Martina E. Daly

High prevalence of a mutation in the factor V gene within the U.K. population: relationship to activated protein C resistance and familial thrombosis

Summary. Recent findings have indicated the importance of factor V (FV) in causing resistance to activated protein C (APC) in a high proportion of patients with venous thrombosis. This prompted us to investigate whether resistance could be due to defective inactivation of FVa by APC. Consequently, we amplified a 3.2 kb fragment of the FV gene sequence encoding the heavy chain APC cleavage site. DNA analysis showed a guanine to adenine transition at nucleotide 1691 in all affected members of two families with inherited APC resistance associated with thrombosis and confirmed suspected homozygosity in two individuals. The mutation, in heterozygous form, was also found in ˜3.5% of our normal population (n = 144) and correlated with low APC resistance. The high prevalence of this mutation suggests that it may be a major contributory factor in early thrombosis.

Common sequence variations in the P2Y12 and CYP3A5 genes do not explain the variability in the inhibitory effects of clopidogrel therapy

The efficacy of the platelet P2Y12 receptor antagonist clopidogrel, which undergoes cytochrome-mediated metabolism to its active form, shows marked inter-individual variability. We investigated whether polymorphic variations in the P2Y12 gene, which have been linked to platelet aggregation phenotypes, or the cytochrome P450 3A5 gene 6986G > A polymorphism, which largely determines CYP3A5 expression, influence the response to clopidogrel therapy. Fifty-four patients listed for elective percutaneous coronary intervention were studied using ADP-induced optical aggregometry, whole-blood single platelet counting (WBSPC) aggregometry, and flow-cytometric analysis of platelet P-selectin expression and platelet-monocyte conjugate formation. Platelet reactivity was measured at baseline, 4 h post clopidogrel 300 mg, and 10 and 28 days following clopidogrel 75 mg daily. A further 55 patients were studied using ADP-induced WBSPC at baseline and 4 h post clopidogrel 600 mg. Patients were genotyped for P2Y12 haplotype and the CYP3A5 6986G > A single nucleotide polymorphism. Neither genotype was found to significantly influence the inhibition of platelet responses by either clopidogrel regimen. In conclusion, common sequence variations within the P2Y12 and CYP3A5 genes do not contribute any major effect to the inter-patient variability in clopidogrel efficacy.

Identification and characterization of a novel P2Y12 variant in a patient diagnosed with type 1 von Willebrand disease in the European MCMDM-1VWD study

We investigated whether defects in the P2Y(12) ADP receptor gene (P2RY12) contribute to the bleeding tendency in 92 index cases enrolled in the European MCMDM-1VWD study. A heterozygous mutation, predicting a lysine to glutamate (K174E) substitution in P2Y(12), was identified in one case with mild type 1 von Willebrand disease (VWD) and a VWF defect. Platelets from the index case and relatives carrying the K174E defect changed shape in response to ADP, but showed reduced and reversible aggregation in response to 10 muM ADP, unlike the maximal, sustained aggregation observed in controls. The reduced response was associated with an approximate 50% reduction in binding of [(3)H]2MeS-ADP to P2Y(12), whereas binding to the P2Y(1) receptor was normal. A hemagglutinin-tagged K174E P2Y(12) variant showed surface expression in CHO cells, markedly reduced binding to [(3)H]2MeS-ADP, and minimal ADP-mediated inhibition of forskolin-induced adenylyl cyclase activity. Our results provide further evidence for locus heterogeneity in type 1 VWD.

The GPIa C807T dimorphism associated with platelet collagen receptor density is not a risk factor for myocardial infarction

The platelet collagen receptor, GPIa/IIa, is an important mediator of platelet adhesion to fibrillar collagens at sites of vascular injury. Recently, a dimorphism at nucleotide 807 of the GPIa cDNA (TTC/TTT in codon 224) was shown to be associated with variation in GPIa/IIa receptor density on the platelet surface. We conducted a case–control study to determine if the 807T allele, linked with increased GPIa/IIa density, contributed to risk of myocardial infarction (MI). DNA from 546 acute MI cases and 507 controls, all aged <75 years, was genotyped for the C807T dimorphism using the TaqManTM system of allelic discrimination. The allelic odds ratio (OR) for MI in the complete cohort was 0.88 (95% CI 0.74–1.05, P = 0.17), indicating that the 807T allele was not associated with an increased risk of MI. There was also no increased risk of MI associated with the homozygous 807TT (P = 0.22) or heterozygous 807CT (P = 0.24) genotypes or for carriers of the 807T allele in any cohort subgroup analysed. We conclude that the GPIa 807T allele is not a risk factor for MI in our population either alone or in combination with other major cardiovascular risk factors.

Evaluation of participants with suspected heritable platelet function disorders including recommendation and validation of a streamlined agonist panel

Light transmission aggregometry (LTA) is used worldwide for the investigation of heritable platelet function disorders (PFDs), but interpretation of results is complicated by the feedback effects of ADP and thromboxane A(2) (TxA(2)) and by the overlap with the response of healthy volunteers. Over 5 years, we have performed lumi-aggregometry on 9 platelet agonists in 111 unrelated research participants with suspected PFDs and in 70 healthy volunteers. Abnormal LTA or ATP secretion test results were identified in 58% of participants. In 84% of these, the patterns of response were consistent with defects in Gi receptor signaling, the TxA(2) pathway, and dense granule secretion. Participants with defects in signaling to Gq-coupled receptor agonists and to collagen were also identified. Targeted genotyping identified 3 participants with function-disrupting mutations in the P2Y(12) ADP and TxA(2) receptors. The results of the present study illustrate that detailed phenotypic analysis using LTA and ATP secretion is a powerful tool for the diagnosis of PFDs. Our data also enable subdivision at the level of platelet-signaling pathways and in some cases to individual receptors. We further demonstrate that most PFDs can be reliably diagnosed using a streamlined panel of key platelet agonists and specified concentrations suitable for testing in most clinical diagnostic laboratories.

