Highlights
In patients with congenital fibrinogen disorders, arterial thrombosis was reported in a meaningful minority overall, including among those with hypofibrinogenemia, but not in the small subgroup carrying fibrinogen αC-region truncation mutations over 286 patient-years of follow-up.
In mice, experimentally induced hypofibrinogenemia alone did not protect against FeCl3-induced carotid artery occlusion, whereas hypofibrinogenemic mice expressing fibrinogen lacking the αC region showed markedly reduced arterial thrombosis.
Platelet GPVI depletion reduced arterial thrombosis in wild-type and siFga-treated hypofibrinogenemic mice, but conferred no additional protection in αC-truncated mice under stronger arterial injury conditions, supporting a functional αC region-GPVI axis.
These findings separate the quantitative effect of low fibrinogen from a qualitative, domain-specific prothrombotic role of the fibrinogen αC region in arterial thrombosis.
Background and Clinical Context
Congenital fibrinogen disorders are rare inherited abnormalities that affect the quantity or quality of fibrinogen, a central substrate in coagulation and fibrin clot formation. Clinically, these disorders can produce bleeding, thrombosis, or both, often in the same family or even in the same individual. This phenotypic paradox has long challenged conventional assumptions that low fibrinogen states are uniformly antithrombotic.
Among congenital fibrinogen disorders, hypofibrinogenemia is characterized by reduced circulating fibrinogen levels, whereas dysfibrinogenemia and related structural variants may alter fibrin assembly, cross-linking, fibrinolysis, or platelet interactions. Prior experimental and clinical work has shown that hypofibrinogenemia can reduce venous thrombosis in some settings. However, whether reduced fibrinogen levels similarly protect against arterial thrombosis has been uncertain. This distinction matters because venous and arterial thrombosis differ biologically: venous thrombi are typically more fibrin- and red cell-rich, while arterial thrombi are more dependent on platelet activation under high shear conditions.
The fibrinogen αC region has emerged as a potentially important structural domain. Located on the Aα chain C-terminus, the αC region contributes to fibrin polymerization, lateral aggregation, clot architecture, mechanical properties, and interactions with cellular receptors. Prior mechanistic work has also implicated cross-talk between fibrin(ogen) and platelet glycoprotein VI, or GPVI, a receptor best known for collagen-mediated platelet activation. Whether the αC region specifically contributes to arterial thrombosis, especially when fibrinogen levels are reduced, has been unclear.
The study by Lee and colleagues addresses this question using a combination of human observational data from congenital fibrinogen disorders and two complementary mouse models that isolate quantitative versus qualitative defects in fibrinogen biology.
Study Design
Human cohort observations
The investigators examined a cohort of 264 patients with congenital fibrinogen disorders. They assessed the occurrence of arterial thrombosis across the cohort and specifically within subgroups with hypofibrinogenemia and with fibrinogen αC-region truncation mutations.
The key descriptive observations were as follows: 19 of 264 patients, approximately 7%, developed arterial thrombosis. Among 41 patients with hypofibrinogenemia, 4 patients, approximately 10%, had arterial thrombosis. By contrast, among 8 patients with fibrinogen αC-region truncation mutations, no arterial thrombotic events were reported over 286 patient-years.
Mouse models
To disentangle the effect of reduced fibrinogen concentration from the effect of losing the αC region, the authors used two distinct murine approaches.
First, wild-type mice were treated with lipid nanoparticles containing small interfering RNA targeting fibrinogen alpha chain, designated siFga. This created an acquired hypofibrinogenemic state without structurally altering the residual fibrinogen molecule.
Second, the investigators studied Fga270/270 mice, a hypofibrinogenemic model expressing fibrinogen with a truncated αC region. This model therefore combines low fibrinogen levels with a specific structural loss of the αC domain.
Arterial thrombosis model
Arterial thrombosis was induced using ferric chloride injury of the carotid artery, a widely used experimental model that causes endothelial damage, oxidative injury, and subsequent platelet-rich thrombus formation. Both 5% and 10% FeCl3 injury conditions were used in selected experiments to probe the robustness of observed antithrombotic effects.
Mechanistic intervention
To test whether any protective effect was linked to fibrinogen αC region interaction with platelet GPVI, platelet GPVI was depleted using the monoclonal antibody JAQ1. The authors then assessed arterial thrombosis responses in wild-type mice, siFga-treated hypofibrinogenemic mice, and Fga270/270 mice.
