Spontaneous 8bp Deletion in Nbeal2 Recapitulates the Gray Platelet Syndrome in Mice

A genetic modifier of venous thrombosis in zebrafish reveals a functional role for fibrinogen AαE in early hemostasis

Plasma fibrinogen molecules comprise 2 copies of Aα, Bβ, and γ chains folded into a hexameric protein. A minor fibrinogen isoform with an extended Aα chain (AαE) is more abundant in newborn human blood than in adults. Larval zebrafish produce predominantly AαE-containing fibrinogen, but its functional significance is unclear. In 3-day-old zebrafish, when hemostasis is reliant on fibrinogen and erythrocyte-rich clotting but is largely thrombocyte-independent, we measured the time to occlusion (TTO) in a laser-induced venous thrombosis assay in 3 zebrafish strains (AB, TU, and AB × TL hybrids). AB larvae showed delayed TTO compared with the TU and AB × TL strains. Mating AB with TU or TL produced larvae with a TU-like TTO. In contrast to TU, AB larvae failed to produce fibrinogen AαE, due to a mutation in the AαE-specific coding region of fibrinogen α-chain gene (fga). We investigated whether the lack of AαE explained the delayed AB TTO. Transgenic expression of AαE, but not Aα, shortened the AB TTO to that of TU. AαE rescued venous occlusion in fibrinogen mutants or larvae with morpholino-targeted fibrinogen α-chain messenger RNA, but Aα was less effective. In 5-day-old larvae, circulating thrombocytes contribute to hemostasis, as visualized in Tg(itga2b:EGFP) transgenics. Laser-induced venous thrombocyte adhesion and aggregation is reduced in fibrinogen mutants, but transgenic expression of Aα or AαE restored similar thrombocyte accumulation at the injury site. Our data demonstrate a genetic modifier of venous thrombosis and a role for fibrinogen AαE in early developmental blood coagulation, and suggest a link between differentially expressed fibrinogen isoforms and the cell types available for clotting.

Dominant negative Gfi1b mutations cause moderate thrombocytopenia and an impaired stress thrombopoiesis associated with mild erythropoietic abnormalities in mice

GFI1B-related thrombocytopenia (GFI1B-RT) is a rare bleeding disorder mainly caused by the presence of truncated GFI1B proteins with dominant-negative properties. The disease is characterized by low platelet counts, the presence of abnormal platelets, a megakaryocytic expansion and mild erythroid defects. However, no animal models faithfully reproducing the GFI1B-RT phenotype observed in patients exist. We had previously generated mice with floxed Gfi1b alleles that can be eliminated by Cre recombinase, but those animals developed a much more severe phenotype than GFI1B-RT patients and were of limited interest in assessing the disease. Using CRISPR/Cas9 technology, we have now established three independent mouse lines that carry mutated Gfi1b alleles producing proteins lacking DNA binding zinc fingers and thereby acting in a dominant negative (DN) manner. Mice heterozygous for these Gfi1b-DN alleles show reduced platelet counts and an expansion of megakaryocytes similar to features of human GFI1B-RT but lacking the distinctively large agranular platelets. In addition, Gfi1b-DN mice exhibit an expansion of erythroid precursors indicative of a mildly abnormal erythropoiesis but without noticeable red blood cell defects. When associated with megakaryocyte-specific ablation of the remaining allele, the Gfi1b-DN alleles triggered erythroid-specific deleterious defects. Gfi1b-DN mice also showed a delayed recovery from platelet depletion, indicating a defect in stress thrombopoiesis. However, injecting Gfi1b-DN mice with romiplostim, a thrombopoietin receptor super agonist, increased platelet numbers even beyond normal levels. Thus, our data support a causal link between DN mutations in GFI1B and thrombocytopenia and suggest that patients with GFI1B-RT could be treated successfully with thrombopoietin agonists.

Stochastic Electrochemical Cytometry of Human Platelets via a Particle Collision Approach

The quantitative analysis of human platelets is important for the diagnosis of various hematologic and cardiovascular diseases. In this article, we present a stochastic particle impact electrochemical (SPIE) approach for human platelets with fixation (F-HPs). Carboxylate-functionalized polystyrene particles (PSPs) are studied as well as a standard platform of SPIE-F-HPs. For SPIE-PSPs (or F-HPs), [Fe(CN)₆]^4- was used as the redox mediator, and electro-oxidation of [Fe(CN)₆]^4- to [Fe(CN)₆]^3- was conducted on a Pt ultramicroelectrode (UME) by applying a constant potential, where the corresponding oxidation current is mass-transfer-controlled. When PSPs (or F-HPs) are introduced into aqueous solution with [Fe(CN)₆]^4-, sudden current drops (SCDs) were observed, which resulted from the partial blockage of a Pt UME by collision of an individual PSP (or F-HP). For SPIE-PSPs (or F-HPs), we found that it is essential to enhance the migration of PSPs (F-HPs) toward a Pt UME by maximizing the steady state current associated with electro-oxidation of [Fe(CN)₆]^4-. This was accomplished by increasing its concentration to the solubility limit. We successfully measured the concentration of F-HPs dispersed in aqueous solution containing [Fe(CN)₆]^4- with a minimum detectable concentration of 0.1 fM, and the size distribution of F-HPs was also estimated from the obtained i_drop distribution based on the SPIE analysis, where i_drop stands for the magnitude of the current drop of each SCD. Lastly, we revealed that HPs without the fixation process (WF-HPs) are difficult to quantitatively analyze by SPIE because of their transient activation process, which results in changes from their spherical shape. The observed difficulty was also confirmed by finite element analysis, which shows that i_drop can be significantly increased, as an elongated WF-HP is adsorbed on the edge of an UME.

