Current genome-editing technologies introduce double-stranded (ds) DNA breaks at a target locus as the first step to gene correction. Although most genetic diseases arise from point mutations, current approaches to point mutation correction are inefficient and typically induce an abundance of random insertions and deletions (indels) at the target locus resulting from the cellular response to dsDNA breaks. Here we report the development of 'base editing', a new approach to genome editing that enables the direct, irreversible conversion of one target DNA base into another in a programmable manner, without requiring dsDNA backbone cleavage or a donor template. We engineered fusions of CRISPR/Cas9 and a cytidine deaminase enzyme that retain the ability to be programmed with a guide RNA, do not induce dsDNA breaks, and mediate the direct conversion of cytidine to uridine, thereby effecting a C→T (or G→A) substitution. The resulting 'base editors' convert cytidines within a window of approximately five nucleotides, and can efficiently correct a variety of point mutations relevant to human disease. In four transformed human and murine cell lines, second- and third-generation base editors that fuse uracil glycosylase inhibitor, and that use a Cas9 nickase targeting the non-edited strand, manipulate the cellular DNA repair response to favour desired base-editing outcomes, resulting in permanent correction of ~15-75% of total cellular DNA with minimal (typically ≤1%) indel formation. Base editing expands the scope and efficiency of genome editing of point mutations.

/pdf/programmable-editing-of-a-target-base-in-genomic-dna-without-53ykhw8ya8.pdf

Programmable editing of a target base in genomic DNA without double-stranded DNA cleavage

DNA mismatch repair (MMR) is a highly conserved biological pathway that plays a key role in maintaining genomic stability. The specificity of MMR is primarily for base-base mismatches and insertion/deletion mispairs generated during DNA replication and recombination. MMR also suppresses homeologous recombination and was recently shown to play a role in DNA damage signaling in eukaryotic cells. Escherichia coli MutS and MutL and their eukaryotic homologs, MutSalpha and MutLalpha, respectively, are key players in MMR-associated genome maintenance. Many other protein components that participate in various DNA metabolic pathways, such as PCNA and RPA, are also essential for MMR. Defects in MMR are associated with genome-wide instability, predisposition to certain types of cancer including hereditary non-polyposis colorectal cancer, resistance to certain chemotherapeutic agents, and abnormalities in meiosis and sterility in mammalian systems.

/pdf/mechanisms-and-functions-of-dna-mismatch-repair-11cevavj74.pdf

Mechanisms and functions of DNA mismatch repair

Small Angle Scattering of X-Rays

DNA mismatch repair: functions and mechanisms.

Background. Colorectal cancer (CRC) is the third most common cancer in the world. In 75% CRC develops sporadically, in 25% hereditary or as a consequence of inflammatory bowel disease. CRC carcinogenesis develops over many years. The cause of CRC in 85% is chromosomal instability (CIN) and in 15% microsatellite instability (MSI-H), where hereditary nonpolyposis colorectal cancer (HNPCC) represents 10-20%. Microsatellite sequences (MS) are repeated sequences of short stretches of DNA all over the genome. Microsatellite stability (MSS) means MS are the same in each cell of an individual, whereas microsatellite instability (MSI-H) means MS differ in normal and cancer cells of an individual. The cause of MSI-H is a damaged mismatch repair mechanism (MMR), with the most important MMR proteins being MSH2, MLH1 and MSH6. Conclusions. MSI-H seems to be an important prognostic factor in CRC and an important predictive factor of CRC chemotherapeutic treatment efficacy. Clinical trials conducted until now have shown contradictory findings in different chemotherapeutic settings, adjuvant and palliative; therefore MSI-H is going to be the object of the future research. The future of cancer treatment is in the individualized therapy based on molecular characteristics of the tumour, such as MSI-H in CRC.

