DNA damage response signaling — MCQs

10 questions

A 33-year-old woman comes to the physician 1 week after noticing a lump in her right breast. Fifteen years ago, she was diagnosed with osteosarcoma of her left distal femur. Her father died of an adrenocortical carcinoma at the age of 41 years. Examination shows a 2-cm, firm, immobile mass in the lower outer quadrant of the right breast. A core needle biopsy of the mass shows adenocarcinoma. Genetic analysis in this patient is most likely to show a defect in which of the following genes?

A 71-year-old man with colorectal cancer comes to the physician for follow-up examination after undergoing a sigmoid colectomy. The physician recommends adjuvant chemotherapy with an agent that results in single-stranded DNA breaks. This chemotherapeutic agent most likely has an effect on which of the following enzymes?

A 3-year-old male child is found to have a disease involving DNA repair. Specifically, he is found to have a defect in the endonucleases involved in the nucleotide excision repair of pyrimidine dimers. Which of the following is a unique late-stage complication of this child's disease?

While performing a Western blot, a graduate student spilled a small amount of the radiolabeled antibody on her left forearm. Although very little harm was done to the skin, the radiation did cause minor damage to the DNA of the exposed skin by severing covalent bonds between the nitrogenous bases and the deoxyribose sugar, leaving several apurinic/apyrimidinic sites. Damaged cells would most likely repair these sites by which of the following mechanisms?

A 54-year-old woman with breast cancer comes to the physician because of redness and pain in the right breast. She has been undergoing ionizing radiation therapy daily for the past 2 weeks as adjuvant treatment for her breast cancer. Physical examination shows erythema, edema, and superficial desquamation of the skin along the right breast at the site of radiation. Sensation to light touch is intact. Which of the following is the primary mechanism of DNA repair responsible for preventing radiation-induced damage to neighboring neurons?

A 5-month-old male infant from a consanguineous marriage presents with severe sunburns and freckling in sun exposed areas. The mother explains that the infant experiences these sunburns every time the infant goes outside despite applying copious amounts of sunscreen. Which of the following DNA repair mechanisms is defective in this child?

A mutant stem cell was created by using an inducible RNAi system, such that when doxycycline is added, the siRNA targeting DNA helicase is expressed, effectively knocking down the gene for DNA helicase. Which of the following will occur during DNA replication?

An 84-year-old man comes to the emergency department because of lower back pain and lower extremity weakness for 3 weeks. Over the past week, he has also found it increasingly difficult to urinate. He has a history of prostate cancer, for which he underwent radical prostatectomy 8 years ago. His prostate-specific antigen (PSA) level was undetectable until a routine follow-up visit last year, when it began to increase from 0.8 ng/mL to its present value of 64.3 ng/mL (N < 4). An MRI of the spine shows infiltrative vertebral lesions with a collapse of the L5 vertebral body, resulting in cord compression at L4–L5. The patient receives one dose of intravenous dexamethasone and subsequently undergoes external beam radiation. Which of the following cellular changes is most likely to occur as a result of this treatment?

An investigator is studying the replication of bacterial DNA with modified nucleotides. After unwinding, the double-stranded DNA forms a Y-shaped replication fork that separates into two strands. At each of these strands, daughter strands are synthesized. One strand is continuously extended from the template strands in a 5′ to 3′ direction. Which of the following is exclusively associated with the strand being synthesized away from the replication fork?

Q10

DNA replication is a highly complex process where replication occurs on both strands of DNA. On the leading strand of DNA, replication occurs uninterrupted, but on the lagging strand, replication is interrupted and occurs in fragments called Okazaki fragments. These fragments need to be joined, which of the following enzymes is involved in the penultimate step before ligation can occur?

