When muscle cells mutiny

The motor neurone disease afflicts muscles but leaves the brain alone

 
Published: Thursday 15 September 1994

-- BRITISH physicist Stephen Hawking, arguably one of the brightest minds of the century and author of the bestseller A Brief History of Time, has been described as a "cerebral man". But his is no mind-over-matter story, nor is he the proverbial Jack-of-all-work-and-no-play. He is a victim of a debilitating affliction called motor neurone disease (MND), which destroys the brawn but leaves the brain ticking normally.

MND -- boffins prefer to call it amyotrophic lateral sclerosis -- is all about the rebellion of muscle-controlling cells against the brain. From sheer laziness, these cells degenerate with time, causing the sufferer great pain and anguish: the victim loses control over the limbs, cannot speak, breathe or swallow easily, and often goes into a muscular spasm.

The majority of MND victims are men between the ages of 40 and 70. But what triggers MND, which grips 1 in 10,000 Indians, is not yet known and this hampers its treatment.

Now, Mark Gurney and his colleagues at the Northwestern University Medical School, Chicago, USA, have linked MND to a defective gene that carries the code for an enzyme called superoxide dismutase (SOD). This enzyme converts superoxide, a harmful form of oxygen, in the cells into a relatively harmless hydrogen peroxide and normal oxygen. The defective gene, conjecture scientists, allows the toxic form of oxygen to accumulate, wreaking havoc on the nerve cells (Science, Vol 264, No 5166).

Gurney's group genetically engineered 2 sets of mice, producing different mutant forms of SOD. The researchers discovered that mice that produced the largest amounts of mutated SOD fell prey to MND, while the less prolific ones didn't. Moreover, they found that mice containing the hyperactive SOD suffered a severe loss of choline acyltransferase -- a key enzyme asociated with spinal cord motor neurons.

Meanwhile, Scottish researchers have reported in the British Medical Journal that they spotted a suspicious-looking virus, known as Coxsackie B virus, in the spinal cords of a number of MND patients. The virus resembles the polio virus and scientists haven't ruled it out as a possible offender. Even diet may determine the occurrence of MND. Scientists have found an unusually high incidence of MND on the island of Guam, in Western New Guinea, and on the Kii peninsula of Japan, which they link to diet.

A team of French scientists headed by V Meininger have reported a drug which is still under clinical trials against MND (New England Journal of Medicine, Vol 330, No 9). Called riluzole, the drug was designed based on the hypothesis that in MND the neurotransmitter chemical, glutamate, accumulates to toxic concentrations, causing the neurons to die. Other hypotheses about the disease are being worked upon to develop drugs that would at least slow down the march of MND, if not eliminate it.

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