**Abstract**

Parkinson disease is a complex disease that has multiple genetic and environmental factors. To achieve the early diagnosis and to be able to modify the disease progression, efforts are being made to identify individuals at risk. About 20 year ago, an evidence of major prevalence of Parkinsonism in patients with Gaucher Disease reported by studies worldwide led to the putative involvement of the *GBA* gene. Nowadays, the link from a rare disease with a common disease is well known and it is confirmed that mutations in the *GBA* gene are the most important genetic risk factor. Apart from rare mutations, genetic association studied appointed common variants in genes well associated with familial cases as *LRRK2* and *SNCA* may also contribute to the increased risk for sporadic cases. Other common variants in the *MAPT* gene were also reported. At least, genetic studies have been observed an excessive burden of relevant variants in genes with lysosomal function. Thus, a synergistic action of variants in genes that codifies proteins involved with the lysosome may be a mean of modulating the risk. In this chapter, we review the most robust genetic risk factor and the relevance of lysosomal function for Parkinson disease.

**Keywords:** Parkinson, GBA, risk factor, lysosome, GWAS

#### **1. Introduction**

Parkinson's disease (PD) is the second most common progressive neurodegenerative disease in humans and it is characterized by motor symptoms as muscular rigidity, resting tremor, bradykinesia, and postural instability and also by nonmotor symptoms (hyposmia, constipation, depression, dementia, and postural hypotension, among others). These symptoms result primarily from the progressive loss of the dopaminergic neurons from the pars compacta of the mesencephalic substantia nigra and subsequent depletion of the dopamine neurotransmitter in the striatum, a central component of the basal ganglia that is responsible for the instigation and coordination of movements (**Figure 1**). The definitive diagnosis of PD is difficult being only confirmed with the presence of Lewy bodies, proteinaceous intracytoplasmic inclusion, in the reminiscent neurons of substantia nigra pars compacta and other regions in the brain postmortem analysis [1].

**Figure 1.**

*Depigmentation of the substantia nigra (SN) (right panel) compared with control (left panel). Adapted from Ref. [1].*

The etiology is not well understand, but PD is considered a complex disease, which counts with multiple genetic and environment factors. The most common is the sporadic PD for which the onset generally is late, after 60 years old. There is a rare form, the familial PD or monogenic PD (~10% of the cases), for which the disease is caused by mutations in a single gene and may present not only a late onset but also an earlier onset (below 45 years old) in some cases. Although less frequent, the study of monogenic forms of PD and their associated genes helps to understand the molecular basis of disease pathogenesis [1, 2].

Segregation studies of mutations in *SNCA* gene in large families with PD cases led to the discovery of the main protein involved in the disease pathogenesis, the α-synuclein. Shortly afterwards, postmortem studies in patients' brains revealed that this protein is the major component of Lewy bodies, in both sporadic and familial PD patients, reinforcing its important role in the development of PD [2–4]. Be it for genetic, environmental or both factors, the fact is that patients' brains do not have the soluble monomeric form of α-synuclein, which is easily degraded by lysosomal function, but the insoluble oligomeric forms [5, 6].

Nowadays, efforts have been directed to identify the individuals at risk of manifesting PD through clinical, genetic and biochemical markers in order to diagnose early and perhaps be possible to modify the progression of disease. For this purpose, genetic variations with the potential to alter the risk for PD have been widely researched. Both disease-causing variants and risk variants in genes associated with PD vary in frequency depending on ethnic background. Certain genetic variants may be a risk factor in an Asian population, but may not be statically significant in a European population, for example. The most robust and consistently replicated results are appointed to variants in the genes *LRRK2, MAPT, SNCA* and *GBA,* the last being the major genetic risk factor highlighting the importance of lysosomal pathway in the pathogenesis of PD [1–8].

#### **2. Genetic risk factors**

More than 20 years ago, Parkinson disease was understood as a disease caused by environmental factors only. It was from genetic analyzes in cases of familial PD that it was discovered that the genetic factor is also important and may even cause certain disease forms. Thus, we have monogenic PD, which can present a pattern

**65**

diverse samples.

