Parkinson’s disease (PD) has become increasingly more prevalent as the population ages. Despite this, little is known about the etiology of the sporadic form of the disease. Considerable research into the etiology of PD, focusing on genetic and environmental factors, has occurred over the past few decades. Although progress has occurred, there is still no single factor that can account for most cases of sporadic PD. Recently, attention has been given to candidate genes, which, it is hypothesized, play a contributing (but not causative) role in sporadic cases of PD.
The aim of this chapter is to provide a synopsis of the current state of knowledge in the genetic etiology of PD. And also, it will be illustrated how to apply this knowledge of genetics in the clinical assessment for the clinicians. Specific attention will be given to the ethnic difference of the prevalence of genetic mutations.
Although an increasing number of genetic factors appear to be associated with PD, we will focus on genes with a Mendelian inheritance (
Table 1), including alpha-synuclein gene (
SNCA, PARK1/4), parkin gene (
Parkin, PARK2), P-TEN-induced putative kinase 1 gene (
PINK1, PARK6), Daisuke-Junko 1 gene (
DJ-1, PARK7), and leucine-rich repeat kinase 2 gene (
LRRK2, PARK8), which have been conclusively proven as a monogenic etiology for familial parkinsonism.
3 Associated contributing genetic factors of PD, focusing on recent genome-wide association studies, will be also reviewed.
Park Genes Following Mendelian Inheritance
The loci, under the name “PARK gene,” have been assigned to facilitate the diagnostic approach for the clinician. The phenotype of some of the PARK genes shows a rather limited resemblance to PD. For example, the
ATP13A2 gene (PARK9) is associated with recessively-inherited early-onset atypical parkinsonism, showing pyramidal signs of dementia and supranuclear gaze palsy.
4 A clinical presentation of the
PLA2G5 gene (PARK14) mutation carriers is adult-onset parkinsonism with dystonia and complicated pyramidal involvement.
5 Although these genes are of monogenic etiology under the PARK loci, we will focus on the conditions presenting symptoms and signs similar to classical parkinsonism.
- SNCA, PARK1/4
SNCA is located in the long arm of chromosome 4, and three missense mutations have been identified in the etiology of dominantly-inherited familial parkinsonism: A53T, E46K, and A30P.
1,
6,
7 Carriers of A53T mutation experience an earlier onset of the disease, often presenting in their mid-forties, and demonstrate levodopa responsiveness, more severe and rapid progression of parkinsonian symptoms, frequent dementia, and prominent autonomic dysfunction in selected cases.
8
Families with E46K mutations experience similar clinical pictures of dementia with Lewy Body disease, but A30P mutation carriers show typical late-onset parkinsonism with relatively mild dementia.
6,
7
Multiplications of
SNCA (PARK4) have also been associated with PD.
9,
10 The clinical phenotype of triplicated cases shows an earlier onset and more rapid course of the disease than that of duplicated families, suggesting a dosage effect of
SNCA.
11 Penetrance in the families of abnormal
SNCA is age-dependent and generally complete, but it appears to be slightly lower in A30P and duplicated cases.
12
Lewy bodies and neurites are key pathological abnormalities of all cases with
SNCA mutations, and they are found not only in the substantia nigra, but also in mesocortical and neo-cortical neurons in the autopsy of A53T cases. This is compatible with a diagnosis of Dementia with Lewy bodies.
8
Alpha-synuclein is a 140-amino acid protein expressed presynaptically in neurons and is suggested to have a role in synaptic plasticity and neurotransmission.
13,
14 Genetic alterations of
SNCA lead to the formation of mutated alpha-synuclein proteins, which are more likely to oligomerise and aggregate, preventing degradation by the ubiquitine-proteasome pathway.
