The contribution of mouse models to the understanding of constitutional thrombocytopenia.

Constitutional thrombocytopenias result from platelet production abnormalities of hereditary origin. Long misdiagnosed and poorly studied, knowledge about these rare diseases has increased considerably over the last twenty years due to improved technology for the identification of mutations, as well as an improvement in obtaining megakaryocyte culture from patient hematopoietic stem cells. Simultaneously, the manipulation of mouse genes (transgenesis, total or conditional inactivation, introduction of point mutations, random chemical mutagenesis) have helped to generate disease models that have contributed greatly to deciphering patient clinical and laboratory features. Most of the thrombocytopenias for which the mutated genes have been identified now have a murine model counterpart. This review focuses on the contribution that these mouse models have brought to the understanding of hereditary thrombocytopenias with respect to what was known in humans. Animal models have either i) provided novel information on the molecular and cellular pathways that were missing from the patient studies; ii) improved our understanding of the mechanisms of thrombocytopoiesis; iii) been instrumental in structure-function studies of the mutated gene products; and iv) been an invaluable tool as preclinical models to test new drugs or develop gene therapies. At present, the genetic determinants of thrombocytopenia remain unknown in almost half of all cases. Currently available high-speed sequencing techniques will identify new candidate genes, which will in turn allow the generation of murine models to confirm and further study the abnormal phenotype. In a complementary manner, programs of random mutagenesis in mice should also identify new candidate genes involved in thrombocytopenia.

[Pathogenesis of thrombotic and hemorrhagic complications in myeloproliferative and myelodysplastic syndromes]. / Patogeneza complicatiilor trombotice si hemoragice în sindroamele mieloproliferative si mielodisplazice.

Chronic myeloproliferative disorders (CMD) and Myelodisplastic Syndromes (MDS) represents a group of clonal pluripotent stem-cell pathologies. During their natural history, the clinical picture reveals both thrombosis and hemorrhage. The thrombosis could affect the microvessels, and also the large vessels, including even less usual territories (suprahepatic veins, porta vein, pulmonary vein). There are many factors contributing to thrombosis in myeloproliferative chronic disorders--the associated comorbidities, the numeric alterations of blood elements and also the disorders of the platelet's function. Thus, there were described quantitative and qualitative anomalies of platelet's receptors: GP Ib, GP IIb/IIIa, GP IV, GP VI, thrombopoietin receptor of the platelet cMPL, the increase of platelet activation; the increase of P selectin and thrombospondin and the increase on GP IIb/IIIa expression--they were all correlated with thrombosis. An important role has been attributed to JAK2 mutation, which affects the platelet receptor for thrombopoietin cMPL. Regarding the hemorrhage in chronic myeloproliferative syndrome, it is favored by many disorders in platelet's function, such as: the decrease of von Willebrand factor's receptor of the platelet, which leads to acquired Bernard Soulier syndrome; quantitative and qualitative disorders of dense granules of the platelet, decrease of the secretion and platelet aggregation after epinephrine, ADP and collagen stimulation. It was also described the acquired von Willebrand syndrome, most frequently type 2.

Trp207Gly in platelet glycoprotein Ibalpha is a novel mutation that disrupts the connection between the leucine-rich repeat domain and the disulfide loop structure and causes Bernard-Soulier syndrome.

BACKGROUND: Bernard-Soulier syndrome (BSS) is a severe inherited bleeding disorder that is caused by a defect in glycoprotein (GP)Ib-IX-V complex, the platelet membrane receptor for von Willebrand factor. PATIENTS: The diagnosis of BSS was made in two members of a Bukharian Jewish family who had life-long thrombocytopenia associated with mucocutaneous bleeding manifestations. METHODS AND RESULTS: Flow cytometry and Western blot analyses showed only trace amounts of GPIb and GPIX on the patients' platelets. Sequence analysis of the GPIbalpha gene revealed a homozygous T > G transversion at nucleotide 709 predicting Trp207Gly substitution in the mature protein. Introduction of the mutation into a mammalian expression construct abolished the surface expression of GPIbalpha in transfected baby hamster kidney cells. The crystal structure of the N-terminus of GPIbalpha (PDB: 1SQ0) indicates that Trp207 is completely buried and located in a disulfide loop structure that interacts with the leucine-rich repeat (LRR) domain. CONCLUSION: A novel mutation, Trp207Gly, causes BSS and predicts disruption of the interaction between a disulfide loop and the LRR domain that is essential for the integrity of GPIbalpha structure.

