Isolated communities are more at risk of rare genetic diseases
- The isolation can be geographic or cultural
ISLAND LIFE is famously idyllic, but it’s long been known that islanders tend to experience disproportionately high rates of some rare genetically transmitted diseases. Faroe islanders, for example, who live on an archipelago in the North Atlantic Ocean, have a much higher-than-average incidence of carnitine transporter deficiency (CTD), a condition that prevents the body from using certain fats for energy. Inhabitants of Gran Canaria, meanwhile, an island off the north-western coast of Africa, are far more likely than average to have familial hypercholesterolaemia, a condition where the liver cannot process cholesterol effectively.
A new paper in Nature Communications provides one more such example. Jim Flett Wilson from the University of Edinburgh, who led the study, reports that people living on the Shetland Islands in northern Scotland have a one-in-41 chance of carrying the gene variant which causes Batten disease, a life-limiting neurodegenerative disease. The comparable rate elsewhere in Britain is one in 300, says Dr Wilson.
Such elevated risk is likely to be the consequence of genetic isolation. When members of a small population overwhelmingly reproduce with their fellows, the probability of children acquiring disease-causing mutations (known as variants) from their parents increases over time. This happens because of a process known as random genetic drift, says Dr Wilson, whereby some genetic variants become more common and others are lost. “This effect is magnified in small populations with little or no inward movement of new people to replenish the genetic pool,” he says.
Such isolation need not only be the product of encircling water. Dr Wilson’s new study also found “genetic islands” on the British mainland. In Lancashire, for example, the researchers found locals were more likely to have ten disease-causing variants—including one associated with Zellweger syndrome, a disease affecting the brain, liver and kidney which can be fatal in the first year of life. Those from the area were 73 times more likely to have the variant. In South Wales, one variant responsible for an inherited predisposition to develop kidney stones later in life was 44 times more common, whereas in Nottinghamshire a variant causing a severe blistering skin disorder was 65 times more common than elsewhere.
Such genetic islands can arise from geography and culture, says Dr Wilson, including a widely shared preference for individuals to pick spouses from the same community they grew up in. Some such islands are already monitored by health authorities. The NHS, for example, runs screening programmes for those of Ashkenazi Jewish descent, as around one in 40 Ashkenazi Jewish people carries harmful variants to the BRCA gene which make them more at risk of breast or ovarian cancer. This compares with around one in 260 people in the general British population.
The incidence of Batten-disease carriers among Shetland islanders is similar to that of the BRCA variant among Jews, says Dr Wilson, and yet no plans exist for a screening programme there. He says that the reliance on the “cascade” model, whereby people are offered testing only after a family member is diagnosed, is only half as reliable at picking up cases as universal testing on demand. Until such screening programmes are put in place, islanders risk being doubly isolated.
© 2024, The Economist Newspaper Ltd. All rights reserved. From The Economist, published under licence. The original content can be found on www.economist.com
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First Published:5 Dec 2024, 08:05 PM IST
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