Enrichment of FLI1 and RUNX1 mutations in families with excessive bleeding and platelet dense granule secretion defects

We analyzed candidate platelet function disorder genes in 13 index cases with a history of excessive bleeding in association with a significant reduction in dense granule secretion and impaired aggregation to a panel of platelet agonists. Five of the index cases also had mild thrombocytopenia. Heterozygous alterations in FLI1 and RUNX1, encoding Friend leukemia integration 1 and RUNT-related transcription factor 1, respectively, which have a fundamental role in megakaryocytopoeisis, were identified in 6 patients, 4 of whom had mild thrombocytopenia. Two FLI1 alterations predicting p.Arg337Trp and p.Tyr343Cys substitutions in the FLI1 DNA-binding domain abolished transcriptional activity of FLI1. A 4-bp deletion in FLI1, and 2 splicing alterations and a nonsense variation in RUNX1, which were predicted to cause haploinsufficiency of either FLI1 or RUNX1, were also identified. Our findings suggest that alterations in FLI1 and RUNX1 may be common in patients with platelet dense granule secretion defects and mild thrombocytopenia.

A novel thromboxane A2 receptor D304N variant that abrogates ligand binding in a patient with a bleeding diathesis.

We investigated the cause of mild mucocutaneous bleeding in a 14-year-old male patient (P1). Platelet aggregation and ATP secretion induced by arachidonic acid and the thromboxane A(2) receptor (TxA(2)R) agonist U46619 were reduced in P1 compared with controls, whereas the responses to other platelet agonists were retained. P1 was heterozygous for a transversion within the TBXA2R gene predictive of a D304N substitution in the TxA(2)R. In Chinese hamster ovary-K1 cells expressing the variant D304N TxA(2)R, U46619 did not increase cytosolic free Ca(2+) concentration, indicating loss of receptor function. The TxA(2)R antagonist [(3)H]-SQ29548 showed an approximate 50% decrease in binding to platelets from P1 but absent binding to Chinese hamster ovary-K1 cells expressing variant D304N TxA(2)R. This is the second naturally occurring TxA(2)R variant to be associated with platelet dysfunction and the first in which loss of receptor function is associated with reduced ligand binding. D304 lies within a conserved NPXXY motif in transmembrane domain 7 of the TxA(2)R that is a key structural element in family A G protein-coupled receptors. Our demonstration that the D304N substitution causes clinically significant platelet dysfunction by reducing ligand binding establishes the importance of the NPXXY motif for TxA(2)R function in vivo.

The prevalence of, and molecular defects underlying, inherited protein S deficiency in the general population

The molecular basis of protein S (PS) deficiency was investigated in seven of eight donors identified with persistently low plasma PS levels from a survey of PS levels in 3788 Scottish blood donors. PROS1 gene analysis identified at least one defect in six donors. Five were heterozygous for the Heerlen polymorphism predicting a Ser460Pro substitution. Haplotype analysis revealed the possibility that this allele was inherited with the same haplotype in four of the five donors, suggesting a founder effect for the Heerlen allele in this population. One Heerlen allele carrier was also heterozygous for a 3 bp deletion 68–72 bp upstream of exon 2. Platelet PROS1 transcript analysis showed no reduction in mRNA expression from the affected allele in this donor. A T to G transversion 3 bp upstream of exon 12 was identified in one donor, which is predicted to reduce the efficiency of PS mRNA splicing. However, PROS1 transcript analysis showed no evidence of exon skipping or cryptic splicing. No PROS1 gene defect was detected in the remaining donor. This genetic information enabled us to refine our estimate of the prevalence of heritable PS deficiency in the Scottish population to between 0·16% and 0·21%, predominantly resulting from the presence of the Heerlen allele.

Determinants of platelet count in humans

An adequate supply of circulating platelets is essential to maintain vascular integrity and to facilitate thrombus formation at sites of vascular injury. The normal platelet count in humans ranges from 150×109/L to 400×109/L. Given that platelets have a circulating lifespan of around 10 days, and

An intact PDZ-motif is essential for correct P2Y12 purinoceptor traffic in human platelets

The platelet P2Y(12) purinoceptor (P2Y(12)R), which plays a crucial role in hemostasis, undergoes internalization and subsequent recycling to maintain receptor responsiveness, processes that are essential for normal platelet function. Here, we observe that P2Y(12)R function is compromised after deletion or mutation of the 4 amino acids at the extreme C-terminus of this receptor (ETPM), a putative postsynaptic density 95/disc large/zonula occludens-1 (PDZ)-binding motif. In cell line models, removal of this sequence or mutation of one of its core residues (P341A), attenuates receptor internalization and receptor recycling back to the membrane, thereby blocking receptor resensitization. The physiologic significance of these findings in the regulation of platelet function is shown by identification of a patient with a heterozygous mutation in the PDZ binding sequence of their P2Y(12)R (P341A) that is associated with reduced expression of the P2Y(12)R on the cell surface. Importantly, platelets from this subject showed significantly compromised P2Y(12)R recycling, emphasizing the importance of the extreme C-terminus of this receptor to ensure correct receptor traffic.