Mechanisms of DNA damage & Repair Pt 1

Which of the following proteins activated in response to DNA damage is phosphorylated first?

A cell's response to DNA damage requires activation of the ATM signaling pathway. ATM, a member of the phosphatidyl-inositol-3-OH kinase-like kinase (PIKK) family, auto-phosphorylates itself first in response to the presence of DNA strand breaks, and then proceeds to phosphorylate Chk2, followed by histone H2AX (to form γH2AX). Other downstream substrates are then phosphorylated (including BRCA1, p53 and others), which in turn activate proteins involved directly in DNA repair and cell cycle checkpoint control.

Which syndrome is caused by a deficiency in the repair-associated protein MRE11?

A deficiency in MRE11 results in an ataxia telangiectasia-like disorder.

Clinical radiosensitivity is associated with the absence of:

ATM gene product.

Which of the following proteins is involved in the activation of the G2/M cell cycle checkpoint in response to DNA damage?

ATM plays a major role in the activation of the G2/M cell cycle checkpoint in response to DNA damage, including that caused by ionizing radiation exposure.

Genetic deficiency in which of the following components of the NHEJ machinery results in severe combined immunodeficiency (SCID) in humans?

Artemis Non-homologous end-joining (NHEJ) through V(D)J recombination is a critical pathway to provide genetic diversity in T cell receptors and immunoglobulins. The RAG nucleases create hairpins in V, D, and J segments that are cleaved by Artemis before end-joining occurs. Artemis deficiency therefore results in severe combined immunodeficiency due to defective V(D)J recombination.

How is ATM activated as a response to radiation-induced DNA double strands?

Auto-phosphorylation converting inactive dimer to an active monomer

Activated CHK1 phosphorylates and inactivates

CDC25 proteins, thereby causing the arrest of the cell cycle.

Which assay can be used to MOST easily measure DNA double strand breaks from clinical tumor samples?

COMET assay The single cell gel electrophoresis assay (SCGE, also known as comet assay) is an uncomplicated and sensitive technique for the detection of DNA damage at the level of the individual eukaryotic cell. It has since increased in popularity as a standard technique for evaluation of DNA damage/repair, biomonitoring and genotoxicity testing. It involves the encapsulation of cells in a low-melting-point agarose suspension, lysis of the cells in neutral or alkaline (pH>13) conditions, and electrophoresis of the suspended lysed cells. The term "comet" refers to the pattern of DNA migration through the electrophoresis gel, which often resembles a comet.

Which radiation source is likely to cause the MOST clustered chromosome breaks?

Carbon ions Carbon ion radiation is high LET radiation and leads to clustered damage in DNA. The high localization of DNA damages from high LET irradiations, are more likely to cause clustered chromosome abnormalities. The increased toxicity of high LET radiation and the extensive genomic rearrangements associated with chromothripsis.

It is known that oxygen levels strongly influence the indirect effects of radiation damage. Which observation would you expect in the following studies.

Cells grown under aerobic conditions exhibit higher levels of DNA strand breaks induced by low-LET radiation, than cells irradiated in hypoxia. High oxygen tensions > 10 mm Hg are effective in producing much higher levels of DNA damage than in low oxygen tension.

Which type of DNA repair inhibitors will likely have the greatest radiosensitization?

DNA double-strand break repair inhibitors Radiation's primary mechanism of action is through DNA double-strand break, thus DNA double-strand break inhibitor will likely be the most potent.

Why are somatic cells typically limited to 40 - 60 divisions?

DNA loss of chromosome terminals Every time a somatic cell divides, DNA is lost from the chromosome terminal because DNA polymerase cannot synthesize new DNA in the absence of an RNA primer. Terminal ends of chromosomes are protected by 1.5 to 150 kilobase repeats called telomeres, which are progressively shortened with each division until reaching critical levels after 40 - 60 divisions. The ability to overcome this limitation and achieve "immortalization" is crucial for carcinogenesis. The reverse transcriptase telomerase can lengthen telomeres, and approximately 90% of human cancer biopsy specimens exhibit telomerase activity, unlike normal somatic tissues (except for stem cells).

critical components of the NHEJ repair pathway.

DNA- PKcs, KU70, KU80, DNAligase IV, and XRCC4

Major protein complexes implicated in the NHEJ pathway

DNA-PK protein kinase and XRCC4/Ligase IV complexes.

base excision repair (BER) is inhibited by PARP, T/F?

