RESEARCH UPDATES
LIMB-GIRDLE, CONGENITAL DYSTROPHIES TIED TO PROTEIN CLUSTER
Limb-girdle muscular dystrophy (LGMD), a type of dystrophy that first affects the hip and shoulder muscles and may move to other muscle groups, is proving to be a disease with many genetic causes.
Last fall, researchers identified two new proteins, beta and gamma sarcoglycan, and found that a lack of either can cause LGMD. The proteins are part of an eight-protein cluster that includes dystrophin, the protein affected in Duchenne and Becker muscular dystrophies. Around each muscle fiber are thousands of these clusters, spanning the fiber's outer covering, its membrane, and anchoring the fiber to other tissue. They probably help protect the fiber from stretch-related damage.
Three MDA-supported teams identified the sarcoglycan proteins as well as the genes that code for them and connected the loss of these proteins to LGMD. The teams included researchers from the University of Iowa in Iowa City, Children's Hospital in Boston, Duke University in Durham, N.C., the University of Pittsburgh and others.
The gene for beta sarcoglycan is on chromosome 4, while the gene for gamma sarcoglycan is on chromosome 13. Defects in these genes lead to defects in or loss of the sarcoglycan proteins. In 1994, MDA researchers identified a chromosome 17 protein that's also part of the membrane cluster. A lack of this protein, now called alpha sarcoglycan, also leads to LGMD. This type of LGMD was originally called severe childhood autosomal recessive muscular dystrophy (SCARMD), and alpha sarcoglycan was originally called adhalin.
Another protein in the membrane cluster is merosin, and it has been linked to a form of congenital muscular dystrophy (CMD). Last fall, researchers in France, Finland and Turkey found that defects in a chromosome 6 gene result in merosin loss and CMD. Another name for merosin is laminin-2.
In addition to vastly improving diagnosis of the muscular dystrophies, understanding genetic and protein defects is a first step in finding treatments such as gene therapy - inserting a normal gene to make up for a defective one.
ATAXIC MICE TO SPEED SCA1 RESEARCH
Researchers at the University of Minnesota in Minneapolis and Baylor College of Medicine in Houston have created mice with some of the symptoms of spinocerebellar ataxia type 1 (SCA1), a disease that involves lack of coordination, difficulty in speaking, vision problems and muscle wasting. The team, which inserted the genetic defect for SCA1 into embryonic mice, included former MDA grantees Huda Zoghbi at Baylor and Harry Orr at Minnesota. In 1993, MDA grantees Zoghbi and Orr were part of a team that found the SCA1-causing genetic defect on chromosome 6.
This type of research, called animal model development, allows scientists to test theories about the causes of a disease and try possible treatments.
GENE THERAPY - DESIGNING VIRUSES
Gene therapy, putting good genes into cells in the body, holds great promise for treating many disorders, including the muscular dystrophies. The challenge is to deliver the genes safely and effectively.
Until recently, researchers have mostly worked either with "naked" genes (DNA), inserting them into human or animal cells directly, or with DNA packaged inside viruses, which are then put into cells. Viruses help genes get into cells, but they have a serious side effect; they can cause the body to attack the very cells the new gene is trying to help.
Now scientists are learning from all these experiments and are using only the barest essentials of the viruses.
Some researchers, such as MDA grantee George Karpati at Montreal Neurological Institute in Canada, are working with the adenovirus, the most commonly used delivery vehicle ("vector") for putting genes in muscle cells. To fit in a new gene (for example, the dystrophin gene, needed for therapy of Duchenne or Becker dystrophy), scientists have had to take out several genes that are part of the adenovirus. But recent studies show that one genetic region in the adenovirus, known as E3, might be important to leave in. This region keeps the body's immune system from noticing the virus, allowing it to slip in like a stealth bomber and deposit its cargo, the new gene.
Other researchers have gone to different viruses for their vector ideas. MDA-supported Richard Bartlett at the University of Miami School of Medicine is working with an adeno-associated virus (AAV). The AAV has a unique property. Unlike adeno- and many other viruses, which stay outside the cell's chromosomes, and unlike retroviruses, which fit themselves into chromosomes in a variety of places, the AAV usually inserts its DNA at a certain place on human chromosome 19. This place may be safe, says Bartlett, because most people already have this viral DNA at this place with no ill effects. It may be effective, he says, because a therapeutic gene, such as dystrophin, would become a permanent part of the cell.
MDA grantee Jon Wolff at the University of Wisconsin in Madison is part of a team that's taking a different approach. They're using genes from a sindbis virus, combining them with the gene they want to put into the cell, and putting the combination into a circular piece of DNA known as a plasmid.
This vector travels naturally to the cell's nucleus, where it takes over part of the cell's functioning and causes it to make many copies of the gene.
Hans Herweijer in Wolff's lab says the system could make cells produce a lot of a therapeutic protein, such as dystrophin. Unfortunately, he says, the body's immune system may attack the plasmid because of the viral genes it contains.
Researchers are still studying whether miniature versions of the dystrophin gene will be good enough to improve muscle function in Duchenne or Becker dystrophies or whether the whole gene should be used.
