Comprehensive Description of Monoallelic GP1BA Variants Associated with Thrombocytopenia

GP1BA encodes the glycoprotein (GP) 1bα subunit of the GP1b/IX/V complex, the platelet receptor for von Willebrand factor (VWF) and a mediator of outside-in signalling that contributes to platelet activation. Biallelic variants in GPIBA that prevent surface expression of GPIB/IX/V cause the recessive disorder Bernard Soulier Syndrome (BSS; OMIM #23100) in which there is often severe bleeding caused by thrombocytopenia and platelet dysfunction. Thrombocytopenia has also been associated with monoallelic missense variants in the GP1bα R loop (residues 227-242) which enhance VWF binding (platelet-type von Willebrand disease (pVWD); OMIM #177820). Other monoallelic GPIBA missense variants such as the Bolzano variant (p.Ala172Val; OMIM #153670) have been associated with dominant thrombocytopenia without altered VWF levels.

In order to better describe the relationship between monoallelic GPIBA variants and thrombocytopenia we evaluated data from the NIHR BioResource for Rare Diseases, (NBR-RD) and the Inherited Platelet Disorders programmes at the Universities of Birmingham (UK), Murcia (Spain) and Toronto (Canada) in which patients with unexplained platelet disorders underwent diagnostic next generation sequencing. Using the BeviMed method for genetic association we compared the genotypes at rare nonsynoymous variants of 105 unrelated thrombocytopenia cases (platelet count (PLT) <130x10⁹/L or the human phenotype ontology term ‘HP 001873 thrombocytopenia’) in the NBR-BR with those of 10,152 unrelated participants without thrombocytopenia. There was a strong statistical association (posterior probability 0.97) between dominantly inherited GPIBA variants and thrombocytopenia. Considering the five variants within the NBR-BR that drove this association alongside genotype data from the other case collections, we identified a total of 21 different monoallelic GPIBA coding variants that were rare (gnomAD allele frequency <1/1000) and had a CADD pathogenicity score >15. These were present in 29 index cases (18 females, median age 37 years) with unexplained thrombocytopenia and included the variant predicting the p.Asn150Ser substitution which was identified in seven independent index cases. The variants comprised 18 missense, two inframe insertions (one with an additional missense change) and one insertion resulting in a frame shift. Seven had been previously associated with thrombocytopenia.

Sixteen of the variants predicted amino acid substitutions or single residue insertions within the GP1bα leucine rich repeat region (LRR; residues 48-200) or in adjacent regions of the LRR N- and C-terminal caps. Cases with variants in the LRR region typically displayed mild mucocutaneous bleeding, abnormal bleeding after minor trauma or surgery and heavy menstrual bleeding. The platelet count was typically 60-100 x10⁹/L and mean platelet volume greater than 11 fL. There were two missense variants in the GP1bα R loop that have been previously associated with pVWD and two further previously unreported missense variants in the cytoplasmic domain. The final variant was a monoallelic single nucleotide insertion predicted to cause a frame shift and absent expression of GP1bα and has been previously reported as a biallelic variant associated with BSS.

These data indicate that in a multicentre cross-sectional cohort of cases with otherwise unexplained thrombocytopenia, monoallelic missense GPIBA variants affecting the LRR or adjacent regions were more prevalent than R loop variants associated with pVWD or variants causing absent GP1bα expression associated with biallelic BSS. The LRR region contains the GP1bα-VWF binding site which is predicted to be disrupted by the missense variants observed in this study. The LRR region is also the location of the GP1bα Bolzano variant and six other monoallelic variants previously associated with thrombocytopenia in previous single case reports. These observations extend the repertoire of GPIBA variants associated with thrombocytopenia and although requiring experimental confirmation of pathogenicity, suggest a common molecular pathogenesis for dominant GPIBA-associated thrombocytopenia in which reduced circulating platelet numbers arises from defective GP1bα-VWF interactions.