15
Familial cases of
SNCA mutation are extremely rare (up to 2.5% of all unrelated carriers), but findings of the alpha-synuclein protein as a principal component of key pathology and the pathogenic role of excessive wild-type alpha-synuclein have provided great insights into the pathogenesis of PD. Pathologic staging, based on the alpha-synuclein pathology, has been made, and it has proposed that the pathology spreads throughout the nervous system, possibly via the alpha-synuclein protein, as a seed molecule aggregated from mutated or excessive protein, like prion protein.
16,
17
- Parkin, PARK2
The
Parkin mutation was first described in young-onset familial parkinsonism in Japan and has been regarded as the most common cause of autosomal-recessive juvenile parkinsonism.
18,
19 The locus is mapped to the telomeric region of the long-arm of chromosome 6. More than 100 mutations are associated with
Parkin, ranging from point mutations and deletions to rearrangements and duplications.
20,
21 Clinically, affected families show parkinsonism similar to sporadic cases, except for the age of onset, which occurs from childhood to 40 years and is rarely seen in individuals over 50 years of age.
22 More symmetric motor symptoms, excellent levodopa responsiveness, frequent dystonia in the legs, hyperreflexia, diurnal fluctuation, sleep benefit, no dementia, mild autonomic symptoms, and severe treatment-related motor complications are also reported, but the general course of the disease looks more benign than typical PD.
19,
23 Initial pathologic studies reported a selective loss of dopaminergic neurons of substantia nigra and a loss of adrenergic neurons of locus ceruleus without Lewy Body, but a few cases of Lewy pathology have been reported recently.
24–
26
The
Parkin protein is a cytosolic protein and is associated with the cell membrane.
27,
28 It is known that the protein functions as an E3 ubiquitin ligase, which tags dysfunctional or excessive proteins for degradation in the ubiquitin proteasome system.
27,
29 The elimination of damaged mitochondria is one of the most important roles of
Parkin protein.
30–
32 PINK1 and
Parkin appears to act together in this pathway.
33,
34 Mutations of
Parkin induce loss or decrease in the role of protein degradation and mitochondrial maintenance, leading to the cytotoxic accumulation of abnormal proteins and mitochondrial dysfunction.
Homozygous
Parkin mutations are found in most patients, but compound heterozygotes have also been reported. Because recessively inherited genetic disorders often appear sporadically, many sporadic cases of juvenile-onset appear to be associated with
Parkin mutations.
35 A significant proportion of sporadic cases have been found to have heterozygous
Parkin mutations, suggesting these mutations may be a disease-modifying risk factor.
36 This is supported by a report of decreased dopamine activity in striatum of mutation carriers.
37 However, a subsequent large case-control study found no significant difference in the frequency of heterozygous
Parkin mutations in patients and controls.
38 Therefore, there is insufficient evidence that
Parkin mutations are a risk for sporadic PD.
- PINK1, PARK6
The
PINK1 mutation is another rare cause of autosomal-recessive early-onset parkinsonism. The
PINK1 gene is located in the short arm of chromosome 1.
39,
40 The majority of the
PINK1 mutations associated with parkinsonism are missense or nonsense mutations, but rare deletion mutations are also reported.
41 Although the age of onset is between 20 and 50 years of age, most affected cases show phenotype of late-on-set cases, such as the mild and slow progression of motor symptoms, good levodopa responsiveness, and mild dementia in some cases.
39,
42,
43 Atypical features, including dystonia and psychiatric disturbances, are also reported in a few families.
40,
41 The pathology of
PINK1-associated parkinsonism is still not known.
PINK1 protein is an active mitochondrial kinase, protecting cells against apoptotic or mitochondrial stressors and maintaining mitochondrial function.
33,
44,
45 The localization and function of
PINK1 in mitochondria is quite interesting in that PD has been assumed to be associated with mitochondrial dysfunction and oxidative stress. Phenotype due to loss of
PINK1 function is rescued by an over-expression of
Parkin, suggesting that both proteins are involved in the same genetic pathway and
PINK1 is upstream of
Parkin. The exact pathogenic mechanism is largely unknown, but recently it has been found that
PINK1 regulates mitochondrial Ca
2+ efflux, and the loss of
PINK1 is associated with mitochondrial Ca
2+ overload and dysfunction of mitochondria.
