ORAI1 Mutations with Distinct Channel Gating Defects in Tubular Aggregate Myopathy

Abstract

Calcium (Ca2+) is a physiological key factor, and the precise modulation of free cytosolic Ca2+ levels regulates multiple cellular functions. Store‐operated Ca2+ entry (SOCE) is a major mechanism controlling Ca2+ homeostasis, and is mediated by the concerted activity of the Ca2+ sensor STIM1 and the Ca2+ channel ORAI1. Dominant gain‐of‐function mutations in STIM1 or ORAI1 cause tubular aggregate myopathy (TAM) or Stormorken syndrome, whereas recessive loss‐of‐function mutations are associated with immunodeficiency. Here, we report the identification and functional characterization of novel ORAI1 mutations in TAM patients. We assess basal activity and SOCE of the mutant ORAI1 channels, and we demonstrate that the G98S and V107M mutations generate constitutively permeable ORAI1 channels, whereas T184M alters the channel permeability only in the presence of STIM1. These data indicate a mutation‐dependent pathomechanism and a genotype/phenotype correlation, as the ORAI1 mutations associated with the most severe symptoms induce the strongest functional cellular effect. Examination of the non‐muscle features of our patients strongly suggests that TAM and Stormorken syndrome are spectra of the same disease. Overall, our results emphasize the importance of SOCE in skeletal muscle physiology, and provide new insights in the pathomechanisms involving aberrant Ca2+ homeostasis and leading to muscle dysfunction.

ORAI1 channel gating and selectivity is differentially altered by natural mutations in the first or third transmembrane domain

“…In accordance with our previous observations (Bohm et al . ), Fura‐2 quench rates were 4–50 times larger in cells expressing TAM channels than WT channels, confirming their basal activity in this cellular model (Fig. B ).…”

“…Upon Ca 2+ readmission, the rates of the evoked Fura‐2 ratio elevations were ∼2.5 and 5 times larger for ORAI1‐T184M and ‐V107M, respectively, compared to WT channels, confirming our earlier findings (Bohm et al . ). The addition of 10 μ m GSK‐7975A efficiently reduced SOCE in all conditions.…”

“…Both the V107M and T184M channels are constitutively active and mediate increased SOCE when expressed in HEK‐293T cells, as inferred from cytosolic Ca 2+ recordings (Bohm et al . ).…”

“…; Bohm et al . ), suggesting that the gating signal is relayed along non‐pore‐lining residues to control ORAI1 channel opening.…”

Mutations in GFPT1-related congenital myasthenic syndromes are associated with synaptic morphological defects and underlie a tubular aggregate myopathy with synaptopathy

“…Furthermore, all patients with GFPT1 mutations containing TAs in ultrastructural and immunohistochemical analyses exhibited significant alterations of NMJs architecture. TAs are frequently associated with congenital neuromuscular diseases resulting in abnormal calcium handling [34] such as tubular aggregates myopathy (TAM) due to mutations in STIM1 and ORAI1 [35][36][37][38] or periodic paralyses due to CACNA1S mutations [39]. This suggests that defective GFPT1 activity results in abnormal calcium homeostasis and TAs occurrence in an unknown way.…”