Parkinson’s disease (PD) is the second most common neurodegenerative disorder and affects 1% of the population aged 65-84 years and 4% above the age of 85.¹ The main symptoms associated with PD are resting tremor, rigidity, bradykinesia and postural instability. Non-motor symptoms are also very common during the course of PD, including autonomic insufficiency, cognitive impairment, depression and sleep disturbances. 

PD results from the progressive degeneration of dopaminergic neurons in the substantia nigra along with the presence of Lewy bodies containing alpha-synuclein in damaged neurons. To date treatment offers only symptomatic relief for patients and there is no existing cure.

Interactive genetic factors

PD is possibly caused by the interaction of genetic factors, environmental exposures and gene-environment interactions. Most people suffering from PD have idiopathic PD, in which the cause of their disease is largely unknown. However, approximately 15% of patients have a form caused by inherited genetic mutations, and suffer from familial PD. This is most common in patients where the age of onset is younger than 50 years of age.²

After the age of 50 the genetic factors were shown to be less important by a study using the WWII Veteran Twin Registrar,³ however, the fact that 20-25% of people with sporadic PD have a first degree relative affected also further supports the genetic contribution to its development.⁴

Synuclein alpha (SNCA), the first gene found to be involved in PD, was discovered through the analysis of a large multigenerational Italian family (the Contursi Kindred), in which PD was inherited in an autosomal dominant pattern.⁵ Since then, 18 more chromosomal loci associated with PD have been discovered. These chromosomal loci have been termed PARK and numbered in chronological order of their identification. 

These genetic mutations were discovered using linkage analysis (PARK 1-15) and genome-wide association studies (PARK 16-18).⁶ PARK 4 was discovered and thought to be a novel chromosomal locus associated with PD, however, it was later found to be identical with PARK 1.⁷

Monogenic forms of the disease

PD genetic research has resulted in the identification of a number of monogenic forms of the disease. These are also the most cogent form of evidence for genetic involvement in PD. Monogenic forms of PD are those forms that are caused by a single mutation in a gene, which can be either dominantly or recessively inherited. Monogenic forms of PD make up 30% of familial disorders and up to 5% of sporadic disorders.⁸ There are six main genes involved in causing monogenic PD, as shown in Table 1.⁸

SNCA is responsible for an early-onset form of the disease, characterised by a rapid progression. This is commonly accompanied by dementia, hallucinations, central hypoventilation, urinary incontinence and cognitive decline. The substantia nigra, hypothalamus and cerebral cortex also become populated with Lewy bodies.⁵ Response to treatment with levodopa is good initially, however, this declines as the pace of disease progression increases. 

Understanding of the mechanism through which SNCA mutations cause neurodegeneration is incomplete. It is thought to be through a gain of function mutation and impaired alpha-synuclein processing.⁹

This causes impaired protein folding and protein aggregation, which in turn leads to Lewy body formation, cellular oxidative stress and depletion of energy.⁹ Over expression of SNCA also affects PD with duplication being associated with late onset, while triplication is associated with early onset.¹⁰

LRRK2 gene

The most common gene mutations that cause monogenic PD are those in the leucine-rich repeat kinase 2 (LRRK2). The LRRK2 gene is responsible for encoding the protein dardarin, a cytoplasmic kinase involved in phosphorylation of alpha synuclein and tau protein.¹¹

Genetic screening studies have found LRRK2 mutations to be responsible for 13% of autosomal-dominant PD in Europe, as well as 1% of sporadic and 6% of familial PD in the US.¹² The form of PD that results from LRRK2 mutations is typically late in onset with a slow progression. Patients usually present with unilateral hand or leg tremor. Dementia and cognitive deficit are uncommon in these patients with the disease being difficult to distinguish from idiopathic PD. 

