Neurogenesis can be inhibited by Cytosine beta-D-arabinofuanoside (Ara-C). How does Ara-C work?

Question

BlackTom AI · Accepted Answer

Cytarabine (Ara-C) is a cytosine nucleoside analog that must be metabolically activated to exert its effects: it is taken up by cells and phosphorylated to Ara-C triphosphate (Ara-CTP).

1) Relevant knowledge points and concepts
- Nucleoside analogs require phosphorylation to nucleoside triphosphates to be incorporated into DNA.
- S-phase specificity: drugs incorporated into DNA preferentially affect dividing cells (e.g., neural progenitors during neurogenesis).
- Incorporation of abnormal nucleotides can stall replication forks and provoke DNA-processing enzymes (such as topoisomerase I) to form stabilized cleavage complexes, producing strand breaks and triggering apoptosis.

2) Step-by-step reasoning process
- Step 1: Cellular uptake and phosphorylation: Ara-C is transported into cells and sequentially phosphorylated by deoxycytidine kinase to Ara-CMP, then to Ara-CDP and finally to Ara-CTP.
- Step 2: Incorporation into DNA: Ara-CTP competes with dCTP and is incorporated by DNA polymerases into the nascent DNA strand during S phase.
- Step 3: Replication disruption: incorporation of the arabinose sugar alters DNA geometry, slows or terminates further chain elongation and stalls replication forks.
- Step 4: Topoisomerase I involvement: stalled forks and altered DNA structures promote formation/stabilization of topoisomerase I cleavage complexes on DNA; these complexes convert into single- and double-strand breaks.
- Step 5: DNA fragmentation and cell death: accumulated DNA breaks activate DNA damage responses and apoptosis, reducing the pool of dividing neural progenitors and thus inhibiting neurogenesis.

3) Intermediate biochemical steps shown above (phosphorylation to Ara-CTP; competition with dCTP; incorporation into DNA; formation of topoisomerase I cleavage complexes).

4) Why the answer is correct
The provided answer states Ara-C is incorporated into DNA and inhibits DNA replication by forming cleavage complexes with topoisomerase I, resulting in DNA fragmentation. That sequence matches the biochemical activation, incorporation, replication stress, topoisomerase I–mediated cleavage complex formation, and ensuing DNA fragmentation that explain Ara-C’s cytotoxicity and inhibition of neurogenesis.

5) Why other mechanisms are incorrect
- It is not primarily a microtubule inhibitor (like vincristine) nor a topoisomerase II poison (like etoposide). 
- It does not act simply by inhibiting methylation or as an antimetabolite without incorporation; its activation to Ara-CTP and incorporation into DNA are required for major cytotoxic effects.

6) Conclusion
Therefore, Ara-C acts by incorporation into DNA, provoking topoisomerase I cleavage complexes and DNA fragmentation, which matches the provided correct answer.

Neurogenesis can be inhibited by Cytosine beta-D-arabinofuanoside (Ara-C). How does Ara-C work?Single choice

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