Possible congenital defect in platelet thromboxane synthetase

Thromboxane synthase mutations in an increased bone density disorder (Ghosal syndrome)

Studying consanguineous families with Ghosal hematodiaphyseal dysplasia syndrome (GHDD), a disorder of increased bone density, we identified mutations in TBXAS1, which encodes thromboxane synthase (TXAS). TXAS, an enzyme of the arachidonic acid cascade, produces thromboxane A(2) (TXA(2)). Platelets from subjects with GHDD showed a specific deficit in arachidonic acid-produced aggregation. We also found that TXAS and TXA(2) modulated expression of TNFSF11 and TNFRSF11B (encoding RANKL and osteoprotegerin (OPG), respectively) in primary cultured osteoblasts.

Inherited bleeding disorder due to familial type 2 platelet cyclo-oxygenase deficiency

Inherited platelet cyclo-oxygenase (COX) deficiency is a rare bleeding disorder. We report here the first case of familial type 2 platelet COX deficiency responsible for a moderate bleeding phenotype. The propositus was admitted in the emergency department for major epistaxis following treatment with aspirin. Epinephrine closure time is very sensitive to drugs which inhibit COX but the test was normal in patients with inherited COX deficiency. This clinical and biological data suggest that the anti-platelet effect of aspirin may be dependent on mechanisms other than the inhibition of COX. Thrombin generation test confirmed mild bleeding phenotype in patients with COX deficiency as they had normal thrombin generating capacity.

Molecular control of megakaryopoiesis and thrombopoiesis

Megakaryopoiesis and subsequent thrombopoiesis occur through complex biologic steps: megakaryocyte precursors that developed from hematopoietic stem cells initially proliferate, then differentiate into mature polyploid megakaryocytes, and finally release platelets. Although a number of growth factors can augment megakaryopoiesis in vitro, thrombopoietin is a physiologic and the most potent regulator of megakaryopoiesis in vitro and in vivo. Thrombopoietin induces the growth of megakaryocyte precursors through activation of multiple signaling cascades, including Ras/mitogen-activated protein kinase (MAPK), signal transducers and activators of transcription 5 (STAT5), phosphatidylinositol 3-kinase (PI3-K)/Akt, and protein kinase C, whereas it induces megakaryocytic maturation primarily through the Ras/MAPK pathway. During the maturation step, megakaryocytes undergo polyploidization characterized by repeated rounds of DNA replication without concomitant cell division. During these rounds of replication, cytokinesis is neglected because of the down-regulated expression of AIM-1, and DNA replication occurs through the increased expression of D-type cyclins. As for transcriptional regulation during megakaryopoiesis, GATA-1 plays a central role in the lineage commitment of hematopoietic stem cells toward erythroid/megakaryocytic lineage and subsequent maturation. p45 NF-E2 is essential for platelet release from terminally differentiated megakaryocytes. At present, mutations of GATA-1, AML1, and HOXA11 genes have been found in hereditary diseases accompanying thrombocytopenia among humans.

Identification of genetic variants in the human thromboxane synthase gene (CYP5A1)

Thromboxane synthase (CYP5A1) catalyzes the conversion of prostaglandin H2 to thromboxane A2, a potent mediator of platelet aggregation, vasoconstriction and bronchoconstriction. It has been implicated in the patho-physiological process of a variety of diseases, such as atherosclerosis, myocardial infarction, stroke and asthma. On the basis of the hypothesis that variations of the CYP5A1 gene may play an important role in human diseases, we performed a screening for variations in the human CYP5A1 gene sequence. We examined genomic DNA from 200 individuals, for mutations in the promoter region, the protein encoding sequences and the 3'-untranslated region of the CYP5A1. Eleven polymorphisms have been identified in the CYP5A1 gene including eight missense mutations R61H, D161E, N246S, L357V, Q417E, E450K, T451N and R466Q. This is the first report of genetic variants in the human CYP5A1 altering the protein sequence. The effect of these variants on the metabolic activity of CYP5A1 remains to be further evaluated.

