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Difficulty: MediumGenetic and Molecular Mechanisms

A 28-year-old man with recently diagnosed Crohn disease comes to the emergency department due to a 3-day history of high fever, chills, and severe fatigue. He also reports painful sores in his mouth. Four weeks ago, his gastroenterologist started him on azathioprine and a tapering dose of prednisone to manage a severe disease flare. His temperature is 39.2∘C39.2^\circ\text{C} (102.6∘F102.6^\circ\text{F}), blood pressure is 110/70 mm Hg110/70\text{ mm Hg}, and pulse is 115/min115/\text{min}. Physical examination shows pale conjunctivae and multiple aphthous ulcers on the buccal mucosa. Laboratory studies show:

Laboratory TestResult
Leukocyte count1,100/mm31,100/\text{mm}^3
Absolute neutrophil count400/mm3400/\text{mm}^3
Hemoglobin7.2 g/dL7.2 \text{ g/dL}
Platelet count42,000/mm342,000/\text{mm}^3

Which of the following genetic mechanisms best explains this patient's current presentation?

  1. Autosomal recessive inheritance of a defective methyltransferase enzymeAnswer
  2. B
    Specific human leukocyte antigen allele variant causing immune-mediated hypersensitivity
  3. C
    Genetic polymorphism altering cytochrome P450-mediated drug clearance
  4. D
    Somatic mutation causing uncontrolled clonal proliferation of myeloid progenitors
  5. E
    X-linked recessive deficiency of an erythrocyte antioxidant enzyme

Answer

Autosomal recessive inheritance of a defective methyltransferase enzyme
The correct answer describes a deficiency in thiopurine methyltransferase (TPMT). Azathioprine is a prodrug that is non-enzymatically converted to 6-mercaptopurine (6-MP). 6-MP is subsequently metabolized through distinct pathways: inactivation by TPMT, inactivation by xanthine oxidase (XO), or activation by hypoxanthine-guanine phosphoribosyltransferase (HGPRT) into 6-thioguanine nucleotides (6-TGN). 6-TGNs are purine analogs that incorporate into cellular DNA, causing cytotoxicity and immunosuppression. Patients with autosomal recessive polymorphisms in the TPMT gene have decreased or absent enzyme activity. When these patients receive normal doses of azathioprine, drug metabolism is heavily shunted toward the HGPRT pathway, resulting in a massive overproduction of cytotoxic 6-TGNs. This leads to profound, dose-dependent bone marrow suppression, presenting clinically as severe pancytopenia (fever, mucosal ulcers, anemia, bleeding). Standard practice requires testing for TPMT enzymatic activity or genotype prior to initiating thiopurine therapy.

Step-by-Step Solution

1
Analyze the clinical presentation and timeline.
The patient developed severe pancytopenia (leukopenia, anemia, thrombocytopenia) and mucosal ulcers four weeks after initiating azathioprine.
Establishing the temporal relationship between a new medication and symptom onset is critical for identifying an adverse drug reaction.
2
Identify the primary metabolic pathway of the suspected medication.
Azathioprine is a prodrug converted to 6-mercaptopurine (6-MP), which is inactivated by thiopurine methyltransferase (TPMT) or xanthine oxidase (XO).
Understanding drug metabolism pathways reveals the molecular vulnerability causing the toxicity.
3
Correlate the toxicity with the underlying genetic mechanism.
An autosomal recessive deficiency in TPMT shunts 6-MP metabolism toward the production of cytotoxic 6-thioguanine nucleotides (6-TGN), causing severe bone marrow suppression.
This confirms that a defective methyltransferase enzyme directly explains the patient's profound pancytopenia.

Key Concept

Patients with a genetic polymorphism resulting in decreased thiopurine methyltransferase (TPMT) activity are at high risk for life-threatening myelosuppression when treated with azathioprine, as the drug is shunted into cytotoxic metabolites.
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Topics

Genetic and Molecular Mechanisms

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