Successes and challenges
Barth syndrome is a rare disease caused by a mutation of the tafazzin protein gene (TAZ), which leads to the development of life-threatening heart failure, as well as skeletal muscle weakness, insufficient immune response and growth disorders (dysmorphic disorder, delayed puberty). Since Barth syndrome is linked to the X chromosome, boys almost always get sick. At the moment, there is no effective treatment for Barth syndrome, doctors' interventions are reduced to symptomatic therapy.
In 2014, William Pu, MD, and his colleagues from Boston Children's Hospital, together with the Wyss Institute, developed a model of Bart's syndrome – a beating heart-on-a-chip. It consisted of heart muscle cells (cardiomyocytes) with a mutation of the TAZ gene obtained from skin cells of patients with Barth syndrome. This work confirmed that the TAZ mutation really underlies cardiac dysfunction: cardiomyocytes could not contract normally, the mitochondria inside them were disorganized, and the heart tissue stretched and thinned. The insertion of a healthy TAZ gene normalized these features, so scientists suggested that gene replacement therapy could form the basis of a promising treatment.
Adenoassociated viral vectors with the normal TAZ gene in the heart tissue (marked in green). Source: Boston Children's Hospital press release.
But in order to better understand the mechanisms of the development of Barth syndrome and its impact on the whole body, not just the heart, Pu and his colleagues needed an animal model of the disease. However, attempts to create a mouse model using traditional methods were unsuccessful.
Modeling of Barth syndrome in mice
Relatively recently, the laboratory of the Douglas Strathdy group at the Cancer Research Institute named after Beatson in the UK created a model of Bart syndrome on animals. In the new work, Pu and colleagues characterized these "knockout" mice, which were of two types: in one type, the TAZ gene was removed in the cells of the whole body; in the other, only in the heart.
Almost all mice with TAZ deletion in whole-body cells died before birth, apparently due to skeletal muscle weakness. But some survived, and these mice developed progressive cardiomyopathy, in which the heart muscle stretches and loses pumping capacity. Their hearts also had scars, and, like patients with dilated cardiomyopathy, the left ventricle of the mice's heart was dilated and had a thin wall.
All mice deprived of TAZ only in the heart tissue survived the prenatal period and showed the same features. Electron microscopy showed that cardiomyocytes in the heart tissue of these mice are poorly organized, as are mitochondria inside cells.
From left to right: mitochondria in cardiomyocytes of a healthy mouse, a mouse with Barth syndrome after using an empty viral vector and a mouse with Barth syndrome treated with a vector carrying the normal TAZ gene. Source: article in Circulation Research.
The researchers used gene therapy to replace the TAZ gene by injecting a viral vector under the skin of newborn mice and intravenously to old mice. Treated mice with TAZ deletion in whole-body cells were able to live to adulthood. TAZ gene therapy also prevented cardiac dysfunction and scarring in newborn mice and reversed already developed cardiac dysfunction in older mice – regardless of whether the mice had TAZ deletions in whole-body cells or just the heart.
To achieve the insertion and preservation of the gene
Further tests showed that TAZ gene therapy provides long-term treatment of cardiomyocytes and skeletal muscle cells of animals, but only if a healthy gene is inserted into at least 70 percent of heart muscle cells.
When switching to research of the method, it will be problematic to achieve such a result in people. Simply increasing the dose of gene therapy will not work: in large animals, including humans, a high dose carries the risk of developing a dangerous inflammatory immune response. The use of several doses of gene therapy will also not work, since neutralizing antibodies to the virus are produced in the body after the first administration.
Another problem is the preservation of cell populations with a corrected genome. In mice, the number of cells with the corrected TAZ gene remained fairly stable in the cardiac tissue, while in the muscle tissue it gradually decreased.
Article S.Wang et al. AAV Gene Therapy Prevents and Reverses Heart Failure in A Murine Knockout Model of Barth Syndrome published in the journal Circulation Research.
Aminat Adzhieva, portal "Eternal Youth" http://vechnayamolodost.ru based on the materials of Boston Children's Hospital: Gene therapy reverses heart failure in mouse model of Barth syndrome.