Adenoviral-mediated gene transfer restores plasma ADAMTS13 antigen and activity in ADAMTS13 knockout mice

Novel mechanisms of action of emerging therapies of hereditary thrombotic thrombocytopenic purpura

INTRODUCTION

Hereditary thrombotic thrombocytopenic purpura (hTTP) is caused by deficiency of plasma ADAMTS13 activity, resulting from ADAMTS13 mutations. ADAMTS13 cleaves ultra large von Willebrand factor (VWF), thus reducing its multimer sizes. Hereditary deficiency of plasma ADAMTS13 activity leads to the formation of excessive platelet-VWF aggregates in small arterioles and capillaries, resulting in hTTP.

AREAS COVERED

PubMed search from 1956 to 2024 using thrombotic thrombocytopenic purpura and therapy identified 3,675 articles. Only the articles relevant to the topic were selected for discussion, which focuses on pathophysiology, clinical presentations, and mechanisms of action of emerging therapeutics for hTTP. Current therapies include infusion of plasma, or coagulation factor VIII, or recombinant ADAMTS13. Emerging therapies include anti-VWF A1 aptamers or nanobody and gene therapies with adeno-associated viral vector or self-inactivated lentiviral vector or a sleeping beauty transposon system for a long-term expression of a functional ADAMTS13 enzyme.

EXPERT OPINION

Frequent plasma infusion remains to be the standard of care in most parts of the world, while recombinant ADAMTS13 has become the treatment of choice for hTTP in some of the Western countries. The success of gene therapies in preclinical models may hold a promise for future development of these novel approaches for a cure of hTTP.

Hereditary Thrombotic Thrombocytopenic Purpura

Hereditary thrombotic thrombocytopenic purpura (hTTP), also known as Upshaw-Schulman syndrome, is a rare genetic disorder caused by mutations in the ADAMTS13 gene that leads to decreased or absent production of the plasma von Willebrand factor (VWF)-cleaving metalloprotease ADAMTS13. The result is circulating ultra-large multimers of VWF that can cause microthrombi, intravascular occlusion and organ damage, especially at times of turbulent circulation. Patients with hTTP may have many overt or clinically silent manifestations, and a high index of suspicion is required for diagnosis. For the treatment of hTTP, the goal is simply replacement of ADAMTS13. The primary treatment is prophylaxis with plasma infusions or plasma-derived factor VIII products, providing sufficient ADAMTS13 to prevent acute episodes. When acute episodes occur, prophylaxis is intensified. Recombinant ADAMTS13, which is near to approval, will immediately be the most effective and also the most convenient treatment. In this review, we discuss the possible clinical manifestations of this rare disease and the relevant differential diagnoses in different age groups. An extensive discussion on prophylaxis and treatment strategies is also presented. Unique real patient cases have been added to highlight critical aspects of hTTP manifestations, diagnosis and treatment.

Clinical Manifestations, Current and Future Therapy, and Long-Term Outcomes in Congenital Thrombotic Thrombocytopenic Purpura

Congenital thrombotic thrombocytopenic purpura (cTTP) is an extremely rare disease characterized by the severe deficiency of a disintegrin and metalloproteinase with thrombospondin type 1 motifs 13 (ADAMTS13), caused by ADAMTS13 mutations. While ADAMTS13 supplementation by fresh frozen plasma (FFP) infusion immediately corrects platelet consumption and resolves thrombotic symptoms in acute episodes, FFP treatment can lead to intolerant allergic reactions and frequent hospital visits. Up to 70% of patients depend on regular FFP infusions to normalize their platelet counts and avoid systemic symptoms, including headache, fatigue, and weakness. The remaining patients do not receive regular FFP infusions, mainly because their platelet counts are maintained within the normal range or because they are symptom-free without FFP infusions. However, the target peak and trough levels of ADAMTS13 to prevent long-term comorbidity with prophylactic FFP and the necessity of treating FFP-independent patients in terms of long-term clinical outcomes are yet to be determined. Our recent study suggests that the current volumes of FFP infusions are insufficient to prevent frequent thrombotic events and long-term ischemic organ damage. This review focuses on the current management of cTTP and its associated issues, followed by the importance of upcoming recombinant ADAMTS13 therapy.

