Quantitative Analysis of VWF Multimer Structure: Discrimination Between VWD Subtypes

Abstract 1215

The diagnosis of von Willebrand disease (VWD) and discrimination between its subtypes includes analysis of VWF:Ag, VWF:RCo, and VWF multimer structure. VWF multimer analysis is qualitative, and therefore a subjective assessment open to interpretation. It is often difficult to assess subtle differences in multimer structure. To address these shortcomings we have developed a quantitative method for analysis of VWF multimers. We have analyzed multimer structure for VWD patients and healthy controls recruited through the Zimmerman Program for the Molecular and Clinical Biology of von Willebrand Disease (ZPMCB-VWD). The patient population includes type 1 and type 2 VWD with well-defined genotypes and phenotypes. Multimer analysis was performed using a 0.65% LiDS-agarose gel electrophoresis system and western blotting with chemilumiscent detection using the Fujifilm LAS-3000 luminescent image analyzer. Densitometry was performed and area-under-the-curve calculated using MultiGauge analysis software. We calculated the percentage of low molecular weight (LMW) multimers defined as bands 1 – 5, mid-molecular weight (MMW) multimers (bands 6 – 10) and high molecular weight (HMW) multimers (bands >10). For healthy controls, the distribution of multimer density (mean ± standard deviation) was 25.3 ± 2.7% HMW, 56.1 ± 4.9% MMW, and 18.6 ± 3.4% LMW. Type 1 VWD (including type 1C) patients had a similar distribution of multimers (22.5 ± 7.6% HMW, 48.5 ± 6.7% MMW, 29.0 ± 7.2 % LMW), although there was a slight shift in distribution to increased LMW. For some type 1C patients with mutations including C1130Y and W1144G, we observed a small loss of HMW multimers (14.2 ± 0.8% HMW, 51.1 ± 1.4% MMW, 34.7 ± 2.3% LMW), as has been previously reported in patients with a C1130F variation. In contrast, some patients with the type 1C “Vicenza” mutation, R1205H, demonstrated increased HMW multimers (32.6 ± 1.0% HMW, 42.2 ± 4.0% MMW, 25.2 ± 3.0% LMW) as previously reported. Although the multimers in the type 1 patients are essentially normal, quantitative analysis reveals subtle abnormalities in structure. In type 2B VWD patients with mutations including V1316M, R1306W, and R1341W, a loss of HMW and MMW multimers was observed (7.1 ± 3.2% HMW, 40.4 ± 8.3% MMW, and 52.5 ± 11.4% LMW). A greater loss of HMW and MMW multimers was observed in patients with type 2A VWD with mutations including Y1349C, R1597W, G1609R, I1628T, G1631D, and G1670S (3.5 ± 6.2% HMW, 19.7 ± 20.4% MMW, and 76.9 ± 26.3% LMW). The type 2A subjects consisted of two groups: those with a virtually complete loss of HMW and MMW (0.0 ± 0% HMW, 4.0 ± 1.0% MMW, and 96.0 ± 1.0% LMW), and those with loss of HMW and decreased MMW (8.7 ± 7.5% HMW, 41.0 ± 14.7% MMW, and 50.3 ± 20.9% LMW). The latter group had a similar multimer distribution to that of type 2B VWD subjects. While most type 2A patients with mutations associated with increased susceptibility to ADAMTS13 proteolysis had severe multimer abnormalities (>95% LMW), some had only moderate abnormalities. Our study demonstrates that quantitative analysis of VWF multimer patterns more clearly distinguishes patients with various subtypes of VWD than subjective analysis. Although one of the two groups of type 2A patients is similar to the type 2B group, the other group is clearly different and is associated with specific genotypes, perhaps eliminating the need for DNA sequence analysis to make a definitive diagnosis for this group. This technique provides an objective measure of VWF structure to better characterize subtle changes observed in the subtypes of VWD and may help to determine the nature of any additional clinical laboratory testing to reach a clear-cut diagnosis.