Nuclear inclusions in oculopharyngeal muscular dystrophy consist of poly(A) binding protein 2 aggregates which sequester poly(A) RNA

Oculopharyngeal muscular dystrophy (OPMD) is an adult-onset disease charact erized by progressive eyelid drooping, swallowing difficulties and proximal limb weakness. The autosomal dominant form of the disease is caused by sho rt (GCG)(8-13) expansions in the PABP2 gene. This gene encodes the poly(A) binding protein 2 (PABP2), an abundant nuclear protein that binds with high affinity to nascent poly(A) tails, stimulating their extension and control ling their length. In this work we report that PABP2 is detected in filamen tous nuclear inclusions, which are the pathological hall mark of OPMD. Usin g both immunoelectron microscopy and fluorescence confocal microscopy, the OPMD-specific nuclear inclusions appeared decorated by anti-PABP2 antibodie s. In addition, the inclusions were labeled with antibodies directed agains t ubiquitin and the subunits of the proteasome and contained a form of PABP 2 that was more resistant to salt extraction than the protein dispersed in the nucleoplasm. This suggests that the polyalanine expansions in PABP2 ind uce a misfolding and aggregation of the protein into insoluble inclusions, similarly to events in neurodegenerative diseases caused by CAG/polyglutami ne expansions. No significant differences were observed in the steady-state poly(A) tail length in OPMD and normal myoblasts, However, the nuclear inc lusions were shown to sequester poly(A) RNA. This raises the possibility th at in OPMD the polyalanine expansions in the PABP2 protein may interfere wi th the cellular traffic of poly(A) RNA.