Azathioprine‐induced severe pancytopenia due to a homozygous two‐point mutation of the thiopurine methyltransferase gene in a patient with juvenile HLA‐B27‐associated spondylarthritis

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Abstract
Severe pancytopenia due to azathioprine (AZA) toxicity in patients with autoimmune diseases is not uncommon. We describe a 14‐year‐old girl with HLA‐B27+ spondylarthritis who was treated with AZA 3 mg/kg/day and who suddenly developed severe pancytopenia in the seventh week of treatment. Analysis of the catabolic pathway of AZA revealed a homozygous deficiency of thiopurine methyltransferase (TPMT) on the basis of a combined 2‐point mutation at nucleotide positions 460 and 719 in the gene for TPMT, causing a toxic level of the metabolic active 6‐thioguanine nucleotides (6‐TGN) (2,394 pmoles/8 × 108 red blood cells). The patient was transfusion dependent and finally recovered 8 weeks after the development of the pancytopenia. At that time, 6‐TGN had already returned to normal therapeutic levels. Family studies revealed another homozygous deficiency in the mother, while the other family members were heterozygous.