Study Notes on Mismatch Repair (MMR) Lecture by Prof. Enni Markkanen

Mismatch Repair (MMR)

Prof. Enni Markkanen
Institute of Veterinary Pharmacology & Toxicology
University of Zürich
Email: enni.markkanen@vetpharm.uzh.ch
Bio 257 – 06.10.2025

Goals of This Lecture

The primary objectives of this lecture include:

• Understanding and explaining:

  • How the accuracy of genome duplication is achieved

  • The role and mechanism of proofreading by DNA polymerases

  • Types of lesions subject to MMR

  • Key criteria for MMR

  • Different steps of MMR in eukaryotes and prokaryotes

  • How MMR can discriminate between new and old strands in both leading and lagging-strand contexts

  • The roles of MMR in cancer

Slide 3 – The Problem of Accurate Duplication of the Genome

Key Concepts and Explanations:

• The DNA replication process must be extremely accurate to preserve genetic information.

• Errors during replication can lead to mutations that may cause diseases, including cancer.

• The error rate of DNA polymerases (Pols) depends on two key factors:

  1. Nucleotide selectivity: The enzyme’s ability to correctly pair nucleotides (A–T, G–C).

  2. Proofreading activity: The capacity to remove incorrectly incorporated bases.

• Proofreading improves accuracy dramatically — by up to 10⁵–10⁶ times.

• Different DNA polymerases (e.g., Pol α, δ, ε) have distinct intrinsic accuracies depending on their roles in replication.

Explanation of Visuals:

• The graph shows error frequency (y-axis) versus DNA polymerase types (x-axis).

• Polymerases with proofreading ability (like Pol δ and Pol ε) have much lower error rates compared to those without (like Pol α).

• The green section emphasizes the improvement in accuracy due to proofreading.

Glossary:

• DNA polymerase (Pol): Enzyme that synthesizes new DNA strands.

• Proofreading activity: Exonuclease function that removes mismatched bases during replication.

• Error rate: Frequency of incorrect nucleotide incorporation.

Key Takeaway:

Slide 4 – DNA Polymerization vs. Proofreading

Key Concepts and Explanations:

• DNA polymerases have two active sites:

  • A polymerase site (for adding nucleotides).

  • An exonuclease site (for proofreading and removing errors).

• During normal replication, the polymerase continuously elongates the new DNA strand.

• When a wrong nucleotide is incorporated:

  • The mismatch distorts the DNA structure.

  • The polymerase pauses and transfers the growing strand to the exonuclease site.

  • The incorrect base is excised (cut out), and the strand is returned to the polymerase site to continue replication.

Explanation of Visuals:

• The figure contrasts two processes:

  • Left: Normal DNA synthesis (smooth elongation).

  • Right: Proofreading step, where the strand is repositioned and the mismatched base is removed.

• Arrows show the movement between polymerase and exonuclease sites.

• The schematic illustrates the balance between speed and accuracy in DNA replication.

Glossary:

• Exonuclease site: Region of the enzyme that removes mismatched nucleotides.

• Mismatch: Incorrect base pairing (e.g., A–C instead of A–T).

• Elongation: Addition of nucleotides to the growing DNA strand.

Key Takeaway:

Slide 5 – Polymerization vs. Proofreading (Molecular View)

Key Concepts and Explanations:

• This slide zooms into the structural mechanism of proofreading within DNA polymerase.

• The enzyme is shaped like a right hand, with domains often referred to as “palm,” “fingers,” and “thumb.”

• Polymerization mode:

  • The active site in the palm adds nucleotides to the growing DNA chain.

• Proofreading mode:

  • When a mismatch occurs, the abnormal geometry of the DNA is recognized.

  • The strand is shifted to the 3′→5′ exonuclease site (Exo), where the incorrect base is removed.

• After correction, the strand returns to the polymerase site for normal synthesis.

Explanation of Visuals:

• Left panel: Correct nucleotide incorporation (smooth base pairing).

• Right panel: Proofreading process (mismatch causes DNA to move from the polymerase site to the exonuclease site).

• Blue arrows indicate the direction of transfer between “Palm” (Pol site) and “Exo” site.

Glossary:

• Palm/Fingers/Thumb: Structural domains of DNA polymerase.

• Exonuclease activity: Enzymatic removal of nucleotides from the DNA end.

• 3′→5′ direction: The backward direction in which proofreading removes bases.

Key Takeaway:

in human cells, δ, ε, and γ are the main polymerases that proofread.

Slide 6 – Effect of Proofreading on Tumorigenesis

Key Concepts and Explanations:

• Proofreading is crucial for preventing mutations that can lead to cancer.

• Mutations in the proofreading domain of DNA polymerase genes (especially Pol δ and Pol ε) cause replication errors to accumulate.

• In mice, defective proofreading (mutations in Pol ε exonuclease domain) leads to:

  • Accelerated mutation rates.

  • Early onset of tumors and reduced survival.

