NBEAL2 related Gray Platelet Syndrome with a combined platelet granules secretion defect in flow cytometry

Introduction: Gray platelet syndrome (GPS, MIM # 139090) is a rare bleeding disorder of mild to moderate severity characterized by moderate macrothrombocytopenia and grayish appearance of the patient´s platelets in the peripheral blood smear due to a severe shortage (or even absence) of α-granules [1] [2]. In 2011 biallelic pathogenic alterations in NBEAL2 have been identified to cause the disease [3] [4] [5]. Besides NBEAL2, alterations in other genes (e.g. GFI1B, GATA1) may also be associated with pale platelets due to a significant reduction in α-granules [6] [7]. NBEAL2 encodes a large scaffolding protein (2754 aa) involved in granule trafficking and the development of α-granules [8] [9]. The protein is expressed in hematopoietic cells, including megakaryocytes, platelets, monocytes, neutrophils, and NK (natural killer) cells. GPS is a syndromic disease affecting not only platelets, but also immune cells. Over time the patients can develop pancytopenia, splenomegaly and bone marrow fibrosis [10]. We report on a 64-year-old female originally from Finland with lifelong thrombocytopenia and bleeding diathesis. The high ISTH-BAT score of 13 points (normal range for women up to 5 points) results from very heavy menstrual bleeding before menopause, which led to a transfusion when she was a teenager. She experienced also bleeding after a tonsillectomy and tooth extractions. Her bleeding tendency was usually successfully treated with tranexamic acid. She had been suspected of having gray platelet syndrome (GPS) as a young adult, but at that time, the genetic correlate of this disease was still unknown.

Method: Blood smear, platelet flow cytometry, NGS

Results: The patient presented with thrombocytopenia (48 x10⁹/L) and a mean platelet volume of 12.1 fl (Ref 7-12 fl). Peripheral blood smear showed partly non-granulated, pale platelets. Interestingly, flow cytometry revealed a combined secretion defect: CD62-P (alpha-granule marker) and CD63 (lysosomal and delta-granule marker) exposure after thrombin activation was severely reduced. We identified a homozygous 5-base pair duplication c.4371_4375dup in NBEAL2 (NM_015175.3) exon 28 leading to a frameshift and premature stop codon (p.Glu1459Alafs*43) resulting in a non-functional protein. The duplication is annotated in gnomAD (MAF 0.0014%) and in ClinVar as likely pathogenic (VCV001705844.1) and has been described compound heterozygous with a splice-site mutation in one patient [3].

Conclusion: A comprehensive diagnosis, including molecular genetic testing, is particularly important in order to provide patients with adequate care and treatment. Especially older GPS patients should be monitored for the development of myelofibrosis.

Publikationsverlauf

Artikel online veröffentlicht:
13. Februar 2025

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