MYOTILIN MAY BE PROTEIN AFFECTED IN LGMD1A

Of 12 known forms of limb-girdle muscular dystrophy (LGMD), the underlying genetic defects have been documented in seven. Now a group of researchers from Finland say they've located a good candidate gene for one of the other forms, LGMD type 1A. This form of LGMD is inherited in a dominant manner and is characterized by onset of muscle weakness in young adulthood and slow progression.

The researchers, led by Paula Salmikangas and Olli Carpen of the University of Helsinki, originally set out to find the genes for skeletal muscle proteins that interact with another muscle protein called alpha-actinin.

One of the genes discovered in this search, myotilin, is on chromosome 5, within an area of the DNA that's thought to contain the genetic defect that causes LGMD1A. Myotilin is a large protein found in the contractile fibers of the muscle cells -- the thick and thin filaments that slide together to cause the muscle cell to contract. Defects in another contractile fiber protein are known to cause a different muscle disease, nemaline myopathy.

So far, the myotilin gene is the only muscle-specific gene that's been found in the region of the chromosome suspected to be involved in LGMD1A. Although this finding makes myotilin a good candidate gene for LGMD1A, the next step will be to see whether people with LGMD1A actually have defects in myotilin. The findings were reported in the July issue of Human Molecular Genetics.

In the United States, MDA researchers Marcy Speer and Michael Hauser of Duke University have established a collaboration with Carpen to screen people suspected of having LGMD1A for defects in the myotilin gene. If you or a family member have a dominant form of LGMD of unidentified genetic origin, Speer and Hauser would like to hear from you. You can contact them at (800) 283-4316. You can also check the Web site at www2.mc.duke.edu/depts/medicine/medgen/ for updates of their research studies.

GENE THERAPY IN MICE SUCCESSFUL FOR ACID MALTASE DEFICIENCY

Acid maltase deficiency, or Pompe's disease, is a metabolic disorder that results in a harmful buildup of glycogen (a form of sugar) in the cells, caused by the lack of acid maltase enzyme. The skeletal and cardiac muscles are particularly affected, leading to heart and respiratory failure.

Yuan-Tsong Chen

Until recently, there were no treatments in development for this disease. Now, there are two clinical trials under way (one by Pharming Pharmaceuticals in Europe and one by MDA grantees Y.T. Chen and Andrea Amalfitano at Duke University in Durham, N.C.) testing whether the missing enzyme can be supplied through injections. This approach has worked well in animal models, but the injections must be maintained throughout life, and manufacturing the enzyme in large enough quantities to treat humans may be difficult.

For these reasons, many researchers believe that, in the long run, it may prove more practical to use gene therapy to allow the cells of the person with the disease to manufacture the missing enzyme directly. In 1998, MDA grantee Paul Kessler of Johns Hopkins University in Baltimore, in collaboration with Barry Byrne at the University of Florida in Gainesville, demonstrated that gene therapy could be used to increase acid maltase production in portions of the cardiac and skeletal muscle of newborn rats through intramuscular and intracardiac injections.

Now Amalfitano and Chen have developed a method for restoring acid maltase activity to all of the cells of mice that lack the enzyme through a single injection into the bloodstream. To accomplish this, they've used a viral vector (carrier) that's taken up efficiently by liver cells to deliver the acid maltase gene. The researchers reasoned that, because the liver has good access to the bloodstream, it might be able to function as an acid maltase production factory -- essentially pumping the therapeutic enzyme continuously into the bloodstream, from which it could be taken up by skeletal and cardiac muscle cells.

Amalfitano and Chen have found that, in acid maltase-deficient mice, the liver took up the gene therapy vector efficiently, and the mice quickly developed high blood levels of acid maltase. The treated mice also had high acid maltase activity in their cardiac and skeletal muscles.

Perhaps most important, once in the various muscle groups, the enzyme was able to reduce the accumulation of glycogen to near normal levels in all muscle tissues tested.

"The heart and diaphragm muscles appeared to be especially responsive to this form of therapy," Amalfitano points out -- an important consideration because the primary cause of illness and/or death in Pompe patients is due to the cardiac or respiratory muscle involvement.

Issues that remain to be worked out include finding ways to maintain production of the enzyme over time, and determining whether there's a significant immune response to either the gene therapy vector or the newly introduced acid maltase enzyme.

The work of Amalfitano and Chen was reported in the August issue of the Proceedings of the National Academy of Sciences, USA.

FIRST MICE WITH MITOCHONDRIAL DNA DISEASE PRODUCED

In order to develop therapies for a human genetic disease, it's often helpful to study animals with similar symptoms, also known as "animal models." In some cases, animals can be found that naturally develop the disease in question, but other times researchers must introduce genetic defects (usually in mice) to create an animal model of a human disease.

Developing mouse models for disorders that affect the tiny energy-producing organelles in our cells called mitochondria, however, has proved problematic. Although the technology exists to produce mice with mutations in the first type of DNA, researchers didn't know how to make mice that have defective mitochondrial DNA. (See "Mitochondrial Disease in Perspective.")

Now researcher David Marchington of the University of Oxford and John Radcliffe Hospital, both in the United Kingdom, has created a line of mice that have many of the qualities of a disease caused by defective mitochondrial DNA. The mice were created with a mutation that can cause the human disease MELAS (mitochondrial encephalomyopathy with lactic acidosis and strokelike episodes).

The cells of the new line of mice contain some mutant mitochondrial DNA and some normal mitochondrial DNA, a situation called heteroplasmy that's a hallmark of mitochondrial DNA disease. However, the mice have only 6 percent mutant mitochondrial DNA, whereas many human mitochondrial diseases have no symptoms unless more than 50 or 60 percent of the mitochondrial DNA is defective. Not surprisingly, the mutant mice have no obvious illness.

