Abstract
Parkinson’s Disease (PD) and vitamin D share a unique link as Vitamin D
deficiency (VDD) prevails in PD. Thus, an in-depth understanding of
Vitamin D biology in PD might be crucial for therapeutic strategies
emphasizing Vitamin D. Specifically, explicating the effect of VDD and
genetic polymorphisms of vitamin D-associated genes in PD, likeVDR (Vitamin D Receptor) or GC (Vitamin D Binding
Protein), may aid the process along with polymorphisms of Vitamin D
metabolizing genes (e.g., CYP2R1 , CYP27A1 ) in PD.
Literature review of single nucleotide polymorphisms (SNPs) related to
Vitamin D levels [GC (GC1-rs7041, GC2-rs4588), CYP2R1 ,
CYP24A1, CYP27B1] and Vitamin D function [VDR (FokI - rs2228570,
ApaI - rs7976091, BsmI-rs1544410, TaqI-rs731236)] was conducted to
explore their relationship with PD severity globally. Furthermore, the
DisGeNET database was utilized to explore the gene-disease associations
in PD, and STRING alongside Cytoscape was utilized to identify critical
genes associated with PD. VDR -FokI polymorphism was reported to
be significantly associated with PD in Hungarian, Chinese, and Japanese
populations, whereas VDR -ApaI polymorphism was found to affect PD
in the Iranian population. However, VDR-TaqI and BsmI polymorphisms had
no significant association with PD severity. Conversely, GC1polymorphisms reportedly affected Vitamin D levels without influencing
the disease severity. CYP2R1 (excluding rs1993116) was also
reportedly linked to clinical manifestations of PD. Genetic
polymorphisms might cause VDD despite enough sunlight exposure and
vitamin D-rich food intake, enhancing inflammation, thereby influencing
PD pathophysiology. Knowledge of the polymorphisms associated with
vitamin D appears promising for developing new therapeutic strategies
against PD.
Introduction
Parkinson’s Disease (PD) came second in terms of prevalence among
neurodegenerative diseases and is projected to be doubled in the coming
three decades [1]. A recent study estimated 6.1 million PD patients
worldwide in 2016, indicating a significant leap from 2.5 million back
in 1990, with a further projected increase soon. However, only the rise
in elderly individuals in the population cannot solely account for the
increase in the prevalence of PD [2]. The specific neurodegenerative
mechanisms in PD are not fully understood. Complex interactions between
genetic and environmental factors, inflammation, oxidative stress,
mitochondrial dysfunction, and immune regulation are thought to play a
role with others [3-5 ].
Altered Vitamin D level is linked to the pathophysiology of different
neurological disorders, including PD [6 ]. Insufficiency and
deficiency of the precursor of Vitamin D (25-hydroxyvitamin D or
calcidiol, abbreviated as 25(OH)D hereafter), at a level of
<30 ng/mL and <20 ng/mL, along with reduced sunlight
exposure, was revealed to be significantly associated with elevated PD
risk in comparison to healthy control having similar sunlight exposure
[7 ]. In the liver and kidney, prohormone Vitamin D (7-
dehydroxycalciferol and ergocalciferol) undergoes a two-step metabolism
to generate the active metabolite, calcitriol (α-1,25, dihydroxy vitamin
D3), that binds to the vitamin D receptor (VDR) to control the
expression of a distinct set of genes [8 ]. The two-step
conversion of vitamin D precursor into its active form is aided by the
action of CYP2R1 (cytochrome P450 2R1) and CYP27A1 (cytochrome P450
oxidase) in the liver to generate calcidiol (25(OH)D) and then into
calcitriol (α-1,25(OH)2D3) by CYP27B1 (cytochrome p450 27B1) in the
kidneys. On the other hand, CYP24A1 (cytochrome P450 family 24 subfamily
A member 1) in the kidneys deactivates calcitriol by converting it into
inactive forms and preventing its formation from calcidiol. VDR first
forms a complex with the retinoid X receptor (RXR), which further binds
to active calcitriol, and the ternary complex is then carried into
circulation. Also, VDR acts as a ligand-inducible transcription factor
and forms a heterodimer with retinoid X receptor (RXR) to bind with
vitamin D-responsive elements (VDREs). VDREs are located in the promoter
regions of the vitamin-D-responsive genes and control the transcription
of these genes (Figure 1) [9 ]. Vitamin D binding protein
(DBP) is involved in the binding, solubilization, and transportation of
vitamin D and its metabolites to various target tissues. Altered DBP
functions have been reported at the onset of diseases
[10-12 ]. Epidemiological studies suggested that
polymorphisms of VDR , GC , and other vitamin D metabolizing
genes might cause an alteration in the level of vitamin D metabolites,
thereby favoring the disease progression [13 ]. Circulating
calcidiol can cross the blood-brain barrier and enter the glial cells
and other neuronal cells to be converted into active calcitriol
[14 ]. Due to the wide availability of VDR and CYP2R1 in
areas of the brain involved in cognition and the formation of new
memories, vitamin D is believed to play a role in neurocognition
[15 ]. Lowered levels of Vitamin D have been directly
correlated with the severity of PD [16] . However, it is
unclear whether Vitamin D levels are related to the early onset or
non-motor symptoms of PD.