*Genetic Risk Factors and Lysosomal Function in Parkinson Disease*

of inheritance defied as autosomal dominant or autosomal recessive. In 1997, the discovery of mutations in the *SNCA* gene as the cause of PD in certain families also helped scientists to better understand the etiopathology of this complex disease through the association between the duplication and triplication of the *SNCA* gene with altered α-synuclein protein expression and the disease progression. Since then, diverse other genes were associated with mutations that cause PD, among them the genes *LRRK2* and *VPS35,* that along with *SNCA* are altered in autosomal dominant cases, and *PINK1*, *PARKIN*, *DJ-1* that are altered in autosomal recessive

However, the genetics of PD are not simply composed of variants that cause the disease. More recently, the focus has been on genetic variants that do not lead to PD alone but increase the risk of developing the disease. Among the genetic risk factors associated with sporadic PD, rare high-impact variants and common low-impact variants have been identified by candidate gene studies and genome-wide association studies (GWAS). The complexity of the PD genetic increases even more due to several risk variants for PD that are heterogeneous and dependent on the genetic background of each population. Genetic variants can be associated with PD in some

Two largest GWAS studies in 2014 and 2017 identified in total 28 independent PD-associated risk loci, mainly in *SNCA*, *LRRK2*, *MAPT* and *GBA*. Genetic risk factors are present in several genes involved in metabolic pathways that may be directly related to α-synuclein metabolism or involved in processes that affect cel-

It is worthy to emphasize the fact that genes that carry rare casual mutations of monogenic PD observed in previous family studies are not excluded of the possibility to also carry common variants that confer risk for developing PD. This is the case

So far, five point mutations (A53T, A30P and E46K) and two copy number variation (duplication and triplication) in the gene *SNCA* are well known to cause autosomal dominant PD indistinguishable of sporadic PD, or an early-onset PD if the triplication of the gene is present [12]. Recently, single nucleotide polymorphisms (SNP) in *SCNA* were reported in non-coding regions, suggesting that those variants play a role in the regulation of the genetic expression through modifications post-transcriptional as interacting with microRNA or altering alternative

In 1999, the association between REP1, a complex polymorphic microsatellite repeat in the promoter region, and PD was pointed out by [13]. Seven years later, Maraganore [14] confirmed this association with a larger meta-analysis study using more than 5000 samples from 11 sites. Further, functional analysis studies provided evidence that the length of alleles affects the protein expression: the 261 bp-long risk allele is associated with an upregulation of α-synuclein expression mimicking *SNCA* locus multiplication, whereas the 259 bp-long protective variant shows

In 2009, Simón-Sánchez et al. [16] used GWAS in a great sample and identified additional signals of association with PD from intro 4 to after the 3′ UTR. One year later, Mata et al. [17] showed possible association between rs356219 in the 3′ UTR region and α-synuclein plasma levels. To definitely ascertain which variants in this region alter the risk for PD, more studies are necessary in large and genetically

*DOI: http://dx.doi.org/10.5772/intechopen.91850*

populations, but not in others [8, 9].

lular homeostasis [10, 11].

of *LRRK2* and *SNCA.*

splicing mechanism [7].

reduced gene expression [15].

**2.1 SNCA**

forms [3, 8].

*Genetic Risk Factors and Lysosomal Function in Parkinson Disease DOI: http://dx.doi.org/10.5772/intechopen.91850*

of inheritance defied as autosomal dominant or autosomal recessive. In 1997, the discovery of mutations in the *SNCA* gene as the cause of PD in certain families also helped scientists to better understand the etiopathology of this complex disease through the association between the duplication and triplication of the *SNCA* gene with altered α-synuclein protein expression and the disease progression. Since then, diverse other genes were associated with mutations that cause PD, among them the genes *LRRK2* and *VPS35,* that along with *SNCA* are altered in autosomal dominant cases, and *PINK1*, *PARKIN*, *DJ-1* that are altered in autosomal recessive forms [3, 8].

