How bad Covid-19 symptoms get could go back to a gene from a Neanderthal in Croatia.
About 50% of the people in South Asia carry it, a study by researchers at the
Max Planck Institute for Evolutionary Anthropology in Germany has found.
While age, gender and co-morbidities were established as risk factors for being infected, they didn't entirely explain why some people got more sick than others. "Thus, genetic risk factors may have a role in disease progression," said the paper, published by 'Nature'. One genomic region (where a gene is located on a chromosome) on chromosome 3, rs35044562, is "significantly associated" with severe Covid-19, a recent study had found. Associated with that is increased risk of respiratory failure and odds of hospitalisation went up 1.6 times for those who carried it. "Although there are other genetic variants, this is the most important one," lead author
Hugo Zeberg told TOI.
Zeberg and his colleague at the Max Planck Institute, Svante Paabo, found this genomic segment, which increased the risk of severe symptoms, was also present in the genome of a 50,000-year-old Neanderthal from Croatia. Then they went over data from the 1000 Genomes Project, an international research project to map genetic diversity.
"In terms of carrier frequencies, 50% of people in South Asia carry at least one copy of the risk haplotype (group of genes from a single parent), whereas 16% of people in Europe and 9% of admixed Americans carry at least one copy of the risk haplotype. The highest carrier frequency occurs in Bangladesh, where more than half the population (63%) carries at least one copy of the Neanderthal risk haplotype," the paper said.
But while the risk is this high in south Asia, it is nearly absent in east Asia. "This extent of difference ... between south and east Asia is unusual and indicates that it may have been affected by selection in the past," the paper said, meaning the genomic segment may have been useful at some point. "The variant is a little bit too frequent in, for example, India for chance alone. It is likely that it has been beneficial in the past and been selected for.
One gene in the region is important for cholera. Could be that this variant protects against cholera, but this is a speculation at this point," Zeberg said. At the same time, negative selection may have reduced the risk variant's frequency in east Asia, "perhaps because of coronaviruses or other pathogens," the paper said.
Does this translate into a greater risk of fatality? Not directly. It makes the cases more severe but when it comes to death rates, there are other factors at work too. "Median age is probably the most important risk factor. Many European countries have a median age 10-20 years above India," Zeberg explained.