Cross-examining candidate genes implicated in multiple system atrophy

The relevance of α-synuclein in MSA pathogenesis is of considerable focus [86]. α-Synuclein is encoded by the SNCA gene located on 4q22.1 [14]. It is concentrated at presynaptic terminals of neurons, where it is postulated to facilitate synaptic activity [14, 17, 59]. In MSA, however, α-synuclein is present in aggregated form as a major component of GCI in oligodendrocytes. Aggregation of α-synuclein appears to cause decreases in the level of neurotrophic factors in oligodendrocytes, reducing the ability of oligodendrocytes to provide axonal stability [79]. It also appears to impact on the ability of oligodendrocytes to regenerate and repair [88].

Mutations in SNCA (A30P, E46K, H50Q, G51D and A53T) are known to cause PD [4, 38, 42, 61, 91]. However, no causal SNCA mutations for MSA have been found to date, and the association between variant α-synuclein and MSA is still unproven. Several studies have investigated the inheritance of SNCA variants in families with MSA [38, 39, 58]. A study of a British family with the SNCA G51D variant revealed neuropathological hallmarks of both MSA and PD [38]. Similar hallmarks were present in two other cases with the same variant [39]. These findings suggest that G51D could be relevant to MSA as well as PD, although it has not been found in cases with only MSA pathology. Similarly, a Finnish family with SNCA A53E variant showed neuropathological hallmarks of both MSA and PD [58]. It should be noted that deposition of α-synuclein in oligodendroglia occurs with greater disease severity and duration in PD [28], suggesting that these mutations may be more aggressive PD mutations rather than relevant for the initiation of MSA. At this stage we cannot rule out the possibility of coexistence of the two diseases. Hence, at this stage, it is difficult to conclude if there is a causal link between SNCA and MSA.

The association of SNCA single nucleotide polymorphisms (SNPs) to MSA has also been studied. Two SNPs, rs3857059 and rs11931074, were found to be significantly associated with increased risk of MSA in an European population [67]. A further two SNPs, rs3822086 and rs3775444, were found to be significantly associated with increased risk of MSA in a different European population [2]. Interestingly, the latter study also found that rs3822086 was significantly associated with MSA-C [2]. In contrast, rs3822086 and rs3775444 were not significantly associated with MSA in a Chinese population[15].

Overall, while current evidence suggests that specific SNCA SNPs are associated with increased risk of MSA in certain populations, no evidence exists for a causal relationship between SNCA and MSA, albeit PD/MSA or MSA-like pathology.

COQ2 encodes the enzyme coenzyme-Q2-polyprenyltransferase in the biosynthetic pathway of coenzyme Q₁₀. Coenzyme Q₁₀ is an integral part of the mitochondrial electron transport chain; it transfers electrons from complex I and II to complex III [21]. Deficiencies in coenzyme Q₁₀ cause mitochondrial dysfunction, oxidative stress and reduced ATP synthesis [62]. Polymorphisms in COQ2 have been associated with MSA in several studies. In one study, functionally-impaired COQ2 V393A variant was shown to be associated with sporadic MSA in a Japanese population [46]. This association was supported by a meta-analysis in a subsequent study, in which V393A was shown to be associated with increased risk of MSA in Han Chinese, Japanese and possibly broader East Asian populations [93]. In contrast, another study showed that V393A was not associated with MSA in 133 Japanese MSA patients [77]. Likewise, V393A was not associated with MSA in Korean MSA patients [35]. Apart from V393A, other COQ2 variants (e.g. L25 V, M128 V, R173H, L402F, A32A and N386I) have been identified in various populations [16, 46, 77, 85]. The pathological link between COQ2 and MSA was strengthened when it was discovered that plasma coenzyme Q₁₀ levels were lower in MSA patients compared to healthy controls [48]. Furthermore, coenzyme Q₁₀ levels were found to be lower in the cerebellum, along with increases in mitochondrial dysfunction and oxidative stress, in MSA cases compared to controls [7]. Interestingly, these changes occurred in MSA cases in the absence of any COQ2 variants associated with MSA [7]. Overall, current evidence cannot fully exclude that specific COQ2 variants are associated with MSA in only certain populations or ethnic groups, but that this pathway may be vulnerable to MSA. Further research is required to determine how COQ2 may contribute to MSA pathogenesis and to reconcile the differences in different ethnic groups.

