Patients’ characteristic of EOPD and LOPD [8].
Abstract
Parkinson’s disease (PD) is divided into early-onset (EOPD) occurring at the age of fewer than 45 years of age and late-onset PD (LOPD) above 45 years of age. EOPD accounts for 5–10% of all the cases with PD. It is thought that occurrence in this age is connected with genetic factors, mutations in e.g. PRKN, PINK1, DJ-1 and changes in proteins it is encoded. The loss of dopaminergic neurons in the nigrostriatal system leads to decreased dopamine (DA) concentrations. Pathogenic PD proteins may affect the DA level. The lower level of DA may be responsible for movement-related symptoms. EOPDs have a slower progression of the disease and a longer disorder duration but tend to develop dyskinesias and motor fluctuations earlier than LOPD. Currently, the diagnosis of PD is based on clinical criteria, supported neuroimaging like MRI or PET. Understanding the pathogenesis of the EOPD may be contributing to improving diagnostics and effectiveness of pharmacotherapy.
Keywords
- molecular factors
- dopamine
- Parkinson’s disease of early onset
1. Introduction
Parkinson’s disease (PD) is one of the most common and spontaneous degenerative disease of the central nervous system (CNS) that is characterized by classical motor symptoms like bradykinesia, muscular rigidity, rest tremor, or postural instability [1]. It is estimated that approximately 5 million people worldwide suffer from PD. The frequency of disease increases with age; there are 1% of people older than 60 years and 5% of people over 85 years [2, 3, 4]. It seems that males suffer more often than females [5]. Furthermore, the estimates indicate that the number of PD patients will maintain increase trend because of population aging.
PD usually develops in the fifth or the sixth decade of life and is called late-onset PD (LOPD), but in a small group of patients, it is diagnosed even before the age of 40 years. The definition of early-onset PD (EOPD) is arbitrary. Some authors defined this disorder with an age of onset (AOO) below 40 years, others even below 50 years, but usually, it refers to age less than 45 [6, 7]. According to the literature data, 5–10% patients suffer from EOPD. EOPD can also be subdivided into the group called juvenile PD with AOO less than 21 years [8].
The main factor in PD pathology is loss or degeneration of dopaminergic neurons in the substantia nigra (SN). Although this disease was described more than 200 years ago, its cause is still not fully understood. It is considered that the pathogenesis depends on both genetic and environmental factors, but genetic changes are main causes in about 5–10% of the PD patients [9]. Some genes and its proteins associated with EOPD like
The phenotype of PD is various and related to AOO. It includes classical motor symptoms and non-motor symptoms such as disorder of mood, cognitive, behavioral, sensory, and autonomic dysfunctions (e.g., orthostatic hypotension and urogenital dysfunction) [10]. Patients’ characteristic of EOPD and LOPD is summarized in Table 1.The study of Wickremaratchi et al. [7] showed that features like tremor, rigidity, response to most common treatment, or presence of dystonia and dyskinesia’s have linear changes (increasing or decreasing). However, dystonia demonstrates the highest risk of occurrence among EOPD and reduction among LOPD patients.
Features | EOPD | LOPD |
---|---|---|
Mean age of onset (years) | 44 | 72 |
Survival from onset (years) | 27 | 10 |
Mean age at death (years) | 71 | 82 |
Tremor at onset, only (%) | 45 | 59 |
Bradykinesia and tremor at onset (%) | 23 | 9 |
Bradykinesia at onset, only (%) | 32 | 25 |
Postural instability at onset (%) | 0 | 7 |
Table 1.
The proper diagnosis of PD is very important. Nowadays, there are a lot of neuroimaging methods that can be used to increase the accuracy of differential diagnosis, but none of them have been endorsed to routine use in clinical practice [11, 12].
2. Dopamine and pathogenesis of Parkinson’s disease
Dopamine (DA) is the organic chemical of the catecholamine family and precursor for noradrenaline. It is synthesized in presynaptic neuron from tyrosine to L-dihydroxyphenylalanine (L-dopa) via tyrosine hydroxylase. Subsequently, aromatic amino acid decarboxylase removes a carboxyl group, form neurotransmitter, which is packed into synaptic vesicles. DA is released into the synapse during stimulation, actives dopaminergic receptors, and evokes a response in the postsynaptic cell [13]. It plays a pivotal role in the generation of normal movements by transmission information from SN to the striatum, where movements are initiated and controlled facility and balance [14].
The pathomechanism of PD is progressive and subsequent degeneration of neurons in SN, which results in the decreased level of DA in the dopaminergic neurons. Further, there is also the presence of Lewy bodies (LBs), intracytoplasmic eosinophilic inclusion bodies, in others neurons in SN. The literature indicates that loss of 60–70% of dopamine neurons in SN is presented as PD motor symptoms [15]. Pathogenesis of PD involves both environmental and genetic factors. It is thought that the pathways involved in PD are impairment of cellular clearance pathways, protein aggregation, oxidative stress, mitochondria dysfunction, and neuroinflammation (Figure 1) [16, 17, 18].

Figure 1.
Association between DA in Parkinson’s disease, EOPD and genetic and biochemical factors. EOPD—early-onset Parkinson’s disease, LB—Lewy bodies, DA—dopamine, ASN—α-synuclein, L-dopa—L-dihydroxyphenylalanine, DAT—dopamine transporter, ROS—reactive oxygen species.
