Questions Flashcards
How long do the radical molecules formed with ionizing radiation last
- 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.
Phosporylation of which Histone is an indicator of DSB formation following exposure to ionizing radiation
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)
Which assays are used to detect DNA double strand breaks (DSB)
- 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)
Which assay is used to detect single strand breaks (SSB)
Alkaline elution
Which assay is used to detect apoptosis
Annexin V-labeling
What is the function of ATR?
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.
Mutations in which genes cause increased sensitivity to ionizing radiation
Nijmegen breakage syndrome (NBS1), familial breast cancer (BRCA1), ataxia telangiectasia (ATM), and ataxia telangiectasia-like disorder (MRE11)
What would be the expected result of a mutation in the XPC gene
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
Calculate DT
DT = Dq + 2.3 D0 (# of logs)
or
DT = Dq + D10 (# of logs)
(remember Dq = 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!
At what dose can chromosomal aberrations be detected in peripheral blood lymphocytes?
0.25 Gy (total body irradiation)
What effect does reducing the dose rate have on the shape of the survival curve?
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)!!
Calculate total cell cycle time
Labeling Index (LI) = λTS/TC
λ = 0.693 to 1 (correction factor)
Must be given TS or TM and LI
Example: LI = 36%, S-phase is 10 hours, λ = 0.693; calculate total cell cycle time
λTS/TC = (0.693 x 10)/TC) = 36%
TC = 19.25 hours
Why is S-phase the most sensitive part of the cell cycle to the effects of Hyperthermia?
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)
Give examples of genomic instability
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)
What are the Phenotypes of Cancer
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
What is Spectral karyotyping (SKY)?
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
Calculate the Hypoxic Fraction
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 fractionoxygenated/survivalfractionhypoxic
Thus the estimate for the fraction of hypoxic cells would be 0.004/0.1 = 0.04.
What is the maximum distance oxygen diffuses in a tumor?
- 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
What are some compounds that can be used to protect mammalian cells from radiation damage?
- 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)
What are some radiation sensitizers?
- Oxygen
- Bromodeoxyuridine (incorporated into cellular DNA in place of thymidine acts as a radiation sensitizer)
How big are apoptotic DNA fragments
Multiples of 185 base pairs (that is why you see DNA laddering on gels)
