Summary
Velocardiofacial syndrome (VCFS) is a common, phenotypically heterogeneous developmental
disorder caused by an interstitial microdeletion within human chromosome 22q11. The
deleted chromosomal region in >90% of VCFS patients includes the GPIbβ gene, encoding for one subunit of the platelet GPIb-V-IX receptor, which is critical
for platelet adhesion under shear, and important in aggregation and thrombin-mediated
activation. Complete loss of GPIb-V-IX due to autosomal recessive inheritance of two
GPIbα,Ibβ or GP9 gene mutations,results in a severe bleeding disorder,Bernard-Soulier syndrome (BSS).
In this study, twenty-one confirmedVCFS patients were analyzed for platelet morphological
and functional alterations, resulting from the heterozygous loss of one GPIbβ gene allele. Compared to unaffected family members,VCFS patients showed a significant
decrease in platelet count; VCFS platelet size and mean platelet volume were increased,
but not as markedly as in BSS. As expected from obligatory heterozygotes for GPIbβ deficiency, VCFS patients showed reduced platelet GPIb-V-IX surface expression and
total GPIb content, but with considerable variation between cases. Platelet function
tested using the PFA-100™ analyzer was impaired in 70% of patients. Platelet aggregation was reduced in response
to a GPIb-dependent agonist, ristocetin, in 50% ofVCFS patients, with 35% showing
a reduced response to thrombin receptor activating peptide. Genomic screening was
performed to exclude mutations of the subunit genes, indicating that these platelet
abnormalities were due to GPIbβ heterozygosity and not spontaneous BSS. In conclusion, many VCFS patients have in-vitro
defects in platelet function that may increase their risk of bleeding during surgery.
Keywords
GPIb - platelet function - inherited platelet disorder - velocardiofacial syndrome