DNA damage response signaling US Medical PG Practice Questions and MCQs

Question 1: A 33-year-old woman comes to the physician 1 week after noticing a lump in her right breast. Fifteen years ago, she was diagnosed with osteosarcoma of her left distal femur. Her father died of an adrenocortical carcinoma at the age of 41 years. Examination shows a 2-cm, firm, immobile mass in the lower outer quadrant of the right breast. A core needle biopsy of the mass shows adenocarcinoma. Genetic analysis in this patient is most likely to show a defect in which of the following genes?

A. BRCA1
B. KRAS
C. TP53 (Correct Answer)
D. Rb
E. PTEN

Explanation: ***TP53*** - This patient's presentation with **early-onset breast cancer**, a history of **osteosarcoma** at a young age, and a father's death from **adrenocortical carcinoma** at 41 years strongly suggests **Li-Fraumeni syndrome**. - Li-Fraumeni syndrome is an autosomal dominant disorder caused by a germline mutation in the **tumor suppressor gene TP53**, increasing the risk for multiple primary cancers at a young age. *BRCA1* - While **BRCA1 mutations** are associated with an increased risk of breast and ovarian cancer, they are not typically linked to osteosarcoma or adrenocortical carcinoma. - The constellation of cancers in this patient is more indicative of Li-Fraumeni syndrome than solely a BRCA1-related cancer syndrome. *KRAS* - **KRAS** is an oncogene commonly mutated in several cancers, including pancreatic, colorectal, and lung cancer, but is not primarily associated with either Li-Fraumeni syndrome or the specific tumors seen in this family history. - Mutations in KRAS are typically somatic mutations acquired during a person's lifetime, not germline mutations causing inherited cancer syndromes like the one suggested here. *Rb* - Mutations in the **retinoblastoma (Rb) gene** are associated with retinoblastoma and an increased risk of osteosarcoma, but not typically with adrenocortical carcinoma or breast cancer as part of a classic inherited syndrome. - The combination of breast cancer, osteosarcoma, and adrenocortical carcinoma points more specifically to TP53. *PTEN* - **PTEN mutations** are associated with Cowden syndrome, which increases the risk for breast cancer, thyroid cancer, and endometrial cancer, along with benign growths. - However, Cowden syndrome does not typically include osteosarcoma or adrenocortical carcinoma as prominent features, making PTEN less likely than TP53 for this specific family history.

Question 2: A 71-year-old man with colorectal cancer comes to the physician for follow-up examination after undergoing a sigmoid colectomy. The physician recommends adjuvant chemotherapy with an agent that results in single-stranded DNA breaks. This chemotherapeutic agent most likely has an effect on which of the following enzymes?

A. DNA polymerase III
B. Topoisomerase I (Correct Answer)
C. Helicase
D. Telomerase
E. Topoisomerase II

Explanation: ***Topoisomerase I*** - **Topoisomerase I** creates **single-stranded DNA (ssDNA) breaks** to relieve torsional stress during DNA replication and transcription. - Many chemotherapeutic agents, such as camptothecin and its derivatives (e.g., irinotecan, topotecan), target topoisomerase I, leading to DNA damage and apoptosis in cancer cells. *DNA polymerase III* - **DNA polymerase III** is primarily involved in bacterial DNA replication, synthesizing new DNA strands in a 5' to 3' direction. - While essential for bacterial survival, it is not the target of chemotherapeutic agents that induce single-stranded DNA breaks in human cells. *Helicase* - **Helicase** is responsible for unwinding the DNA double helix during replication and transcription, separating the two strands. - While its function is critical for DNA processes, it does not directly create DNA breaks as its primary mechanism of action. *Telomerase* - **Telomerase** is an enzyme that maintains telomere length at the ends of chromosomes, particularly active in cancer cells. - Inhibitors of telomerase aim to shorten telomeres, leading to cellular senescence or apoptosis, but they do not primarily cause single-stranded DNA breaks. *Topoisomerase II* - **Topoisomerase II** creates **double-stranded DNA (dsDNA) breaks** to untangle and decatenate DNA. - Though also a target for chemotherapy (e.g., etoposide, doxorubicin), its mechanism involves double-stranded breaks, not single-stranded breaks as specified in the question.

