Gene Therapy for Cardiovascular Disorders - Applications of Biotechnology in Cardiovascular Therapeutics - page 249

into short, 21-23 nucleotide dsRNAs termed small interfering RNAs (siRNAs)
that are used in a sequence-specific manner to recognize and destroy comple-
mentary RNAs. RNAi is described in more detail in a special report on this topic
(Jain 2011a ).
During the past few years, major conceptual and technical advances have been
made toward the therapeutic modulation of cardiac gene expression for the treat-
ment of cardiac diseases. These include the identification of new molecular therapy
targets in cardiac disorders, often derived from genetic animal models and a better
understanding of the molecular and cellular determinants of cardiac gene transfer
in animal models as well as in clinical trials. The development of severe heart fail-
ure in the genetic MLP(−/−) animal model could be completely abolished by the
targeted ablation of phospholamban (PL), a key regulator of cardiac calcium
homeostasis. This effect of permanent germ-line PL ablation provides, in conjunc-
tion with former important work on disturbed calcium handling in the failing
human heart, a rationale for the suppression of PL by antisense strategies (antisense
RNAs, ribozymes, RNAi) or PL variants (Poller et al. 2004 ).
RNAi for Hypercholesterolemia
Hepatic ABCA1 contributes to HDL plasma levels and influences lipemia follow-
ing meals. An adenovirus-mediated RNAi approach has been used to test the effi-
ciency of plasmid-based siRNA-induced knockdown of co-transfected murine ATP
binding cassette transporter A1 (ABCA1). The most effective plasmid was used to
generate a recombinant adenovirus (Ad) as a tool to selectively downregulate
ABCA1 expression in mouse liver (Ragozin et al. 2005 ). In comparison to controls,
Ad-anti-ABCA1 infected mice showed an approximately 50% reduction of endog-
enous ABCA1 and a clear upregulation of apolipoprotein E.
A chemically modified siRNAs was shown to silence an endogenous gene
encoding apolipoprotein B (apoB) after intravenous injection in mice (Soutschek
et al. 2004 ). Administration of chemically modified siRNAs resulted in silencing
of the apoB mRNA in liver and jejunum, decreased plasma levels of apoB protein,
and reduced total cholesterol. These siRNAs were also shown to silence human
apoB in a transgenic mouse model. In the in vivo study, the mechanism of action
for the siRNAs was proven to occur through RNAi-mediated mRNA degradation,
and cleavage of the apoB mRNA occurred specifically at the predicted site. These
findings demonstrate the therapeutic potential of siRNAs for the treatment of dis-
ease. siRNA-mediated therapeutic efficacy was demonstrated in an animal model
of hypercholesterolemia. Temira Pharmaceuticals is developing ApoB SNALPs
(stable nucleic acid-lipid particles), which consist of a SNALP-encapsulated
siRNA designed to silence ApoB. ApoB SNALPs are delivered with high effi-
ciency into the liver hepatocytes, the cells which produce ApoB, where the siRNA
acts to knock down the precursor mRNA coding for ApoB protein. The resulting
decrease in circulating VLDL and LDL results in significant reductions in LDL