Key Findings
Human data suggest that low fibrinogen alone does not eliminate arterial thrombotic risk
The clinical observations are modest in scale but directionally important. Arterial thrombosis occurred in about 7% of the overall congenital fibrinogen disorder cohort and in about 10% of those with hypofibrinogenemia, indicating that a reduced fibrinogen level does not necessarily protect against arterial events. This is consistent with prior clinical experience showing that inherited fibrinogen disorders can be thrombosis-prone despite bleeding risk.
By contrast, no arterial thrombosis was observed among the 8 patients with αC-region truncation mutations over 286 patient-years. This subgroup is small, so the finding should not be overinterpreted as definitive proof of protection. Nonetheless, it aligns strikingly with the experimental animal data and provides a clinically relevant signal that qualitative loss of the αC region may matter more than fibrinogen deficiency itself in arterial thrombogenesis.
Acquired hypofibrinogenemia did not suppress occlusive carotid thrombosis
In the siFga model, wild-type mice rendered hypofibrinogenemic still developed occlusive carotid thrombi similarly to control mice. This is one of the central findings of the paper. It indicates that lowering fibrinogen levels alone, at least to the extent achieved in this model, is insufficient to block arterial occlusion after ferric chloride injury.
From a pathobiological perspective, this makes sense. Arterial thrombosis depends strongly on platelet adhesion, activation, and aggregation under shear. Even reduced quantities of fibrinogen may be adequate to support platelet bridging and fibrin formation once the thrombotic process is initiated. Alternatively, arterial thrombus formation in this injury model may be sufficiently amplified by platelet-driven pathways that quantitative reduction in fibrinogen alone does not become rate-limiting.
αC-region truncation reduced arterial thrombosis
In contrast, Fga270/270 mice displayed suppressed carotid thrombosis following FeCl3 challenge. Because these mice are also hypofibrinogenemic, the comparison with siFga-treated mice is especially informative. The differing phenotypes argue that the protective effect is not simply caused by lower fibrinogen levels. Instead, loss of the αC region itself appears to impair an arterial thrombosis-promoting function.
This distinction is mechanistically important. It suggests that fibrinogen is not only a bulk coagulation substrate but also a structurally modular mediator of thrombus formation. Certain domains may selectively influence venous versus arterial thrombosis, clot growth versus stabilization, or interactions with platelets versus fibrin polymerization.
GPVI depletion protected wild-type mice under moderate arterial injury
When platelet GPVI was depleted with JAQ1, wild-type mice were protected from arterial thrombosis after 5% FeCl3 injury but not after 10% FeCl3 injury. This result underscores two points. First, GPVI contributes meaningfully to thrombus formation in this model. Second, the magnitude of protection is injury dependent. Under more severe vascular injury, parallel prothrombotic pathways may overcome GPVI depletion.
This dose-response feature is relevant when considering translational implications. It suggests that targeting the αC region-GPVI axis may be most effective in settings where thrombosis remains receptor-dependent and not fully driven by overwhelming tissue damage.
GPVI depletion suppressed thrombosis in siFga-treated mice but not in αC-truncated mice under strong injury
A particularly elegant part of the study is the differential effect of JAQ1 across models. In siFga-treated hypofibrinogenemic mice, JAQ1 administration further suppressed arterial thrombosis following 10% FeCl3 challenge. Thus, even when fibrinogen quantity is reduced, GPVI-dependent mechanisms remain operational and therapeutically targetable.
In Fga270/270 mice, however, JAQ1 did not enhance protection after 10% FeCl3 injury. The most plausible interpretation is that truncation of the αC region had already removed a key component of the GPVI-linked prothrombotic pathway. In other words, the αC region may be functionally upstream of, or closely integrated with, platelet GPVI-mediated arterial thrombosis in this setting.
Mechanistic Interpretation
The study supports a model in which the fibrinogen αC region facilitates arterial thrombosis through interactions that involve platelet GPVI. Although GPVI is classically described as a collagen receptor, accumulating evidence indicates that it can also engage fibrin and fibrinogen-derived structures, thereby amplifying platelet activation and thrombus consolidation. The present work suggests that the αC region is an important structural determinant of that interaction under hypofibrinogenemic conditions.
Several mechanistic possibilities deserve mention. The αC region may directly contribute to GPVI binding, or it may alter fibrin network formation in a way that exposes or stabilizes GPVI-reactive motifs. It may also affect local thrombus architecture, fibrin density, or resistance to embolization. Because arterial thrombosis requires rapid platelet-fibrin crosstalk in a high-shear environment, even subtle structural changes in fibrinogen can have outsized functional effects.
