Questions

Question 1

How long do the radical molecules formed with ionizing radiation last

Answer 1

• The initial ionization process (absorption) takes approximately 10^-15 second.
• The primary radicals produced by the ejection of an electron typically have a lifetime of 10^-10 second. The resulting hydroxyl radical has a lifetime of approximately 10^-9 second.
• The DNA radicals subsequently produced have a lifetime of approximately 10^-5 second.
• Biology takes hours, days, months for cell killing, years for carcinogenesis, generations for heritable effects.

Question 2

Phosporylation of which Histone is an indicator of DSB formation following exposure to ionizing radiation

Answer 2

Production of DNA double-strand breaks (DSBs) by ionizing radiation leads to the rapid phosphorylation of histone H2AX on serine 139 (γ-H2AX) by ATM (can be visualized using a phospho-specific antibody)

Question 3

Which assays are used to detect DNA double strand breaks (DSB)

Answer 3

• Neutral comet assay (single-cell gel electrophoresis
• PFGE (pulsed-field gel electrophoresis)
• Radiation induced nuclear foci (easy and newer way of detecting using antibody binding, with a second antibody with fluorescent tag enabling quantifiable fluorescence microscopy)

Question 4

Which assay is used to detect single strand breaks (SSB)

Answer 4

Alkaline elution

Question 5

Which assay is used to detect apoptosis

Answer 5

Annexin V-labeling

Question 6

What is the function of ATR?

Answer 6

ATR is activated during every S-phase to regulate the firing of replication origins, the repair of damaged replication forks and to prevent the premature onset of mitosis.

Question 7

Mutations in which genes cause increased sensitivity to ionizing radiation

Answer 7

Nijmegen breakage syndrome (NBS1), familial breast cancer (BRCA1), ataxia telangiectasia (ATM), and ataxia telangiectasia-like disorder (MRE11)

Question 8

What would be the expected result of a mutation in the XPC gene

Answer 8

XPC is a gene whose product is involved in nucleotide excision repair. Mutations in XPC result in the human genetic disease xeroderma pigmentosum, characterized by extreme sensitivity to ultraviolet light

Question 9

Calculate D_T

Answer 9

D_T = D_q + 2.3 D₀ (# of logs)

or

D_T = D_q + D₁₀ (# of logs)

(remember D_q = D₀ Ln (n) where n = the extrapolation #)

of logs is the power of the initial number of cells (i.e. 1 x 10⁸ initial cells in tumor = 8 logs to get you to 1 cell remaining alive!)

If it asks how much to get a 90% chance of complete kill, add 1 more log (to get to 0.1)….its the chance you kill that last remaining cell!

If it asks how much to get a 99% chance of complete kill, add 2 more logs (to get to 0.01); its the chance you kill that last remaining cell!

Question 10

At what dose can chromosomal aberrations be detected in peripheral blood lymphocytes?

Answer 10

0.25 Gy (total body irradiation)

Question 11

What effect does reducing the dose rate have on the shape of the survival curve?

Answer 11

A decreased dose rate flattens out the curve (less bending toward the X-axis) because there is a loss of B type damage (so the damage you get becomes more and more just alpha type damage or linear type damage)!!

Question 12

Calculate total cell cycle time

Answer 12

Labeling Index (LI) = λT_S/T_C

λ = 0.693 to 1 (correction factor)

Must be given T_S or T_M and LI

Example: LI = 36%, S-phase is 10 hours, λ = 0.693; calculate total cell cycle time

λT_S/T_C = (0.693 x 10)/T_C) = 36%

T_C = 19.25 hours

Question 13

Why is S-phase the most sensitive part of the cell cycle to the effects of Hyperthermia?

Answer 13

Protein denaturing!

(S-phase is the most sensitive, while G₂/M and G₁ are more resistant… this is the opposite of cell cycle sensitivity to radiation…thus the potential for synergism between hyperthermia and radiation!!!). Heat can make PLD (and SLD) into lethal damage!!

Heat Shock Proteins (HSP’s), which refold proteins, mediate thermotolerance (transcription can skyrocket after even 1 application of hyperthermia)

Question 14

Give examples of genomic instability

Answer 14

1) Nucleotide Instability (NIN): fixed via Nucleotide Excision Repair (NER)
2) Microsatellite Instability (MIN): fixed via Mismatch repair
3) Chromosomal Instability (CIN): Fixed via HR or NHEJ
- can also manifest due to improper karyokinesis or absent tumor suppressor genes (TSG’s)

Question 15

What are the Phenotypes of Cancer

Answer 15

1) Limitless replicative potential
2) Self-sufficiency of growth signals
3) Insensitivity to anti-growth signals
4) Evade Apoptosis
5) Tissue Invasion
6) Sustained Angiogenesis

Question 16

What is Spectral karyotyping (SKY)?

Answer 16

Spectral karyotyping (SKY): uses fluorescence staining of chromosomes employing uniquely-colored probes specific for individual chromosomes, thus allowing them to be distinguished from each other on the basis of color. Stable translocations are revealed using SKY as a single chromosome that appears to be multi-colored

Question 17

Calculate the Hypoxic Fraction

Answer 17

Hypoxic Fraction (HF): The fraction of cells in a tumor that are hypoxic. This can be estimated using the paired survival curve method. This corresponds to the surviving fraction of cells irradiated in normally oxygenated tumors divided by the surviving fraction of cells from a tumor made fully hypoxic by asphyxiating the host with nitrogen immediately prior to irradiation; this is assumed to render all of the tumor cells radiobiologically hypoxic.

Example: 10 Gy of X-rays reduces the tumor cell survival fraction to 0.004 in a well oxygenated animal, and to 0.1 in an animal irradiated under nitrogen.

Hypoxic Fraction = survival fraction_oxygenated/survivalfraction_hypoxic

Thus the estimate for the fraction of hypoxic cells would be 0.004/0.1 = 0.04.

Question 18

What is the maximum distance oxygen diffuses in a tumor?

Answer 18

• Maximum oxygen diffusion: 70μm
• Maximum thickness of viable tumor: 100-180μm
  
  • no matter how large a tumor is, this is pretty constant,
  • the rest of the tumor is hypoxic / necrotic / not growing

Question 19

What are some compounds that can be used to protect mammalian cells from radiation damage?

Answer 19

• WR-638 (Cystaphos)
• WR-2721 (Amifostine/trade name Ethyol)
• Cysteine (sulfhydryl-containing compound that scavenges radiation-induced free radicals; it therefore acts as a radioprotector and reduces cell killing. Also can donate a hydrogen atom to facilitate direct chemical repair at sites of DNA damage)
• WR-1065 (activated form of Amifostine)

Question 20

What are some radiation sensitizers?

Answer 20

• Oxygen
• Bromodeoxyuridine (incorporated into cellular DNA in place of thymidine acts as a radiation sensitizer)

Question 21

How big are apoptotic DNA fragments

Answer 21

Multiples of 185 base pairs (that is why you see DNA laddering on gels)