Methyl CpG-binding proteins induce large-scale chromatin reorganization during terminal differentiation.

Pericentric heterochromatin plays an important role in epigenetic gene regulation. We show that pericentric heterochromatin aggregates during myogenic differentiation. This clustering leads to the formation of large chromocenters and correlates with increased levels of the methyl CpG-binding protein MeCP2 and pericentric DNA methylation. Ectopic expression of fluorescently tagged MeCP2 mimicked this effect, causing a dose-dependent clustering of chromocenters in the absence of differentiation. MeCP2-induced rearrangement of heterochromatin occurred throughout interphase, did not depend on the H3K9 histone methylation pathway, and required the methyl CpG-binding domain (MBD) only. Similar to MeCP2, another methyl CpG-binding protein, MBD2, also increased during myogenic differentiation and could induce clustering of pericentric regions, arguing for functional redundancy. This MeCP2- and MBD2-mediated chromatin reorganization may thus represent a molecular link between nuclear genome topology and the epigenetic maintenance of cellular differentiation.

Uncoupling the replication machinery: replication fork progression in the absence of processive DNA synthesis.

The precise coordination of the different steps of DNA replication is critical for the maintenance of genome stability. We have probed the mechanisms coupling various components of the replication machinery and their response to polymerase stalling by inhibition of the DNA polymerases in living mammalian cells with aphidicolin. We observed little change in the behaviour of proteins involved in the initiation of DNA replication. In contrast, we detected a marked accumulation of the single stranded DNA binding factor RPA34 at sites of DNA replication. Finally, we demonstrate that proteins involved in the elongation step of DNA synthesis dissociate from replication foci in the presence of aphidicolin. Taken together, these data indicate that inhibition of processive DNA polymerases uncouples the initiation of DNA replication from subsequent elongation steps. We, therefore, propose that the replication machinery is made up of distinct functional sub-modules that allow a flexible and dynamic response to challenges during DNA replication.

Identification and characterization of KAT, a novel gene preferentially expressed in several human cancer cell lines.

We describe the molecular characterization of a novel human gene on chromosome 1q23.3, termed KAT, which is highly conserved among mammals. The KAT gene spans a genomic region of approximately 1.6 kilobases and consists of 4 exons encoding a 115 amino acid protein with a molecular mass of about 12.5 kDa. The gene is expressed in several human tissues, including kidney, liver, skeletal muscle, heart, colon, thymus, spleen, placenta and lung. We identified an alternatively spliced form, lacking exon 2, in human and mouse tissues. In silico analysis of expressed sequence tags, derived from different types of human tumors, revealed another splice variant. This transcript is characterized by retention of the third intron, leading to a truncated translation product. The KAT protein is localized around the nuclear membranes. It was found to be expressed in several breast, colon and lung carcinoma cell lines, but not in normal breast epithelial cell lines. In addition, KAT protein was detected in invasive ductal carcinoma, but not in adjacent tissues. This suggests a role of this gene in tumorigenesis.