However, the genetics of PD are not simply composed of variants that cause the disease. More recently, the focus has been on genetic variants that do not lead to PD alone but increase the risk of developing the disease. Among the genetic risk factors associated with sporadic PD, rare high-impact variants and common low-impact variants have been identified by candidate gene studies and genome-wide association studies (GWAS). The complexity of the PD genetic increases even more due to several risk variants for PD that are heterogeneous and dependent on the genetic background of each population. Genetic variants can be associated with PD in some populations, but not in others [8, 9].

Two largest GWAS studies in 2014 and 2017 identified in total 28 independent PD-associated risk loci, mainly in *SNCA*, *LRRK2*, *MAPT* and *GBA*. Genetic risk factors are present in several genes involved in metabolic pathways that may be directly related to α-synuclein metabolism or involved in processes that affect cellular homeostasis [10, 11].

#### **2.1 SNCA**

*Methods in Molecular Medicine*

**Figure 1.**

*Ref. [1].*

The etiology is not well understand, but PD is considered a complex disease, which counts with multiple genetic and environment factors. The most common is the sporadic PD for which the onset generally is late, after 60 years old. There is a rare form, the familial PD or monogenic PD (~10% of the cases), for which the disease is caused by mutations in a single gene and may present not only a late onset but also an earlier onset (below 45 years old) in some cases. Although less frequent, the study of monogenic forms of PD and their associated genes helps to understand

*Depigmentation of the substantia nigra (SN) (right panel) compared with control (left panel). Adapted from* 

Segregation studies of mutations in *SNCA* gene in large families with PD cases led to the discovery of the main protein involved in the disease pathogenesis, the α-synuclein. Shortly afterwards, postmortem studies in patients' brains revealed that this protein is the major component of Lewy bodies, in both sporadic and familial PD patients, reinforcing its important role in the development of PD [2–4]. Be it for genetic, environmental or both factors, the fact is that patients' brains do not have the soluble monomeric form of α-synuclein, which is easily degraded by

Nowadays, efforts have been directed to identify the individuals at risk of manifesting PD through clinical, genetic and biochemical markers in order to diagnose early and perhaps be possible to modify the progression of disease. For this purpose, genetic variations with the potential to alter the risk for PD have been widely researched. Both disease-causing variants and risk variants in genes associated with PD vary in frequency depending on ethnic background. Certain genetic variants may be a risk factor in an Asian population, but may not be statically significant in a European population, for example. The most robust and consistently replicated results are appointed to variants in the genes *LRRK2, MAPT, SNCA* and *GBA,* the last being the major genetic risk factor highlighting the importance of lysosomal

More than 20 years ago, Parkinson disease was understood as a disease caused by environmental factors only. It was from genetic analyzes in cases of familial PD that it was discovered that the genetic factor is also important and may even cause certain disease forms. Thus, we have monogenic PD, which can present a pattern

the molecular basis of disease pathogenesis [1, 2].

pathway in the pathogenesis of PD [1–8].

**2. Genetic risk factors**

lysosomal function, but the insoluble oligomeric forms [5, 6].

**64**

It is worthy to emphasize the fact that genes that carry rare casual mutations of monogenic PD observed in previous family studies are not excluded of the possibility to also carry common variants that confer risk for developing PD. This is the case of *LRRK2* and *SNCA.*

So far, five point mutations (A53T, A30P and E46K) and two copy number variation (duplication and triplication) in the gene *SNCA* are well known to cause autosomal dominant PD indistinguishable of sporadic PD, or an early-onset PD if the triplication of the gene is present [12]. Recently, single nucleotide polymorphisms (SNP) in *SCNA* were reported in non-coding regions, suggesting that those variants play a role in the regulation of the genetic expression through modifications post-transcriptional as interacting with microRNA or altering alternative splicing mechanism [7].

