Transcobalamin II deficiency mimicking myelodysplastic syndrome in a child: a case report

Transcobalamin II (TCN2) deficiency is a rare autosomal recessive metabolic disorder that impairs vitamin B12 transport and can present with megaloblastic anemia and neutropenia, often mimicking hematologic diseases such as myelodysplastic syndrome (MDS). We report a 5-year-old male presenting with pallor, fatigue, and fever. On admission, he had acute anemia crisis and neutropenia, with chest CT revealing a lung infection, while serum folate and vitamin B12 levels were normal. Bone marrow smears and flow cytometry suggested possible MDS; however, genetic testing identified compound heterozygous pathogenic variants in the TCN2 gene, confirming TCN2 deficiency. Following parenteral methylcobalamin therapy, his symptoms improved and blood counts gradually normalized. Oral maintenance therapy was then initiated, with stable hematologic parameters during follow-up. This case highlights the importance of considering rare metabolic disorders such as TCN2 deficiency in pediatric patients with unexplained anemia and neutropenia, and underscores the value of early recognition, genetic diagnosis, and long-term management. These findings contribute to the limited literature on pediatric TCN2 deficiency and reinforce clinician awareness when evaluating children with MDS-like presentations.