Question 3: A 3-year-old male child is found to have a disease involving DNA repair. Specifically, he is found to have a defect in the endonucleases involved in the nucleotide excision repair of pyrimidine dimers. Which of the following is a unique late-stage complication of this child's disease?

A. Telangiectasia
B. Colorectal cancer
C. Malignant melanoma (Correct Answer)
D. Lymphomas
E. Endometrial cancer

Explanation: **Malignant melanoma** - The described condition is **xeroderma pigmentosum**, an autosomal recessive disorder characterized by a defect in **nucleotide excision repair (NER)**, specifically the inability to remove **pyrimidine dimers** caused by **UV radiation**. - This severely impaired DNA repair leads to an extreme predisposition to **UV-induced skin cancers**, including basal cell carcinomas, squamous cell carcinomas, and, most aggressively, **malignant melanoma**, which is a unique and life-threatening late-stage complication. *Telangiectasia* - **Telangiectasias** are dilated small blood vessels that appear on the skin or mucous membranes and can be associated with various conditions. - While skin abnormalities are prevalent in xeroderma pigmentosum due to sun damage, **melanoma** is a more specific and severe late-stage complication directly resulting from the DNA repair defect. *Colorectal cancer* - **Colorectal cancer** is typically associated with other DNA repair defects, such as those in the **mismatch repair system**, as seen in conditions like **Lynch syndrome**. - It is not a primary or most significant late-stage complication of xeroderma pigmentosum, which is primarily characterized by skin cancers. *Lymphomas* - **Lymphomas** are cancers of the lymphatic system, often linked to immune deficiencies or specific genetic translocations. - While individuals with genetic syndromes can have increased cancer risks, **lymphoma** is not the hallmark late-stage complication of xeroderma pigmentosum; skin cancers are the predominant concern. *Endometrial cancer* - **Endometrial cancer** is a gynecological cancer often associated with hormonal factors or genetic predispositions like Lynch syndrome, which involves mismatch repair defects. - This type of cancer is not a characteristic or unique late-stage complication of xeroderma pigmentosum, whose pathology is centered on **UV-induced DNA damage** and subsequent skin malignancies.

Question 4: While performing a Western blot, a graduate student spilled a small amount of the radiolabeled antibody on her left forearm. Although very little harm was done to the skin, the radiation did cause minor damage to the DNA of the exposed skin by severing covalent bonds between the nitrogenous bases and the deoxyribose sugar, leaving several apurinic/apyrimidinic sites. Damaged cells would most likely repair these sites by which of the following mechanisms?

A. Nucleotide excision repair
B. Nonhomologous end joining repair
C. Homologous recombination
D. Mismatch repair
E. Base excision repair (Correct Answer)

Explanation: **Base excision repair** - This mechanism is specifically involved in correcting **single-base DNA damage** or **modified bases**, such as **apurinic/apyrimidinic (AP) sites**. - It involves removing the damaged base by a **DNA glycosylase**, creating an AP site, which is then processed by an **AP endonuclease** to cleave the phosphodiester backbone, followed by DNA polymerase and ligase. *Nucleotide excision repair* - Primarily repairs **bulky DNA lesions**, such as **thymine dimers** caused by UV radiation, or damage from chemical adducts that distort the DNA helix. - It involves excising a larger oligonucleotide containing the damage, not just a single base. *Nonhomologous end joining repair* - This pathway is used to repair **double-strand DNA breaks**, where both strands of the DNA molecule are broken. - It is a "quick-and-dirty" repair mechanism that ligates the broken ends together, often leading to small insertions or deletions. *Homologous recombination* - A repair mechanism for **double-strand DNA breaks** that uses a homologous DNA template (e.g., sister chromatid) to accurately repair the break. - This process is highly accurate but occurs only when a homologous template is available, typically during the S and G2 phases of the cell cycle. *Mismatch repair* - Corrects **base-pair mismatches** and **small insertions/deletions** that occur during DNA replication, which were not corrected by DNA polymerase proofreading. - It targets newly synthesized DNA strands based on methylation patterns in the parental strand.

