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995 Aberrant Activity of the Calcium Sensor STIM1 Underlies Congenital Platelet Disorders and Myeloproliferative Neoplasms

Program: Oral and Poster Abstracts
Type: Oral
Session: 631 Myeloproliferative Syndromes and Chronic Myeloid Leukemia: Basic and Translational: Oncogenic Drivers and Genetic Models
Hematology Disease Topics & Pathways:
Research, MPN, Genetic Disorders, Chronic Myeloid Malignancies, Diseases, Myeloid Malignancies
Monday, December 11, 2023: 5:30 PM

Molly Brakhane, MSc1, Vittorio Abbonante, PhD2*, Angelo B. A. Laranjeira3*, Christian Di Buduo, PhD2*, Brooke Sadler, PhD4, Katrina Ashworth5*, Tim Kong, MSc3, Fan He, PhD3, Mary C. Fulbright3*, Dong Chen, MD, PhD6, Regina Clemens, MD7*, Elena Pegoraro8*, Michele P. Lambert, MD, MTR9, Kandace Gollomp, MD9, Jorge Di Paola, MD5, Alessandra Balduini, MD2 and Stephen T. Oh, MD, PhD10

1Division of Hematology, Washington University at St. Louis, Saint Louis, MO
2University of Pavia, Pavia, ITA
3Division of Hematology, Department of Medicine, Washington University School of Medicine, St Louis, MO
4Pediatrics, Washington University School of Medicine, Saint Louis, MO
5Department of Pediatrics, Washington University School of Medicine, St Louis, MO
6Division of Hematopathology, Mayo Clinic, Rochester, MN
7Departmnet of Pediatrics, Washington University at St Louis, St Louis, MO
8University of Padova, Padova, ITA
9Division of Hematology, Children's Hospital of Philadelphia, Philadelphia, PA
10Department of Medicine, Washington University School of Medicine, Saint Louis, MO

MB and VA equal contributors

AB and STO co-corresponding authors

This study was initiated following evaluation of a 32 year-old woman who presented with a history of thrombocytopenia identified in childhood who subsequently developed features of myelofibrosis (MF). A bone marrow biopsy demonstrated hypercellularity in conjunction with megakaryocyte hyperplasia and marked reticulin fibrosis. Molecular testing for JAK2, CALR, and MPL mutations was negative. Given the unusual association between congenital thrombocytopenia and MF in this patient, exome sequencing was performed, revealing a heterozygous R304W mutation in the coiled coil (CC) domain of STIM1. Activating mutations in the CC and EF hand domains of STIM1 have been associated with Stormorken syndrome, a rare congenital platelet disorder associated with abnormal store operated calcium entry (SOCE). We subsequently identified a second patient with MF associated with a STIM1 activating mutation. This individual was found to have severe thrombocytopenia at birth, and exome sequencing revealed a heterozygous S88G mutation in the EF hand of STIM1. A bone marrow biopsy obtained at 6 months of age revealed atypical megakaryocytes and grade 1-2 MF. A repeat biopsy at age 2 showed persistence of stable MF.

The unusual finding of MF in these two patients suggested the possibility of altered calcium signaling as a shared mechanism driving congenital platelet disorders such as Stormorken syndrome and myeloproliferative neoplasms (MPNs) including MF. In support of this notion, we identified elevated STIM1 expression in MF vs normal megakaryocyte progenitors, as well as in platelets from patients with essential thrombocythemia (ET) vs healthy controls. Additionally, we found that STIM1 expression was significantly elevated in CD34+ hematopoietic stem/progenitor cells (HSPCs) from both ET and MF patients vs healthy controls. Notably, STIM1 expression was increased in both JAK2 and CALR-mutant MF patients. Collectively, these findings provide evidence of aberrant STIM1 expression in MPN patient cells.

To determine the functional role of STIM1 in MPN disease development, colony assays and patient-derived xenograft (PDX) experiments were performed with MF patient CD34+ cells subjected to CRISPR ablation of STIM1. Strikingly discordant results were observed with JAK2 vs CALR-mutant patient samples. Abrogation of STIM1 in CALR-mutant CD34+ cells led to decreased colony formation, and NSGS mice engrafted in parallel with STIM1-targeted cells exhibited decreased human CD45+ cell engraftment in conjunction with prolonged survival. These findings suggest an important role for STIM1 in CALR-mutant MPN disease phenotypes. In contrast, targeting of STIM1 in JAK2-mutant CD34+ cells led to increased colony formation and exacerbated disease phenotypes in vivo, as manifested by enhanced human CD45+ cell engraftment, worsened splenomegaly, and early lethality. Similar results were obtained in experiments utilizing pharmacologic inhibitors of SOCE activity. Taken together, these findings indicate that the consequences of aberrant STIM1 activity may be context-dependent relating to specific MPN driver mutations.

To expand these observations, we identified a separate cohort of 9 family members in Italy with Stormorken syndrome and confirmed STIM1 EF hand mutations. In ex vivo megakaryocytic differentiation assays, cells from affected individuals exhibited a defect in proplatelet formation. These observations were corroborated by initial analyses of a newly generated Stim1 R304W conditional knock-in mouse which recapitulated the characteristic thrombocytopenia found in patients with Stormorken syndrome.

In summary, this study represents the first demonstration of MF development in patients with Stormorken syndrome, thereby uncovering a previously unrecognized hallmark of altered calcium signaling via aberrant STIM1 activation underlying Stormorken syndrome and MPNs. Our findings suggest distinct mechanisms relating to the interaction between JAK2 vs CALR mutation and altered STIM1 activity. Further studies of these relationships may have important ramifications for potential therapeutic approaches targeting these pathways.

Disclosures: Lambert: Octapharma: Membership on an entity's Board of Directors or advisory committees, Research Funding; Dova: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Principia: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Rigel: Consultancy, Membership on an entity's Board of Directors or advisory committees; Shionogi: Consultancy, Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Research Funding; Argenx: Consultancy, Research Funding; Sobi: Consultancy; Sanofi: Consultancy; Janssen: Consultancy; Sysmex: Research Funding. Di Paola: CSL Behring: Consultancy; Alnylam: Consultancy. Oh: CTI BioPharma, Bristol Myers Squibb, Disc Medicine, Blueprint Medicines, PharmaEssentia, Constellation/MorphoSys, Geron, AbbVie, Sierra Oncology/GSK, Cogent, Incyte, Morphic, Protagonist: Consultancy.

*signifies non-member of ASH