Functional Substitution by TAT-Utrophin in Dystrophin-Deficient Mice

{{Summary
 * title=Functional Substitution by TAT-Utrophin in Dystrophin-Deficient Mice
 * authors=Kevin J. Sonnemann, Hanke  Heun-Johnson, Amy J.  Turner, Kristen A.  Baltgalvis, Dawn A.  Lowe, James M.  Ervasti
 * journal=PLoS Med
 * pub_date=2009/06
 * url=http://dx.doi.org/10.1371/journal.pmed.1000083
 * doi=10.1371/journal.pmed.1000083
 * subject=Medicine
 * summary=-

Background
Muscular dystrophies are genetic (inherited) diseases in which the body's muscles gradually weaken and degenerate. The commonest and most severe muscular dystrophy—Duchenne muscular dystrophy—affects 1 in 3,500 boys (girls can be carriers of the disease but rarely have any symptoms). At birth, these boys seem normal but the symptoms of their disease begin to appear in early childhood. Affected children may initially have difficulty walking or find it to hard to sit or stand independently. As they age, their muscle strength progressively declines and most affected boys are confined to a wheelchair by the time they are 12 years old. The muscles involved in breathing also weaken and the heart muscle becomes enlarged. Few boys with Duchenne muscular dystrophy live beyond their early 20 s, usually dying from breathing or heart problems. At present there is no cure for Duchenne muscular dystrophy. However, physical therapy and treatment with steroids can prolong the ability of patients to walk, and assisted ventilation can help with their breathing.

Why Was This Study Done?
In all muscular dystrophies, one of the proteins needed to build and maintain healthy muscles is missing or nonfunctional because of a genetic change (mutation). In Duchenne muscular dystrophy the mutation is in dystrophin, a protein that is involved in the formation of the dystrophin–glycoprotein complex. This complex normally sits in the membranes that surround muscle fibers and protects these membranes from damage during muscle contraction. Consequently, in Duchenne muscular dystrophy, the muscle fiber membranes become damaged and eventually the muscle fibers die. Thus, if functional dystrophin could be introduced into the muscles of patients with Duchenne muscular dystrophy, it might be possible to reduce their symptoms and prolong their lives. Indeed, the effects of dystrophin deficiency in the dystrophin-deficient mdx mouse can be reduced by the introduction of an artificial gene that expresses dystrophin or the closely related protein utrophin. Unfortunately, this gene therapy approach has not yet been effectively demonstrated in humans. In this study, therefore, the researchers investigate whether utrophin protein can be introduced directly into dystrophin-deficient mouse muscles by exposing the muscle cells to utrophin fused to the protein transduction domain of the HIV-1 TAT protein. Most proteins will not cross cell membranes, but proteins fused to this cell-penetrating domain readily enter many cell types, including muscle cells.

What Did the Researchers Do and Find?
The researchers injected full-length utrophin fused to the TAT protein transduction domain (TAT-Utr) and a short, “micro” version of utrophin fused to the same domain (TAT-μUtr) into the abdomens of mdx mice and looked to see where the proteins ended up. After two injections, both proteins were present in a wide range of tissues and organs, including several types of muscle. However, the levels of TAT-Utr were much lower than those of TAT-μUtr. Next, the researchers injected another group of mdx mice with TAT-μUtr six times over three weeks. Again, TAT-μUtr was present in all the tissues that the researchers examined. Furthermore, μUtr–glycoprotein complexes formed in the TAT-μUtr injected mdx mice and the membrane integrity and overall health of the dystrophin-deficient muscles of the mdx mice improved compared to mdx mice treated with saline. Finally, the researchers report, TAT-μUtr injections greatly improved the contractile performance of the muscles of the mdx mice.

What Do These Findings Mean?
These findings provide the first demonstration that injection of TAT-utrophin protein fusions may provide a way to treat muscular dystrophies caused by the loss of dystrophin. However, although this direct protein-replacement therapy looks hopeful, approaches that work in animals do not necessarily work in people. In particular, for this approach to work in patients with muscular dystrophy, it would be necessary to give frequent, high-dose injections of the TAT-μUtr fusion protein, a process that could eventually trigger a deleterious immune response. Nevertheless, the researchers suggest that by combining this novel approach with other approaches that also increase utrophin expression, it might be possible to prevent or delay the development of the symptoms of Duchenne muscular dystrophy.

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 * journal_volume=6
 * pub_open_access=Yes