Genetic Analysis of Von Willebrand Disease By Using the Exome Sequencing Approach

Background: Von Willebrand disease (VWD) is the most common inherited bleeding disorder characterized by quantitative and/or qualitative von Willebrand factor (VWF) abnormalities. VWF gene is a large gene consists of 52 exons, wheras its pseudogene corresponds to exons 23-34 that makes the analysis much complicated. In this study, we performed genetic analyses of the VWF gene from 20 Japanese VWD patients, aimed to find the causative genetic mutations by using various techniques including the exome sequencing approach, and to explore the diversity of yet unknown VWD pathogenetic mechanisms.

Methods: Patients' samples were collected after the written informed consent has been obtained. First, we analyzed exon 28 of VWF genes from peripheral blood genomic DNA of the patients by PCR using allele-specific primers, and analyzed DNA sequences of the patients by Sanger sequencing method. If patients show the type 3 phenotype, the possibility of deletion or insertion of the gene was taken into consideration, then MLPA method was selected. In cases the gene mutation was not detected by exon 28 sequencing or MLPA, we analyzed all exons of VWF by Sanger method. In selected patients, the exome sequencing was employed by using the next-generation sequencer Hi-Seq 2500 (Illumina) and exomes were captured using SureSelect XT Human All Exon V5+UTRs kit (Agilent Technologies).

Results: Twenty VWD cases were studied. As a result of the VWF exon 28 sequencing, we identified causative gene mutation in 12 cases (10 cases were type 2, 2 cases were type 1). Gene mutations found in exon 28 were p.R1308C, p.R1315C, p.V1316M, p.R1334W, p.A1461D, p.L1503R, p.S1506L, p.R1597Q, p.G1609R. In one case and her sister of type 3, MLPA could not detect the large deletions or insertions. Sanger sequencing showed the they were compound heterozygous for an p.E2341X (c.7021G>T) mutation in exon 41 and a p.Y2631X (c.7892-7893insA) mutation in exon 48. In another case of type 3, we used exome sequencing and found a novel homozygous gene mutation p.G2752S (c.8254G>A) in exon 52. The mutation was confirmed by Sanger sequencing and the propositus was homozygote for the mutation, while the parents and two sons had the same mutation in the heterozygote. In the multimer analysis of plasma, multimers larger than the dimer in size were hardly visible, but there was a faint monomer band was present in the plasma. Multimeric patterns of his family were normal.

Discussion: We analyzed the VWF genes of 20 VWD cases and identified causative gene mutations in 14 cases including three novel mutations (p.E2341X, p.Y2631X, p.G2752S). We previously demonstrated BOECs established from the type 3 patient (p.E2341X and p.Y2631X) reproduced the phenotype of the disease (ASH 2014). Meanwhile, G2752S mutation is located in CTCK domain, therefore possibly it affects the VWF dimer formation. However, the causative gene mutations were not detected in 6 cases (5 cases were type 1, 1 case was type 2) by exome sequencing. Although previous observations have pointed out that causative VWF gene mutations are not detectable in about 30 % of entire type 1 cases, a newer approach would be needed to elucidate molecular pathogenesis of VWD.