Etiology and pathogenesis of Parkinson disease

Historical introduction

Parkinson disease (PD) was first described in 1817 with the publication by James Parkinson of a small book entitled An Essay on the Shaking Palsy ( ). In it, he described six individuals with the clinical features that have come to be recognized as a disease entity. One of the people was followed in detail over a long period; the other five consisted of brief descriptions, including two of whom he had met walking in the street and another whom he had observed at a distance. Such distant observations without a medical examination demonstrates how readily distinguishable the condition is. The physical appearance of flexed posture, resting tremor, and shuffling gait are easily recognizable. Parkinson’s opening description has the key essentials: “Involuntary tremulous motion, with lessened muscular power, in parts not in action and even when supported; with a propensity to bend the trunk forward, and to pass from a walking to a running pace: the senses and intellects being uninjured.” In his small monograph, Parkinson provided a detailed description of the symptoms and also discussed the progressive worsening of the disorder, which he called “the shaking palsy” and also its Latin term “paralysis agitans.” On the 200th anniversary of Parkinson’s publication, a group of scholars summarized many aspects of current knowledge of PD, including a brief description of James Parkinson, the person, and of the clinical features of the disorder that he described and also some features, particularly nonmotor features, that he missed ( ).

After the publication of Parkinson’s Essay, the disease was widely accepted in the medical community. It took 70 years for the disorder to be referred to as “Parkinson’s disease,” as recommended by the French neurologist , who argued against the term “paralysis agitans” (see , for English translation) and recommended the disorder be named after James Parkinson. Charcot argued that there is no true paralysis but rather “lessened muscular power” that is today called akinesia, hypokinesia, or bradykinesia; all three terms are often used interchangeably by clinicians, although these three terms specifically refer to lack of movement, small movement, and slow movement, respectively. These terms represent a paucity of movement not resulting from weakness or paralysis. Similarly, Charcot emphasized that tremor need not be present in the disorder, so “agitans” and “shaking” are not appropriate as part of the name of the disorder.

The clinical features of PD and its differential diagnosis are presented in a separate chapter, Chapter 4 .

Incidence, prevalence, and mortality

Although PD can develop at any age, it begins most commonly in older adults, with a peak age at onset of around 60 years. The likelihood of developing PD increases with age, with a lifetime risk currently of 1 in 15 ( ). A positive family history doubles the risk for developing PD to about 4%. A summation from seven population-based studies in various European countries found the overall prevalence of PD in people aged over 65 to be 1.8%, with an increase from 0.6% for persons aged 65 to 69 years to 2.6% for people aged 85 to 89 years ( ). Twin studies indicate that PD with an onset under the age of 50 years is more likely to have a genetic relationship than for patients with an older age at onset ( ). Males have higher prevalence (male-to-female ratio of 3:2) and incidence rates than females ( Fig. 5. 1 ) ( ), but the age-specific incidence rates have not varied over the last seven decades ( Fig. 5.2 ) ( ). The incidence rates vary among studies, but average between 11.0 and 13.9 per 100,000 population per year ( ). In a Northern California study, the incidence rates varied among ethnic groups, being highest in Hispanics, then non-Hispanic whites, then Asians, and lowest in blacks ( ). Most of the speculation of the lower prevalence and incidence rates in females centered on hormonal differences between the genders. But an X-chromosome-wide association study has found two gene loci on the X-chromosome may contributes to PD genetic risk ( ).

The prevalence rates of PD have varied in different studies and in different countries ( ). The figure of 187 per 100,000 population given by is a reasonable estimate in the United States. However, if the population is restricted to adults older than 39 years of age, the prevalence rate is 347 per 100,000 ( ) because both prevalence and incidence rates increase with age. At age 70, the prevalence is approximately 550 per 100,000, and the incidence is 120 per 100,000 per year. At the present time, approximately 850,000 individuals in the United States have PD, although one estimate is less than this ( ). The number is expected to grow as the population ages ( ). Advancing age is the single greatest risk factor for developing sporadic PD. The world-wide prevalence of PD is growing at an exponential rate, and it is the fastest growing of all neurologic diseases ( ; ). The incidence rate is particularly increasing in men over 70 years of age ( ). A younger age at onset of PD is associated with a slower rate of progression and a greater compensatory capacity ( ).

In the pre-levodopa era, mortality was reported to be 3-fold greater in patients with PD ( ). The mortality rate was reduced to 1.6-fold greater than age-matched non-PD individuals after the introduction of levodopa ( ; ). Today, patients with PD can live 20 or more years, depending on the age at onset ( ). Death in PD is usually the result of some concurrent unrelated illness or the effects of decreased mobility, aspiration, or increased falling with subsequent physical injury. The Parkinson-plus syndromes typically progress at a faster rate and often cause death within 9 years. Thus, the diagnosis of PD versus other forms of parkinsonism is of prognostic importance and therapeutic significance because it almost always responds to at least a moderate degree with levodopa therapy, whereas the Parkinson-plus disorders usually do not.

