Dominant mutations are inevitably more complex to understand than recessive mutations because there are more mechanisms by which dominance can occur. These include those centered around increased or altered function (gain or persistence of normal function or gain of novel toxic function) and those where loss is important (dominant negative or haploinsufficiency). For the two dominant genes that are most similar to sporadic PD, α-synuclein and LRRK2, human genetic information suggests that they may work by different mechanisms.
Several kindreds have been reported with point mutations in α-synuclein. The clinical phenotype generally resembles sporadic PD although with extensive cortical involvement reminiscent of diffuse Lewy body disease at least in some cases (50
). There are also whole gene duplications (52
) and triplications (54
) (Fig. ). Interestingly, the phenotypes of triplication cases tend to be earlier onset, more severe and with more cortical involvement compared with duplication cases, which is nicely illustrated in a large Swedish pedigree (55
). Collectively, these results show that wild-type α-synuclein has properties that can trigger neurodegeneration when expression levels are increased by relatively modest amounts. Perhaps not the only interpretation, but certainly the simplest, is that α-synuclein mutations cause disease by increased function.
Figure 2. Genes associated with dominant Lewy body diseases. α-Synuclein and LRRK2 are shown, not to scale, with pathogenic mutations above the protein organization in red. α-Synuclein has a series of imperfect repeats of the general sequence KTEGV (more ...)
Polymorphisms around the SNCA
locus are the strongest hits in two recent genome-wide association studies of sporadic PD (7
). This suggests strongly that altered α-synuclein function is important in both familial and sporadic PD, a contention further supported by the presence of the same protein in Lewy bodies (2
). Whether the increased PD risk driven by the SNPs nominated in GWAS is related to dosage of α-synuclein is not yet known, but some early studies suggest a correlation between risk alleles and higher expression levels (56
α-Synuclein is a small protein that is inherently prone to misfold a nd to aggregate. Therefore, one of the leading hypotheses is that protein misfolding is critical to the toxic effects of this protein and a property shared by point mutations or by increasing protein load (57
). Several animal models have been built around overexpression of mutant or wild-type α-synuclein (58
) and, while it is difficult to directly compare different models, there seems to be an approximate relationship between levels of expression and strength of effects. One limitation with most of the available mouse models is that there are relatively modest effects on nigral neuron survival, although viral models in rats and monkeys have generally shown more robust effects.
A dosage effect is not seen with mutations in LRRK2. As discussed elsewhere (60
), there are a series of mutations in different domains of LRRK2 clustering around the two enzymatic GTPase and kinase domains (Fig. ). One mutation, G2019S in the kinase domain, is relatively common in populations from North Africa and in these populations there are individuals who are homozygous for the G2019S mutation. Interestingly, the disease in these individuals is clinically identical to heterozygotes (62
). This data do not support a gain of normal function, so presumably the disease in LRRK2 mutation carriers is caused by another genetic mechanism. Possible mechanisms include a dominant negative effect, a gain of novel function or that the overall LRRK2 activity simply has to exceed a pathogenic threshold for the patient to express disease.
One way to try to understand the mechanism of LRRK2 mutations is to compare overexpression with knockout in experimental models. Knockout of the LRRK homologue in worms affects sensitivity to dopaminergic neurotoxins (63
). Variable results have been reported in flies (64
). LRRK2 knockout mice have normal numbers of dopaminergic neurons, do not display any behavioral abnormalities and live to adulthood (66
). In vitro
analyses suggest that the loss of LRRK2 changes dopamine neurogenesis via effects on the cell cycle (67
). Overall, the evidence that LRRK2 is required for dopamine neuron genesis or survival in adult animals is therefore modest and while not enough to exclude an important role under some circumstances, the available data do not favor this conclusion.
