To the editor:
During the last few years, the Ca2+ sensor stromal interaction molecule 1 (STIM1) and the channel protein Orai1 have emerged as critical components of store operated Ca2+ entry in platelets.1 In human platelets, both proteins were detected in dense granules and lysosome-related organelles.2,3 However, it remains unclear whether Orai1 and STIM1 are required for human platelet function in vivo. Platelet activation requires agonist-induced elevation of intracellular Ca2+, which is released from intracellular stores and enters from the extracellular space. As essential second messenger, Ca2+ regulates fundamental cellular processes in platelets such as coagulant activity and granule release.4 Platelets from Stim1-deficient mice showed a marked defect in agonist-induced Ca2+ responses and impaired thrombus formation.5 However, Stim1−/− mice showed only mild bleeding time prolongation. Mice lacking Stim1 in platelets formed unstable platelet-rich thrombi and had delayed and reduced fibrin generation in injured arterioles.6 Integrin-αIIβ3-mediated aggregation was not impaired in Stim1-deficient platelets.4,6
Patients bearing mutations in STIM1 present with immunodeficiency, autoimmune disorders, congenital myopathy, and ectodermal dysplasia.7 In 5 published patients, enhanced bleeding diathesis was not reported,8-10 however, it seems that these patients were not challenged by surgeries in mucocutaneous areas (ie, tonsillectomy). The sixth patient presented with mucocutaneous bleeding symptoms.9 The prognosis of patients lacking functional Orai1 or STIM1 is poor unless treated by hematopoietic stem cell transplantation.
Here, we investigated platelet function of a patient with a homozygous R429C mutation in STIM1. The girl presented with recurrent autoimmune hemolytic anemia and thrombocytopenia, recurrent bacterial and viral infections, an enamel defect of her teeth, and mild muscular hypotonia.10 Until the age of 5 years, she did not show increased bleeding symptoms. However, she had never been challenged by a surgery in mucocutaneous areas.
Platelet aggregation after stimulation with epinephrine was slightly impaired (Figure 1A). Platelet aggregation/agglutination after stimulation with adenosine 5'-diphosphate, collagen, and ristocetin was within normal limits. Bleeding time was within the upper normal range (patient: 5.5 minutes; normal: 6.0 minutes). Flow cytometry analyses of patient's platelets revealed that platelet α (CD62P)-granule and δ (CD63)-granule secretion in response to thrombin stimulation was impaired (Figure 1B-C). Surface expression of GPIb/IX and GPIIb/IIIa, ristocetin-induced binding of Von Willebrand factor, and binding of soluble fibrinogen were normal, as well as hemoglobin, platelet count, platelet size, and lactate dehydrogenase.
Platelet analyses of the father (heterozygous STIM1 R429C mutation) revealed a reduced Ca2+- store content, a partially impaired store-regulated Ca2+ entry, as well as a secretion defect, in agreement with his heterozygous status.
These data extend findings in mice and show that STIM1 also plays an important role in human platelet physiology. We identified a granule secretion defect affecting α- and δ-granules in platelets from the patient with a STIM1-mutation. This storage pool defect seems similar to that observed in patients with familial hemophagocytic lymphohistiocytosis type 5 who present with mucocutaneous bleedings.11 Although no major bleeding symptoms in patients with STIM1-mutations have been reported so far, our findings show that a mild bleeding diathesis seems to be an additional feature of the complex clinical syndrome associated with STIM1-deficiency.
Acknowledgments: This work was supported by the German Federal Ministry of Education and Research (BMBF 01 EO 0803).
Contribution: L.N. performed platelet studies and analyzed data,; K.S.-L., S.E., J.H., and B.Z. designed research, analyzed data and wrote the paper; and C.S., T.V., and M.B. took care of the patient.
Conflict-of-interest disclosure: The authors declare no competing financial interests.
Correspondence: Barbara Zieger, Department of Pediatrics and Adolescent Medicine, University Medical Center Freiburg, Mathildenstr. 1, 79106 Freiburg, Germany; e-mail: firstname.lastname@example.org.
L.N. and K.S.-L. contributed equally to this study.