Functional analyses ofSTIM1mutations reveal a common pathomechanism for tubular aggregate myopathy and Stormorken syndrome

Abstract

22Tubular aggregate myopathy (TAM) is a progressive disorder essentially involving muscle 23 weakness, cramps, and myalgia. TAM clinically overlaps with Stormorken syndrome 24 (STRMK), associating TAM with miosis, thrombocytopenia, hyposplenism, ichthyosis, short 25 stature, and dyslexia. TAM and Stormorken syndrome arise from gain-of-function mutations in 26 STIM1 or ORAI1, both encoding key regulators of Ca 2+ homeostasis, and mutations in either 27 gene results in excessive Ca 2+ entry. The pathomechanistic similarities and differences of TAM 28 and Stormorken syndrome are only partially understood. Here we provide functional in cellulo 29 experiments demonstrating that STIM1 harboring the TAM D84G or the STRMK R304W 30 mutation similarly cluster and exert a dominant effect on the wild-type protein. Both mutants 31 recruit ORAI1 to the clusters, induce major nuclear import of the Ca 2+ -dependent transcription 32 factor NFAT, and trigger the formation of circular membrane stacks. In conclusion, the 33 analyzed TAM and STRMK mutations have a comparable impact on STIM1 protein function 34 and downstream effects of excessive Ca 2+ entry, highlighting that TAM and Stormorken 35 syndrome involve a common pathomechanism. 36 37 Keywords: STIM1, tubular aggregate myopathy, Stormorken syndrome, SOCE, calcium 38 39 40 41 42 Functional analysis of STIM1 mutations 3 43 44 Tubular aggregate myopathy (TAM) and Stormorken syndrome (STRMK) are spectra of the 45 same disease affecting muscle, platelets, spleen, and skin (1, 2). The majority of the TAM 46 patients primarily present with muscle weakness, cramps, and myalgia with a heterogeneous 47 age of onset and disease severity (3-6). Additional features including thrombocytopenia, 48 hyposplenism, miosis, ichthyosis, short stature, hypocalcemia, or dyslexia can be seen as well, 49 and the occurrence of the totality of the multi-systemic signs constitutes the diagnosis of 50Stormorken syndrome (7-18). 51Both TAM and Stormorken syndrome are characterized by the presence of densely packed 52 membrane tubules in muscle fibers, and investigations on muscle sections by 53 immunofluorescence have shown that these tubular aggregates contain various sarcoplasmic 54 reticulum (SR) proteins, suggesting that they originate from the SR (4, 7, 15, 16, 19). The 55 tubular aggregates are the histopathological hallmark of TAM and Stormorken syndrome, and 56 were also described in hypokalemic periodic paralysis, myasthenic syndrome, malignant 57 hyperthermia, inflammatory or ethyltoxic myopathy, and accumulate in normal muscle with 58 age (20-25). 59TAM and Stormorken syndrome are caused by heterozygous missense mutations in STIM1 (4, 60 10-12) (OMIM #605921) or ORAI1 (12) (OMIM #610277), both encoding key factors of store-61 operated Ca 2+ entry (SOCE). SOCE is a major mechanism regulating Ca 2+ homeostasis and 62 thereby drives a multitude of Ca 2+ -dependent cellular functions including muscle contraction. 63STIM1 has a single transmembrane domain and is primarily localized at the 64 endo...