Anatomic pathology of PD and Lewy bodies

It was many years after Parkinson’s original description before the basal ganglia were recognized by as being involved in disorders of abnormal movements. It was not until 1895 that the substantia nigra (SN) was suggested to be affected in PD. suggested this on the basis of a report by of a tuberculoma in that site that was associated with a contralateral hemiparkinsonian tremor. These authors were careful to point out that the pyramidal tract and the brachium conjuctivum above and below the level of the lesion contained no degenerating fibers. The importance of the SN was emphasized by Tretiakoff in his doctoral thesis in 1919 ( ; ). He studied the SN in 54 brains, including nine cases of PD, one case of hemiparkinsonism, and three cases of postencephalitic parkinsonism, finding neuronal loss in this nucleus in all 13 cases. With the hemiparkinsonian case, Tretiakoff found a lesion in the nigra on the opposite side, concluding that the nucleus served the motor activity on the contralateral side of the body. The SN, so named because of its normally high content of neuromelanin pigment, was noted to show depigmentation, loss of nerve cells, and gliosis. These findings remain the principal and essential histopathologic features of the disease. In his study, Tretiakoff also found Lewy bodies in the SN, expanding the earlier observation of , who had discovered the presence of these cytoplasmic inclusions in the substantia innominata and the dorsal vagus nucleus in PD. Lewy bodies are located in the neuronal cytoplasm and have an eosinophilic core surrounded by a clear halo ( Fig. 5.3 ). They are now widely recognized as a major pathologic hallmark of the disorder. Lewy bodies have since been seen in autonomic ganglia, the peripheral nervous system ( ), and other regions of the central nervous system (CNS), including the cerebral cortex ( ) and olfactory cells ( ). Biopsies of peripheral tissue, such as the gastrointestinal tract and submandibular gland, can reveal Lewy pathologic findings, and this approach has been proposed as a potential biomarker for PD ( ; ; ; ).

made a detailed study of the pathologic findings of PD in 1925 and found that the most constant and severe lesions are in the SN. Since then many workers, including and , have confirmed these findings and added other observations, including involvement of other brainstem nuclei such as the locus ceruleus (LC) and the raphe nuclei. The pigmented cells of the LC contain neuromelanin; these cells are also lost in PD, with many of those remaining containing Lewy bodies. The asymmetry of clinical signs in PD is reflected by the asymmetrical and more severe contralateral loss of substantia nigra pars compacta (SNc) neurons. Neuronal loss extends beyond loss in SNc, LC, and raphe, with loss of neurons in the dorsal motor vagal nucleus, hypothalamus, nucleus basalis of Meynert, and sympathetic ganglia ( ; ). There is also neurodegeneration of glutamatergic projection neurons from thalamus to the basal ganglia ( ) and glutamatergic projection neurons from the presupplementary motor cortex to the premotor cortices ( ).

PD and the Parkinson-plus syndromes have in common a degeneration of SNc dopaminergic neurons, with a resulting deficiency of striatal dopamine as a result of loss of these nigrostriatal neurons. The average number of pigmented (dopamine-containing) neurons in an 80-year-old human is 550,000; in those with PD, there is a reduction by 66% ( ). The average total number of nonpigmented neurons is 260,000 in controls and reduced by 24% in the patients. Accompanying this neuronal loss is an increase in glial cells in the nigra and a loss of the neuromelanin normally contained in the dopaminergic neurons. There is also a reduction of nigral neurons and striatal dopamine with aging ( ; ; ) ( Fig. 5. 4 ), and although PD is associated with increasing age, the greater rate ( Fig. 5. 5 ) and the pattern of cell loss in the SN differ between those in aging and those in PD ( Figs. 5.6 and 5.7 ) ( ; ). Still, a case has been made that aging plays a primary role as a risk factor for PD, based on studies in nonhuman primates ( ). Moreover, there is an inverted U-shape curve of neuromelanin content in the human SN over the life span ( Fig. 5. 8 ) ( ), supporting the loss of dopaminergic neurons with age. Clinical features begin to emerge when approximately 80% of striatal dopamine content (or 60% of nigral dopaminergic neurons) is lost ( ). The course of clinical decline is associated with the progressive reduction of striatal dopamine ( ). There is mounting evidence that the loss of the dopaminergic nigrostriatal neurons starts at the nerve terminals in the striatum and then dies back to involve the axons and cell bodies in the SN ( ; ; ; ; ; ). With special magnetic resonance imaging (MRI) techniques, neuromelanin can now be assessed ( ), and these MRI studies show reduced neuromelanin in the SN ( ) and LC ( ).

