DCTN1 genetic variability has been implicated in three major neurodegenerative phenotypes: parkinsonism, MND, and ALS/FTD. The present study describes a comprehensive sequence analysis of 286 patients and the identification of 36 novel DCTN1 variants. Furthermore, all the identified coding variants, novel noncoding variants located within 25 bases from an exon-intron boundary, and those reported pathogenic for MND, ALS, FTD, and Perry syndrome were analyzed in three independent case–control series of PD, FTLD, and ALS. This analysis revealed that all the variants identified are very rare and do not associate with disease susceptibility. Nonsegregation within families confirmed the lack of involvement of DCTN1 variants in disease development, although most pedigrees are small and segregation is therefore equivocal.
Of note, one of the originally reported pathogenic mutations for ALS (p.T1249I) was identified in three control individuals, five patients with PD, one patient with FTLD, and five patients with ALS, which in conjunction with lack of segregation in a large pedigree with a high incidence of PD further weakens the pathogenicity evidence of this variant (). All other reported mutations for MND (p.G59S), ALS/FTD (p.M571T, p.R785W, p.R1101K), and Perry syndrome (p.G71R/A/E, p.T72P, p.Q74P) were not identified in this study.3,6,7,9
To date, all proven pathogenic DCTN1
mutations seem to cluster in the N-terminal cytoskeleton-associated protein-glycine-rich (CAP-Gly) domain of dynactin p150Glued
. Functionally, this domain has been shown to be involved in direct binding to microtubules, with the interaction mediated by the 67
motif. The p.G59S mutation identified in one family with MND is located in the center of the CAP-Gly domain, whereas all Perry syndrome mutations are in (p.G71) or immediately adjacent to (p.T72 and p.Q74) the 67
motif. Previous studies performed by our group and others demonstrate reduced binding affinity of mutant p150Glued
(CAP-Gly domain) for microtubules.3,5,9
Pathogenicity has not been established for any of the coding changes identified in other domains of the p150Glued
Variant p.R785W implicated in ALS was identified in a small pedigree with two affected and two unaffected carriers, p.M571T was observed in one familial ALS sample alone, and p.R1101K in a sibpair with one carrier presenting ALS and the other FTD. In addition to this lack of segregation and replication, functional evidence does not support pathogenicity for these variants outside the CAP-Gly domain.12
However, if confirmed pathogenic, it may reflect the biologic importance of p150Glued
and the dynactin complex. Mutations affecting the dimerization of the p150Glued
backbone, its interaction with other subunits of the dynactin complex, or a putative cargo might have detrimental effects on the cellular homeostasis and thus prove fatal for the cell.
Interestingly, the p150Glued
protein possesses a second microtubule binding domain (basic domain amino acids 116–145). In contrast to the CAP-Gly domain, which firmly attaches dynactin to microtubules, this second domain moves progressively along microtubules even in the absence of molecular motors. Due to this second domain, pathogenic mutations in the CAP-Gly domain may not completely ablate microtubule binding, but disrupt organized movement of the dynactin complex, leading to disease.13
However, this scenario does not explain the selective vulnerability of different neuronal populations encountered in Perry syndrome, ALS, and FTD. A number of other protein interactions have also been mapped to the CAP-Gly domain, including CLIP170, α-tubulin, and end-binding (EB1) proteins.14–16
Apart from reducing affinity for microtubule binding, it is conceivable that the disruption of the interaction with one of these or an as yet unknown binding partner could also be responsible for disease. Although it appears unlikely that common mutations in DCTN1
play a major role in these neurodegenerative disorders, they nonetheless provide the first direct link between selective neuronal vulnerability observed in MND, FTD, and PD, cellular transport, and postmortem TDP-43 proteinopathy.