/pdf/microsatellite-instability-in-colorectal-cancer-4gx0ndbtm5.pdf

Microsatellite instability in colorectal cancer

MutLα (MLH1–PMS2) is a latent endonuclease that is activated in a mismatch-, MutSα-, proliferating cell nuclear antigen (PCNA)-, replication factor C (RFC)-, and ATP-dependent manner, with nuclease action directed to the heteroduplex strand that contains a preexisting break. RFC depletion experiments and use of linear DNAs indicate that RFC function in endonuclease activation is limited to PCNA loading. Whereas nicked circular heteroduplex DNA is a good substrate for PCNA loading and for endonuclease activation on the incised strand, covalently closed, relaxed circular DNA is a poor substrate for both reactions. However, covalently closed supercoiled or bubble-containing relaxed heteroduplexes, which do support PCNA loading, also support MutLα activation, but in this case cleavage strand bias is largely abolished. Based on these findings we suggest that PCNA has two roles in MutLα function: The clamp is required for endonuclease activation, an effect that apparently involves interaction of the two proteins, and by virtue of its loading orientation, PCNA determines the strand direction of MutLα incision. These results also provide a potential mechanism for activation of mismatch repair on nonreplicating DNA, an effect that may have implications for the somatic phase of triplet repeat expansion.

https://www.pnas.org/content/pnas/107/37/16066.full.pdf

PCNA function in the activation and strand direction of MutLα endonuclease in mismatch repair

DNA Mismatch Repair: Functions and Mechanisms

DNA mismatch repair is a conserved antimutagenic pathway that maintains genomic stability through rectification of DNA replication errors and attenuation of chromosomal rearrangements. Paradoxically, mutagenic action of mismatch repair has been implicated as a cause of triplet repeat expansions that cause neurological diseases such as Huntington disease and myotonic dystrophy. This mutagenic process requires the mismatch recognition factor MutSβ and the MutLα (and/or possibly MutLγ) endonuclease, and is thought to be triggered by the transient formation of unusual DNA structures within the expanded triplet repeat element. This review summarizes the current knowledge of DNA mismatch repair involvement in triplet repeat expansion, which encompasses in vitro biochemical findings, cellular studies, and various in vivo transgenic animal model experiments. We present current mechanistic hypotheses regarding mismatch repair protein function in mediating triplet repeat expansions and discuss potential therapeutic approaches targeting the mismatch repair pathway.

/pdf/dna-triplet-repeat-expansion-and-mismatch-repair-fhobij7fci.pdf

DNA triplet repeat expansion and mismatch repair

The hemimethylated d(GATC) sequence that directs Escherichia coli mismatch repair can reside on either side of a mismatch at a separation distance of 1,000 bp or more. Initiation of repair involves the mismatch-, MutS-, and MutL-dependent activation of MutH endonuclease, which incises the unmethylated strand at the d(GATC) sequence, with the ensuing strand break serving as the loading site for the appropriate 3′-to-5′ or 5′-to-3′ excision system. However, the mechanism responsible for the coordinated recognition of the mismatch and a hemimodified d(GATC) site is uncertain. We show that a protein roadblock (EcoRIE111Q, a hydrolytically defective form of EcoRI endonuclease) placed on the helix between the two DNA sites inhibits MutH activation by 70–80% and that events that escape inhibition are attributable, at least in part, to diffusion of EcoRIE111Q away from its recognition site. We also demonstrate that a double-strand break located within the shorter path linking the mismatch and a d(GATC) site in a circular heteroduplex abolishes MutH activation, whereas a double-strand break within the longer path is without effect. These findings support the idea that initiation of mismatch repair involves signaling along the helix contour.

https://www.pnas.org/content/pnas/104/31/12709.full.pdf

Anna Pluciennik

Papers

DNA mismatch repair: functions and mechanisms.

PCNA function in the activation and strand direction of MutLα endonuclease in mismatch repair

DNA Mismatch Repair: Functions and Mechanisms

DNA triplet repeat expansion and mismatch repair

Protein roadblocks and helix discontinuities are barriers to the initiation of mismatch repair