Calcium Leaking from the Endoplasmic Reticulum Caused by a Human ITPR3 R2524C Variant Causes Immunodeficiency, Ectodermal Dysplasia, and Charcot-Marie-Tooth Syndrome

Patients with mutations in various inositol tris-phosphate receptor (ITPR) family members display diverse clinical phenotypes. We report on two unrelated individuals with the ITPR3 p.Arg2524Cys variant who have growth retardation, immunodeficiency, partial anhidrosis, and peripheral neuropathy. To delineate the mechanisms leading to these phenotypes, a mouse line was developed to genocopy human ITPR3 p.Arg2524Cys. Mice heterozygous for the homologous Itpr3 p.Arg2523Cys variant were small, had severe B cell lymphopenia, milder T cell loses, sweat defects, ectodermal dysplasia, and sciatic nerve transcript alterations. B cell development was blocked at the pre–B cell stage, leading to low peripheral B cells. T cell development was impacted at the single-positive stage. Structural modeling suggested the ITPR3 p.Arg2524Cys change may result in a “leaky calcium channel,” although some reports suggested a dominant negative effect. Our ability to perform calcium analyses in immediately ex vivo cells revealed that endoplasmic reticulum calcium stores were depleted in the Itpr3 p.Arg2523Cys mice relative to littermate controls. This calcium depletion diminished extracellular calcium influx following T cell receptor and co-receptor stimulations, consistent with poor calcium uptake following ionomycin treatment (calcium ionophore). In addition to immune cells, the ITPR3 R2524C variant reduced water release by sweat glands. The leaky calcium pool from the ER mediated by the ITPR3 variant and the clinical phenotypes was consistent with selected cell types being more severely impacted by the single allelic p.Arg2524Cys substitution. Our findings suggest clinical strategies to improve immune functions in affected patients.