A novel gain‐of‐function mutation in ORAI1 causes late‐onset tubular aggregate myopathy and congenital miosis

Abstract

We present three members of an Italian family affected by tubular aggregate myopathy (TAM) and congenital miosis harboring a novel missense mutation in ORAI1. All patients had a mild, late onset TAM revealed by asymptomatic creatine kinase (CK) elevation and congenital miosis consistent with a Stormorken-like Syndrome, in the absence of thrombocytopathy. Muscle biopsies showed classical histological findings but ultrastructural analysis revealed atypical tubular aggregates (TAs). The whole body muscle magnetic resonance imaging (MRI) showed a similar pattern of muscle involvement that correlated with clinical severity. The lower limbs were more severely affected than the scapular girdle, and thighs were more affected than legs. Molecular analysis revealed a novel c.290C>G (p.S97C) mutation in ORAI1 in all affected patients. Functional assays in both human embryonic kidney (HEK) cells and myotubes showed an increased rate of Ca entry due to a constitutive activation of the CRAC channel, consistent with a 'gain-of-function' mutation. In conclusion, we describe an Italian family harboring a novel heterozygous c.290C>G (p.S97C) mutation in ORAI1 causing a mild- and late-onset TAM and congenital miosis via constitutive activation of the CRAC channel. Our findings extend the clinical and genetic spectrum of the ORAI1-related TAM.

Mapping the functional anatomy of Orai1 transmembrane domains for CRAC channel gating

“…Active CRAC Channels. Several recent studies have described GOF missense Orai1 mutations in TMs 2-4, with some linked to tu-bular aggregate myopathy with additional symptoms including thrombocytopenia and miosis (12)(13)(14)(15). The identification of GOF mutations in the non-pore-lining segments suggests that these domains of Orai1 may play a greater role in channel activation than previously appreciated.…”

“…2 and SI Appendix, Table S1). These included two loci that have been linked to tubular aggregate myopathy with congenital miosis-S97C on the non-pore-lining face of TM1 (13) and P245C in TM4 (12)-as well as H134C in TM2, F187C in TM3, and A235C in TM4 (Figs. 1 and 2 and SI Appendix, Table S1).…”

“…6C), suggesting that mutations that increase the hydrophobicity of the serine ridge may force an interaction with a Contacts from TM1 to TM2/3 Ring TM2/3 Ring Inner Surface hydrophobic region on TM2/3 ring instead of its usual polar interaction partners. This possibility is intriguing because a recent report has described a GOF human Orai1 mutation S97C that causes a Stormorken-like syndrome with tubular aggregate myopathy and congenital miosis (13). The molecular basis of the GOF phenotype of this mutant is currently not understood, but such a switching mechanism could potentially explain its constitutive activity.…”

“…Although not considered in most early studies, there is increasing evidence from genetic disease association studies in human patients that the transmembrane domains of Orai1 may be critically involved in this process. In particular, several reports have described patients with tubular aggregate myopathy, thrombocytopenia, and congenital miosis caused by gain-of-function (GOF) Orai1 mutations located within the TM domains (12)(13)(14)(15). In addition, recent structure-function analysis of several GOF mutations including that of a putative "hinge" at the base of TM4 (16), a Pro residue in TM4 (17), and several residues cataloged in a cancer genomics database (18), provide more direct support for involvement of the non-pore-lining TMs in Orai1 gating.…”

Pore opening mechanism of CRAC channels

“…Human studies have shown that loss of CRAC channel function leads to a devastating immunodeficiency along with additional symptoms of autoimmunity, ectodermal dysplasia, and muscle defects, highlighting the vital importance of CRAC channels for human health [7–9]. Conversely, constitutive channel activation from gain-of-function mutations in CRAC channel proteins are linked to pathologies such as tubular aggregate myopathy and thrombocytopenia [10–12]. In several instances, the molecular mechanisms of how these loss- and gain-of-function mutations disrupt CRAC channels remains unclear, underscoring the need for a better understanding of the operational mechanisms of CRAC channels including their gating mechanism.…”