-associated ATP13A2 protein. J BiolChem 286: 29654-29662.
79. Prokhortchouk E and PADefossez (2008)The cell biology of DNA methylation in mammals. BiochimBiophysActa 1783: 2167-2173.
80. Cao JX, HP Zhang, and LX Du (2013) [Influence of environmental factors on DNA methylation]. Yi chuan= Hereditas/Zhongguoyichuanxuehuibianji 35: 839-846.
81. SmithZD and A Meissner (2013) DNA methylation: roles in mammalian development. Nat Rev Genet 14: 204-220.
82. Laffita-Mesa JM and P Bauer (2014) Herenciaepigenética (metilacióndelácidodesoxirribonucleico): contextoclínico en neurodegeneraciones y gen ATXN2. MedicinaClínica 143: 360-365.
83. Kouzarides T (2007) Chromatin modifications and their function. Cell128: 693-705.
84. Huertas D, RSendra and P Munoz (2009) Chromatin dynamics coupled to DNA repair. Epigenetics 4: 31-42.
85. Luco RF et al. (2010) Regulation of alternative splicing by histone modifications. Science 327: 996-1000.
86. Peschansky VJ and C Wahlestedt (2014) Non-coding RNAs as direct and indirect modulators of epigenetic regulation. Epigenetics 9: 3-12.
87. Ogino S et al. (2013) Molecular pathological epidemiology of epigenetics: emerging integrative science to analyze environment, host, and disease. Mod Pathol 26: 465-484.
88. AmmalKaidery, NS Tarannum, and B Thomas (2013) Epigenetic landscape of Parkinson's disease: emerging role in disease mechanisms and therapeutic modalities. Neurotherapeutics 10: 698-708.
89. Coppede F (2012) Genetics and epigenetics of Parkinson's disease. ScientificWorldJournal 2012: 489830.
90. AiSX et al. (2014)Hypomethylation of SNCA in blood of patients with sporadic Parkinson's disease. Journal of the Neurological Sciences 337: 123-128.
91. Schmitt I et al. (2015) L-dopa increases alpha-synuclein DNA methylation in Parkinson's disease patients in vivo and in vitro. MovDisord
92. Desplats P et al.(2011) Alpha-synuclein sequesters Dnmt1 from the nucleus: a novel mechanism for epigenetic alterations in Lewy body diseases. J BiolChem 286: 9031-9037.
93. Landgrave-GJ, OM-Gomez and RG Guzman (2015) Epigenetic mechanisms in neurological and neurodegenerative diseases. Front Cell Neurosci 9: 58.
94. Sharma S and RTaliyan (2015) Targeting histone deacetylases: a novel approach in Parkinson's disease. Parkinsons Dis 2015: 303294.
95. Cardo L et al. (2013) Profile of microRNAs in the plasma of Parkinson’s disease patients and healthy controls. Journal of Neurology260: 1420-1422.
96. Khoo SK et al.(2012) Plasma-based circulating microRNA biomarkers for Parkinson’s disease. J. Parkinsons Dis 2: 321-331.
97. Zhao N et al. (2014) Serum microRNA-133b is associated with low ceruloplasmin levels in Parkinson's disease. Parkinsonism & Related Disorders 20: 1177-1180
98. Cheng TF,  SChoudhuri and KMJacobs (2012) Epigenetic targets of some toxicologically relevant metals: a review of the literature. Journal of Applied Toxicology. 32: 643-653.
99. Jose CC et al.(2014) Epigenetic dysregulation by nickel through repressive chromatin domain disruption. Proceedings of the National Academy of Sciences 111: 14631-14636.
100. Ryu HW et al.(2015) Influence of toxicologically relevant metals on human epigenetic regulation. Toxicol Res 31: 1-9.
101. Wang L et al. (2013) Aberration in epigenetic gene regulation in hippocampal neurogenesis by developmental exposure to manganese chloride in mice. ToxicolSci136: 154-165.
102. Cantone L et al. (2011) Inhalable Metal-Rich Air Particles and Histone H3K4 Dimethylation and H3K9 Acetylation in a Cross-sectional Study of Steel Workers. Environmental Health Perspectives 119: 964-969.
103. Bollati V et al. (2010) Exposure to Metal-Rich Particulate Matter Modifies the Expression of Candidate MicroRNAs in Peripheral Blood Leukocytes. Environmental Health Perspectives 118: 763-768.
104. Hou LF et al. (2011) Ambient PM exposure and DNA methylation in tumor suppressor genes: a cross-sectional study. Particle and Fibre Toxicology 8.