Everything You Need to Know About DNA Repair Mechanisms for Step 1

DNA is constantly being damaged—by UV light, ionizing radiation, reactive oxygen species, and replication errors. The USMLE loves DNA repair because the mechanisms are high-yield, testable by clues, and tightly linked to classic diseases (e.g., xeroderma pigmentosum, Lynch syndrome, ataxia-telangiectasia, Fanconi anemia).

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Big-Picture Definition (What is DNA Repair?)

DNA repair mechanisms are cellular pathways that detect and correct DNA damage to maintain genomic stability. Failure of repair → mutations, cancer predisposition, cell death, and developmental defects.

Step 1 framing:

• Identify the type of damage (UV dimers? mismatch? double-strand break?)
• Map it to the repair pathway
• Connect to associated disease, cancer risk, and clinical clues

First Aid cross-reference: Biochemistry → Nucleic acid structure and function / DNA repair (often also echoed in Pathology → Neoplasia for mismatch repair and cancer syndromes).

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High-Yield Classification: Match Damage → Repair Pathway

1) Nucleotide Excision Repair (NER) — “Cut out bulky lesions”

Fixes: bulky, helix-distorting lesions (classically UV-induced pyrimidine dimers)
Mechanism (Step-style):

• Recognize distortion
• Endonuclease cuts on both sides
• Remove damaged segment
• DNA polymerase fills
• DNA ligase seals

Classic association: Xeroderma pigmentosum (XP)

• Defect: NER
• Clinical presentation:
  • Severe sunburns and photosensitivity
  • Freckling at early age
  • Markedly increased risk of skin cancers (SCC, BCC, melanoma)
• Diagnosis clues:
  • History of extreme sun sensitivity + early skin malignancies
• Treatment/management (testable concepts):
  • Strict UV avoidance, protective clothing
  • Aggressive dermatologic surveillance and early excision of lesions

USMLE pearl: UV damage → think pyrimidine dimers → think NER → think XP.

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2) Base Excision Repair (BER) — “Fix small, non–helix-distorting damage”

Fixes: small base changes that don’t massively distort the helix
Examples:

• Deamination (e.g., cytosine → uracil)
• Oxidation (ROS damage; 8-oxoG)
• Alkylation

Mechanism:

• DNA glycosylase removes the abnormal base → creates AP (abasic) site
• AP endonuclease cuts backbone
• DNA polymerase fills
• Ligase seals

Clinical relevance: not usually tested as a single named syndrome in Step 1 the way NER/MMR are, but frequently tested conceptually:

• ROS damage increases with radiation/oxidative stress
• BER is a “housekeeping” pathway for daily spontaneous lesions

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3) Mismatch Repair (MMR) — “Fix replication errors”

Fixes: replication errors that escape proofreading:

• Base-base mismatches
• Insertion/deletion loops (especially in microsatellites)

Core concept: identifies the newly synthesized strand and repairs to match the template.

Classic association: Lynch syndrome (Hereditary Nonpolyposis Colorectal Cancer, HNPCC)

• Defect: MMR genes (classically MLH1, MSH2, also MSH6, PMS2)
• Pathophysiology:
  • Failure to repair replication errors → microsatellite instability
  • Increased mutations in growth-regulatory genes
• Clinical presentation (HY):
  • Colon cancer often proximal/right-sided
  • Also endometrial, ovarian, gastric cancers (and others)
  • Fewer polyps than FAP, but high malignant potential
• Diagnosis (Step-relevant):
  • Tumor testing: microsatellite instability or loss of MMR proteins on IHC
  • Family history patterns (early onset, multiple related cancers)
• Treatment/management:
  • Increased surveillance (colonoscopy earlier/more frequently)
  • Cancer treatment per staging; some tumors respond to immunotherapy in real practice (checkpoint inhibitors), but for Step 1 focus on mechanism + associations

USMLE pearl: Microsatellite instability = MMR failure = Lynch.

First Aid cross-reference: DNA repair + colorectal cancer genetics (also appears in GI pathology sections).

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4) Double-Strand Break (DSB) Repair

Double-strand breaks are among the most lethal forms of DNA damage.

A) Nonhomologous End Joining (NHEJ) — “Quick and dirty”

When: throughout cell cycle (especially G1)
How: directly ligates broken ends
Risk: error-prone → small insertions/deletions

Clinical tie-in (conceptual):

• Important in immune system gene rearrangement (V(D)J), but Step questions often emphasize that defects increase genomic instability.

B) Homologous Recombination (HR) — “Accurate template-based repair”

When: S/G2, when a sister chromatid is available
How: uses the intact sister chromatid as a template
More accurate than NHEJ

High-yield cancer association: BRCA1/BRCA2 involved in HR repair

• Increased risk: breast and ovarian cancers (also prostate, pancreatic)

First Aid cross-reference: Tumor suppressors (BRCA) and DNA repair.

