Autosomal dominant PD: Heterozygous variants in the SNCA, LRRK2, VPS35, CHCHD2 and EIF4G1 genes cause autosomal dominant forms of Parkinson disease. Generally autosomal dominant forms tend to manifest at a later age compared to autosomal recessive forms. SNCA is the first gene in which a mutation was identified in Parkinson disease and this gene codes for alpha- synuclein, which is the primary protein found in Lewy bodies. Disease causing variants in SNCA could be single nucleotide variants or gene duplications and triplications. The p.Gly2019Ser variant in LRRK2 accounts for 5-7% of autosomal dominant forms (Nichols et al.,2005).

Autosomal recessive PD: Autosomal recessive forms have an earlier onset of disease, mild non-motor symptoms and a slow progression. They are caused due to biallelic variations in PRKN which codes for Parkin, PINK1, ATP13A2, DNAJC6, SYNJ1 etc.

X-linked PD: PARK 12 is the only locus that has been shown to demonstrate X-linked transmission. ATP6AP2 is the gene that has been implicated in X-linked Parkinsonism (Korvatska et al., 2013).

5 Pathogenesis of Parkinson disease

The genes identified as causing idiopathic Parkinsonism are shown to affect four different processes: synaptic transmission, endosomal trafficking, lysosomal autophagy and energy metabolism.

5.1 Synaptic transmission

Alpha- synuclein, which is found in presynaptic terminals in the central and autonomous nervous system, is involved in exocytosis and synaptic release of neurotransmitters and it is the main component of Lewy bodies. Triplications of the SCNA gene which codes for alpha synuclein lead to earlier onset of symptoms compared to duplications, implicating a dosage effect in the pathogenesis. The exact mechanism by which alpha synuclein leads to neuronal death and spreads throughout the CNS still remains unexplained. There are various theories regarding the spread of alpha synuclein pathology in the brain, like ‘selective vulnerability hypothesis’ and ‘pathogenic spread hypothesis’ (Lill, 2016).

LRRK2 codes for a protein kinase, which regulates glutamate transmission and striatal signal transduction (Lin et al., 2014). DNAJ6, a biallelic mutation of which causes autosomal recessive forms of Parkinsonism, encodes a protein auxilin, which aids in clathrin mediated synaptic vesicle recycling. Synaptojamin, a protein coded by SYNJ1, forms complexes with auxilin and has been implicated in autosomal recessive Parkinsonism.

5.2 Endosomal trafficking

This is a complex process by which the receptors or vesicles are internalized and then recycled in the Golgi complex or degraded in the lysosomes. VPS35 and DNAJC13 are implicated in endosomal trafficking causing Parkinsonism.

5.3 Lysosomal autophagy

Alpha synuclein, which gets accumulated in cells in Parkinsonism, is not degraded by lysosomes and it is not clear whether this is the cause or effect of dysfunction in that pathway. Accumulation of intracellular alpha synuclein is found in many disorders like neuronal ceroid lipofuscinosis, Gaucher disease and Neimann-Pick type C. Heterozygous carriers of mutations in GBA, which in the homozygous state cause Gaucher disease, have an increased prevalence of Parkinsonism and Lewy body-associated dementia. It is postulated that accumulation of glucosylceramide due to decreased GBA enzyme activity, results in decreased lysosomal degradation of alpha synuclein (Mazzulli et al.,2011). ATP6AP2 which is implicated in X-linked Parkinsonism and ATP13A2 code for lysosomal proteins and when mutated cause impairment in lysosomal autophagy.

5.4 Energy metabolism in mitochondria

Mitochondrial dysfunction has been implicated in the pathogenesis of Parkinsonism and several mutations in genes in the common pathway in mitophagy in mitochondria cause Parkinsonism. The most important among them are PARK2, PINK1, FBXO7 and DJ1.

6 Genetic testing for Parkinsonism

Three-generation pedigree, detailed family history and evaluation need to be done in every family to determine whether the cases are simplex or familial. Age of presentation of affected individuals and their relevant medical records should be collected and noted in detail. No formal guidelines have been formulated to regulate genetic testing in Parkinson disease (Klein et al., 2012).

6.1 Whom to test?

1.

Early onset PD with atypical features and/ or family history

2.

Any patient with juvenile onset of Parkinson disease irrespective of family history

3.