False Methoxyamine inhibits BER. PARP stimulates BER and repairs SSBs.

Non-homologous end-joining requires the involvement of a sister chromatid, T/F?

False Non-homologous end joining does not require a sister chromatid.

The BLM protein serves to coat single stranded DNA regions to prevent their degradation. T/F?

False RPA serves to coat single stranded DNA regions generated during homologous recombination to prevent their degradation. The BLM protein is a helicase. RecQ genes encode for DNA helicases that unwind DNA 3' to 5' in an ATP-dependent manner and also have 3' to 5' exonuclease activity. The main function of RecQ helicases is to monitor the DNA replication fork for damage, acting as intermediaries between replication and recombination. RecQ helicases are found at DNA replication forks and protect them from stalling. In fact, RecQ expression is the highest in S phase of the cell cycle.

The yield of initial DNA double-strand breaks produced in an irradiated mammalian cell will be influenced by absence of RAD51, T/F

False The absence of RAD51, which is a recombinase that plays a role in homologous recombinational repair, may affect the repairability of DNA double-strand breaks, but not their initial yield. however, absence of histone proteins, lack of oxygen and presence of amifostine during irradiation will affect the initial yield. Nuclear proteins play a critical role in protecting DNA from radiation damage. Thus removal of histones greatly enhances the sensitivity of mammalian cells to radiation damage.

Artemis is primarily responsible for ligating broken DNA ends, T/F

False The main role for Artemis is to cleave (through its nuclease activity) any residual DNA loops or hairpins that form during non-homologous end-joining. It prepares the ends for ligation by ligase.

Breast and ovarian cancers are the most common cancer in people diagnosed with ataxia telangiectasia. T/F

False, Patients with ataxia telangiectasia are predisposed to the development of leukemia and lymphoma.

Which method can be used to identify symmetrical translocations?

Fluorescence in-situ hybridization (FISH) cytogenics They are better visualized using FISH cytogenics with chromosome-specific DNA probes. Classical karyotyping can be used to identify other forms of chromosome aberrations such as dicentrics and rings. However, these aberrations are lethal at mitosis and cannot be passed to daughter cells. They can be visible after ionizing radiation for the lifespan of peripheral lymphocytes (about 1,500 days), but decline afterwards.

an adaptor/bridging protein involved in the activation of RAS?

GRB2 Growth factor receptor-bound protein 2 also known as Grb2 is an adaptor protein involved in signal transduction/cell communication.

Which of the following organisms is correctly ordered from most radiosensitive to most radioresistant? E. coli, HIV, HeLa cells, S. cerevisiae

HeLa cells < E. coli < S. cerevisiae < HIV Since mammalian cells (HeLa) have the largest DNA content compared to bacteria (E. coli), yeast (S. cerevisiae), and viruses (HIV), they have the largest target for radiation damage; therefore, they are the most radiosensitive. Other factors that affect the inherent radiosensitivity of cells from different organisms include the ability to sense and repair DNA damage, the prevalent mode(s) of cell death, the cell cycle duration and phase, the degree of differentiation, etc.

what proteins phosphorylate histone H2AX in response to DNA damage?

Histone H2AX is phosphorylated at the serine 139 amino acid moiety in response to DNA double strand breaks produced by ionizing radiation exposure. ATM and ATR, members of the PI3K-like kinase family, phosphorylate H2AX to form γH2AX. This, in turn, plays a critical role in the recruitment of DNA repair proteins to sites of DNA double strand breaks.

The mechanism of DNA double strand break repair in mammalian cells that involves the pairing of sister chromatids is:

Homologous recombination

Chemotherapeutic agents frequently produce DNA double-strand breaks by causing stalling and collapse of DNA replication forks. Which of the following pathways is required for the repair of replication-associated double-strand breaks?

Homologous recombination (HR)

Which of these compounds related to ionizing radiation is the least stable?

H₂O⁺ (water ion radical) Ionization of a water molecule produces the water ion radical (H₂O⁺) which is both charged and has an unpaired electron, is very unstable (lifetime of 10⁻¹⁰ seconds). This reacts with another water molecule to form a hydronium ion (H₃O⁺) and a hydroxyl radical , with a lifetime of about 10⁻⁹ seconds. Hydroxyl radicals can transfer their free electron to organic molecules such as DNA to form organic radicals. Organic radicals have a lifetime of about 10⁻⁵ second before being reduced by oxygen to an organic peroxide, which causes irreversible damage.

