4.2 Radiation-induced damage to nucleic acids

Radiation can wreak havoc on our DNA, causing various types of damage. From simple base modifications to complex double-strand breaks, these lesions can have serious consequences for cells if left unrepaired.

Our bodies have evolved sophisticated repair mechanisms to deal with radiation-induced DNA damage. However, when these systems fail, the long-term effects can be severe, including genomic instability, cellular senescence, and even cancer.

Radiation-induced DNA lesions

Types and Frequencies of DNA Lesions

• Radiation induces various DNA lesions including base modifications, sugar modifications, DNA-protein crosslinks, single-strand breaks (SSBs), and double-strand breaks (DSBs)
• Base modifications occur most frequently at ~1000 lesions per Gy of ionizing radiation
  • Involve oxidation, deamination, and alkylation of DNA bases
• Sugar modifications happen at 600-1000 lesions per Gy
  • Include oxidation and fragmentation of the deoxyribose backbone
• DNA-protein crosslinks form at 150 lesions per Gy
  • Result from radiation-induced covalent bonds between DNA and nearby proteins
• Single-strand breaks (SSBs) occur at ~1000 per Gy
• Double-strand breaks (DSBs) happen less often at ~40 per Gy
• Lesion frequencies vary based on radiation type/energy, cellular environment, and DNA conformation

Factors Influencing DNA Damage

• Radiation type impacts damage patterns
  • High linear energy transfer (LET) radiation (alpha particles) produces more complex DSBs
  • Low LET radiation (X-rays, gamma rays) causes more dispersed damage
• Cellular oxygen levels affect damage
  • Higher oxygen increases free radical production and oxidative DNA damage
  • Hypoxic conditions can make cells more radioresistant
• DNA conformation influences damage susceptibility
  • Tightly packed heterochromatin protects DNA more than loose euchromatin
  • Actively transcribed genes may be more vulnerable to radiation damage
• Cell cycle phase affects damage and repair capacity
  • S phase cells are most radiosensitive due to open chromatin during replication
  • G0/G1 phase cells are more radioresistant

DNA Breaks: Formation and Consequences

Single-Strand Break (SSB) Formation and Effects

• SSBs form when radiation breaks one DNA strand
  • Caused by direct ionization or free radical attack
• SSBs can lead to replication fork collapse during DNA synthesis
  • May result in more severe double-strand breaks if unrepaired
• SSB repair involves several steps:
  1. Detection by PARP1 enzyme
  2. End processing to remove damaged nucleotides
  3. Gap filling by DNA polymerase
  4. Nick sealing by DNA ligase
• Unrepaired SSBs can cause:
  • Stalled replication forks
  • Transcription blockage
  • Eventual conversion to DSBs

Double-Strand Break (DSB) Formation and Consequences

• DSBs involve breakage of both DNA strands
  • Can occur simultaneously or from two nearby SSBs on opposite strands
• DSBs are considered most lethal form of DNA damage
  • Lead to chromosomal aberrations, cell cycle arrest, and apoptosis if unrepaired
• DSB complexity varies
  • Simple DSBs have clean DNA ends
  • Complex DSBs have additional damage within 1-2 helical turns (clustered lesions)
• High-LET radiation produces more complex, difficult-to-repair DSBs
• DSBs trigger DNA damage response (DDR) signaling
  • Activate ATM and DNA-PK kinases
  • Phosphorylate histone H2AX to mark damage sites
  • Recruit repair factors like 53BP1 and BRCA1
• Unrepaired DSBs can result in:
  • Chromosomal translocations
  • Large deletions or duplications
  • Mitotic catastrophe and cell death

DNA Repair Mechanisms

Excision Repair Pathways

• Base Excision Repair (BER) addresses small base modifications
  1. Glycosylase removes damaged base
  2. AP endonuclease cleaves DNA backbone
  3. DNA polymerase fills gap
  4. DNA ligase seals nick
• Nucleotide Excision Repair (NER) handles bulky lesions and crosslinks
  1. XPC-RAD23B or UV-DDB recognizes damage
  2. TFIIH unwinds DNA around lesion
  3. XPF-ERCC1 and XPG nucleases excise damaged region
  4. DNA polymerase and ligase fill and seal gap
• Mismatch Repair (MMR) corrects base mismatches and small loops
  1. MutS proteins recognize mismatch
  2. MutL proteins recruit exonuclease
  3. Exonuclease removes error-containing strand
  4. DNA polymerase and ligase restore correct sequence

Double-Strand Break Repair Pathways

• Homologous Recombination (HR) uses sister chromatid template
  1. MRN complex and CtIP resect DNA ends
  2. RPA coats single-stranded DNA
  3. BRCA2 loads RAD51 to form nucleoprotein filament
  4. RAD51 filament invades homologous template
  5. DNA synthesis and resolution of intermediates
• Non-Homologous End Joining (NHEJ) directly ligates broken ends
  1. Ku70/80 binds DNA ends
  2. DNA-PKcs is recruited and activated
  3. Artemis processes DNA ends if necessary
  4. XRCC4-XLF-Ligase IV complex seals break
• Choice between HR and NHEJ influenced by:
  • Cell cycle phase (HR in S/G2, NHEJ throughout)
  • Chromatin structure
  • Complexity of DNA damage

Unrepaired DNA Damage: Long-Term Effects

Genomic Instability and Cellular Senescence

• Unrepaired/misrepaired DNA damage leads to genomic instability
  • Increased mutations, chromosomal aberrations, and aneuploidy
• Persistent DNA damage triggers cellular senescence
  • Permanent cell cycle arrest contributing to tissue aging
  • Senescent cells secrete inflammatory factors (SASP)
• Radiation-induced bystander effects amplify damage
  • Non-irradiated cells show DNA damage and instability
  • Mediated by secreted factors and gap junctions
• Epigenetic alterations persist long-term after exposure
  • Changes in DNA methylation patterns
  • Altered histone modifications (acetylation, methylation)

Carcinogenesis and Hereditary Effects

• Misrepaired DSBs can cause chromosomal rearrangements
  • Translocations activate oncogenes (BCR-ABL in leukemia)
  • Deletions inactivate tumor suppressors (p53)
• Radiation-induced mutations in critical genes promote cancer
  • DNA repair genes (BRCA1/2, MLH1)
  • Cell cycle regulators (RB1, CDKN2A)
  • Apoptosis mediators (BAX, BCL2)
• Transgenerational effects may occur through:
  • Epigenetic modifications in germ cells
  • Persistent genomic instability in offspring
• Increased cancer risk in irradiated populations
  • Atomic bomb survivors show elevated rates of leukemia and solid tumors
  • Radiotherapy patients have higher risk of secondary cancers