The authors have declared that no competing interests exist.
Heart failure(HF) is a disease with high morbility and mortality. The benefits of current pharmacological and device therapy for survival outcomes of patients with HF are limited. Gene therapy represents a novel promising strategy in treating HF, as it can theoretically normalize the aberrantly expressed genes and their regulatory mechanisms permanently. However, the translation of gene therapy for HF from bench to bedside has been less successful. There are many challenges ahead for gene therapy, especially in the areas of selection of the optimal targets, the needs for developing delivery systems and the improvement in design of clinical trials. In this review, we summarize the most promising gene targets which have been used in experimental and clinical studies for treating HF, highlighting the results from several clinical trials. We also review the latest development in gene therapy vectors and delivery methods, aiming to provide directions for future studies.
Heart failure(HF) is a global public health problem. There are about 38 million people diagnosed with HF and its prevalence will increase with the ageing of the population.
A number of preclinical studies in animal models of HF have suggested benefits of gene transfer in managing HF. However, the translation of gene therapy to clinical trials has been less successful. In this review, we will show the latest developments of targets, vectors, and delivery methods in gene therapy for HF, highlight the results from studies in clinical trials and provide future perspectives for HF by gene transfer.
With the increased understanding of the molecular changes in HF, various targets have been identified. Several targets have been employed in clinical trials of gene transfer. The transgenes include Sarcoplasmic/endoplasmic reticulum calcium ATPase 2a (SERCA2a), adenylyl cyclase 6 (AC6) and stromal cell-derived factor-1 (SDF-1).
Ca2+ mishandling is an important pathophysiological mechanism of HF and is associated with both systolic and diastolic dysfunction of the failing heart.
SERCA2a is a key Ca2+ handling protein located in sarcoplasmic reticulum (SR). During cardiac diastole, it plays a role in moving Ca2+ from cytoplasm to sarcoplasmic reticulum (SR).
In the pilot CUPID 1 trial, an adeno-associated viral (AAV1) vector encoding SERCA2a (AAV1.SERCA2a) was used in a small number of patients. Patients treated with AAV1.SERCA2a showed improvements in several efficacy parameters and in clinical outcomes of the patients.
AC6 is part of β-adrenergic signaling, playing a role in converting adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP) and pyrophosphate.
Recently, Hammond et al. reported the results of a randomized, double-blind, placebo-controlled, phase 2 clinical trial for patients with HF using adenovirus 5 encoding human AC6 (Ad5.hAC6).
SDF-1 gene transfer is another strategy that has been studied in HF.
SDF-1 gene therapy was shown to increase vasculogenesis and improve phase I study using three doses (5, 15, or 30mg) of SDF-1 plasmid treatment for patients with ischemic HF, SDF-1 gene therapy showed potential efficacy.
Apart from SERCA2a, targeting phospholamban (PLN) and S100A1 are among the most promising strategies for improving Ca2+-handling in gene therapy for HF. PLN plays a role in inhibiting the affinity of SERCA2a to Ca2+. Phosphorylation of PLN relieves its inhibitory effects.
S100A1 is a small protein, belonging to a family of Ca2+-regulated proteins. It enhances systolic and diastolic function of the heart through regulating SERCA2A/PLN and ryanodine receptor (RyR) function. S100A1 was found to be reduced in HF.
Another target is involved in the β-adrenergic system. In HF, downregulation and desensitization of β-adrenergic receptor (βAR) occur partly due to increased G-protein-coupled receptor kinase 2 (GRK2) activity.
Apart from selecting the right targets, efficient gene delivery system is another assurance of successful gene therapy. The main components of gene delivery system are the vectors and delivery methods, which have been studied for many years to increase efficacy of transduction.
The delivery methods that have been used in clinical trials of cardiovascular gene therapy mainly include direct intramyocardial injection and peripheral intravenous injection.
Intramyocardial injection can be accomplished by using catheter techniques or during open-heart surgery. It has been widely used both in animal studies
Peripheral intravenous injection includes antegrade intracoronary injection and retrograde injection through the coronary sinus into the venous system of the heart. Retrograde injection will increase exposure time and subsequently improve the transduction efficacy.
The techniques employed in the antegrade intracoronary injection is similar with that used in percutaneous coronary interventions, which is safer and easier. In addition, the method allows for homogeneous distribution of vectors. However, the disadvantage of antegrade delivery is that it often results in relatively low vector transfection efficiency. Myocardial uptake is influenced by virus concentration, exposure time to the virus, coronary flow rate, vascular permeability, perfusion pressure and so on.
Various vectors have been developed for gene transfer in fundamental and clinical studies. Each kind of the vector has its own advantages and disadvantages. Selecting the right vector is an important element for the success of gene therapy.
Vehicles used in cardiovascular gene therapy mainly include non-viral vectors and viral vectors. To date, non-viral gene vectors often refer to naked plasmid DNA, which is easy to produce and has no limitation of DNA size. Furthermore, naked plasmid DNA also has the advantage of lack of a significant immune response, resulting in low biosafety risk.
As the limitations of non-viral vectors, the majority of studies targeting HF have applied viral vectors, which are able to provide higher transduction efficiency of the transgene than non-viral vectors. Over the past years, great efforts have been made to improve the capacity of virus to deliver genes to the target organs. Various kinds of viral vectors have been used for gene therapy, among which adenovirus, adeno-associated virus (AAV), and lentivirus vectors have been the interests of researchers.
Adenovirus can be produced in large quantity and are capable of delivering materials to almost all kinds of cardiac cell types efficiently.
However, the application of adenovirus in clinical trials is limited. Adenoviral mediated transgene expression is transient, only with a duration of expression from days to 2 weeks.
Lentiviral vectors can integrate into the host genome and are able to provide long-term transgene expression both in animals and human.
AAV is a nonpathogenic human virus and was first discovered in adenovirus preparations, belonging to the parvoviridae family.
There is a great demand to develop novel strategies for treating HF. Gene therapy is among the most promising approaches of reversing the fundamental abnormalities in failing cardiomyocytes. However, though a majority of preclinical studies have demonstrated promising results, the successful experience of translating gene therapy for HF to clinical trials is absent. Detailed analysis of the completed clinical trials will help improve gene therapy strategies.
In the CUPID 2 trial, the amount of vector DNA within the available myocardium from 7 patients were an approximate median of 43 copies per μg DNA, representing the lower end of the threshold for dose-response curves (<500 copies per μg DNA) in pharmacology studies.
Another challenge is that there is currently no uniform strategy for selecting patients and endpoints, which are pivotal to the success of gene therapy.
In many clinical trials of gene therapy, patients with advanced HF are often screened. However, gene therapy may be effective in only some subgroups of patients. It is urgent to develop methods to select the most suitable patients for each clinical trial. Various endpoints (e.g. survival, exercise tolerance, biomarkers, recurrent HF hospitalizations) have been used in previous clinical trials. There is a clear need to develop validated endpoints in future clinical trials.
Finally, current clinical trials of gene therapy for HF have shown no safety concerns. This should encourage the conduction of more clinical trials to test therapeutic effects of new targets and improved gene delivery systems.