'Gapmer Antisense' Stops Disease in MMD1 Mice

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An experimental treatment for type 1 myotonic muscular dystrophy (MMD1, or DM1) has corrected several aspects of the disease in an MMD1 mouse model.

A year after treatment with an experimental antisense oligonucleotide dubbed ASO 445236 ended, positive effects remained apparent in the mice.

Thurman Wheeler at the University of Rochester (N.Y.) and colleagues announced their findings Aug. 2, 2012, in the prestigious journal Nature.

MDA supported this work through a development grant to Masayuki Nakamori, then at the University of Rochester.

'Gapmer antisense' destroys toxic RNA in three steps

Wheeler and colleagues used a strategy called gapmer antisense to destroy the genetic defect that causes MMD1. In humans with the disease, the defect is an abnormally expanded section of DNA in a gene called DMPK. (In research mice, the defect is often inserted into a different gene.)

The expanded section is made up of the nucleotide sequences CTG (cytosine, thymine, guanine) repeated many more times than usual. When the DNA is transcribed into RNA, the repeats become CUG repeats (cytosine, uracil, guanine).

The expanded RNA takes on toxic properties, largely through trapping proteins and keeping them from carrying out roles needed for a cell to function.

The gapmer antisense strategy is a three-step process that involves:

• targeting a strand of RNA for destruction;
• attracting an enzyme called RNase H1, which partially digests the strand and breaks it into fragments; and
• attracting other enzymes to destroy the remaining fragments of RNA.

An advantage of this strategy for MMD1 is that RNase H1 acts primarily in the cell nucleus, which is where the expanded RNA is located in MMD1-affected cells. (Some other antisense strategies do not utilize RNase H1.)

Prolonged benefit seen in MMD1 mice

The systemic injection of gapmer antisense compound ASO44536 (one of three compounds tested) into mice with CUG repeats in the actin gene had several benefits, which lasted for a year after stopping four weeks of treatment with a total of eight injections.

The effects included:

• reduction of CUG repeats in skeletal muscle;
• correction of some of the negative effects of the CUG repeats on muscle tissue, including prevention of shrinkage of muscle fibers;
• correction of the errors in gene activity (how much RNA is made from DNA) and protein synthesis (how RNA instructions are spliced together) seen with the presence of CUG repeats; and
• elimination of myotonia, the prolonged muscle contraction that occurs in MMD1 mice and people with MMD1.

Muscle damage avoided

A previous MDA-supported study of a different gapmer antisense compound in a mouse model of MMD1, published in February 2012, found many beneficial effects but also some damage to muscles, which was not seen in the current set of experiments.

The authors of this current study say their strategy appears not to cause muscle damage. The strategy utilizes subcutaneous injection (a systemic, or system-wide, route of administration) instead of direct injection into muscle; and the compound targets an area of RNA adjacent to the repeats instead of the repeats themselves, which may reduce undesired effects.

Positive effects also seen when repeats were in DMPK gene

Most previous MMD1 rodent studies have involved a mouse model in which the DNA expansion defect is inserted into the actin gene, a convenient approach for technical reasons.

However, the investigators in this new study conducted experiments in both the actin mutant mouse and in mice with a DNA expansion in the DMPK gene, the gene that's actually affected in humans with MMD1.

After four weeks of twice-weekly subcutaneous injections into mice with CUG repeats in the DMPK gene, an antisense compound produced significant reduction of CUG repeat RNA in muscle tissue in a leg.

Results inspire optimism, but challenges remain

In an accompanying editorial published in the same issue of Nature, Peter Todd and Henry Paulson, both in the Department of Neurology at the University of Michigan, commented on these results.

They said the findings "inspire optimism that previous challenges faced by researchers looking at antisense oligonucleotide therapies for DM1 [MMD1] and other neuromuscular diseases are surmountable — although significant hurdles remain regarding safety and delivery to affected tissues other than skeletal muscle, such as the heart and brain."

Todd and Paulson caution that "care must be taken in oligonucleotide design to avoid potentially deleterious off-target effects" and that "therapeutic success in a mouse model is still a long way from effective application in humans."

They end by saying, "However, the path to success now seems clearly visible."

More information

To learn more about antisense in myotonic dystrophy, check out these other resources:

• Targeting Nuclear RNA for in Vivo Correction of Myotonic Dystrophy — summary of a scientific paper in Nature, published Aug. 2, 2012
• Drug Discovery: Kill the Messenger Where It Lives — summary of an editorial on the above paper, published Aug. 2, 2012
• A Promising Step Forward Toward Muscular Dystrophy Treatment — lay-language news story from the University of Rochester Medical Center, published Aug. 1, 2012
• MMD Research: Seeking to Free Proteins from a 'Toxic Web' — Quest magazine feature about research in myotonic muscular dystrophy, published July 2012
• 'Gapmer Antisense' Targets MMD1 Defect for Destruction — Quest News Online, Feb. 29, 2012