A patient has a germline mutation causing a functional deficiency in the DNA damage response protein Artemis. Other than sensitivity to ionizing radiation, what other feature would you most expect this patient to exhibit?

Immunodeficiency

Homologous recombinational repair of DNA double strand breaks is most likely to occur:

In late S phase Homologous recombinational repair requires the presence of a homologous DNA template, and is therefore most likely to occur following DNA replication in late S phase (when a sister chromatid is available as a template) or G₂ phases of the cell cycle.

Under normal conditions, what are the dominant modes of DNA damage for γ-rays versus α-particles?

Indirect action for γ-rays but direct action for α-particles the higher the LET of a radiation beam, the greater the fraction of cellular damage produced by direct action. For low-LET radiation, about 70% of the damage occurs through the indirect effect, barring cellular hypoxia or an unusually high concentration of radical scavengers.

A clustered lesion

Involves the formation of several DNA lesions/damages within a highly localized region of DNA

Which order, from greatest to least, best represents the following radiation-induced DNA lesions contributing to lethality? 1 - Thymine glycol; 2 - Double-strand DNA break; 3 - Single-strand DNA break; 4 - Multiply damaged sites; and 5 - Two opposing single-strand DNA breaks separated by 10 base pairs;

4, 2, 5, 3, 1 Conventional wisdom states that the multiply damaged site is the most lethal lesion following irradiation. It is the most difficult to repair. Logically, the #5 lesion would be less of a problem than #2 because of the stability (even if small) from the intervening strand that is paired and the possibility of repair between the two events. Thymine glycols and single-strand breaks are not considered to be lethal.

What proportion of patients with advanced ovarian cancer have homologous recombination repair (HR) deficiency?

50% Over half of patients with advanced ovarian cancer have homologous recombination repair (HR) deficiency. Across multiple PARP inhibitor trials, it was this subset of patients (which includes patients with BRCA mutations) that drove the progression-free survival benefit for maintenance therapy. The myChoice HRD assay was used on multiple trials to determine HR status.

Place the following in order of radiolysis events, from first to last: Peroxyradical formation, Radical formation, Ionization, Damage

Ionization > Radical formation > Peroxyradical formation > Damage The order of events in radiolysis include ionization (10⁻¹⁵ seconds), radical formation (10⁻⁹ seconds), peroxyradicals (if oxygen present), followed lastly by the formation of damages. Chemical species generated by ionizing radiation include hydroxyl radicals (most common), superoxide radicals, hydrogen peroxide, hydrated electrons.

How is the Multi-Target/Single-Hit model for cell survival superior to the linearquadratic model?

It provides a better fit to the data at very large single doses where the survival curve is exponential.

Cells are most radioresistant in what phase of the cell cycle?

Late S Cell are most radiosensitive in the M and G₁ phases of the cell cycle. Cells are most radioresistant in the late S phase.

Which of the following syndromes has the largest D₀? Li Fraumeni, LIG4, Bloom and Lynch

Lynch Radiosensitive cells have a low D0; while radioresistant cells have a high D₀ (need to give high dose). XP, HNPCC/Lynch, Cockayne syndrome, and Progeria are all syndromes without radiosensitivity, so they have large D₀s. LIG4 syndrome has a defect in DNA ligase IV enzyme which is important in NHEJ. Bloom syndrome has mild radiation sensitivity. Li Fraumeni has a p53 or CHK2 mutation. Patient with Lu Fraumeni have an increased incidence of sarcoma, breast and brain cancer.

Which of the following microenvironmental or treatment-related conditions would inhibit sublethal damage recovery?

Maintenance of cells at or below 5°C or above about 40°C) or severe hypoxia (oxygen concentration below about 3 mm Hg).

Which of the following describes the role of ɤH2AX in DNA DSB repair?

Marks sites of DNA damage Phosphorylation of H2AX can occur via ATM; ATM promotes phosphorylation and recruits and activates other repair factors. ATM is the primary kinase, which phosphorylates H2AX, although ATR and DNA-PKcs also perform this action.

Which of the following types of chromosome aberrations is most responsible for the formation of micronuclei observed after irradiation?

Micronuclei are created due to the presence of acentric fragments, which form in the progeny of irradiated cells that undergo mitosis in the presence of one or more asymmetrical chromosome aberrations.

Which double-strand break DNA repair system does not involve DNA end-resection?