COVID-19 compelled people towards more sedentary lifestyles, causing
reduced exposure to sunlight which might have affected their vitamin D
levels in the body. A pan-India study showed that motor symptoms of PD,
like stiffness, rigidity, slow movement, tremors, freezing of gait, and
non-motor symptoms like fatigue, depression, pain, constipation, and
anxiety in post-pandemic times worsened. 35.4% of the respondents
reported having sleep disturbances, while 23.9% reported new onset or
worsening of the condition [17 ]. Assay of the Vitamin D
status of the population at the time of pandemic restrictions could have
helped establishing a correlation between altered vitamin D levels and
PD progression. The situation may get aggravated if there is a genetic
predisposition through polymorphisms in genes linked to vitamin D
functioning.
The global PD population of 9.4 million in 2020 is estimated to increase
to 12.9 million by 2040 [18 ]. VDR and GCpolymorphisms’ effect on PD varies in different ethnic populations. No
report exists for India, where the geriatric population was 138 million
(67 million males and 71 million females) in 2021, as per Population
Projection Report 2011-2036 formed by the National Population
Commission. 40-99 % of the Indian population is Vitamin D deficient
[19 ]. PD prevalence in the Indian population is lower than
that of the Caucasian population; the possible causes may include a
lower percentage of the aged population in India and some protective
environmental or ethnic factors [20] . Studies have shown
that GC polymorphism (rs7041) is correlated with low Vitamin D levels.
However, some other studies showed that subjects with polymorphic GC
genes had a high level of Vitamin D [10] . Vitamin D level
is not the sole determinant of PD or any diseases associated with it.
Genetic profiling of SNPs related to vitamin D activation,
metabolization or transport, and allele variation should also be
considered. Polymorphic alleles of VDR , GC , CYP2R1 ,CYP27A1 , and CYP27B1 might cause altered Vitamin D
functioning and hence varied drug response among PD patients. The
objective of this review is to explore the effect of vitamin D in PD.
Also, to understand the association between vitamin D-related gene
polymorphisms and PD.
Methods
Search Strategy Overview
The connection between Vitamin D status, VDR polymorphism, and PD was
searched on platforms including PubMed, Scopus, and ResearchGate using
the keywords ”Parkinson’s Disease”, ”Vitamin D”, “Vitamin D
Receptor”, “Vitamin D receptor polymorphism”, “Vitamin D receptor
polymorphism and Parkinson’s disease”, “Vitamin D binding protein”,
“GC polymorphism and Parkinson’s disease”, “Vitamin D metabolizing
enzymes (CYP2R1 , CYP27A1 , CYP27B1 , CYP24A1 )
and Parkinson’s Disease”, “polymorphism of CYP genes in Parkinson’s
Disease” till September 2022. Further, a stepwise strategic search was
carried out to find various restriction sequences of the Vitamin
D-linked genes involved with PD in diverse populations.