In parallel, several groups have made mice that express mutant LRRK2 using varying promoters at different levels of overexpression. These include a bacterial artificial chromosome (BAC) with the R144G mutation at 5–10-fold increased expression (68
), an R1441C knock-in that is at endogenous expression levels (69
). Of the different mouse models, the R1441G BAC mice produced the most dramatic phenotype, with akinesia that is partially reversible by treatment with L-Dopa or the direct dopamine agonist apomorphine, accompanied by impaired dopamine release. No effect of a BAC expressing wild-type LRRK2 was reported. In contrast, there were no abnormalities in spontaneous motor behavior in homozygous R1441C KI mice, although there was evidence of impaired D2-receptor-mediated function. No cell death was noted in either model, although some alteration in dopamine cell bodies and accumulation of tau pathology was reported in the BAC mouse model (68
). Collectively, these results suggest that mutant LRRK2 might have a role in dopamine release in the striatum, although the differences in spontaneous phenotypes raise a question about the possible role of overexpression in these models, especially in light of the lack of dosage effect in humans discussed above.
Expressing a different LRRK2 mutant, G2019S, from a tetracycline responsive Calcium/calmodulin-dependent protein kinase II-alpha (CaMKII) promoter even at high levels produced only mild neurodegeneration in regions (striatum and forebrain), where CaMKII is active (70
). However, crossing these to animals expressing A53T α-synuclein from the same promoter exacerbated the abnormal accumulation of α-synuclein aggregates and caused more impressive neurodegeneration. Importantly, there was no apparent neurodegeneration caused by A53T α-synuclein if LRRK2 was removed by knockout.
These data, along with the fact that most human LRRK2 cases have α-synuclein positive Lewy bodies (71
), suggest that LRRK2 and α-synuclein are in the same pathogenic pathway. There are some limitations of all the animal models to date, especially that none have dramatic nigral degeneration and there is perhaps some question about mechanisms that appear to be dose-dependent when the human disease appears not to be. However, they set the scene for more detailed explorations of why LRRK2 affects α-synuclein and how, which will almost certainly influence the PD field for the next few years.
Do mutations in LRRK2 or α-synuclein affect mitochondrial function, as we saw for recessive parkinsonism? Overexpression of human WT LRRK2, but not overexpression of LRRK2 mutant or kinase dead LRRK2, protects C. elegans
after exposure to mitochondrial toxins (63
). Drosophila overexpressing human mutant LRRK2 are also more susceptible to rotenone than those overexpressing wt LRRK2 (72
). However, LRRK2
knockout mice are not more sensitive to another mitochondrial toxin, MPTP (66
). Co-expression of mutant LRRK2 with A53T α-synuclein results in mitochondrial structural and functional abnormalities in double transgenic mice (70
). Increased expression of parkin can limit the toxic effects of α-synuclein (73
) or LRRK2 (72
). However, the reciprocal experiment of parkin knockout does not make phenotypes worse, at least for an α-synuclein model that does not have nigral degeneration (76
The difficulty with some of these data is that it is difficult to understand if the effects on mitochondrial function for mutant LRRK2 or α-synuclein are specific and not simply related to cell death. Given that the human data suggest that recessive parkinsonism and dominant PD overlap in that they both have nigral degeneration, it is possible that there are multiple pathways that lead to a common outcome. Further work is needed to understand the details of these pathways, to see if they naturally combine, and this will be discussed below.
We should also ask which, if any, of the different pathways are relevant for sporadic PD. As discussed earlier, several lines of evidence nominate α-synuclein as a link between familial and sporadic Lewy body diseases. Part of the evidence is recent GWAS studies, and it is therefore interesting that the next strongest effect in populations with European ancestry is MAPT
, which codes for the microtubule protein Tau. These variants also increase expression of Tau mRNA (8
), so presumably MAPT
can influence PD risk by a dosage effect as seen for α-synuclein. Mutations in MAPT
produce parkinsonism as part of their phenotype (77
) and some LRRK2 cases that are clinically similar to sporadic PD have Tau pathology (71
). There is a weak signal for LRRK2 in GWAS in different populations (7
). Collectively, these results suggest that a pathway for dominant PD, which may include MAPT
/Tau as a risk factor, does play a role in sporadic disease. Whether the recessive parkinsonism genes are important in the same process is less clear. None of the autosomal recessively inherited PD genes (Parkin, PINK1 or DJ-1) were identified as PD risk loci in the recent GWAS, but such studies are only powered to find relatively common alleles and rare variants may have been missed. Future studies with deeper sequencing approaches are therefore needed to re-assess whether rare variants in these genes are important in sporadic PD.