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ATM, Cell Cycle Checkpoints, and Radiation: A Classic Step 1 Cluster

Ataxia-telangiectasia (ATM mutation)

ATM helps sense DNA damage (especially double-strand breaks) and coordinates cell cycle checkpoint arrest (notably at G1/S) and repair.

Clinical presentation (HY):

• Progressive cerebellar ataxia
• Telangiectasias
• Recurrent sinopulmonary infections (often due to IgA deficiency and broader immune dysfunction)
• Increased risk of malignancies (especially leukemia/lymphoma)
• Radiation sensitivity

Diagnosis clues:

• Child with ataxia + telangiectasias + recurrent infections
• Lab patterns may show immunoglobulin abnormalities; Step often tests the triad and radiation sensitivity

USMLE pearl: Radiation sensitivity and cancer predisposition point to DNA repair/checkpoint disorders.

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Interstrand Crosslink Repair: Fanconi Anemia (Very High Yield)

Fanconi anemia

Defect: DNA repair of interstrand crosslinks (classically from agents like chemotherapy; crosslinks block replication/transcription)
Pathophysiology: bone marrow failure + cancer risk from chromosomal instability.

Clinical presentation (HY mnemonic-style features):

• Pancytopenia / aplastic anemia (bone marrow failure)
• Increased AML risk
• Congenital anomalies: abnormal thumbs/radii, short stature, café-au-lait spots (congenital malformation pattern is commonly tested)

Diagnosis (Step-style):

• Chromosomal breakage tests may be referenced
• Clinical picture: child with physical anomalies + pancytopenia

Treatment/management:

• Supportive care (transfusions, infection management)
• Hematopoietic stem cell transplant is definitive for marrow failure

First Aid cross-reference: Often appears under hematology (aplastic anemia) as well as biochemistry DNA repair.

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Putting It Together: Common Step 1 Presentations (Clinical Vignettes)

Vignette 1

Child with extreme sun sensitivity, early skin cancers.
Answer: Xeroderma pigmentosum → nucleotide excision repair defect (UV pyrimidine dimers).

Vignette 2

Right-sided colon cancer + endometrial cancer in family; tumor has microsatellite instability.
Answer: Lynch syndrome → mismatch repair defect.

Vignette 3

Child with ataxia, telangiectasias, recurrent infections; radiation sensitivity.
Answer: ATM mutation (checkpoint/DSB response).

Vignette 4

Child with pancytopenia + thumb abnormalities.
Answer: Fanconi anemia → crosslink repair defect → marrow failure, AML risk.

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Diagnosis: What Tests/Clues Matter for Step 1?

You usually won’t be asked to order an exhaustive workup, but you will be tested on hallmark findings:

• XP: early skin cancers + UV sensitivity (mechanism: NER)
• Lynch: microsatellite instability / loss of MMR proteins; proximal colon cancer
• ATM: ataxia + telangiectasia + recurrent infections + radiation sensitivity
• Fanconi: aplastic anemia + congenital anomalies; increased AML risk
• BRCA: familial breast/ovarian cancer pattern; HR defect

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Treatment/Management: Step-Relevant Principles

While Step 1 is mechanism-heavy, keep these high-yield management anchors in mind:

• UV-related repair defects (XP): strict sun avoidance, frequent dermatologic checks
• Lynch: earlier and more frequent cancer screening (colon/endometrial awareness)
• Fanconi: supportive care; stem cell transplant for marrow failure
• ATM: supportive management; reduce radiation exposure (radiation sensitivity)
• BRCA-associated cancers: screening and risk reduction strategies may be mentioned; core testable concept is DNA repair (HR) defect

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Rapid High-Yield Table (Memorize This)

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First Aid Cross-References (Where This Shows Up Again)

Even if the primary entry is in Biochemistry, DNA repair repeats across FA in multiple contexts:

• Biochemistry: DNA repair mechanisms (NER/BER/MMR, etc.)
• Pathology (Neoplasia): tumor suppressors and genomic instability (e.g., BRCA, MMR/Lynch)
• Dermatology/Oncology: UV-associated skin cancers (XP concept)
• Hematology: aplastic anemia and marrow failure (Fanconi anemia)
• Immunology: recurrent infections in ATM (immune dysfunction)

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Ultra–High-Yield Facts to Nail Before Test Day

• UV pyrimidine dimers → NER → xeroderma pigmentosum
• Microsatellite instability → mismatch repair defect → Lynch syndrome
• ATM mutation → ataxia-telangiectasia → radiation sensitivity + infections + cancer
• Fanconi anemia → crosslink repair defect → aplastic anemia + AML risk + thumb anomalies
• BRCA1/2 → homologous recombination defect → breast/ovarian cancer predisposition