One of the surprising developments of recent years was the recognition of the relatively high proportion of patients with early-onset parkinsonism caused by recessive mutations in several genes (Table 1). So far, three have been identified: parkin (PARK2), PINK1 (PARK6), and DJ-1 (PARK7). Again, the study of the function of these genes has provided valuable insight into the molecular mechanisms of dopaminergic degeneration.
Juvenile cases of parkinsonism in siblings were first recognized in Japan. The first genetic locus for autosomal-recessive juvenile parkinsonism (AR-JP), as this form of PD was called, was mapped to chromosome 6. Mutations were then identified in a large gene in that region that was called parkin.2 Clinically, these patients suffer from L-dopa-responsive parkinsonism and often develop early and severe L-dopa-induced motor fluctuations and dyskinesias. Some show diurnal fluctuations, with symptoms becoming worse later in the day. Dystonia at onset of the disease is common.
Parkin mutations turned out to be a common cause of parkinsonism with early onset, particularly in individuals with evidence of recessive inheritance. Nearly 50% of families from a population of sibling pairs with PD had parkin mutations.44 Also, parkin mutations are responsible for the majority of sporadic cases with very early onset (before age 20), and are still common (25%) when onset is between 20 and 35. Prevalence is almost certainly well below 5% in those with onset later than 45.
Several studies have described the clinical spectrum of parkin-associated parkinsonism. Mean age at onset in a European population was 32 years; progression of the disease was usually relatively slow, but L-dopa-associated fluctuations and dyskinesias occurred frequently. Dystonia (usually in a lower extremity) at disease onset was found in about 40% of patients, and brisk reflexes of the lower limbs were present in 44%.44 Psychiatric abnormalities have been recognized in PD patients with parkin mutations45 but there are no systematic studies to determine whether this is a characteristic feature associated with parkin-mutations. Phenotype–genotype studies implicate that the type of mutation may influence the clinical phenotype to a certain degree: patients with at least one missense mutation showed a faster progression of the disease with a higher UPDRS (United Parkinson's Disease Rating Scale) motor score than carriers of truncating mutations. Missense mutations in functional domains of the parkin gene resulted in earlier onset.46
It is still controversial whether heterozygous mutations in the parkin gene can cause parkinsonism or can confer an increased susceptibility for typical late-onset PD. There is evidence from imaging studies47 that heterozygous carriers of parkin mutations have reduced uptake of fluorodopa in the basal ganglia. Furthermore, families with heterozygous mutation carriers manifesting symptoms of PD have been described.48, 49 On the other hand, the frequency of heterozygous mutations in the parkin gene was found to be similar in elderly healthy individuals, as compared to a cohort with late-onset typical PD,50 and in a large family reported recently, 12 heterozygous carriers of a particular parkin mutation (ex3delta40) were asymptomatic.51 Also, in a group of families with PD showing anticipation (late-onset PD in the parent generation and early-onset PD in the offspring) genotyping results did not support the explanation that the presence of single or compound heterozygous parkin-mutations contribute to this phenomenon.52 Therefore, at present the data are still insufficient to confidently judge the role of single heterozygous parkin mutations in the development of PD.
Knowledge on the neuropathology of molecularly confirmed cases of AR-JP is still based on only a few cases. Severe and rather selective degeneration of neurons in the substantia nigra and the locus coeruleus, usually with absence of Lewy bodies, has been described.53 However, a later publication found typical LBs in a single patient.49 In another case with parkin mutations, tau was found in the inclusions,54 and most recently, another case was reported showing α-SYN positive inclusions that resemble LBs in some respects, but differ in some of their staining properties.55 It is unclear whether these differences may be attributed to differential effects of parkin mutations on its E3-ubiquitin ligase function.