Thromboxanes: synthase and receptors

Thromboxane A2 is a biologically potent arachidonate metabolite through the cyclooxygenase pathway. It induces platelet aggregation and smooth muscle contraction and may promote mitogenesis and apoptosis of other cells. Its roles in physiological and pathological conditions have been widely documented. The enzyme that catalyzes its synthesis, thromboxane A2 synthase, and the receptors that mediate its actions, thromboxane A2 receptors, are the two key components critical for the functioning of this potent autacoid. Recent molecular biological studies have revealed the structure-function relationship and gene organizations of these proteins as well as genetic and epigenetic factors modulating their gene expression. Future investigation should shed light on detailed molecular signaling events specifying thromboxane A2 actions, and the genetic underpinning of the enzyme and the receptors in health and disease.

Multiple factors regulating the expression of human thromboxane synthase gene

Characterization of the 5.5 kb promoter of human thromboxane synthase (TS) gene revealed a proximal positive regulatory sequence (PPRS, -90 to -25 bp) and several distal repressive elements. The maximal promoter activity was found to reside within the first 285 bp, approximately 75% of which was contributed by the PPRS. The sequence between -365 and -665 bp exerted a strong repressive effect (approximately 55%) on reporter gene expression independent of orientation and position, consistent with properties expected for a silencer. The sequence upstream of -665 bp to -5.5 kb contains mainly repressive elements which further reduce the promoter activity by 30%. The 65 bp PPRS worked in an orientation-independent, but position-dependent, manner and could be further divided into two independent elements, PPRS1 (-90 to -50 bp) and PPRS2 (-50 to -25 bp). While similar nuclear factor(s) from different cell types interact with PPRS2, those interacting with PPRS1 exhibit cell specificity. Internal sequence deletion and oligonucleotide competition established that a binding sequence for NF-E2 in PPRS1 (-60 tgctgattcat -50) was important for enhancing TS promoter activity in HL-60 cells. The presence of NF-E2 mRNA in HL-60 cells was demonstrated by reverse-transcription PCR amplification of the cDNA and Northern blot analysis. A 9-fold transactivation of luciferase (luc) reporter gene expression had been detected when NF-E2 cDNA was co-expressed with a TS promoter/luc construct. Despite the fact that NF-E2 and the cis-elements could alter the efficiency of TS transcription, they were not sufficient for restricting cell-specific TS expression. Analysis of the methylation status at the TS promoter in several human cell lines reveals cell-specific patterns of methylation that might correlate with TS expression. Taken together, these results suggest that the expression of human TS gene is modulated by multiple factors including cis-elements, trans-activator(s), and possibly genomic methylation.

Genomic structure and polymorphism of the human thromboxane synthase-encoding gene

Thromboxane synthase (TS) is a cytochrome P-450 (CYP450) enzyme catalyzing the conversion of prostaglandin endoperoxide (PGH2) into thromboxane A2 (TxA2) which plays a crucial role in hemostasis and cardiovascular diseases. Twelve genomic clones containing the DNA encoding the human TS gene (hTS) were isolated and characterized to determine the exon/intron boundaries and restriction maps of the nearly contiguous structure of the gene. The hTS contains 13 exons spanning more than 150 kb. Its first five exons, divided by relatively large introns, spread over 100 kb, but encode less than one third of the full-length TS transcript. Southern analysis indicates that the human haploid genome contains a single copy of the TS gene. Although multiple transcription start points (tsp) are utilized, transcription of hTS is primarily TATA-independent, as determined by promoter-directed reporter gene expression in transfected cells. A dinucleotide (CA) repetitive sequence identified in the ninth intron of the gene exhibits allelic polymorphism. At least four distinctive alleles, containing from 13 to 20 copies of the CA repeats, have been detected.

