Debunking myths on genetics and DNA

Sunday, February 9, 2014

Gene therapy for the heart

My post today is about state-of-the-art gene therapy that delivers genes straight to the heart, where the genes activate proteins critical in restoring cardiac tissue in people affected by heart failure. The technique, developed at the Cardiovascular Research Center at Icahn School of Medicine at Mount Sinai, is undergoing clinical trial.

Cardiovascular disease is the leading cause of death worldwide. Heart failure--a condition by which the heart weakens and no longer pumps blood efficiently throughout the body--is one of the manifestations of cardiovascular disease. According to the CDC, heart failure affects about 5.1 million people in the US, and about half of the people who develop heart failure die within 5 years of diagnosis.

A lot is going on at the cellular level when muscles contract and release. Calcium ions work like a "switch" that allows the contraction to start. Therefore, it is of vital importance, for the correct functioning of the muscle, that the calcium ions are released at the right time and then reabsorbed at the end of the contraction. When this flow of calcium ions is impaired heart failure can occur.

Calcium is normally stored in an organelle of the cell called sarcoplasmic reticulum. Muscle contraction is carried on thanks to the interaction of two proteins, actin and myosin. At rest, these two proteins are separated by a molecule called troponin. When the neurons send a stimulus to the muscle to contract, calcium is released from the sarcoplasmic reticulum into the cytoplasm where it binds to the troponin molecule, shifting the conformation of the complex, and making actin and myosin interact and initiate the contraction. Upon termination, calcium pumps regulate the uptake of calcium back into the sarcoplasmic reticulum. Troponin gets back between actin and myosin and the contraction stops.

Therefore, muscle cells need to (1) store large amounts of calcium ions, and (2) make sure the calcium ions are free to flow during release and uptake. The release, uptake and intake of calcium ions in the cells of cardiac muscle is regulated by two proteins, SUMO-1 and SERCA2a. Reduced levels of SUMO-1 cause SERCA2a levels to drop too, and low levels of both proteins have been associated to heart failure. The genes that encode these two proteins are down-regulated in patients suffering from heart failure, causing calcium ions to "linger" in the cells instead of flowing in and out as required for proper muscular contractions.

Researchers from the Cardiovascular Research Center at Icahn School of Medicine at Mount Sinai have been studying this process in animal models and demonstrated that heart function can be substantially restored through a single dose of SUMO-1 and/or SERCA2a gene transfer [1]. Following these promising results in animals, a clinical trial started and, according to a press release from last November, the single dose gene therapy is already showing very promising results:
"The new long-term follow-up results from their initial Calcium Up-Regulation by Percutaneous Administration of Gene Therapy In Cardiac Disease (CUPID 1) clinical trial found a one-time, high-dose injection of the AAV1/SERCA2a gene therapy results in the presence of the delivered SERCA2a gene up to 31 months in the cardiac tissue of heart failure patients. In addition, study results show clinical event rates in gene therapy patients are significantly lower three years later compared to those patients receiving placebo. Also, patients experienced no negative side effects following gene therapy delivery at three-year follow-up."
The one dose gene therapy is delivered directly to the heart through a catheter, and the SERCA2a genes are inserted inside a modified adeno-associated virus (AAV). I've discussed viral vectors for gene therapy in the past (see this post and this one). What I didn't know at the time is that there's a new family of viral vectors fine tuned for cardiac gene therapy: they are called cardiotropic vectors [2].

AAV has been historically used in gene therapy because it is found in 80% of the human population and it is often asymptomatic, meaning that it is well tolerated in the population (basically, it is harmless). This makes it a safe means to deliver genes. However, it preferentially transfers genetic material to the liver, not the heart. Among the various things that can make gene therapy go wrong is of course, delivering the genes to the wrong target. In [2] authors Yang and Xiao discuss how by introducing specific mutations to the AAV genome they were able to construct an AAV mutant specific to the cardiac muscle tissue. These techniques make use of bioinformatic methods to reshuffle the AAV genes and introduce mutations according to prediction models to generate new variants that are then tested in mice models for organ specificity. This is quite exciting as we can foresee a future where we will have a vector for every possible tissue we need to target with gene therapy.

[1] Tilemann L, Lee A, Ishikawa K, Aguero J, Rapti K, Santos-Gallego C, Kohlbrenner E, Fish KM, Kho C, & Hajjar RJ (2013). SUMO-1 gene transfer improves cardiac function in a large-animal model of heart failure. Science translational medicine, 5 (211) PMID: 24225946

[2] Yang L, & Xiao X (2013). Creation of a cardiotropic adeno-associated virus: the story of viral directed evolution. Virology journal, 10 PMID: 23394344

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