Platelet functional alterations in a Bernard-Soulier syndrome patient with filamin A mutation
© Li et al. 2015
Received: 4 June 2015
Accepted: 11 June 2015
Published: 2 July 2015
Defects in filamin A (FLNA) gene could lead to low platelet counts and decreased surface expression of glycoprotein (GP) Ibα. Here, we report and investigate the FLNA genomic alteration of a case with Bernard-Soulier syndrome (BSS), a rare hereditary bleeding disorder caused by quantitative or qualitative abnormalities in the GP Ib-IX-V receptor. DNA sequencing analysis reveals the presence of a GP Ibα c.987G > A mutation and a FLNA c.1582 G > A mutation in this patient. Transient transfection studies show that GP Ibα c.987G > A mutation abolishes the surface expression of GP Ibα on the transfected CHO cells. On the other hand, abnormal responses to collagen, including the platelet aggregation, secretion, and GP VI signaling pathways, are associated with FLNA c.1582G > A mutation. Our findings confirm a central role for FLNA in platelet-adhesive functions. The interaction between FLNA and GP Ibα in platelets deserves to be investigated.
KeywordsBernard-Soulier syndrome Platelets Glycoprotein Ib-IX complex Thrombocytopenia Mutation
To the Editor
Filamin A (FLNA), a dimeric actin crosslinking protein, anchors the platelet adhesion glycoprotein (GP)Ib-IX-V receptor to actin cytoskeleton. The GP Ibα-FLNA interaction is essential for the platelet adhesion to von Willebrand factor that binds the GP Ib-IX-V receptor, for normal signal transduction reactions involved in platelet activation, and for maintaining normal platelet shape and integrity . The absence of or mutations in the GP Ib-IX-V receptor or FLNA gene is responsible for Bernard-Soulier syndrome (BSS) or macrothrombocytopenia , respectively. We described here a female BSS case with FLNA mutation, showing her platelet functional defects in the platelet aggregation, secretion, and GP VI signaling pathways.
To establish that GP Ibα c.987G > A mutation was responsible for the lack of GP Ibα surface expression in the patient’s platelets, we expressed in Chinese hamster ovary (CHO) cells the mutant or normal GP Ibα cDNA together with normal human cDNAs of GP Ibβ and GPIX. Only trace amounts of GP Ibα and GPIX was detected on CHO cells harboring the mutation, but truncated GP Ibα and normal GPIX was present in the CHO cell lysates (Fig. 1c). Together, these data could suggest shorten GP Ibα and GPIX were synthesized, but failed to be anchored and inserted into the plasma membrane.
A question raised by our observation was that patient’s platelet responses to collagen were impaired, despite a normal amount of platelet GPVI and FcRγ chain in her platelets. Several lines of evidence demonstrated that FLNA acted as a signaling scaffold for GPVI through interaction with tyrosine kinase Syk . In our study, the normal level and regular distribution of FLNA was detected in the patient’s platelets (Fig. 1d). However, the phosphorylation of Syk was low (41 % of control) in the platelet signaling pathway of GPVI induced by Cvx (400 pmol/L). Similarly, the phosphorylation of LAT, a direct substrate of Syk, was decreased (45 % of control) (Fig. 1e). FLNA could bind to the tyrosine kinase Syk through its immunoglobulin-like repeat 3–5 in platelets . In our patient, the identified FLNA c.1582G > A, an Ig repeat 3 mutation, interfered with the Ig repeats engaged in signaling of GPVI–collagen interaction. In turn, platelet aggregation and ATP secretion on collagen was interrupted. We thus conclude that abnormal responses to collagen were associated with FLNA c.1582G > A mutation.
To the best of our knowledge, this was the first report of a FLNA mutation causing abnormal response to collagen in a BSS patient. According to our results, we demonstrated that the synergistic effect of both GP Ibα c.987G > A mutation and FLNA c.1582G > A mutation could lead to the platelet functional alteration, including aggregation, secretion, and the GPVI signaling pathway. The relationship between GP Ibα and FLNA should constitute an area of interest for future studies.
This work was funded by the Jiangsu Provincial Special Program of Medical Science (BL2012005), the Jiangsu Province’s Key Medical Center (ZX201102), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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