Acquired hemophilia A and delta storage pool deficiency in a patient with indolent non-Hodgkin lymphoma.

B-cell lymphoproliferative diseases may be associated with acquired hemostasis disorders, such as acquired hemophilia A (AHA) caused by autoantibodies that neutralize factor VIII activity, and δ-storage pool deficiency, an abnormality of platelet function due to defective dense granules and impaired secretion. We describe the case of a 67-year-old man in whom these two acquired bleeding disorders were concomitantly present as the first clinical manifestation of an indolent non-Hodgkin lymphoma. Immunosuppressive therapy with prednisone was initially started to eradicate anti-FVIII antibodies, subsequently boosted with cyclophosphamide and rituximab, these medications being also chosen to treat the associated indolent lymphoma. Bleeding symptoms were first tackled with limited benefit by using rFVIIa and then rescued using recombinant porcine FVIII. After a 6 month's follow-up lymphoma and AHA were in remission and platelet function was improved. This case underlines the need of multiple and complex diagnostic and therapeutic approaches to rare acquired bleeding disorders associated with lymphoproliferative diseases.

Novel variant in HPS3 gene in a patient with Hermansky Pudlak syndrome (HPS) type 3.

Hermansky-Pudlak syndrome (HPS) is a rare autosomal recessive disorder caused by defects in 10 human HPS genes, characterized by oculocutaneous albinism (OCA) and bleeding diathesis associated to platelet δ-storage pool defect (SPD). We report a case of 4-year-old boy from non-consanguineous parents with OCA and negative personal and familiar hemorrhagic history, referred to us for severe bleeding after mild trauma. His platelet function, studied by lumi-aggregometry, showed normal first wave of aggregation in response to exogenous agonists and impaired second wave with defective ATP release. This, in combination with impaired platelet δ-granules content (serotonin, ATP, ADP) and the OCA phenotype suggested the HPS diagnosis. HPS3: sequencing revealed a novel pathogenic homozygous variant (NM_032383.4:c.7>T, p.Gln3*) resulting in a premature stop codon at the amino acid 3. Moreover, our report highlights the importance of evaluating platelet function in children with OCA without bleeding diathesis to identify HPS early and prevent bleeding complications.

Evaluation of platelet function in essential thrombocythemia under different analytical conditions.

Background. Studies of platelet aggregation (PA) in essential thrombocythemia (ET) reported contrasting results, likely due to differences in analytical conditions.Objective. We investigated platelet aggregation using different techniques and analytical conditions.Patients and Methods. PA was studied by light-transmission aggregometry (LTA) in platelet-rich plasma (PRP) and impedance aggregometry in PRP and whole blood (WB). ADP, collagen, thrombin receptor activating peptide (TRAP-14) and adrenaline were used as agonists. Since ET patients (n = 41) were on treatment with aspirin (100 mg/d), healthy controls (n = 29) were given aspirin (100 mg/d) for 5 days before testing: therefore, thromboxane A2-independent PA was tested in all subjects. Blood samples were collected in citrate (C) [low Ca2+] or lepirudin (L) [physiological Ca2+]; platelet count was adjusted to 250 x 109/L in a set of C-PRP (adjusted C-PRP) and left unmodified in the other samples.Results. Results of PA in 17 ET patients who were poor responders to aspirin (high serum thromboxane B2 levels) were not included in the analysis. With LTA, PA in ET was lower than in controls in adjusted C-PRP and normal in native C-PRP and L-PRP. With impedance aggregometry, PA in L-PRP and L-WB tended to be higher in ET than in controls. Platelet serotonin and ADP contents were reduced in ET. The percentages of circulating platelets expressing P-selectin and platelet-leukocyte hetero-aggregates were higher in ET.Conclusions. Analytical conditions dramatically affect in vitro PA of ET patients, which appears defective under the least physiological conditions and normal/supranormal under conditions that are closer to the physiological.

Complications of whole-exome sequencing for causal gene discovery in primary platelet secretion defects.

