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

Abstract

The store-operated Ca(2+) release-activated Ca(2+) (CRAC) channel is activated by diminished luminal Ca(2+) levels in the endoplasmic reticulum and sarcoplasmic reticulum (SR), and constitutes one of the major Ca(2+) entry pathways in various tissues. Tubular aggregates (TAs) are abnormal structures in the skeletal muscle, and although their mechanism of formation has not been clarified, altered Ca(2+) homeostasis related to a disordered SR is suggested to be one of the main contributing factors. TA myopathy is a hereditary muscle disorder that is pathologically characterized by the presence of TAs. Recently, dominant mutations in the STIM1 gene, encoding a Ca(2+) sensor that controls CRAC channels, have been identified to cause tubular aggregate myopathy (TAM). Here, we identified heterozygous missense mutations in the ORAI1 gene, encoding the CRAC channel itself, in three families affected by dominantly inherited TAM with hypocalcemia. Skeletal myotubes from an affected individual and HEK293 cells expressing mutated ORAI1 proteins displayed spontaneous extracellular Ca(2+) entry into cells without diminishment of luminal Ca(2+) or the association with STIM1. Our results indicate that STIM1-independent activation of CRAC channels induced by dominant mutations in ORAI1 cause altered Ca(2+) homeostasis, resulting in TAM with hypocalcemia.

Tubular aggregate myopathy caused by a novel mutation in the cytoplasmic domain of STIM1

“…This result is in marked contrast to the previous reports, in which increased Ca 21 influx caused by constitutive activation of SOCE was observed. 4,7 The transcript of CTID mutant was present in the affected skeletal muscle of our patient, suggesting that nonsense-mediated mRNA decay is not induced. We speculate that the CTID mutant may inhibit the oligomer formation of STIM1 necessary to activate the ORAI1 channels, leading to the decreased intracellular Ca 21 influx.…”

“…Ca 21 measurements were carried out as previously described 7 with minor modifications. HEK293 cells were cotransfected with plasmids expressing STIM1 and pIRES2-mCherry.…”

“…The immunolabeling of STIM1 and ORAI1, a calcium channel in plasma membrane, colocalized to the tubular aggregates visualized by Gomori trichrome staining (figure 2D), which is consistent with previous reports. 4,7 Because premature termination codons often result in nonsense-mediated decay of mRNA, we analyzed mRNA transcript in the muscle biopsy specimen.…”

Ca2+ handling abnormalities in early-onset muscle diseases: Novel concepts and perspectives

“…Interestingly, most STIM1 mutations identified in patients are located within the NH 2 terminal EF hand Ca 2+ binding domain of Stim1, leading to a constitutively active molecule that oligomerizes and clusters at ER/PM junctions independently of Ca 2+ store depletion. Theoretically, such Stim1 molecules should activate Ca 2+ influx independently of how full the intracellular Ca 2+ stores are, a hypothesis confirmed in cells carrying STIM1 mutations, which exhibit dysregulation of Ca 2+ homeostasis characterized by a high resting [Ca 2+ ] and Ca 2+ entry independent of store depletion [84,93,94,96].…”

“…Specifically, they have been identified in several patients with childhood or adult onset tubular aggregate myopathy [82][83][84]93,94] as well as patients with Stormorken syndrome [95,96] a rare disorder characterized by prolonged bleeding, thrombocytopenia/thrombocytopathy, asplenia, intellectual disability, mild hypocalcemia, muscle fatigue and tubular aggregate myopathy. Muscle weakness is generally only mild in the adultonset cases and some of the patients with an ORAI1 mutation presented only with exertional cramps [97].…”