• This demonstrates the protective role of proofreading against genome instability and cancer development.

Explanation of Visuals:

• The survival curve compares wild-type mice (normal proofreading) vs. Pol ε mutants (defective proofreading).

• The mutant line shows a steep decline in survival, indicating early tumor development.

• The x-axis shows age (months), and the y-axis shows survival (%).

Glossary:

• Tumorigenesis: Formation of tumors caused by uncontrolled cell growth.

• Pol ε (DNA polymerase epsilon): Major enzyme for leading-strand synthesis during DNA replication.

• Exonuclease domain: Region responsible for proofreading and error correction.

Key Takeaway:

Slide 7 – Effect of Proofreading on Tumorigenesis (continued)

Key Concepts and Explanations:

• This slide reinforces that defective proofreading in DNA polymerase ε (Pol ε) dramatically increases tumor formation in mice.

• Proofreading mutations (e.g., Pol ε exonuclease mutants) prevent the removal of replication errors, allowing mutations to accumulate with every cell division.

• Mice carrying such mutations show rapid tumor development and reduced lifespan, confirming that proofreading is a key tumor-suppressive mechanism.

Explanation of Visuals:

• The survival curve compares wild-type (normal proofreading) and Pol ε mutants (defective proofreading).

• The mutant (green line) shows a sharp decline in survival within 12 months, indicating early death from tumor burden.

• The x-axis represents age in months, and the y-axis represents survival (%).

Glossary:

• Proofreading-defective mutant: A polymerase with an inactive exonuclease domain.

• Pol ε (DNA polymerase epsilon): Major polymerase for the leading strand during replication.

• Tumorigenesis: The process of forming malignant tumors due to mutation accumulation.

Key Takeaway:

Slide 8 – The Problem of Accurate Duplication of the Genome (Revisited)

Key Concepts and Explanations:

• DNA polymerases achieve remarkable accuracy — an average of one error per 10⁷ nucleotides replicated.

• This high fidelity depends on:

  1. Nucleotide selectivity

  2. Proofreading

  3. Mismatch repair (MMR) — the third key correction system.

• If any of these three mechanisms fail, replication errors can persist, leading to genomic instability and cancer predisposition (e.g., Lynch syndrome due to MMR defects).

Explanation of Visuals:

• The figure shows how each level of error correction (selectivity, proofreading, mismatch repair) progressively reduces the replication error rate.

• Color gradient (orange to blue) illustrates decreasing error frequency as additional fidelity mechanisms act.

• The text box emphasizes that MMR defects, particularly in mismatch repair genes, are strongly linked to Lynch syndrome.

Glossary:

• MMR (Mismatch Repair): System that corrects base-pair mismatches left after replication.

• Lynch Syndrome: Inherited cancer predisposition caused by MMR gene mutations.

• Replication fidelity: Accuracy of DNA copying during cell division.

Key Takeaway:

Slide 9 – Mismatches Are Atypical “DNA Lesions” (1)

Key Concepts and Explanations:

• Unlike UV or chemical lesions, mismatches are not damaged bases — they are normal bases incorrectly paired (e.g., G–T or A–C).

• Mismatches arise when replication errors escape proofreading.

• They are transient: they only exist while the two DNA strands are still annealed (before the next replication cycle).

• Mismatch repair (MMR) must act immediately after replication, while the original (parental) strand can still be identified.

Explanation of Visuals:

• Examples of base-pair mismatches are shown: G–G, G–A, T–A, C–A, etc.

• These are labeled as “undamaged bases” that form incorrect pairings.

• The text box below highlights that MMR must occur before the strands separate, otherwise the mutation becomes permanent.

Glossary:

• Mismatch: Incorrectly paired bases that do not follow normal Watson–Crick rules.

• Annealed strands: Paired complementary DNA strands before replication is completed.

• MMR: Pathway recognizing and fixing base mismatches and small insertions/deletions.

Key Takeaway:

Slide 10 – Mismatches Are Atypical “DNA Lesions” (2)

Key Concepts and Explanations:

• Mismatches also include insertions and deletions (IDLs) that occur in repetitive DNA regions, such as microsatellites.

• These regions are prone to DNA polymerase slippage during replication, causing frameshifts.

• When MMR fails to correct these, microsatellite instability (MSI) arises — a hallmark of MMR-deficient tumors.

• MMR acts while both DNA strands are still paired, detecting distortions caused by small loops or misaligned bases.

Explanation of Visuals:

• Diagrams show:

  • Base-pair mismatches and small loops formed by insertions or deletions.

  • Repetitive “A” or “T” tracts where slippage occurs.

• The boxed note highlights microsatellite instability as a diagnostic feature of MMR-deficient cancers.

Glossary:

• IDL (Insertion–Deletion Loop): Small DNA loop formed when bases are mistakenly added or skipped.

• Microsatellites: Short, repetitive DNA sequences prone to slippage.

