Study identifies three enzymes required for building sugar superstructure involved in muscular dystrophies
Thursday, August 8, 2013
Kevin Campbell, PhD,
Kevin Campbell

The central protein in CMDs is dystroglycan (DG). "It looks like at least 10 to 15 genes encode proteins that contribute to the glycan superstructure that makes DG effective," says Campbell, professor and head of molecular physiology and biophysics at the UI Carver College of Medicine, and an investigator of the Howard Hughes Medical Institute. "Our goal is to figure out the whole pathway by which the glycan structure is built, since defects in any of the proteins can potentially lead to disease. Knowing which genes are involved is expected to help us develop clinical tests for these dystrophies, and also ways to screen for potential therapeutic agents."

Normally, DG is modified with a unique sugar chain that acts like glue, allowing DG to attach to other proteins and, by doing so, to reinforce cell membranes in many tissues—including muscle and brain. DG does not function properly without this sugar modification, and glitches in the construction of the glycan cause the progressive muscle dysfunction and the brain abnormalities that characterize many forms of muscular dystrophy.

Almost a dozen genetic mutations are now known to cause DG-related CMDs, which include Fukuyama Congenital Muscular Dystrophy, Walker-Warburg Syndrome, Muscle-Eye-Brain disease, and certain types of limb-girdle muscular dystrophy. All of these mutations affect proteins (enzymes) that are responsible for building DG’s unique sugar chain.

The new study assigns a role to three of these causative mutations, showing that the three affected enzymes act sequentially to build an early section of DG's critical glycan. When any of these proteins are mutated, the sugar chain is not constructed correctly and the DG protein loses its function. The first author of the study was Takako Yoshida-Moriguchi, Ph.D., a UI research assistant professor in Campbell's lab

The three enzymes connect a series of sugars together to form the glycan -- like stringing beads together to make a necklace. The starting point of the chain is a mannose sugar, which is attached to the backbone of the DG protein. The first enzyme analyzed in the study, called GTDC2, links a glucosamine (GlcNAc) sugar to this starting mannose. The second enzyme, B3GALNT2, then adds a galactosamine sugar to the GlcNAc. Only when this disaccharide is complete can a third enzyme—an unusual type of kinase called SGK196—add a phosphate group to the mannose at the beginning of the chain.

Earlier work from Campbell's lab has shown that this phosphate link is required for other enzymes to build the final section of the sugar chain—the part that actually allows dystroglycan to do its job.

"What I find really exciting is that even with the whole genome having been described, we are still finding novel enzymes that carry out functions we didn't know about even two or three years ago," Campbell says.

Identifying and understanding the roles of these enzymes may eventually provide leads for developing therapies to treat CMD and other muscle diseases, Campbell adds.

In addition to Campbell and Yoshida-Moriguchi, the study team included UI scientists Tobias Willer, Ph.D., Mary Anderson, David Venzke, and Liping Yu, Ph.D., as well as Tamieka Whyte, and Francesco Muntoni, from Great Ormond Street Hospital for Children in London, U.K., and Hane Lee, and Stanley Nelson at the David Geffen School of Medicine, UCLA.

This study was funded in part by a National Institutes of Health grant (1U54NS053672) for the Senator Paul D. Wellstone Muscular Dystrophy Cooperative Research Center at the UI, the Muscular Dystrophy Campaign, and the Great Ormond Street Hospital Children Charity.