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'Extreme Inbreeding' Identified in Small Subset of UK Biobank Participants

NEW YORK – By digging into data from the UK Biobank project, Australian researchers have identified a subset of participants with inbreeding in their recent ancestry.

For the study, published online today in Nature Communications, the team tapped into SNP profiles produced for the large population-based study of individuals with European ancestry to search for telltale runs of homozygosity (ROH) stretching more than 1.5 million bases apiece. These ROHs typically stem from mating between first-degree relatives (parents and offspring or full siblings) or second-degree relatives (half siblings, grandparents and grandchildren, and so on), the authors explained.

Though this "extreme inbreeding" was rare, it turned up in roughly one in every 3,650 — or around 0.03 percent — of the participants profiled, providing an opportunity for researchers to look for potential associations with traits such as stature or cognitive ability.

"In most human societies, there are taboos and laws banning mating between first- and second-degree relative, but actual prevalence and effects on health and fitness are poorly quantified," co-senior and co-corresponding author Peter Visscher, a researcher at the Institute for Molecular Bioscience at the University of Queensland, and his colleagues wrote.

Using pedigree inbreeding prediction guidelines developed by the American College of Medical Genetics and Genomics, the researchers searched for extreme inbreeding in 456,414 consented individuals with European ancestry for whom they had 301,412 quality-controlled SNPs available.

Based on the presence of autosomal ROHs extending beyond 1.5 megabases, the team identified 60 females and 65 males predicted to have been born to first- or second-degree relatives. The search led to 115 such individuals when the group used a stricter definition of inbreeding, based on even longer ROHs.

The authors noted that those estimates were in the ballpark — or, at least, within the same order of magnitude — as incest offences reported in England and Wales between 2002 and 2017, though most UK Biobank participants were born far earlier. Based on those patterns, they suggested that "prevalence of [extreme inbreeding] is relatively unchanged over time" in this region, "although mean inbreeding coefficients have significantly decreased over the years."

With the data on hand, the researchers looked at everything from the predicted relationships between the inbred individuals' parents to the distribution of the ROHs found in individuals with a background of extreme inbreeding.

The investigators were also able to start delving into traits or conditions that might correspond with inbreeding between first- or second-degree relatives using phenotypic data collected for the UK Biobank effort. There, they saw hints that extreme inbreeding may affect a wide range of traits — from height and strength to lung function, years of education, and broad disease susceptibility.

Based on the available data, though, the authors noted that "we found no evidence that [extreme inbreeding] is more prevalent in health-deprived families nor that low education contributes to increase the likelihood of [extreme inbreeding] in the population."

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