• Microsatellite instability (MSI): Condition where microsatellite lengths vary due to defective MMR.

Key Takeaway:

Slide 11 – Specific Problems of IDLs

Key Concepts and Explanations:

• Strand slippage during DNA replication in repetitive sequences creates IDLs.

• Two outcomes depend on whether the polymerase’s exonuclease proofreading is active:

  • Exonuclease activated: The slippage is recognized, and the error is removed.

  • Exonuclease inactive: The slippage persists, forming an IDL that must be corrected by MMR.

• In MMR-deficient cells, these IDLs accumulate, causing microsatellite instability (MSI) and frameshift mutations.

• MSI is therefore a diagnostic marker for MMR-defective tumors (e.g., in colorectal or endometrial cancers).

Explanation of Visuals:

• Two DNA strands are shown with repetitive “A” sequences (microsatellite regions).

  • Left: Proofreading active — slippage recognized and corrected.

  • Right: Proofreading inactive — IDL persists.

• Yellow text emphasizes that MMR-defective cells cannot efficiently fix single-base IDLs, leading to instability.

Glossary:

• Strand slippage: Temporary misalignment of DNA strands during replication.

• Frameshift mutation: Insertion or deletion that alters the reading frame of a gene.

• MSI (Microsatellite Instability): Variable microsatellite lengths caused by replication errors not repaired by MMR.

Key Takeaway:

Slide 12 – G:T Mispair

Key Concepts and Explanations:

• A G:T mispair occurs when guanine (G) is incorrectly paired with thymine (T) instead of cytosine (C).

• This mismatch arises during DNA replication due to tautomeric shifts — temporary changes in base structure that alter pairing properties.

• If unrepaired before the next replication cycle, this mispair becomes a point mutation (G:C → A:T transition).

• Such mismatches are not chemically damaged bases — they are normal nucleotides in the wrong configuration.

Explanation of Visuals:

• The figure compares:

  • Normal base pair (G:C) with three hydrogen bonds.

  • Mispair (G:T) with incorrect hydrogen bonding and distorted geometry.

• The distortion alerts repair systems such as proofreading or mismatch repair (MMR).

Glossary:

• Mispair: Incorrect base pairing between non-complementary nucleotides.

• Tautomeric shift: Temporary change in hydrogen-bonding pattern of a base.

• Transition mutation: A purine-to-purine or pyrimidine-to-pyrimidine substitution.

Key Takeaway:

Slide 13 – Proofreading vs. Mismatch Repair (MMR)

Key Concepts and Explanations:

• Proofreading and MMR are two sequential defense systems that correct replication errors, but they act at different stages:

  1. Proofreading:

     • Occurs immediately during replication.

     • Corrects mismatches when the DNA strands are still unannealed (open).

  2. Mismatch Repair (MMR):

     • Acts after replication once the strands are fully annealed and sealed.

     • Repairs mismatches that escaped proofreading.

Explanation of Visuals:

• The upper line (Proofreading): shows mismatch correction occurring during strand synthesis.

• The lower line (MMR): shows mismatch correction after replication, when both strands are intact but one carries the new error.

• Text color differentiates timing — blue for MMR, red for proofreading.

Glossary:

• Annealed DNA: Double-stranded, base-paired DNA after replication.

• Replication fork: Region where DNA is actively being synthesized.

• Post-replicative repair: DNA correction processes occurring after synthesis.

Key Takeaway:

Slide 14 – Criteria for MMR

Key Concepts and Explanations:
To function properly, the MMR system must fulfill three critical criteria:

1. Mismatch recognition: It must accurately identify various types of mismatches (e.g., G:T, A:C, insertion–deletion loops).

2. Timing: Repair must occur before the next replication, while the parental (template) and daughter strands are still together.

3. Strand discrimination: The system must determine which strand is newly synthesized and contains the error.

Explanation of Visuals:

• The diagram shows different mismatches (G:T, A:A, C:T, etc.) and how MMR identifies them.

• Blue and red arrows illustrate strand discrimination — MMR must remove the incorrect base from the new strand, not the template.

• A note emphasizes that strand discrimination often depends on temporary DNA signals (e.g., nicks or methylation status).

Glossary:

• Strand discrimination: Ability to identify and correct the newly made strand.

• Hemimethylation: Temporary state where only the parental DNA strand is methylated.

• IDLs: Insertion–deletion loops corrected by MMR.

Key Takeaway:

Slide 15 – Prokaryotic MMR in Action

Key Concepts and Explanations:

• The slide refers to a video demonstration of mismatch repair in bacteria (E. coli).

• In prokaryotes, MMR relies on a well-characterized system involving MutS, MutL, and MutH proteins:

  1. MutS detects the mismatch.

  2. MutL acts as a linker between recognition and cleavage steps.

  3. MutH identifies the unmethylated (new) strand and introduces a cut.

• The nicked DNA is then degraded past the mismatch and resynthesized correctly by DNA polymerase.

