MTHFD1 is a trifunctional protein containing 10-formyltetrahydrofolate synthetase, 5,10-methenyltetrahydrofolate cyclohydrolase and 5,10-methylenetetrahydrofolate dehydrogenase activities. It is encoded by MTHFD1 and functions in the cytoplasmic folate cycle where it is involved in de novo purine synthesis, synthesis of thymidylate and remethylation of homocysteine to methionine. Since the first reported case of severe combined immunodeficiency resulting from MTHFD1 mutations, seven additional patients ascertained through molecular analysis have been reported with variable phenotypes, including megaloblastic anemia, atypical hemolytic uremic syndrome, hyperhomocysteinemia, microangiopathy, infections and autoimmune diseases. We determined the level of MTHFD1 expression and dehydrogenase specific activity in cell extracts from cultured fibroblasts of three previously reported patients, as well as a patient with megaloblastic anemia and recurrent infections with compound heterozygous MTHFD1 variants that were predicted to be deleterious. MTHFD1 protein expression determined by Western blotting in fibroblast extracts from three of the patients was markedly decreased compared to expression in wild type cells (between 4.8 and 14.3% of mean control values). MTHFD1 expression in the fourth patient was approximately 44% of mean control values. There was no detectable methylenetetrahydrofolate dehydrogenase specific activity in extracts from any of the four patients. This is the first measurement of MTHFD1 function in MTHFD1 deficient patients and confirms the previous molecular diagnoses.

MTHFD1是一种三功能蛋白，包含10-甲酰基四氢叶酸合成酶，5,10-亚甲基四氢叶酸环化酶和5,10-亚甲基四氢叶酸脱氢酶活性。它由MTHFD1编码，并在细胞质叶酸循环中起作用，在该循环中它参与从头嘌呤合成，胸苷酸合成以及高半胱氨酸再甲基化为蛋氨酸。自从第一个报道的由MTHFD1引起的严重联合免疫缺陷病例以来据报道，通过分子分析确定的另外7名患者具有可变的表型，包括巨幼细胞性贫血，非典型溶血性尿毒症综合征，高同型半胱氨酸血症，微血管病，感染和自身免疫性疾病。我们确定了三位先前报道的患者以及巨幼细胞性贫血和复合杂合性MTHFD1反复感染患者的培养成纤维细胞中细胞提取物中MTHFD1表达的水平和脱氢酶比活性。被预测为有害的变体。与野生型细胞中的表达相比，在三名患者的成纤维细胞提取物中通过蛋白质印迹法测定的MTHFD1蛋白表达显着降低（在平均对照值的4.8％至14.3％之间）。在第四位患者中，MTHFD1表达约为平均对照值的44％。在四名患者中任何一位患者的提取物中均未检测到亚甲基四氢叶酸脱氢酶的比活。这是MTHFD1缺乏症患者中MTHFD1功能的首次测量，证实了先前的分子诊断。