Disease relevance of PABPN1

• Oculopharyngeal muscular dystrophy (OPMD) is an adult-onset dominant genetic disease caused by the expansion of a GCG trinucleotide repeat that encodes the polyalanine tract at the N-terminus of the nuclear poly(A)-binding protein (PABPN1) [1].
• A Drosophila model of oculopharyngeal muscular dystrophy reveals intrinsic toxicity of PABPN1 [2].
• We generated transgenic mice expressing either the wild type or the expanded form of human PABPN1, and transgenic animals with the expanded form showed clear signs of abnormal limb clasping, muscle weakness, coordination deficits, and peripheral nerves alterations [3].
• Oculopharyngeal muscular dystrophy (OPMD) is an autosomal dominant disorder of middle age presenting as progressive dysphagia and eyelid ptosis, due to short expansions of the GCG trinucleotide repeat (from GCG6 to GCG8-13) in the polyadenylate binding-protein nuclear 1 (PABPN1) gene [4].
• Oculopharyngeal muscular dystrophy (OPMD) is a late-onset autosomal dominant muscular dystrophy that results from small expansions of a polyalanine tract in the PABPN1 gene [5].

High impact information on PABPN1

• Pathological expansions of the polyalanine tract may cause mutated PABP2 oligomers to accumulate as filament inclusions in nuclei [6].
• Short GCG expansions in the PABP2 gene cause oculopharyngeal muscular dystrophy [6].
• We isolated the poly(A) binding protein 2 gene (PABP2) from a 217-kb candidate interval on chromosome 14q11 (B.B. et al., manuscript submitted) [6].
• Complex-loaded mRNA is thought to undergo a pioneer round of translation when still bound by cap-binding proteins CBP80 and CBP20 and poly(A)-binding protein 2 [7].
• With the exception of PABPN1, which codes for a poly(A)-binding protein, all these genes encode transcription factors that play important roles during development [8].

Chemical compound and disease context of PABPN1

• Here, we show that trehalose reduces aggregate formation and toxicity of mutant PABPN1 in cell models [9].
• This suggests that the polyalanine expansions in PABP2 induce a misfolding and aggregation of the protein into insoluble inclusions, similarly to events in neurodegenerative diseases caused by CAG/polyglutamine expansions [10].

Biological context of PABPN1

• Here, we show that PABPC1, like the major nuclear poly(A) binding protein PABPN1, associates with nuclear pre-mRNAs that are polyadenylated and intron containing [11].
• A concomitant reduction of cell death in drug-treated mutant PABPN1 expressing cells was also observed [12].
• This was achieved by inactivating the mutant PABPN1 nuclear localization signal and by generating full-length mutant PABPN1 fused to a strong nuclear export sequence [13].
• This indicated larger genetic heterogeneity and showed that unequal crossing-over and not replication slippage must be the underlying mechanism of elongation.We performed sequencing of the PABPN1 gene in 30 German OPDM index patients to determine the exact genotype [14].
• PABP2 binds to the growing poly(A) tail, stimulating its extension during the polyadenylation process, and limits the length of the newly synthesized poly(A) tail [15].

Anatomical context of PABPN1

• Presence of intranuclear inclusions (INIs) containing PABPN1 aggregates in the skeletal muscles is the hallmark of OPMD [1].
• Mutant PABPN1 aggregates as nuclear inclusions in OMPD patient muscles [2].
• We show here that exposure to moderate levels of ZnSO4, 8-hydroxyquinoline, ibuprofen and indomethacin produced a robust stress response resulting in the induction of HSP70 in HeLa cells expressing the mutant PABPN1 as a green fluorescent protein (GFP) fusion protein [12].
• The antibody reagents specifically detect endogenous PABPN1 in cell lysates on western blot and label PABPN1 in cultured cells and muscle sections [16].
• PABPN1 is found on BR mRNPs within the nucleoplasm as well as on mRNPs docked at the nuclear pore [17].

Associations of PABPN1 with chemical compounds

• Strikingly, the polyalanine tract is not absolutely required for muscle degeneration, whereas another domain of PABPN1, the RNA-binding domain and its function in RNA binding are required [2].
• To elucidate the possible role of PABP2 in skeletal muscle, we established the stable C2 cell lines expressing human PABP2 [15].
• The interaction of PABP2 and SKIP was confirmed by glutathione S-transferase-pulldown assay and immunoprecipitation [15].

Physical interactions of PABPN1

• Furthermore, Ski-interacting protein (SKIP) was isolated as a binding protein for PABP2 using the yeast two-hybrid system [15].
• Our results suggest that PABPN1 binds to the polymerase before, at, or shortly after the start of transcription, and that the assembly of PABPN1 onto the poly(A) tail may be coupled to transcription [17].

Regulatory relationships of PABPN1

• The reporter assays showed that PABP2 co-operated with SKIP to synergistically activate E-box-mediated transcription through MYOD: Moreover, both PABP2 and SKIP were directly associated with MyoD to form a single complex [15].
• In vitro the NS1A protein inhibits the ability of PABII to stimulate the processive synthesis of long poly(A) tails catalyzed by poly(A) polymerase (PAP) [18].

Other interactions of PABPN1

• The disorder is caused by trinucleotide (GCG) expansions in the N-terminal part of the poly(A)-binding protein 1 (PABPN1) that result in the extension of a 10-alanine segment by up to seven more alanines [19].
• PABPN1 is involved in the synthesis of poly (A) tails, increasing the processivity of poly (A) polymerase and contributing to defining the length of a newly synthesized poly (A) tail [20].
• Intriguingly, among FRG1P-associated proteins are SMN and PABPN1, both being involved in neuromuscular disorders, possibly through RNA biogenesis-related processes [21].

Analytical, diagnostic and therapeutic context of PABPN1

• Wildtype PABPN1 protein is expressed ubiquitously and was shown to be mostly concentrated in discrete nuclear domains called 'speckles'. Using an established cell- culture model, we show that most mutant PABPN1- positive (alanine expanded form) intranuclear aggregates are structures distinct from intranuclear speckles [13].
• METHODS: Detailed family history and clinical assessment of the OPMD patient were followed by mutation analysis of the PABPN1 gene by direct DNA sequencing and by our newly developed method, fluorescent PABPN1 polymerase chain reaction (PCR) product (flPPP) method [22].
• METHODS: Sequence analysis of exon 1 of the PABPN1 gene was undertaken on 202 patients referred for a possible diagnosis of OPMD but negative for the triplet repeat expansion mutation [23].
• Using both immunoelectron microscopy and fluorescence confocal microscopy, the OPMD-specific nuclear inclusions appeared decorated by anti-PABP2 antibodies [10].
• No animal models of OPMD have been discovered in nature; therefore, we generated transgenic mice expressing human PABPN1 (hPABPN1) using a chicken beta-actin (CAG) promoter [24].

References