Primary platelet secretion defects constitute a heterogeneous group of functional defects characterized by reduced platelet granule secretion upon stimulation by different agonists. The clinical and laboratory heterogeneity of primary platelet secretion defects warrants a tailored approach. We performed a pilot study in order to develop DNA sequence analysis pipelines for gene discovery and to create a list of candidate causal genes for platelet secretion defects. Whole-exome sequencing analysis of 14 unrelated Italian patients with primary secretion defects and 16 controls was performed on Illumina HiSeq. Variant prioritization was carried out using two filtering approaches: identification of rare, potentially damaging variants in platelet candidate genes or by selecting singletons. To corroborate the results, exome sequencing was applied in a family in which platelet secretion defects and a bleeding diathesis were present. Platelet candidate gene analysis revealed gene defects in 10/14 patients, which included ADRA2A, ARHGAP1, DIAPH1, EXOC1, FCGR2A, ITPR1, LTBP1, PTPN7, PTPN12, PRKACG, PRKCD, RAP1GAP, STXBP5L, and VWF The analysis of singletons identified additional gene defects in PLG and PHACTR2 in two other patients. The family analysis confirmed a missense variant p.D1144N in the STXBP5L gene and p.P83H in the KCNMB3 gene as potentially causal. In summary, exome sequencing revealed potential causal variants in 12 of 14 patients with primary platelet secretion defects, highlighting the limitations of the genomic approaches for causal gene identification in this heterogeneous clinical and laboratory phenotype.

Acquired platelet dysfunction and overproduction of platelet cyclic AMP in two patients with myeloid malignancies.

The pathophysiology of impaired platelet function in acquired disorders is often poorly understood. We report two unrelated patients with hematologic malignancies associated with acquired severe bleeding diathesis, and complex platelet function abnormalities, including overproduction of the physiological inhibitor cyclic-AMP (cAMP). Patient 1, with mild macrocytic anemia and thrombocytopenia (100 x 109/L), was diagnosed with chronic myelomonocytic leukemia a few months after the onset of her bleeding diathesis and our analysis of platelet function. Patient 2, with bleeding diathesis of recent onset, was studied when his myelodysplastic syndrome with excess blasts had already progressed to acute myeloid leukemia. In both patients, platelet aggregation/ATP secretion, serum thromboxane B2, intraplatelet content of ADP, ATP, serotonin, and fibrinogen were severely impaired. Baseline platelet cAMP levels were mildly elevated and markedly increased after stimulation by prostaglandin E1. In conclusion, these are the first patients with myeloid malignancies associated with acquired severe platelet dysfunction and overproduction of cAMP.

Identification of a new dysfunctional platelet P2Y12 receptor variant associated with bleeding diathesis.

Defects of the platelet P2Y12 receptor (P2Y12R) for adenosine diphosphate (ADP) are associated with increased bleeding risk. The study of molecular abnormalities associated with inherited qualitative defects of the P2Y12R protein is useful to unravel structure-function relationships of the receptor. We describe the case of 2 brothers, sons of first cousins, with lifelong history of abnormal bleeding, associated with dysfunctional P2Y12R and a previously undescribed missense mutation in the encoding gene. ADP (4-20 µM)-induced aggregation of patients' platelets was markedly reduced and rapidly reversible. Other agonists induced borderline-normal aggregation. Inhibition of vasodilator-stimulated phosphoprotein phosphorylation and prostaglandin E1-induced increase in cyclic adenosine monophosphate (cAMP) by ADP was impaired, whereas inhibition of cAMP increase by epinephrine was normal. [(3)H]PSB-0413, a selective P2Y12R antagonist, bound to a normal number of binding sites; however, its affinity, and that of the agonists ADP and 2-methylthio-adenosine-5'-diphosphate, was reduced. Patients' DNA showed a homozygous c.847T>A substitution that changed the codon for His-187 to Gln (p.His187Gln). Crystallographic data and molecular modeling studies indicated that His187 in transmembrane 5 is important for agonist and nucleotide antagonist binding and located in a region undergoing conformational changes. These studies delineate a region of P2Y12R required for normal function after ADP binding.

Platelet mitochondrial dysfunction in critically ill patients: comparison between sepsis and cardiogenic shock.

