Researchers at Baylor College of Medicine in Houston and Isis Pharmaceuticals in Carlsbad, Calif., have announced encouraging results for their antisense-based strategy in development for the treatment of type 1 myotonic muscular dystrophy (DM1, or MMD1).
|MDA research grantee Thomas Cooper at Baylor College of Medicine coordinated the research team that developed the new gapmer antisense compound.|
The strategy, known as gapmer antisense, attracts a naturally occurring cellular enzyme called RNase H to the site of the genetic defect in MMD1-affected cells, where it destroys toxic excess genetic material. RNase H is located in the cell nucleus, which is where the MMD1 defect is located.
MDA research grantee Thomas Cooper, a professor in the Department of Pathology and Immunology at Baylor, coordinated the study team, which published its findings online Feb. 27, 2012, in Proceedings of the National Academy of Sciences-United States.
"There is a great deal of work ahead to develop this approach into a viable therapy," Cooper said, "but the results from this study and from other labs using antisense approaches provide a general direction for future research. An important component is the collaboration of biotech companies that have the expertise and facilities for the development and production of a variety of antisense compounds. One should always anticipate that there are biological roadblocks ahead, but indications are that this is a productive approach."
Blocking or destroying extra genetic material known as CUG repeats in MMD1 is the primary goal of many therapeutic strategies in development, including the one Cooper's group is developing.
It is believed that doing either or both of these would improve the functioning of nerve and muscle cells and thereby inprove the functioning of muscles and other organs in people with MMD1.
The underlying cause of MMD1, a complex, multisystem disease affecting skeletal muscles, as well as the heart, brain and other organs, is an expanded stretch of a repeated DNA sequence on chromosome 19 known as a DNA triplet repeat.
In MMD1, the triplet repeat is composed of three nucleotides (chemical components of DNA) — cytosine, thymine and guanine — which are abbreviated CTG.
Normally, there are between three and 37 CTG triplet repeats on chromosome 19. But in MMD1, there can be hundreds or even thousands of such repeats.
When the DNA is converted to RNA (a natural process), the triplet repeat becomes cytosine, uracil, guanine, or CUG, and it known as a CUG repeat.
CUG repeat expansions cause a lot of problems for MMD1-affected nerve and muscle cells. For instance, they:
In MMD1-affected cells, extra DNA leads to the creation of extra RNA, which forms clumps, ensnares the MBNL1 protein and sets in motion a cascade of problems affecting many organs. The extra RNA contains sequences known as CUG repeats, which the experimental strategy called gapmer antisense targets and destroys.
Cooper and his colleagues conducted several experiments with their gapmer antisense compounds in cells in a lab dish that contained large CUG repeat expansions in their nuclei; and in mice with this genetic defect that develop an MMD1-like disease.
The investigators injected the muscle of one hind leg of each mouse with the gapmer antisense compound and the opposite leg with a different substance, which they expected to be inactive, as a comparison (control).
They found that the gapmer antisense compounds, known in these experiments as CAG gapmers, had several desired effects. For example, they:
The CAG gapmers and the inactive control gapmers caused some abnormalities to appear in the injected mouse skeletal muscles. The researchers propose that this damage may have been caused by a process called electroporation, which they used in the mice following the gapmer injections.
Electroporation makes the cells more permeable to injected compounds, but it may have damaged them in the process and probably would not be used in humans.
Cooper and colleagues performed additional experiments in the MMD1 mice using a combination of the CAG gapmer and another type of antisense-based experimental treatment for MMD1 known as CAG25. This compound, known as a morpholino antisense, blocks the interaction of the expanded CUG repeat with an important muscle protein called MBNl1, freeing MBNL1 to perform its normal role in the cell.
CAG gapmer antisense by itself reduced the number of RNA stretches containing 960 CUG repeats by 50 percent, but when CAG25 was added, the reduction was 75 percent.
"There are several systemically administered RNase H-based antisense drugs in clinical trials, suggesting bright prospects in the future," the investigators write in their Feb. 27 paper. "The growing interest in therapeutic [antisense] will accelerate development of novel nucleotide modifications and delivery methods addressing issues of toxicity and [distribution]."