PO03 | Enlarged platelets with reduced GP IB/IX can indicate disorders other than Bernard-Soulier syndrome

Abstract

Background and Aims: Evaluation of platelet morphology is a cornerstone of the diagnostic work-up of patients with suspected inherited platelet disorders (IPD). We have established an immunofluorescence microscopy-based method on the peripheral blood smear in combination with light-microscopy as a screening-tool for IPD. The technique has been validated for 9 disorders displaying typical changes of platelet structure. As a quality-control of this approach, we regularly perform a blinded investigation of subjects, who had previously undergone genetic testing. Methods: Blood smears were stained using 13 primary antibodies against different platelet structures (alpha and dense granule, surface glycoproteins, cytoskeletal components) and 2 fluorescence-labelled secondary antibodies. Exclusively based on the morphologic evaluation, we formulated a possible diagnosis. After uncovering patients’ information, we compared the anticipated defect with molecular outcome. Results: Over the last 12 months, we enrolled to this study 30 subjects. Based on the finding of platelet macrocytosis and reduced expression of the surface platelet glycoprotein (GP) Ib/IX, we predicted a diagnosis of Bernard Soulier syndrome (BSS) in 4 individuals. After unblinding molecular results, we could confirm the suspicion only in one case, in whom a pathogenic heterozygous deletion in GP1BB gene (c.236_244del [p.Pro79_Leu81del]) consistent with dominant-inherited BSS was found. In two subjects belonging to the same pedigree, a novel homozygous variant in the gene encoding for UDP-N-acetylglucosamine 2-epimerase (GNE): c.1516G>A [p.Gly506Ser] was found. In one individual, we found two novel variants in compound heterozygosis in the gene encoding for uridine diphosphate (UDP)-galactose-4-epimerase (GALE): c.382G>A [p.Val128Met] and c.590T>C [p.Ile197Thr]. All the three patients suffered from severe thrombocytopenia. In the patient with GALE mutations, a syndromic picture including mental retardation, mitral valve prolapse and hip malformation was also apparent. Conclusions: Platelet macrocytosis with reduced expression of GPIb/IX can indicate inherited thrombocytopenias due mutations in GALE or GNE. These two genes play a role at different levels of platelet production and clearance. In both disorders, the reduced externalization of GPIb/IX is a consequence of impaired protein glycosylation. Immunofluorescence microscopy on the peripheral blood smear and genetic testing complement each other.

Keywords

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