Targeted gene therapy for the treatment of heart failure

Abstract

Chronic heart failure is one of the leading causes of morbidity and mortality in Western countries and is a major financial burden to the health care system. Pharmacologic treatment and implanting devices are the predominant therapeutic approaches. They improve survival and have offered significant improvement in patient quality of life, but they fall short of producing an authentic remedy. Cardiac gene therapy, the introduction of genetic material to the heart, offers great promise in filling this void. In-depth knowledge of the underlying mechanisms of heart failure is, obviously, a prerequisite to achieve this aim. Extensive research in the past decades, supported by numerous methodological breakthroughs, such as transgenic animal model development, has led to a better understanding of the cardiovascular diseases and, inadvertently, to the identification of several candidate genes. Of the genes that can be targeted for gene transfer, calcium cycling proteins are prominent, as abnormalities in calcium handling are key determinants of heart failure. A major impediment, however, has been the development of a safe, yet efficient, delivery system. Nonviral vectors have been used extensively in clinical trials, but they fail to produce significant gene expression. Viral vectors, especially adenoviral, on the other hand, can produce high levels of expression, at the expense of safety. Adeno-associated viral vectors have emerged in recent years as promising myocardial gene delivery vehicles. They can sustain gene expression at a therapeutic level and maintain it over extended periods of time, even for years, and, most important, without a safety risk.

Figure 1

Adeno-associated viral (AAV) endocytosis and intracellular trafficking. The entry of AAV vectors into the cell involves several steps, each of which can affect transduction efficiency. The first step involves the binding to the receptor, co-receptors, or attachment factors (1). Endocytosis, which is primarily receptor mediated, occurs in distinct membrane compartments, identified as clathrin-coated pits, although use of alternative pathways has not been excluded (2). Following endocytosis the AAV vectors are compartmentalized into early endosomes (3), which then mature to late endosomes (4). Acidification of the late endosome is a prerequisite for the release of the AAV (5a). Alternatively, late endosomes fuse with lysosomes resulting in degradation of the vector (5b). The AAVs that escape the endosomal compartment traffic to the nucleus (6), where viral uncoating leads to the single-stranded DNA release (7). The ssDNA is then converted to dsDNA (8) and finally it either forms concatamers (9) or integrated into the host genome (10).

Figure 2

Calcium signalling during excitation-contraction coupling. AC, adenylyl cyclase; ATP, adenosine triphosphate; β-AR, β-adrenergic receptor; cAMP, cyclic adenosine monophosphate; GRK, G protein–coupled receptor kinase; Gs, stimulatory G protein; I-1, (protein phosphatase) inhibitor-1; K⁺ Channel, voltage-gated potassium channel; LTTC, voltage-operated L-type Ca² channels of the T tubule; NCX, sarcolemmal sodium/calcium exchanger; Na⁺/K⁺ Exchanger, sodium/potassium exchanger; PKA, protein kinase A; PLN, phospholamban; PP1, protein phosphatase 1; RyR, ryanodine receptor; SERCA, sarcoplasmic/endoplasmic reticulum calcium ATPase; S100A1, S100 calcium-binding protein A1; SR, sarcoplasmic reticulum; TnI, troponin I.