Another rare form of SCID is caused by mutation in a gene encoding Orai1, a component of the CRAC channel
(see Chapter 7). Antigen receptor signaling leads to the activation of the y isoform of phospholipase C (PLCy) and the inositol trisphosphate (IP3)-dependent release of calcium ions from the endoplasmic reticulum and mitochondria (see Chapter 7). The released calcium is replenished by store-operated CRAC channels that facilitate an influx of extracellular calcium. This process is crucial for lymphocyte activation, and it is defective in cells with mutant ORAI1 . A similar phenotype is observed in patients with mutations in STIM1, which encodes an endoplasmic reticulum protein that senses the depletion of calcium stores and contributes to the opening of the CRAC channel. Patients with ORAI1 and STIM1 mutations do not exhibit a defect in T cell development, but their T cells cannot be properly activated.
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