Usefulness of the Total Thrombus-Formation Analysis System (T-TAS) in the diagnosis and characterization of von Willebrand disease

INTRODUCTION

The heterogeneity of von Willebrand disease (VWD) makes its diagnosis a difficult task.

METHODS

We report here on the usefulness of a microchip-based flow-chamber system, the total thrombus-formation analysis system (T-TAS), in the identification and characterization of VWD. Thirty VWD patients and 20 healthy subjects were studied with the T-TAS platelet (PL) and atherome (AR) microchips developed for the in vitro assessment of platelet thrombus formation and fibrin-rich platelet thrombus formation respectively.

RESULTS

Samples from severe type 1 VWD, characterized by von Willebrand factor (VWF) levels below 10 U dL^-1 , failed to occlude either the PL or the AR chip capillaries, while the occlusion times were normal in patients with mild type 1 VWD (VWF above 25 U dL^-1 ). PL and/or AR chip occlusion occurred, but took longer than normal, for samples from type Vicenza and type 1 VWD patients, whose VWF levels ranged between 10 and 25 U dL^-1 . No PL or AR chip capillary occlusion was seen for samples from patients with type 2A or 2B VWD featuring the absence of large VWF multimers, whereas no abnormalities emerged for type 2B patients with normal multimer patterns.

CONCLUSION

The T-TAS appears to be sensitive mainly to plasma VWF concentration and the presence of large multimers. Failure of the PL and AR chips to become occluded points to a lack of large VWF multimers, or type 1 VWD with VWF levels below 10 U dL^-1 . Although the T-TAS does not assure a precise VWD diagnosis, it does point us in the right direction, and thus seems a useful global preliminary test.

ARMC5 mutation analysis in patients with primary aldosteronism and bilateral adrenal lesions

Idiopathic hyperaldosteronism (IHA) due to bilateral adrenal hyperplasia is the most common subtype of primary aldosteronism (PA). The pathogenesis of IHA is still unknown, but the bilateral disease suggests a potential predisposing genetic alteration. Heterozygous germline mutations of armadillo repeat containing 5 (ARMC5) have been shown to be associated with hypercortisolism due to sporadic primary bilateral macronodular adrenal hyperplasia and are also observed in African-American PA patients. We investigated the presence of germline ARMC5 mutations in a group of PA patients who had bilateral computed tomography-detectable adrenal alterations. We sequenced the entire coding region of ARMC5 and all intron/exon boundaries in 39 patients (37 Caucasians and 2 black Africans) with confirmed PA (8 unilateral, 27 bilateral and 4 undetermined subtype) and bilateral adrenal lesions. We identified 11 common variants, 5 rare variants with a minor allele frequency <1% and 2 new variants not previously reported in public databases. We did not detect by in silico analysis any ARMC5 sequence variations that were predicted to alter protein function. In conclusion, ARMC5 mutations are not present in a fairly large series of Caucasian patients with PA associated to bilateral adrenal disease. Further studies are required to definitively clarify the role of ARMC5 in the pathogenesis of adrenal nodules and aldosterone excess in patients with PA.

Severe, recessive type 1 is a discrete form of von Willebrand disease: the lesson learned from the c.1534-3C>A von Willebrand factor mutation

Type 1 von Willebrand disease (VWD) is transmitted mainly as a dominant trait - especially in forms involving von Willebrand factor (VWF) levels below 20 U/dL - and less frequently as a recessive trait. In the latter case, mutations at heterozygous level may be associated with type 3 carrier status, while mutations at homozygous or compound heterozygous level often coincide with type 3 VWD. Here we present a recessive, severe type 1 form as a distinct type of VWD. Eight patients with severe type 1 VWD belonging to 7 unrelated families were studied. They had VWF levels below 10 U/dL, FVIII higher than 10 U/dL, and a significantly lower than normal platelet VWF content. All patients were homozygous or compound heterozygous for the c.1534-3C>A VWF mutation, that simultaneously induces the skipping of exon 14, the activation of a cryptic splice site, and a normal VWF gene transcription. This means that one of the three different mRNA generated assures the synthesis of normal VWF. The probands' relatives who were heterozygous for the c.1534-3C>A mutation always had low platelet VWF levels, sometimes with circulating VWF levels within normal range. This finding confirms the utility of measuring platelet VWF content to identify an abnormal VWF synthesis. Because the c.1534-3C>A mutation impairs, but does not abolish normal mRNA processing, it may never cause type 3 VWD. We propose a model of severe recessive type 1 VWF defect associated with mutations that sporadically go undetected by the cells' molecular machinery, as the c.1534-3C>A VWF mutation.

BULLET POINTS

What is known about this topic? - Type 1 VWD is transmitted mainly as a dominant trait. - Recessive type 1 mutations at homozygous or compound heterozygous level are often associated with type 3 VWD, and at heterozygous level with type 3 VWD carrier status. What does this paper add? - There are quantitative VWF mutations, such as c.1534-3C>A, that impair, but do not abolish normal mRNA processing. - The c.1534-3C>A VWF mutation simultaneously induces the skipping of exon 14, the activation of a cryptic splice site, and a normal VWF gene transcription. - The c.1534-3C>A mutation is the archetype of mutations that cause severe recessive type 1 VWD, but never type 3 VWD. - Recessive, severe type 1 appears to be a distinct form of VWD.

Loss of cysteine 584 impairs the storage and release, but not the synthesis of von Willebrand factor

Cysteines play a key part in von Willebrand factor (VWF) dimerisation and polymerisation, and their loss may severely affect VWF structure and function. We report on three patients with type 3 von Willebrand disease carrying the new c.1751G>T missense mutation that induces the substitution of cysteine 584 by phenylalanine (C584F), and the deletion of seven nucleotides in exon 7 (c.729_735del), producing a premature stop codon at position 454 (E244Lfs*211). VWF was almost undetectable in the patients' plasma and platelets, while a single, poorly represented, oligomer emerged on plasma VWF multimer analysis. No post-DDAVP increase in VWF and factor VIII was observed. Expressing human recombinant C584F-VWF in HEK293T cells showed that C584F-VWF was synthesised and multimerised but not secreted - apart from the first oligomer, which was slightly represented in the conditioned medium, with a pattern similar to the patients' plasma VWF. The in vitro expression of the E244Lfs*211-VWF revealed a defective synthesis of the mutated VWF, with a behavior typical of loss of function mutations. Cellular trafficking, investigated in HEK293 cells, indicated a normal C584F-VWF content in the endoplasmic reticulum and Golgi apparatus, confirming the synthesis and multimerisation of C584F-VWF. No pseudo-Weibel Palade bodies were demonstrable, however, suggesting that C584F mutation impairs the storage of C584F-VWF. These findings point to cysteine 584 having a role in the release of VWF and its targeting to pseudo-Weibel Palade bodies in vitro, as well as in its storage and release by endothelial cells in vivo.