NHEJ is responsible for the fast component of DNA repair, and is the dominant mechanism for DSB repair. Classical NHEJ does not involve DNA end-resection.

What are the 2 most common DNA double-strand break repair pathways in mammalian cells?

Non-homologous end joining (NHEJ) and Homologous Recombination (HR) NHEJ is the predominant pathway in the G0/G1-phases of the cell cycle, while HR increases during S/G2, when a sister chromatid becomes available as a template for repair.

In which part of the radiation damage response does DNA-dependent protein kinase (DNA-PKcs) primarily function?

Non-homologous end joining repair NHEJ is more important during G₁ and early S phase

Which of the following mechanisms removes bulky adducts from DNA?

Nucleotide excision repair

Which DNA repair process is generally not considered important in radiosensitivity?

Nucleotide excision repair Nucleotide excision repair is generally not considered important in radiosensitivity. Mutations in NER results in UV sensitivity syndromes, but no impact on radiation sensitivity. NER removes bulky add-ons from UV or cross-links from chemotherapy (cisplatin). In NER, an endonuclease breaks DNA on either side of a lesion and removes a chunk of DNA → DNA Polymerase synthesizes → Ligase links together the nick

Errors in which DNA repair pathways results in Xeroderma Pigmentosum?

Nucleotide excision repair Xeroderma Pigmentosum, an autosomal recessive disorder, is characterized by impaired nucleotide excision repair - mainly removes bulky adducts from DNA such as UV-induced pyrimidine dimers and chemical adducts

Which of the following assays is commonly used for measurement of radiation-induced DNA double-strand breaks?

Pulsed-field gel electrophoresis Alkaline elution is used for detection of single-strand breaks, while High-pressure liquid chromatography (HPLC) is used to detect base damage. Northern blotting is used for analysis of RNA.

The biological effects resulting from exposure to ultraviolet (UV) radiation are due primarily to the formation of:

Pyrimidine dimers Ultraviolet radiation is non-ionizing; The major types of DNA damage produced in cells by exposure to UV radiation include cyclobutane pyrimidine dimers and pyrimidine (6-4) pyrimidine photoproducts. In both cases, two pyrimidines, located next to each other, react to form a dimer following excitation of atoms in DNA. DNA-protein crosslinks are also important lesions in cells exposed to UV radiation. Crosslinks are particularly disruptive, as they occur mostly in the area of the chromosome that is undergoing replication. Thymol glycol [5,6-dihydroxy-5,6-dihydrothymine] and oxidized guanine [8-oxo-7,8-dihydroguanine (8-oxo-G)] are DNA base lesions present in clustered DNA damage induced in cells by ionizing radiation.

What proteins coat single strand DNA to prevent replication fork collapse in HRR?

RPA is the single strand DNA binding protein that coats single strand DNA during replication and protects DNA during strand insertion in HRR.

Normal tissues from which syndrome shows the highest level of sensitivity to ionizing radiation?

Radiation sensitivity is greatest in a person with ataxia telangiectasia. People with this syndrome are very sensitive to ionizing radiation due to the absence of functional ATM protein, which plays a central role in the repair of DNA double strand breaks and regulation of the cell cycle following irradiation.

Which of the following best describes the action of an exonuclease enzyme?

Removes nucleotides from the ends of DNA strands

Homologous recombination is thought to be most active in which phase of the cell cycle?

S-Phase HR is most active when a sister chromatid is available as a template for repair, which is in the late S-phase of the cell cycle. Sister chromatids are available in G₂ and early M-phase as well, but the chromatin is compacted and may not be as suitable for HR repair.

What is the primary reason that an 8 Gy x-ray dose delivered at 1 Gy/h is less toxic than the same dose delivered at 1 Gy/min?

SLD repair occurs during irradiation.

Which of the following is a guanine nucleotide exchange factor (GEF) involved in the activation of RAS?

SOS The GEF SOS1 activates Ras, whose target is the kinase Raf. Raf is a proto-oncogene because mutations in this protein have been found in many cancers. The Rho GTPase Vav1, which can be activated by the GEF receptor, has been shown to promote tumor proliferation in pancreatic cancer. GEFs represent possible therapeutic targets as they can potentially play a role in regulating these pathways through their activation of GTPases.

Which double-strand break DNA repair system is most likely to result in large deletions following irradiation?

Single strand annealing is most likely to result in large deletions following irradiation because it involves the larges regions of homology, and because it looks for these regions, it can result in large areas of deletions.

