Aberrations

Question 1

Chromosome aberrations

Answer 1

Result from early interphase irradiation before duplication
Break is in a single strand of chromatin
The break is then replicated in the sister chromatid
Result from defective NHEJ, HR
Lethal = dicentric, ring
Non-lethal = symmetric translocation (Visualize w/ FISH, Burkitt lymphoma oncogene activation), small interstitial deletions (can lead to cancer if tumor suppressor gene lost), terminal deletion

Question 2

Chromatid aberrations

Answer 2

Result from late interphase irradiation after duplication
One chromatid contains a break while the other is intact
Result from defective HR
• Anaphase bridge (dicentric) lethal

Question 3

Lymphocyte aberrations

Answer 3

Days to weeks after total body irradiation, the number of asymmetric aberrations (dicentrics and rings) in human lymphocytes can be used to estimate dose
Lymphocytes are stimulated to divide and arrested at metaphase
The number of aberrations per cell is a linear-quadratic function of dose
Can detect a recent total body exposure as low as 0.25 Gy
Lymphocyte lifespan 1500 days, therefore lethal aberrations decline months to years after irradiation (‘unstable’ aberrations) and are best used for recent exposures

Symmetric translocations are stable and can be used for recent exposure and 50+ years later to estimate total body dose

Question 4

What evidence supports that cell killing by ionizing radiation is mainly due to DNA damage

Answer 4

• Cell death after polonium (alpha particle emitter) injected into nuclei not cytoplasm or plasma membranes
• Cell death after irradiation correlates with 3H thymidine (preferentially localizes to nucleus) not 125I concanavalin (preferentially localizes to cytoplasm and plasma membranes)

Question 5

of DNA lesions per cells after 1 Gy of photons

Answer 5

• > 1000 damages to DNA bases
• 1000 single-strand breaks
• 20-40 double-strand breaks

Question 6

D0

Answer 6

• D0 is the radiation dose that induces an average of 1 lethal event per cell, leaving 37% of cells viable (Radiobiology for the Radiologist)
• D0 for mammalian cells is about 1-2 Gy for photons

Question 7

Measurement of DSB

Answer 7

• Pulsed-field gel electrophoresis = Can analyze thousands to millions of cells with a single measurement for a population of cells
• Single-cell gel electrophoresis (comet assay) = Detects DNA damage and repair at a single cell level e.g. for a tumor biopsy. Alkaline DSB + SSB, Neutral DSB only
• DNA damage-induced nuclear foci = Phosphorylated H2AX (γH2AX) and 53BP1 form foci at the sites of DSB. Increases in γH2AX and phosphorylated 53BP1 can also be quantified using flow cytometry

Question 8

DNA damage sensors

Answer 8

MRN, ATM, ATR, ATRIP, Ku70, Ku80, DNA-PKcs
Proteins that recognize specific lesions in DNA and initiate DNA damage response cascade
Damage sensors are specific to the type of damage
• Base lesions: glycosylases
• Single-strand breaks: PARPs
• Double-strand breaks: MRN complex (MRE11, RAD50, *NBS1)
• Double-strand breaks: Ku proteins (Ku70, Ku80)

**Nijmegen breakage syndrome is caused by mutation of NBS1, which leads to radiosensitivity