47 As with
Parkin, heterozygous
PINK1 mutations may also be a genetic risk factor, based on the increased frequency of
PINK1 mutations in sporadic cases and decreased dopaminergic activity in healthy carriers, as shown in functional-imaging study.
48–
50 Its exact role in sporadic PD also remains elusive, demanding further studies.
- DJ-1, PARK7
DJ-1-associated parkinsonism is found in only a few families and is another rare cause of recessively-inherited parkinsonism.
51 The locus is mapped to the short arm of chromosome 1, and studies have found that missense mutations and whole exonic deletion are associated with parkinsonism.
51,
52 The age of onset is from 20 to 40 years in most families, and the clinical phenotype of affected cases is similar to that of
Parkin and
PINK1 cases.
53 In addition to parkinsonism, some patients showed peculiar characteristics, such as psychiatric symptoms, short statue, and brachydactyly.
54 Currently, there is no pathologic report of
DJ-1-associated parkinsonism.
DJ-1 protein has H
2O
2 responsiveness, functioning as a sensor for oxidative stress, and is an antioxidant.
55,
56 It has been suggested that
DJ-1 could be a part of novel E3 ligase complex with
Parkin and
PINK1, but the pathogenic role of
DJ-1 mutations is still unknown.
57 Both homozygous and compound heterozygous mutations are found in affected families, and the possibility of
DJ-1 as a risk factor for sporadic disease has also been suggested like other recessive genetic etiologies.
58
- LRRK2, PARK8
LRRK2 is another genetic cause of dominantly-inherited familial parkinsonism, and it is located in the short arm of chromosome 12.
59 LRRK2 protein is a large, multi-domain protein, and over 40 pathologic mutations have been reported, although only five missense mutations have been proven to have a causative role (R1441C, R1441G, Y1699C, G2019S and I2020T).
60–
66 G2019S mutation is the most common, and the phenotype in families is similar to late onset sporadic cases. Onset occurs in patients aged in their 60s, with asymmetry of motor symptoms, and levodopa responsiveness is seen in two-thirds of cases, with slightly more frequent resting tremor (75%).
67 A more complex presentation of dystonia, amyotrophy, postural tremor, and restless legs syndrome is also described.
68 Most families with other mutations of
LRRK2 show a similar phenotype to typical sporadic cases.
69 However, the age at onset, as well as the severity of motor symptoms, may be highly variable, even within families.
The penetrance of the
LRRK2 mutation is age-dependent, but it is quite different according to mutations. The penetrance seems incomplete in the mutation of G2019S, which was reported at 28% at the age of 59 years, 51% at 69, 74% at age 79, and more than 90% at the age of 75 in R1441C.
69,
70 It also appears to be varied between the ethnicities and is higher in Arab Berber than Ashkenazi Jews.
71,
72 The penetrance reported in initial family-based studies may be overestimated, and the corrected overall penetrance is 67%.
73
Although limited pathologic studies of
LRRK2 mutation carriers reported Lewy pathology, in most cases different
LRRK2 mutations showed variable results.
74,
75 Autopsy of G2019S-mutations cases showed a relatively consistent pattern of neurodegeneration with Lewy bodies, but others (R1441C, Y1699C, and I2020T) showed pure nigral degeneration without Lewy pathology or pleomorphic findings, including tau pathology.
59,
66,
76,
77 LRRK2 is placed genetically upstream of deposited proteins, such as alpha-synuclein or tau, so mutations of
LRRK2 might induce different outcomes depending on the course the disease takes.
12,
78 Although
LRRK2 protein is reported to have kinase and GTPase features, its pathogenic role in parkinsonism is largely unknown.
79,
80 Evidences suggest that the mutant
LRRK2 impacts the morphology and the possible function of the neuritic/synaptic compartment.
81–
83