Question 5: A 54-year-old woman with breast cancer comes to the physician because of redness and pain in the right breast. She has been undergoing ionizing radiation therapy daily for the past 2 weeks as adjuvant treatment for her breast cancer. Physical examination shows erythema, edema, and superficial desquamation of the skin along the right breast at the site of radiation. Sensation to light touch is intact. Which of the following is the primary mechanism of DNA repair responsible for preventing radiation-induced damage to neighboring neurons?

A. Homology-directed repair
B. Base excision repair
C. Nonhomologous end joining repair (Correct Answer)
D. DNA mismatch repair
E. Nucleotide excision repair

Explanation: ***Nonhomologous end joining repair*** - This pathway is crucial for repairing **double-strand DNA breaks**, which are a major form of damage caused by **ionizing radiation**. - It directly ligates the broken DNA ends without requiring a homologous template, making it an efficient but potentially error-prone repair mechanism. *Homology-directed repair* - This pathway is also used to repair **double-strand DNA breaks** but requires a **homologous DNA template** (usually a sister chromatid) for accurate repair. - While highly accurate, it is typically active during the S and G2 phases of the cell cycle and is generally slower and less dominant than NHEJ for immediate radiation-induced damage in non-dividing cells like neurons. *Base excision repair* - This mechanism primarily corrects damage to individual DNA bases, such as **oxidative damage**, alkylation, or deamination. - It is not the primary mechanism for repairing the **double-strand breaks** induced by ionizing radiation. *DNA mismatch repair* - This pathway corrects errors that arise during **DNA replication**, specifically mismatched base pairs or small insertions/deletions. - It is not involved in repairing radiation-induced DNA damage like **double-strand breaks**. *Nucleotide excision repair* - This pathway repairs bulky DNA lesions, such as those caused by **UV radiation** (e.g., pyrimidine dimers) or chemical mutagens. - It removes a segment of DNA containing the damage but is not the primary repair mechanism for **double-strand breaks** caused by ionizing radiation.

Question 6: A 5-month-old male infant from a consanguineous marriage presents with severe sunburns and freckling in sun exposed areas. The mother explains that the infant experiences these sunburns every time the infant goes outside despite applying copious amounts of sunscreen. Which of the following DNA repair mechanisms is defective in this child?

A. Non-homologous end joining
B. Homologous recombination
C. Base excision repair
D. Mismatch repair
E. Nucleotide excision repair (Correct Answer)

Explanation: ***Nucleotide excision repair*** - The symptoms of **severe sunburns** and **freckling in sun-exposed areas** are classic manifestations of **Xeroderma Pigmentosum (XP)**. - XP is caused by a defect in **nucleotide excision repair (NER)**, which is crucial for removing **UV-induced DNA damage**, such as **pyrimidine dimers**. *Non-homologous end joining* - This mechanism repairs **double-strand DNA breaks** by directly ligating the broken ends, often with some loss of genetic information. - Defects in non-homologous end joining are associated with conditions like **immunodeficiency** and increased cancer risk, but not with UV sensitivity like XP. *Homologous recombination* - This high-fidelity repair pathway uses a **homologous DNA template** to accurately repair **double-strand breaks** and interstrand crosslinks. - Impaired homologous recombination is linked to conditions like **Fanconi anemia** and increased risk of certain cancers, but not primarily to UV hypersensitivity. *Base excision repair* - **Base excision repair (BER)** is responsible for removing **damaged or modified bases** from DNA, such as oxidized or alkylated bases. - Defects in BER can lead to increased spontaneous mutagenesis and cancer, but do not explain the specific sensitivity to UV light seen in this infant. *Mismatch repair* - **Mismatch repair (MMR)** corrects errors that occur during DNA replication, such as **base mismatches** or small insertions/deletions. - Defective MMR is strongly associated with **hereditary nonpolyposis colorectal cancer (Lynch syndrome)**, but not with severe reactions to sun exposure.