Levodopa is the preferred treatment in this subgroup and response is good. The mechanisms by which the mutations in this gene lead to its pathogenesis are unclear. It is thought that since it is a large protein with multiple domains, the mutations may affect the domains and thus its protein-protein interactions.¹³

Parkin (PRKN) was the first of the autosomal recessive mutations to be discovered and since then over 100 mutations in the Parkin gene have been identified. PRKN is a protein product expressed in the substania nigra, Lewy bodies and other brain regions. It is responsible for ubiquitination and degradation of proteins by proteasomes.¹⁴

Early in life

When this gene is mutated it loses its proteasome activity, resulting in quickened cell death due to its inability to clear accumulating protein. This form of PD tends to begin affecting patients early in their lives, with it being the leading cause of juvenile onset PD. It is slow in progression and has an excellent response record to levodopa. Like iPD, it may present with tremor-predominant and akinetic rigid forms. However, distinguishing features include young age of onset, dystonia, hyperreflexia, and symmetrical signs and symptoms.⁸ The diagnosis of PRKN does not appear to infer an increased risk of dementia.⁹

PTEN-induced putative kinase 1 (PINK1) mutations are the second most common cause of autosomal recessive PD of early onset, with frequencies varying greatly in different ethnic groups.¹⁵ Slow progression and excellent responsiveness to levodopa are associated with this form of PD.⁸ DJ-1 mutations are autosomal recessive in inheritance and are extremely rare, accounting for approximately 2% of early onset cases of PD.¹⁵ This form of PD is slow in progression and responds well to levodopa treatment.¹⁷

While PRKN, DJ-1 and PINK1 mutations can produce a clinical phenotype closely resembling idiopathic PD, there are also a number of other recessively-inherited mutations that present as an atypical parkinsonism. 

ATP13A2 mutations are autosomal recessive in inheritance and are responsible for the PD condition called Kufor-Rakeb syndrome. This is a quite severe form of PD with symptoms including early onset, fast progression, dementia, supranuclear gaze palsy and pyramidal signs.¹⁸

Risk factors

Genetic research into PD has led to the identification of a number of risk factors for the development of the disease. These include variations in the three genes LRRK2, SNCA and microtubule-associated protein tau (MAPT) and loss of function mutations in glucocerebrosidase (GBA) gene. MAPT and GBA gene mutations have not been denoted PARK. These risk factors have been identified through gene mapping techniques such as linkage analysis and genome-wide association studies.¹⁹ In SNCA polymorphic length and single nucleotide variations have been shown to be the leading PD risk factors. These are followed closely by missense single nucleotide polymorphisms in the LRRK2 gene. 

GBA gene encodes the enzyme GBA. Loss of function mutations in this gene results in the accumulation of glucocerebroside. This is known as Gaucher disease, an autosomal recessive glycoprotein storage disorder that involves the liver, spleen, bone marrow, lungs and nervous system.²⁰ Small subsets of these patients develop PD with brain stem or Lewy body pathology.²¹ There has also been an increased incidence of PD in relatives of patients with Gaucher disease.²²

These observations led to the development of a number of case-control analysis studies. In 2009, a definitive study on this topic was published by Sidransky et al, in which they analysed 5,691 patients with PD and 4,898 controls from 16 centres in 12 countries. 

In the subset of participants in which the entire GBA coding region was screened, loss-of-function mutations were found in 6.9% of patients and 1.3% of controls.²³ GBA mutations have been found in 8-14% of autopsy proven diagnosis of PD.^24,25

It was also observed that the clinical characteristics of the patients with and without the mutations were very similar. It is suggested that these mutations produce a misfolded GBA which will contribute to neurodegeneration by inducing lysosomal insufficiency, by impairing autophagic pathways necessary for degrading alpha-synuclein, or by overwhelming the ubiquitin-proteasome pathway.²⁶

MAPT mutations

MAPT gene encodes the microtubule-associated protein tau, which is expressed in neurons and plays a key role in the organisation and integrity of the cytoskeleton.²⁷ Filamentous neuronal tau inclusions define a set of neurodegenerative diseases referred to as the ‘tauopathies’ which include Alzheimer disease, corticobasal degeneration and frontotemporal dementia with Parkinsonism. 

Due to the shared clinical features of the tauopathies and PD, a number of studies have been carried out on the relationship between MAPT mutations and the risk of developing PD. Two large case-control studies published in 2007 provided strong evidence that the MAPT H1 haplotype is associated with PD risk. In addition, a meta-analysis of 18 studies yielded an odds ratio for PD of 1.4.²⁸ The MAPT H1 haplotype has been confirmed as a PD risk factor in all four of the genome-wide association studies conducted in populations of European origin in the past five years.^29,30,31,32.

