A 15-year effort has led to a gene discovery that could one day wipe out an unusual "slow motion" disease known to affect only certain people in Quebec.

The deadly ailment, formally known as the French-Canadian type of Leigh's syndrome, springs from a genetic defect passed down to the people of the Saguenay region from the 21 families who first settled the area in the 1840s.

Yet, interestingly, descendants of these founding families did not start developing the disease in noticeable numbers until the late 1980s, said Brian Robinson, head of the metabolism program at Toronto's Hospital for Sick Children.

That is because it took more than a century for the mutated gene, which is not prevalent among people anywhere else on the planet, to spread widely through the population.

Those afflicted fall ill as babies. They suffer brain lesions and cellular defects that leave their bodies starving for energy. They can be left walking and talking at molasses-like speeds; they can be mentally slow. Sometimes a toxic overload of lactic acid has deadly results. Often the neurometabolic disorder kills them as toddlers.

But finding the culprit gene -- a hunt that researchers from Canada and the U.S. pursued -- means people in the region can now undergo screening to find out if they are carriers.

Carrying one copy of the mutated gene has no ill effects. But children who inherit two copies, one from each parent, all develop the incurable disease. Parents will also be able to learn through prenatal tests conducted at 10 weeks whether the fetus has inherited the disorder, which could lead them to terminate the pregnancy.

One parent who lost two children to the disease said that psychologically, it may be easier to end a pregnancy than to lose a child to this disorder.

Tom Hudson, director of the Montreal Genome Centre at McGill University and co-author of the report, published today in the Proceedings of the National Academy of Sciences, said: "The idea is that people will have the information they need to make informed choices."

Quebec has beaten back genetic disorders before. A screening program that began in Montreal-area high schools in the 1970s has reduced the incidence of two by 95 per cent.

Young people who learned they carried genes for the blood disorder thalassemia, which can be found in people of Mediterranean descent, or Tay-Sach's, a devastating neurodegenerative disease prevalent among Jews, either chose not to marry another carrier or underwent prenatal testing if they did.

Dr. Hudson, who credited affected parents for organizing the research efforts, said plans are now under way to offer wide screening for the French-Canadian type of Leigh's syndrome in the Saguenay, where the disease is commonly known as lactic acidosis.

Most of the 300,000 people in Saguenay-Lac-Saint-Jean and Charlevois, 250 kilometres north of Quebec City, descended from a small founding population. As a result, they have higher rates of five specific inherited disorders considered rare in most other places. With this report, genes for all five conditions have been identified.

Local pediatrician Charles Morin took note of children coming down with the perplexing slow-energy disorder in the 1980s. Tests showed no sign of an infection, and the children were sent to St. Justine Hospital in Montreal for further tests.

Their blood was then shipped to Dr. Robinson at Sick Children's. "You could see something was wrong in the mitochondria of their cells" where food is converted to energy, Dr. Robinson said.

Researchers eventually learned that their cells lacked an enzyme necessary to turn oxygen into water. That function is controlled by a gene on Chromosome 2 that helps regulate mitochondria and that researchers at McGill and the Whitehead Institute at the Massachusetts Institute of Technology have now pinpointed.

Dr. Robinson said the disorder likely began as a sporadic mutation in at least one person who was a member of the 21 families who travelled north of Quebec City in 1840 to find fertile farmland.

For years, he said, doctors may have mistaken the occasional child who suffered from the disease as having some other malady. The disorder did not receive its formal name as a syndrome until 1993.

Unlike the classic form of Leigh's syndrome, the type that strikes the people of the Saguenay -- where one in every 23 people is a carrier -- develops more slowly. A few children there, for example, have survived well into their teens.