As a newly emerging virus, all people are susceptible to SARS-CoV-2 infection, though the nature and severity of COVID-19 vary significantly among cases. Notably, reported disease burden and case fatality rates differ considerably from one country to another (36). The exact influence of host genetic makeup in this variation has remained mostly unknown. The importance of host genetics' contribution in differential responses to SARS-CoV-2 is highlighted by a modeling study, revealing that 50% of the variance of the 'predicted COVID-19' phenotype is due to genetic factors (6). For example, studies reported that the variable expression pattern and genetic variation in angiotensin-converting enzyme 2 (ACE2), as a functional receptor for SARS-CoV-2, might be associated with the susceptibility to infection and severity of the disease (37). Notably, a recent genome-wide association study revealed that critical illness in COVID-19 is related to host antiviral defense mechanisms (IFNAR2 and OAS genes) and mediators of inflammatory organ damage (DPP9, TYK2, and CCR2) (38). Thus, genes related to immune responses are of particular interest for our understanding of predisposition to severe COVID-19 because of the immunopathology of SARS-CoV-2.
The EC system has received much attention due to its regulatory roles in immune response and its effects on immune-associated disease progression (39). There is evidence supporting the system's specific involvement in respiratory viral associated immunopathology and in modulating inflammation following infection. We previously provided evidence that the EC system plays an essential role during respiratory syncytial virus (RSV) infection in humans and mice (17). Other studies also showed that the lack of cannabinoid receptors could increase inflammation and tissue damage following influenza virus infection, and their activation can impair immune responses induced by the virus (40–42). Such data supported the idea of using cannabinoids as a potential therapeutic approach in COVID-19 patients (16, 43). The rationale behind the current study was based on SARS-CoV-2 immunopathology, together with the data available on the immune regulatory role of CB2 signaling.
The CB2-Q63R polymorphism is caused by two missense mutations in the CNR2 gene changing CAA to CGG, which lead to a substitution of an uncharged polar amino acid (glutamine) with a positively charged polar amino acid (arginine) at position 63 located at the first intracellular loop of the CB2 receptor (24). Studies indicated that the CB2-63R variant has less functions than 63Q in the modulation of immune responses, especially T-cell proliferation (25). While the exact mechanism is unknown, a study reported that the signal intensity caused by 63R activation is relatively weaker than that caused by 63Q activation (44). The present study reports for the first time, the association between the CB2 receptor and COVID-19 severity. A significant difference in the Q63R allele and genotype distributions was found between COVID-19 expired and discharged patients (Table 2). The co-dominant, recessive, and additive inheritance models showed a significant association between Q63R and COVID-19 severity (Table 3). According to the co-dominant model, RR subjects showed a risk for developing severe COVID-19 more than thrice of QQ subjects.
The association between the CB2-Q63R variation and autoimmune conditions such as thrombocytopenic purpura (45), celiac disease (23), juvenile idiopathic arthritis (19), inflammatory bowel disease (46), and rheumatoid arthritis (22) has been reported. The data reported by the current authors have been implied the involvement of the Q63R variation in susceptibility to multiple sclerosis in Iranian patients (18). Interestingly, our previous study showed that the inflammatory response to virus is more inhibited in cases with QQ variants, allowing the virus to replicate and induce severe infection (17). In the case of SARS-CoV-2 infection, while a robust innate immune response is essential to eliminate viral pathogens, a prolonged or dysregulated/exuberant manner can damage the respiratory tract (47). The current results are consistent with previous studies that showed a reduced EC-induced modulation of the immune system in human subjects carrying the RR variant of CB2 compared with those having the QQ variant (2, 19, 23, 45, 46).
Importantly, experimental data from previous competition ligand binding assay showed that the binding affinity of 2-AG and CB2-63R is similar to CB2-Q63, but CB2-63R had a significantly lower maximum response after binding to 2-AG compared with Q63 type (24). Our molecular docking results confirm that the CB2-63R induces a more inadequate response to ligand binding. The biological effects of cannabinoids are mediated through the activation of G-protein-coupled cannabinoid receptors (48). G-proteins act as adaptors that link G-protein-coupled receptors (GPCRs) to other signaling and regulatory proteins to operate or modulate intracellular signaling pathways (15). The molecular docking results showed that the predicted structures of mutant CB2 could not bind to G-protein in the correct position, resulting in EC signaling dysfunction. This data is consistent with Wang et al., finding that CB2-63R induces lower signaling transduction than CB2-63Q in human primary T-cells (44). The CB2 receptor is predominantly expressed in the immune and immune-derived cells, and its activation indirectly affects viral infections by altering host immune responses, particularly inflammation, along different signaling pathways (49, 50).