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

Bloom syndrome DNA helicase mitigates mismatch repair-dependent apoptosis

Generation of O6-methylguanine (O6-meG) by DNA-alkylating agents such as N-methyl N-nitrosourea (MNU) activates the multiprotein mismatch repair (MMR) complex and the checkpoint response involving ATR/CHK1 and ATM/CHK2 kinases, which may in turn trigger cell cycle arrest and apoptosis. The Bloom syndrome DNA helicase BLM interacts with the MMR complex, suggesting functional relevance to repair and checkpoint responses. We observed a strong interaction of BLM with MMR proteins in HeLa cells upon treatment with MNU as evidenced by co-immunoprecipitation as well as colocalization in the nucleus as revealed by dual immunofluorescence staining. Knockout of BLM sensitized HeLa MR cells to MNU-induced cell cycle disruption and enhanced expression of the apoptosis markers cleaved caspase-9 and PARP1. MNU-treated BLM-deficient cells also exhibited a greater number of 53BP1 foci and greater phosphorylation levels of H2AX at S139 and RPA32 at S8, indicating the accumulation of DNA double-strand breaks. These findings suggest that BLM prevents double-strand DNA breaks during the MMR-dependent DNA damage response and mitigates O6-meG-induced apoptosis.

Realistic aspects behind the application of the rat model of chemically-induced mammary cancer: Practical guidelines to obtain the best results

Cancer is one of the most important public health problems worldwide. Despite the great contribution of in-vitro studies for biomedical research, animals are essential to study diseases' biopathology and diagnosis, and searching for new preventive and therapeutic strategies. Breast cancer is currently the most common cancer globally, accounting for 12.5% of all new annual cancer cases worldwide. Although the rat model of mammary cancer chemically-induced is widely used to study this disease, there is a lack of standardization in procedures for cancer induction, sample collection, and analysis. Therefore, it is important to provide a practical guide for researchers aiming to work with this model to make the analysis of results more uniform. Thus, in this review, we provide the researchers with a detailed step-by-step guide to implement a rat model of mammary cancer, based on our wide experience in this field, to obtain the best results, maximum throughput of each experiment, and easy comparison among researches.

Rad5 and Its Human Homologs, HLTF and SHPRH, Are Novel Interactors of Mismatch Repair

DNA mismatch repair (MMR) repairs replication errors, and MMR defects play a role in both inherited cancer predisposition syndromes and in sporadic cancers. MMR also recognizes mispairs caused by environmental and chemotherapeutic agents; however, in these cases mispair recognition leads to apoptosis and not repair. Although mutation avoidance by MMR is fairly well understood, MMR-associated proteins are still being identified. We performed a bioinformatic analysis that implicated Saccharomyces cerevisiae Rad5 as a candidate for interacting with the MMR proteins Msh2 and Mlh1. Rad5 is a DNA helicase and E3 ubiquitin ligase involved in post-replicative repair and damage tolerance. We confirmed both interactions and found that the Mlh1 interaction is mediated by a conserved Mlh1-interacting motif (MIP box). Despite this, we did not find a clear role for Rad5 in the canonical MMR mutation avoidance pathway. The interaction of Rad5 with Msh2 and Mlh1 is conserved in humans, although each of the Rad5 human homologs, HLTF and SHPRH, shared only one of the interactions: HLTF interacts with MSH2, and SHPRH interacts with MLH1. Moreover, depletion of SHPRH, but not HLTF, results in a mild increase in resistance to alkylating agents although not as strong as loss of MMR, suggesting gene duplication led to specialization of the MMR-protein associated roles of the human Rad5 homologs. These results provide insights into how MMR accessory factors involved in the MMR-dependent apoptotic response interact with the core MMR machinery and have important health implications into how human cells respond to environmental toxins, tumor development, and treatment choices of tumors with defects in Rad5 homologs.

