Michal Zucker

Patients with severe factor XI deficiency have a reduced incidence of deep-vein thrombosis

Factor XI (FXI) plays a dual role in haemostasis and thrombosis. It contributes to thrombin generation and promotes inhibition of fibrinolysis. Severe FXI deficiency was shown to confer protection against arterial and venous thrombosis in animal models without compromising haemostasis. We have previously shown that patients with severe FXI deficiency have a low incidence of ischaemic stroke, but display the usual incidence of myocardial infarction. In the present study, we compared the incidence of deep-vein thrombosis (DVT) in 219 unrelated patients with severe FXI deficiency aged 20-94 to the incidence in a large population-based study. No cases of DVT were observed in the FXI-deficient cohort, a result that is significantly lower than the expected number (4.68) computed from the population-based study. The low incidence remains statistically significant when compared to three other population-based studies. These data suggest that severe FXI deficiency provides protection against DVT.

Abnormal plasma clot structure and stability distinguish bleeding risk in patients with severe factor XI deficiency

Factor XI (FXI) deficiency is a rare autosomal recessive disorder. Many patients with even very low FXI levels (< 20 IU dL−1) are asymptomatic or exhibit only mild bleeding, whereas others experience severe bleeding, usually following trauma. Neither FXI antigen nor activity predicts the risk of bleeding in FXI‐deficient patients.

Characterization of high-affinity [3H]TBZOH binding to the human platelet vesicular monoamine transporter

The present study indicates that human platelets can be used as an accessible peripheral model not only for the plasma membrane serotonin transporter, but also for the vesicular monoamine transporter. The vesicular monoamine transporter (VMAT2) is responsible for the accumulation of monoamines in the synaptic vesicles. VMAT2 differs from the plasma membrane transporters in its capability to recognize serotonin, histamine, norepinephrine and dopamine with almost the same affinity. Dihydrotetrabenazine (TBZOH) is a very potent inhibitor of VMAT2 that binds with high affinity to this transporter. [3H]TBZOH has been used as a ligand to label VMAT2 in human, bovine and rodent brain. In this study we characterized the pharmacodynamic and pharmacokinetic parameters of [3H]TBZOH binding in human platelets as compared to rat brain. The density (Bmax) and affinity (Kd) of [3H]TBZOH specific binding was assessed by Scatchard analysis. Association and dissociation rate constants (k(on), K(off)) were assessed by kinetic binding studies. In this study high-affinity and saturable binding sites for [3H]TBZOH were demonstrated in human platelets. Both the affinity of [3H]TBZOH to its binding site in platelets (Kd = 3.2+/-0.5 nM) and the kinetic rate constants (K(on) = 2.8 x 10(7) M(-1) min(-1); K(off) = 0.099 min(-1)) were similar to that in rat brain (Kd(striatum) = 1.5 nM; Kd(cerebral cortex) = 1.35 nM; K(on) = 2 x 10(7) M(-1) min(-1); K(off) = 0.069 min(-1)). Only the VMAT2 blockers tetrabenazine and reserpine inhibited [3H]TBZOH specific binding.

Elevated platelet vesicular monoamine transporter density in untreated patients diagnosed with major depression

The intraneuronal uptake of monoamines into brain synaptic vesicles is mediated by the vesicular monoamine transporter (VMAT2). This transporter plays a major role in monoamine storage and quantal release. Recently we demonstrated a high degree of similarity between the pharmacodynamic characteristics of platelet and brain VMAT2. In the present study we measured the VMAT2 density, using [3H]dihydrotetrabenazine ([3H]TBZOH) as a ligand, in platelets of untreated patients diagnosed with major depressive disorder (MDD) (n=10; three with recurrent depression and seven with first episode depression) compared to sex- and age-matched healthy control subjects (n=23). A significant elevation in the VMAT2 density (B(max)) was observed in the platelets of untreated MDD patients (+24%) compared to healthy control subjects. No significant change was found in the affinity constant (K(d)). The increased platelet VMAT2 density may reflect depression-related enhancement of the capacity to accumulate monoamines in the vesicles in the presence of lower monoamine turnover.

Repeated swim stress leads to down-regulation of vesicular monoamine transporter 2 in rat brain nucleus accumbens and striatum

We assessed the impact of chronic swim stress in rats (daily for 3 weeks) on vesicular monoamine transporter 2 (VMAT2) in the nucleus accumbens and striatum. Exposure to repeated swim stress resulted in significant reduction in VMAT2 density in nucleus accumbens (20%, p<0.01) and striatum subregions (21-38%, p<0.001). The down-regulation of VMAT2 in this dopaminergic regions may serve as an adaptatory mechanism in the response to prolonged stress, and may be relevant to chronic stress-induced depression.