An acquired form of Bernard Soulier syndrome associated with acute myeloid leukemia.

OBJECTIVE: To investigate glycoprotein-1b (GP-1b) expression on platelets from patients with acute myeloid leukemia (AML). METHODS: Purified platelets, obtained from AML-patients and normal control subjects, were examined for surface membrane GP1b-expression by flow cytometry and GP1b-mediated aggregation responses by aggregometry. The level of elastase in plasma from patients and controls was measured by enzymed-linked immunosorbent assay. The whole of this work was carried out at the University of Liverpool, Liverpool, United Kingdom during the period of 1994-2001. RESULTS: Platelets from the majority of AML-patients showed reduced GP1b-expression and reduced GP1b-mediated aggregation responses. Reduction in platelet GP1b-expression was associated with increased plasma elastase levels. CONCLUSION: The present study suggests that elastase, released from leukemic blasts, degrades platelet GP1b, resulting in dysfunctional circulating platelets in AML-patients. These results could explain the bleeding disorders observed in these patients.

MYH9 spectrum of autosomal-dominant giant platelet syndromes: unexpected association with fibulin-1 variant-D inactivation.

The autosomal-dominant giant platelet syndromes (Fechtner, Epstein, and Sebastian platelet syndromes and May-Hegglin anomaly) represent a group of disorders characterized by variable degrees of macrothrombocytopenia with further combinations of neutrophil inclusion bodies and Alport-like syndrome manifestations, namely, deafness, renal disease, and eye abnormalities. The disease-causing gene of these giant platelet syndromes was previously mapped by us to chromosome 22. Following their successful mapping, these syndromes were shown to represent a broad phenotypic spectrum of disorders caused by different mutations in the nonmuscle myosin heavy chain 9 gene (MYH9). In this study, we examined the potential role of another gene, fibulin-1, encoding an extracellular matrix protein as a disease modifier. Eight unrelated families with autosomal-dominant giant platelet syndromes were studied for DNA sequence mutations and expression of the four fibulin-1 splice variants (A-D). A mutation in the splice acceptor site of fibulin-1 exon 19 was found in affected individuals of the Israeli Fechtner family, whereas no MYH9 mutations were identified. Unexpectedly, fibulin-1 variant D expression was absent in affected individuals from all eight families and coupled with expression of a putative antisense RNA. Transfection of the putative antisense RNA into H1299 cells abolished variant D expression. Based on the observation that only affected individuals lack variant D expression and demonstrate antisense RNA overexpression, we suggest that these autosomal-dominant giant platelet syndromes are associated, and may be modified, by aberrant antisense gene regulation of the fibulin-1 gene.

Autosomal dominant macrothrombocytopenia in Italy is most frequently a type of heterozygous Bernard-Soulier syndrome.

A form of autosomal dominant macrothrombocytopenia is characterized by mild or no clinical symptoms, normal platelet function, and normal megakaryocyte count. Because this condition has so far received little attention, patients are subject to misdiagnosis and inappropriate therapy. To identify the molecular basis of this disease, 12 Italian families were studied by linkage analysis and mutation screening. Flow cytometry evaluations of platelet membrane glycoproteins (GPs) were also performed. Linkage analysis in 2 large families localized the gene to chromosome 17p, in an interval containing an excellent candidate, the GPIbalpha gene. GPIbalpha, together with other proteins, constitutes the plasma von Willebrand factor (vWF) receptor, which is altered in Bernard-Soulier syndrome (BSS). In 6 of 12 families, a heterozygous Ala156Val missense substitution was identified. Platelet membrane GP studies were performed in 10 patients. Eight were distinguished by a reduction of GPs comparable to that found in a BSS heterozygous condition, whereas the other 2, without the Ala156Val mutation, had a normal content of platelet GPs. In conclusion, the current study provides evidence that most (10 of 12) patients with an original diagnosis of autosomal dominant macrothrombocytopenia shared clinical and molecular features with the heterozygous BSS phenotype. The remaining 2 affected subjects represented patients with "true" autosomal dominant macrothrombocytopenia; the GPIb/IX/V complex was normally distributed on the surface of their platelets. Thus, the diagnosis of heterozygous BSS must always be suspected in patients with inherited thrombocytopenia and platelet macrocytosis.

Novel heterozygous missense mutation in the platelet glycoprotein Ib beta gene associated with isolated giant platelet disorder.

