An atypical genetic haemorrhagic disease with increased concentration of a natural inhibitor of prothrombin consumption

Comprehensive functional characterization of a novel ANO6 variant in a new patient with Scott syndrome

BACKGROUND

Scott syndrome is a mild platelet-type bleeding disorder, first described in 1979, with only 3 unrelated families identified through defective phosphatidylserine (PS) exposure and confirmed by sequencing. The syndrome is distinguished by impaired surface exposure of procoagulant PS on platelets after stimulation. To date, platelet function and thrombin generation in this condition have not been extensively characterized.

OBJECTIVES

Genetic and functional studies were undertaken in a consanguineous family with a history of excessive bleeding of unknown cause.

METHODS

A targeted gene panel of known bleeding and platelet genes was used to identify possible genetic variants. Platelet phenotyping, flow adhesion, flow cytometry, whole blood and platelet-rich plasma thrombin generation, and specialized extracellular vesicle measurements were performed.

RESULTS

We detected a novel homozygous frameshift variant, c.1943del (p.Arg648Hisfs∗23), in ANO6 encoding Anoctamin 6, in a patient with a bleeding history but interestingly with normal ANO6 expression. Phenotyping of the patient's platelets confirmed the absence of PS expression and procoagulant activity but also revealed other defects including reduced platelet δ granules, reduced ristocetin-mediated aggregation and secretion, and reduced P-selectin expression after stimulation. PS was absent on spread platelets, and thrombi formed over collagen at 1500/s. Reduced thrombin generation was observed in platelet-rich plasma and confirmed in whole blood using a new thrombin generation assay.

CONCLUSION

We present a comprehensive report of a patient with Scott syndrome with a novel frameshift variant in AN06, which is associated with no platelet PS exposure and markedly reduced thrombin generation in whole blood, explaining the significant bleeding phenotype observed.

Inherited haemorrhagic disease with abnormal prothrombin consumption

The propositus is a 4-year-old boy who presented with a history of excessive bleeding after surgical procedures as well as haematomas and epistaxis. The defect in haemostasis consisted in an anomaly of the prothrombin consumption tests as the only abnormality while all the other conventional coagulation and fibrinolysis tests as well as platelet function tests were normal. The father of the propositus had no previous history of excessive bleeding but was found to have an abnormal prothrombin consumption index. The reaction to prothrombin conversion, normal at onset, slowed down to less than normal and did not reach completion until 24 h. The in vivo studies suggest that the effect does not act on the interaction between platelet phospholipid and plasma. The factor II dosage and the electrophoretic mobility of prothrombin of the plasma were normal; nevertheless when studying the purified prothrombin by means of crossed immunoelectrofocusing there appeared an anomaly of pI. This result suggests the possible existence of an abnormal prothrombin molecule responsible for a slow prothrombin conversion.

Distribution of antithrombin III and glucosylceramide in human plasma lipoproteins and lipoprotein deficient plasma

We have investigated the distribution of antithrombin-III and glucosylceramide (Glc-Cer) in human plasma, plasma lipoproteins and lipoprotein-deficient plasma. Antithrombin III activity was measured employing immunochemical and biological assays. Glc-Cer was quantified by gas liquid chromatography (GLC). Whole plasma contained 145 micrograms antithrombin III/ml plasma, all of which was associated with the lipoprotein-deficient plasma (d greater than 1.25 g/ml). Whereas, most if not all the plasma GlcCer was associated with plasma low density lipoproteins (LDL) (d-1.022-1.055 g/ml) and high density lipoproteins (HDL) (d-1.063-1.25). GlcCer was not found in the lipoprotein-deficient plasma. We conclude that GlcCer on lipoproteins does not contribute to antithrombin III activity. Moreover, the absence of GlcCer in lipoprotein-deficient plasma does not impair antithrombin-III activity.

