Different effects of histone H1 on de novo DNA methylation in vitro depend on both the DNA base composition and the DNA methyltransferase

Protection of DNA against direct radiation damage by complex formation with positively charged polypeptides

Radioprotection of DNA from direct-type radiation damage by histones has been studied in model systems using complexes of positively charged polypeptides (PCPs) with DNA. PCPs bind to DNA via ionic interactions mimicking the mode of DNA-histone binding. Direct radiation damage to DNA in films of DNA-PCP complexes was quantified as unaltered base release, which correlates closely with DNA strand breaks. All types of PCPs tested protected DNA from radiation, with the maximum radioprotection being approximately 2.5-fold compared with non-complexed DNA. Conformational changes of the DNA induced by PCPs or repair of free radical damage on the DNA sugar moiety by PCPs are considered the most feasible mechanisms of radioprotection of DNA. The degree of radioprotection of DNA by polylysine (PL) increased dramatically on going from pure DNA to a molar ratio of PL monomer:DNA nucleotide approximately 1:2, while a further increase in the PL:DNA ratio did not offer more radioprotection. This concentration dependence is in agreement with the model of PCP binding to DNA that assumes preferential binding of positively charged side groups to DNA phosphates in the minor groove, so that the maximum occupancy of all minor-groove PCP binding sites is at a molar ratio of PCP:DNA = 1:2.

DNA methylation, methyltransferases, and cancer

The field of epigenetics has recently moved to the forefront of studies relating to diverse processes such as transcriptional regulation, chromatin structure, genome integrity, and tumorigenesis. Recent work has revealed how DNA methylation and chromatin structure are linked at the molecular level and how methylation anomalies play a direct causal role in tumorigenesis and genetic disease. Much new information has also come to light regarding the cellular methylation machinery, known as the DNA methyltransferases, in terms of their roles in mammalian development and the types of proteins they are known to interact with. This information has forced a new view for the role of DNA methyltransferases. Rather than enzymes that act in isolation to copy methylation patterns after replication, the types of interactions discovered thus far indicate that DNA methyltransferases may be components of larger complexes actively involved in transcriptional control and chromatin structure modulation. These new findings will likely enhance our understanding of the myriad roles of DNA methylation in disease as well as point the way to novel therapies to prevent or repair these defects.

Human DNA-(cytosine-5) methyltransferase-PCNA complex as a target for p21WAF1

DNA-(cytosine-5) methyltransferase (MCMT) methylates newly replicated mammalian DNA, but the factors regulating this activity are unknown. Here, MCMT is shown to bind proliferating cell nuclear antigen (PCNA), an auxiliary factor for DNA replication and repair. Binding of PCNA requires amino acids 163 to 174 of MCMT, occurs in intact cells at foci of newly replicated DNA, and does not alter MCMT activity. A peptide derived from the cell cycle regulator p21(WAF1) can disrupt the MCMT-PCNA interaction, which suggests that p21(WAF1) may regulate methylation by blocking access of MCMT to PCNA. MCMT and p21(WAF1) may be linked in a regulatory pathway, because the extents of their expression are inversely related in both SV40-transformed and nontransformed cells.