The glycoprotein (GP) Ib/IX/V complex plays an important role in primary hemostasis, serving as the platelet receptor for von Willebrand factor (vWF). Recent studies have shown that the phenotype caused by mutations in the subunits of the GPIb/IX complex spans a wide spectrum; from the normal phenotype, to isolated giant platelet disorders (GPD), and to the full-blown bleeding disorder, the Bernard-Soulier syndrome (BSS). We characterize here a novel missense mutation of the GPIb beta gene associated with isolated GPD. In the patient's platelets, the expression level of the GPIb/IX complex was moderately reduced compared with that of the GPIIb/IIIa complex, whereas the latter was expressed at higher levels than in a normal control. Immunoblot analysis showed normal electrophoretic mobility of GPIb alpha, GPIb beta, and GPIX. However, the amount of GPIb beta was approximately 66% of the normal value. DNA sequencing analysis revealed a novel heterozygous missense mutation in the GPIb beta gene that converts Arg (CGC) to Cys (TGC) at residue 17. Transient transfection studies demonstrated that mutant GPIb beta protein was not detected in transfected 293T cells. These findings indicated that null expression of the abnormal GPIb beta causes decreased expression of the complex and results in the GPD phenotype in the patient, and suggested that homozygosity of the mutation may lead to a BSS phenotype in vivo.

[Bernard-Soulier thrombocytopenia: clinical significance of a rare disorder]. / Thrombozytopathie Bernard-Soulier: klinische Bedeutung einer seltenen Erkrankung.

We present 5 cases with thrombocytopenia and abnormal platelet function. The diagnosis of Bernard-Soulier syndrome was suspected in some subjects of advanced age on the ground of morphologic changes in the thrombocytes and of low platelet counts with or without prolonged bleeding time. The platelets showed normal aggregation with adrenalin, ADP and collagen but abnormal agglutination with ristocetine. All patients had normal von Willebrand factor levels in plasma. Flow cytometry demonstrated on thrombocytes lack of GP Ib expression of varying degree in comparison to normal controls, using various anti-GP Ib-antibodies (CD42b). The combination of these findings confirmed the diagnosis of Bernard-Soulier syndrome with varying expression of GP Ib. Flow cytometry and the use of specific monoclonal antibodies may be a rapid and reliable diagnostic tool. Differential diagnosis and treatment strategies are discussed. A congenital thrombopathy should always be considered in patients with thrombocytopenia of unknown origin and abnormal platelet morphology.

Acquired pseudo-pseudo Bernard-Soulier syndrome complicating Gaucher's disease.

AIMS: To investigate the abnormality in platelet function in two patients with type I Gaucher's disease causing a chronic bleeding tendency despite normalisation of the platelet count after spleen removal. METHODS: Routine laboratory methods were used to assess baseline coagulation. Platelet aggregometry was used to assess platelet responses to a range of agonists, and abnormalities were further assessed in mixing experiments using washed platelets and patients' plasma. RESULTS: Platelets from both patients with Gaucher's disease failed to agglutinate to ristocetin, despite normal platelet surface glycoprotein (GP) Ib and plasma von Willebrand factor activity. The agglutination of normal washed platelets was abolished by incubation in patient plasma. The inhibitory activity did not lie in the IgG fraction of patient plasma, and was found to be loosely associated with the patient platelet surface. CONCLUSIONS: The inhibition of ristocetin induced platelet agglutination in patients with Gaucher's disease causes a prolonged skin bleeding time. This could be due to the accumulated glucocerebroside in the plasma coating the platelet membrane. It is suggested that the term pseudo-pseudo Bernard-Soulier syndrome would be appropriate, as on initial screening, the abnormality has the features of Bernard-Soulier syndrome, but further investigation shows normal plasma von Willebrand activity and platelet surface GP Ib concentrations. The inhibitory activity is not due to a platelet specific antibody as is the case in pseudo-Bernard Soulier syndrome.

[Platelet membrane glycoproteins and the molecular mechanisms of hereditary platelet dysfunction].

Glycoproteins, present on the outer surface of platelets, function in the aggregating and releasing reactions of these cells. Some of the hereditary diseases of platelet dysfunction, such as, thrombasthenia and Bernard-Soulier syndrome, have been shown to result from defects in some of these glycoproteins. Recent advances in molecular biology have enabled us to analyze the genes which encode the defective glycoproteins in these patients. The information from these studies also helps us to understand in detail the function of the glycoproteins. At present, the major glycoproteins, GP II b/IIIa and GP I b/IX, have been studied extensively and we know the details of the mechanisms through which they function. However, as to the minor glycoproteins, GP I a/II a, GP IV and GP VI, details remain to be elucidated.