Pathologically, almost all patients with PD have Lewy bodies in the SN and LC. Juvenile PD ( ; ), now recognized as being the result mainly of homozygous and compound heterozygous PRKN mutations, is a major exception. In addition, only about two-thirds of PD patients with the LRRK2 G2019S mutation have Lewy bodies ( ). Other gene mutations in the LRRK2 gene, including the R1441G mutation ( )], are less likely to have Lewy bodies ( ). In one case of young-onset PD with PINK1 mutation, the autopsy showed the presence of Lewy bodies ( ). The pathologic processes have not yet been described in juvenile PD patients with homozygous mutations in DJ-1 genes, but there is one case of heterozygotic DJ-1 mutation with rapid, young-onset PD developing dementia and not responding to levodopa, and who had widespread Lewy bodies ( ). Clinically, this is not similar to the juvenile onset, slowly progressive form of homozygous recessive DJ-1 PD. There are no Lewy bodies in the Parkinson-plus syndromes or in postencephalitic parkinsonism ( ). In drug-induced parkinsonism, Lewy bodies are not seen except in some individuals with incidental Lewy bodies ( ). In PD patients who develop dementia, Lewy bodies are seen in the cerebral cortex and in the hippocampus ( ).

The presence of Lewy bodies in the SNc and the LC plus the clinical features of PD (without the characteristic clinical features of some other form of parkinsonism) are usually used to make the pathologic diagnosis of PD, but there is no complete agreement among neuropathologists about the pathologic criteria for the diagnosis of PD ( , ). Some patients with clinical PD die with nigral degeneration without Lewy bodies ( , ). In fact, as mentioned previously, patients with juvenile PD usually do not have Lewy bodies ( ), especially those with homozygous or compound heterozygous PRKN mutations ( ). Thus, Lewy bodies are a critical pathologic marker confirming the diagnosis of PD, but they are not necessarily present in all cases of PD, and their presence is not pathognomonic for PD ( ).

Lewy bodies are found in 4% to 6% of routine autopsies ( ), the incidence rate increases with age as does PD ( ) ( Table 5.1 ), and people dying with such incidental Lewy bodies are considered to have a presymptomatic state of PD ( ; ; ). The site of regional nigral neuronal loss in incidental Lewy body disease is the lateral region in the ventral tier, the same as for PD (see Fig. 5.6 ) ( ). These brains show a reduction in striatal dopaminergic markers (e.g., tyrosine hydroxylase and vesicular monoamine transporter 2) but not as severe as those with clinical PD ( ; ). Cortical Lewy bodies in patients with dementia and no parkinsonism could be a separate disease or a variant in the presentation of the same disorder that causes PD.

Lewy bodies consist of a dense granular inner core surrounded by a radiating filamentous outer zone ( Fig. 5. 9 ) and without a clear demarcating membrane separating them from the cytoplasm ( ; ); they consist of a crowded aggregated protein-lipid complex ( ).

The outer zone contains the protein, alpha-synuclein ( ), and antibodies to this protein are commonly used by neuropathologists to locate and identify Lewy bodies in brain and other tissues. Other proteins are also found in Lewy bodies, including ubiquitin and heat shock proteins ( ). However, the process of Lewy Body formation, rather than simply α-synuclein fibrillization, has now been directly implicated as one of the major drivers of neurodegeneration ( ). Lewy neurites are aggregates of alpha-synuclein localized in neuronal processes. Although alpha-synuclein oligomers and fibrils are clearly toxic (discussed in more detail later in the section of the rogue protein hypothesis), there is debate and uncertainty as to whether the Lewy body is also toxic or is a means of protection by the neuron against the toxic fibrils ( ). A diagram describing the relationship of natural alpha-synuclein, mostly a soluble protein in the nerve terminal cytosol, alpha-synuclein oligomers and fibrils, and Lewy bodies is presented in Fig. 5.10 .

Biochemical pathologic processes of PD

The pigmented neurons of both the SNc and the ventral tegmental area (VTA) (medial to the SN in the midbrain) contain dopamine. The former neurons project to the neostriatum, the latter to the limbic system and the neocortex. In PD, the mesolimbic and mesocortical neurons are relatively spared, whereas the nigrostriatal neurons are progressively lost. As a result, there is a corresponding decrease of dopamine content in both the nigra and the striatum, with the innervation of the posterior putamen affected first and most severely, as can be detected in 6-[ ¹⁸ F]-fluoro-l-dopa (FDOPA) positron emission tomography (PET) scans ( Fig. 5. 11 ) and in dopamine transporter (DAT) single-photon emission computed tomography (SPECT) scans. Whereas dopamine is reduced initially in the posterior striatum in PD, over time, as the disease progresses, all striatal subregions are affected to a similar extent ( ).

