Enzyme block reduces death in mice with myotonic dystrophy

HOUSTON - (Nov. 10, 2009) – Using drugs to block the action of an enzyme called protein kinase C can reduce or even eliminate heart problems in mice with myotonic dystrophy, said researchers from Baylor College of Medicine in a report that appears today in the Journal of Clinical Investigation.

“The disease is very severe in these mice,” said Dr. Thomas Cooper, professor of pathology and molecular and cellular biology at BCM and senior author of the report. “Eighty percent of them die within three weeks of turning on the mutation. If we give an inhibitor of protein kinase C, less than 20 percent die.”

Myotonic dystrophy type 1 is the second most common form of skeletal muscle disease, behind Duchenne muscular dystrophy. While most people think of it only as affecting skeletal muscle, it also adversely affects the heart and brain, said Cooper.

Myotonic dystrophy is caused by a mutation that contains hundreds and even thousands of repeats of the nucleotides CTG within a gene called DM kinase protein gene or DMPK. [Cytosine (C), thymine (T), guanine (G) and adenine (A) are all nucleotides that make up DNA. C, G, A, and uracil (U) make up RNA.]  In the mouse that Cooper and his colleagues specially bred, the mutated gene can be turned on in heart, skeletal muscle and brain tissue at any age. Turning on the mutation means that the repeats are copied into RNA as in humans, and the mice reproduce disease features that are seen in humans.

The RNA repeats alter the functions of two proteins. One is called muscleblind-like, “which is sopped up by the RNA repeats, causing problems,” said Cooper.

The other is CUGBP1 (CUG triplet repeat, RNA binding protein 1), which controls alternative splicing – the process that determines which proteins a cell makes. The repeat-containing RNA causes protein kinase C to add a phosphate molecule to CUGBP1, activating it and causing levels to go up, altering the way genetic information found in other genes is interpreted.

“When we give the mice the inhibitor, protein kinase C is inhibited and CUGBP1 comes back to normal,” said Cooper.

He and Dr. Muge N. Kuyumcu-Martinez, an instructor in pathology at BCM, and one of the paper’s first authors, are quick to point out that giving the protein kinase C inhibitors is not a perfect treatment. 

“It is a therapy option,” said Cooper. Other researchers are looking at molecules that can prevent muscleblind-like protein from binding RNA in the nucleus. Combinations of the two might be more effective, he said.

“For us, it is a proof of principle,” he said. “It shows protein kinase C’s importance in this disease. It helps us understand how repeat RNA can activate a signaling pathway.”

Cooper plans to continue studying the disease in mice with a federal Grand Opportunity award to his group and one at MIT in Cambridge. 

“We will do deep sequencing of the messenger RNA (the template from proteins are synthesized in the cell) to look at the whole transcriptome of the heart and muscle tissues in patients with myotonic dystrophy,” he said. The groups also plan to look at the sequences in the mice that reproduce the disease. 

Cooper said his group plans to work with the myotonic dystrophy mouse they study because the disease can be turned on at any time and progression of the disease can be followed in molecular detail. “This approach allows us to look at the primary and secondary events in the disease,” he said.

Others who took part in the study include Guey-Shin Wang (also a first author) Satyam Sarma, Nitin Mathur and Xander H.T. Wehrens, all of BCM. 

Funding for this work came from the National Institutes of Health, the Muscular Dystrophy Association and the American Heart Association.

An abstract of the report can be found at http://www.jci.org/.