Molecular pathways in dystonia

Abstract

The hereditary dystonias comprise a set of diseases defined by a common constellation of motor deficits. These disorders are most likely associated with different molecular etiologies, many of which have yet to be elucidated. Here we discuss recent advances in three forms of hereditary dystonia, DYT1, DYT6 and DYT16, which share a similar clinical picture: onset in childhood or adolescence, progressive spread of symptoms with generalized involvement of body regions and a steady state affliction without treatment. Unlike DYT1, the genes responsible for DYT6 and DYT16 have only recently been identified, with relatively little information about the function of the encoded proteins. Nevertheless, recent data suggest that these proteins may fit together within interacting pathways involved in dopaminergic signaling, transcriptional regulation, and cellular stress responses. This review focuses on these molecular pathways, highlighting potential common themes among these dystonias which may serve as areas for future research. This article is part of a Special Issue entitled "Advances in dystonia".

Figure 1. Proteins encoded by DYT1, DYT6, and DYT16 genes

Schematics of the three proteins are shown with dystonia-related mutations indicated relative to functional domains. Missense mutations are shown above each protein's sequence, while deletions/truncating mutations are shown below. (A) The DYT1 protein, torsinA, is a member of the AAA⁺ superfamily of molecular chaperones. Key functional motifs include: an N-terminal signal sequence (SIG), N-linker (N) at the boundary of the AAA⁺ cassette (AAA), Walker A (A) and B (B) domains, and sensor 1 (S1) and 2 (S2) characteristic of members of this protein family. Five known mutations are shown, of which only one, delE302/303 has been clearly established as pathogenic. (B) The DYT6 protein, THAP1, is a zinc-finger protein consisting of a conserved DNA binding module (THAP) , a central proline-rich region (PRO), and a coiled-coil domain (COILED) which includes a nuclear localization signal (NLS). Most mutations are predicted to disrupt key residues in either the DNA binding region or the NLS. (C) The DYT16 protein, PACT, regulates activity of protein kinase R, through three modular domains (M1-3). Domains M1 and M2 are involved in kinase binding while M3 is required for activation. The P222L mutation falls near the boundary of M3, while the frameshift mutation occurs in M1 and may potentially truncate the protein.

Figure 2. Potential interactions between torsinA, THAP1, and components of dopaminergic neurotransmission

Dopamine (DA) receptors can be classified into 5 subtypes, D1-D5, all of which signal through heterotrimeric G-proteins that act on adenylate cyclase (AC). Activation of D1 and D5 receptors stimulates AC to increase cAMP production, while D2 receptor activation inhibits AC and decreases cAMP levels. D3 and D4 are thought to function similarly to D2 receptors. Trafficking of DA receptors to the cell surface begins in the ER and depends on molecular chaperones. Given that DYT1 patients and mouse models both show decreased D2 receptor availability at the cell surface, it is possible that torsinA participates in the processing of D2 receptors in the ER and that the mutant form, torsinAΔE, loses this ablity, producing a trafficking defect. THAP1 interacts directly with Par-4, which competes with calmodulin for binding to a cytosolic regulatory site on the D2 receptor. A loss of Par-4 results in increased binding by calmodulin and a decrease in D2 activity. It is not known whether and how DYT6 mutations in THAP1 affect Par-4 and its interaction with D2. THAP1 has been recently shown to negatively regulate torsinA expression. DYT6 mutations in THAP1 which lose this activity could potentially result in increased torsinA levels, which in turn could create imbalances in the ER which also perturb chaperone activity. The predicted net effect of a D2 defect is a loss of inhibitory input to AC which results in aberrant cAMP-related signaling during DA neurotransmission. Antagonists of adenosine A2A receptors could potentially compensate for decreased D2 activity and normalize transmission.

Figure 3. DYT6 mutations in DNA binding module of THAP1

NMR solution structure of THAP1's DNA binding domain as determined by Bessiere et al., (2008). Positions are highlighted to indicate: (a) four zinc coordinating residues (dark blue) with zinc ion (light blue); (b) residues which, when individually mutated, were shown to significantly decrease or even abolish DNA binding (yellow); and (c) known DYT6 mutants (red).

Figure 4. Potential interactions between torsinA, PACT, and THAP proteins in endoplasmic reticulum stress signaling

Components of the unfolded protein response (UPR). The UPR chaperone, BiP, is recruited to misfolded proteins, resulting in activation of three key ER stress mediators, IRE-1, ATF6, and PERK. Activated IRE-1 results in splicing of the mRNA encoding XBP-1, producing a mature transcription factor that translocates to the nucleus and upregulates expression of molecular chaperones. Activated ATF6 is transported to the Golgi, where it is cleaved to produce a mature transcription factor that also upregulates chaperone expression. Activated PERK phosphorylates eIF2α, which transiently decreases global protein synthesis to ease the burden in the ER. TorsinA is believed to function at some level in this cascade, while torsinAΔE may impair the cell's ability to respond to misfolded proteins. PACT responds to cytosolic and ER stresses to activate PKR, which also phosphorylates eiF2α to decrease protein synthesis. Mutations in PACT which prevent PKR activation would also impair the cell's ability to respond to ER and other stress stimuli. PKR is normally inhibited by p58IPK (p58), although this inhibition is removed during stress signaling by THAP0, a close relative of THAP1. It is not yet known whether THAP1 itself participates directly in the p58IPK/PKR pathway.