Histone Methylation by SETD1A Protects Nascent DNA through the Nucleosome Chaperone Activity of FANCD2.

Components of the Fanconi anemia and homologous recombination pathways play a vital role in protecting newly replicated DNA from uncontrolled nucleolytic degradation, safeguarding genome stability. Here we report that histone methylation by the lysine methyltransferase SETD1A is crucial for protecting stalled replication forks from deleterious resection. Depletion of SETD1A sensitizes cells to replication stress and leads to uncontrolled DNA2-dependent resection of damaged replication forks. The ability of SETD1A to prevent degradation of these structures is mediated by its ability to catalyze methylation on Lys4 of histone H3 (H3K4) at replication forks, which enhances FANCD2-dependent histone chaperone activity. Suppressing H3K4 methylation or expression of a chaperone-defective FANCD2 mutant leads to loss of RAD51 nucleofilament stability and severe nucleolytic degradation of replication forks. Our work identifies epigenetic modification and histone mobility as critical regulatory mechanisms in maintaining genome stability by restraining nucleases from irreparably damaging stalled replication forks.

Tuning the Multifunctionality of Iron Oxide Nanoparticles Using Self-Assembled Mixed Lipid Layers.

We present an approach to tuning the multifunctionality of iron oxide nanoparticles (IONs) using mixed self-assembled monolayers of cationic lipid and anionic polyethylene glycol (PEG) lipid. By forming stable, monodispersed lipid-coated IONs (L-IONs) through a solvent-exchange technique, we were able to demonstrate the relationship between surface charge, the magnetic transverse relaxivity (r2 from T2-weighted images), and the binding capacity of small interfering ribonucleic acids (siRNAs) as a function of the cationic-to-anionic (PEG) lipid ratio. These properties were controlled by the cationic charge and the PEG conformation; relaxivity and siRNA binding could be varied in the mushroom and brush regimes but not at high brush densities. In vitro results combining cell viability, uptake, and transfection efficiency using HeLa cells suggest that the functional physicochemical and biological properties of L-IONs may be best achieved using catanionic lipid coatings near equimolar ratios of cationic to anionic PEG-lipids.

Cyclic peptide-capped gold nanoparticles for enhanced siRNA delivery.

Previously, we have reported the synthesis of a homochiral l-cyclic peptide [WR]5 and its use for delivery of anti-HIV drugs and biomolecules. A physical mixture of HAuCl4 and the peptide generated peptide-capped gold nanoparticles. Here, [WR]5 and [WR]5-AuNPs were tested for their efficiency to deliver a small interfering RNA molecule (siRNA) in human cervix adenocarcinoma (HeLa) cells. Flow cytometry investigation revealed that the intracellular uptake of a fluorescence-labeled non-targeting siRNA (200 nM) was enhanced in the presence of [WR]5 and [WR]5-AuNPs by 2- and 3.8-fold when compared with that of siRNA alone after 24 h incubation. Comparative toxicity results showed that [WR]5 and [WR]5-AuNPs were less toxic in cells compared to other available carrier systems, such as Lipofectamine.

FANCD2 Binding to H4K20me2 via a Methyl-Binding Domain Is Essential for Efficient DNA Cross-Link Repair.

Fanconi anemia (FA) is an inherited disease characterized by bone marrow failure and increased cancer risk. FA is caused by mutation of any 1 of 22 genes, and the FA proteins function cooperatively to repair DNA interstrand cross-links (ICLs). A central step in the activation of the FA pathway is the monoubiquitination of the FANCD2 and FANCI proteins, which occurs within chromatin. How FANCD2 and FANCI are anchored to chromatin remains unknown. In this study, we identify and characterize a FANCD2 histone-binding domain (HBD) and embedded methyl-lysine-binding domain (MBD) and demonstrate binding specificity for H4K20me2. Disruption of the HBD/MBD compromises FANCD2 chromatin binding and nuclear focus formation and its ability to promote error-free DNA interstrand cross-link repair, leading to increased error-prone repair and genome instability. Our study functionally describes the first FA protein chromatin reader domain and establishes an important link between this human genetic disease and chromatin plasticity.

Understanding the Histone DNA Repair Code: H4K20me2 Makes Its Mark.

Chromatin is a highly compact structure that must be rapidly rearranged in order for DNA repair proteins to access sites of damage and facilitate timely and efficient repair. Chromatin plasticity is achieved through multiple processes, including the posttranslational modification of histone tails. In recent years, the impact of histone posttranslational modification on the DNA damage response has become increasingly well recognized, and chromatin plasticity has been firmly linked to efficient DNA repair. One particularly important histone posttranslational modification process is methylation. Here, we focus on the regulation and function of H4K20 methylation (H4K20me) in the DNA damage response and describe the writers, erasers, and readers of this important chromatin mark as well as the combinatorial histone posttranslational modifications that modulate H4K20me recognition. Finally, we discuss the central role of H4K20me in determining if DNA double-strand breaks (DSB) are repaired by the error-prone, nonhomologous DNA end joining pathway or the error-free, homologous recombination pathway. This review article discusses the regulation and function of H4K20me2 in DNA DSB repair and outlines the components and modifications that modulate this important chromatin mark and its fundamental impact on DSB repair pathway choice. Mol Cancer Res; 16(9); 1335-45. ©2018 AACR.