Labile anticoagulant in a patient with lymphoma

A patient with diffuse lymphocytic lymphoma and monoclonal IgM kappa immunoglobulin was found to have an unstable circulating anticoagulant. The anticoagulant inhibited phospholipid-dependent plasma coagulation reactions. Unlike previously described anticoagulants of this type, this plasma inhibitor was neutralized in vitro by products of platelet lysis but not by exogenous addition of other phospholipids or by intact platelets. Inhibitory activity seemed dependent on the monoclonal immunoglobulin, but isolated immunoglobulin fractions lacked anticoagulant activity, suggesting that the inhibitory function was dependent on an easily disrupted macromolecular aggregate. Recognition and characterization of other similar anticoagulants may provide a means of studying the role of phospholipids in normal hemostasis.

Purification and characterization of rat plasma antithrombin III

Antithrombin III was purified from rat plasma in 70% yield by salting out with (NH4)2SO4, affinity chromatography on heparin-Sepharose 4B, and ion-exchange chromatography on DE-52. The preparation was homogeneous as judged by polyacrylamide gel electrophoresis in the presence and absence of sodium dodecyl sulfate, by analytical ultracentrifugation, and by immuno-chemical analysis. It was composed of a single polypeptide chain whose molecular weight was estimated to be about 64 000 both by sodium dodecyl sulfate polyacrylamide gel electrophoresis and by sedimentation equilibrium analysis. Antithrombin III was a glycoprotein containing 3.6% glucosamine, 0.2% fucose, 2.5% mannose, 1.6% galactose and 3.9% sialic acid. Isoelectric focusing in polyacrylamide gel revealed four bands in the range of pH 4.7--4.9, indicating the microheterogeneity. Rat antithrombin III inhibited bovine alpha-thrombin by forming an equimolar complex. Kinetics of this reaction were studied by gel electrophoresis in sodium dodecyl sulfate. When the inhibitor was allowed to react with an excess amount of thrombin, the initial equimolar complex with a molecular weight of 110 000 was cleaved gradually to a product with a molecular weight of 97 000, which was further cleaved to a second product with a molecular weight of 85 000.

Familial haemostatic defect associated with reduced prothrombin consumption

Ten individuals from three unrelated families suffered from an usual inherited haemorrhagic disorder. The defect was associated with reduced prothrombin conversion as shown by a grossly abnormal prothrombin consumption index. All known plasma coagulation factors were present in adequate concentrations but in the two families tested the results were consistent with a defect of the interaction between plasma and platelets or phospholipid. Some of the affected family members exhibited a mild bleeding tendency characterized by post-operative or post-partum haemorrhage but none suffered spontaneous bleeding. In therapeutic trials in one of the patients both plasma and platelet transfusions were needed in order to correct the abnormality. This finding together with in vitro and other in vivo studies indicated that the abnormality is associated with an inhibitor of the interaction between plasma and phospholipid during blood coagulation. The results suggest the possibility of an abnormality of the gamma-carboxyglutamic acid residues of one or more vitamin K dependent coagulation factors but immunological and electrophoretic studies have so far failed to detect this. The findings underline the importance of a test of prothrombin conversion in screening procedures for an otherwise unexplained bleeding disorder.

The Passovoy defect: further characterization of a hereditary hemorrhagic diathesis

We studied a coagulation abnormality present in 12 members of five kindreds who bruised easily and bled excessively after minor trauma. Their activated partial thromboplastin times were between 32 and 39 seconds (normal, 22.8 to 28.8 seconds). Prothrombin times, thrombin times, platelet-function tests and the levels of factors XII, XI, IX, VIII, prekallikrein and high-molecular-weight kininogen were normal. Within these kindreds inheritance of prolonged partial thromboplastin times followed an autosomal and probably dominant pattern. The prolonged thromboplastin times were corrected by normal plasma and by normal plasma adsorbed with celite, but there was no mutual correction between plasmas of the patients. These subjects shared a common defect in the intrinsic pathway of coagulation that we designate by the proband's surname, Passovoy.

Hageman factor (factor XII) deficiency in marine mammals

Hematologic and coagulation studies were conducted on Atlantic bottlenose dolphins and killer whales. Hematologic values were similar to those in man. These animals differed from other mammals in that the Hageman factor (factor XII) was absent and this absence caused marked prolongation of coagulation. Levels of factors VIII and V were high and those of VII and X were low compared with levels in man.