Blocking enzyme benefits mice with myotonic dystrophy
By Ruth SoRelle, M.P.H.
Mice with myotonic dystrophy suffer severe, deadly heart dysfunction – a little known effect of the inherited disease that can threaten its human sufferers as well.
In fact, myotonic dystrophy type 1 – the second most common form of muscular dystrophy – affects not only skeletal muscle but also heart and brain, said Dr. Thomas A, Cooper, professor of pathology and molecular and cellular biology at Baylor College of Medicine and senior author of a report that appeared recently in the Journal of Clinical Investigation.
Severe disease
"The disease is very severe in these mice," said Cooper. "Eighty percent of them die within three weeks of turning on the mutation." However, a drug that blocks or inhibits the activity of an enzyme called protein kinase C reduces the death rate to no more than 20 percent.
Myotonic dystrophy is caused by a mutation that contains hundreds and even thousands of repeats of the nucleotides CTG within the 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.
Repeats hinder function
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 of the associated protein 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.
Not a perfect treatment
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.
Enzyme is important
"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.
Mouse proves good model
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.