Explanation of Visuals:

• Although the slide mainly references a YouTube link, it conceptually illustrates:

  • Mismatch detection → strand incision → resynthesis → ligation.

• This forms the template for understanding eukaryotic MMR, which uses MutS and MutL homologs but lacks MutH.

Glossary:

• MutS/MutL/MutH: Core prokaryotic proteins for mismatch repair.

• Dam methylation: DNA adenine methylation system that distinguishes new from old strands.

• Exonuclease: Enzyme removing DNA from the nick to the mismatch.

Key Takeaway:

Slide 16 – Mismatch Recognition: MutS

Key Concepts and Explanations:

• MutS is the key recognition protein in bacterial mismatch repair.

• It forms a homodimer (two identical subunits) that scans DNA for mismatches.

• Upon detecting a distortion in the DNA helix, MutS:

  1. Binds the mismatch site tightly.

  2. Undergoes a conformational change using ATP binding.

  3. Recruits MutL to initiate downstream repair.

• In eukaryotes, MutS has evolved into MutS homologs (MSH) that form heterodimers:

  • MSH2–MSH6 (MutSα): Repairs single base mismatches and small IDLs.

  • MSH2–MSH3 (MutSβ): Repairs larger IDLs.

Explanation of Visuals:

• Structural images show MutS dimers bound to mismatched DNA (A–D: bacterial MutS; E–F: eukaryotic MSH homologs).

• The right diagram illustrates how MutS slides along DNA like a clamp, powered by ATP.

• The labels identify the sliding clamp conformation and mismatch recognition pocket.

Glossary:

• MutS homodimer: Bacterial mismatch recognition complex.

• MSH (MutS Homolog): Eukaryotic protein equivalent to bacterial MutS.

• Sliding clamp: Protein configuration that encircles DNA and moves along it.

Key Takeaway:

Slide 17 – MutL: Molecular Matchmaker

Key Concepts and Explanations:

• MutL acts as a molecular coordinator (or “matchmaker”) in mismatch repair.

• It links mismatch recognition (MutS) to strand incision and excision.

• In bacteria, MutL forms a homodimer, while in eukaryotes it exists as MutL homologs (heterodimers), each specialized for distinct repair functions:

  • MutLα (MLH1–PMS2): Major endonuclease in MMR.

  • MutLβ (MLH1–PMS1): Minor or regulatory role.

  • MutLγ (MLH1–MLH3): Functions mainly in meiotic recombination but may assist in MMR.

Mechanism:

1. MutL binds to the MutS–DNA complex after mismatch recognition.

2. Upon ATP binding, MutL undergoes a conformational change forming a sliding clamp, which can move along DNA.

3. MutL then activates the endonuclease PMS2 (in MutLα), initiating strand incision.

Explanation of Visuals:

• The figure shows MutL clamp formation upon ATP binding.

• The schematic highlights how MutL couples mismatch recognition (MutS) with downstream strand excision.

Glossary:

• MutL: Protein linking mismatch recognition to incision.

• PMS2: Subunit of MutLα with endonuclease activity.

• Sliding clamp: Circular protein complex that encircles DNA and moves along it.

Key Takeaway:

Slide 18 – The Strand Discrimination Problem in Eukaryotes (1)

Key Concepts and Explanations:

• In eukaryotes, identifying the newly synthesized DNA strand (the one containing the error) is a critical challenge — unlike bacteria, eukaryotic DNA is not methylated.

• Instead, strand discrimination relies on replication-associated structures:

  • Lagging strand: Identified by Okazaki fragment nicks.

  • Leading strand: Recognized through temporary gaps near the replication fork involving PCNA (the sliding clamp).

• MutLα (MLH1–PMS2) is recruited to these sites, where it introduces a nick into the new strand.

Mechanism Overview:

1. Mismatch recognized by MutSα (MSH2–MSH6).

2. MutLα moves toward the nearest strand discontinuity (nick or gap).

3. The error-containing fragment is excised by EXO1 (exonuclease 1).

4. DNA polymerase δ fills the gap, and DNA ligase seals it.

Explanation of Visuals:

• Diagram shows leading and lagging strands with mismatches and associated repair sites.

• Blue and red arrows indicate directionality and the role of MutLα and EXO1 during excision.

Glossary:

• Okazaki fragment: Short DNA fragment synthesized on the lagging strand.

• PCNA: Sliding clamp that coordinates replication and repair enzymes.

• EXO1: 5′→3′ exonuclease that removes mismatched DNA.

Key Takeaway:

Slide 19 – The Strand Discrimination Problem in Eukaryotes (2)

Key Concepts and Explanations:

• This slide continues explaining strand discrimination mechanisms for both leading and lagging strands:

  • Lagging strand:

    • Already contains nicks at Okazaki fragment junctions.

    • MutLα introduces an additional nick if necessary.