INTRODUCTION: Platelet mitochondrial respiratory chain enzymes (that produce energy) are variably inhibited during human sepsis. Whether these changes occur even during other acute critical illness or are associated with impaired platelet aggregation and secretion (that consume energy) is not known. The aims of this study were firstly to compare platelet mitochondrial respiratory chain enzymes activity between patients with sepsis and those with cardiogenic shock, and secondly to study the relationship between platelet mitochondrial respiratory chain enzymes activity and platelet responsiveness to (exogenous) agonists in patients with sepsis. METHODS: This was a prospective, observational, case-control study. Platelets were isolated from venous blood of 16 patients with severe sepsis or septic shock (free from antiplatelet drugs) and 16 others with cardiogenic shock, within 48 hours from admission to Intensive Care. Platelet mitochondrial respiratory chain enzymes activity was measured with spectrophotometry and expressed relative to citrate synthase activity, a marker of mitochondrial density. Platelet aggregation and secretion in response to adenosine di-phosphate (ADP), collagen, U46619 and thrombin receptor activating peptide were measured with lumiaggregometry only in patients with sepsis. In total, 16 healthy volunteers acted as controls for both spectrophotometry and lumiaggregometry. RESULTS: Platelets of patients with sepsis or cardiogenic shock similarly had lower mitochondrial nicotinamide adenine dinucleotide dehydrogenase (NADH) (P < 0.001), complex I (P = 0.006), complex I and III (P < 0.001) and complex IV (P < 0.001) activity than those of controls. Platelets of patients with sepsis were generally hypo-responsive to exogenous agonists, both in terms of maximal aggregation (P < 0.001) and secretion (P < 0.05). Lower mitochondrial NADH (R (2) 0.36; P < 0.001), complex I (R (2) 0.38; P < 0.001), complex I and III (R (2) 0.27; P = 0.002) and complex IV (R (2) 0.43; P < 0.001) activity was associated with lower first wave of aggregation with ADP. CONCLUSIONS: Several platelet mitochondrial respiratory chain enzymes are similarly inhibited during human sepsis and cardiogenic shock. In patients with sepsis, mitochondrial dysfunction is associated with general platelet hypo-responsiveness to exogenous agonists. TRIAL REGISTRATION: ClinicalTrials.gov NCT00541827 . Registered 8 October 2007.

Flow-chamber device (T-TAS) to diagnose patients suspected of platelet function defects.

BACKGROUND: Patients suspected of platelet function defects represent a diagnostic challenge for the clinical laboratory, mainly due to the complexity and poor standardization of screening methods. We compared a new flow-based chip-equipped point-of-care (T-TAS) device with lumi-aggregometry and other specific tests. MATERIALS AND METHODS: The study included 96 patients suspected of platelet function defects and 26 patients referred to hospital for an evaluation of residual platelet function while on antiplatelet therapy. RESULTS: Forty-eight of 96 patients displayed abnormal platelet function by lumi-aggregometry, and 10 of them had defective granule content and were classified as δ-storage pool disease (δ-SPD). T-TAS compared favorably with lumi-aggregometry in detecting the most severe forms of platelet function defects (i.e., δ-SPD) [test agreement (lumi-light transmission aggregometry [lumi-LTA] vs T-TAS) for the δ-SPD subgroup was 80% and K CHOEN 0.695. T-TAS was less sensitive to milder platelet function defects (i.e., primary secretion defects [PSD]). Concerning patients on antiplatelets, test agreement (lumi-LTA vs T-TAS) in detecting patients who were responders to this therapy was 54%; K CHOEN 0.150. DISCUSSION: The results indicate that T-TAS can detect the more severe forms of platelet function defects such as δ-SPD. There is limited agreement of T-TAS with lumi-aggregometry in identifying responders to antiplatelets. However, this poor agreement is commonly shared by lumi-aggregometry and other devices owing to the lack of test specificity and of prospective data from clinical trials linking platelet function with therapeutic efficacy.

Comprehensive investigation of platelet function in patients with cirrhosis.