The pigmented neurons of the LC contain norepinephrine, and these neurons project widely in the CNS. A third set of monoaminergic neurons are those containing serotonin (5-hydroxytryptamine [5-HT]), located in the raphe of the pons and medulla. In PD a progressive loss of all three types of monoaminergic cells occurs, most severely in the dopaminergic cells. So, in addition to a depletion of striatal dopamine, there is also a reduction in brain norepinephrine and 5-HT ( ; ) ( Table 5.2 ). There is also reduction in other neurotransmitters ( ) and enzyme activities for the synthesis of other neurotransmitters (see Table 5.2 ), indicating the biochemical changes in PD extends beyond the loss of only the monoamines.

In addition to the cardinal motor features of PD, such as rest tremor, bradykinesia, rigidity, and postural instability that define the clinical diagnosis, there are also a host of nonmotor symptoms, some occurring before the motor symptoms and some occurring after (see Chapter 8 ). The early motor symptoms of bradykinesia and rigidity and tremor are associated with monoaminergic cell and neurotransmitter loss. The later motor symptoms of flexed posture, loss of postural reflexes, and the freezing phenomenon appear to correlate poorly with dopaminergic deficit, and are likely associated with involvement of other brain structures. For example, the pedunculopontine nucleus has been suggested to play a role in freezing of gait ( ). The nonmotor symptoms probably are the result of loss of neuronal function other than dopamine. Catecholamine reduction in PD is seen in the autonomic nervous system and accounts for the reduction in 123I-metaiodobezyguanidine (MIBG) SPECT scan labeling in the heart in PD because of loss of postganglionic myocardial sympathetic nerve fibers ( ). Other imaging techniques to detect impairment of the autonomic nervous system have also been studied ( ). A noradrenergic transporter PET ligand showed reduced uptake in PD and even more so in patients who also had rapid eye movement (REM) sleep behavior disorder (RBD) ( ). A neuropathological study combined with good clinical data found no correlation of clinical orthostaic hypotension in people with PD and the degree of neuronal loss in the SN or LC ( ).

Among the neurotransmitter changes is the reduction of brain acetylcholine ( ). Acetylcholinesterase (AChE), which serves as a marker for cholinergic neurons, can be measured by PET scanning ( ). A reduction of AChE begins early in PD ( ). Reduced thalamic AChE activity correlates with falling in PD ( ), and in part represents decreased cholinergic output of the pedunculopontine nucleus (PPN), which appears to be important for gait. Cortical loss of acetylcholine probably contributes to the dementia seen in PD ( ; ; ). Overall, reduced AChE is more widespread and profound in both PD with dementia and in dementia with Lewy bodies ( ).

With the loss of striatal dopamine, there are compensatory changes, such as supersensitivity of dopamine receptors, so that symptoms of PD are first encountered only when there is about an 80% reduction of dopamine concentration in the putamen (or a loss of 60% of nigral dopaminergic neurons) ( ). Another compensatory mechanism is an increase in neurotransmitter turnover, as detected by an increased ratio of homovanillic acid (HVA) to dopamine. In a major review, correlated loss of dopamine concentration in the striatum with severity of bradykinesia and rigidity in PD. With further loss of dopamine concentration, parkinsonian bradykinesia becomes more severe. The progressive loss of the dopaminergic nigrostriatal pathway can be detected during life using PET and SPECT scanning (see Fig. 5.9 ); these show a continuing reduction of FDOPA and DAT ligand binding in the striatum that correlates with the bradykinesia score in the Unified Parkinson’s Disease Rating Scale (UPDRS) ( ; ; ; ; ; ; ). Using special statistical techniques, fluorodeoxyglucose (FDG) PET also shows a correlation between worsening bradykinesia and increase of lentiform metabolism ( ). In fact, using FDG PET demonstrates a metabolic network characteristic of PD compared with other forms of parkinsonism ( ; ).

Interconnected pathogenic mechanisms of PD

The best correlation of symptoms with progressive loss of striatal dopamine are those of bradykinesia and rigidity, which relate to striatal dopamine deficiency and loss of SNc dopaminergic neurons and can be correlated with a progressive decrease of dopaminergic imaging by PET or SPECT (see earlier discussion). These symptoms are two of the cardinal features of the disease; therefore, research efforts have concentrated on the pathogenic mechanisms that cause loss of the nigrostriatal dopamine system, and this will be reflected in this review. Similar mechanisms might involve the other monoaminergic systems (noradrenergic and serotonergic). It seems likely that loss of these other monoamines might be instrumental in the high rate of depression ( ) and anxiety in patients with PD. There is little knowledge of the pathogenesis of neuronal loss of nonaminergic neurons, except that toxic forms of alpha-synuclein, referred to here as a rogue protein, can spread from cell to cell and eventually cause cell death, mostly by a death process called parthanatos (described later). In addition, little is known about the anatomic or biochemical associations for most clinical features of the disease, including motoric features of tremor, freezing, flexed posture, loss of postural reflexes, and the multitude of nonmotor features.