As mutations in parkin cause parkinsonism, in all likelihood by a loss-of-function mechanism, the study of the normal function of parkin should provide insight into the molecular pathogenesis of the disorder. Several groups have now shown that parkin, a protein found in the cytosol but also associated with membranes, functions in the cellular ubiquitination/protein degradation pathway as a ubiquitin ligase.56 It has been hypothesized that the loss of parkin function may lead to the accumulation of a nonubiquitinated substrate that is deleterious to the dopaminergic cell but, due to its nonubiquitinated nature, does not accumulate in typical Lewy bodies. Several proteins have been shown to interact with parkin. However, the putative toxic protein, which has been hypothesized to accumulate due to the lack of parkin in patients (or in knock-out animals, for that matter) has not yet been identified.
However, novel functions of parkin are being identified, and it is possible that they may be of equal or even greater relevance to the pathogenesis of PD. For example, it has been shown that parkin does not only mediate the well-studied ubiquitinylation via lysin48 (K48), which directs ubiquitinylated proteins for proteasomal degradation, but also via lysin63 (K63), which may play a role intracellular signaling processes and also in Lewy body formation.57 Additional clues to possible other relevant functions of parkin have been derived from the proteomic analysis of parkin−/− mice. A recent study revealed a decreased abundance of a number of proteins involved in mitochondrial function or oxidative stress, accompanied by a reduction in respiratory capacity of striatal mitochondria, a decreased serum antioxidant capacity and increased protein and lipid peroxidation.58 This corresponds well to recent analyses of Drosophila parkin−/− models. Greene et al. extended an earlier study, that had detected mitochondrial pathology, apoptotic muscle degeneration, and locomotor defects in Drosophila parkin−/− mutants32 and found that these changes are associated with a profound increase of expression of genes involved in the defense against oxidative stress59 and that loss-of-function mutations in genes for oxidative stress components enhance the parkin mutant phenotypes. These novel findings indicate that proteasomal dysfunction, although supported by several lines of evidence, may not be the sole mechanism contributing to neurodegeneration in parkin-related disease.
Whatever the mechanism, increasing evidence suggests an important role of parkin for dopamine neuron survival. Overexpression of wildtype rat parkin could protect against the toxicity of mutated human A30P α-SYN in a rat lentiviral model of PD. The parkin-mediated neuroprotection was associated with an increase in hyperphosphorylated α-SYN inclusions, suggesting a key role for parkin in the genesis of Lewy bodies.60
Recently, mutations in the PINK1-gene (PARK6) have been identified as another cause for autosomal-recessive early-onset parkinsonism.6 This gene is particularly interesting within the context of the findings linking PD to mitochondrial dysfunction and oxidative stress, as PINK1 encodes a mitochondrially located protein. Mutations in the PINK1-gene are much less common that parkin mutations, and probably account for only 1–2% of early-onset cases.61–64 Again the question of the role of single heterozygous mutations is unsettled. In 5 of 100 patients studied by Valente et al.61 only a single mutation was identified. Age at onset in the heterozygotes was in the fourth to fifth decade (range, 37–47 years). Two of 200 healthy control individuals also carried one heterozygous missense mutation. Together with previous positron emission tomography studies demonstrating nigrostriatal abnormalities in clinically asymptomatic PARK6 carriers, this observation argues that haploinsufficiency of PINK1 may represent a susceptibility factor toward parkinsonism, but the question is certainly not settled.
Mutations in the DJ-1 gene (PARK7) are another rare cause of autosomal-recessive parkinsonism.7, 65, 66 The clinical picture with early-onset and slow progression is similar to the other recessive Parkinson syndromes. Following the initial discovery of two mutations in an Italian and a Dutch family,7 only a few additional bona fide pathogenic mutations (one homozygous67 and one compound heterozygous68 have been identified.
The normal function of DJ-1 and its role in dopamine cell degeneration is unknown, but there is evidence that links DJ-1 to oxidative stress response and mitochondrial function. Canet-Aviles et al. have shown that, in the presence of oxidative stress, wildtype DJ-1 translocates to the outer mitochondrial membrane and is associated with neuroprotection.69 Interestingly, DJ-1 is expressed mostly in astrocytes in normal and PD brain, stressing the importance of glial-neuronal interaction in PD.70
Again, the pathogenic role of several single heterozygous sequence variants detected in cohorts of early-onset PD is unclear.66, 68