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Somatic MECOM mosaicism in a patient with congenital bone marrow failure without a radial abnormality

To the editor: Radial bone abnormalities are frequently observed in various congenital diseases including bone marrow failure syndrome. Bilateral radial hypoplasia in thrombocytopenic infants is known as thrombocytopenia-absent radius (TAR) syndrome (OMIN #27400), while radioulnar synostosis with amegakaryocytic thrombocytopenia (RUSAT) includes thrombocytopenia (possible pancytopenia) andproximal radioulnar synostosis without thumb abnormality.1 HomeoboxA11 (HOXA11), previously reported in a case of RUSAT1 (OMIN #605432), is thought to be a causative gene for RUSAT.2 Recently, a report3 showed heterozygousmutations inmyelodysplasia syndrome 1 (MDS1) and ecotropic virus integration site 1 (EVI1) complex locus (MECOM) as the cause of RUSAT2 (OMIN #616738). In their report, Niihori et al. reported missense mutations in MECOM in three cases, all of which had thrombocytopenia and multiple anomalies including radial synostosis. However, we report herein a case of congenital bone marrow failure with a germline mutation in MECOM without radial abnormality, which might be explained by somatic mosaicism. The patient was a female born at 40 weeks of gestation by vaginal delivery with no significant family history. Immediately after birth, the infant suffered massive pulmonary bleeding and developed respiratory insufficiency requiring intubation and mechanical ventilation for 9 days. A blood test at birth revealed severe thrombocytopenia (platelets 9,000/mm3) and anemia (Hb 7.2 g/dl), but the leukocyte count was normal. At birth, radiography and echography disclosed no skeletal abnormalities such as radial dysplasia, nor any other abnormalities besides patent ductus arteriosus. Her leukocyte count decreased and eventually progressed to pancytopenia with severe neutropenia (neutrophil<100/mm3) at 2months of age. Bonemarrow aspiration showed a hypocellular marrow without dysplasia. The karyotyping was normal, and no chromosome fragility was observed in mitomycin C test. As she suffered sepsis twice, and transfusion dependency persisted, we performed allogeneic cord blood transplantation at 5 months of age using human leukocyte antigen 1 allele mismatched cord blood. The preconditioning regimen consisted of fludarabine (125 mg/m2), melphalan (140 mg/m2), and total body irradiation (3 Gy). Graft versus host disease prophylaxis consisted of tacrolimus with short-term methotrexate. Neutrophil engraftment was achieved at day 18 after transplantation. Currently at 11 months