Expanding the clinical and genetic spectrum of pathogenic variants in STIM1

Activation mechanisms and structural dynamics of STIM proteins

The family of stromal interaction molecules (STIM) includes two widely expressed single-pass endoplasmic reticulum (ER) transmembrane proteins and additional splice variants that act as precise ER-luminal Ca2+ sensors. STIM proteins mainly function as one of the two essential components of the so-called Ca2+ release-activated Ca2+ (CRAC) channel. The second CRAC channel component is constituted by pore-forming Orai proteins in the plasma membrane. STIM and Orai physically interact with each other to enable CRAC channel opening, which is a critical prerequisite for various downstream signalling pathways such as gene transcription or proliferation. Their activation commonly requires the emptying of the intracellular ER Ca2+ store. Using their Ca2+ sensing capabilities, STIM proteins confer this Ca2+ content-dependent signal to Orai, thereby linking Ca2+ store depletion to CRAC channel opening. Here we review the conformational dynamics occurring along the entire STIM protein upon store depletion, involving the transition from the quiescent, compactly folded structure into an active, extended state, modulation by a variety of accessory components in the cell as well as the impairment of individual steps of the STIM activation cascade associated with disease.

Tubular aggregate myopathy causing progressive fatiguable weakness

Tubular aggregate myopathies comprise a rare group of disorders with characteristic pathological findings and heterogeneous phenotypes, including myasthenic syndrome. We describe a patient with tubular aggregate myopathy who presented with fatiguable weakness improving with pyridostigmine, respiratory involvement and possible cardiac manifestations. We highlight the utility of muscle biopsy in atypical myasthenic syndrome.

ORAI1 inhibition as an efficient preclinical therapy for tubular aggregate myopathy and Stormorken syndrome

Tubular aggregate myopathy (TAM) and Stormorken syndrome (STRMK) are clinically overlapping disorders characterized by childhood-onset muscle weakness and a variable occurrence of multisystemic signs, including short stature, thrombocytopenia, and hyposplenism. TAM/STRMK is caused by gain-of-function mutations in the Ca2+ sensor STIM1 or the Ca2+ channel ORAI1, both of which regulate Ca2+ homeostasis through the ubiquitous store-operated Ca2+ entry (SOCE) mechanism. Functional experiments in cells have demonstrated that the TAM/STRMK mutations induce SOCE overactivation, resulting in excessive influx of extracellular Ca2+. There is currently no treatment for TAM/STRMK, but SOCE is amenable to manipulation. Here, we crossed Stim1R304W/+ mice harboring the most common TAM/STRMK mutation with Orai1R93W/+ mice carrying an ORAI1 mutation partially obstructing Ca2+ influx. Compared with Stim1R304W/+ littermates, Stim1R304W/+Orai1R93W/+ offspring showed a normalization of bone architecture, spleen histology, and muscle morphology; an increase of thrombocytes; and improved muscle contraction and relaxation kinetics. Accordingly, comparative RNA-Seq detected more than 1,200 dysregulated genes in Stim1R304W/+ muscle and revealed a major restoration of gene expression in Stim1R304W/+Orai1R93W/+ mice. Altogether, we provide physiological, morphological, functional, and molecular data highlighting the therapeutic potential of ORAI1 inhibition to rescue the multisystemic TAM/STRMK signs, and we identified myostatin as a promising biomarker for TAM/STRMK in humans and mice.

The Ca2+ Sensor STIM in Human Diseases

The STIM family of proteins plays a crucial role in a plethora of cellular functions through the regulation of store-operated Ca2+ entry (SOCE) and, thus, intracellular calcium homeostasis. The two members of the mammalian STIM family, STIM1 and STIM2, are transmembrane proteins that act as Ca2+ sensors in the endoplasmic reticulum (ER) and, upon Ca2+ store discharge, interact with and activate the Orai/CRACs in the plasma membrane. Dysregulation of Ca2+ signaling leads to the pathogenesis of a variety of human diseases, including neurodegenerative disorders, cardiovascular diseases, cancer, and immune disorders. Therefore, understanding the mechanisms underlying Ca2+ signaling pathways is crucial for developing therapeutic strategies targeting these diseases. This review focuses on several rare conditions associated with STIM1 mutations that lead to either gain- or loss-of-function, characterized by myopathy, hematological and immunological disorders, among others, and due to abnormal activation of CRACs. In addition, we summarize the current evidence concerning STIM2 allele duplication and deletion associated with language, intellectual, and developmental delay, recurrent pulmonary infections, microcephaly, facial dimorphism, limb anomalies, hypogonadism, and congenital heart defects.