  • Leading strand:

    • Requires coordination with MutSα, MutLα, PCNA, and replication factor C (RFC) to identify the newly synthesized strand.

    • MutLα’s endonuclease is activated upon interaction with PCNA, introducing a controlled nick.

Stepwise process:

1. MutSα detects mismatch → recruits MutLα.

2. MutLα endonuclease (PMS2 subunit) introduces a single nick.

3. EXO1 degrades the strand containing the mismatch in a 5′→3′ direction.

4. DNA polymerase δ resynthesizes the excised region.

5. DNA ligase I seals the final nick.

Explanation of Visuals:

• Two diagrams illustrate how MMR components handle mismatches on both leading and lagging strands.

• Color-coded proteins (MutSα, MutLα, PCNA, EXO1) show coordinated repair actions.

Glossary:

• MutSα: MSH2–MSH6 heterodimer recognizing mismatches.

• MutLα: MLH1–PMS2 heterodimer acting as endonuclease.

• RFC: Replication factor C, loads PCNA onto DNA.

Key Takeaway:

Slide 20 – Eukaryotic MMR in Action

Key Concepts and Explanations:

• This slide references an animation (by Josef Jiricny) illustrating the dynamic coordination of eukaryotic MMR.

• Key sequential steps shown in the animation:

  1. Mismatch recognition by MutSα (MSH2–MSH6).

  2. Recruitment of MutLα (MLH1–PMS2) and formation of the repair complex.

  3. Activation of MutLα endonuclease and incision near the mismatch.

  4. EXO1-mediated excision of the mismatched region.

  5. DNA resynthesis and ligation, completing repair.

Explanation of Visuals:

• The slide likely shows a placeholder (“MMR animation”) — meant to be accompanied by a video demonstration of the process in real time.

Glossary:

• MMR animation: Visualization tool to show the ordered recruitment and activity of proteins involved in mismatch repair.

• MMR complex: Combined group of MSH, MLH, EXO1, PCNA, and polymerase proteins executing repair.

Key Takeaway:

Slide 21 – Mismatch Repairisome

Key Concepts and Explanations:

• The MMR “repairisome” represents the complete set of proteins working together to execute mismatch repair.

• Each protein complex has a specific, coordinated function:

  • MutSα (MSH2–MSH6): Recognizes single-base mismatches and small IDLs.

  • MutSβ (MSH2–MSH3): Recognizes larger IDLs.

  • MutLα (MLH1–PMS2): Introduces strand incision (endonuclease).

  • EXO1: Removes mismatched segment.

  • RPA: Stabilizes single-stranded DNA.

  • PCNA: Sliding clamp guiding polymerase and repair proteins.

  • DNA polymerase δ: Fills the repair gap.

  • DNA ligase I: Seals the nick.

  • RFC: Loads PCNA onto DNA to coordinate synthesis.

Explanation of Visuals:

• The table lists MMR proteins with their subunits and cellular functions, summarizing the full pathway.

• It provides a quick reference overview of all major players in eukaryotic mismatch repair.

Glossary:

• Repairisome: Functional assembly of multiple proteins performing DNA repair collaboratively.

• IDLs: Insertion–deletion loops repaired by MMR.

• RPA: Single-stranded DNA-binding protein stabilizing excised regions.

Key Takeaway:

Slide 22 – MMR: Overview in E. coli and Eukaryotes

Key Concepts and Explanations:

• This slide summarizes and compares the mismatch repair (MMR) mechanisms in prokaryotes (E. coli) and eukaryotes.

• While both systems share the same logic — recognition, strand discrimination, excision, resynthesis, and ligation — their molecular details differ.

Prokaryotic MMR (E. coli):

1. Recognition: MutS binds to the mismatch.

2. Recruitment: MutL interacts with MutS and activates MutH.

3. Strand discrimination: MutH identifies the unmethylated daughter strand (via GATC sites).

4. Excision: The nicked DNA is degraded past the mismatch.

5. Resynthesis: DNA polymerase III fills the gap; ligase seals it.

Eukaryotic MMR:

1. Recognition: MutSα (MSH2–MSH6) or MutSβ (MSH2–MSH3) recognizes mismatches.

2. Recruitment: MutLα (MLH1–PMS2) binds to the site and introduces a strand nick.

3. Strand discrimination: Based on replication nicks and PCNA rather than methylation.

4. Excision: Carried out by EXO1, directed by RPA and PCNA.

5. Resynthesis: DNA polymerase δ fills the gap, and DNA ligase I completes the process.

Explanation of Visuals:

• The left column shows bacterial steps involving MutS, MutL, MutH, and DNA methylation.

• The right column shows the eukaryotic pathway with MutSα, MutLα, PCNA, EXO1, polymerase δ, and ligase.

• Both pathways end with accurate DNA restoration.

Glossary:

• MutH: Bacterial endonuclease responsible for strand incision at unmethylated sites.