Genetic screening

Genetic screening for PD would be beneficial in the following cases:

• Juvenile onset PD, irrespective of family history
• Early onset PD with atypical features, with or without a family history of the disease
• Late onset PD with a strong family history of the disease. 
• In these situations genetic screening would give the patients a greater understanding of their disease and what to expect as it progresses. It would also enable the clarification of treatment regimes and help with family planning in the future.⁹ The European Federation of the Neurological Sciences published guidelines that recommend:
• LRRK2 screening for mutations in Europeans showing dominant inheritance of PD
• Screening for the LRRK2 p.G2019S mutation in familial and sporadic cases of PD in specific populations
• Analysis of PRKN, PINK1, and DJ-1 in patients over the age of 35 with recessively-inherited PD.³³

The majority of patients with early onset PD are strongly in favour of genetic screening as it will enable them to make more informed life decisions and increase their understanding of the disease.³⁴ However, genetic screening is not something that should be recommended lightly to patients. The decision to carry out genetic testing should be a well-informed one and must be offered in the framework of genetic counselling.⁸

New treatments for PD are currently in clinical trials and these involve the use of gene-therapeutic approaches to either compensate for the loss of dopamine or to protect dopaminergic neurons from further degeneration. The overall goal is to restore function.³⁵ This form of treatment is appealing as only a limited area will require treatment in the brain and long-term gene expression will follow a single treatment. 

Stem cells

The earliest attempts at gene therapy in PD involved the use of embryonic stem cells. Foetal nerve cells were transplanted into PD patients and a subset of these patients experienced palliative relief for many years.^36,37,38 Some of these patients upon autopsy were found to display PD pathology in the grafted tissue suggesting that the local area in the brain brings about de novo PD.^39,40

One of the biggest obstacles for this form of therapy is the acquisition of foetal tissue. The development of viral vector technology has provided an efficient method for the delivery of genetic material without the need for human transplanted cells. Adeno-associated virus (AAV) and lentivirus are currently in clinical trials.⁴¹

AAV a member of the parovirus family is non-pathogenic and the majority of patients that have been exposed to it have shown low immunity.⁴² Over 10 recombinant AAV serotypes have been engineered into vectors and of these rAAV2 is the most frequently used in gene therapy trials. 

This can be attributed to its long-term expression, efficient transduction in neurons and diminished pro-inflammatory and immune responses. In comparison, lentivirus vectors are capable of accommodating a larger transgene load, and so are used for multigene PD treatments. Lentivirus vectors are RNA retroviruses capable of chromosomal integration and stable long-term expression.⁴³

One approach of PD gene therapy is to increase dopamine production through the direct delivery of genes involved in neurotransmitter synthesis to the pathological areas in the brain. Both Human L-Amino Acid Decarboxylase 2 and ProSavin are currently in clinical trials for this approach. 

A second approach is designed to change the neuronal phenotype, as a result bypassing the need for dopamine. The loss of dopaminergic neurons in the subatantia nigra pars compacta results in a change in input to the internal globus pallidus and the substantia nigra pars reticulate through disinhibition of the subthalamic nucleus. Conversion of the excitatory neurons to an inhibitory phenotype is an alternative approach to silence the overactive subthalamic nucleus.⁴⁴

The motor symptoms of PD appear after substantial loss of dopaminergic neurons and so protection of these neurons is a therapeutic goal. This has been achieved in animal models of dopaminergic neuron death following treatment with proto typical neurotrophic factor GDNF.⁴⁵

Although genetic forms of PD are rare, they are important in leading to enhanced understanding of idiopathic forms of the disease. Genetic research into PD has enabled the identification of six genes:

• SNCA
• LRRK2
• Parkin
• DJ1
• PINK1
• ATP13A2.

These six genes are linked to monogenic forms of the disease, most of which result in a form that closely resembles idiopathic PD. Variations in the genes LRRK2, SNCA, and MAPT along with loss of function mutations in the GBA gene are now considered well-established risk factors for the disease. 

Genetic screening will allow the identification of individuals that are at risk of developing the disease at the earliest possible stage. Genetic research into PD has also led to the development of gene-cell therapy using viral vectors, which are currently in clinical trials. Although important progress has been made, the mechanism through which these genetic mutations lead to the development of PD is still poorly understood.

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