Characterization of seven novel mutations causing factor XI deficiency

Background and Objectives Factor XI (FXI) deficiency is a rare autosomal recessive disorder, the main manifestation of which is injury-related bleeding. The disorder is rare in most populations, but common among Jews in whom two mutations, E117X and F283L, account for 98% of cases. Other mutations, C38R and C128X, are prevalent in French Basques and Britons, respectively. Additional sporadic mutations have been described in most parts of the world. The objective of this study was to identify the mutations in 15 unrelated FXI-deficient patients and characterize missense mutations by expression in baby hamster kidney (BHK) cells. Design and Methods Clinical and laboratory information and DNA samples were obtained from the patients and mutations were identified by sequencing. Missense mutations were expressed in BHK cells and their effect on FXI secretion and dimerization was assessed using enzyme-linked immunosorbent assay and immunoblotting. Results Of 16 mutations detected, seven are novel including two deletions, one splice site and four missense mutations. Expression of the four novel missense mutations (C58Y, Y427C, C527Y and V20A) in cells revealed no secretion of FXI-C58Y, Y427C and C527Y and secretion of only 22% of normal in the medium for FXI-V20A. Secretion of FXI from BHK cells harboring a previously reported E297K substitution cells was also impaired (4.5% of wild-type). Homodimerization was normal for all five mutants. Interpretation and Conclusions Defective homodimerization of FXI was previously recognized as a major mechanism for defective secretion of FXI from producing cells. In this study, five FXI missense mutations (four novel) were associated with impaired secretion albeit normal dimerization, underscoring the existence of other mechanisms for defective secretion.

Changes in vesicular monoamine transporter (VMAT2) and synaptophysin in rat substantia nigra and prefrontal cortex induced by psychotropic drugs

We investigated the regulatory effect of the dopaminergic agent L-dopa, the mood stabilizer lithium and the nonselective monoamine oxidase inhibitor phenelzine on brain vesicular monoamine transporter (VMAT2) expression. Rats were treated chronically (21 days) with the three psychoactive drugs. VMAT2 gene expression at the protein level was assessed in the prefrontal cortex and striatum by autoradiography with high-affinity [3H]dihydrotetrabenazine ([3H]TBZOH) binding and at the mRNA level in the substantia nigra pars compacta by in situ hybridization. In addition, the effect of various treatments on the synaptophysin mRNA level was determined in the substantia nigra by in situ hybridization. Chronic administration of L-dopa resulted in a significant decrease (28%, p < 0.05) in the density of [3H]TBZOH binding in the prefrontal cortex but had no effect on VMAT2 and synaptophysin mRNA levels in the substantia nigra. Lithium treatment increased [3H]TBZOH-specific binding in the prefrontal cortex (23%, p < 0.05) but had no effect on VMAT2 and synaptophysin mRNA levels. Phenelzine did not modulate VMAT2 gene expression but reduced the synaptophysin mRNA level (19%, p < 0.05). The modulatory activities of these drugs, although relatively weak, may be relevant to the drug-induced synaptic and neuronal plasticity as well as to the molecular and cellular pathophysiology of monoamine-related neuropsychiatric disorders.

Increased platelet vesicular monoamine transporter density in adult schizophrenia patients.

Vesicular monoamine transporter (VMAT2) plays a major role in the synaptic accumulation and release of monoamines. In the present study, we assessed high affinity [(3)H]dihydrotetrabenazine binding to platelet VMAT2 in a group of treated (n = 9) and untreated (n = 4) patients with schizophrenia and age- and sex-matched controls. Significantly elevated platelet VMAT2 density (B(max)) (53%, P<0.0001) was observed in the mixed population of schizophrenia patients. The affinity of the ligand (K(d)) to platelet VMAT2 was similar in both groups. The increased platelet VMAT2 density may indicate a schizophrenia-related hyperactivity of the monoaminergic system or an adaptive response to chronic drug treatment.

Point mutations regarded as missense mutations cause splicing defects in the factor XI gene

See also Duga S, Asselta R. Mutations in disguise. This issue, pp 1973–6.

Type I mutation in the F11 gene is a third ancestral mutation which causes factor XI deficiency in Ashkenazi Jews

Factor XI (FXI) deficiency is one of the most frequent inherited disorders in Ashkenazi Jews (AJ). Two predominant founder mutations termed type II (p.Glu117Stop) and type III (p.Phe283Leu) account for most cases.