Cirrhosis presents with thrombocytopenia and possibly thrombocytopathy. Previous studies exploring platelet function gave conflicting results and most controversies are explained by the variety of methods employed for investigation. We sought to assess in-vitro the overall platelet function in cirrhosis. We investigated 34 patients by using the following tests. (i)Aggregometry. (ii)Measurement of the content of platelet granules. (iii)Cytometric platelet activation. (iv)Plasmatic markers of in-vivo platelet activation. (v)Platelet procoagulant activity by thrombin generation (TG) in platelet-rich plasma (PRP). TG measured in PRP for patients and controls was similar. Platelets from patients with cirrhosis showed reduction of aggregation and secretion of ATP. Similar results were observed for platelet activation parameters such as P-selectin expression and PAC-1 platelet binding. Plasma levels of ßeta-thromboglobulin and soluble P-selectin, were increased in patients-vs-controls. In contrast, there were no patients-vs-controls differences for plasmatic platelet-factor-4. Results are consistent with a state of in-vivo platelet activation and decreased in-vitro aggregation. Since bleeding events following invasive procedures are uncommon in cirrhosis, we speculate that in-vitro aggregometry testing does not reflect the situation occurring in-vivo. Results of the study and pathophysiological considerations support the conclusion that platelet function in cirrhosis as determined by aggregometry, although somewhat impaired, may support the overall hemostatic potential, which is needed for most invasive interventions. These conclusions are in line with the recommendations of international guidelines, warning against indiscriminate use of prophylactic preprocedural administration of platelets before invasive procedures. Decision on platelet support should not be made based on in-vitro laboratory testing for platelet function.

Congenital fibrinogen disorders: a retrospective clinical and genetic analysis of the Prospective Rare Bleeding Disorders Database.

ABSTRACT: Congenital fibrinogen deficiency (CFD) is a rare bleeding disorder caused by mutations in FGA, FGB, and FGG. We sought to comprehensively characterize patients with CFD using PRO-RBDD (Prospective Rare Bleeding Disorders Database). Clinical phenotypes, laboratory, and genetic features were investigated using retrospective data from the PRO-RBDD. Patients were classified from asymptomatic to grade 3 based on their bleeding severity. In addition, FGA, FGB, and FGG were sequenced to find causative variants. A total of 166 CFD cases from 16 countries were included, of whom 123 (30 afibrinogenemia, 33 hypofibrinogenemia, 55 dysfibrinogenemia, and 5 hypodysfibrinogenemia) were well characterized. Considering the previously established factor activity and antigen level thresholds, bleeding severity was correctly identified in 58% of the cases. The rates of thrombotic events among afibrinogenemic and hypofibrinogenemic patients were relatively similar (11% and 10%, respectively) and surprisingly higher than in dysfibrinogenemic cases. The rate of spontaneous abortions among 68 pregnancies was 31%, including 86% in dysfibrinogenemic women and 14% with hypofibrinogenemia. Eighty-six patients received treatment (69 on-demand and/or 17 on prophylaxis), with fibrinogen concentrates being the most frequently used product. Genetic analysis was available for 91 cases and 41 distinct variants were identified. Hotspot variants (FGG, p.Arg301Cys/His and FGA, p.Arg35Cys/His) were present in 51% of dysfibrinogenemia. Obstetric complications were commonly observed in dysfibrinogenemia. This large multicenter study provided a comprehensive insight into the clinical, laboratory, and genetic history of patients with CFDs. We conclude that bleeding severity grades were in agreement with the established factor activity threshold in nearly half of the cases with quantitative defects.

The platelet P2Y(12) receptor under normal and pathological conditions. Assessment with the radiolabeled selective antagonist [(3)H]PSB-0413.

Various radioligands have been used to characterize and quantify the platelet P2Y(12) receptor, which share several weaknesses: (a) they are metabolically unstable and substrates for ectoenzymes, (b) they are agonists, and (c) they do not discriminate between P2Y(1) and P2Y(12). We used the [(3)H]PSB-0413 selective P2Y(12) receptor antagonist radioligand to reevaluate the number of P2Y(12) receptors in intact platelets and in membrane preparations. Studies in humans showed that: (1) [(3)H]PSB-0413 bound to 425 ± 50 sites/platelet (K (D) = 3.3 ± 0.6 nM), (2) 0.5 ± 0.2 pmol [(3)H]PSB-0413 bound to 1 mg protein of platelet membranes (K (D) = 6.5 ± 3.6 nM), and (3) competition studies confirmed the known features of P2Y(12), with the expected rank order of potency: AR-C69931MX > 2MeSADP ≫ ADPßS > ADP, while the P2Y(1) ligand MRS2179 and the P2X(1) ligand α,ß-Met-ATP did not displace [(3)H]PSB-0413 binding. Patients with severe P2Y(12) deficiency displayed virtually no binding of [(3)H]PSB-0413 to intact platelets, while a patient with a dysfunctional P2Y(12) receptor had normal binding. Studies in mice showed that: (1) [(3)H]PSB-0413 bound to 634 ± 87 sites/platelet (K (D) = 14 ± 4.5 nM) and (2) 0.7 pmol ± 0.3 [(3)H]PSB-0413 bound to 1 mg protein of platelet membranes (K (D) = 9.1 ± 5.3 nM). Clopidogrel and other thiol reagents like pCMBS or DTT abolished the binding both to intact platelets and membrane preparations. Therefore, [(3)H]PSB-0413 is an accurate and selective tool for radioligand binding studies aimed at quantifying P2Y(12) receptors, to identify patients with P2Y(12) deficiencies or quantify the effect of P2Y(12) targeting drugs.