A variety of pathogenic mechanisms have been uncovered for the loss of dopamine neurons, and probably more will be uncovered. The reader is referred to reviews on this topic for details ( ; ; ; ; ; ; ). With the development of genetic causes of PD, a multiple-hit hypothesis was proposed by and is discussed later in this chapter. Evidence has accumulated over decades from pathologic and biochemical findings that implicate oxidative stress, mitochondrial dysfunction, excitotoxicity, inflammation, and apoptosis as taking place in the SNc ( Fig. 5.12 ).

More recently, alpha-synuclein aggregation has taken center stage both in the progressive worsening of PD spreading from the SN to involve other parts of the brain and in the mechanism of cell death ( ). The mechanism of cell death in PD was previously considered to be by the process of apoptosis, but is currently considered to be mainly by parthanatos (David et al., 2009), which is discussed later. Alpha-synuclein can convert from a soluble monomer to form toxic oligomers and fibrils, and then Lewy bodies and Lewy neurites. New research on alpha-synuclein has placed a major emphasis on the accumulation of toxic protein as perhaps the most important pathogenic factor. The spread of the toxic fibrils of alpha-synuclein likely contributes to the worsening of PD, with more symptoms developing over time as more neurons are affected and die. Alpha-synuclein fibrils has become a rogue protein, or prion-like, in that when it leaves one neuron and enters an adjacent one, the rogue protein can convert natural alpha-synuclein of the newly invaded cell into a toxic form, hence considered prion-like. Each of the factors in Fig. 5.12 cross-interacts with the others to add to the pathogenesis of cell death. The understanding of alpha-synuclein becoming a rogue protein has led to the development of potential disease-modifying strategies ( ).

How are the toxic forms of alpha-synuclein initially generated? One mechanism is by the mutated gene (SNCA) that generates a form of alpha-synuclein that cannot be autophagotized, so it accumulates ( ). Other mechanisms are by excessive synthesis (by duplication and triplication of the SNCA gene) ( ). Excessive monomeric alpha-synuclein can oligermize starting the process to generate the toxic forms of the protein. There could be a deficit of insufficient degradation (i.e., inadequate autophagy) of alpha-synuclein ( ). There is also much interest that alpha-synuclein enters the brain through the olfactory system or the gut via the vagus nerve terminals there. This chapter will provide more details on these processes of pathogenesis.

Of all the etiologic and pathogenic factors discussed so far and others to be discussed later, why is the nigrostriatal dopaminergic neuron particularly affected in PD, so-called selective vulnerability? There are a number of unique factors with these neurons, a leading one being high exposure to oxidative stress due to the enzymatic and auto-oxidation of dopamine,

These neurons are autonomously paced around 2 Hz, so they are continuously using a high amount of energy. These neurons are extremely highly branched with huge numbers of nerve terminals for each neuron, and the neuron is unmyelinated, which may make it more susceptible. These factors are discussed in the next section.

Oxidative stress

Why is the nigrostriatal dopaminergic neuron particularly affected in PD? A common source of oxidative stress in all cells is via normal mitochondrial function. A key source of additional oxidative stress in monoaminergic neurons is via monoamine metabolism and auto-oxidaction (discussed in more detail later in the Endogenous Factors section). Because most research in PD neurodegeneration has been applied to dopaminergic neurons, we will focus on dopamine. Another factor leading to increased oxidative stress in SNc dopaminergic neurons is the influx of calcium into the cytosol because of the opening of L-type calcium channels during autonomous pacemaking, resulting in greater production of reactive oxygen species by the mitochondria ( ) (discussed in more detail later in the Endogenous Factors section). An important element of the nigrostriatal dopamine neuron, making it quite susceptible to oxidative damage, is the neuron’s unique anatomic features of being poorly myelinated and with very long, highly branched axons and terminals ( Fig. 5. 13 ) ( ).

Antioxidant defenses protect cells, and one of the leading antioxidants is reduced glutathione (GSH), and this is diminished in the SNc of PD patients at postmortem ( ; ). This reduction of GSH is specific to PD brains and is not seen in atypical parkinsonisms with nigral degeneration. The reduction of GSH likely reflects an excess usage of this reducing agent, implying a high degree of oxidative stress taking place there. The decrease in GSH in the SNc occurs in incidental Lewy bodies and in PD, suggesting that oxidative stress has occurred before nerve cell loss. Some evidence suggests that GSH depletion itself may play an active role in PD pathogenesis ( ). It should be mentioned that intranasal but not intravenous GSH administration can enter the brain, but a small clinical trial showed no clinical benefit ( ). Iron accumulation in the nigra also contributes to oxidative stress ( ). Oxidized products of lipids, DNA, and protein are seen in PD nigra ( ; ), providing postmortem evidence of oxidative stress. Neuromelanin in dopamine neurons is derived mainly from the condensation of oxidized dopamine products and thus represents a protective mechanism by the cell to defend against oxidative stress (Sulzer et al., 2004). One of the mutant genes that can cause PD, DJ-1, functions normally to protect against oxidative stress and is discussed later in the section on Genetics.

