A noncanonical splicing variant c.875-5 T > G in von Willebrand factor causes in-frame exon skipping and type 2A von Willebrand disease.

INTRODUCTION: A novel variant involving noncanonical splicing acceptor site (c.875-5 T > G) in propeptide coding region of von Willebrand factor (VWF) was identified in a patient with type 2A von Willebrand disease (VWD), who co-inherited with a null variant (p.Tyr271*) and presented characteristic discrepancy of plasma level of VWF antigen and activity, and a selective reduction of both intermediate-molecular-weight (IMWMs) and high-molecular-weight VWF multimers (HMWMs). MATERIALS AND METHODS: VWF mRNA transcripts obtained from peripheral leukocytes and platelets of the patients were investigated to analyze the consequence of c.875-5 T > G on splicing. The impact of the variant on expression and multimer assembly was further analyzed by in vitro expression studies in AtT-20 cells. The intracellular processing of VWF mutant and the Weibel-Palade bodies (WPBs) formation was evaluated by immunofluorescence staining and electron microscopy. RESULTS: The mRNA transcript analysis revealed that c.875-5 T > G variant led to exon 8 skipping and an in-frame deletion of 41 amino acids in the D1 domain of VWF (p.Ser292_Glu333delinsLys), yielding a truncated propeptide. Consistent with the patient's laboratory manifestations, the AtT-20 cells transfected with mutant secreted less VWF, with the VWF antigen level in conditioned medium 47 % of wild-type. A slight retention in the endoplasmic reticulum was observed for the mutant. Almost complete loss of IMWMs and HMWMs in the medium and impaired WPBs formation in the cell, indicating truncated VWF propeptide lost its chaperon-like function for VWF multimerization and tubular storage. CONCLUSIONS: The VWF splicing site variant (c.875-5 T > G) causes propeptide truncation, severely compromising VWF multimer assembly and tubular storage.

A novel GATA1 variant p.G229D causing the defect of procoagulant platelet formation.

INTRODUCTION: GATA1 is one of the master transcription factors in hematopoietic lineages development which is crucial for megakaryocytic differentiation and maturation. Previous studies have shown that distinct GATA1 variants are associated with varying severities of macrothrombocytopenia and platelet dysfunction. OBJECTIVE: To determine the underlying pathological mechanisms of a novel GATA1 variant (c. 686G > A, p. G229D) in a patient with recurrent traumatic muscle hematomas. METHODS: Comprehensive phenotypic analysis of the patient platelets was performed. Procoagulant platelet formation and function were detected using flow cytometry assay and thrombin generation test (TGT), respectively. The ANO6 expression was measured by qPCR and western blot. The intracellular supramaximal calcium flux was detected by Fluo-5N fluorescent assay. RESULTS: The patient displayed mild macrothrombocytopenia with defects of platelet granules, aggregation, and integrin αIIbß3 activation. The percentage of the procoagulant platelet formation of the patient upon the stimulation of thrombin plus collagen was lower than that of the healthy controls (40.9 % vs 49.0 % ± 5.1 %). The patient platelets exhibited a marked reduction of thrombin generation in platelet rich plasma TGT compared to the healthy controls (peak value: ∼70 % of the healthy controls; the endogenous thrombin potential: ∼40 % of the healthy controls). The expression of ANO6 and intracellular calcium flux were impaired, which together with abnormal granules of the patient platelets might contribute to defect of procoagulant platelet function. CONCLUSIONS: The G229D variant could lead to a novel platelet phenotype characterized by defective procoagulant platelet formation and function, which extended the range of GATA1 variants associated platelet disorders.

Tandem and inverted duplications in haemophilia A: Breakpoint characterisation provides insight into possible rearrangement mechanisms.

INTRODUCTION: Approximately half of patients with severe haemophilia A are caused by structural variants in the F8 gene. Unlike inversions or deletions directly impairing the integrity of F8, some duplications do not completely disrupt the open reading frame or even retain an intact F8 copy. Currently, only a few duplication breakpoints were precisely characterized, and the corresponding rearrangement mechanisms and clinical outcomes remain to be further investigated. AIM: Establishing an effective strategy for breakpoint characterization of duplications and revealing their rearrangement mechanisms. METHODS: AccuCopy is used for the detection of duplications, long-distance PCR for the characterization of tandem duplications, genome walking technique and whole genome sequencing for the characterization of inverted duplications. RESULTS: Four F8 duplication rearrangements were successfully characterized at the nucleotide level: one tandem duplication (exons 7-11) and three inverted duplications (exons 7-22, exons 2-26, and exons 15-22). Two shared features of inverted duplication were found after carefully analysing our results and breakpoint information in the literature: 1, an inverted fragment was inserted into the original chromosome via two junctions; 2, one junction is mediated by a pair of inverted repetitive elements, while the other consists of two breakpoints with microhomology. CONCLUSION: Similar breakpoint features motivated us to propose a DNA replication-based model to explain the formation of duplication rearrangements. Based on our model, we further divide the inverted duplications into three basic types: type I with a DEL-NOR/INV-DUP pattern, type II with a DUP-NOR/INV-DUP pattern and type III with a DUP-TRP/INV-DUP pattern.

Congenital (hypo-)dysfibrinogenemia and bleeding: A systematic literature review.

Ranging from bleeding to thrombosis, the clinical features of congenital fibrinogen qualitative disorders, including dysfibrinogenemia and hypodysfibrinogenemia, are highly heterogeneous. Although the associations between some specific fibrinogen mutations and the thrombotic phenotypes have been well elucidated, the underlying mechanism between fibrinogen variants and bleeding events remains underestimated. After systematically reviewing the literature of (hypo-)dysfibrinogenemia patients with bleeding phenotypes, we identified several well-characterized bleeding-related fibrinogen variants in those patients. Several possible pathomechanisms are proposed to explain the genotype-phenotype associations: 1, mutations in the NH2-terminal portion of the Aα chain hamper fibrinogen fitting into the active site cleft of thrombin and drastically slow the conversion of fibrinogen into monomeric fibrin; 2, mutations adding new N-linked glycosylation sites introduce bulky and negatively charged carbohydrate side chains and undermine the alignment of fibrin monomers during polymerization; 3, mutations generating unpaired cysteine form extra disulfide bonds between the abnormal fibrinogen chains and produce highly branched and fragile fibrin networks; 4, truncation mutations in the fibrinogen αC regions impair the lateral fibril aggregation, as well as factor XIII crosslinking, endothelial cell and platelet binding. These established relationships between specific variants and the bleeding tendency will help manage (hypo-)dysfibrinogenemia patients to avoid adverse bleeding outcomes.