• PCNA: Eukaryotic sliding clamp coordinating repair synthesis.

• EXO1: Exonuclease removing the mismatched segment.

Key Takeaway:

Slide 23 – Goals of This Lecture

Key Concepts and Explanations:

• This slide summarizes the learning objectives and what students should now understand after completing the lecture on MMR and replication fidelity.

• By this point, you should be able to:

  1. Explain how DNA replication accuracy is achieved.

  2. Describe the proofreading mechanism of DNA polymerases.

  3. Identify which DNA lesions are repaired by MMR.

  4. List the key criteria for MMR (mismatch recognition, timing, and strand discrimination).

  5. Outline the steps of MMR in both prokaryotes and eukaryotes.

  6. Understand how MMR distinguishes between old and new strands.

  7. Discuss the role of MMR in cancer development (to be covered in group work).

Explanation of Visuals:

• The slide uses a checklist format with blue highlights and check marks (✓) for completed topics.

• The only remaining open question (“What roles MMR plays in cancer”) is highlighted in a blue box as the next discussion focus.

Glossary:

• Lesion: Any DNA alteration, including mismatches or structural distortions.

• MMR: Mismatch Repair system ensuring post-replicative DNA fidelity.

• Proofreading: Enzymatic removal of misincorporated nucleotides during replication.

Key Takeaway:

Slide 24 – Group Work (Overview and Task 1)

Key Concepts and Explanations:

• The group work section encourages active discussion and application of MMR concepts to real biological contexts.

• The work is divided into two phases:

  1. Reading task (5 min): Highlight the essential information in the text.

  2. Discussion phase (5 min): Reflect on the key points and address remaining questions together.

Main Tasks:

• Task 1 – MMR and Cancer:

  • Explain how deficiency in MMR affects cell function.

  • Defective MMR leads to mutation accumulation, microsatellite instability, and tumorigenesis (especially colorectal and endometrial cancers).

  • Without MMR, cells fail to correct replication errors, allowing mutations in tumor suppressor or DNA repair genes to persist.

Explanation of Visuals:

• Text-based layout outlining the two phases and task instructions.

• The focus question for Task 1 (“MMR and cancer”) sets the stage for linking molecular mechanisms to clinical outcomes.

Glossary:

• MMR deficiency (dMMR): Loss of function in one or more MMR genes (e.g., MLH1, MSH2, MSH6, PMS2).

• Microsatellite instability (MSI): Repetitive sequence length variation due to failed mismatch correction.

• Tumorigenesis: Process of tumor formation due to accumulated mutations.

Key Takeaway:

Slide 25 – Group Work (Tasks 2 and 3)

Key Concepts and Explanations:

• Task 2 – MMR, Microsatellite Instability, and Cancer:

  • Explore how MMR defects lead to microsatellite instability (MSI) and how this instability contributes to tumor evolution.

  • Explain the mechanistic link:

    • MMR fails → IDLs accumulate → MSI → mutations in key genes (e.g., TGF-βRII, BAX) → cancer progression.

  • MSI is a key diagnostic marker for Lynch Syndrome and other MMR-deficient tumors.

• Task 3 – Methylation, DNA Damage, and Chemotherapy Resistance:

  • Discuss how DNA methylation, damage, and MMR deficiency are interconnected.

  • MLH1 promoter hypermethylation can silence MMR genes, causing MMR loss without genetic mutations.

  • Chemotherapy resistance arises because MMR-deficient cells cannot recognize and trigger apoptosis in response to DNA-damaging drugs (e.g., cisplatin, temozolomide).

  • Thus, MMR-deficient tumors are mutation-prone but therapy-resistant.

Explanation of Visuals:

• Simple text layout listing the three tasks.

• Key phrases like “MMR and cancer”, “microsatellite instability”, and “chemotherapy resistance” are highlighted, linking molecular mechanisms to clinical implications.

Glossary:

• MLH1 promoter methylation: Epigenetic silencing mechanism in sporadic MSI cancers.

• Chemotherapy resistance: Failure of cancer cells to die after DNA damage due to defective repair signaling.

• Lynch Syndrome: Inherited MMR-deficiency cancer predisposition syndrome.

Key Takeaway:

Slide 26 – Task 1: MMR-Deficient Tumors

Key Concepts and Explanations:

• MMR-deficient tumors arise when key mismatch repair genes (e.g., MLH1, MSH2, MSH6, PMS2) are inactivated by mutation or methylation.

• Loss of MMR causes accumulation of replication errors, leading to:

  • Microsatellite instability (MSI) — variations in short repetitive sequences.

  • Increased mutation rates in oncogenes and tumor suppressor genes.

• These tumors exhibit a hypermutator phenotype, meaning their mutation frequency is much higher than in MMR-proficient (syngeneic) cells.

• The resulting genomic instability drives rapid tumor evolution and heterogeneity.