Which double-strand break DNA repair systems involves use of the longest stretches of homology?

Single-strand annealing Single-strand annealing involves use of the longest stretches of homology. Single strand annealing (SSA) is a process that is initiated when a double strand break is made between two repeated sequences oriented in the same direction. Single stranded regions are created adjacent to the break that extend to the repeated sequences such that the complementary strands can anneal to each other. This annealed intermediate can be processed by digesting away the single stranded tails and filling in the gaps.

Which technique(s) are used to characterize DNA repair?

Southern blotting is used to detect preferential DNA repair of active, compared to inactive, genes. Alkaline filter elutions are used to detect the total number of DNA strand breaks. Neutral filter elutions are used to measure double strand breaks. The comet assay is an electrophoresis separation of DNA fragments from whole cells and this measures strand breaks. PFGE is used to separate DNA pieces of relatively high molecule weight.

Which radiation-induced chromosome aberration may lead to the activation of fusion oncogenes in daughter cells?

Symmetric translocations, while not necessarily lethal to the affected cell, can create new fusion proteins which lead to carcinogenesis. A classic example is the t(9;22) Philadelphia chromosome which contains the BCR-ABL fusion oncogene which is associated with ALL.

Which one of the following radiation-induced chromosome aberrations is a "single hit" type? Acentric ring, Terminal deletions, Anaphase bridge, Dicentric ring

Terminal deletion as opposed to an interstitial deletion which requires two chromosomal breaks.

5 Gy of X-rays is delivered to mammalian cells in vitro using a megavoltage linear accelerator. What is the MOST likely observation 6 hours after irradiation?

The BRCA1 protein localizes to a subset of the total number of radiation-induced subnuclear foci

Which key kinase regulator links the initial recognition of DNA double-strand breaks with signaling for cell cycle arrest and break repair?

The ataxia-telangiectasia-mutated (ATM) protein kinase is rapidly and specifically activated in response to DNA double-strand breaks in eukaryotic cells. MRN complex is required for ATM activation after double-strand breaks (DSBs). Activated ATM then plays a role in cell cycle delay after DNA damage, especially after double-strand breaks (DSBs). ATM together with NBS1 act as primary DSB sensor proteins. These modified mediator proteins then amplify the DNA damage signal and transduce the signals to downstream effectors such as CHK2 and p53

What is the lowest single dose of radiation that can be readily detected by scoring aberrations in peripheral blood lymphocytes?

The lowest single dose of radiation that can be readily detected by scoring aberrations in peripheral blood lymphocytes is 0.25 Gy.

Molecular analysis indicates that the majority of ionizing radiation-induced HPRT mutations in human cell lines are the result of:

The majority of radiation-induced gene mutations are large deletions of the type that would produce frank chromosomal aberrations visible at the next cell division.

What is the ratio of the number of single strand breaks to double strand DNA breaks detected in a cell immediately following exposure to the D₀ dose?

The ratio of single-strand breaks to double-strand DNA breaks after exposure to the D₀ dose is 25. The number of single-strand and double-strand breaks detected immediately following the D₀ dose are 1000 and 40, respectively.

Which of the following curves correlates with mitosis?

The survival curve for cells in mitosis (and G₂) is steep and has no shoulder. Cell in late S phase have a shallower survival curve and a large shoulder, with G₁ and early S phase in between.

Which type of radiation-induced DNA damage is most important for cell killing caused by exposure to ionizing radiation?

The type of radiation-induced DNA damage most implicated in cell killing is the double-strand break.

How many double strand breaks are produced per gray per cell?

There are 1,000 single strand breaks per grey per cell. There are 40 double strand breaks produced per grey per cell. There are 100,000 ionizations per gray per cell. 2,000 - 3,000 base damages are produced per grey per cell.

The frequencies of which chromosome biomarker is stable and can be used in retrospective dosimetry?

Translocation Unlike unstable dicentrics, chromosome translocations are expected to be stable and their frequencies can be used in retrospective dosimetry. Radiation-induced rings are also unstable and therefore not unsuitable for retrospective dosimetry. Acentric fragments can be stable or lost from the cell during cell division and are therefore not unsuitable for retrospective dosimetry.

BRCA1 and BRCA2 predominantly regulate homologous recombination as opposed to non-homologous end joining; T/F?