MAPT is located on chromosome 17q21.31 and encodes the protein tau [9]. The role of tau in microtubule assembly and stability has long been established [84]. When phosphorylated, tau forms aggregates and detaches from the microtubules causing microtubule instability and degradation [9, 36]. Tau has been implicated in numerous neurodegenerative diseases including Alzheimer’s disease (AD), frontotemporal dementia (FTD), corticobasal degeneration (CBD), progressive supranuclear palsy (PSP), Pick’s disease (PiD) and DLB, all of which are considered as either primary or secondary tauopathies [25, 82]. There is some evidence that tau could be involved in MSA pathogenesis. Although tau is present in GCI [12], it is unclear whether this is due to tau playing an active role in MSA pathogenesis or whether this occurs downstream in the disease process.

Two haplotypes have been identified for MAPT. The H1 haplotype is associated with increased risk of PSP, CBD and PD [6, 18, 90], whereas the H2 haplotype rs870723-G allele is associated with decreased risk of late-onset AD [3]. The MAPT haplotypes also appear to be associated with MSA risk. In a study consisting of 61 pathologically-confirmed cases of MSA, the H1 haplotype was shown to be a risk factor for MSA [80]. In another study consisting of 127 pathologically-confirmed cases of MSA, H2 and H1E were associated with decreased risk of MSA, whereas H1x and H1J were associated with increased risk of MSA [44]. There are six isoforms of tau that exist in the human brain, and different isoforms have been shown to have different pathological effects. Increased levels of the 4R isoform have been implicated in FTD [9, 11], whereas the 3R isoform has shown to be pathologically important in PiD, and both 3R and 4R deposit in AD [82]. Currently, the role of tau isoforms in MSA is unknown.

GBA1 encodes beta-glucocerebrosidase, which is an enzyme that cleaves glucocerebroside in lysosomes. Mutations in GBA1 cause accumulation of glucocerebroside in cells and certain tissues, and can cause Gaucher’s disease (GD) [1]. GBA1 mutations are also a common risk factor for PD and DLB [8, 92]. There is some evidence that GBA1 could also be linked to MSA. Sequencing of coding regions and flanking splice sites of the GBA1 gene in 969 MSA patients, sub-divided into Japanese, European and North American cohorts, revealed that the carrier frequency of GD-linked mutations in MSA was 1.75% across all groups [47]. Interestingly, there was a significant association between GBA1 mutations and MSA-C [47]. In another study consisting of 17 pathologically-confirmed MSA and 82 AD cases, GBA1 mutations – N370S, T369 M and R496 – were present in 4 MSA cases [73]. Importantly, the MSA group had a higher frequency of GBA1 mutations compared to the AD group. However, in a study consisting of 108 pathologically-confirmed MSA cases, GBA1 mutations were not associated with MSA [72]. And, the GBA1 L444P mutation was not associated with MSA in 54 Chinese MSA patients [75]. Furthermore, no association was found with 54 lysosomal storage disorder genes in 375 MSA patients analyzed [60]. With these conflicting findings, there is still much uncertainty over whether GBA1 is linked to MSA.

LRRK2 encodes an enzyme that functions as both a kinase and a GTPase in various cellular processes [33]. LRRK2 variants are associated with increased risk of PD [55, 96]. LRRK2 G2019S is the most common mutation and accounts for 3–10% of familial PD and 1–8% of sporadic PD in European populations [32]. LRRK2 mutations, particularly G2019S, have been investigated in MSA, however, to date, no evidence of a causal link between LRRK2 mutations and MSA has been found [54]. Furthermore, no associations were found between other LRRK2 variants (R1628P and G2385R) and MSA in 318 MSA patients of Han Chinese origin [89]. However, a study consisting of 177 pathologically-confirmed MSA cases from American and British cohorts revealed that M2397 T was a protective haplotype for MSA [31]. In a very recent study, the G2019S LRRK2 mutation was found in a neuropathology-confirmed MSA case [64]. Much more research is required to determine whether LRRK2 is linked to MSA.