It is of interest that an endogenous substance, uric acid, which has antioxidant properties, has been correlated with a reduction in developing PD ( ). Subsequent studies evaluating urate levels in plasma in the PRECEPT ( ) and cerebrospinal fluid (CSF) in the DATATOP (Ascherio et al., 2009) clinical trials found that higher urate levels in men were associated with a slower rate of progression of PD. Treatment with urate’s precursor, inosine, was tested in a controlled clinical trial as a therapeutic agent for PD. Unfortunately, as announced in a press release, inosine did not slow the progression of PD ( https://www.michaeljfox.org/foundation/news-detail.php?parkinson-inosine-trial-ending-early ). Furthermore, the De Novo Parkinson Cohort (DeNoPa) study showed that elevated uric acid, in addition to other factors, was associated with faster disease progression in PD ( ). Other antioxidants have been studied, but most have been ineffective, including high-dosage tocopherol (vitamin E) ( ) and high-dosage coenzyme Q10 ( ).

Of all the antioxidant type drugs tested, only the type B monoamine inhibitors, such as selegiline and rasagiline, have shown the ability to reduce PD signs and symptoms, and this topic is covered in Chapter 6 .

Mitochondrial dysfunction

Mitochondria appear to play an important role in the pathogenesis of PD ( ). The finding that 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication causes parkinsonism ( ), and the discovery that MPTP selectively destroys dopamine neurons and impairs complex 1 activity in the mitochondria ( ) led to studying mitochondria in PD patients ( Fig. 5. 14 ). Decreased complex 1 activity is seen in the SNc of PD brains and not elsewhere nor in other forms of parkinsonism ( ; ). Another complex 1 toxin, rotenone, which is a commonly used pesticide, also damages SNc neurons in animals ( ). We will see later, when discussing the role of gene mutations in PD, that the proteins from four genes, PRKN, PINK1, DJ-1, and VPS13C, related to PD, help maintain the integrity and healthy function of the mitochondria, and loss of function of these genes results in PD. Mitochondrial dysfunction impairs adenosine triphosphate (ATP) production, which hinders energy-dependent mechanisms in cells.

Mitochondrial dysfunction can be both a cause and a consequence of oxidative stress. Deregulation of mitochondrial respiration leads to generation of reactive oxygen species, contributing to oxidative stress, and oxidative and nitrosative stress deteriorate mitochondrial function. An early event in MPTP toxicity is oxidative stress, which is the consequence of an inability to transport electrons because of the inhibition of mitochondrial complex 1. The accumulating electrons are a source of oxidative stress ( ). Besides their role in electron transport and oxidative phosphorylation, mitochondria are a major cellular source of free radicals, affect calcium homeostasis, and instigate cell-death pathways via apoptosis ( ; ).

PARP and cell death by parthanatos

In addition to necrosis and apoptosis, parthanatos is another form of cell death. The term was coined to combine PAR and Thanatos, the personification of death in Greek mythology ( ). PAR is the shorthand name for poly(adenosine diphosphate [ADP]-ribose), a polymer required in this form of cell death (along with another requirement, apoptosis-inducing factor).

When mitochondria are damaged, as discussed in the previous section, they release a mitochondrial protein called apoptosis-inducing factor (AIF). AIF enters the cytosol and then into the cell’s nucleus to trigger a caspase-independent pathway of cell death, known as parthanatos. (The programmed cell death pathway known as apoptosis is caspase-dependent and is triggered by the release of cytochrome c from the mitochondria). AIF is activated in the nucleus by PAR to initiate DNA fragmentation and chromatin condensation, leading to cell death. This is a common cell death pathway in stroke, diabetes, and myocardial infarction ( ). It is now considered the most common cell death pathway in PD. PAR is a polymer of various lengths and can be linear or branched. PAR is generated by the nuclear enzyme poly(ADP-ribose) polymerase-1 (PARP-1). PARP-1 catalyzes the transfer of ADP-ribose units from nicotinamide adenine dinucleotide (NAD) to protein targets. A normal function of PARP-1 is the repair of DNA breaks. Parthanatos also can be triggered by DNA damage, initiating activation of PARP-1 to generate PAR. PAR then leaves the nucleus to enter the cytosol and then into mitochondria, where it causes the release of AIF. Thus, there is a reciprocal mechanism between PAR and AIF. PAR is degraded by the enzyme poly(ADP-ribose) glycohydrolase (PARG) ( ).

