Hisanobu Oda

We identified seven alternatively spliced forms of human 8-oxoguanine DNA glycosylase (OGG1) mRNAs, classified into two types based on their last exons (type 1 with exon 7: 1a and 1b; type 2 with exon 8: 2a to 2e). Types 1a and 2a mRNAs are major in human tissues. Seven mRNAs are expected to encode different polypeptides (OGG1-1a to 2e) that share their N terminus with the common mitochondrial targeting signal, and each possesses a unique C terminus. A 36-kDa polypeptide, corresponding to OGG1-1a recognized only by antibodies against the region containing helix-hairpin-helix-PVD motif, was copurified from the nuclear extract with an activity introducing a nick into DNA containing 8-oxoguanine. A 40-kDa polypeptide corresponding to a processed form of OGG1-2a was detected in their mitochondria using antibodies against its C terminus. Electron microscopic immunocytochemistry and subfractionation of the mitochondria revealed that OGG1-2a locates on the inner membrane of mitochondria. Deletion mutant analyses revealed that the unique C terminus of OGG1-2a and its mitochondrial targeting signal are essential for mitochondrial localization and that nuclear localization of OGG1-1a depends on the NLS at its C terminus.

Histone post-translational modifications impact many aspects of chromatin and nuclear function. Histone H4 Lys 20 methylation (H4K20me) has been implicated in regulating diverse processes ranging from the DNA damage response, mitotic condensation, and DNA replication to gene regulation. PR-Set7/Set8/KMT5a is the sole enzyme that catalyzes monomethylation of H4K20 (H4K20me1). It is required for maintenance of all levels of H4K20me, and, importantly, loss of PR-Set7 is catastrophic for the earliest stages of mouse embryonic development. These findings have placed PR-Set7, H4K20me, and proteins that recognize this modification as central nodes of many important pathways. In this review, we discuss the mechanisms required for regulation of PR-Set7 and H4K20me1 levels and attempt to unravel the many functions attributed to these proteins.

PR-Set7/Set8/KMT5A is the sole enzyme known to catalyze monomethylation of histone H4 lysine 20 (H4K20) and is present only in multicellular organisms that compact a large fraction of their DNA. We found that mouse embryos that are homozygous null mutants for the gene PR-Set7 display early embryonic lethality prior to the eight-cell stage. Death was due to the absence of PR-Set7 catalytic activity, since microinjection of the wild type, but not a catalytically inactive version, into two-cell embryos rescued the phenotype. A lack of PR-Set7 activity resulted not only in depletion of H4K20me1 but also in reduced levels of the H4K20me2/3 marks catalyzed by the Suv4-20h1/h2 enzymes, implying that H4K20me1 may be essential for the function of these enzymes to ensure the dimethylated and trimethylated states. Embryonic stem cells that were inducibly deleted for PR-Set7 passed through an initial G(2)/M phase, but the progeny were defective at the subsequent S and G(2)/M phases, exhibiting a delay in their cell cycle, accumulation at G(2)/M, massive DNA damage, and improper mitotic chromosome condensation. Cell cycle analysis after synchronization indicated that the defects were a consequence of decreased H4K20me1 due to the absence of PR-Set7. Most importantly, the lack of H4K20me1 also resulted in defects in chromosome condensation in interphase nuclei. These results demonstrate the critical role of H4K20 monomethylation in mammals in a developmental context.