ADAMTS13 Deficiency Shortens the Life Span of Mice With Experimental Diabetes

In patients with diabetes, impaired activity of ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 repeats, member 13), the plasma metalloprotease that cleaves highly thrombogenic von Willebrand factor multimers, is a major risk factor of cardiovascular events. Here, using Adamts13^-/- mice made diabetic by streptozotocin, we investigated the impact of the lack of ADAMTS13 on the development of diabetes-associated end-organ complications. Adamts13^-/- mice experienced a shorter life span than their diabetic wild-type littermates. It was surprising that animal death was not related to the occurrence of detectable thrombotic events. The lack of ADAMTS13 drastically increased the propensity for ventricular arrhythmias during dobutamine-induced stress in diabetic mice. Cardiomyocytes of diabetic Adamts13^-/- mice exhibited an aberrant distribution of the ventricular gap junction connexin 43 and increased phosphorylation of Ca²⁺/calmodulin-dependent kinase II (CaMKII), and with the consequent CaMKII-induced disturbance in Ca²⁺ handling, which underlie propensity for arrhythmia. In vitro, thrombospondin 1 (TSP1) promoted, in a paracrine manner, CaMKII phosphorylation in murine HL-1 cardiomyocytes, and ADAMTS13 acted to inhibit TSP1-induced CaMKII activation. In conclusion, the deficiency of ADAMTS13 may underlie the onset of lethal arrhythmias in diabetes through increased CaMKII phosphorylation in cardiomyocytes. Our findings disclose a novel function for ADAMTS13 beyond its antithrombotic activity.

Animal models for thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a puzzling disorder in many ways. The disease is difficult to diagnose as analogous symptoms are also found in other microangiopathic disorders. Although ADAMTS13 deficiency is generally required to develop TTP, only some patients with severe ADAMTS13 deficiency do spontaneously develop this disease. It is therefore assumed that environmental and/or genetic factors are needed to cause acute TTP. Nevertheless, acute TTP-like symptoms have also been observed in patients with moderate or normal levels of ADAMTS13. The development of animal models for TTP has allowed a closer look at the specific need for ADAMTS13 deficiency and the necessity for additional triggers in the pathophysiology of TTP. Mouse models for congenital TTP and a baboon model for acquired TTP have been generated. These animal models have also proven to be extremely valuable in developing new treatment strategies for TTP. In the current review, we discuss current animal models for TTP, what we have learned from them and how they were used to test new treatment strategies.

AAV-mediated expression of an ADAMTS13 variant prevents shigatoxin-induced thrombotic thrombocytopenic purpura

Severe deficiency of plasma ADAMTS13 activity causes thrombotic thrombocytopenic purpura (TTP), a life-threatening syndrome for which plasma is the only effective therapy currently available. As much as 5% of TTP cases are hereditary, resulting from mutations of the ADAMTS13 gene. Here, we report the efficacy and safety of recombinant adeno-associated virus serotype 8 (AAV8)-mediated expression of a murine ADAMTS13 variant (MDTCS), truncated after the spacer domain, in a murine model of TTP. Administration of AAV8-hAAT-mdtcs at doses greater than 2.6 × 10(11) vg/kg body weight resulted in sustained expression of plasma ADAMTS13 activity at therapeutic levels. Expression of the truncated ADAMTS13 variant eliminated circulating ultralarge von Willebrand factor multimers, prevented severe thrombocytopenia, and reduced mortality in Adamts13(-/-) disease-prone mice triggered by shigatoxin-2. These data support AAV vector-mediated expression of a comparable truncated ADAMTS13 variant as a novel therapeutic approach for hereditary TTP in humans.

Convenient and reproducible in vivo gene transfer to mouse parotid glands

OBJECTIVES

Published studies of gene transfer to mouse salivary glands have not employed the parotid glands. Parotid glands are the likely target tissue for most clinical applications of salivary gene transfer. The purpose of the present study was to develop a convenient and reproducible method of retroductal gene transfer to mouse parotid glands.

METHODS

The volume for vector delivery was assessed by infusion of Toluidine Blue into Stensen's ducts of Balb/c mice after direct intraoral cannulation. Recombinant, serotype 5 adenoviral vectors, encoding either firefly luciferase or human erythropoietin (hEpo), were constructed and then administered to parotid glands (10(7) vector particles/gland). Transgene expression in vivo was measured by enzyme activity (luciferase) or an enzyme-linked immunosorbent assay (hEpo). Vector biodistribution was measured by real-time quantitative (Q) PCR.

RESULTS

The chosen volume for mouse parotid vector delivery was 20μL. Little vector was detected outside of the targeted glands, with both QPCR and luciferase assays. Transgene expression was readily detected in glands (luciferase, hEpo), and serum and saliva (hEpo). Most secreted hEpo was detected in saliva.

CONCLUSION

These studies show that mouse parotid glands can be conveniently and reproducibly targeted for gene transfer, and should be useful for pre-clinical studies with many murine disease models.