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Immunodeficiency associated with a novel functionally defective variant of SLC19A1 benefits from folinic acid treatment


Insufficient dietary folate intake, hereditary malabsorption, or defects in folate transport may lead to combined immunodeficiency (CID). Although loss of function mutations in the major intestinal folate transporter PCFT/SLC46A1 was shown to be associated with CID, the evidence for pathogenic variants of RFC/SLC19A1 resulting in immunodeficiency was lacking. We report two cousins carrying a homozygous pathogenic variant c.1042 G > A, resulting in p.G348R substitution who showed symptoms of immunodeficiency associated with defects of folate transport. SLC19A1 expression by peripheral blood mononuclear cells (PBMC) was quantified by real-time qPCR and immunostaining. T cell proliferation, methotrexate resistance, NK cell cytotoxicity, Treg cells and cytokine production by T cells were examined by flow cytometric assays. Patients were treated with and benefited from folinic acid. Studies revealed normal NK cell cytotoxicity, Treg cell counts, and naive-memory T cell percentages. Although SLC19A1 mRNA and protein expression were unaltered, remarkably, mitogen induced-T cell proliferation was significantly reduced at suboptimal folic acid and supraoptimal folinic acid concentrations. In addition, patients’ PBMCs were resistant to methotrexate-induced apoptosis supporting a functionally defective SLC19A1. This study presents the second pathogenic SLC19A1 variant in the literature, providing the first experimental evidence that functionally defective variants of SLC19A1 may present with symptoms of immunodeficiency.

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Fig. 1: G348R substitution does not alter SLC19A1 protein expression.
Fig. 2: G348R substitution in SLC19A1 reduces T cell proliferation in nonoptimal folate concentrations.
Fig. 3: G348R substitution in SLC19A1 leads to methotrexate resistance.
Fig. 4: Normal NK cell cytotoxicity, normal Treg cell numbers, but increased GM-CSF and IL-17A producing T cells in both patients.

Data availability

All the raw data are available upon reasonable request from the corresponding authors.


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This study was financially supported by Erciyes University BAP grant (TCD-2021-10863) to EÜ, and Turkish Academy of Science GEBIP and Science Academy BAGEP awards to AE.

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Authors and Affiliations



Conceptualization was performed by AE, EÜ. Methodology was performed by ŞE, YH, SB, KEB, CA, AE, MFC. Software was performed by AB. Investigation was performed by VG, ŞE, AÖ, EY, HP, TP, AE, EÜ. Validation was done by AB, MK, AE. Supervision was performed HC, AE, EÜ. Resources—Writing—original draft were written by VG, ŞE, YH, AE. Writing—review & editing were performed by VG, YH, SB, AÖ, EY, CA, MK, HC, HP, TP, AE, EÜ. Funding acquisition by AE, EÜ. All authors have read and approved the manuscript.

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Correspondence to Ahmet Eken or Ekrem Ünal.

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All experimental procedures were approved by Erciyes University institutional review board (#2021/17) and conducted according to regulations. Parents of the patients signed the informed consents for the immunological study and provided consent for the publication of data. Controls were selected randomly from amongst the age and sex-matched healthy donors.

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Gök, V., Erdem, Ş., Haliloğlu, Y. et al. Immunodeficiency associated with a novel functionally defective variant of SLC19A1 benefits from folinic acid treatment. Genes Immun 24, 12–20 (2023).

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