Mechanism:

1. Without MMR, replication errors (base mismatches and small insertions/deletions) persist.

2. Errors accumulate especially in microsatellite regions → MSI.

3. Mutations affect genes controlling cell growth and apoptosis (e.g., TGF-βRII, BAX, PTEN).

4. Cells evade growth control → tumor formation.

Explanation of Visuals:

• The figure shows tumor growth curves comparing:

  • Syngeneic (MMR-proficient) tumors — slower growth.

  • MMR-deficient tumors — rapid expansion.

• Graphs illustrate mutation load and instability patterns increasing over time.

• The label “syngeneic” denotes genetically matched controls with normal MMR.

Glossary:

• Hypermutator phenotype: Extremely high mutation rate due to repair deficiency.

• Syngeneic: Genetically identical or MMR-proficient control line.

• Genomic instability: State of frequent genetic alterations driving cancer.

Key Takeaway:

Slide 27 – Task 2: Microsatellite Instability (MSI)

Key Concepts and Explanations:

• Microsatellite instability (MSI) is a hallmark of MMR-defective cancers.

• Microsatellites are short tandem DNA repeats (e.g., AAAAA or CACACACA) that are prone to slippage during replication.

• When MMR fails to correct slippage errors, the repeat length changes — creating detectable insertions or deletions.

Mechanism:

1. During replication, DNA polymerase may slip on repetitive sequences.

2. In normal cells, MMR detects and repairs the insertion/deletion loops.

3. In MMR-deficient cells, these loops remain → length variation between alleles.

4. The cumulative effect causes microsatellite instability, visible as shifts in fragment sizes when analyzed by PCR.

Explanation of Visuals:

• The slide shows PCR fragment analysis:

  • Top panel: Stable microsatellite pattern (normal tissue).

  • Middle and bottom panels: MMR-deficient samples with increasing MSI — seen as multiple peaks or altered fragment lengths.

• The label “increasing ratio of PCR products” reflects more instability and higher mutation load in MMR-deficient cells.

Glossary:

• Microsatellites: Repetitive DNA sequences (mono-, di-, or trinucleotide repeats).

• Slippage: Replication error causing repeat expansions or contractions.

• MSI testing: PCR-based diagnostic method to detect instability in microsatellite loci.

Key Takeaway:

Slide 28 – Task 3: Methylation and DNA Damage

Key Concepts and Explanations:

• This slide focuses on the connection between DNA methylation, DNA damage, and chemotherapy response.

• Certain chemotherapeutic drugs (e.g., temozolomide) work by methylating DNA bases, creating lesions that require MMR for repair.

• The enzyme MGMT (O⁶-methylguanine-DNA methyltransferase) removes such methyl groups to protect cells from cytotoxicity.

Mechanism:

1. Temozolomide (TMZ) adds methyl groups to guanine at the O⁶ position, producing O⁶-methylguanine (O⁶-meG) lesions.

2. During replication, O⁶-meG mispairs with thymine (T), creating G:T mismatches.

3. In MMR-proficient cells, these mismatches trigger repair cycles → excessive repair attempts lead to cell death (basis for TMZ efficacy).

4. In MMR-deficient cells, mismatches persist unchecked → drug resistance, as the cell no longer undergoes repair-induced apoptosis.

5. MGMT can directly reverse the methylation, also conferring resistance by removing the drug’s effect.

Explanation of Visuals:

• The chemical structure of temozolomide and O⁶-methylguanine are shown.

• The lower text identifies MGMT as the enzyme that counteracts methylation damage.

• Listed chemotherapies (temozolomide, streptozotocin, dacarbazine, procarbazine) act by DNA methylation, targeting rapidly dividing cancer cells.

Glossary:

• MGMT: Enzyme repairing methylated guanine by transferring the methyl group to itself (“suicide reaction”).

• Temozolomide: Alkylating agent used in glioblastoma and melanoma treatment.

• Chemotherapy resistance: Failure of cancer cells to respond to DNA-damaging agents due to repair defects.

Key Takeaway:

Slide 29 – 6-Methylguanosine Mispairing

Key Concepts and Explanations:

• This slide explains how DNA methylation damage leads to base mispairing and mutagenesis.

• The lesion discussed is O⁶-methylguanine (O⁶-meG), formed when a methyl group is added to the oxygen atom at the 6th position of guanine — commonly caused by alkylating agents (e.g., temozolomide, dacarbazine).

Mechanism:

1. Normal pairing: Guanine (G) correctly pairs with cytosine (C).

2. After methylation: O⁶-meG structurally resembles adenine, so during replication it pairs incorrectly with thymine (T) instead of cytosine.

3. The result is a G:C → A:T transition mutation, a frequent mutational signature of methylating damage.

Explanation of Visuals:

• The left panel shows normal base pairing (G–C) with three hydrogen bonds.

• The right panels show:

  • O⁶-meG–T mispairing (incorrect hydrogen bonding).

  • O⁶-meG–C (the original correct pair).