α-Synuclein (ASN) is a major component of LB [19]. Aggregation of ASN is considered to be engaged in the pathogenesis of PD in consequence of the cellular clearance pathway like ubiquitin-proteasome and autophagy-lysosome [20]. The literature indicates that ASN modulates dopamine transporter (DAT) activity. DAT is responsible for removing DA from the synaptic cleft. It is showed that the polymorphisms in gene coding DAT (
Oxidative stress is a disturbance in the balance between prooxidant and antioxidant homeostasis and production of reactive oxygen species (ROS) [24]. The main mitochondrial site of generation ROS is complex I [25]. There is a direct relationship between mitochondrial dysfunction and decreased activity of complex I among PD patients [26]. Moreover, it is known involvement of such genes like
DAQs are electrophilic species, very reactive toward cellular nucleophiles, which effect damage of cells. DAQs can bind to Parkin and promote its aggregation. Thus, this protein losses its function. It seems that DAQs are more responsible for inactivation of Parkin than ROS [14]. The study of Bisaglia et al. [27] shows that DAQs interact with ASN by inhibition of ASN fibrilization and stabilizing ASN/DAQ oligomers. It seems that DAQs can also modify the structure of DJ-1 through modifications in cysteine residues of its protein [28].
Kitada et al. [29] show that mutations in
3. Genetic risk factors for early-onset Parkinson’s disease
The etiology of EOPD is not completely explained. It seems that genetic factors, environmental factors, or both of them may play an important role in the pathogenesis of this disease. There have been identified several genes and their mutations associated with EOPD, but new loci are still being identified. Most of these genes are inherited autosomal recessive, for example,
3.1. PRKN gene
One of the most important genes involved in the pathogenesis of EOPD is
3.2. PINK1 gene
Another gene which mutations are involved in the occurrence of EOPD is phosphate and tensin homolog (PTEN)-induced putative kinase 1 (
Mutations in
3.3. DJ-1 gene
The third gene associated with EOPD is
The
3.4. GBA gene
The
Moreover, homozygous mutations in
3.5. SNCA gene
One of the most common mutations in
4. The phenotype of early-onset Parkinson’s disease
Patients with EOPD are characterized as younger AOO and longer disease duration than patients with LOPD [81]. Some symptoms vary among patients (Table 2), but classical motor symptoms are mainly affected.
Gene (locus) | Location | Selected mutations in EOPD patients | Inheritance | Clinical phenotype |
---|---|---|---|---|
6q26 | Q171X, R275W, G284R, T425N, A82Q, L174L, L261L | Recessive | Tremor, bradykinesia, urinary dysfunctions | |
1p36.12 | Q456X, Y258X, R276X, M318L, A427E, R312R, A339T, D391D, G411S, T420T, D525N, S576S | Recessive | Foot dystonia, gait impairment, excellent L-dopa responsiveness | |
1p36.23 | D149A, A104T, L10P, D24A, F162 L, E64D | Recessive | Similar to |
|
1q22 | N370S, L444P, H255Q, E326K, D409H, R329H | Recessive | Mild Gaucher’s symptoms, cognitive impairment | |
4q22.1 | A53T, A30P, E46K | Dominant | Rigidity, rapid progression |
EOPD with
Most of EOPD patients with mutations in
The phenotype of
The phenotype of
5. Neuroimaging of early-onset Parkinson’s disease
A lot of neuroimaging techniques are used to diagnose PD properly, follow the progress, and also get to know the neurobiology mechanism involved in revealing the disease. The most commonly used methods are magnetic resonance imaging (MRI), positron emission tomography (PET), single-photon emission computed tomography (SPECT), magnetic resonance spectroscopy (MRS), and transcranial sonography (TCS). There are also multimodal neuroimaging techniques that combine imaging with complementary modalities to increase the benefits of examination.
PET imaging is a technique using radiolabeled agents like 11C, 18F, and 15O. It is more sensitive and presents a better special resolution in comparison to SPECT, which employs radioisotopes 123I or 99mTc. It is thought that SPECT is cheaper, more widely available, and a valuable imaging modality for many PD applications [97]. It seems that Technetium99m-labeled tropane derivative (99mTc-TRODAT-1) can be used to reveal dysfunction of dopaminergic system by binding DAT [98]. It was also showed that striatal DAT-binding potential was 34% lower among EOPD than LOPD patients [99]. The study of Shyu et al. [100] identified lower uptake of 99mTc-TRODAT-1 in the putamen, but normal in the caudal nucleus among patients with
MRS is a kind of magnetic resonance for identifying many endogenous compounds involved in the pathomechanism of PD like DA, γ-aminobutyric acid (GABA), and glutamate, so it gives an opportunity for probing biochemical systems [102, 103]. It allows research neurochemicals directly, without invasion and radiation exposure.
There is also another kind of resonance MRI in patients with EOPD. MRI creates images of the human body by detecting spin properties of nuclei [97]. MRI is not able to directly image dopaminergic neuronal loss, but it can provide complementary data to those obtained with nuclear tracer imaging [104]. The study of Wang et al. [105] shows that pathological asymmetry between both hemispheres in NG pathways in the early stage of EOPS using an MRI method.
TCS is another technique used in PD. It is a noninvasive, validated ultrasound method for demonstrating characteristic alterations of deep brain regions especially SN, but also lenticular nucleus (NL) or ventricles [106]. It is less expensive than the previously described tools, that is why it can be an important advantage of its application [97]. The literature indicates that TCS-MRI fusion allows analyzing SN and NL echogenicity as highly sensitive and specific markers for EOPD [107].
There are also multimodal imaging for imaging structure and metabolism like PET/CT. Using this method, the study of Shi et al. [108] shows the unequal radioactive distribution of 18F-2-deoxy-D-glucose among patients with compound mutations in the
6. Summary
The occurrence of EOPD is associated with molecular factors both genetic and biochemical ones. The presence of various genetic variants such as
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