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R-Loops Focus Story

Learning How to Read the Book of Life

Research in the Gene Expression & Regulation Program at Wistar continues to reveal new knowledge on RNA and its functions to regulate how our genes are expressed and how that can go awry.

The advent of mRNA vaccines for COVID-19 — touted as the next-generation tool in vaccinology — brought RNA to the fore, giving popularity to this once less-publicized cousin of DNA.

In high school biology, we learned that our genes are the repositories of the blueprint to make all of our proteins. Our genes carry out most of the functions in our cells. RNA is the carrier of information from DNA to ribosomes — the machines that manufacture proteins.

The process of reading and executing the instruction book of life involves strict oversight and multiple levels of regulation to allow a relatively small number of genes to orchestrate all the functions of our body. Control of gene expression plays a critical role in determining what proteins are present in a cell and in what amounts at any given time.

It is becoming abundantly clear that this control process happens both during and after RNA transcription.

Wistar scientists have pioneered the study of RNA biology, discovering new RNA types and unraveling some of the mechanisms that modify RNA to regulate its functions for gene expression. Following along that path, labs in the Gene Expression and Regulation Program continue to delve deep into the RNA world and make exciting discoveries related to RNA structure and functions.

R-LOOPS: Friend-foes

Dr. Kavitha Sarma, assistant professor, focuses on particular nucleic acid structures called R-loops that contain both DNA and RNA and form during transcription, the first step of gene expression.

In our DNA book, consider genes as the individual words and nucleotides as the letters that make up those words. When a DNA template is "transcribed" into messenger RNA (mRNA), the sequence of letters that form each gene gets "read" and copied into an RNA molecule that will leave the nucleus and travel to the cytoplasm, where words will be read by ribosomes to provide instructions for making proteins.

Sometimes during transcription, the newly synthesized RNA molecule sticks to its template DNA strand, forming a stable DNA/RNA hybrid that appears like a loop when visualized by electron microscopy, hence the name R-loop.

This is a normal occurrence — R-loops are constantly formed and removed throughout the genome and their presence can be beneficial for transcriptional regulation. However, accumulation of R-loops can cause DNA damage, chromosome rearrangements and genomic instability and underlie a host of diseases from cancer to neurodegenerative disorders and possibly autism.

The Sarma lab is interested in R-loops for their potential in causing disease and in serving as new therapeutic targets. They have been busy developing new, improved techniques to detect R-loops to study the contributions of these structures in gene regulation and the consequences of their accumulation in the cell1.

Thanks to these technological advances, Dr. Sarma and her colleagues were able to identify new factors that regulate R-loops and are now closing in on their function in glioblastoma and colon cancer.

The lab received funding from the W.W. Smith Charitable Trust to study the role of R-loops in brain cancer and with support from the Basser Center for BRCA and the Margaret Q. Landenberger Research Foundation they are dissecting the correlation between R-loop formation and BRCA1/2 gene mutations in breast and ovarian cancer to eventually use R-loops for novel diagnostic and therapeutic applications. The Simons Foundation supports the lab’s work elucidating the consequences of unregulated R-loops in autism spectrum disorders.

EDITING RNA TO RESOLVE R-LOOPS

Dr. Kazuko Nishikura, professor, has published a new function of R-loops2 in preserving the integrity of our chromosome ends — the telomeres.

Dr. Nishikura has been a pillar of Wistar science for almost four decades with a career overlapping with the rise and expansion of the RNA biology field. She was one of the first to characterize a process called RNA editing and its multiple functions in the cell, and to discover the enzyme ADAR1 that is responsible for it.

RNA editing changes one or more letters in RNA "words," allowing cells to make discrete modifications to an RNA molecule. RNA editing is a good example of how our cells make the most of their genes and create different protein products from a single gene by slightly modifying the RNA sequence.

With support from grants from the National Institutes of Health and Emerson Collective, the Nishikura lab recently showed that ADAR1 helps the cells resolve R-loops formed at the chromosome ends and prevents their accumulation by facilitating degradation of the RNA strand.

Nishikura and colleagues found that depletion of a particular form of the ADAR1 protein leads to extensive telomeric DNA damage and arrested proliferation specifically in cancer cells, indicating this process as a new target for cancer therapy.

ALTERNATIVE POLYADENYLATION: Tell Me What Your APA Is and I Will Tell You Where to Go

An important level of mRNA regulation involves modifying its structure, especially at the tail end of the sequence, termed 3’ end. A process called polyadenylation adds a stretch of specific nucleotides to protein-coding mRNAs to regulate their stability, transportation from nucleus to cytoplasm and translation into proteins.

Dr. Bin Tian, professor, and his lab study this process to understand regulatory mechanisms and to identify new drug targets. They have contributed important knowledge on polyadenylation in normal and diseased conditions, including the discovery that alternative polyadenylation (APA) is widespread across genes. 

This is a dynamic mechanism of gene regulation that generates different 3′ ends in mRNA molecules, resulting in multiple mRNAs from the same gene, which scientists call isoforms.

The lab’s latest research is uncovering the role APA plays in facilitating protein production in certain sites within the cell where those proteins are most needed.

When mRNAs leave the nucleus and move to the cytoplasm, they need to be properly directed to reach the ribosomes and be translated into proteins. Although too small to be seen with the naked eye, a cell is a huge space for something as tiny as an mRNA molecule that has to find its way. Imagine finding yourself in a baseball stadium and not knowing how to get to your seat.

The Tian lab discovered that some mRNAs possess specific properties in their sequence and structure that enable them to associate with the endoplasmic reticulum (ER), a network of tubes that build, package and transport proteins and where a large fraction of ribosomes in the cell are located3

These mRNAs tend to encode for proteins involved in cell signaling, the process that allows the cells to communicate with neighboring cells by sending, receiving and processing signals to respond to changes in their environment. 

Dr. Tian and his team hypothesize that association with the ER anchors certain mRNA isoforms in specific cellular locations where important signaling events happen, making the whole process more efficient. According to this model, the ER would serve a new function as a scaffold to keep proteins at hand where they are needed, representing a platform that provides venues for signaling events to happen quickly and effectively. 

The lab also creates computer-based data mining tools to analyze APA using large data sets, such as those from The Cancer Genome Atlas (TCGA) program.


The extraordinary biological complexity of human life is a reflection of the many sophisticated ways in which gene expression can be fine-tuned. 

The cutting-edge science underway at Wistar pushes the limits of RNA research to advance our understanding of how the human genome is decoded, how the messengers of genetic information are guided, and how accidental mistakes that happen while reading and interpreting the DNA book can be fixed, all of which may enable researchers to develop novel and more precise ways to treat diseases. 

1 A nuclease- and bisulfite-based strategy captures strand-specific R-loops genome-wide, Elife 2021
2 ADAR1 RNA editing enzyme regulates R-loop formation and genome stability at telomeres in cancer cells, Nature Communications 2021
3 Alternative 3’UTRs play a widespread role in translation-independent mRNA association with endoplasmic reticulum, Cell Reports 2021