Novel plasmids for the fluorescence-based evaluation of DNA mismatch repair in human cells

Mismatch repair (MMR) is a highly conserved DNA repair pathway that corrects mismatched bases during DNA replication. The biological significance of MMR in human cells is underscored by the fact that dysfunction of the MMR pathway results in Lynch syndrome, which is associated with a genetic predisposition to different cancer types. We have previously established a reporter mismatch plasmid to evaluate MMR using fluorescent proteins in living cells. However, the preparation of these plasmids requires significant amounts of time and money, which reduces their broad applicability. To overcome the abovementioned limitations, we produced in this study a novel reporter plasmid, pBSII NLS-MC-EGFP-tdTomato (pBET2), that can be used in the oligo swapping method. In this method, a nicking endonuclease produces a single-stranded DNA gap on a double-stranded DNA plasmid that can be replaced by ligation with synthetic oligonucleotides. It is significantly easier and more user-friendly than previous assays, which require in vitro DNA synthesis with single-stranded plasmid DNA and purification using ultracentrifugation in cesium chloride-ethidium bromide gradients. The plasmid also contains a nicking site that allows the MMR repair machinery to efficiently distinguish the newly synthesized strand as a target for repair. In addition, a nuclear localization signal facilitates green fluorescent protein expression in the nucleus, which helps to verify the effectiveness of MMR using fluorescence microscopy. Similar to the previous reporter plasmid, this construct facilitates the assessment of MMR proficiency in human living cells via the expression of fluorescent proteins while overcoming many of the negative aspects of the previous protocol.

FAN1 controls mismatch repair complex assembly via MLH1 retention to stabilize CAG repeat expansion in Huntington's disease

CAG repeat expansion in the HTT gene drives Huntington's disease (HD) pathogenesis and is modulated by DNA damage repair pathways. In this context, the interaction between FAN1, a DNA-structure-specific nuclease, and MLH1, member of the DNA mismatch repair pathway (MMR), is not defined. Here, we identify a highly conserved SPYF motif at the N terminus of FAN1 that binds to MLH1. Our data support a model where FAN1 has two distinct functions to stabilize CAG repeats. On one hand, it binds MLH1 to restrict its recruitment by MSH3, thus inhibiting the assembly of a functional MMR complex that would otherwise promote CAG repeat expansion. On the other hand, it promotes accurate repair via its nuclease activity. These data highlight a potential avenue for HD therapeutics in attenuating somatic expansion.

DNA Mismatch Repair and its Role in Huntington's Disease

DNA mismatch repair (MMR) is a highly conserved genome stabilizing pathway that corrects DNA replication errors, limits chromosomal rearrangements, and mediates the cellular response to many types of DNA damage. Counterintuitively, MMR is also involved in the generation of mutations, as evidenced by its role in causing somatic triplet repeat expansion in Huntington’s disease (HD) and other neurodegenerative disorders. In this review, we discuss the current state of mechanistic knowledge of MMR and review the roles of key enzymes in this pathway. We also present the evidence for mutagenic function of MMR in CAG repeat expansion and consider mechanistic hypotheses that have been proposed. Understanding the role of MMR in CAG expansion may shed light on potential avenues for therapeutic intervention in HD.

FAN1, a DNA Repair Nuclease, as a Modifier of Repeat Expansion Disorders

FAN1 encodes a DNA repair nuclease. Genetic deficiencies, copy number variants, and single nucleotide variants of FAN1 have been linked to karyomegalic interstitial nephritis, 15q13.3 microdeletion/microduplication syndrome (autism, schizophrenia, and epilepsy), cancer, and most recently repeat expansion diseases. For seven CAG repeat expansion diseases (Huntington's disease (HD) and certain spinocerebellar ataxias), modification of age of onset is linked to variants of specific DNA repair proteins. FAN1 variants are the strongest modifiers. Non-coding disease-delaying FAN1 variants and coding disease-hastening variants (p.R507H and p.R377W) are known, where the former may lead to increased FAN1 levels and the latter have unknown effects upon FAN1 functions. Current thoughts are that ongoing repeat expansions in disease-vulnerable tissues, as individuals age, promote disease onset. Fan1 is required to suppress against high levels of ongoing somatic CAG and CGG repeat expansions in tissues of HD and FMR1 transgenic mice respectively, in addition to participating in DNA interstrand crosslink repair. FAN1 is also a modifier of autism, schizophrenia, and epilepsy. Coupled with the association of these diseases with repeat expansions, this suggests a common mechanism, by which FAN1 modifies repeat diseases. Yet how any of the FAN1 variants modify disease is unknown. Here, we review FAN1 variants, associated clinical effects, protein structure, and the enzyme's attributed functional roles. We highlight how variants may alter its activities in DNA damage response and/or repeat instability. A thorough awareness of the FAN1 gene and FAN1 protein functions will reveal if and how it may be targeted for clinical benefit.