STIM1 and ORAI1 related disorders

The regulation of gene expression in many mammalian tissues depends on the Ca²⁺– regulated transcription factor family NFAT (nuclear factor of activated T cells) (Fig. 9).

Intracellular Ca²⁺ rises are sensed by CaM and transmitted to the serine/threonine protein phosphatase calcineurin (CN). The activation of CN mediates NFAT dephosphorylation, a step that prompts the nuclear translocation of the transcription factor and the consequent expression of the genes of interest. In the nucleus, NFAT is deactivated by specific kinases and returns to the cytosol (Hogan et al., 2003; Kar et al., 2011). To sustain gene expression, the NFAT nuclear import/export cycle necessitates repeated Ca²⁺ elevations. These oscillations are supported by STIM1 and ORAI1 proteins (Lewis and Cahalan, 1989; Dolmetsch et al., 1998), illustrating their central role in the function of many cellular systems. Therefore, gain or loss–

of–function mutations in either STIM1 or ORAI1 genes lead to multisystemic disorders (Fig. 10) (Lacruz and Feske, 2015).

Figure 9. Nuclear factor of activated T cells (NFAT) cycle.

1) SOCE mediated by STIM1 and ORAI1 proteins results in cytosolic Ca²⁺ elevations.

2) Ca²⁺-bound CaM activates the calcineurin (CN) phosphatase.

3) Activated CN dephosphorylates NFAT, inducing the factor’s conformational change and the exposure of its nuclear localization signal (red).

4) NFAT translocation into the nucleus triggers gene expression and consequent cell activation, proliferation or differentiation.

5) Nuclear phosphorylation of NFAT prompts the factor’s export back into the cytosol.

During prolonged or repetitive Ca²⁺

signaling events, NFAT nuclear import/export cycle resumes.

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Immune disorders

Ca²⁺ influx in T lymphocytes almost entirely rely on ORAI1 channels. Actually, the identification of ORAI1 as the channel subunit of SOCE/CRAC came from the combination of a genome–wide RNAi screen with positional cloning in a family affected by a serious immune disorder named SCDI (severe combined immunodeficiency, or “bubble baby disease”) (Feske et al., 2005; Feske et al., 2006). SCDI patients suffer from T lymphocytes, B lymphocytes and natural killer cells deficiency. The absence of Ca²⁺ but also Na⁺ currents in these patients’ T lymphocytes was due to a homozygous ORAI1 mutation (R91W) that fully abrogated channel pore opening and compromised T cell gene expression, proliferation and cytokine release.

SOCE and ICRAC were rescued in T lymphocytes by the transient expression of the wild–type (WT) ORAI1. Severe immunodeficiency was also observed in patients with a nonsense mutation in the STIM1 gene producing an incomplete dysfunctional protein (Picard et al., 2009).

Unfortunately, the prognosis of such immune diseases is extremely poor in the absence of bone marrow transplantation, as patients undergo recurrent uncontrolled infections.

Figure 10. Diseases associated with gain and loss-of-function mutations in STIM1 and ORAI1 genes.

From Lacruz and Feske (2015).

Defects observed in patients with STIM1 or ORAI1 gain or loss-of-function mutations. Arrows point toward tissues affected by both gain and loss-of-function mutations and resulting in comparable clinical manifestations. Tubular aggregate myopathy (TAM), York and Stormorken syndromes are partly overlapping diseases (see section 3.2). SCID: severe combined immunodeficiency, AIHA:

autoimmune hemolytic anemia.

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Furthermore, the conditional invalidation of STIM1 gene in mouse lymphoid or myeloid lineages induced the impairment of T cell, B cell, mast cells and macrophages cytokines secretion, migration, and phagocytosis (Baba et al., 2008; Oh-Hora et al., 2008; Beyersdorf et al., 2009; Braun et al., 2009a). Dendritic cells cross presentation and neutrophils phagocytosis or ROS production were also impaired (Nunes et al., 2012; Zhang et al., 2014; Demaurex and Nunes, 2016; Nunes-Hasler et al., 2017).