Question 7: A mutant stem cell was created by using an inducible RNAi system, such that when doxycycline is added, the siRNA targeting DNA helicase is expressed, effectively knocking down the gene for DNA helicase. Which of the following will occur during DNA replication?

A. The RNA primer is not created
B. DNA is not unwound (Correct Answer)
C. The two melted DNA strands reanneal
D. DNA supercoiling is not relieved
E. Newly synthesized DNA fragments are not ligated

Explanation: ***DNA is not unwound*** - **DNA helicase** is essential for unwinding the **double-stranded DNA** helix, separating it into two single strands. This process creates the **replication fork**. - Without functional DNA helicase due to **gene knockdown**, the DNA helix cannot be unwound, thus halting DNA replication. *The RNA primer is not created* - **RNA primers** are synthesized by **primase**, an enzyme distinct from DNA helicase. - While unwinding is necessary for primer synthesis, the *creation* of the primer itself is a function of primase. *The two melted DNA strands reanneal* - **Reannealing** of DNA strands is prevented by **single-strand binding proteins (SSBs)**, which bind to the separated single strands. - While helicase unwinds, SSBs specifically keep the strands apart to allow DNA polymerase access. *DNA supercoiling is not relieved* - **DNA supercoiling** is relieved by **topoisomerases**, enzymes that cut, unwind, and religate DNA strands to reduce torsional stress. - This is a distinct function from DNA helicase, which focuses on breaking hydrogen bonds between strands. *Newly synthesized DNA fragments are not ligated* - **Ligation** of newly synthesized **Okazaki fragments** on the lagging strand is performed by **DNA ligase**. - This process occurs downstream from the unwinding step facilitated by DNA helicase.

Question 8: An 84-year-old man comes to the emergency department because of lower back pain and lower extremity weakness for 3 weeks. Over the past week, he has also found it increasingly difficult to urinate. He has a history of prostate cancer, for which he underwent radical prostatectomy 8 years ago. His prostate-specific antigen (PSA) level was undetectable until a routine follow-up visit last year, when it began to increase from 0.8 ng/mL to its present value of 64.3 ng/mL (N < 4). An MRI of the spine shows infiltrative vertebral lesions with a collapse of the L5 vertebral body, resulting in cord compression at L4–L5. The patient receives one dose of intravenous dexamethasone and subsequently undergoes external beam radiation. Which of the following cellular changes is most likely to occur as a result of this treatment?

A. Intercalation of neighbouring DNA base pairs
B. Disruption of microtubule assembly
C. Formation of DNA crosslinks
D. Generation of hydroxyl radicals (Correct Answer)
E. Formation of pyrimidine dimers

Explanation: ***Generation of hydroxyl radicals*** - **External beam radiation** primarily causes cellular damage through the **ionization of water molecules**, leading to the formation of highly reactive **hydroxyl radicals**. - These radicals directly damage **DNA**, proteins, and cell membranes, leading to **cell death or apoptosis**, especially in rapidly dividing cells like cancer cells. *Intercalation of neighbouring DNA base pairs* - This mechanism is characteristic of certain **chemotherapeutic agents** (e.g., **doxorubicin**, **daunorubicin**) that insert themselves between stacked DNA base pairs. - This process distorts the DNA helix, interfering with replication and transcription, but it is **not the primary mechanism of radiation therapy**. *Disruption of microtubule assembly* - **Microtubule inhibitors** (e.g., **vincristine**, **paclitaxel**) disrupt the formation or disassembly of microtubules, which are essential for cell division and intracellular transport. - While this is a common mechanism of action for some **chemotherapeutic drugs**, it is **not how radiation therapy works**. *Formation of DNA crosslinks* - **Alkylating agents** (e.g., **cyclophosphamide**, **cisplatin**) form covalent bonds within or between DNA strands, creating crosslinks that prevent DNA replication and transcription. - Though highly damaging to DNA, this is a distinct mechanism of action typically associated with **chemotherapy**, not direct radiation. *Formation of pyrimidine dimers* - **Ultraviolet (UV) radiation** causes the formation of **pyrimidine dimers** (e.g., thymine dimers) in DNA. - This type of DNA damage is characteristic of UV light exposure and is **not the primary mechanism of action for external beam radiation therapy**, which uses higher-energy ionizing radiation.

