High frequency of rare genetic diseases found in Quebec's Saguenay-Lac-St-Jean

Quebec's Saguenay-Lac-St-Jean region is known for its spectacular fjord and rugged outdoor landscapes. But its isolation has also made the region north of Quebec City home to something else: a higher than normal presence of more than two dozen rare genetic diseases, including some whose frequency in the region have recently been measured for the first time.

A team of medical and genetics specialists associated with the Saguenay-Lac-St-Jean health authority recently added 11 rare diseases or conditions to the list of 14 that had already been identified as occurring more frequently in the region than elsewhere in the world.

Those added to the list include five conditions that could be potentially lethal, three that are associated with intellectual deficiency and another associated with hearing loss.

Their findings were published Feb. 14 in the American Journal of Medical Genetics, under the title, "Portrait of autosomal recessive diseases in the French-Canadian founder population of Saguenay-Lac-Saint-Jean."

Dr. Tania Cruz Marino, the first author listed on the study, said the idea for the research came after doctors in the region started seeing cases of extremely rare diseases cropping up in people from separate families.

"Why do we see such a rare variant for a disease that's almost unknown in the world, repeating itself in people that aren't related?" she said in an interview.

But the results of the study, which was a collaboration between medical professionals in a clinical setting and researchers analyzing years worth of genetic data, surprised even the researchers.

Cruz Marino said one disease — Oculogastrointestinal neurodevelopmental syndrome — is so rare that only 10 people in the world have it, and five of them are in Saguenay-Lac-St-Jean.

Luigi Bouchard, also one of the study's authors and professor in biochemistry and genomics at Université de Sherbrooke, said the carrier frequency for the various diseases documented in the study ranged from one in 15 people to one in 124.

That number refers to people who carry the defective genes that cause illness, but who are not necessarily sick themselves.

"In all cases, it's higher than what we could expect in the rest of the world," he said.

The higher frequency of these rare diseases is the result of a "genetic lottery," Bouchard said.

The French-speaking Saguenay-Lac-St-Jean population was fairly isolated until recent decades, had a high birthrate and descended from a small pool of common ancestors, which means any hereditary diseases they carried would tend to occur more frequently in subsequent generations, he said.

This "founder effect" means that some genetic diseases that are extremely rare elsewhere are more common in the region, while others may be completely absent, he said.

All of the diseases are recessive — both parents have to be carriers of the defective gene in order for their offspring to have a chance of developing the condition, Bouchard said. A child with two carrier parents has only a 25 per cent chance of developing a disease, meaning that diagnoses are extremely rare even in the region.

Cruz Marino and Bouchard say that while the results may sound scary for some people, their research could have a positive contribution to the screening, identification and treatment of rare diseases in Saguenay-Lac-St-Jean.

In particular, they're calling on the province's Health Department to expand a program that offers free genetic screening tests to would-be parents with at least one biological grandparent born in the Saguenay—Lac-Saint-Jean, Charlevoix or Haute-Côte-Nord regions.

The program currently screens for four diseases that were previously identified, but Bouchard and Cruz Marino say they think between five and eight more should now be added to the list. They also believe newborns in the region should be given hearing tests early on to screen for the condition causing hearing loss, which has a carrier frequency of one out of 32 people.

They're hoping the study will raise awareness of the importance for couples to get screened — ideally before they get pregnant — because a healthy person has no way of knowing whether they're a carrier.

The study included 82 French-Canadian individuals affected with genetic disorders, as well as 1,059 people who were healthy but who accessed the screening program to see if they were carriers.

This report by The Canadian Press was first published Feb. 28, 2023.