In 1999, the association between REP1, a complex polymorphic microsatellite repeat in the promoter region, and PD was pointed out by [13]. Seven years later, Maraganore [14] confirmed this association with a larger meta-analysis study using more than 5000 samples from 11 sites. Further, functional analysis studies provided evidence that the length of alleles affects the protein expression: the 261 bp-long risk allele is associated with an upregulation of α-synuclein expression mimicking *SNCA* locus multiplication, whereas the 259 bp-long protective variant shows reduced gene expression [15].

In 2009, Simón-Sánchez et al. [16] used GWAS in a great sample and identified additional signals of association with PD from intro 4 to after the 3′ UTR. One year later, Mata et al. [17] showed possible association between rs356219 in the 3′ UTR region and α-synuclein plasma levels. To definitely ascertain which variants in this region alter the risk for PD, more studies are necessary in large and genetically diverse samples.

#### **2.2 LRRK2**

In the region of chromosome 12 is localized the *LRRK2* gene where several genetic variants have been found; however, segregation in families with monogenic PD and case-control studies demonstrated that only seven point variants (R1441G, R1441C, R1441H, Y1699C, G2019S, I2020T and N1437H) have enough evidence to be defined as cause of PD. This gene encodes a protein of the same name composed of domains with kinase activity, GTPase and several domains of interaction with other proteins, suggesting that its function changes depending on which proteins form complexes, the type of cells and the stage of development. Mutations in *LRRK2* gene are the most common cause of PD familial cases. The mutation G2019S is the most prevalent worldwide, and it is present in 4% of familial PD and is associated with an indistinguishable phenotype from the clinical manifestations of sporadic PD [12–18].

Besides the prevalence in rare monogenic PD, this mutation can also confer risk in the sporadic PD, being found in 1% of the cases. G2019S has a penetrance variable, and its carrier's risk to develop PD depends on age and ethnic background. The age-related risk has been estimated to be 28% at age 59, 51% at 69, and 74% at 79 years. The frequency is higher in North African, Middle Eastern and Ashkenazi Jewish PD patients [18, 19].

G2019S is frequent in most populations worldwide, but it is very rare in the Asian population where it accounts for less than 1% of *LRRK2* mutations. In contrast, most common genetic variants SNPs G2385R and R1628P are more frequent in Asian populations than in Caucasian populations. Those SNPs are associated with an increased risk of 2.2 fold and 1.84 fold to develop PD, respectively [20, 21]. Lately, regions close to *LRRK2* have been appointed by GWAS as increasing by 1.2 fold the risk for PD. This fact alerts that the regulation of the gene expression is important to develop this disease [8].

Those works reinforce the idea that genes can carry both rare disease-causing variants and common variants that increase the risk for PD, as seen in *SNCA* and *LRRK2*. Additionally, common variants present in those genes enhance the importance of their proteins' role in the disease and implicate that there is a common neurodegenerative process between sporadic and familial PD.

#### **2.3 MAPT**

The gene *MAPT* is frequently associated with other neurodegenerative diseases as Alzheimer disease and frontotemporal dementia (FDT). *MAPT* encodes for the microtubule-associated protein tau, whose role is to regulate microtubule dynamics and assemble microtubules into parallel arrays within axons, essential for normal axonal transport in neurons. Polymorphisms in this gene have been found to be an indisputable risk factor of the synucleinopathy. The H1 and H2 haplotypes represent two distinct clades of subhaplotypes ensued from an inversion of ∼900 kb on chromosome 17q21, spanning the entire *MAPT* coding region, and are tagged, among others, by genotypes at two SNPs: rs9468 and rs1800547 [4–8].

The H1 haplotype and its subhaplotype H1c have been significantly associated with an increased risk for a number of neurodegenerative diseases. Several studies proposed the most common H1 haplotype as susceptibility factor for PD with an odds ratio of 1.5. Recent studies that investigated the association between H1 and specific PD clinical manifestation also observed the higher prevalence of H1 in patients with cognitive defects, as dementia and H1 homozygous PD patients showed an increased risk to manifest non-tremor dominant subtype, which is a worse clinical prognosis [22, 23].

**67**

*SNCA* [30].

changed [25, 26].

