SMARCAD1-mediated recruitment of the DNA mismatch repair protein MutLα to MutSα on damaged chromatin induces apoptosis in human cells

SMARCAD1 is an ATP-dependent histone octamer exchange factor with de novo nucleosome assembly activity

The adenosine 5′-triphosphate (ATP)–dependent chromatin remodeler SMARCAD1 acts on nucleosomes during DNA replication, repair, and transcription, but despite its implication in disease, information on its function and biochemical activities is scarce. Chromatin remodelers use the energy of ATP hydrolysis to slide nucleosomes, evict histones, or exchange histone variants. Here, we show that SMARCAD1 transfers the entire histone octamer from one DNA segment to another in an ATP-dependent manner but is also capable of de novo nucleosome assembly from histone octamer because of its ability to simultaneously bind all histones. We present a low-resolution cryo–electron microscopy structure of SMARCAD1 in complex with a nucleosome and show that the adenosine triphosphatase domains engage their substrate unlike any other chromatin remodeler. Our biochemical and structural data provide mechanistic insights into SMARCAD1-induced nucleosome disassembly and reassembly.

Exploiting DNA Endonucleases to Advance Mechanisms of DNA Repair

The earliest methods of genome editing, such as zinc-finger nucleases (ZFN) and transcription activator-like effector nucleases (TALENs), utilize customizable DNA-binding motifs to target the genome at specific loci. While these approaches provided sequence-specific gene-editing capacity, the laborious process of designing and synthesizing recombinant nucleases to recognize a specific target sequence, combined with limited target choices and poor editing efficiency, ultimately minimized the broad utility of these systems. The discovery of clustered regularly interspaced short palindromic repeat sequences (CRISPR) in Escherichia coli dates to 1987, yet it was another 20 years before CRISPR and the CRISPR-associated (Cas) proteins were identified as part of the microbial adaptive immune system, by targeting phage DNA, to fight bacteriophage reinfection. By 2013, CRISPR/Cas9 systems had been engineered to allow gene editing in mammalian cells. The ease of design, low cytotoxicity, and increased efficiency have made CRISPR/Cas9 and its related systems the designer nucleases of choice for many. In this review, we discuss the various CRISPR systems and their broad utility in genome manipulation. We will explore how CRISPR-controlled modifications have advanced our understanding of the mechanisms of genome stability, using the modulation of DNA repair genes as examples.

DNA mismatch repair in the chromatin context: Mechanisms and therapeutic potential

DNA mismatch repair (MMR) maintains genomic stability primarily by correcting replication errors. Defects in MMR lead to cancers and cause resistance to many chemotherapeutic drugs. Emerging evidence reveals that MMR is coupled with replication and precisely regulated in the context of chromatin; strikingly, tumors defective in MMR are highly responsive to immune checkpoint blockade therapy. As a tribute to Dr. Samuel Wilson for his many scientific contributions to the field of DNA repair and his leadership as Editor-in-Chief of the journal DNA Repair, we summarize recent developments in research on MMR at the chromatin level, its implications for tumorigenesis, and its therapeutic potential.

Mismatch Repair Pathway, Genome Stability and Cancer

The acquisition of genomic instability is one of the key characteristics of the cancer cell, and microsatellite instability (MSI) is an important segment of this phenomenon. This review aims to describe the mismatch DNA repair (MMR) system whose deficiency is responsible for MSI and discuss the cellular roles of MMR genes. Malfunctioning of the MMR repair pathway increases the mutational burden of specific cancers and is often involved in its etiology, sometimes as an influential bystander and sometimes as the main driving force. Detecting the presence of MSI has for a long time been an important part of clinical diagnostics, but has still not achieved its full potential. The MSI blueprints of specific tumors are useful for precize grading, evaluation of cancer chance and prognosis and to help us understand how and why therapy-resistant cancers arise. Furthermore, evidence indicates that MSI is an important predictive biomarker for the application of immunotherapy.

MLH1-mediated recruitment of FAN1 to chromatin for the induction of apoptosis triggered by O6 -methylguanine

O⁶ -Methylguanines (O⁶ -meG), which are produced in DNA by the action of alkylating agents, are mutagenic and cytotoxic, and induce apoptosis in a mismatch repair (MMR) protein-dependent manner. To understand the molecular mechanism of O⁶ -meG-induced apoptosis, we performed functional analyses of FANCD2 and FANCI-associated nuclease 1 (FAN1), which was identified as an interacting partner of MLH1. Immunoprecipitation analyses showed that FAN1 interacted with both MLH1 and MSH2 after treatment with N-methyl-N-nitrosourea (MNU), indicating the formation of a FAN1-MMR complex. In comparison with control cells, FAN1-knockdown cells were more resistant to MNU, and the appearances of a sub-G₁ population and caspase-9 activation were suppressed. FAN1 formed nuclear foci in an MLH1-dependent manner after MNU treatment, and some were colocalized with both MLH1 foci and single-stranded DNA (ssDNA) created at damaged sites. Under the same condition, FANCD2 also formed nuclear foci, although it was dispensable for the formation of FAN1 foci and ssDNA. MNU-induced formation of ssDNA was dramatically suppressed in FAN1-knockdown cells. We therefore propose that FAN1 is loaded on chromatin through the interaction with MLH1 and produces ssDNA by its exonuclease activity, which contributes to the activation of the DNA damage response followed by the induction of apoptosis triggered by O⁶ -meG.