Practical-Haemostasis.com

A Practical Guide to Laboratory Haemostasis

 

Von Willebrand Factor: Multimer Analysis



Introduction

VWF multimeric analysis is a method for analysing the concentration and distribution of VWF multimers present in plasma. VWF multimer analysis is usually performed after functional and immunological VWF assays indicate an abnormality and rarely if ever in isolation.
VWF is synthesised as a 2050 amino acid monomer [pro-vWF] and the pro-VWF subunits undergo dimersation in the endoplasmic reticulum through disulphide bonding of their C-terminal ends. The VWF dimers then move to the Golgi apparatus where the dimers are assembled by crosslinking of cysteine residues at their N-terminal ends to form a series of VWF multimers of increasing molecular weight.
The largest multimers are found in the in endothelial cell Weibel–Palade bodies and platelet α-granules [which serve to store VWF prior to its export in the circulation] but are detected only transiently in plasma. Whilst the high molecular weight multimers have the greatest haemostatic capacity, they are also potentially thrombogenic - see ADAMTS13 Assays and TTP.
The catabolism of vWF is mediated primarily by ADAMTS13 [a disintegrin-like and metalloprotease with thrombospondin type 1 motif no. 13.] ADAMTS13 is a metalloproteinase which cleaves VWF between tyrosine 842 and methionine 843 in the VWF A2 domain generating a series of smaller multimers. It is this multimeric pattern that is analysed in the laboratory. In the absence of ADAMTS13 the cleavage of VWF does not occur and ultra large VWF multimers appear in the plasma. The absence of ADAMTS13 and the generation of ultra large VWF multimers is associated with Thrombotic Thrombocytopaenic Purpura [TTP.]

Principles & Method

The first approach to an analysis of VWF multimers involved 2 Dimensional Crossed Immunoelectrophoresis [2D-CIE.]

In this gel, VWF has been stained with Coumassie Blue following 2D-CIE. The migration index [MI] can be calculated from the ratio of B:A. In general the MI is <1.25.

Multimeric analysis of VWF is carried out by electrophoresis of plasma samples using non-reducing agarose gels in the presence of sodium dodecyl sulphate (SDS). Gels may be of varying concentration e.g. 1.5% for high resolution and 1% of low resolution. Higher gel concentrations are useful to provide enhanced discrimination of smaller multimers and individual sub bands. For most diagnostic purposes, low-resolution gels are used.

Following electrophoresis the multimers are visualised:
1. The gels are fixed, washed and dried and then reacted with an anti-human VWF 125I-labelled antibody followed by autoradiography.
2. Western blotted and then probed with either enzyme-labelled antibodies and subsequent visualization of VWF multimers using enzyme substrates or enhanced chemiluminescence followed by autoradiography.

Interpretation

In normal plasma, VWF multimers appear as a series of bands separated by the mass of 2 subunits - see below. Higher resolution gels reveal the presence of three bands comprising each multimer (a major band with leading and trailing sub-bands), the result of proteolytic cleavage in the circulating blood.

An exception is Vicenza variant VWD with ultra large VWF multimers and low VWF. VWF Vicenza [also known as Type 1C VWD] arises from increased clearance of the mutant VWF from the plasma. For more information about VWF multimer findings in type 2 VWD variants, see pages 13–15 and associated references.

This LINK takes you to the Sheffield ISTH-SSC VWF Online Database and will show you the patterns of multimers obtained in various subtypes of VWD

VWD Type Multimer Analysis
1 All multimers are present including high molecular weight [HMW] forms but all are present in reduced concentration
2A Absence of large and intermediate forms
2B

Platelet-type VWD
Absence of large forms

Platelet-type VWD shows similar results to 2B VWD with a loss of the HMW multimers
2M All sizes present
2N All sizes present
3 Absent multimers as no VWF
VWD Vicenza Ultra-large VWF multimers present


Reference Ranges

There are no clear reference ranges for VWF multimer analysis as it is primarily a qualitative test. However, densitometric scanning can be performed to quantify the intensity of each multimer and hence the relative concentration of each multimeric band. By comparison against a control it can be seen which multimers are missing or reduced in concentration.

What Test Next

Multimer analysis is performed as part of a series of tests of VWF and rarely in isolation.

Data Interpretation

Click HERE to go to the Data Interpretation Exercises.