Question 9: An investigator is studying the replication of bacterial DNA with modified nucleotides. After unwinding, the double-stranded DNA forms a Y-shaped replication fork that separates into two strands. At each of these strands, daughter strands are synthesized. One strand is continuously extended from the template strands in a 5′ to 3′ direction. Which of the following is exclusively associated with the strand being synthesized away from the replication fork?

A. Reverse transcriptase activity
B. Repeated activity of ligase (Correct Answer)
C. Elongation in the 3'→5' direction
D. Synthesis of short RNA sequences
E. 5' → 3' exonuclease activity

Explanation: ***Repeated activity of ligase*** - The lagging strand, synthesized away from the replication fork, is made in fragments (**Okazaki fragments**) due to the 5' to 3' synthesis direction of DNA polymerase. - **DNA ligase** repeatedly joins these Okazaki fragments together, forming a continuous strand. *Reverse transcriptase activity* - **Reverse transcriptase** synthesizes DNA from an RNA template, which is not involved in normal bacterial DNA replication. - This enzyme is characteristic of **retroviruses** and certain eukaryotic telomere maintenance. *Elongation in the 3'→5' direction* - DNA polymerases only synthesize new DNA strands in the **5' to 3' direction**. - While reading the template strand in the 3' to 5' direction, the daughter strand is always built from 5' to 3'. *Synthesis of short RNA sequences* - **RNA primers** are synthesized by **primase** on both the leading and lagging strands to initiate DNA synthesis. - This process is not exclusive to the strand synthesized away from the replication fork; the leading strand also requires an initial RNA primer. *5' → 3' exonuclease activity* - The **5' to 3' exonuclease activity** of DNA polymerase I in bacteria is responsible for removing RNA primers. - This activity occurs on both the leading and lagging strands, as both require primer removal.

Question 10: DNA replication is a highly complex process where replication occurs on both strands of DNA. On the leading strand of DNA, replication occurs uninterrupted, but on the lagging strand, replication is interrupted and occurs in fragments called Okazaki fragments. These fragments need to be joined, which of the following enzymes is involved in the penultimate step before ligation can occur?

A. DNA gyrase
B. DNA ligase
C. DNA helicase
D. DNA polymerase I (Correct Answer)
E. DNA polymerase III

Explanation: **DNA polymerase I** - **DNA polymerase I** plays a crucial role in removing the **RNA primers** from the Okazaki fragments on the lagging strand. - After primer removal, it fills the resulting gaps with **deoxyribonucleotides** before DNA ligase seals the nicks. *DNA gyrase* - **DNA gyrase** (a type of **topoisomerase**) is involved in relieving **supercoiling** ahead of the replication fork. - It does not directly participate in the joining of Okazaki fragments, but rather in maintaining DNA topology during replication. *DNA ligase* - **DNA ligase** is responsible for the **final sealing** of the nicks between adjacent Okazaki fragments. - It forms a **phosphodiester bond** between the 3'-hydroxyl end of one fragment and the 5'-phosphate end of the next, following primer removal and gap filling. *DNA helicase* - **DNA helicase** unwinds the double-stranded DNA helix, separating the two strands at the **replication fork**. - This enzyme is essential for initiating replication but does not participate in processing Okazaki fragments. *DNA polymerase III* - **DNA polymerase III** is the primary enzyme responsible for the **elongation of new DNA strands** in both leading and lagging strand synthesis. - It synthesizes the actual Okazaki fragments but does not directly remove primers or fill the gaps.

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