The ongoing pandemic of coronavirus disease 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has led to over 119 million confirmed cases and over 2.6 million deaths so far. Even as vaccines are being rolled out in many countries, supply challenges and cold-chain obstacles are likely to slow down the pace of universal immunization, favoring the emergence of novel vaccine-resistant strains.
The ability to detect potentially fatal illness early on in the course of the disease would enable a swift prioritization of these high-risk individuals for treatment. A new preprint on the medRxiv* server presents an array of biomarkers that may help achieve this laudable goal.
Study: Proteome-wide Mendelian randomization identifies causal links between blood proteins and severe COVID-19. Image Credit: NIAID / Flickr
Cytokine storm and other immunologic responses
The researchers exploited the knowledge that much of the disease process in severe COVID-19 is due to a dysregulated immune response, also known as a cytokine storm. Recent work suggests that still more is involved in the abnormal and damaging host response.
On the level of individual genetics, the innate immune host response varies between individuals. The alleles of these genes could therefore be crucial in shaping the course of the disease, since genetic, other host factors, and environmental factors interact to produce the disease outcome.
GWAS a good choice
Since prior habits such as viral exposure, dose of virus inoculum, smoking, obesity or overweight, can lead to varying levels of inflammation in the body, the researchers preferred not to study the expression of these genes in COVID-19 patients themselves, but in healthy subjects.
These have been reported in many genome-wide association studies (GWASs) in the community. The current study used GWAS data from the COVID-19 Host Genetics Initiative on COVID-19 subtypes. They looked at the relationship between blood proteins known to modulate immunological responses and severe COVID-19 infection using Mendelian proteomics.
The advantage of this approach is that the inheritance of these alleles is random, allowing environmental factors to be discounted in the disease outcome.
The researchers looked at approximately 3,800 associations between immunomodulatory blood proteins and severe COVID-19. They found that five blood protein markers were linked to a much higher risk for severe COVID-19, including TNN13, PRTN3, and HLA-DQA2, while two were associated with lower odds – MHCIA and NCR3.
The five high-risk markers were TNNI3 (troponin I3, cardiac type), PRTN3 (proteinase 3), HLA-DQA2 (major histocompatibility complex (MHC), class II, DQ alpha 2), C4a-C4b (the complement components C4a and C4b heterodimer), and LRPAP1 (low-density lipoprotein [LDL] receptor-related protein-associated protein 1).
The magnitude of enhancement of the odds of severe disease in COVID-19 varied from 6.7-fold higher for TNNI3 to 1.7-fold higher for LRPAP1. Put another way, the odds of severe COVID-19 were almost 600% higher if the level of TNN13 was increased by one standard deviation. For PRTN3, it was 150% increased.
Interestingly, none of these are the conventional immunological or inflammatory proteins, such as C-reactive protein (CRP) or interleukin-6 (IL-6), indicating that if even more proteins are explored, more upstream protein markers could be identified.
The 600% increase in the odds of severe COVID-19 with an increase of one standard deviation in TNNI3 led to a closer examination of this biomarker. This protein may affect the contraction of the cardiac muscle. This may have a negative impact on multiple biological functions of the nervous, respiratory and digestive systems.
TNNI3 is a very specific cardiac marker, encoding troponin I. It regulates the sliding of thick and thin sarcomeres in cardiac muscle during contraction.
Higher levels of TNNI3 correlate with worse outcomes in many conditions. In a study from Wuhan, China, about 40% of deaths in a COVID-19 cohort were due to heart muscle damage and cardiac failure. Moreover, patients with COVID-19 who have acute cardiac injury have a higher death rate.
Up to 28% of patients with COVID-19 with high TNNI3 will need intensive care, have higher odds of death in hospital, and will have residual myocardial damage. Many cardiovascular complications have been reported with COVID-19 mortality, and troponin may thus predict mortality odds in COVID-19 independent of inflammation.
TNNI3 is also linked to the increased expression of the virus receptor, angiotensin-converting enzyme 2 (ACE2), in patients with obstructive hypertrophic cardiomyopathy and binds to this receptor.
With HLA-DQA2, C4a-C4b and LRPAP1, the odds of severe disease rose by 130%, 76% and 73%, respectively. C4a-C4b is associated with schizophrenia, which in turn increases the risk for SARS-CoV-2 infection seven-fold, and the risk of death in COVID-19 almost three-fold.
The genetic markers of schizophrenia failed to show any significant link with severe COVID-19, however.
PRTN3 is a serine protease found in neutrophils and is increased almost 30-fold in nasal samples from COVID-19 patients. Neutrophil degranulation is hyperactivated with this infection. Since this molecule is also found to be overexpressed in a dysregulated immune response, in sepsis and acute kidney injury, it is worth exploring the utility of targeting this enzyme to modulate immune phenomena in COVID-19.
The low-risk markers, with the odds of severe COVID-19 reduced by 40% and 50%, respectively, were MHCIA (MHC class I polypeptide-related sequence A) and NCR3 (natural cytotoxicity triggering receptor 3).
HLA proteins are key to coordinating the immune responses to antigenic stimuli and thus engaging cellular and humoral immunity. These molecules help the body distinguish self- from non-self-proteins so as to mount an immune response only when it is appropriate.
MHCIA is expressed on epithelial and dendritic cells that present antigens to other innate immune cells. Its expression rises in situations of cellular stress, such as viral infection.
MHCIA binds to and activates natural killer cells of the subtype NKG2D and other T cells that are involved in the antiviral response. Since NKG2D cell frequencies are lower in severe COVID-19, the study suggests that “a genetic propensity for higher blood MHC1A may protect against severe COVID-19, potentially through activating cytolytic cells.”
NCR3 is also involved in NK activation in response to the viral antigen presented by dendritic cells, and is upregulated in response to interferon-gamma. It is also implicated in antiviral responses, and a genetic predisposition for higher NCR3 levels may protect against severe COVID-19.
What are the implications?
Our results highlight the utility of applying large scale Mendelian randomization analyses to identify blood markers that may be causal for severe COVID-19.”
The study shows the increased risk associated with several upstream blood proteins and two protective markers.
The high-risk markers are part of the major histocompatibility complex, or implicated in contraction of cardiac myocytes, or in the innate immune system. Their identification may help predict those who are at risk of the worst outcome.
The findings also open up avenues for therapeutic interventions to possibly prevent disease progression.
Levosimendan is a positive inotrope, which binds to a myocardial complex of cardiac troponin C and troponin I. This binding leads to an ATP-dependent influx of potassium along with increased sensitivity to the effects of calcium ions.
This drug is currently approved for use in cardiac and kidney failure and for SARS. The researchers suggest it may have potential in treating severe COVID-19 as well and recommend further study of this application.
Similar interventions may be evaluated for each of the high-risk markers, helping to manage such patients better.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
- Palmos, A. B et al. (2021). Proteome-wide Mendelian randomization identifies causal links between blood proteins and severe COVID-19. medRxiv preprint. doi: https://doi.org/10.1101/2021.03.09.21253206, https://www.medrxiv.org/content/10.1101/2021.03.09.21253206v1