In 1999 the Dawson group discovered that MPTP-treated mice developed a high level of PARP in the brain ( ). In a subsequent experiment, they genetically engineered mice to disable the production of PARP and found that the ability of MPTP to destroy dopamine neurons was virtually eliminated. The investigators suggested that PARP inhibitors might be a useful treatment for people with PD. Japanese investigators in 2004 tested a PARP inhibitor in the MPTP mouse model and showed striking ability to reduce the degeneration of nigral neurons ( ). Around the same time, postmortem studies showed that people with PD had increased levels of PARP in the brain ( ). In 2018 the Dawson group reported that in the animal model in which preformed alpha-synuclein fibrils are injected to induce the spread of the toxic alpha-synuclein to cause neuronal death via parthanatos and loss of dopamine neurons, there is activation of PARP-1 and generation of PAR. Both PARP inhibitors and genetic deletion of PARP-1 prevented this pathologic process ( ). They also showed that the presence of PAR converted pathologic alpha-synuclein to a more toxic strain, which allows for a vicious cycle whereby toxic fibrils induce PARP-1 activation, which generates PAR that in turn eventually makes the toxic fibrils even more toxic ( Fig. 5.15 ). The investigators also measured PAR levels in autopsied brains and in the CSF of patients with PD and found elevated levels in both tissues. This suggests that PARP activation plays a role in PD pathogenesis. PARP inhibitors, already used in cancer therapy, should be tested in PD, but those currently on the market do not penetrate into the brain. Appropriate drugs will need to be developed ( ).

Familial PD

Families with PD occurring in several members have been recognized over the years, with approximately 10% of newly diagnosed patients reporting someone else in the family having PD. was one of the first to describe the occurrence of PD in family members. Perhaps the largest pedigree of PD described to date and with many generations affected was that reported by Mjönes (1949) of a Swedish and Swedish American family. He found an autosomal dominant inheritance pattern in this family with 60% penetrance. Subsequently, the genetic alterations in this family were found to be triplications and duplications of the synuclein gene, SNCA ( ). But, generally, because most PD patients have a sporadic occurrence without a positive family history, the disease was for over 100 years not thought to have a genetic cause.

Twin Studies

To study the possibility of a genetic cause of PD, though, evaluated twins with PD. They found zero concordance in 12 monozygotic twin pairs and concluded that “genetic factors appear not to play a major role in the etiology of PD.” This group ( ) continued to analyze twin pairs and subsequently in 43 monozygotic (MZ) and 19 dizygotic (DZ) pairs, with the index case having definite PD, the frequency of PD in MZ twins was similar to that expected in an unrelated control group matched for age and sex. The authors again concluded that “the major factors in the etiology of PD are nongenetic.” A Finnish twin study 5 years later ( ) also found low concordance in MZ twin pairs and also concluded that PD appears to be “an acquired disease not caused by a hereditary process.” However, Bill Johnson, then at Columbia University, began to question the conclusions of the twin studies ( ), saying they were too small to be statistically conclusive and recommended that linkage studies be conducted. A subsequent twin study found that twins with a young onset of PD had a higher concordance rate in MZ twins than in dizygotic twins, but the difference was lost in twins with an older age at onset ( ). One possible explanation of discordance in MZ twins was the finding there was decreased mitochondrial functionality and a tendency for a higher number of somatic mtDNA variants among the affected MZ twins ( ).

The momentum toward a genetic cause of PD

In 1990, Golbe, Duvoisin, and colleagues described a large kindred with autosomal dominant PD originating in Contoursi, Italy, with some of the family having emigrated to the United States ( ). This led Duvoisin to rethink his previous conclusions that PD is largely nongenetic (Duvoisin and Johnson, 1991). At this time there were also reports that FDOPA PET scans can detect decreased FDOPA uptake in some nonaffected relatives, including twins with an affected co-twin ( ). A large PET study in twins indicated that the concordance for decreased striatal FDOPA uptake in PD twins is greater than previously realized ( ). With the availability of the Contoursi kindred and with the tools of linkage analysis, the stage was set for finding a gene mutation in familial PD.

Finding the first gene mutation in PD

Linkage analysis was carried out on the Contoursi kindred, and after several years of searching, linkage to chromosome 4q21-q23 was found ( ). By the next year, identified the mutated gene, SNCA, for the protein alpha-synuclein. The Contoursi family actually originated in Greece and immigrated to Italy. The mutation seen in this family (Ala53Thr) was also found in three small, unrelated Greek families. Subsequently, five other mutations were found in SNCA, all of which also caused autosomal dominant PD, namely Ala30Pro, in a German family ( ), Glu46Lys in a Spanish family ( ), Gly51Asp in a French family ( ) and a British family ( ), His50Gln in two unrelated people of English descent (Appel-Cresswell et al., 2013; ), and Ala53Glu in Finnish families because of a founder effect ( , ).

