More than 2,500 genes exhibit significant sex differences in expression in mouse alveolar type II cells (AT2s), important for keeping the lungs functioning, potentially explaining sex biases in the prevalence and severity of lung diseases. In particular, very high numbers of X-linked genes escape transcriptional silencing in lung alveolar type 2 (AT2s) cells, researchers report January 12th in the journal Stem Cell Reports.
“Our study is the first to compare male and female AT2 cells for gene expression, and our findings suggest that there are likely sex differences in lung repair following viral-induced injury,” says co-senior author Montserrat Anguera, an associate professor at the University of Pennsylvania.
Sex differences exist for many lung diseases, but the mechanistic basis for this remains unclear. “We started this project during the beginning of the pandemic, because we were curious about the sex bias with COVID-19 disease, where more older men have increased morbidity, and wondered whether X-chromosome inactivation (XCI) might contribute to this sex bias,” Anguera says. “We realized that the SARS-CoV2 virus first encounters AT2 cells in the lung, and that the virus enters cells through the angiotensin-converting enzyme 2 (Ace2) receptor, which is located on the X chromosome.”
XCI is a process by which one of the copies of the X chromosome is inactivated in female mammals. The inactive X chromosome is silenced by being packaged into a transcriptionally inactive structure called heterochromatin. XCI prevents female mammals from having twice as many X-chromosome gene products as males, who only possess a single copy of the X chromosome.
In the new study, Anguera and co-senior author Andrew Vaughan, an assistant professor at the University of Pennsylvania, investigated XCI maintenance and sex-specific gene expression profiles using male and female AT2s. The results showed that approximately 68% of expressed X-linked genes in mouse AT2s escape XCI. These genes include Ace2, which serves as the entry point into cells for SARS-CoV-2, but is also involved in lung repair.
There were genome-wide expression differences between male and female AT2s, possibly contributing to sex differences in lung injury and repair in multiple settings. Taken together, the findings demonstrate that AT2 cells have the highest levels of XCI escape for mouse cells reported to date and support a renewed focus on AT2s as a potential contributor to sex-biased differences in lung disease.
In addition, the results showed that AT2 cells, similar to immune cells, do not strictly follow the classic rules of XCI. “We were surprised to find that female AT2 cells lack canonical epigenetic modifications that are typically enriched on the inactive X as a result of XCI. These include the long noncoding RNA Xist and heterochromatic histone modifications H3K27me3 and H2AK119-ubiquitin,” Anguera says. “Because the inactive X in female AT2 cells has fewer epigenetic marks, this enables more gene expression chromosome wide, including the Ace2 gene.”
For now, it remains an open question whether ACE2 escapes XCI in human AT2 cells. to The authors say this is a likely scenario because there are significantly higher numbers of XCI escape genes in human cells compared to mouse cells.
Moving forward, the authors plan to investigate how expression from the inactive X in AT2 cells is affected by SARS-CoV2 infections. They also will continue to study how expression from the X chromosome is regulated in other cell types that do not exhibit conventional XCI maintenance. “Our findings open the door to future work investigating the genetic and epigenetic basis, residing within the X chromosome, of sex differences in immune responses to inhaled viruses,” Anguera says.