Gene-disease association (GDA) analysis and
protein-protein interaction (PPI) network of top target genes in
Parkinson’s disease
PD-linked genes were explored through the DisGeNET database (v7.0)[21] , and based on the GDA score (as generated by the
server) top 500 genes were selected. Next, these genes were subjected to
PPI analyses via STRING server (v11.5.) and Cytoscape (v3.9.1) by
selecting a minimum confidence score of 0.7. The PPI network was then
further analyzed for the hub or core genes (top 5) within the network
using the cytoHubba plugin (by selecting the MCC algorithm) of the
Cytoscape [22]. The top five genes linked to PD (as
obtained from Cytoscape) were further searched through the STRING server
for any association with VDR and GC .
- Results All the reports included vitamin D, Parkinson’s disease, GC ,VDR , and activating and metabolizing enzymes of vitamin D likeCYP2R1 , CYP27B1 , CYP27B1 , and CYP24A1 were
studied and included in this review. A search for PD-linked genes in
the database (DisGeNET) showed that VDR is one of the top 300
associated genes. The top 5 genes that emerged from this association
study were IL-6 , TNF , STAT3 , IL1B, andCXCL8 chronologically: all of which are either proinflammatory
markers or aid in the inflammatory pathway. The result further
justifies our search for the relationship between Vitamin D and PD, as
Vitamin D is known for its anti-inflammatory effects.
- Vitamin D in the brain
Vitamin D has been found to be present in the brain, and VDRis mainly expressed in the astrocytes [23] . Vitamin D
was found to help brain function in preclinical research and human
population studies. The role of Vitamin D in various
neurodevelopmental and neuropsychiatric conditions is established,
with deficiency leading to impaired neurocognition[24] . Circulating calcidiol can cross the blood-brain
barrier, enter the glial and other neuronal cells, and get converted
to active calcitriol [14]. Due to the wide occurrence ofVDR and CYP27B1 in areas of the brain involved in
cognition and the formation of new memories, Vitamin D influences
neurocognition [15] . In a rat model, calcitriol has
been reported to partially restore the expression of tyrosine
hydroxylase in substantia nigra, thereby promoting the conversion of
tyrosine to dopamine, further justifying the role of Vitamin D in PD[25] .
- VDR, GC, Hydroxylase gene polymorphisms in Parkinson’s
Disease
Despite adequate sunlight exposure and sufficient dietary intake of
vitamin D, a significant section of the world population is still
suffering from VDD. This might be due to a deficiency in binding
proteins like VDR and DBP, activating enzymes CYP2R1, CYP27A1, and
CYP27B1, and/or deactivating proteins like CYP24A1. To understand
the possible role, polymorphisms of these genes were studied to
check their association with PD. VDR gene polymorphism has
been shown to affect PD patients in Korean, Hungarian, Taiwanese,
Chinese, Iranian, and Japanese populations and Faroe Islanders
[26-33 ].VDR-FokI (rs2228570)
polymorphism was found to be significantly correlated with PD
patients in Hungary, China, and Japan, but not in the Korean,
Taiwanese, and Iranian populations. VDR-ApaI (rs7976091)
polymorphism was reported to have a significant association in the
Iranian population but not in the populations of Korea, Japan,
China, Hungary, and the Faroe Islands. However, VDR-BsmI andVDR-TaqI polymorphisms were not found to have any correlation
with PD in the above-mentioned populations. Also, SNPs of VDR
affected vitamin D levels and influenced the risk factor of PD onset
variedly in different ethnic groups (Tables 1 and 2). The only
report on the effect of GC polymorphisms showed lowered
vitamin D levels but not disease severity (Tables 3a and 3b).CYP2R1 gene variants (except rs1993116) were reportedly
associated with the initial clinical motor features of PD [34].
- Vitamin D and Inflammation in PD
Inflammatory changes are believed to be one of the regulating
factors of disease progression in PD patients. In PD, both the
central and peripheral inflammations are believed to trigger
astrocytes and brain microglial cells to switch from their
neuroprotective roles to pathogenesis, contributing to the disease
onset and progression [35] . They start to produce
proinflammatory cytokines in response to inflammatory stimulations
of IL-1β, LPS, and TNF-α [36 ]. TNF . IL6 ,IL1B , and CXCL8 produce proinflammatory cytokines, andSTAT3 , an indicator of inflammation, regulates astrogliosis
and triggers apoptosis. VDR directly interacts with TNF,
thereby modulates the TNF-IL1B-CXCL8-IL6-STAT3 circuit (Figure 2C)
to influence the inflammatory response in PD. Thus, any polymorphism
in VDR might likely result in VDD, which in turn has been
intricately linked to inflammation. It has been shown that vitamin D
ameliorates inflammation in mice models. A significant decrease in
the proinflammatory cytokines IL-1β and TNF-α and an increase in the
anti-inflammatory cytokines IL-10, TFG-β, and IL-4 were reported in
the brain of vitamin D-treated mice [37 ].
