Thursday, 18 April 2019

New disease surveillance tool


A new computational method called 'CATCH' designs molecular 'baits' for any virus known to infect humans and all their known strains, including those that are present in low abundance in clinical samples, such as Zika. The approach can help small sequencing centers around the globe conduct disease surveillance, which is crucial for controlling outbreaks.

Scientists have been able to detect some low-abundance viruses by analyzing all the genetic material in a clinical sample, a technique known as "metagenomic" sequencing, but the approach often misses viral material that gets lost in the abundance of other microbes and the patient's own DNA.
Another approach is to "enrich" clinical samples for a particular virus. To do this, researchers use a kind of genetic "bait" to immobilize the target virus's genetic material, so that other genetic material can be washed away. Scientists in the Sabeti lab had successfully used baits, which are molecular probes made of short strands of RNA or DNA that pair with bits of viral DNA in the sample, to analyze the Ebola and Lassa virus genomes. However, the probes were always directed at a single microbe, meaning they had to know exactly what they were looking for, and they were not designed in a rigorous, efficient way.


What they needed was a computational method for designing probes that could provide a comprehensive view of the diverse microbial content in clinical samples, while enriching for low-abundance microbes like Zika.

Short for "Compact Aggregation of Targets for Comprehensive Hybridization," CATCH allows users to design custom sets of probes to capture genetic material of any combination of microbial species, including viruses or even all forms of all viruses known to infect humans.

To run CATCH truly comprehensively, users can easily input genomes from all forms of all human viruses that have been uploaded to the National Center for Biotechnology Information's GenBank sequence database. The program determines the best set of probes based on what the user wants to recover, whether that's all viruses or only a subset. The list of probe sequences can be sent to one of a few companies that synthesize probes for research. Scientists and clinical researchers looking to detect and study the microbes can then use the probes like fishing hooks to catch desired microbial DNA for sequencing, thereby enriching the samples for the microbe of interest.

Tests of probe sets designed with CATCH showed that after enrichment, viral content made up 18 times more of the sequencing data than before enrichment, allowing the team to assemble genomes that could not be generated from un-enriched samples. They validated the method by examining 30 samples with known content spanning eight viruses. The researchers also showed that samples of Lassa virus from the 2018 Lassa outbreak in Nigeria that proved difficult to sequence without enrichment could be "rescued" by using a set of CATCH-designed probes against all human viruses. In addition, the team was able to improve viral detection in samples with unknown content from patients and mosquitos.

See: Hayden C. Metsky, Katherine J. Siddle, Adrianne Gladden-Young et al. Capturing sequence diversity in metagenomes with comprehensive and scalable probe design. Nature Biotechnology, 2019; 37 (2): 160 DOI: 10.1038/s41587-018-0006-x

Posted by Dr. Tim Sandle, Pharmaceutical Microbiology

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