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.

Immune dysfunction, including severe combined immunodeficiency, has been described in genetic disorders affecting the metabolism of the vitamins cobalamin (vitamin B12) and folate. We have reviewed reports of clinical findings in patients with a number of inborn errors of cobalamin or folate metabolism, specifically looking for immune problems. There is little evidence that immune function is affected in most of the disorders. Exceptions are Imerslund-Gräsbeck syndrome and hereditary folate malabsorption (affecting intestinal absorption of cobalamin and folate, respectively), transcobalamin deficiency (affecting transport of cobalamin in blood and cellular cobalamin uptake), and methylenetetrahydrofolate dehydrogenase 1 deficiency (catalyzing cytoplasmic interconversion of reduced folate coenzyme derivatives). Although some inborn errors of cobalamin or folate can be associated with immune dysfunction, the degree and type of immune dysfunction vary with no obvious pattern.

Methylenetetrahydrofolate dehydrogenase (MTHFD1) deficiency has recently been reported to cause a folate-responsive syndrome displaying a phenotype that includes megaloblastic anemia and severe combined immunodeficiency. To describe our investigative approach to the molecular diagnosis and evaluation of immune dysfunction in a family with MTHFD1 deficiency. The methods used were exome sequencing and analysis of variants in genes involved in the folate metabolic pathway in a family with 2 affected siblings. Routine laboratory and research data were analyzed to gain an in-depth understanding of innate, humoral, and cell-mediated immune function before and after folinic acid supplementation. Interrogation of exome data for concordant variants between the siblings in the genes involved in folate metabolic pathway identified a heterozygous mutation in exon 3 of the MTHFD1 gene that was shared with their mother. In view of highly suggestive phenotype, we extended our bioinformatics interrogation for structural variants in the MTHFD1 gene by manual evaluation of the exome data for sequence depth coverage of all the exons. A deletion involving exon 13 that was shared with their father was identified. Routine laboratory data showed lymphopenia involving all subsets and poor response to vaccines. In vitro analysis of dendritic cell and lymphocyte function was comparable to that in healthy volunteers. Treatment with folinic acid led to immune reconstitution, enabling discontinuation of all prophylactic therapies. Exome sequencing demonstrated MTHFD1 deficiency as a novel cause of a combined immunodeficiency. Folinic acid was established as precision therapy to reverse the clinical and laboratory phenotype of this primary immunodeficiency.