Interestingly, increased calcitriol and normal calcidiol levels have
been found in PD patients under sustained inflammation and elevated
inflammatory cytokine levels. Inflammation and low vitamin D are
somewhat believed to have a cause-effect relationship with each
other [38 ]. Vitamin D also decreased the mRNA
expression of proinflammatory cytokines in specific areas of the
brain, thereby preventing neuroinflammation [37 ].
Overall, optimum Vitamin D action in the brain might be crucial to
prevent or delay PD onset.
- Discussion
PD is a primary healthcare concern worldwide, being the second most
common neurodegenerative disease. Recent studies claim that VDD is
prevalent among PD patients, which might be due to lifestyle or
insufficient dietary intake. Since the pathophysiology of PD is unknown,
this study aimed to find the role of Vitamin D in the onset and
progression of PD. We also tried deciphering the role of SNPs of
the VDR, GC , and CYP2R1 genes on vitamin D levels and
disease severity in PD patients. Vitamin D is abundant in the brain,
exerting its role in neurodevelopment, neurocognition, preventing
neuroinflammation, etc. It modulates cerebral activity in developing and
adult brains by aiding connections of neural circuitry for movement,
emotions, and reward-dependent behavior [39]. VDD is one of
the most common neuroinflammation causes, an elementary response for
protecting neurons and neuronal damage. Prolonged or unresolved
inflammation might lead to neurotoxicity and neuronal damage
[40].
One of the most common findings is that the serum level of vitamin D is
inversely associated with motor severity in PD [41 ]. The
cause of VDD has often been attributed to the inadequate functioning of
vitamin D-associated proteins. It is tempting to believe that it might
be due to the SNPs of these genes. As shown in Table 1 and Table 3a,
SNPs of both VDR and GC increased the risk of PD by
altering the serum levels of vitamin D. VDR -FokI was reported to
be significantly higher in PD patients than in healthy controls in
Chinese, Japanese, and Hungarian populations (Table 2). Furthermore,
allele variations like FokIA in the Chinese, FokICC in Japanese, and
FokIC allele in the Hungarian population, had increased risk for PD
(Table 2). Conversely, studies in other populations reported no
significant difference, which indicated that the effect of VDR
polymorphisms varied depending on ethnicity, with effects of
polymorphism varying with different restriction sites
[26-33 ]. The role of SNPs of Hydroxylase/CYP genes which
regulate vitamin D synthesis (Figure 1), is yet to be studied in PD.
Cytochrome P450 2R1 (25-hydroxylase) is responsible for the formation of
25 (OH)D, and variants of this allele might also be attributed to VDD
[42]. Gene variants of CYP2R1 cause low utilization of vitamin D,
thereby contributing to the clinical manifestations of PD
[34 ].
One of the instigating factors of neuroinflammation in PD is the altered
gut microbiota stimulating the release of proinflammatory cytokines,
causing an elevation of these molecules in PD [43 ]. The
two-way hypothesis by Braak [44 ] proposed two possible
routes of entry of microbes into the human system: one through the nasal
path and the other through the gut, which ultimately triggers PD
pathogenesis. Gut-dysbiosis due to microbial invasion plays a crucial
role in PD prognosis by triggering inflammatory pathways
[45 ]. Astrocytes and M1 microglial cells increase the
permeability of the blood-brain allowing the entry of macrophages and T
cells into the brain. This causes inflammation which finally leads to
the loss of Dopaminergic neurons in PD [46]. Vitamin D is
known to down-regulate the production of proinflammatory cytokines by
exerting its anti-inflammatory role on monocytes and aiding microglial
transition. However, the functioning of vitamin D depends on the
expression of vitamin D-linked genes,
especially VDR [47] . The involvement of
inflammatory pathways in PD got validated through our network
analysis (Figure 2), where the top five PD-linked genes
(IL-6, TNF, STAT3, IL1B, and CXCL8) turned out to be
related to inflammation. The analysis further revealed a direct link
between VDR with TNF (Figure 2C).
