Distinct Immunological Phenotype in Galactosemia Type 3 Patients with Biallelic GALE Mutations

Galactosemia involves different forms of inborn errors of metabolism affecting galactose processing. Different from type 1 and type 2, respectively, related to GALT and GALK1 defects, type 3 galactosemia is due to biallelic mutations in UDP-galactose-4-epimerase (GALE), a transferase enzyme involved in the Leloir pathway. Here, we investigated two fraternal twins presenting with a syndromic disease characterized by intellectual disability, mild thrombocytopenia, fluctuating immunoglobulin levels, and T and B cell lymphopenia. The patients were diagnosed with type 3 galactosemia based on whole-exome sequence-detected compound heterozygous GALE variants, c.449C>T p.(T150M) and c.551G>A p. (R184H). Patients’ peripheral blood mononuclear cells showed markedly reduced GALE protein abundance. In vitro overexpression studies demonstrated that the previously unreported GALE p. (R184H) variant showed increased ubiquitination and degradation, which, together with the previously reported/disease-causing p.(T150M) mutation, contribute to their disease. The patients’ T cell phenotype was abnormal, showing low recent thymic emigrants as well as naive CD4+ and CD8+ T cells. CD4+ and CD8+ T cells also failed to properly proliferate after mitogen or T cell receptor stimulation, in addition to exhibiting increased apoptosis. The patients’ B cell immunophenotyping in peripheral blood showed very low B cell numbers with relatively increased plasmablasts, increased apoptosis, and a markedly reduced CD19 mean fluorescence intensity (MFI), while CD20 MFI was within normal ranges. Patient-derived EBV-transformed B cells also exhibited reduced GALE abundance, with a hyperglycosylated/higher molecular weight CD19 expression pattern. Overall, our studies indicate that type 3 galactosemia due to GALE mutations can affect T cell thymic output, proliferation, and survival, altogether contributing to T cell lymphopenia. In terms of B cells, glycosylation defects on critical lineage-specific molecules, such as CD19, are affected, likely impacting B cell numbers, function, and/or trafficking. Further studies are underway for the in-depth evaluation of the GALE-specific immune glycosylated molecules/pathways involved (e.g., mass spectrometry, RNA-seq) and comparison with type 1 and type 2 galactosemia patients, none of whom are reported to have associated immune defects.