Heterozygous loss of platelet glycoprotein (GP) Ib-V-IX variably affects platelet function in velocardiofacial syndrome (VCFS) patients

SummaryVelocardiofacial 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.

Two Functional Copies of the DGCR6 Gene Are Present on Human Chromosome 22q11 Due to a Duplication of an Ancestral Locus

The DGCR6 (DiGeorge critical region) gene encodes a putative protein with sequence similarity to gonadal(gdl), a Drosophila melanogaster gene of unknown function. We mapped the DGCR6 gene to chromosome 22q11 within a low copy repeat, termed sc11.1a, and identified a second copy of the gene, DGCR6L, within the duplicate locus, termed sc11.1b. Both sc11.1 repeats are deleted in most persons with velo-cardio-facial syndrome/DiGeorge syndrome (VCFS/DGS), and they map immediately adjacent and internal to the low copy repeats, termed LCR22, that mediate the deletions associated with VCFS/DGS. We sequenced genomic clones from both loci and determined that the putative initiator methionine is located further upstream than originally described, but in a position similar to the mouse and chicken orthologs.DGCR6L encodes a highly homologous, functional copy ofDGCR6, with some base changes rendering amino acid differences. Expression studies of the two genes indicate that both genes are widely expressed in fetal and adult tissues. Evolutionary studies using FISH mapping in several different species of ape combined with sequence analysis of DGCR6 in a number of different primate species indicate that the duplication is at least 12 million years old and may date back to before the divergence of Catarrhines from Platyrrhines, 35 mya. These data suggest that there has been selective evolutionary pressure toward the functional maintenance of both paralogs. Interestingly, a full-length HERV-K provirus integrated into the sc11.1a locus after the divergence of chimpanzees and humans.