G.O.1: Dominant mutations in ORAI1 cause tubular aggregate myopathy with hypocalcemia via constitutive activation of store-operated Ca2+ channels

Abstract

Tubular aggregate myopathy (TAM) is a hereditary muscle disorder that is pathologically characterized by the presence of tubular aggregates. The responsible genes have not been clarified for many individuals affected by TAM. In order to identify the genetic cause of TAM, we performed whole exome sequencing in genomic DNA from 2 families affected by dominant TAM with hypocalcemia. We identified one common variant in both families, heterozygous missense mutation, c.292G>A (Gly98Ser) in ORAI1. We also found an additional heterozygous missense mutation, c.412C>T (p.Leu138Phe) in ORAI1, in the third TAM family by Sanger sequencing. The mutations were not listed in dbSNP137, 1000genomes, or Human Genetic Variation Database collecting Japanese genetic variations. Orai1 is a cell-surface transmembrane protein which acts as a store-operated Ca 2+ channel, is highly expressed in the skeletal muscles. The store-operated Ca 2+ channel is activated by diminished luminal Ca 2+ levels in the sarcoplasmic reticulum via the activation of stromal interaction molecule 1 (STIM1) sensing of Ca 2+ depletion. We demonstrated that skeletal myotubes from an affected individual and HEK293 cells expressing mutated Orai1 proteins displayed spontaneous extracellular Ca 2+ entry into cells independent of STIM1 activation. Our results indicated that calcium entry into myofibers via store-operated Ca 2+ channels were constitutively activated by dominant mutations in ORAI1 that cause altered Ca 2+ homeostasis, resulting in tubular aggregate myopathy with hypocalcemia. Recently, dominant mutations in STIM1 have been identified to cause tubular aggregate myopathy. Together, these results strongly suggest that the constitutive extracellular Ca 2+ entry caused by a mutation in either the STIM1 or ORAI1 is associated with a certain type of tubular aggregate myopathy.