Two Cases of Juvenile Myelomonocytic Leukemia With Identical Somatic <i>PTPN11</i> Mutations in Children After In Utero Exposure to Thiopurine‐Containing Chemotherapy

Juvenile myelomonocytic leukemia (JMML) is a rare pediatric malignancy without defined environmental triggers. We report two cases of JMML with identical somatic PTPN11 mutations in children exposed in utero to thiopurine-containing chemotherapy. One mother underwent antileukemic treatment including thioguanine during early pregnancy; the other received 6-mercaptopurine for Crohn's disease throughout gestation. Both children were diagnosed with JMML at 34 and 37 months, respectively, with pathogenic PTPN11 exon 3 (c.226G>A, p.E76K) mutation. Each underwent allogeneic hematopoietic stem cell transplantation and is currently in remission. These findings suggest a possible link between prenatal thiopurine exposure and leukemogenesis.