Germline de novo mutations and linkage markers vs. DNA sequencing for carrier detection in von Willebrand disease

Origin and timing of de novo variants implicated in type 2 von Willebrand disease

Very few studies have shown the real origin and timing of de novo variants (DNV) implicated in von Willebrand disease (VWD). We investigated four families with type 2 VWD. First, we conducted linkage analysis using single nucleotide variant genotyping to recognize the possible provenance of DNV. Second, we performed amplification refractory mutation system‐quantitative polymerase chain reaction to confirm the real origin of variant (~0% mutant cells) or presence of a genetic mosaic variant (0%–50% mutant cells) in three embryonic germ layer‐derived tissues and sperm cells. Then, three possible timings of DNV were categorized based on the relative likelihood of occurrence according to the number of cell divisions during embryogenesis. Two each with type 2B VWD (proband 1 p.Arg1308Cys, proband 4 p.Arg1306Trp) and type 2A VWD (proband 2 p.Leu1276Arg, proband 3 p.Ser1506Leu) were identified. Variant origins were identified for families 1, 2 and 3 and confirmed to originate from the mother, father and father, respectively. However, the father of family 4 was confirmed to have isolated germline mosaicism with 2.2% mutant sperm cells. Further investigation confirmed the paternal grandfather to be the origin of variant. Thus, we proposed that DNV originating from the two fathers most likely occurred at the single sperm cell, the one originating from the mother occurred at the zygote during the first few cellular divisions; alternatively, in family 4, the DNV most likely occurred at the early postzygotic development in the father. Our findings are essential for understanding genetic pathogenesis and providing accurate genetic counselling.

De novo mutation and somatic mosaicism of gene mutation in type 2A, 2B and 2M VWD

Background

Von Willebrand disease (VWD) is not uncommon in Taiwan. In type 2 or type 3 VWD hemorrhagic symptoms are severer and laboratory data relatively more distinctive. De novo mutation and somatic mosaicism of type 2 VWD gene were rarely reported. Therefore clinical, laboratory and genetic studies of only type 2A, 2B and 2M VWD will be presented and issues of de novo mutation and somatic mosaicism will be explored.

Methods

Fifty-four patients belonging to 23 unrelated families from all around the country in whom type 2 VWD exclusive of type 2N has been diagnosed not only by clinical and routine laboratory studies but also by genetic confirmation during 1990–2015 were investigated. A novel technique named amplification refractory mutation system-quantitative polymerase chain reaction (ARMS-qPCR) was used to confirm the presence of somatic mosaicism. Informed consent was obtained for study.

Results

De novo mutation was identified in 4 families among 15 families (26.7 %) in whom family members including parents were available for examination. All their parents were free from bleeding symptoms and had no similar mutation as their respective affected daughter.

An interesting example of somatic mosaicism of VWF gene mutation was found in a large family with type 2A VWD. The father carrying a mutated VWF gene, p.Arg1597Trp, transmitted this mutation to his 3 daughters, 1 son, 3 granddaughters and 2 grandsons. However, the father had normal laboratory findings and experienced no abnormal bleeding, while his offspring who inherited the mutation showed abnormal laboratory findings compatible with type 2A VWD and had history of abnormal bleedings. ARMS-qPCR revealed that the father had only 25.5 % mutant in his blood cells and 31.1 % mutant in his oral mucosal cells, while all his offspring had about 49 % mutant in their blood cells.

Conclusion

De novo mutation of type 2 VWD gene was identified in 4 out of 15 families (26.7 %) examined. Since only one child was affected in each family, germline mosaicism was not likely. A somatic mosaicism of type 2A VWD gene was documented in a big family by a newly in-house developed technique ARMS-qPCR.