Disorders of platelet function

Qualitative platelet disorders are described and reviewed above. The acquired platelet function defects are very common, and sometimes result in hemorrhage, especially in association with trauma or surgery. However, the specific biochemical defect is absent, and no characterized platelet abnormalities have been recognized. On the other hand, the hereditary qualitative platelet defects are rare, but the platelet abnormalities are characteristic. The study of these patients had led to an increased understanding of the normal primary hemostatic mechanism. Recently, the molecular basis analysis of the platelet defects has been developed. This will help us understand the molecular events involved in platelet adhesion and aggregation.

The porcine thromboxane synthase-encoding cDNA: sequence, mRNA expression and enzyme production in Sf9 insect cells

A full-length cDNA encoding porcine thromboxane synthase (TS) was isolated and sequenced. The open reading frame encodes a 534-amino acid (aa) protein (M(r) 60,451) which shares more than 75% identity with TS from other species and is 30% homologous to several enzymes of the cytochrome P-450 III family. Sequence comparison among porcine (p), human (h), and murine (m) TS indicated conservation of eight Cys residues and one putative N-glycosylation site. Several highly conserved regions were identified at the near N terminus, middle and C terminus. The most divergent region lies at aa residues 290-325, within which a Lys308 residue was unique to pTS. Between aa residues 70 and 90, considerable divergence was observed in mTS. Northern analysis showed that the pTS gene was expressed as a 2.3-kb transcript primarily in lung, kidney and thymus. A high-titer recombinant (re-) baculovirus containing pTS cDNA was developed to conduct a time course study of enzyme production in Spodoptera frugiperda (Sf9) cells. TS activity was detectable in the microsomes of Sf9 cells 12-h post-infection and reached maximum by 48 h. The produced TS resembles purified pTS in catalysis, as well as inhibition by a substrate analog inhibitor.

Disorders of haemostasis in diabetes mellitus

The frequent occurrence of vascular complications in diabetes mellitus has prompted extensive research into possible mechanisms which may relate to these complications. Recently, most emphasis has been directed towards the platelet and there is now good evidence of disordered platelet function, platelet biochemistry and platelet vessel wall interaction in many diabetic patients. In addition to these disturbances of platelet behaviour, other haemostatic abnormalities are not uncommon in the diabetic patient. These include evidence of coagulation activation, impaired fibrinolysis and altered blood viscosity. All of these changes could theoretically predispose to a thrombotic state, but the precise relationship between the observed abnormalities and the development and progression of the vascular complications remains to be established.

Epinephrine potentiation of arachidonate-induced aggregation of cyclooxygenase-deficient platelets

Evaluation of platelet physiology, biochemistry, ultrastructure, and function in a young woman without history of hemorrhagic problems revealed that her platelets were deficient in cyclooxygenase activity. Her citrate platelet-rich plasma (C-PRP) responded monophasically when stirred with aggregating agents in the same manner as aspirin-treated normal C-PRP, but could be irreversible aggregated by high concentrations of thrombin, collagen, and ADP. Her platelets did not aggregate when stirred with AA at concentrations as high as 2 mM. Ultrastructure and levels of serotonin and adenine nucleotides wer normal. Amounts of 14C-AA released after stirring with thrombin were similar to normal cells. However, evaluation of prostaglandin synthesis after stirring with 14C-AA revealed no evidence of endoperoxide or thromboxane production, although products of the lipoxygenase pathway were produced in normal amounts. Aggregation in response to AA was completely corrected after mixing with 10% normal C-PRP. However, equal volumes of her C-PRP and normal aspirin-treated C-PRP did not respond to AA, whereas 10% normal platelets combined with aspirin-treated cells corrected aggregation to AA. Since epinephrine pretreatment corrects the response of dog platelets that are not aggregated by AA, we evaluated the influence of epinephrine on her platelets. Preexposure to 5 micro M epinephrine, a concentration that gave only primary waves of aggregation, resulted in normalization of her response to AA, even though correction was not associated with the generation of endoperoxides or thromboxanes. The results may explain why patients with platelet cyclooxygenase deficiency have mild or absent bleeding symptoms.