True

Re: Base excision repair (BER), when defective, may increase mutation rate, but usually does not dramatically alter cellular radiosensitivity, T/F

True Defects in base excision repair may increase mutation rate, but generally do not alter cell survival after ionizing radiation, with the exception of mutation in the XRCC1 gene which confers a slight increase in radiation sensitivity. Defects in nucleotide excision repair (NER) increase sensitivity to UV, but not to ionizing radiation

A deficiency in MRE11 (which makes up the MRE11-RAD50-NBS1 complex) results in an ataxia telangiectasia-like disorder. T/F?

True The protein plays a role in both homologous recombination and NHEJ.

As chromatin compaction increases, the amount of radiation-induced DNA damage is

decreased. The complexing of DNA with histone proteins and the resulting condensation of the chromatin serve important roles in terms of regulating DNA's "accessibility" for the purposes of transcription, replication, and repair. By the same token, the DNA associated with highly compacted chromatin is less accessible to the radiation-induced free radicals that might otherwise damage it.

operational definition of PLD repair?

describes changes in survival with altered post-irradiation conditions. If survival is increased PLD is said to be repaired while if there is a decrease in survival PLD is said to be fixed as "fixed in place". The classic example of PLDR repair occurs when survival increases after the delayed post-irradiation subculture of plateau phase cells.

Lethal chromosomal aberrations include

dicentric, ring and anaphase bridge. the dicentric and the ring are chromosome aberrations; while the anaphase bridge is a chromatid aberration.

How will the removal of histones affect radiation-induced DNA damage?

enhance radiosensitivity DNA in cells are more resistant to damage by radiation than would be expected from studies on free DNA. There are two reasons for this: the presence in cells of low-molecular-weight scavengers that mop up some of the free radicals produced by ionizing radiation, and the physical protection afforded the DNA by packaging with proteins such as histones. The removal of histones enhances sensitivity to radiation.

Which results would you expect from laboratory studies with regard to influence of oxygen on the response of lung tumor cells to radiation therapy?

hypoxic cells are more resistant (higher D₀) to X-rays than aerobic cells Cells at very low oxygen tension (<2 mm Hg) are 2.5 - 3.0 times more resistant to radiation than aerated cells.

Nuclear proteins play a critical role in protecting DNA from radiation damage. Thus removal of histones greatly enhances the sensitivity of mammalian cells to radiation damage.

immune deficiency. V(D)J recombination is the process whereby segments of the immunoglobulin genes and the T-cell receptor are rearranged during development of the vertebrate immune system. V(D)J recombination is initiated by the introduction of a single strand DNA break at specific sites in the DNA by the RAG1/RAG2 protein complex. This single strand break is converted to a DNA double strand break, which is repaired during the process of V(D)J recombination by the nonhomologous end-joining (NHEJ) machinery

RAD51 and BRCA2 function together:

in homologous recombination.

Mutations which render the DNA mutation repair mammalian pathway MOST sensitive to ionizing radiation are:Ref-4911

non-homologous end-joining (NHEJ)

Double-strand DNA breaks caused by ionizing radiation trigger the transcription of DNA damage response genes. Which of the following proteins is a transcriptional transactivator? p53, CHK1, p21, ATM

p53 In response to various forms of DNA damage including double-strand breaks, p53 is stabilized and binds to the promoters of numerous target genes, including p21, activating their transcription. This transcriptional transactivation by p53 is an important component of the cellular DNA damage response.

initiating step for homologous recombination repair

processing of the 3` end of the DNA break by the NBS1/MRE11/RAD50 protein complex.

These major rad proteins are involved with homologous rejoining?

rad50, rad51, rad57 Homologous repair requires rad50, rad51, rad 52, rad54, rad55, rad57, ATM, XRCC2, XRCC3, NBS1 and MRE11, as well as other proteins to recognize, signal and initiate repair of DNA damage.

If there is no _____ occurring, the impact of a single fraction versus two equal fractions would be negligible.

repair The experimental definition of SLD is that there is an increase in cell survival if a given dose of radiation is split into two equal fractions separated in time. If there is no repair occurring, the impact of a single fractions, versus two equal fractions, would be negligible (assuming that repopulation, reoxygenation and resassortment are held constant).

Which radiation-induced chromosome aberrations is most difficult to identify using classical karyotyping techniques?

symmetric translocation Symmetrical translocations are difficult to identify using classical karyotyping techniques as the affected chromosomes may not appear structurally abnormal. Fluorescence in-situ hybridization (FISH) using chromosome specific DNA probes make identification of symmetric translocations much easier. Dicentric chromosomes, ring chromosomes, and anaphase bridges are readily identifiable with classical karyotyping techniques as the affected chromosomes appear structurally abnormal.