Strikingly, autoimmune reactions leading to hemolytic anemia, neutropenia and thrombocytopenia were also associated with STIM1 and ORAI1 defects (Feske, 2010). This SOCE–related lymphoproliferative disorder is thought to result from the alteration of regulatory T cells, supposed to promote immune tolerance (Schuhmann et al., 2010).

Skeletal muscle disorders

Although skeletal muscle contraction depends on voltage–gated Ca²⁺ channels and Ca²⁺

release by the sarcoplasmic RyR (Wang et al., 2001; Protasi, 2002; Endo, 2009), SOCE was shown to be essential for SR refilling and prolonged muscle contraction (Launikonis and Rios, 2007; Kiviluoto et al., 2011). In addition, STIM1 orchestrates myoblast differentiation and maturation into myotube (Darbellay et al., 2009). Patients with STIM1 or ORAI1 deficiency display a loss of fast twitch (type II) fibers, and experience hypotonia and increased fatigue (Feske, 2010). Gain–of–function mutations of STIM1 and ORAI1 are associated with a muscular disorder named tubular aggregate myopathy (TAM) that causes muscular pain, weakness and contractures, heightened by exercise. TAM is combined with extramuscular symptoms (thrombocytopenia, miosis, anemia, asplenia, and ichthyosis) in Stormorken syndrome (see section 3.2., (Bohm et al., 2013; Misceo et al., 2014; Nesin et al., 2014; Endo et al., 2015; Bohm et al., 2017)). The cause of the early lethality observed in STIM1 or ORAI1 knockout (KO) mice models remains uncertain, but was attributed to an abnormal muscle differentiation leading to congenital myopathy and respiratory failure (Feske et al., 2010).

Other disorders

Additionally, hypocalcification of dental enamel, ichthyosis and anhydrosis, all ectodermal dysplasias (affecting skin, hair, nails, teeth, and sweat glands) were described in patients lacking functional STIM1 or ORAI1 (Lacruz and Feske, 2015). Increased bleeding is also frequently reported. In addition to autoimmune thrombocytopenia, defective Ca²⁺ influx in platelets following collagen, thrombin or ADP–mediated activation reduces the probability of

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platelets aggregation and natural thrombosis (Varga-Szabo et al., 2008; Braun et al., 2009b).

As many functions of the central nervous system such as synaptic remodeling or fusion of neurotransmitters vesicles rely on SOCE, some patients with STIM1 or ORAI1 deficiency presented neurodevelopmental and neurodegenerative diseases (Kraft, 2015). Finally, in many human cancers, high expression levels of STIM1 and/or ORAI1 is associated with critical prognosis and metastasis. In vivo studies in mouse models of breast cancer highlighted the implication of STIM1 and ORAI1 in aberrant remodeling of focal adhesions, tumor growth and increased migration (Yang et al., 2009; McAndrew et al., 2011). And several gain-of-function mutations in the ORAI1 gene were identified using the cBioPortal cancer database and were associated with increased NFAT nuclear translocation (Frischauf et al., 2017).

Given the extensive expression of STIM1 and ORAI1 in human tissues, the variety of diseases caused by STIM1 or ORAI1 dysfunction remains quite narrow. This support the idea that most cell systems developed compensatory mechanisms to assure Ca²⁺ homeostasis, and redundancy and heteromerization of STIM and ORAI isoforms might be one of them. Partial penetrance or heterozygosity might also balance the severity of some alterations. Moreover, particular defects might only be revealed in certain pathological conditions. For instance, STIM1 and ORAI1 expression levels affect cardiac and vascular myocytes only in murine models of hypertrophic cardiomyopathy or hypertension (Voelkers et al., 2010; Souza Bomfim et al., 2017). Besides, the fact that gain or loss–of–function disorders in humans do not always correlate with mouse phenotypes adds a layer of complexity to understand the role of STIM1 and ORAI1 in diseases.