*Genetic Risk Factors and Lysosomal Function in Parkinson Disease*

The underlying biological mechanisms that link the *MAPT* locus (and tau protein) to neurodegeneration are not yet adequately characterized. Through the functional characterization of variants in the *MAPT* gene, some theories of the tau protein effect in PD involve increased tau expression; altered gene splicing promoting aggregation; and altered 4/3 repeated transcript ratio. The emerging concept of H1 pathogenicity points to the role of each tau isoforms expressed rather than the

According to this model, the H1 haplotype is associated with an underexpression

Diverse studies identified *MAPT* locus variants as a risk factor for PD, but it may not be true to any population. In the Caucasian population, there was this association, while it was absent in the Japanese population. This observation has potential implications for the analysis of complex traits across populations such as genetic heterogeneity, particularly at minor risk loci, highlighting the power of comparing

Despite having a modest effect (less than 30% of the change in risk), these common variants can have a considerable impact when combined. Results from a 2015 study revealed that patients at an early age of onset of symptoms had a higher polygenic combination of risk variants than patients with a late onset. This demonstrates the possible effect of the synergistic value of the changes caused by these variants to modulate the PD clinic, such as the age of onset of symptoms [12].

The *GBA* gene was initially described in association with a rare lysosomal storage disease (LSD) called Gaucher disease (GD). When mutated in homozygosis, depending on the mutation present, the resulting enzyme is malformed or even no enzyme is synthesized leading to enzyme glucocerebrosidase (GCase) partial or total deficiency and glucosylceramide (GlcCer) accumulation. The symptoms are multisystemic, with the brain, spleen, liver and bone marrow being the main organs affected. The presence and intensity of those symptoms differ between the three types of GD (GD1, GD2 and GD3). The heterozygous individuals do not present any clinical manifestation; however, in the last years, this perspective has

Further a number of studies have recorded the occurrence of parkinsonian manifestations in patients with GD and their relatives [27, 28]. In ref. [29] was showed that *GBA* mutations homozygous individuals have 21.4 fold increased risk to develop PD with probability of 9–12% to manifest motor symptoms before 80 years old. Despite being low, this risk is considerably higher than in the same age group in general population, 3%. The *GBA* and PD association was confirmed in the Jewish Ashkenazi, which showed a prevalence of *GBA* mutations in heterozygosis and homozygosis individuals in the PD population that by far outweighs the reported prevalence of mutations in other susceptibility genes for PD, as *Parkin* and

Researchers worldwide have attempted to validate the same association in populations from many different genetic backgrounds [31–41]. In 2009, an international

**3. GBA: the principal genetic risk factor for Parkinson disease**

of a protective isoform and an overexpression of the detrimental variant, which lead to a subtle neuronal dysfunction that accumulates over the years and induces or accelerates cellular degeneration [22–24]. However this locus harbors many genes and the extended linkage disequilibrium means that the tau protein may be not the cause of neurodegeneration and its DNA sequence is just close to the casual locus. Thus, while *MAPT* is a candidate, we cannot be certain that this is the true biologi-

*DOI: http://dx.doi.org/10.5772/intechopen.91850*

overall number of transcripts.

cal mediator of risk [7–16].

GWAS across different populations [9–16].

*Methods in Molecular Medicine*

sporadic PD [12–18].

Jewish PD patients [18, 19].

important to develop this disease [8].

worse clinical prognosis [22, 23].

In the region of chromosome 12 is localized the *LRRK2* gene where several genetic variants have been found; however, segregation in families with monogenic PD and case-control studies demonstrated that only seven point variants (R1441G, R1441C, R1441H, Y1699C, G2019S, I2020T and N1437H) have enough evidence to be defined as cause of PD. This gene encodes a protein of the same name composed of domains with kinase activity, GTPase and several domains of interaction with other proteins, suggesting that its function changes depending on which proteins form complexes, the type of cells and the stage of development. Mutations in *LRRK2* gene are the most common cause of PD familial cases. The mutation G2019S is the most prevalent worldwide, and it is present in 4% of familial PD and is associated with an indistinguishable phenotype from the clinical manifestations of

