A single recurrent dominant variant in ITPR3 leads to a syndromic immunodeficiency
Intracellular calcium levels are finely tuned through intricate actions of a number of channels and transporters, including those at key endocellular stores, e.g. endoplasmic reticulum (ER), lysosomes, and mitochondria. Along with the highly homologous genes Inositol 1,4,5-trisphosphate (IP3) receptor type 1 (ITPR1) and 2 (ITPR2), ITPR3 encodes the IP3 receptor (IP3R), a key player in intracellular calcium release in animals. Here we report the first cases of ITPR3 defects in man leading to a primarily dysimmune phenotype. In four unrelated patients of diverse ethnicity, and suffering from a complex immunodeficiency syndrome, we report the same de novo pathogenic variant - c.7570C>T; p.Arg2524Cys - in ITPR3. Clinically, recurrent severe infectious episodes of viral and bacterial origins, features of ectodermal dysplasia and that of Charcot-Marie-Tooth disease were paramount. The identified variant does not affect gene transcription, yet it was structurally predicted and biologically proven to disrupt proper protein folding and function in vivo. This eventually leads to defective Calcium flux in patient cells, dysregulation of mitochondrial function and a broad dysimmune phenotype characterized primarily by a profound CD4 T cell lymphopenia associated with quasi absence of naïve CD4 and CD8 cells, itself mirrored by an increase in cognate memory cells. The Calcium signaling defect was recapitulated ex vivo through the introduction of this single variant in Jurkat cells. Moreover, site-directed mutagenesis displayed the exquisite sensitivity of Arg2524 to any amino acid change. In conclusion, a single unique recurrent de novo variant in ITPR3 leads to a novel syndromic immunodeficiency.