Introduction
Circular DNA is a blind spot for anyone who works with sequencing data: standard pipelines assume linear molecules and lose the circles during library prep and assembly. This post walks through a recent study where that gap turned out to matter, and shows how the right assembly strategy brought the missing sequence back into view.
An Unexpected Discovery Inside Stress Granules
A jolt of heat or a drop in available nutrients can stall protein synthesis. When that happens, cells sweep mRNA and protein into dense cytoplasmic bodies called stress granules. These granules have been studied for decades as assemblies of RNA and protein. A new study the National Heart, Lung, and Blood Institute (NHLBI), published in eLife, adds a component nobody expected: DNA.
We’re pleased to highlight the work conducted for our client NHLBI, for whom Bridge Informatics performed partial analysis. The study found that more than half of the nucleic acid in stress granule cores is not RNA but circular, double-stranded DNA, most of it extrachromosomal circular DNA (eccDNA). Using CRISPR to target those circles in yeast, the authors showed they are required for granules to form, and that disrupting them slows how well a cell Circular DNA is close to invisible to conventional sequencing workflows:
Circular DNA is close to invisible to conventional sequencing workflows: standard library prep shears DNA, and standard callers expect linear molecules with defined ends, so the circles drop out before anyone looks at the data. Recovering them takes two separate moves. The study paired two sequencing platforms, Illumina for base-level accuracy and Oxford Nanopore for the long reads that resolve larger and repetitive structures. Bridge Informatics then took those reads and assembled them into candidate DNA circles, reconciling the calls from each platform into a high-confidence set. That cross-platform step is what separated genuine circles from artifacts of any single method.
This is a good example of work we perform time and time again for our clients: taking a dataset that resists off-the-shelf tools and building the analysis that surfaces the result. Hybrid short- and long-read integration, eccDNA detection, and separating true signal from artifact are the kinds of problems a general-purpose pipeline tends to fumble.
The biology has plenty of open questions ahead. eccDNA already carries weight in cancer and in immune signaling. A functional role in the cytoplasmic stress response raises new ones: how these circles leave the nucleus, and what else they help organize once they are there. For anyone working on membraneless organelles, DNA-RNA interactions, or eccDNA, the paper rewards a full read.
Read the reviewed preprint in eLife:
https://elifesciences.org/reviewed-preprints/111336
This is the kind of problem we love to take on at Bridge Informatics: data that off-the-shelf tools can’t crack, with a real result inside. Click here to schedule a free introductory call with a member of our team.