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Difficulty: MediumInfectious Disease Mechanisms

A 52-year-old woman is evaluated in the hospital for progressive fatigue, shortness of breath, and visual disturbances over the past week. She underwent a deceased-donor renal transplantation 6 months ago and is currently maintained on tacrolimus, mycophenolate mofetil, and prednisone. Temperature is 38.1°C (100.6°F), pulse is 96/min, respirations are 18/min, and blood pressure is 132/78 mm Hg. Physical examination reveals crackles at both lung bases. Fundoscopy of the right eye shows extensive retinal hemorrhages and white, fluffy perivascular exudates. Laboratory studies show a leukocyte count of 2,400/mm³ and a platelet count of 90,000/mm³. The patient is started on the standard first-line intravenous antiviral therapy for her presumed infection. After 14 days, her symptoms persist, and a quantitative viral load shows progressive systemic infection. Her antiviral regimen is switched to an intravenous pyrophosphate analog, which results in a gradual clinical improvement. Which of the following molecular mechanisms most likely accounts for the failure of the initial antiviral therapy in this patient?

  1. Inability of the virus to catalyze the initial phosphorylation of the drugAnswer
  2. B
    Conformational change in the viral DNA polymerase preventing drug binding
  3. C
    Loss of viral thymidine kinase expression
  4. D
    Accelerated hepatic metabolism of the drug via cytochrome P450 induction
  5. E
    Amino acid substitution in a viral reverse transcriptase

Answer

Inability of the virus to catalyze the initial phosphorylation of the drug
The patient's clinical presentation of fever, pneumonitis, pancytopenia, and retinitis (hemorrhages and cotton-wool spots) 6 months post-transplant is classic for severe Cytomegalovirus (CMV) infection. The standard first-line treatment is intravenous ganciclovir, a guanosine analog. To become active, ganciclovir must undergo initial phosphorylation catalyzed by the CMV-specific UL97 protein kinase, followed by subsequent phosphorylations by host cellular kinases. The active triphosphate form then competitively inhibits the viral DNA polymerase (UL54). The most common mechanism of CMV resistance to ganciclovir is a mutation in the UL97 gene, which prevents the crucial first phosphorylation step, rendering the drug inactive. The patient was successfully treated with foscarnet, a pyrophosphate analog that directly inhibits the viral DNA polymerase without requiring prior activation by viral kinases. Foscarnet's efficacy confirms that the viral DNA polymerase was still functional and susceptible, pinpointing the resistance mechanism to the initial phosphorylation step.

Step-by-Step Solution

1
Identify the primary infectious syndrome and causative pathogen.
The patient has pneumonitis, pancytopenia, and characteristic retinitis ('pizza pie' appearance with hemorrhages and exudates) 6 months post-renal transplant. This classic presentation strongly points to tissue-invasive Cytomegalovirus (CMV).
Transplant patients on profound immunosuppression are at high risk for CMV reactivation, classically presenting with multi-organ involvement including the retina, lungs, and bone marrow.
2
Determine the first-line therapy and the salvage therapy used.
The standard first-line therapy for severe CMV is intravenous ganciclovir. The salvage therapy described as a 'pyrophosphate analog' is foscarnet.
Understanding the pharmacologic sequence is required to deduce the mechanism of resistance.
3
Deduce the mechanism of resistance to the initial drug that leaves the salvage drug effective.
Ganciclovir requires initial phosphorylation by the CMV UL97 protein kinase to become active. Foscarnet does not require phosphorylation. Therefore, a mutation in UL97 prevents ganciclovir activation but preserves foscarnet susceptibility.
Because foscarnet bypasses the initial viral phosphorylation step and directly inhibits the viral DNA polymerase (UL54), it remains effective against UL97-mutated CMV strains.

Key Concept

Cytomegalovirus (CMV) resistance to ganciclovir typically occurs via mutations in the viral UL97 protein kinase, which prevents the drug's essential initial phosphorylation step.
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Topics

Infectious Disease Mechanisms

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