Functional evidence implicating a novel TOR1A mutation in idiopathic, late-onset focal dystonia

Abstract

Background: TOR1A encodes a chaperone-like AAA-ATPase whose Delta GAG (Delta E) mutation is responsible for an early onset, generalised dystonia syndrome. Because of the established role of the TOR1A gene in heritable generalised dystonia (DYT1), a potential genetic contribution of TOR1A to the more prevalent and diverse presentations of late onset, focal dystonia has been suggested.

Results: A novel TOR1A missense mutation (c.613T-->A, p.F205I) in a patient with late onset, focal dystonia is reported. The mutation occurs in a highly evolutionarily conserved region encoding the AAA-ATPase domain. Expression assays revealed that expression of F205I or Delta E, but not wildtype TOR1A, produced frequent intracellular inclusions.

Conclusions: A novel, rare TOR1A variant has been identified in an individual with late onset, focal dystonia and evidence provided that the mutation impairs TOR1A function. Together these findings raise the possibility that this novel TOR1A variant may contribute to the expression of dystonia. In light of these findings, a more comprehensive genetic effort is warranted to identify the role of this and other rare TOR1A variants in the expression of late onset, focal dystonia.

Figure 1

(A) Genomic structure of TOR1A and relative location of novel mutation. Exons are represented by rectangles and coding exonic regions are shown in black. Below is a representation of TOR1A protein with location of ATPase functional domain relative to full length protein shown. For reference, the location of the ΔE(302/303) mutation is shown with an asterisk. (B) Alignment of TOR1A amino acid sequences with residues 191–225 of the human protein. Mutated residue is boxed and surrounding identical residues are shaded grey. Alignments are among Torsin family 1, member A predicted proteins, except for C. elegans (tor-1) and D. melanogaster (torp4a).

Figure 2

Novel F205I mutation produced inclusions in BHK cell expression studies. (A) Under identical transfection conditions, BHK cells expressing TOR1A-EGFP exhibited fluorescent inclusions in 44% of F205I, 10% of wildtype and 79% of ΔE (*p< 0.05, n= 200 cells per construct from a minimum of two experimental replications). (B) Mean pixel size of TOR1A-EGFP inclusions (* p < 0.05, n= 60 WT, 107 F205I, and 187 ΔE puncta from a minimum of 3 experimental replications). (C, D) Representative images of confocal immunocolocalization of TOR1A-EGFP in BHK cells. LaminB1 and KDEL are nuclear envelope and endoplasmic reticulum markers, respectively. Scale bar indicates 25 microns.