4 DISCUSSION
In this study, to the best of our knowledge, we used data from the largest GWASs of serum sex hormones and COVID-19 outcomes to date, as well as the MR analysis technique, to systematically investigate the potential sex-specific causal associations of serum sex hormones with COVID-19 susceptibility and severity. Androgen signaling inhibition has been debated regarding its potential to reduce the susceptibility and severity of COVID-19 in men; however, accumulating evidence does not support this potential 12, which is consistent with the MR analysis results in our study. Interestingly, our results suggest that serum estradiol and bioavailable testosterone may serve as potential intervention or therapeutic targets for preventing SARS-CoV-2 infection in women.
The role of sex hormones in modulating COVID-19 susceptibility and severity has generated intense research interest given that sex-specific differences in these COVID-19 outcomes have been reported globally31-33. However, inconsistent conclusions have been drawn from various studies. For testosterone, it has been reported that lower levels of testosterone are associated with a higher risk of developing severe COVID-19 disease in men 11,34,35. Moreover, men who had severe COVID-19 or needed intensive care have higher total testosterone levels than the mild COVID-19 or the control groups 36. In contrast, estradiol levels were elevated in men with severe COVID-19 compared to the controls, but no significant difference was observed in women with critical COVID-1937.
Interestingly, the findings of a previous MR analysis study did not support the causal effects of estradiol levels in men and women on COVID-19 susceptibility and severity 38, where a total of 10 SNPs were selected to genetically predict the estradiol levels in both men and women. In our study, we conducted a stringent strategy and LD clumping procedure for genetic instruments selection. Finally, two genome-wide significant SNPs (i.e., rs45446698 and rs16991615) for estradiol levels in women and seven SNPs for estradiol levels in men, respectively, were used for MR analysis (see eTable 2 for details). GWAS summary data on binary and continuous estradiol levels were used in our analysis 18. It is worth noting that our results suggested a protective effect of estradiol levels in women on COVID-19 infection. Moreover, suggestive causal effects of elevated estradiol levels in men on COVID-19 hospitalization and critical illness were observed. These findings may be explained by previous studies and animal experiments that show estrogens have anti-inflammatory effects and can affect immune systems 39-41. Experiments revealed the suppressive function of 17β-estradiol (E2) on the production of proinflammatory cytokines by macrophages and monocytes, and its ability to inhibit chemokines to prevent innate immune cells, such as neutrophils and monocytes, from concentrating into inflamed areas 42. Additionally, E2 could stimulate CD4+ T-helper cells to produce anti-inflammatory cytokines and produce antibodies by affecting B cells 39,42,43.
In addition, the angiotensin-converting enzyme 2 (ACE2 ) and the transmembrane serine protease 2 (TMPRSS2 ) in the lung have been well-studied as the entry receptor and for the spike (S) protein priming of SARS-CoV-2 44. Recently, researchers found that elevated estradiol levels downregulate the expression of ACE2 andTMPRSS2 messenger ribonucleic acid (mRNA) in lung epithelial cells 14,45,46. This may support the protective role of increased estradiol levels on COVID-19 infection and critical illness revealed in our MR analysis.
Although several ongoing or completed clinical trials are targeting sex hormone pathways, for example, estrogen therapy (NCT04539626), estradiol (NCT04853069), estradiol and progesterone therapy (NCT04865029) and non-steroidal androgen receptor antagonist (NCT04870606), hormone therapy has not been authorized for COVID-19 treatment. A recent clinical trial did not support the use of androgen suppression treatment for improving severe COVID-19 illness and hospitalization in men10. This finding was consistent with our results, which showed that total and bioavailable testosterone levels in men did not causally associate with the risk of COVID-19 infection or severe illness. These results are also supported by previous MR studies38,47. The potential implications of our results for COVID-19 prevention/ treatment warrant further validation in larger clinical trials.
There were several limitations in our study. Firstly, due to the lack of large-scale sex-specific COVID-19 GWAS, we utilized sex-combined GWAS summary statistics of COVID-19 susceptibility and severity as the outcomes in the MR analysis. Secondly, the findings in this study were generated using GWAS data of European ancestry. Therefore, the conclusions of this study may not be generalizable to populations of other ancestries. Thirdly, due to the lack of individual data, we were unable to test the potential non-linear causal associations of serum sex hormones with COVID-19 susceptibility and severity.
In summary, our MR analysis revealed novel causal associations of serum estradiol and bioavailable testosterone with SARS-CoV-2 infection in women but not men, except for a suggestive inverse causal association of estradiol levels with COVID-19 severity in men. This suggests a sex-specific causal nature of sex hormones in relation to COVID-19 outcomes and their potential roles as therapeutic targets for preventing SARS-CoV-2 infection and improving patient outcomes. Further validation in larger clinical trials is warranted.