Genome-wide linkage analysis and whole-exome sequencing identifies an ITGA2B mutation in a family with thrombocytopenia

Hereditary thrombocytopenias can be subclassified based on mode of inheritance and platelet size. Here we report a family with autosomal dominant (AD) thrombocytopenia with normal platelet size. Linkage analysis and whole exome sequencing identified the R1026W substitution in ITGA2B as the causative defect. The same mutation has been previously reported in 7 Japanese families/patients with AD thrombocytopenia, but all of these patients had macrothrombocytopenia. This is the first report of a family with AD thrombocytopenia with normal platelet size resulting from mutation in ITGA2B. ITGA2B mutations should therefore be included in the differential diagnosis of this latter disorder.

Whole exome sequencing of ENU-induced thrombosis modifier mutations in the mouse

Although the Factor V Leiden (FVL) gene variant is the most prevalent genetic risk factor for venous thrombosis, only 10% of FVL carriers will experience such an event in their lifetime. To identify potential FVL modifier genes contributing to this incomplete penetrance, we took advantage of a perinatal synthetic lethal thrombosis phenotype in mice homozygous for FVL (F5L/L) and haploinsufficient for tissue factor pathway inhibitor (Tfpi+/-) to perform a sensitized dominant ENU mutagenesis screen. Linkage analysis conducted in the 3 largest pedigrees generated from the surviving F5L/L Tfpi+/- mice ('rescues') using ENU-induced coding variants as genetic markers was unsuccessful in identifying major suppressor loci. Whole exome sequencing was applied to DNA from 107 rescue mice to identify candidate genes enriched for ENU mutations. A total of 3,481 potentially deleterious candidate ENU variants were identified in 2,984 genes. After correcting for gene size and multiple testing, Arl6ip5 was identified as the most enriched gene, though not reaching genome-wide significance. Evaluation of CRISPR/Cas9 induced loss of function in the top 6 genes failed to demonstrate a clear rescue phenotype. However, a maternally inherited (not ENU-induced) de novo mutation (Plcb4R335Q) exhibited significant co-segregation with the rescue phenotype (p = 0.003) in the corresponding pedigree. Thrombosis suppression by heterozygous Plcb4 loss of function was confirmed through analysis of an independent, CRISPR/Cas9-induced Plcb4 mutation (p = 0.01).

Sensitized mutagenesis screen in Factor V Leiden mice identifies thrombosis suppressor loci

Factor V Leiden (F5^L ) is a common genetic risk factor for venous thromboembolism in humans. We conducted a sensitized N-ethyl-N-nitrosourea (ENU) mutagenesis screen for dominant thrombosuppressor genes based on perinatal lethal thrombosis in mice homozygous for F5^L (F5^L/L ) and haploinsufficient for tissue factor pathway inhibitor (Tfpi^+/- ). F8 deficiency enhanced the survival of F5^L/LTfpi^+/- mice, demonstrating that F5^L/LTfpi^+/- lethality is genetically suppressible. ENU-mutagenized F5^L/L males and F5^L/+Tfpi^+/- females were crossed to generate 6,729 progeny, with 98 F5^L/LTfpi^+/- offspring surviving until weaning. Sixteen lines, referred to as "modifier of Factor 5 Leiden (MF5L1-16)," exhibited transmission of a putative thrombosuppressor to subsequent generations. Linkage analysis in MF5L6 identified a chromosome 3 locus containing the tissue factor gene (F3). Although no ENU-induced F3 mutation was identified, haploinsufficiency for F3 (F3^+/- ) suppressed F5^L/LTfpi^+/- lethality. Whole-exome sequencing in MF5L12 identified an Actr2 gene point mutation (p.R258G) as the sole candidate. Inheritance of this variant is associated with suppression of F5^L/LTfpi^+/- lethality (P = 1.7 × 10^-6), suggesting that Actr2^p.R258G is thrombosuppressive. CRISPR/Cas9 experiments to generate an independent Actr2 knockin/knockout demonstrated that Actr2 haploinsufficiency is lethal, supporting a hypomorphic or gain-of-function mechanism of action for Actr2^p.R258G Our findings identify F8 and the Tfpi/F3 axis as key regulators in determining thrombosis balance in the setting of F5^L and also suggest a role for Actr2 in this process.