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Zorluk: OrtaInfectious Disease Mechanisms

A 38-year-old man with a history of advanced human immunodeficiency virus infection is evaluated in the infectious disease clinic for a two-week history of worsening blurred vision and "floating spots" in his right eye. His current CD4+ T-cell count is 32 cells/mm³. Vital signs show a temperature of 37.1°C (98.8°F), blood pressure of 118/76 mmHg, and heart rate of 78/min. Funduscopic examination reveals extensive yellow-white retinal infiltrates with associated perivascular hemorrhage. He is immediately initiated on intravenous ganciclovir therapy. After three weeks of compliant treatment, serial examinations indicate continued progression of the retinal lesions. Subsequent genotypic resistance testing of the responsible pathogen identifies a significant mutation in the UL97 gene. Which of the following best describes the direct molecular consequence of this specific genetic alteration?

  1. Inability to perform the initial phosphorylation of the prescribed antiviral agentCevap
  2. B
    Structural alteration of the viral DNA polymerase preventing analog incorporation
  3. C
    Upregulation of membrane efflux pumps leading to rapid drug extrusion
  4. D
    Loss of viral thymidine kinase activity resulting in failed drug activation
  5. E
    Modification of a viral protease enzyme preventing functional polypeptide cleavage

Cevap

The direct molecular consequence of the UL97 gene mutation is the inability to perform the initial phosphorylation of ganciclovir, which prevents the drug from becoming activated.
The patient's presentation of painless vision loss with 'floating spots' and funduscopic findings of retinal infiltrates with hemorrhage in the setting of advanced HIV (CD4+ count < 50 cells/mm³) is classic for Cytomegalovirus (CMV) retinitis. The primary treatment is ganciclovir, an antiviral guanosine analog. To exert its effect, ganciclovir must be converted to its active triphosphate form. The crucial first step in this activation pathway is the initial monophosphorylation catalyzed by a specific viral kinase encoded by the CMV UL97 gene. Once monophosphorylated, host cellular kinases complete the di- and tri-phosphorylation steps. The active ganciclovir triphosphate then competitively inhibits the viral DNA polymerase (encoded by the UL54 gene). A mutation in the UL97 gene structurally alters the viral kinase, leading to an inability to perform this initial phosphorylation. As a result, the prodrug cannot be activated, conferring clinical resistance to ganciclovir.

Adım Adım Çözüm

1
Identify the patient's underlying opportunistic infection based on the clinical presentation.
The patient has advanced HIV with a CD4+ T-cell count of 32 cells/mm³ and presents with painless vision loss and retinitis (yellow-white retinal infiltrates, perivascular hemorrhage), which is highly characteristic of Cytomegalovirus (CMV) retinitis.
Accurate identification of the pathogen is required to understand the corresponding antiviral mechanism.
2
Determine the mechanism of action of the initial antiviral therapy.
The patient was treated with ganciclovir. Ganciclovir is a guanosine analog that requires initial monophosphorylation by the CMV UL97 kinase, followed by di- and tri-phosphorylation by host cellular kinases, to become an active inhibitor of viral DNA polymerase.
Understanding the drug's physiological activation pathway is necessary to identify how resistance develops.
3
Correlate the identified genetic mutation with the molecular mechanism of resistance.
Genotypic testing reveals a mutation in the UL97 gene. This mutation impairs the function of the viral kinase, directly preventing the necessary initial phosphorylation step of ganciclovir and rendering the drug clinically inactive.
Linking the specific mutated gene to its normal physiologic function confirms the direct consequence of the mutation.

Anahtar Kavram

Cytomegalovirus (CMV) resistance to ganciclovir is most commonly due to a mutation in the viral UL97 gene, which encodes the kinase responsible for the initial phosphorylation and activation of the drug.
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Infectious Disease Mechanisms

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