‘LARGE’ PROTEIN CORRECTS DEFECT
IN SOME MD-AFFECTED CELLS

A protein known as “LARGE” may have the capacity to restore normal structure and function to cells in several forms of muscular dystrophy, say MDA-supported researchers who published their findings in the July issue of the journal Nature Medicine.

MDA grantees Rita Barresi, Steven Moore and Kevin Campbell, all at the University of Iowa in Iowa City, were on the team, which also included researchers from Canada, Sweden and Japan.

The investigators found that LARGE, an enzyme that attaches sugar molecules to proteins -- a glycosyltransferase -- can correct the molecular defect in several muscular dystrophies in which the attachment of sugars (glycosylation) to a protein in the cell membrane is faulty.

The muscular dystrophies that result from these glycosylation defects include several forms of congenital MD -- Fukuyama MD, muscle-eye-brain disease, Walker-Warburg syndrome and type 1C and 1D congenital MDs. In several of these, both muscle and brain cells are affected.

Another disorder, the type 2I form of limb-girdle MD, is also caused by this type of glycosylation defect.

In each case, an enzyme (a protein that allows chemical reactions to take place) that’s responsible for attaching sugar molecules to alpha-dystroglycan, a cell membrane protein, is missing or abnormal, leading to less than adequate attachment of the sugars and to serious consequences for the cell.

Most of the other muscular dystrophies, Campbell notes, are due to defects in proteins that form parts of the cell’s physical structure. It’s harder to replace or compensate for those, he says.

“When you have enzymatic activity, you don’t have to produce a lot of the protein; a little will probably do,” Campbell noted.

The researchers studied mice lacking the LARGE protein because of a genetic defect in the gene for LARGE and found that giving them a working version of that gene via a viral delivery system returned the biochemistry, structure and function of their muscle fibers nearly to normal.

They then added the LARGE gene to cells from people with Fukuyama MD, muscle-eye-brain disease and Walker-Warburg syndrome and found that adequate numbers of sugar molecules were attached to the alpha-dystroglycan protein.

Further study of some of the cells revealed that the laminin protein, which must attach to the sugars on alpha-dystroglycan and can’t “dock” without them, was properly attached.

The authors call LARGE “an attractive target for the design of therapies intended to manipulate alpha-dystroglycan glycosylation.” They were pleasantly surprised that the protein worked in cells affected by a variety of genetic defects, not just defects in LARGE itself.

“We’re looking at testing different compounds in cells to see if we can increase LARGE’s activity,” Campbell said, noting that all patients except those with type 1D congenital MD already have the LARGE protein but apparently not enough of it to compensate for their other enzymatic defect.

That’s good news, he says, because it means adding LARGE is unlikely to generate an undesirable immune response.

“There are different things we’re pursuing,” he added. One strategy is a gene delivery system for LARGE that could potentially be used in people. “We’re making AAV-LARGE [using the adeno-associated virus to deliver the LARGE gene] to test in mice and then in patient cells.”