Stormorken syndrome caused by STIM1 mutation: A case report and literature review

The aim of the present case study was to identify the genetic cause of a patient with a clinical presentation of tubular aggregate myopathy (TAM)/Stormorken syndrome (STRMK) and review the published clinical data of patients with TAM/STRMK. A child with thrombocytopenia and hyperCKemia at the Children's Hospital of Soochow University were recruited in the study. Peripheral blood samples of the infant and her parents were collected, and then whole-exome sequencing was performed. Detection of the stromal interaction molecule 1 (STIM1) level of the child was performed using western blot analysis. In addition, a literature review was performed based on a thorough retrieval of published literature from the PubMed database, as well as domestic databases. In the present study, the c.326A>G mutation in a STIM1 allele (p.H109R) was identified only in the child, as opposed to the unaffected parents. The level of STIM1 was not decreased in the child. Among the mutation sites identified in previous studies, there were 46 cases across 30 families of STIM1 EF-hand mutations, 21 cases across 14 families of STIM1 CC1 mutations and 20 cases across 8 families of calcium release-activated calcium channel protein 1 mutations, in which 7 parents had the same mutation site as the patient described herein. On the whole, it is demonstrated that TAM/STRMK is an extremely rare disease with autosomal dominant inheritance. Patients often have multisystemic signs. Gene detection at an early stage is helpful for diagnosis. Long-term exercise training may also have a certain curative effect.

Case Report: Novel STIM1 Gain-of-Function Mutation in a Patient With TAM/STRMK and Immunological Involvement

Gain-of-function (GOF) mutations in STIM1 are responsible for tubular aggregate myopathy and Stormorken syndrome (TAM/STRMK), a clinically overlapping multisystemic disease characterised by muscle weakness, miosis, thrombocytopaenia, hyposplenism, ichthyosis, dyslexia, and short stature. Several mutations have been reported as responsible for the disease. Herein, we describe a patient with TAM/STRMK due to a novel L303P STIM1 mutation, who not only presented clinical manifestations characteristic of TAM/STRMK but also manifested immunological involvement with respiratory infections since childhood, with chronic cough and chronic bronchiectasis. Despite the seemingly normal main immunological parameters, immune cells revealed GOF in calcium signalling compared with healthy donors. The calcium flux dysregulation in the immune cells could be responsible for our patient’s immune involvement. The patient’s mother carried the mutation but did not exhibit TAM/STRMK, manifesting an incomplete penetrance of the mutation. More cases and evidence are necessary to clarify the dual role of STIM1 in immune system dysregulation and myopathy.

CIC-39Na reverses the thrombocytopenia that characterizes tubular aggregate myopathy

Store-Operated Ca2+-Entry is a cellular mechanism that governs the replenishment of intracellular stores of Ca2+ upon depletion caused by the opening of intracellular Ca2+-channels. Gain-of-function mutations of the two key proteins of Store-Operated Ca2+-Entry, STIM1 and ORAI1, are associated with several ultra-rare diseases clustered as tubular aggregate myopathies. Our group has previously demonstrated that a mouse model bearing the STIM1 p.I115F mutation recapitulates the main features of the STIM1 gain-of-function disorders: muscle weakness and thrombocytopenia. Similar findings have been found in other mice bearing different mutations on STIM1. At present, no valid treatment is available for these patients. In the present contribution, we report that CIC-39Na, a Store-Operated Ca2+-Entry inhibitor, restores platelet number and counteracts the abnormal bleeding that characterizes these mice. Subtle differences in thrombopoiesis were observed in STIM1 p.I115F mice, but the main difference between wild-type and STIM1 p.I115F mice was in platelet clearance and in the levels of platelet cytosolic basal Ca2+. Both were restored upon treatment of animals with CIC-39Na. This finding paves the way to a pharmacological treatment strategy for thrombocytopenia in tubular aggregate myopathy patients.