Prostaglandins and ischemic heart disease

There is an abundance of information suggesting that prostaglandins are involved in the development and clinical expression of atherosclerosis. Many studies demonstrate a relationship between prostaglandins and the risk factors for peripheral and coronary artery disease. Thus, part of the mechanism by which hyperlipidemia, diabetes mellitus, smoking, hypertension, sex hormones, age, heredity, emotional stress and diet contribute to the development and progression of atherosclerosis may be through an imbalance between thromboxane A2 and prostaglandin I2. Recent studies show a temporal relationship between acute ischemic events (specifically, unstable angina) and a transcardiac increase in thromboxane B2, while others demonstrate a salutary effect of disaggregatory and vasodilatory prostaglandins in such patients. If prostaglandins and thromboxane prove important in ischemic vascular disease, attention will be directed at the correction of their pathologic imbalance. This may be accomplished by dietary manipulation as well as by the development of prostaglandin receptor antagonists or inhibitors of specific prostaglandin pathways.

Familial bleeding tendency with partial platelet thromboxane synthetase deficiency: reorientation of cyclic endoperoxide metabolism

Three family members from three successive generations presented with a moderate bleeding tendency and a functional platelet defect. They had absent aggregation with arachidonic acid (0.6--3 microM), reversible aggregation with ADP (4 microgram) and cyclic endoperoxide analogues, single wave aggregation only with adrenaline (5.4 microgram) and a prolonged template bleeding time (> min). Malondialdehyde formation was reduced after N-ethylmaleimide stimulation (2--6 nmol/10(9) platelets; control values 8--12 nmol) and serum thromboxane B2 values were reduced (33--101 ng/ml; control values 200--700 ng/ml). When the platelets were incubated with [3H]arachidonic acid the final metabolite of the lipoxygenase pathway (HETE) was produced in normal amounts but the production of thromboxane B2 and HHT was decreased whereas prostaglandin F2a, and E2 and probably D2 were increased. Evidence for enhanced production of prostaglandin D2 was also provided by the rise in the patient's platelet cyclic AMP levels following stimulation with arachidonic acid. The patient's washed platelets stimulated the production of 6-keto PGF 1a by aspirin-pretreated cultured bovine endothelial cells. The plasma levels of 6-keto PGF1a (439--703 pg/ml; normal 181 +/- 46 pg/ml) were raised. The decreased production of thromboxane B2, HHT and malondialdehyde and increased formation of prostaglandin F2a, E2, D2 and of 6-keto PGF1a are compatible with a partial platelet thromboxane synthetase deficiency and reorientation of cyclic endoperoxide metabolism. The markedly prolonged bleeding time would result not only from reduced formation of thromboxane A2 but also from increased production of the aggregation inhibiting prostaglandins PGI2 and PGD2.

Congenital deficiency of cyclo-oxygenase in a woman with generalized atherosclerosis

The case of a 52-year-old woman with congenital cyclo-oxygenase deficiency, signs of generalized atherosclerosis and a moderate bleeding tendency is reported. Secondary platelet aggregation was absent. Platelet aggregation induced by arachidonic acid failed totally while that induced by calcium ionophore was normal. No malondialdehyde formation could be detected in her platelet-rich-plasma. The life-long deficiency of cyclo-oxygenase had not protected her from progressive vascular disease. This case suggests that the chronic intake of large doses of aspirin cannot prevent arterial disorders.