Still, the new line of mice should prove useful in studying the way mutant mitochondrial DNA is distributed in different tissues and how the mutation is passed on to the next generation. Also, researchers suggest that they may be able to increase the proportion of defective mitochondrial DNA with a drug treatment. These findings were reported in the August issue of Nature Medicine.

CLINICAL TRIAL IN DERMATOMYOSITIS OPENS IN PHOENIX

MDA clinic co-director Kumaraswamy Sivakumar at St. Joseph's Hospital and Medical Center in Phoenix will conduct a study of the drug TNFR:Fc (brand name Enbrel) in people with dermatomyositis (DM) who haven't responded well to other treatments.

Enbrel acts by blocking a natural body chemical called tumor necrosis factor, which is thought to cause inflammation and tissue destruction in some diseases. The study will test the safety and effectiveness of Enbrel and also examine the immune responses associated with DM.

The researchers are looking for men and women ages 18 to 60 with DM and no other neuromuscular illness, diabetes or infection. Candidates for the study will need to have been on the same treatment regimen for at least two months prior to the study; they should be resistant to or unable to tolerate their present immunosuppressive medications. Other requirements will be described at the time of application.

To apply for the study, call nurse Mary Harrigan at (602) 406-6651 or e-mail her at mharrig@chw.edu.

UNAFFECTED VOLUNTEERS NEEDED FOR ROCHESTER MUSCLE DISORDERS STUDY

Richard Moxley

Neurologist Richard Moxley of the University of Rochester (N.Y.) Medical Center is investigating the causes of the muscle wasting, weakness and metabolic abnormalities seen in myotonic muscular dystrophy (MMD) and proximal myotonic myopathy.

The study, sponsored by the National Institutes of Health, has been open since 1997 and is comparing findings in people with myotonic dystrophy and proximal myotonic myopathy to those in people with other neuromuscular diseases and with unaffected volunteers. The researchers hope to identify specific alterations in body function that could eventually lead to new therapies.

They're looking for participants with myotonic muscular dystrophy, proximal myotonic myopathy, facioscapulohumeral muscular dystrophy and Charcot-Marie-Tooth disease (type 1A only), as well as volunteers with no neuromuscular disorder (unaffected volunteers).

There's a special need for unaffected volunteers to participate so that the researchers can identify specific metabolic alterations that are unique to the muscle diseases under study. The researchers are particularly interested in unaffected family members.

All participants must be between 21 and 60 years old, nonsmokers, not obese and able to walk.

The study requires three inpatient stays at the University of Rochester General Clinical Research Center. The first visit is five days and the second and third visits are three days. For information, call research nurse Cheryl Barbieri at (716) 275-5409 or e-mail neuro_research@URMC.rochester.edu.

UPDATE ON CARDIAC PROBLEMS IN MD

New research suggests some of the cardiac problems seen in muscular dystrophy could be caused by loss of skeletal muscles, blood-vessel muscles or heart muscle itself.

It's long been known that cardiac problems occur in muscular dystrophies. Since the 1980s, when it became possible to identify the genetic flaws and missing proteins in several forms of MD, experts have, for the most part, assumed that the same missing proteins that cause skeletal muscle degeneration are missing in the heart and therefore lead directly to cardiac muscle degeneration. (See "The Heart is a Muscle, Too," Part 1 and Part 2.)

That's probably the main cause of cardiac degeneration in some forms of MD. But MDA grantee Kevin Campbell of the University of Iowa has contributed to a growing body of knowledge that suggests factors outside the heart itself may play a major role in the cardiac problems associated with certain muscular dystrophies.

In a series of experiments, Campbell found that mice missing the delta-sarcoglycan protein fared far worse with respect to heart muscle dysfunction than mice missing the alpha-sarcoglycan protein. Both proteins are important in cardiac and skeletal muscle, and both, when absent, lead to limb-girdle muscular dystrophy (LGMD) in mice and humans.

The difference in the fates of the hearts in the two types of LGMD, Campbell says, may lie in the fact that delta-sarcoglycan seems to be needed in "smooth" muscle, the type that surrounds blood vessels and allows them to dilate and constrict. Alpha-sarcoglycan is important for cardiac and skeletal muscle contraction but apparently isn't important to the smooth muscles in blood vessels, he says.

In a paper published in the Aug. 20 issue of Cell, Campbell and his fellow authors say the heart abnormalities in delta-sarcoglycan-deficient mice are likely due to the dysfunction in blood vessels and, therefore, to a disruption in blood supply to the heart. They found a blood vessel-relaxing drug helpful in preventing damage in these mice.

In another study, MDA grantees R. Mark Grady and Joshua Sanes of the University of Washington in St. Louis showed that defects in a cellular signaling mechanism that helps control blood vessel dilation probably occur when the muscle membrane is disrupted, as it is in most muscular dystrophies. Their paper is in the August issue of Nature Cell Biology.

And, earlier this year, MDA-supported researchers Michael Rudnicki and Lynn Megeney, then both at McMaster University in Canada, found that skeletal muscle degeneration plays a major role in determining the severity of heart muscle degeneration in mice with Duchenne muscular dystrophy. The mechanism isn't clear but it could be related to changes in blood flow, biochemical signals or both, they say. (See Quest, vol. 6, no. 2, "Research Updates.")

With better understanding of the specific mechanisms responsible for the problem in each form of muscular dystrophy, treatment of cardiac complications in MD could improve.