Late onset disease with a strong family history

6.2 Which genes to test?

Testing strategy can be stepwise single gene testing or multigene panel testing. In families with autosomal dominant inheritance, the European Federation of Neurological Sciences recommends screening for mutations in LRRK2. In specific populations with familial and sporadic PD, the same federation recommends screening for the LRRK2 mutation - p.G2019S. Testing for Parkin, PINK1 and DJ1 is indicated in families with autosomal recessive PD.

6.3 Ethical concerns regarding molecular testing

Without appropriate pretest counseling by a trained Medical Geneticist and/ or genetic counselor, molecular testing for Parkinson disease should not be attempted. Direct- to- consumer testing is available and is being used by patients and healthy at-risk individuals. With many susceptible loci being identified without ample evidence to prove causality, genetic counseling is crucial before molecular testing is ordered. Susceptibility testing should be strongly discouraged, especially in healthy individuals. Providing prenatal diagnostic testing for an adult onset disease is still debatable.

7 Genetic counseling

A family with an affected individual should be counseled regarding the environmental, epigenetic and genetic factors, which can cause Parkinson disease. Since Parkinson disease is a common neurodegenerative condition, the lifetime risk of developing this condition is 1-2% (Elbaz et al., 2002). The empiric risk of recurrence in a family with a sporadic case of late onset classical PD is 3-7%. (Elbaz et al., 2002). In monogenic forms, depending on the pattern of inheritance, the risk of recurrence will vary.

8 Therapeutic strategies

The treatment strategies being tried for Parkinson disease include pharmacologic therapy, therapies based on molecular mechanisms of disease, cell-based therapy and gene therapy.

a. Pharmacological therapy: The main intention of pharmacological methods is to achieve symptom control by normalizing dopamine levels. This includes monotherapy with dopaminergic drugs like levodopa, combination of levodopa-carbidopa, monoamine oxidase B inhibitors (MAO B inhibitors) and Catechol-o-methyl transferase inhibitors, which increase the levels of dopamine. Dopamine agonists like pramipexole and ropinirole can also be tried in early stages of disease. Non-dopaminergic drugs, which are useful, include anticholinergic compounds, antiviral drugs like amantadine, norepinephrine and serotonergic receptor and muscarinic related compounds. The main drawbacks of these pharmacological agents are that they cannot alleviate non-motor symptoms like dementia and mood disorders and they do not correct the abnormalities in cholinergic and serotonergic pathways.

b. Therapeutic strategies based on molecular mechanism of disease:

1.

Small molecular therapy: Table 2 shows the various small molecules that have been tried in Parkinson disease.

2.

Cell based therapy: Attempts to improve dopamine levels in the brain by transplanting fetal midbrain tissues and adrenal medullary tissues to mice models started as early as 1980s (Parmer, 2018). These transplants were later shown to induce transplant related side effects (Barker et al., 2015). Moreover due to limited supply of human fetal ventral mesencephalic tissue and related ethical concerns, grafting to human brain had limited clinical utility. With the discovery of human embryonic stem (hESC) cells in 1998, attempts to use them for producing dopaminergic neurons began. Several stem cell sources for grafting were considered and these were pluripotent embryonic stem cells, induced pluripotent stem cells (iPSCs), mesenchymal stem cells and induced neurons obtained by reprograming somatic cells (Barker et al., 2015). Many research groups have completed preclinical trials with GMP (Good Manufacturing Practice) level cell manufacturing and now clinical trials are on their way (Barker et al., 2017).

3.

Gene therapy-based approach: Viral vector mediated approach using lenti virus, adeno virus and recombinant adeno virus has been tried in animal models. (Maiti et al., 2017). Triple gene therapy by delivering the genes required for the three enzymes to produce dopamine from L Dopa has also been tried. RNA interference to silence SNCA, Parkin and PINK is another approach. CRISPR-cas9 mediated genome editing has been used to develop a stable cell line, which expresses SNCA. More studies in animal and cellular models are required before gene-based therapy can be tried on human beings.

c. Surgical therapy: The two surgical approaches that are used for Parkinsonism include deep brain stimulation and pallidotomy or thalamotomy.

9 Conclusion

Parkinson disease is a common neurodegenerative condition, which occurs due to interplay between environmental, epigenetic and genetic factors. Only 5-10% of Parkinson disease is due to monogenic causes. Genetic testing for Parkinson disease should be attempted with utmost care only after appropriate pretest counseling. Newer modalities of treatment for Parkinsonism like cell-based therapy are on the horizon with clinical trials being conducted now.

References