Neutrophil specific granule and NETosis defect in Gray Platelet Syndrome

Gray Platelet Syndrome (GPS) is an autosomal recessive bleeding disorder characterized by a lack of α-granules in platelets and progressive myelofibrosis. Rare loss of function variants in NBEAL2, a member of the family of BEACH genes, are causal of GPS. The gene is involved in fusion, fission and trafficking of vesicles and granules. Whether NBEAL2 controls the ontogeny of granules of myeloid cells remains disputed. We found that neutrophils obtained from the peripheral blood from GPS patients have a normal distribution of azurophilic granules, but show a deficiency of specific granules, as confirmed by immuno-electron microscopy and mass spectrometry proteomics analyses. In cultures from peripheral CD34+ hematopoietic stem cells (HSCs) into mature neutrophils, the time dynamics showed concordance of NBEAL2 and specific granule protein expression at transcriptional and protein level, which were discordant in neutrophils obtained GPS-HSCs. This is indicative of normal granulopoiesis in GPS and identifies NBEAL2 as an important regulator of granule release (similar to platelets) which is suggested to occur upon egress into the blood stream. Patient neutrophil functions, including production of reactive oxygen species, chemotaxis and killing of bacteria and fungi were intact. Since GPS patients do not excessively suffer from infections, the consequence of the reduced specific granule content and lack of NET formation for innate immunity remains to be explored.