Corona: let's go digital

How is it possible that people who contract a serious lung disease such as COVID-19 quickly develop other problems outside the lungs? Why do people die from serious complications and do doctors see damage in many different organs upon autopsy?

A more comprehensive answer now seems to be coming from an American group of researchers (Sydney Stein and colleagues; the results are described in a preprint). They performed an extensive autopsy on 44 people who died from COVID-19. These individuals all had symptoms of so-called 'long COVID', the serious physical symptoms that occur and last for a long time as a result of the infection with the corona virus.

To obtain their results, the researchers performed a number of techniques, including a combination of digital PCR and in situ hybridization and immunohistochemistry. To put it simply, they used a PCR method that detects individual DNA molecules (PCR is also done in testing to see if you are infected or not, but that is a different form of PCR: Q-PCR) . The result you get from this method is not a curve based on a number of points, but a simple yes/no answer; hence the name digital PCR – think of the 0 or 1 situation in the digital world. The method is extremely powerful, because you can, for example, pick up 1 particle of HIV - the virus that causes AIDS - from a whole blood sample, as was already established in 2012.

After Corona particles were identified by digital PCR, in situ hybridization and immunohistochemistry were done for confirmation. In situ hybridization is a method in which you detect DNA or RNA in place (hence 'in situ') in the tissue. This is done by binding a piece of DNA or RNA that carries a dye. If the DNA or RNA is present, you will see staining. Immunohistochemistry works differently: with this method you detect specific proteins in the tissue with antibodies. These antibodies are subsequently stained. Thus, in a positive immunohistochemistry stain, a particular molecule, such as the virus 'spike', is present. The in situ hybridization reveals the viral RNA encoding the spike.

Another nice example of applied technology: single copy sequencing was used in the study. This allows you to show whether the DNA of the virus is the same or different in certain tissues or whether there is 'heterogeneity' within a certain tissue. You look at a piece of RNA of the virus of which only 1 so-called copy is present, in this case the RNA coding for the spike. It turned out that other variants of the virus were present in the brain compared with the lungs or other organs. They may have originated there.

The results showed that the patients had virus particles in virtually all tissues in the body. The virus had also managed to cross the so-called 'blood-brain' barrier and settle in brain cells. A few numbers to show how convincing the data seem to be: 43 of 44 patients had the virus in respiratory tissue at death; in cardiovascular tissue it occurred in 35 out of 44 patients; in the genitals it was detected in 17 of 40 patients and in muscle, skin, fat or nervous tissue the virus was detected in 30 of 44 patients. The gastrointestinal tract did not remain virus-free either: the virus was detected there in 32 of the 44 patients. In 11 patients, the brain tissue was investigated thoroughly: in 10 of them the virus was detected in this tissue. All in all, this indicates that the virus is certainly not limited to the lungs and is found throughout the body.

The researchers indicate that this is not the first study of the presence of the virus outside the lungs, but it is the most comprehensive. They explain the long-term presence of the virus in tissues outside the lungs due to a lower immune system activity in these tissues. In the lungs, invaders are eliminated faster, something we would definitely like to see throughout the whole body to counteract the serious effects of lung COVID.