Evaluation of an automated platelet aggregation method for detection of congenital or acquired platelet function defects.

BACKGROUND: Light transmission aggregometry (LTA) is the most widely used laboratory method for an initial screening of patients with a suspected platelet function defect (PFD), and its use has also been proposed for assessing the efficacy of antiplatelet treatment (APT). An automated LTA method has been developed by Sysmex (Kobe, Japan) on a routine coagulation analyzer (CS-2400), together with a new research parameter called PAL (platelet aggregation level) to evaluate patients on APT. MATERIALS AND METHODS: We evaluated the performance of CS-2400 compared to a stand-alone lumi-dual-aggregometer device in the diagnosis of PFD and in assessing the efficacy of APT. For these purposes, the study population was represented by a cohort of 23 patients with a previous diagnosis of PFD and a cohort of 28 patients on APT. RESULTS: Compared to healthy volunteers, patients with PFD showed a statistically significant reduction (p<0.05) in the maximal %light transmission, irrespective of the agonist used, both with the CS-2400 and the lumi-dual-aggregometer. As regards PFD patients, CS-2400 was effective in identifying the more severe defects, with a good sensibility and specificity, but less effective in identifying milder forms of PFD, such as platelet secretion defects. Patients on APT showed a statistically significant (p=0.001) reduced median %light transmission and PAL scores compared to healthy controls. DISCUSSION: Thanks to this LTA technology, CS-2400, a routine coagulation analyzer widely available in routine laboratories, could prove useful for initial assessment of patients with a suspected PFD. Moreover, the PAL scores were a fairly accurate reflection of the platelet response to APT.

Characterization of Immunogenicity and Safety of COVID-19 mRNA-1273 in HIV-Positive Italian Patients with Hemophilia: A Prospective Single-Center Cohort Study.

To characterize the immunogenicity of mRNA-1273 (Moderna, Cambridge, MA, USA) vaccine in HIV-positive hemophilic patients during the third COVID-19 wave in Italy and to investigate biomarkers of coagulation and endothelial perturbation before and after complete vaccination schedule, twenty-three consecutive adult HIV-positive patients with hemophilia were included. Blood was collected before and two weeks after vaccination. We measured anti-SARS-CoV-2 spike protein antibodies to assess immunogenicity; circulating biomarkers of coagulation (protein C and D-dimer), endothelial perturbation (von Willebrand factor (VWF)) and anti-Platelet Factor 4 (PF4) antibodies were analyzed. Flow-based analysis of thrombus formation was performed in nine patients using a flow-chamber device. Two weeks after completing the vaccination schedule, all patients had anti-spike antibodies values consistent with an effective immunization. Mean (±standard deviation) basal values of protein C and VWF (106 ± 21% and 171 ± 45%, respectively) were not significantly different from data obtained two weeks after the second dose (103 ± 20%, 162 ± 43%, respectively). D-dimer median values (interquartile range) were not significantly different at baseline (442 (603-142) ng/mL) and after the second dose (477 (654-262) ng/mL). Anti-PF4 antibodies were detected in three patients with no associated clinical manifestations. No significant differences were found in flow-based analysis of thrombus formation. Our data demonstrate that in HIV-positive patients with hemophilia, SARS-CoV-2 vaccination is effective and safe, with no effects on coagulation and endothelial perturbation.

Constitutively released adenosine diphosphate regulates proplatelet formation by human megakaryocytes.