Ataxia telangiectasia (AT) cells are characterized by:

the inability to repair PLD. Cells from patients who are homozygous for defective or lost ATM genes do not repair PLD, are exquisitely sensitive to ionizing radiation (not UV radiation), and are characterized by x-ray survival curves with little or no shoulder. The defect leading to this phenotype is not a DNA repair deficiency per se, but rather, a defect in the signaling pathways that coordinate damage recognition, recruitment of repair complexes, and cell cycle checkpoint control.

What is the optimal radiation LET to obtain the highest RBE for DNA double strand breaks production?

100 keV/µm

What is the approximate lifetime of DNA radicals formed either by direct ionization or reaction with hydroxyl radicals?

10⁻⁵ seconds The approximate lifetime of DNA radicals formed either by direct ionization or reaction with OH (hydroxyl) radicals is 10⁻⁵ seconds. The OH radical has an approximate lifetime of 10⁻⁹ seconds, the primary radicals from ejection of an electron is 10⁻¹⁰ seconds, and the initial ionization may take only10⁻¹⁵ seconds.

Which is the most abundant type of DNA damage caused by low LET ionizing radiation?

Base damage Typically, there are 1000-2500 base damages, 40 double strand breaks, 1000 single strand breaks and 800-1600 sugar damages per Gray of low LET radiation.

What repair mechanism is responsible for the greatest number of radiation induced damage?

Base excision repair Base alterations and base damage are the most abundant type of alteration caused by photons. The oxidation of guanine (8-oxo-guanine) is the most common base damage.

Which of the following human genetic diseases is characterized by an increased level of sister chromatid exchanges?

Bloom syndrome The mutated gene responsible for Bloom Syndrome is a RecQ helicase that is involved in homologous recombination repair, specifically by repairing damage at stalled replication forks. Werner syndrome and bloom syndrome both play a role in the RecQ helicases which are involved in HRR. This defect leads to exchanges with undamaged templates in either homologous chromosomes or sister chromatids

In response to radiation-induced DNA damage, p53 increases the expression of which of the following gene products?

DNA damage-induced G1 arrest is governed by p53-mediated transactivation of p21, which inhibits G1 cyclins and therefore prevents movement of cells from G1 into S phase.

The Ku proteins involved in NHEJ are:

DNA binding proteins. The eukaryotic Ku protein is a heterodimer of Ku70 (XRCC6) and Ku80 (XRCC5) that participates in NHEJ, although is not a DNA repair protein per se. Instead, Ku functions as a molecular scaffold to which other proteins involved in NHEJ can bind, including the main repair protein, DNA-PKcs. Ku binds to and forms a sheath around the DNA that is capable of sliding along the helix as repair progresses. It also acts to stabilize and align broken DNA ends so that they don't unwind or mis-rejoin.

Non-targeted, radiation-induced bystander effects are associated with:

Effects in non-irradiated cells co-cultured with irradiated cells

Ku70/80 (XRCC6/XRCC5) a known substrate for ATM? T/F

False

6-4 photoproducts are produced by exposure to ionizing radiation? T/F

False 6-4 photoproducts are produced by UV and not ionizing radiation.

Asymmetrical chromosomal aberrations are more likely to be carcinogenic than fatal. T/F

False An asymmetrical chromosomal aberration results in the formation of an acentric fragment that is often lost from the cell at the subsequent mitosis, usually leading to cell death. Symmetrical aberrations result in an even exchange of chromosomal material with nothing lost, and as such, tend to be less fatal, longer-lived, and potentially more carcinogenic.

What best characterizes a radiation-induced chromosomal translocation?

Two-hit non-lethal aberration Translocations require two chromosomal breaks, but it tends to be non-lethal

Regarding sublethal damage repair (SLDR) and mammalian cell survival:

a D₀ of 10 Gy indicates a high capacity for SLDR The shoulder region of the cell survival curve represents sublethal damage repair, and that is related to the beta component of cell kill.

The NBS1/MRE11/RAD50 protein complex

acts in both HR and NHEJ pathways and also in maintenance of telomeres.

Non-homologous recombination

can result in error-prone (i.e., low-fidelity) repair of DNA double-strand breaks and predominates in the G1 phase of the cell cycle