The findings also refine the concept of thrombosis risk in congenital fibrinogen disorders. A low fibrinogen level should not be assumed to confer protection against arterial events. Rather, thrombotic phenotype may depend on which fibrinogen domain is altered, how much fibrinogen remains, and whether the predominant vascular setting is venous, arterial, or microvascular.
Clinical Relevance
For clinicians caring for patients with congenital fibrinogen disorders, the paper offers an important practical message: bleeding tendency and low fibrinogen do not exclude arterial thrombosis risk. This is especially relevant during pregnancy, surgery, acute inflammation, immobilization, replacement therapy, and other prothrombotic states in which clinical vigilance may be reduced because the patient is perceived as “anticoagulated by nature.”
The study also raises the possibility that structural fibrinogen variants may stratify thrombotic risk more accurately than fibrinogen concentration alone. If validated in larger registries, genotype-phenotype correlations involving the αC region could eventually inform surveillance, counseling, and perhaps the design of individualized replacement or antithrombotic strategies.
At a therapeutic level, the work supports continued interest in GPVI and related platelet-fibrin interfaces as antithrombotic targets. A major goal in arterial thrombosis therapeutics is to inhibit pathologic thrombosis while preserving hemostasis. Domain-specific targeting of fibrinogen-platelet interactions could, in theory, offer a narrower and potentially safer approach than broad inhibition of coagulation or platelet function. However, this remains speculative and requires much deeper validation.
Strengths and Limitations
Strengths
The study has several notable strengths. It combines human observational data with mechanistic murine experiments, allowing clinical signals to be tested in biologically controlled systems. The use of two different hypofibrinogenemic models is especially powerful because it separates quantitative fibrinogen deficiency from qualitative αC-region loss. The GPVI depletion experiments add functional depth and strengthen the causal interpretation of the αC region-GPVI axis.
Limitations
The main limitation on the clinical side is sample size. The subgroup with αC-region truncation mutations included only 8 patients, and absence of arterial events in such a small cohort cannot establish protection definitively. Event ascertainment details, competing bleeding risks, concomitant treatments, and potential confounders are not fully captured in an abstract-level analysis.
On the experimental side, ferric chloride injury is a useful but artificial model of arterial thrombosis. It does not perfectly recapitulate human plaque rupture, erosion, or complex atherothrombotic biology. The relevance of these findings to spontaneous myocardial infarction or ischemic stroke therefore remains inferential.
Further, GPVI depletion by JAQ1 is a robust experimental tool but is not directly equivalent to clinically deployable GPVI modulation. The study also does not fully resolve whether the αC region directly binds GPVI or indirectly modifies fibrin structure and platelet signaling. Additional biophysical and imaging studies will be needed.
Implications for Future Research
Several next steps emerge from this work. First, larger international registries of congenital fibrinogen disorders should examine arterial events according to molecular genotype, with specific attention to αC-region variants. Second, structural studies should define how αC truncation alters fibrin(ogen)-GPVI engagement. Third, complementary arterial thrombosis models, including laser injury, mechanical plaque disruption, or atherosclerosis-associated thrombosis systems, would help establish broader biological relevance.
It will also be important to determine whether αC-region effects are unique to hypofibrinogenemia or whether they influence arterial thrombosis even at normal fibrinogen levels. Finally, translational studies should assess whether selective interference with fibrinogen-GPVI signaling can dissociate antithrombotic efficacy from bleeding liability better than current antiplatelet approaches.
Conclusion
This study advances the field by showing that hypofibrinogenemia alone does not necessarily reduce arterial thrombosis, whereas loss of the fibrinogen αC region does. In both mouse experiments and supportive human observational data, the αC region emerges as a domain-specific promoter of arterial thrombus formation, likely through a pathway involving platelet GPVI. The work is a reminder that thrombotic biology depends not only on how much fibrinogen is present, but also on which parts of the molecule remain functionally intact. For clinicians and investigators, that is an important conceptual shift with implications for risk assessment, mechanism-based therapy, and the broader understanding of inherited coagulation disorders.
Funding and Trial Registration
The abstract provided does not list funding details or a ClinicalTrials.gov registration number. As this was a mechanistic translational study involving patient cohort observations and mouse models rather than a registered interventional clinical trial, a ClinicalTrials.gov identifier may not apply. Readers should consult the full Blood article for complete funding and disclosure information.
References
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