Structural obstruction to full DNA replication in terminally differentiated skeletal muscle cells.

Terminal cell differentiation is often associated with permanent withdrawal from proliferation, termed the postmitotic state. Though widespread among vertebrates and determinant for their biology, the molecular underpinnings of this state are poorly understood. Postmitotic skeletal muscle myotubes can be induced to reenter the cell cycle; however, they generally die as a result of their inability to complete DNA replication. Here, we explore the causes of such incompetence. Genomic hybridization of newly synthesized DNA shows that the replicative failure does not concern specific genomic regions, but can stochastically affect any of them. Myoblast and myotube nuclei are incubated in replicative Xenopus egg extract, which provides a full DNA replication machinery. While myoblast nuclei attain complete DNA replication, those from myotubes, even in these conditions, duplicate less than half of their genomes, strongly indicating that the structure of myotube chromatin obstructs DNA replication. Furthermore, disassembling and disorganizing chromatin with a strong salt treatment does not modify the replicative differences between the two types of nuclei, suggesting that they are rooted in the core structure of chromatin.