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Difficulty: HardGenetic and Molecular Mechanisms

A 34-year-old man comes to the emergency department due to a 3-day history of high fever, severe sore throat, and easy bruising. Four weeks ago, he was diagnosed with severe ileocolonic Crohn disease and was started on a tapering dose of oral prednisone and azathioprine. His temperature is 39.2°C (102.6°F), blood pressure is 105/65 mm Hg, and pulse is 115/min. Physical examination shows pale conjunctivae, pharyngeal erythema with exudates, and scattered petechiae over the lower extremities.

Laboratory studies show:
Leukocyte count: 1,100/mm³
Absolute neutrophil count: 200/mm³
Hemoglobin: 8.2 g/dL
Platelet count: 35,000/mm³

The patient's condition is most likely due to an underlying genetic defect in which of the following molecular processes?

  1. Autosomal codominant mutation impairing S-methylation of a purine analogAnswer
  2. B
    Gene duplication resulting in ultrarapid cytochrome P450 oxidation
  3. C
    Homozygous promoter variant decreasing glucuronidation of a drug metabolite
  4. D
    Autosomal recessive defect in hepatic N-acetylation of an amine compound
  5. E
    X-linked recessive mutation decreasing generation of reduced glutathione

Answer

The correct underlying mechanism is an autosomal codominant mutation impairing S-methylation of a purine analog, which represents thiopurine methyltransferase (TPMT) deficiency.
Azathioprine is an immunosuppressant prodrug that is rapidly converted to 6-mercaptopurine (6-MP). 6-MP is subsequently metabolized via three parallel pathways: activation by hypoxanthine-guanine phosphoribosyltransferase (HGPRT) into active, cytotoxic thioguanine nucleotides; and inactivation by either xanthine oxidase (to 6-thiouric acid) or thiopurine methyltransferase (TPMT). TPMT inactivates 6-MP via S-methylation. Patients with a genetic deficiency in TPMT (inherited in an autosomal codominant manner) cannot adequately metabolize 6-MP via this degradation pathway. Consequently, 6-MP is disproportionately shunted toward the HGPRT pathway, resulting in a toxic accumulation of thioguanine nucleotides. This leads to severe, life-threatening myelosuppression, presenting as global pancytopenia typically within weeks of starting the drug. Prior to initiating azathioprine or 6-MP, patients should undergo TPMT phenotyping or genotyping.

Step-by-Step Solution

1
Identify the patient's acute clinical presentation and offending agent.
The patient has severe pancytopenia (evidenced by fever, bleeding, and profoundly low white blood cells, hemoglobin, and platelets) occurring 4 weeks after initiating azathioprine.
Recognizing the specific drug-adverse effect pairing is the first essential step to determining the underlying molecular mechanism.
2
Determine the metabolic pathway and activation/inactivation steps of the offending drug.
Azathioprine is a prodrug nonenzymatically converted to 6-mercaptopurine (6-MP). 6-MP is inactivated by thiopurine methyltransferase (TPMT) via S-methylation and by xanthine oxidase, while it is activated into cytotoxic thioguanine nucleotides by HGPRT.
Understanding the competing enzymatic pathways is necessary to identify which specific deficiency leads to toxic accumulation of the drug.
3
Correlate the observed toxicity with the precise molecular and genetic defect.
A genetic deficiency in TPMT forces 6-MP metabolism down the activation pathway, causing massive accumulation of thioguanine nucleotides and resulting in life-threatening bone marrow suppression. This defect is due to an autosomal codominant mutation affecting S-methylation.
This establishes the definitive pharmacogenomic mechanism asked in the question.

Key Concept

Pharmacogenomics of thiopurines (azathioprine and 6-mercaptopurine) and the role of thiopurine methyltransferase (TPMT) deficiency in causing severe myelosuppression due to impaired S-methylation.
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Topics

Genetic and Molecular Mechanisms

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