Besides the prevalence in rare monogenic PD, this mutation can also confer risk in the sporadic PD, being found in 1% of the cases. G2019S has a penetrance variable, and its carrier's risk to develop PD depends on age and ethnic background. The age-related risk has been estimated to be 28% at age 59, 51% at 69, and 74% at 79 years. The frequency is higher in North African, Middle Eastern and Ashkenazi

G2019S is frequent in most populations worldwide, but it is very rare in the Asian population where it accounts for less than 1% of *LRRK2* mutations. In contrast, most common genetic variants SNPs G2385R and R1628P are more frequent in Asian populations than in Caucasian populations. Those SNPs are associated with an increased risk of 2.2 fold and 1.84 fold to develop PD, respectively [20, 21]. Lately, regions close to *LRRK2* have been appointed by GWAS as increasing by 1.2 fold the risk for PD. This fact alerts that the regulation of the gene expression is

Those works reinforce the idea that genes can carry both rare disease-causing variants and common variants that increase the risk for PD, as seen in *SNCA* and *LRRK2*. Additionally, common variants present in those genes enhance the importance of their proteins' role in the disease and implicate that there is a common

The gene *MAPT* is frequently associated with other neurodegenerative diseases as Alzheimer disease and frontotemporal dementia (FDT). *MAPT* encodes for the microtubule-associated protein tau, whose role is to regulate microtubule dynamics and assemble microtubules into parallel arrays within axons, essential for normal axonal transport in neurons. Polymorphisms in this gene have been found to be an indisputable risk factor of the synucleinopathy. The H1 and H2 haplotypes represent two distinct clades of subhaplotypes ensued from an inversion of ∼900 kb on chromosome 17q21, spanning the entire *MAPT* coding region, and are tagged,

neurodegenerative process between sporadic and familial PD.

among others, by genotypes at two SNPs: rs9468 and rs1800547 [4–8].

The H1 haplotype and its subhaplotype H1c have been significantly associated with an increased risk for a number of neurodegenerative diseases. Several studies proposed the most common H1 haplotype as susceptibility factor for PD with an odds ratio of 1.5. Recent studies that investigated the association between H1 and specific PD clinical manifestation also observed the higher prevalence of H1 in patients with cognitive defects, as dementia and H1 homozygous PD patients showed an increased risk to manifest non-tremor dominant subtype, which is a

**2.2 LRRK2**

**66**

**2.3 MAPT**

The underlying biological mechanisms that link the *MAPT* locus (and tau protein) to neurodegeneration are not yet adequately characterized. Through the functional characterization of variants in the *MAPT* gene, some theories of the tau protein effect in PD involve increased tau expression; altered gene splicing promoting aggregation; and altered 4/3 repeated transcript ratio. The emerging concept of H1 pathogenicity points to the role of each tau isoforms expressed rather than the overall number of transcripts.

According to this model, the H1 haplotype is associated with an underexpression of a protective isoform and an overexpression of the detrimental variant, which lead to a subtle neuronal dysfunction that accumulates over the years and induces or accelerates cellular degeneration [22–24]. However this locus harbors many genes and the extended linkage disequilibrium means that the tau protein may be not the cause of neurodegeneration and its DNA sequence is just close to the casual locus. Thus, while *MAPT* is a candidate, we cannot be certain that this is the true biological mediator of risk [7–16].

Diverse studies identified *MAPT* locus variants as a risk factor for PD, but it may not be true to any population. In the Caucasian population, there was this association, while it was absent in the Japanese population. This observation has potential implications for the analysis of complex traits across populations such as genetic heterogeneity, particularly at minor risk loci, highlighting the power of comparing GWAS across different populations [9–16].

Despite having a modest effect (less than 30% of the change in risk), these common variants can have a considerable impact when combined. Results from a 2015 study revealed that patients at an early age of onset of symptoms had a higher polygenic combination of risk variants than patients with a late onset. This demonstrates the possible effect of the synergistic value of the changes caused by these variants to modulate the PD clinic, such as the age of onset of symptoms [12].
