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Identification of transcobalamin deficiency with two novel mutations in the TCN2 gene in a Chinese girl with abnormal immunity: a case report Shihong Zhan1, Fangfang Cheng2, Hailong He3, Shaoyan Hu3* and Xing Feng1*

Abstract Background: Transcobalamin (TC) transports vitamin B12 from blood into cells. TC II deficiency is a rare autosomal recessive disorder. It is characterized by failure to thrive, diarrhoea, pallor, anaemia, pancytopenia or agammaglobulinemia. It is usually confirmed by molecular analysis of the TCN2 gene. We report a 2-month-old girl with two novel mutations, which were first reported in humans. Case presentation: We present a 2-month-old Chinese girl with pancytopenia, severe combined immunodeficiency disease, and megaloblastic anaemia. Targeted next-generation sequencing (NGS) was performed, which detected compound heterozygous variants in exon 7 of the TCN2 gene (Mutation 1: c.1033 C > T; Mutation 2: c.10171031delinsGTAACAGAGATGGTT). These mutations result in stop codons in TCN2. The c.1033C > T mutation causes a stop at codon 345 (p.Gln345Ter), and the c.1017-1031delinsGTAACAGAGATGGTT mutation causes a stop at codon 340 (p.Leu340Ter). After being diagnosed, she was treated with intramuscular 1 mg hydroxycobalamin (OH-Cbl) every day for 2 months. The CBC value returned to normal after half a month. The peripheral blood lymphocyte subsets and immunoglobulin recovered after 2 months. Then, the dosage of OH-Cbl was gradually reduced. Conclusions: TC II deficiency is a serious complication that requires lifelong treatment. Its diagnosis is difficult due to the lack of clearly identifiable symptoms. Genetic testing should be performed as early as possible if this disease is suspected. The specific observations of this case report make a considerable contribution to the literature and provide a reference for the diagnosis and treatment of future cases. Keywords: Cobalamin, deficiency, transcobalamin, megaloblastic anaemia, case report

Background Transcobalamin (TC), a vitamin B12 (cobalamin, Cbl) binding protein in plasma, promotes the cellular uptake of vitamin B12 by receptor-mediated endocytosis. Inherited TC II deficiency is an autosomal recessive disorder characterized * Correspondence: [email protected]; [email protected] 3 Hematology-Oncology Department, Children’s Hospital of Soochow University, No. 92 Zhongnan Road, 215000 Suzhou, Jiangsu Province, China 1 The Neonatal Department, Children’s Hospital of Soochow University, No. 92 Zhongnan Road, 215000 Suzhou, Jiangsu Province, China Full list of author information is available at the end of the article

by megaloblastic anaemia caused by cellular vitamin B12 depletion [1, 2]. It may be accompanied by neurological complications, including a delay in psychomotor and mental development. Sometimes severe immune deficiency, including abnormal humoral and cellular immunity, is also observed. The protein (TC II) is encoded by the TCN2 gene, which spans 18 kb and contains 9 exons on chromosome 22q12. b. TC II deficien

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