Current observations suggest that the coronavirus SARS-CoV-2 causes severe symptoms mainly in elderly patients with chronic disease. However, when two pairs of previously healthy young brothers from two families required mechanical ventilation at the intensive care unit in rapid succession, doctors and researchers at Radboud University Medical Center were inclined to consider that genetic factors had a key role in compromising their immune system.
Their research identified the gene TLR7 as an essential player in the immune response against SARS-CoV-2. A finding with potentially major consequences for understanding and possibly treatment of COVID-19.
During the wave of COVID-19 patients that flooded Dutch hospitals in the first half of 2020, two young brothers became seriously ill with the SARS-CoV-2 virus and had to be mechanically ventilated in the ICU. One of them died from the consequences of the infection, the other recovered. The severe course of disease in otherwise healthy young brothers was a relatively rare occurrence, especially because the virus mainly affects the elderly. This observation triggered the curiosity of an attentive physician from the MUMC+ department of clinical genetics. She contacted her colleagues in Nijmegen who then investigated why these two young brothers were so severely affected.
Genetic factors
“In such a case, you immediately wonder whether genetic factors could play a role,” says geneticist Alexander Hoischen. “Getting sick from an infection is always an interplay between – in this case – the virus and the human immune system. It may be a mere coincidence that two brothers from the same family become so severely ill. But it is also possible that an inborn error of the immune system has played an important role. We investigated this possibility, together with our multidisciplinary team at Radboudumc.”
One X-chromosome
All genes (collectively called the exome) of both brothers were sequenced, after which the investigators combed through the data searching for a possible shared cause. Cas van der Made, PhD student and resident at the department of Internal Medicine: “We mainly looked at genes that play a role in the immune system. We know that several of these genes are located on the X-chromosome, and with two brother pairs affected X-chromosomal genes were most suspicious. Women carry two X-chromosomes, while men possess a Y-chromosome apart from the X. Therefore, men have only one copy of the X-chromosomal genes. In case men have a defect in such a gene, there is no second gene that can take over that role, as in women.”
Gene identification
That search quickly revealed mutations in the gene encoding for the Toll-like receptor 7, TLR7 for short. There are multiple TLR-genes, which belong to a family of receptors with an important role in the recognition of pathogens (such as bacteria and viruses) and the activation of the immune system.