Families with SNCA mutations have a younger age at onset (usually in their 40s), a more rapidly worsening course of PD, and also some early cognitive impairment. So, the gene mutation, on average, causes a more severe form of the disease than the typical adult-onset sporadic case. The SNCA gene was originally labeled PARK1.

Although all these SNCA mutations are rare in causing all worldwide cases of both familial and sporadic PD, the protein alpha-synuclein has taken on a premier and most important role as being highly likely involved in the disease’s pathogenesis, including the sporadic cases. Immediately after the gene mutation was discovered ( ), found that fibrillary alpha-synuclein is a major component of the Lewy body, a finding confirmed the following year ( ). By good fortune, the Goedert laboratory of protein chemistry had been studying alpha-synuclein and even had generated antibodies against it. When the Polymeropoulos paper was published, the Goedert laboratory immediately decided to apply their alpha-synuclein antibody to autopsied tissue from patients with PD and discovered that Lewy bodies contain alpha-synuclein ( ).

In fact, staining microscopic brain slices with the antibody for alpha-synuclein has become the pathologist’s tool to detect Lewy bodies and probably accounts for the recognition that diffuse Lewy body disease (DLBD), also known as dementia with Lewy bodies (DLB) and Lewy body dementia (LBD), is the second most common cause of dementia after Alzheimer disease (AD). Before staining for alpha-synuclein, ubiquitin antibodies had been used because ubiquitin is another protein found within the Lewy body. Ubiquitin is located in the central core and alpha-synuclein in the surrounding halo ( Fig. 5.16 ). Autopsies on patients with mutated or excessive amounts of alpha-synuclein show an abundance of Lewy bodies ( ).

Phosphorylation of alpha-synuclein at Ser129 promotes insoluble fibril formation, and the alpha-synuclein found in Lewy bodies is phosphorylated at Ser129 ( ). It appears that when cytosolic alpha-synuclein becomes phosphorylated, it eventually becomes incorporated into Lewy neurites and Lewy bodies.

Phosphorylation of alpha-synuclein at tyrosine 39 is promoted by the tyrosine kinase c-Abl, Overexpression of c-Abl in a mouse model of PD accelerates alpha-synuclein aggregation, neuropathologic conditions, and neurobehavioral deficits, whereas deletion of its gene reduced these events ( ). The c-Abl kinase inhibitor, nilotinib, was shown to induce alpha-synuclein protein degradation via the autophagy and proteasome pathways ( ). C-Abl is activated in the brain of PD patients and in MPTP-mice, where it inhibits parkin through tyrosine phosphorylation leading to the accumulation of parkin substrates and neuronal cell death ( ). In MPTP mice, nilotinib was found to be protective against loss of dopamine neurons ( ). An open-label trial in six patients with PD was reported to improve the patients ( ), but subsequently two controlled clinical trials failed to find benefit.

Wild-type alpha-synuclein has several functions in neurons, related to synaptic vesicle function ( ). It is important for vesicle trafficking, vesicle docking and priming, vesicle fusion, and neurotransmitter release involving the SNARE protein complex assembly ( ). alpha-Synuclein has also been found to play a role in endocytosis ( ; ).

Other gene mutations in PD labeled as PARK

PARK4

The evolution of the gene identification for PARK4 is a fascinating story. described a kindred with autosomal dominant Lewy body parkinsonism in four generations, with an early age at onset (PARK4). The kindred was extended ( ; ; ), and action tremor similar to essential tremor and dementia were prominent within the family and widespread Lewy bodies were seen in the cerebral cortex. Linkage was initially reported to chromosome 4p, but this haplotype also occurred in individuals in the pedigree who did not have clinical Lewy body parkinsonism but rather had postural tremor only ( ). A second genome-wide search in this family found linkage on chromosome 4q and the mutation to be a triplication of a region of the chromosome that includes the SNCA gene ( ), the same gene that is mutated in PARK1. Instead of a mutation, though, this triplication produces a doubling of the normal, wild-type alpha-synuclein protein. A second PD family (Swedish American) with PD-dementia with triplication of SNCA also has been reported ( ). The large Swedish family reported originally by Mjönes (1949) was genetically analyzed and found to consist of duplications and triplications of normal SNCA ( ). The remarkable thing is that the Mjönes family has an alpha-synuclein duplication in one branch, and a triplication in the other, with a correspondingly different clinical picture (late versus early onset, respectively). The fact that excess normal alpha-synuclein protein can cause PD is extremely important because it indicates that too much alpha-synuclein, and not just mutations, can be a causative factor. Thus alpha-synuclein protein can be considered to play a central role in the pathogenesis of PD. Both making too much alpha-synuclein protein and blocking its degradation could give rise to an excessive amount of alpha-synuclein.

A comparison of age at onset of PD among triplications and duplications and the missense mutations of the SNCA gene showed that duplications and triplications resulted in an earlier age at onset compared with mutated SNCA ( Fig. 5.17 ).