Immune cell responses and immune-regulatory responses are prevalent in
PD [48 ]. IL6 was found to be elevated in PD patients, and
the cytokine levels were correlated with the disease’s severity
[49 ]. STAT3 works as a signaling molecule in most
immune-regulatory pathways in PD. The gene DJ-1which is directly
associated with PD, regulates astrogliosis via STAT3 in cases of brain
injury [50 ]. In PD, activated microglias secrete a diverse
range of neurotoxic mediators and proinflammatory molecules like
superoxide, TNF-α, IL-1β, IL-6, and NO [51 ]. These
cytokines mediate their actions by binding with Toll-like receptor 4
(TLR 4), which induces activation of STAT1 and STAT3 [52 ].
It pushes dopaminergic neurons towards apoptosis by activating the
transcription of genes responsible for cellular death encoding proteins
like Bcl-xL, Fas, and TNF-related apoptosis-inducing ligands and
caspases. Therefore, STAT3 activation in microglial cells causes
functional changes like attenuation of dopaminergic neurons, occurring
due to auto phagocytosis in an IL-1-dependent manner. Dopaminergic
neuronal loss is a characteristic feature of PD [53 ].
Vitamin D is known to decrease the production of proinflammatory
molecules like IL-6 and TNFα and increase the production of
anti-inflammatory markers like IL-10, TGF-β, etc.
[54]. Active vitamin D3 induces the tolerogenic phenotype of
dendritic cells, and this transition activates the IL6-JAK2-STAT3
pathway as JAK2 –mediated phosphorylation of STAT3 requires vitamin D
stimulation. VDR interacts with phosphorylated STAT3 and
methylcytosinedioxygenase TET2 to produce complexes providing immunity
and enhancing tolerance properties to dendritic cells via regulatory T
cells [55,56].
Thus, VDD triggers the inflammatory pathway, which might lead to PD. The
multiple causes of VDD might include inadequate dietary intake, low
sunlight exposure, and polymorphism of vitamin D-linked genes
like VDR, GC , and CYPs . Incompetent VDR might also lead to
VDD and increase the production of calcitriol from extra-renal tissues,
decreasing precursor molecule calcidiol and having a positive feedback
effect with enhanced inflammation. VDD leads to increased pro-cytokine
release and a decrease in anti-inflammatory cytokine production.
Microbial infection may also lead to the production of inflammatory
cytokines, gut dysbiosis, and upregulation of α-synuclein
[45] . Vitamin D analogues have been reported to decrease
intracellular-free Ca (II) and downregulate the expression of
calbindin-D28k to reduce a-synuclein
aggregation [57], thereby prohibiting inflammatory
responses leading to PD. Gut dysbiosis often results in the production
of lipopolysaccharides (LPS). Increased inflammation, upregulated
α-synuclein, lipopolysaccharides (LPS), and downregulation of
calbindin-D28k contribute to α-synuclein aggregation, which might induce
dopaminergic neuronal activity apoptosis leading to neurodegeneration
and, ultimately, PD (Figure 3).
Conclusion
This review attempts to summarize the roles of vitamin D, VDD, and
polymorphisms of vitamin D-associated genes in PD. Although VDR
polymorphisms were found to affect vitamin D levels and the severity of
PD, the result varied in a population-specific manner. Thus, in a
country like India, where diverse ethnic populations are present,
in-depth and extensive studies are required to establish the
relationship between Vitamin D and PD. Since reports
on GC and CYP2R1 polymorphism in PD are also limited,
further genetic studies are necessary to check for any association
between the two. Virtually no reports exist regarding the role of
polymorphisms of other genes involved in vitamin D metabolism in PD, so
they might also be included to obtain a clearer picture of the
mechanisms of VDD in PD. Populations predisposed to such polymorphisms
may be included in genetic counseling to delay or prevent the onset of
neurodegenerative diseases like PD. After extensive research, vitamin D
supplementation can also be considered a potent therapeutic mechanism in
such diseases.