- Factor IX is essential for blood clotting. Deficiency of Factor IX could be corrected by delivering factor IX gene using viral vectors.
In an experiment the gene for Factor IX was cloned appropriately into Adenovirus (AV), Adeno-associated virus (AAV) and Retrovirus (RV). Retrovirus integrate the gene into the dividing cells. AAV integrates partially into non-dividing cells. AV does not integrate the gene but transfects both dividing and non-dividing cells.
Following expression profile for Factor IX was observed when the three vectors were injected intra-muscularly into three groups of experimental mice. Which one of the following outcomes is correct?(1) a by RV; b by AV; c by AAV
(2) a by AV; b by AAV; c by RV
(3) a by AA V; b by RV; c by AV
(4) a by AV; b by RV; c by AAVThe correct matching is (4) a by AV; b by RV; c by AAV.
Key properties of the three viral vectors
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Retrovirus (RV)
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Integrates only into dividing cells.
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Integration → stable, long‑term expression.
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On intramuscular injection, expression starts slowly (cells must divide) but then plateaus and remains high.
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Adenovirus (AV)
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Remains episomal, does not integrate.
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Infects dividing and non‑dividing cells.
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Gives very high but transient expression; immune clearance and loss of episomes make levels fall quickly.
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Adeno‑associated virus (AAV)
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Infects dividing and non‑dividing cells; mostly episomal with some integration.
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Expression rises more slowly than AV but can persist for months, though it may gradually decline as episomes are diluted.
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These kinetic features determine how Factor IX appears in serum over time.
Interpreting the three curves
From the graph:
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Curve a – sharp early high peak then rapid fall.
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Typical of adenovirus: strong initial transgene expression followed by immune‑mediated loss.
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Curve b – gradual rise to a plateau that remains relatively stable.
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Matches retrovirus, where integration into dividing cells leads to stable, long‑term expression.
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Curve c – moderate rise to a lower peak than a, then slow decline over time.
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Fits AAV, giving long‑lasting but eventually decreasing expression.
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So:
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a = AV, b = RV, c = AAV → option (4).
Why other options are wrong
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a by RV; b by AV; c by AAV
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Assigns the transient peak (a) to RV and the stable curve (b) to AV, which contradicts their known biology.
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a by AV; b by AAV; c by RV
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Gives stable high b to AAV and slowly declining c to RV, again inconsistent with RV’s integrating, long‑term behavior.
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a by AAV; b by RV; c by AV
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Puts the sharp transient peak on AAV instead of AV and slow decline on AV instead of AAV; reversed.
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Only option (4) correctly reflects the expected serum Factor IX expression kinetics for each viral vector.
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