Current targeted therapeutic strategies for oculopharyngeal muscular dystrophy: from pharmacological to RNA replacement and gene editing therapies

OPMD Therapy Progress 2 ARC Publishing Introduction Oculopharyngeal muscular dystrophy (OPMD) is a midlife onset (~45 years) primarily autosomal dominant muscular disease with minor neuronal involvement [1]. It is characterized by progressive swallowing difficulties, eyelid drooping and serious proximal limb weakness at later stages. The pathological hallmark of the disease is the presence of intranuclear inclusions (INI) in the muscle biopsies of patients [2, 3]. The highest prevalence of OPMD is in the French-Canadian population, where 1/1,000 people are at risk, though the disease is found worldwide. In 1990, our group began collecting samples from affected families and, in 1998, we identified the poly(A) binding protein nuclear 1 protein (PABPN1) gene as causative [4, 5]. OPMD results from short expansions of a 5’ trinucleotide alanine coding repeat located at the N-terminus end of the protein. The normal PABPN1 protein has 10 alanines and the mutated (expPABPN1) has 11-18 [4]. The PABPN1 protein has 306 amino acids. In addition to the polyalanine stretch, PABPN1 contains a proline-rich region in the N-terminus, an RNA-binding domain in the central region and an arginine-rich C-terminus. PABPN1’s well-recognized role in mRNA polyadenylation and transport is attributed to its ability to bind poly(A) tails with high affinity [6, 7]. A number of recent studies have revealed additional roles for the protein, including transcription regulation and poly(A) RNA export [8, 9]. PABPN1 appears to control gene expression in various tissues, including muscles [10, 11]. The depletion of PABPN1 in mouse primary muscle cells leads to myogenic defects and reduced differentiation [11]. PABPN1 expression in human cells has also been reported to be autoregulated through a mechanism of controlled intron retention coupled to pre-mRNA decay [12]. Moreover, PABPN1 was shown to suppress alternative cleavage, polyadenylation sites [13] and the regulation of long non-coding RNAs (lncRNA) [8]. Since our publication of the first PABPN1 mutations in 1998, a striking array of cellular and molecular mechanisms has been proposed to contribute to the pathogenesis of OPMD (Figure 1). Proposed mechanisms include sequestration of cellular factors by the mutant protein [14, 15], defects in the potential clearance pathway (i.e. chaperones, and ubiquitin-proteasome pathway UPP) [14, 15], alterations in transcription, histone acetylation alteration [16], aging-associated factors [17], Wnt pathway perturbation [18], and aberrant protein-protein interactions [19]. The toxic gain-of-function mechanism of OPMD supports the hypothesis that disease onset and progression is dependent upon the expansion of the polyalanine tract. The mutant protein becomes misfolded due to the presence of expanded tract as it allows a transition toward a distinct β-sheet conformation that may cause toxicity in several ways. It may exert toxicity as a monomer or it may self-associate to form toxic oligomers. The oligomers can assemble into larger aggregated species and ultimately they are deposited in macromolecular intracellular inclusions. The context of the polyalanine tract within the PABPN1 protein has an essential role in the disease process. The composition of regions flanking the repeats can alter the biochemical and biophysical properties of expPABPN1. Differences in the production and/or clearance of expPABPN1 in eyelid and pharyngeal muscles might be determinants of age-related muscle vulnerability in OPMD. These might be used as therapeutic targets. For example, as polyalanine expansions lead to misfolded PABPN1 protein, structural refolding assisted by enhanced chaperone activity might be beneficial. An increased degradation of expPABPN1 and aggregates via proteasomes could reduce the amounts of toxic species inside the cell. Despite our understanding of the normal functions of PABPN1 protein, and the proposed mechanisms contributing to OPMD pathogenesis, the mechanisms whereby mutated expPABPN1 is toxic to muscle is still a mystery. Alanine is a highly hydrophobic amino acid, often found in the cores of proteins [20], and polyalanine oligomers are extremely resistant to chemical denaturation and enzymatic degradation [21]. Our initial hypothesis was that expanded polyalanine tract of expPABPN1 forms β-pleated sheets that give rise to toxic INI. As our work progresses, we are discovering that soluble expPABPN1 may be the most critical species for toxicity [22]. OPMD shares many common features with other polyalanine diseases, polyglutamine diseases, muscle dystrophic diseases, and diseases caused by RNA binding proteins (Figure 2). In addition to OPMD, at least eight other inherited diseases are now caused by genes with polyalanine expansions [23]. All the affected genes in polyalanine diseases, except PABPN1, code for transcription factors that play important roles in early development [23]. OPMD was the first polyalanine disorder identified that leads to INI in muscle nuclei of patients[2]. Later studies demonstrated the presence of protein aggregates in several polyalanine diseases [24-26]. Hydrophobic polyalanine tracts in the normal range are considered to be flexible spacers Figure 1. A model of molecular mechanisms behind OPMD pathology. The indicated pathways point to possible sites for therapeutic intervention.