BACKGROUND: The interaction of adenosine diphosphate with its P2Y(1) and P2Y(12) receptors on platelets is important for platelet function. However, nothing is known about adenosine diphosphate and its function in human megakaryocytes. DESIGN AND METHODS: We studied the role of adenosine diphosphate and P2Y receptors on proplatelet formation by human megakaryocytes in culture. RESULTS: Megakaryocytes expressed all the known eight subtypes of P2Y receptors, and constitutively released adenosine diphosphate. Proplatelet formation was inhibited by the adenosine diphosphate scavengers apyrase and CP/CPK by 60-70% and by the P2Y(12) inhibitors cangrelor and 2-MeSAMP by 50-60%, but was not inhibited by the P2Y(1) inhibitor MRS 2179. However, the active metabolites of the anti-P2Y(12) drugs, clopidogrel and prasugrel, did not inhibit proplatelet formation. Since cangrelor and 2-MeSAMP also interact with P2Y(13), we hypothesized that P2Y(13), rather than P2Y(12) is involved in adenosine diphosphate-regulated proplatelet formation. The specific P2Y(13) inhibitor MRS 2211 inhibited proplatelet formation in a concentration-dependent manner. Megakaryocytes from a patient with severe congenital P2Y(12) deficiency showed normal proplatelet formation, which was inhibited by apyrase, cangrelor or MRS 2211 by 50-60%. The platelet count of patients with congenital delta-storage pool deficiency, who lack secretable adenosine diphosphate, was significantly lower than that of patients with other platelet function disorders, confirming the important role of secretable adenosine diphosphate in platelet formation. CONCLUSIONS: This is the first demonstration that adenosine diphosphate released by megakaryocytes regulates their function by interacting with P2Y(13). The clinical relevance of this not previously described physiological role of adenosine diphosphate and P2Y(13) requires further exploration.

Metformin overdose causes platelet mitochondrial dysfunction in humans.

INTRODUCTION: We have recently demonstrated that metformin intoxication causes mitochondrial dysfunction in several porcine tissues, including platelets. The aim of the present work was to clarify whether it also causes mitochondrial dysfunction (and secondary lactate overproduction) in human platelets, in vitro and ex vivo. METHODS: Human platelets were incubated for 72 hours with saline or increasing doses of metformin (in vitro experiments). Lactate production, respiratory chain complex activities (spectrophotometry), mitochondrial membrane potential (flow-cytometry after staining with JC-1) and oxygen consumption (Clark-type electrode) were then measured. Platelets were also obtained from ten patients with lactic acidosis (arterial pH 6.97 ± 0.18 and lactate 16 ± 7 mmol/L) due to accidental metformin intoxication (serum drug level 32 ± 14 mg/L) and ten healthy volunteers of similar sex and age. Respiratory chain complex activities were measured as above (ex vivo experiments). RESULTS: In vitro, metformin dose-dependently increased lactate production (P < 0.001), decreased respiratory chain complex I activity (P = 0.009), mitochondrial membrane potential (P = 0.003) and oxygen consumption (P < 0.001) of human platelets. Ex vivo, platelets taken from intoxicated patients had significantly lower complex I (P = 0.045) and complex IV (P < 0.001) activity compared to controls. CONCLUSIONS: Depending on dose, metformin can cause mitochondrial dysfunction and lactate overproduction in human platelets in vitro and, possibly, in vivo. TRIAL REGISTRATION: NCT 00942123.

Role of MRP4 (ABCC4) in platelet adenine nucleotide-storage: evidence from patients with delta-storage pool deficiencies.

We previously showed that the MRP4 (ABCC4) transporter is expressed in human platelet delta-granules and may be involved in ADP transport. We now demonstrate by immunoblotting and immunofluorescence microscopy that platelet MRP4 is absent in two patients with a platelet delta-storage pool deficiency (delta-SPD)-like phenotype with reduced platelet adenine nucleotide (AN) but normal serotonin levels, whereas their other membrane marker proteins of platelet granules were normally expressed and localized. In these patients, MRP4 was present in lymphocytes, and the coding region of their MRP4/ABCC4 gene did not show any mutation that explained the lack of expression. In platelets with "classic" delta-SPD (low AN and serotonin levels), MRP4 was quantitatively (immunoblot) normal, but, like other delta-granules membrane marker proteins (eg, LAMP2), was mostly displaced from delta-granules to patches at the plasma membrane, suggesting that platelets with classic delta-SPD have an abnormality that impairs the assembly of normal delta-granules. Thus, defective expression of platelet MRP4 is associated with selective defect in AN storage. The genetic basis of the new delta-SPD phenotype remains to be elucidated.