Expansion of GGGGCC repeat sequence in the C9orf72 gene has been implicated in both amyotrophic lateral sclerosis and FTD [20, 63]. C9orf72 has been investigated in MSA, however no association, as yet, has been found between C9orf72 and MSA. In a cohort of 100 British and American pathologically-confirmed MSA cases, there was no association between C9orf72 expansion and MSA [68]. This finding was verified in subsequent studies of Chinese MSA cases [13, 76], European and American MSA cases [69]. Based on current evidence, it appears that C9orf72 is not linked to MSA.

A number of other genes has been investigated in MSA, all of which have not been substantially verified. One of the genes examined is CHCHD2, which encodes coiled-coil-helix-coiled-coil-helix domain containing 2 protein that is involved in mitochondrial metabolism. CHCHD2 V66 M mutation was identified in a MSA patient of Italian heritage [51]. The function of this mutation is unknown. In another study consisting of 89 Chinese MSA patients, it was shown that no CHCHD2 variants were found and there were no associations between CHCHD2 and MSA [87]. Two recent studies examined the CAG repeat expansion length of spinal cerebellar ataxia (SCA) genes in MSA. In one, it was reported that the SCA1 CAG expansions were more common in MSA-C compared to MSA-P or controls in an Italian cohort [49]. In another, it was reported that there were no association between SCA1 CAG expansion and MSA in Chinese MSA patients, rather ataxin-2 (ATXN2) was suggested as a risk factor for MSA [95]. The rs1799964 SNP in tumor necrosis factor-α (TNF-α) and the rs16944 SNP in interleukin1β (IL1β) have also been suggested to be potential risk factors for MSA in Han Chinese population [94].

In 2016, a genome-wide associated study (GWAS) comprising 918 MSA cases and 3,864 controls of European ancestry was carried out [65], which is the largest genetic study into MSA to date. No genes were found to be significantly associated (p < 5 × 10^− 8) with MSA after stringent multiple corrections. However, there were four genes with a p value < 1 × 10^− 6; these were F-box protein other 47 (FBXO47), elongation of very long-chain fatty acid 7 (ELOVL7), endothelin-1 (EDN1) and MAPT [65]. MAPT was the only gene that has been previously implicated in MSA. Neither SNCA nor COQ2 was found to be significantly associated with MSA. These results suggest that genes other than those previously associated with neurodegeneration could be involved in MSA. The SNPs identified from the GWAS were tested in another study consisting of 906 MSA cases and 941 controls of Chinese origin, however no significant association was found between these SNPs and MSA [27], once again indicating potential ethnic differences in MSA genetics. It is important to note that in both of these two studies, not all of the MSA cases analyzed were pathologically confirmed.

Copy number variations (CNVs) are DNA structural rearrangements, such as deletions, duplications, inversions and translocations, all of which can affect gene dosage [45]. The role of CNVs in the pathomechanisms of AD [19] and PD [43, 78] have been extensively studied. The first indication of a link between CNVs and MSA came from a Japanese study, in which it was reported that 32% of MSA patients examined were heterozygous for deletions in the Src homology 2 domain containing-transforming protein 2 (SHC2) gene, suggesting that SHC2 copy number is important in MSA [66]. In contrast, however, it was found that changes in SHC2 copy number were insignificant in a group of American MSA patients [24]. A recent study of genome-wide CNV in a Japanese cohort identified 311 CNVs related to MSA, of which three were significantly different in MSA compared to controls [29]. These CNVs were located on CTD small phosphatase-like (CTDSPL), polypeptide N-acetlgalactosaminyltransferase-like 6 (GALNTL6) and small nuclear ribonucleoprotein polypeptide N (SNRPN). Interestingly, all of these CNVs were located in introns, suggesting that the control of expression of these genes is at the level of transcription.