A baby boy born with markedly outsized calves on his legs mayexcite pride in his papa, who envisions having sired a future track staror wrestler.
In fact, the true object of this parent's admiration is not a pair of well-developed gastrocnemius (calf) muscles, but the actual replacementof that musculature by fat deposits and connective tissue.
This deception is a harbinger of impending Duchenne's musculardystrophy (DMD). The overt disease usually makes its appearancearound the child's third year of life, when he starts to stumble, has ahard time climbing stairs, tires easily and waddles when he walks _which he may do on tip-toes.
These early disabilities of DMD move up gradually from lower legmuscles to thighs to diaphragm to heart and respiratory muscles. Byage 12, a DMD boy is usually confined to a wheelchair. By 20 or so,death intervenes, from heart or lung failure.
DMD's fatal, progressive muscle wastage arises from an inheritedlack of dystrophin, a protein crucially involved in stabilizing amuscle cell's membrane during muscular contraction.
At 2.7 million base pairs in length, the dystrophin gene on the humanX chromosome is the largest disease-related gene known in man. Thisvast dimension exposes its DNA as a huge target of opportunity forany number of sporadic mutation hits.
Because females possess two X chromosomes, girls almost neverincur DMD; like a spare tire, their second X takes over from itsdefective counterpart. But by this token, mothers are carriers of thesex-linked, recessive disease, which they may pass on to a son.
The disease strikes one in 3,500 live male births. Some 1,100 newcases a year occur in the U.S, one-third of them from new mutations.
A faithful animal model of dystrophin-lacking myopathy is the mdxmouse. After years of experimenting on this DMD-mimicking rodent,neuroscientists and molecular geneticists have in the 1990s attemptedtherapeutic trials in Duchenne patients.
Molecular geneticist Kay Davies at Oxford University, a pioneer inthe field, told BioWorld Today: "A lot of investigators triedtransplanting myoblasts _ functioning muscle cells containingdystrophin _ into the muscles of DMD boys, with minimalefficiency. One problem," she added, "is being able to deliver thecells to all muscles in the patient's body, which for DMD is an almostimpossible challenge."
An alternative strategy being widely tried is gene therapy. "JimWilson's group [at the University of Pennsylvania] in particular,"Davies said, "has been introducing dystrophin genes into adenoviralvectors, and demonstrated some efficiency. Here, the major hurdle is:How can you avoid an immune response? "
In the current issue of Nature, dated Nov. 28, Davies reportscircumventing both of these roadblocks to DMD therapy, but so faronly in the animal model. Her paper bears the title: "Amelioration ofthe dystrophic phenotype of mdx mice using a truncated utrophintransgene."
In 1989, Davies and her co-authors discovered a one-million-base-pair gene on human chromosome 6, and named it "utrophin."
"In normal muscle," she said, "utrophin acts to stabilize the structuresthat allow transmission of signals from nerves to muscles."
But what led to her novel therapeutic strategy was the fact that "earlyin human fetal life, utrophin and dystrophin both are found at themuscle membrane, and work hand in hand. That gave us a clue thatmaybe it would be possible to replace the one with the other."
A Serendipitous DMD Patient _ With A Difference
"In 1990," Davies recounted, "we discovered a man who was verymildly affected by muscular dystrophy. In fact, he didn't show anysymptoms until his 40s. He's still alive today, in his 70s."
That man's dystrophin gene turned out to have a large portionmissing in the middle. Davies' group designed an equivalenttruncated minigene for utrophin by removing 40 percent of its DNAsequence.
"We got both business ends of the molecule," she said; "one thatinteracts with the internal part of the muscle cell, and one thatinteracts with the proteins at the membranes."
Hooking the gene up to a customized promoter, the team injected thispackage into early mdx mouse embryos, "so that utrophin wasexpressed at high levels in muscles.
In the transgenic progeny, "the muscle and its condition," she added,"look as if the utrophin compensates extremely well for the missingdystrophin _ in fact, spectacularly well."
She noted that "because utrophin is expressed ubiquitously in thebody, the immune system is accustomed to it, whereas a the majorityof DMD patients have never seen dystrophin.
"Having proved the hypothesis," Davies said, "we now need toscreen for compounds in the conventional pharmaceutical sense, toupregulate utrophin gene expression in patients. So we'recollaborating with Oncogene Science Inc. [of Uniondale, N.Y.] toscreen hundreds and thousands of small-molecule compounds, usingrobotics, in order to find one that might be able to do the trick."
Oncogene Takes On Small-Molecule Drug Screening
Britain's Medical Research Council has worldwide patents pendingon Davies' minigene and promoter. Oncogene is discussing with theCouncil licensing one or more of these inventions.
The company's vice president of drug discovery, Arthur Bruskin, toldBioWorld Today: "Using the information that comes from KayDavies' lab . . . and our automated transcriptional-based drugdiscovery technology, we will look at a promoter-reporter constructin a muscle cell line for compounds that increase the expression ofthe reporter gene, luciferase, attached to the utrophin promoter."
"Our objective," Bruskin said, "is to identify an orally activecomponent that upregulates utrophin in muscle cells."
But he emphasized that "this is a very early-stage program; wehaven't really started screening yet." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.