An 8-year-old boy with Duchenne muscular dystrophy (DMD)is making medical history. According to last week's issue ofNature Genetics, his is "the first case of a novel and extremelysubtle type of mutation never before associated with DMD."
This editorial comment introduced a paper titled "A missensemutation in the dystrophin gene in a Duchenne musculardystrophy patient" by Thomas Prior et al. of Ohio StateUniversity.
This unique patient is one of an estimated 10,000 boyssuffering from Duchenne, the most severe form of musculardystrophy. It afflicts one in 3,500 newborn males in the U.S,almost none of whom live beyond their 20s.
Their lethal enemy is a large defective gene for a mysteriousmuscle-cell protein, dystrophin.
The gene resides on the X chromosome, which means that onlyboys can inherit DMD; their sisters may only be carriers.
In two-thirds of those boys, the dystrophin gene has beenwarped by either deletion or duplication of important stretchesof DNA. This results in either no dystrophin at all or a non-functional version of the protein being transfected. Thesedisrupted sequences are readily detected and the diseasediagnosed by direct DNA testing.
The remaining third are victims of de novo point mutations, thetrue troublemakers of DMD diagnosis. These single-nucleotideslip-ups are called "nonsense" mutations; they involveswitching one DNA base -- A, C, G or T -- for another. Theresulting mix-up in the genetic code causes the dystrophingene to shut down cold.
What's worse, such point mutations are peculiar to individualDMD victims, so they cannot be detected by conventionallinkage analysis. But the absence of functioning dystrophin hasbeen tested as the absolute hallmark of DMD.
Not any more.
The boy in the current issue of Nature Genetics has a pointmutation of a new and different type. Instead of "nonsense,"which acts as a stop sign for dystrophin gene transcription, hisdefect is "missense." It permits some dysfunctional protein tobe made, thus confounding diagnosis.
In this hitherto unseen mutational mistake, Prior and his co-workers discovered, the T (thymine) nucleotide traded placeswith the G (guanine) base. This led disastrously to transcriptionof the amino acid arginine instead of leucine.
"The replacement of a neutral amino acid by a polar amino acidmay disrupt the overall conformation of the actin-bindingdomain," Prior's paper said, "... and therefore cause a relativelysevere phenotype."
"What is of great significance," Prior, a clinical pathologist atOhio State School of Medicine, explained to BioWorld, "is thatthe switch took place in a part of the gene, the actin-bindingdomain, that's very important for muscle function." He added:"It made more of a statement about the basic biochemicalnature of dystrophin. If you screw up the actin-binding domainof the protein, you've probably done something major."
What dystrophin normally does in the body, he declared, is "amillion-dollar question," which discoveries such as his missenseshould in time help elucidate.
There had not been a known case of DMD in the boy's ancestry,so this single swap of just one of dystrophin's 3,685 aminoacids for another, which diminished his muscle supply ofdystrophin to 20 percent, brought on the disease.
Studies of the affected child's family strongly suggest that themutation originated in his mother. She passed theunprecedented missense gene to her daughter as well as herson. Thus, the boy's sister, though free of symptoms, is a DMDcarrier.
The Ohio State team made the missense discovery using aheteroduplex technique and sequencing PCR-amplifiedproducts to screen its non-deletion/duplication population of105 patients for point mutations.
"If you have a mutation that just shuts the protein down, youdon't learn much about it," Prior said. "But if you have amutation that allows the protein to be made -- as this missensedoes -- and yet the patient still has DMD, you learn a little bitmore about the function of dystrophin."
Since finding the missense mutation last winter, Prior haslocated another 13 gene alterations, most of them nonsense."We've been able to extend carrier diagnostics to probablyseven or eight families now, which would have had to rely onlinkage analysis."
He added, ruefully: "My goal was to find a hot spot for pointmutations so we could help families out. I'm sorry we don'thave that hot spot. So at this particular time, there's not muchwe can do for that 33 percent of DMD patients with newspontaneous point mutations. It's still an individual-by-individual basis."
His point-mutation hunt is funded by the Muscular DystrophyAssociation Inc. of Tucson, Ariz., with a $180,000, three-yeargrant through 1994. The MDA's director of researchdevelopment, Norine Stirpe, told BioWorld that the discoveryreported in the current Nature Genetics, besides helping thatone family screen its female members for carrier status, "helps(us) understand the functions of this huge dystrophin proteinthat are involved in various activities, such as how it attacheswithin the muscle cell."
Stirpe cited theories tackling the million-dollar question of theprotein's role in muscle. They suggest that it is part of a cellularstructure that maintains the integrity of the muscle membrane.She pointed out that "muscle cells and fibers as a whole areunder a lot of stress with the amount of movement they do.The theory is that if dystrophin is missing, as in DMD, itsassociated proteins have nothing to attach to, and themembrane is more susceptible to damage.
The MDA is actively supporting gene therapy research aimed atreplacing the missing dystrophin. "We know," she said, "that ifwe replace it, we'll probably be able to reform the entirestructure that dystrophin is part of, with the associatedproteins and actin.
"But we need to know what parts of the protein are vital forassociating with actin and the other proteins within the cell,"she added. "Prior's paper gives us more data on the functions ofdifferent areas of dystrophin."
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.