Sonia Gobeil can trace her ancestors to the pioneers who first settled the isolated Saguenay region northeast of Quebec City, but she had never heard of the devastating brain disease they passed on through their genes until her son was diagnosed with it.

The disease, an inherited form of ataxia, affects the part of the brain that co-ordinates movement. It can skip generations and miss entire branches of family trees, only to pop up according to the laws of genetics and chance, leaving most patients in a wheelchair by the time they are 40. It is more common in the Saguenay and Charlevoix regions of Quebec than anywhere else in the world.

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The history of the disease there begins with a small population of immigrants from France who first settled the Charlevoix region in the 17th century. Researchers suspect that several of those people carried a copy of a rare genetic mutation that on its own is benign, but can wreak havoc in the brains of those unlucky enough to inherit two copies – one from each parent.

The population grew and, 100 years later, the migrants pushed the government to open the territory around the Saguenay River for settlement. There, they carved out a society in the forested wilderness, largely cut off from the rest of the province and generating their own distinctive genetic map. For generations, families were large – 11 children a household on average in the early 1900s – and marriages outside the Catholic faith were discouraged by the church. The Saguenay also got few newcomers. The first passable road to Quebec City, 200 kilometres away, was opened in 1951.

Today, the Saguenay remains strikingly homogeneous: 98 per cent of its 273,000 residents are francophone Quebeckers, and immigrants make up less than 1 per cent of the population.

The remoteness has produced a form of proud self-reliance, a trait that’s now being put to the service of probing the region’s genetic history and dealing with its legacy. In addition to autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS), which Ms. Gobeil’s two sons have, three other genetic diseases are much more common in the region than elsewhere in the world. One causes a buildup of lactic acid in blood and can be deadly; another is an enzyme deficiency that can lead to liver and kidney failure. The third causes a birth defect in which the structure that connects the two hemispheres of the brain is missing or only partially formed. Since 2010, couples have had access to free genetic screening. About 1,800 people have been screened so far for the diseases, which affect one in five in the region. A local group goes into elementary and high schools to teach children about hereditary diseases, using simple devices like beaded necklaces to explain DNA and genetics.

Ms. Gobeil’s first born, François, was three when his pre-school teachers noticed that his gait was awkward when he ran. She and husband Jean Groleau didn’t think the problem was serious. They were shocked when doctors diagnosed the boy, now nine, with ARSACS. Their second son, Laurent, who is now six, also has the disease. François can’t skate or play hockey, which he loves. Both boys have a stiff left leg.

After François was diagnosed, the couple learned there was no treatment and no research under way to find one.

“Knowing there was no research being done was like a punch in the stomach. But at the same time, we saw it as an open door.” The condition is hereditary; we can’t change that they have it. But with research, we can do something about it and try to change the path of the future,” said Ms. Gobeil, a lawyer in Montreal.

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In Saguenay, a family takes on a disease that haunts the region