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). Presence of intranuclear inclusions (INIs) containing PABPN1 aggregates in the skeletal Muscles is the hallmark of OPMD. Here, we show that ectopic expression of the mutant PABPN1 produced INIs in a muscle Cell Culture model and reduced expression of several muscle-specific proteins including a-actin, slow troponin C, muscle creatine kinase, and two myogenic transcription factors, myogenin and MyoD. However, the levels Of two upstream regulators of the MyoD gene, the Myf-5 and Pax3/7, were not affected, but both proteins co-localized with the PABPN1 aggregates in the mutant PABPN1 overexpressing cells. In these cells, although myogenin and MyoD levels were reduced, these two transcription factors did not co-localize With the Mutant PABPN1 aggregates. Therefore, sequestration of Myf5 and Pax3/7 by the mutant PABPN1 aggregates was a specific effect on these factors. Our results suggest that trapping of these two important myogenic determinants may interfere with an early step in myogenesis. (c) 2005 Elsevier Inc. All results reserved.