Researchers at Mount Sinai's Icahn School of Medicine have shed valuable light on the nuanced functions and complex regulatory pathways of RNA editing, a crucial mechanism underlying brain development and disease.
In a study published June 26 in Nature communicationThe team reported that large differences were found between postmortem and living brain tissue in the prefrontal cortex, as these are associated with one of the most common RNA modifications in the brain, known as adenosine-to-inosine (A-to-I) editing. This discovery will play a major role in shaping the development of diagnostics and therapies for brain diseases.
While DNA contains the genetic blueprint for humans, RNA actually carries out its instructions to create functional proteins that play a key role in how the body functions, including the complex functions of the central nervous system. The function and stability of RNA are determined by many modifications, each of which has a specific purpose. These modifications, known as RNA editing, are an ongoing process that occurs in all of our cells and tissues, made possible by enzymes known as ADAR. This process can continue in individual cells for some time after the person to whom they belonged has died.
The conversion of adenosine nucleosides to inosine (A-to-I) is a common and well-studied RNA modification and is orchestrated by proteins of the ADAR family, primarily ADAR1 and ADAR2. In the mammalian brain, thousands of highly regulated A-to-I editing sites have been discovered in anatomical regions and cell types, some by researchers at Mount Sinai. These sites are known to be involved in neuronal maturation and brain development. Aberrant regulation of A-to-I assembly has been linked to neurological disorders.
“Until now, research on A-to-I editing and its biological significance in the mammalian brain has been limited to the analysis of postmortem tissues. By using fresh samples from living individuals, we were able to detect significant differences in RNA editing activity discover that previous studies, which relied only on postmortem samples, may have overlooked,” said Michael Breen, PhD, co-senior author of the study and assistant professor of psychiatry, and genetics and genomic sciences, at Icahn Mount Sinai “We were particularly surprised to find that levels of RNA editing were significantly higher in postmortem brain tissue compared to living tissue, which is likely due to postmortem changes such as inflammation and hypoxia that do not occur in living brains. Furthermore, we found that RNA editing in living tissue tends to involve evolutionarily conserved and functionally important sites that are also dysregulated in human disease, highlighting the need to study both living and postmortem samples for a comprehensive understanding of the brain biology.
After death, the lack of oxygen rapidly damages brain cells, triggering an irreversible cascade of damage that can alter ADAR expression and A-to-I editing. “We hypothesized that molecular responses to postmortem-induced hypoxic and immune responses could significantly alter the landscape of A-to-I editing. This could lead to misunderstandings about RNA editing in the brain if we only study postmortem tissues,” said Miguel Rodríguez de los Santos, PhD, co-first author of the study and a postdoctoral researcher in the Department of Psychiatry at Mount Sinai. “Studying living brain tissue gives us a clearer picture of the biology of RNA editing in the human brain.”
To investigate this, the research team based their study on the Living Brain Project, in which dorsolateral prefrontal cortex (DLPFC) tissues are obtained from living people during neurosurgical procedures for deep brain stimulation, an elective treatment for neurological disease. For comparison, a cohort of postmortem DLPFC tissues from three brain banks was constructed to match the living cohort for key demographic and clinical variables. The team examined multiple genomic data types from the Living Brain Project, including bulk tissue RNA sampling, single-nuclei RNA sequencing and whole-genome sequencing. The generation of this data is described in several upcoming Living Brain Project manuscripts.
The researchers identified more than 72,000 sites where A-to-I editing was more frequent or different in postmortem than living DLPFC brain tissue. They found higher levels of the enzymes ADAR and ADARB1, which are responsible for increased editing patterns in postmortem brain tissue. Interestingly, they also found hundreds of sites with higher levels of A-to-I editing in living brain tissue. These sites are primarily found in the connections between neurons (called synapses) and are typically conserved through evolution, suggesting they play important roles in brain activity. Some known A-to-I editing sites were highly edited in living brains, suggesting they may be involved in crucial neuronal processes such as synaptic plasticity, which is essential for learning and memory. However, many other A-to-I editing sites found in living brain tissue have unclear functions, and more research is needed to understand their impact on brain health.
“Using fresh brain tissue from living human donors provided us with the opportunity to examine the brain without the confusion inherent in postmortem tissue analysis,” said Alexander W. Charney, MD, PhD, co-senior author of the study and associate professor in psychiatry. Genetic and Genomic Sciences, Neuroscience and Neurosurgery at Icahn Mount Sinai and co-leader of the Living Brain Project. “In doing so, we have revealed more precise insights into the prevalence and role of A-to-I editing in the human brain. It is critical to note that our findings do not negate, but instead provide, missing context for the use of postmortem brain tissue in research. Understanding these differences will help improve our knowledge of brain function and disease through the lens of RNA editing modifications. , potentially leading to better diagnostic and therapeutic approaches.”
The research team will further analyze the RNA editing data to better understand its implications and identify potential therapeutic targets for Parkinson's disease. They are also expanding the research with emerging work from this cohort that focuses on gene expression, proteomics and multi-omics of the living brain.
“By leveraging the unique, transdisciplinary nature of the Living Brain Project, we can transform a groundbreaking clinical care modality such as deep brain stimulation into a platform for unprecedented insights into the biology of the human brain that will give rise to new therapeutic opportunities,” said Brian Kopell. , MD, co-first author of the study, director of the Center for Neuromodulation at Mount Sinai and co-leader of the Living Brain Project
