To investigate the genetic etiology, clinical diagnosis and treatment of a child with pancytopenia, failure to thrive and pulmonary infection.

Peripheral blood samples of the child and her parents were collected. Genomic DNA was extracted. Genetic variants associated with hematological diseases were detected by high-throughput sequencing.

Three variants ofTCN2 gene were found, one of which located in exon 5 upstream(c.581-8A>T), the parents has carried this variant ; one in exon 6 (c.924_927del), the variant was originated from the mother; one in exon 7 (c.973C>T), the variant has ocurredde novo. The variants pathogenic analysis combined with clinical manifestation, pancytopenia, the increase in methylmalonic acid level and increased homocysteine, the child was diagnosed with transcobalaminⅡdeficiency.The patient presented with respiratory infection, which was confirmed to be pneumocystosis by lung radioscopy and pathogenic high throughput sequencing of broncho-alveolar lavage fluid. The patient presented with acute respiratory distress syndrome during the treatment with intramuscular injection of vitamin B₁₂, and improved after anti-infection with compound sulfamethoxazole and symptomatic support treatment.

We reported a case of Chinese child with TCNⅡ deficiency due to novel gene variant, and analyzed the pathogenicity of the three variants. The treatment of TCNⅡ deficiency with cobalamin should be individualized.

TranscobalaminⅡdeficiency; Vitamin B₁₂; TCN2 gene; Pneumocystosis