Low dose aspirin, platelet function and prostaglandin synthesis: influence of epinephrine and alpha adrenergic blockade

The present investigation has evaluated the effects of low doses of oral aspirin on platelet prostaglandin synthesis and function. Whole (80 mg) or half (40 mg) tablets of baby aspirin given to adults had no effect on the response of their platelets to thrombin, ADP and epinephrine, but selectively inhibited aggregation induced by threshold concentrations of arachidonate 16-20 hours after ingestion. Larger amounts of arachidonate overcame the inhibition imposed by low dose aspirin, but not by adult aspirin tablets (600 mg). Epinephrine, in concentrations too low to cause aggregation, restored the sensitivity of aspirin-treated platelets to arachidonate. Studies with alpha-adrenergic agonists, antagonists and calcium channel blockers demonstrated that the corrective effect of epinephrine was mediated by an alpha-adrenergic receptor influence on calcium modulation of the platelet membrane.

Hereditary bleeding disorder due to a primary defect in platelet release reaction

A large family with a hereditary bleeding disorder was investigated. Easy bruising, epistaxis and menorrhagia were noted in seven members of three generations and at least one member in each generation was affected. Platelet function abnormalities were characterized by reduced 14C-serotonin release, absent second wave aggregation in response to ADP or epinephrine and reduced aggregation in response to collagen. Bleeding time was prolonged in three individuals and platelet factor 3 availability was abnormal in four. Platelet count, morphology, adhesiveness and clot retraction were normal in all. Platelet ADP and ATP as well as ATP to ADP ratio were normal. This family probably represents the first documented instance of hereditary platelet primary release disorder. To elucidate the pathogenetic mechanism, further functional studies were performed. No appreciable shape change, 14C-serotonin release of aggregation was observed when the propositus' platelets were stimulated with sodium arachidonate or a PGH2 analogue. By contrast, platelets responded normally to ionophore A23187, thrombin and ristocetin. The findings indicate that the hereditary primary release disorder is probably due to a reduced thromboxane A2 production secondary to thromboxane synthetase deficiency. Alternatively, it may be due to platelet membrane abnormalities which render platelets unresponsive to thromboxane A2.

Determination of the formation of thromboxane B2 (TxB2), 12L-hydroxy-5,8,10 heptadecatrienoic acid (HHT) and 12L-hydroxy-5,8,10,14 eicosatrienoic acid (HETE) from arachidonic acid and of the TxB2 :HHT, TxB2 :HETE and (TxB2 +HHT) :HETE ratio in human platelets. Possible use in diagnostic purposes

The formation of thromboxane B2 (TxB2), 12L-hydroxy-5,8,10 heptadecatrienoic acid (HHT) and 12L-hydroxy-5,8,10,14 eicosatrienoic acid (HETE) was determined in the platelets of normal human males and of patients with disorders in which an abnormal platelet aggregation occurs. Platelets were labelled with [1-14C] arachidonic acid. After Aggregation and extraction the metabolites wee separated by TLC and determined. In the platelets of normal males, TxB2 values were in the range 4.4-12.4%, expressed as a percentage of total radioactivity. Curves were constructed for the following ratios: TxB2 :HHT, TxB2 :HETE and (TxB2 + HHT) :HETE. These ratios were fairly contant. A comparison was made with the ratios obtained in the platelets of a small number of patients with either an enhanced or a diminished aggregation.

Thrombocytopathy in preleukaemia: association with a defect of thromboxane A2 activity

Platelet aggregation and the platelet prostaglandin pathway have been investigated in two patients with preleukaemic states who had a haemorrhagic tendency but a normal platelet count. In both patients platelet aggregation induced by collagen adenosine diphosphate (ADP) and arachidonic acid (AA) were abnormal. Malonyldiadehyde (MDA) production from exogenous AA was normal in both patients thus excluding cyclo-oxygenase deficiency. The platelet aggregating and rabbit aorta contracting activities of thromboxane A2 (TxA2) were very low in both patients. Production of thromboxane B2 (TxB2) assessed by thin layer chromatographic separation of the metabolites of [1(-14)C]AA and by radioimmunoassay, was normal. These abnormalities of platelet function appear to be due to the production of TxA2 with a low biological activity.