• Red labels emphasize the formation of incorrect G:T base pairs following methylation.

Glossary:

• O⁶-methylguanine (O⁶-meG): A mutagenic DNA lesion resulting from guanine methylation.

• Alkylating agents: Chemicals that add alkyl or methyl groups to DNA bases.

• Transition mutation: Substitution of one purine–pyrimidine pair for another (e.g., G:C → A:T).

Key Takeaway:

Slide 30 – Futile MMR

Key Concepts and Explanations:

• Futile MMR describes a damaging repair cycle that occurs when methylated DNA lesions (like O⁶-meG) persist despite MMR attempts to fix them.

• MMR repeatedly recognizes the mispair (O⁶-meG:T), excises the newly synthesized strand, and resynthesizes it — but the lesion remains on the template strand.

• This leads to endless repair cycles, known as futile repair, causing:

  • Replication fork collapse

  • Double-strand breaks (DSBs)

  • Apoptosis (programmed cell death)

Mechanism:

1. DNA polymerase inserts thymine opposite O⁶-meG.

2. MMR recognizes this mismatch and removes the T.

3. During repair synthesis, DNA polymerase reinserts T again, since the template lesion (O⁶-meG) remains.

4. The process repeats — creating a futile cycle that stalls replication and triggers cell death.

Explanation of Visuals:

• The schematic shows:

  • A methylated G (meG) mispaired with T.

  • The MMR machinery continuously reinitiating repair on the same site, symbolized by the repetitive loop-like arrow between mispair and re-synthesis.

Glossary:

• Futile repair cycle: Repeated but unsuccessful DNA repair due to an unremovable template lesion.

• Replication fork collapse: Stalling of DNA replication leading to double-strand breaks.

• Apoptosis: Programmed cell death to eliminate damaged cells.

Key Takeaway:

Slide 31 – MGMT Levels and Cancer Therapy

Key Concepts and Explanations:

• MGMT (O⁶-methylguanine-DNA methyltransferase) is the enzyme responsible for directly removing methyl groups from O⁶-meG lesions.

• The effectiveness of methylating chemotherapies (e.g., temozolomide) depends heavily on the tumor’s MGMT status and MMR functionality.

• MGMT (O⁶-methylguanine-DNA methyltransferase) is both a guardian against cancer and, paradoxically, a problem in cancer therapy — because it can make tumors resistant to certain chemotherapy drugs.

💊 2⃣ Why MGMT is a problem in cancer therapy

Some chemotherapy drugs — especially alkylating agents — kill tumor cells by creating these same types of DNA lesions (like O⁶-methylguanine).

Common examples:

• Temozolomide (TMZ) — used in glioblastoma (brain cancer)

• Carmustine (BCNU) and Lomustine (CCNU) — nitrosoureas used in brain tumors and lymphomas

These drugs work by damaging tumor DNA, triggering cell death.
But if the tumor cell has high MGMT activity, MGMT quickly removes the methyl group → the damage never accumulates → the drug doesn’t work.

So:

Mechanism and Clinical Scenarios:

1. Tumors with low or absent MGMT (ideal for therapy):

• No MGMT → O⁶-meG lesions persist.

• MMR recognizes these lesions, initiating futile repair cycles → cell death.

• Such tumors are sensitive to methylating agents.

• Example: Some colon cancers respond well to methylating chemotherapy if MGMT is low.

2. Tumors with high MGMT expression:

• MGMT rapidly removes methyl groups from O⁶-meG → no lesions remain.

• MMR never gets activated → drug resistance.

• These patients do not benefit from methylating agents and may experience unnecessary toxicity.

3. Tumors deficient in MMR:

• MMR fails to recognize or repair O⁶-meG:T mismatches.

• No futile repair cycle → cells survive despite DNA damage.

• Leads to chemoresistance and continued mutation accumulation.

Explanation of Visuals:

• The top section shows a histological image with MGMT expression staining in tumor tissue.

• Text highlights treatment implications:

  • Blue: low MGMT = therapy effective.

  • Yellow: high MGMT or MMR deficiency = resistance to treatment.

Glossary:

• MGMT: Enzyme that directly reverses O⁶-methylguanine lesions.

• Chemotherapy resistance: Failure of cancer cells to respond to treatment due to efficient repair or MMR loss.

• Temozolomide: Alkylating chemotherapy agent used in glioblastoma and colorectal cancers.

Key Takeaway:

• Low MGMT + functional MMR → effective treatment (cell death).

• High MGMT or MMR deficiency → therapy resistance and tumor survival.

✅ Overall Summary of Slides 26–31 (Key Message):
MMR and MGMT are central to how cells respond to DNA methylation damage.

• MMR deficiency leads to microsatellite instability and cancer progression.

• MGMT loss sensitizes tumors to methylating chemotherapy.

• High MGMT or MMR loss confer drug resistance, explaining why patient-specific molecular profiling is crucial for effective cancer treatment.