Negar Sadeghi,1,*
1. Department of Cell and Molecular Biology & Microbiology, Faculty of Biological Science and Technology, University of Isfahan, Isfahan, Iran
Introduction: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a member of the coronavirus family that causes an infectious disease called the Coronavirus disease 2019 (COVID-19). The angiotensin-converting enzyme 2 (ACE2) type I membrane receptor which acts as a passway for the entrance of the virus to the host cell is a member of the Renin-Angiotensin pathway. The expression level of ACE2 is high in the lung, and the spike protein on the SARS-CoV-2 can bind to this receptor. The regulation of expression levels of the ACE2 gene is dependent on key regulatory elements for chromatin modification and transcription factors. Epigenetics is the changes in gene expression that do not involve changes to the underlying DNA sequence, and it happens at the level of histone proteins, RNA and DNA. The epigenetic changes in DNA methylation of the ACE2 gene and histone proteins, located near the ACE2 promoter, affect SARS-CoV-2 infection outcomes in different ages and genders. Epigenetic compounds such as Valproic acid (VPA), which shows antiviral effects through Histone Deacetylase (HDAC) inhibition, represent promising candidates for use as COVID-19 treatments.
Methods: In a bioinformatic study, four available genome-wide DNA methylation human datasets were examined to recognize the relation between DNA methylation profiling related to ACE2 and susceptibility risk for COVID-19 related to age and gender. Another study using molecular docking and bioinformatics showed that VPA targets and inhibits the active sites of the HDACs from classes I and IIa specifically. In another study, for investigation of VPA effect on inhibition of SARS-CoV-2 infection through decreasing ACE2 expression, first VPA was used for Huh-7 and HK-2 cell lines treatment, and then the cells were infected with SARS-CoV-2 which had been obtained from a nasal swab. Then, the ACE2 expression level in these cells was measured by RT-PCR and western blot. To verify the hypothesis of downregulation of the ACE2 through HDAC inhibition with VPA, several HDAC inhibitors with the same and different functions rather than VPA were applied in HK-2 and Huh-7 cell lines. Then, the expression level of ACE2 was analyzed.
Results: Higher levels of ACE2 expression are correlated with greater susceptibility to SARS-CoV-2 infection and viral loads. Based on a comparative analysis of DNA methylation in various tissues, the cause of high expression of ACE2 in the lung is the hypomethylation of three CpG sites around the ACE2 promoter. Moreover, during aging DNA methylation level is decreased and led to differential methylation patterns of several genes. Thus, the probability of contracting Covid-19 increases with aging. Also, analysis of DNA methylation at two CpG sites related to the ACE2 gene shows that females are significantly hypomethylated compared to males. Moreover, the ACE2 gene is located on the X chromosome that is under X-inactivation in females. However, this gene can escape from this inactivation. Therefore, it may explain the higher ACE2 expression observed in females and the higher rate of infection in females. In addition, histone modifier enzymes such as HDACs seem to be related to enhanced levels of ACE2 expression in COVID-19 patients. HDACs normally repress transcription by removing acetyl groups from histones and resulting in chromatin compression, so their inhibition is usually associated with increased gene expression. However, recent studies have reported that HDACs can cause transcription stimulation. In the case of COVID-19, free ACE2 on the surface of the cells is reduced due to binding to spike protein of the virus, and following Angiotensin (Ang) II accumulation, Angiotensin Receptor (ATR) 1 is activated and sends a signal into the nucleus to stimulate the HDACs. These enzymes, in turn, lead to ACE2 upregulation. Decreased expression of the ACE2 at mRNA and protein levels due to using HDAC inhibitors as same as VPA shows the fact that the ACE2 downregulation is caused by HDAC class I inhibition.
Conclusion: Epigenetic regulation of the ACE2 gene, including DNA methylation and histone modifications, has a significant impact on the outcome of SARS-CoV-2 infection. Gender and age-related differences in ACE2 DNA methylation observed in the respiratory system cause pathological differences in patients with COVID-19. Epigenetic medicines such as VPA, which affects ACE2 expression through HDAC inhibition, will play a promising role in COVID-19 therapy.
Keywords: Epigenetics, DNA Methylation, ACE2, COVID-19