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Difficulty: Very hardNeoplastic Processes

A 42-year-old woman is evaluated for a recurrence of high-grade serous ovarian carcinoma. She has a strong family history of breast and ovarian malignancies and is known to carry a deleterious germline mutation in the BRCA1 gene. Her oncologist initiates therapy with olaparib, an oral targeted agent that exploits the principle of synthetic lethality. Arrange the following molecular and cellular events in the correct chronological sequence that mediates the antineoplastic efficacy of this pharmacological intervention.

  1. 1Pharmacologic inhibition of base excision repair leading to the persistence of endogenous single-strand DNA breaks.
  2. 2Stalling and collapse of replication forks during the S phase, converting single-strand breaks into double-strand DNA breaks.
  3. 3Failure of the high-fidelity homologous recombination repair pathway due to an underlying genetic deficiency.
  4. 4Compensatory reliance on highly error-prone mechanisms, primarily non-homologous end joining, to repair DNA damage.
  5. 5Massive accumulation of chromosomal aberrations culminating in mitotic catastrophe and tumor cell apoptosis.

Answer

The correct sequence begins with the pharmacologic inhibition of base excision repair, followed by the conversion of single-strand breaks to double-strand breaks during replication, the failure of homologous recombination, the forced reliance on error-prone non-homologous end joining, and ultimately, fatal genomic instability.
Synthetic lethality occurs when the simultaneous disruption of two genes/pathways leads to cell death, whereas the disruption of either alone is non-lethal. Here, the tumor has a baseline defect in homologous recombination (HR) due to the BRCA1 mutation. When olaparib inhibits PARP (blocking base excision repair), single-strand DNA breaks accumulate. During DNA replication, these single-strand breaks stall replication forks, converting them into double-strand breaks. Because the tumor lacks HR, it cannot repair these double-strand breaks accurately. It relies on the error-prone non-homologous end joining (NHEJ) pathway, which results in devastating chromosomal rearrangements, mitotic catastrophe, and apoptosis.

Step-by-Step Solution

1
Identify the primary molecular target of the drug.
Olaparib inhibits PARP, blocking base excision repair and causing single-strand DNA breaks to accumulate.
Understanding the direct pharmacodynamic action of the drug establishes the first event in the cascade.
2
Determine the downstream consequence of the primary target inhibition.
Single-strand breaks are converted into double-strand breaks during the S phase of the cell cycle.
Single-strand breaks are not immediately lethal; they become lethal double-strand breaks when replication forks collapse.
3
Relate the patient's genetic profile to the molecular cascade.
The BRCA1 mutation prevents the cell from using homologous recombination to repair the newly formed double-strand breaks.
This step defines the tumor-specific vulnerability required for synthetic lethality.
4
Identify the cellular compensatory mechanism.
The cell utilizes error-prone non-homologous end joining as a desperate attempt to repair the DNA.
Cells possess backup repair pathways, which are activated when the primary high-fidelity pathway is defective.
5
Determine the ultimate cellular outcome.
Error-prone repair causes massive genomic instability, leading to mitotic catastrophe and apoptosis.
This is the final terminal event that achieves the clinical antineoplastic effect.

Key Concept

Mechanisms of synthetic lethality, specifically PARP inhibition in homologous recombination-deficient (BRCA-mutated) neoplasms.
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Topics

Neoplastic Processes

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