Reverse transcription PCR shaped the global diagnostic response to SARS-CoV-2, yet its reliance on thermal cycling, uninterrupted power, and trained personnel exposed clear limitations. In contrast, the adoption of recombinase-based isothermal amplification promising faster, simpler workflows, remained uneven. In the first session of our Isothermal Masterclass Series, Dr. Till Adhikary demonstrated how a smartly engineered RT-RPA/RAA workflow can reach analytical performance levels suitable for real-world diagnostics, while operating at little more than body temperature.
What emerged from his presentation was not a pandemic anecdote, but a repeatable methodology: a set of biochemical principles, design constraints, and empirical tests that together produce a rapid, low-complexity assay suitable for decentralized settings.
How recombinase-driven amplification enables rapid, low-temperature diagnostics
RPA and RAA systems were built to bypass thermal cycling. Instead of heat-driven denaturation, they rely on:
- Recombinase to form primer–protein filaments that scan double-stranded DNA
- Single-stranded DNA binding protein to stabilize displaced strands
- Strand-displacing polymerase to extend primers without high-temperature separation
Optimal exponential amplification occurs at 37-42 °C. Lower temperatures are possible.
The advantage is obvious: amplification anywhere, with minimal hardware.
The challenge is equally fundamental: at these temperatures, primer specificity decreases. Without the high-temperature stringency of PCR, primers may bind non-selectively.
For Dr. Adhikary, this constraint reframes assay design. “Probe cleavage, not annealing temperature, enforces specificity in RPA,” he explained. “Everything begins with getting the probe right”.
The nfo probe: the key specificity determinant
In RPA–nfo assays, the probe defines the true specificity of the reaction. The probe contains a 5′ reporter label (e.g., FAM), an internal tetrahydrofuran (THF) abasic site, and a 3′ blocking group. When the probe hybridizes correctly to its target sequence, endonuclease IV (Nfo) recognizes the THF within the duplex and cleaves it. This cleavage removes the 3′ block and generates a free 3′-OH group.
The exposed 3′-OH allows Bsu polymerase to extend the downstream probe fragment. Extension does not introduce new labels; instead, it incorporates the cleaved probe fragment into the amplicon. Because the reverse primer carries biotin, the resulting product contains both detection tags, FAM from the probe and biotin from the primer, allowing recognition on lateral-flow strips.
If the probe does not bind, Nfo does not cleave, extension does not occur, and no dual-labeled amplicon is produced. This THF-dependent cleavage makes the probe the primary driver of assay specificity.
Primer architecture: narrow constraints, major impact
RPA primers differ from PCR primers in both length and structural requirements:
- 30–35 nt, needed for stable recombinase loading
- GC content 30–70%
- 100–200 bp amplicon for fast reaction kinetics
- Minimal primer–primer or primer–probe complementarity
Tools like Primer3 (http://frodo.wi.mit.edu/cgi-bin/primer3/primer3_www.cgi) and MFEprimer (https://www.mfeprimer.com/) support initial screening, but Dr. Adhikary emphasized their limits. Some interactions only reveal themselves under real reaction conditions, particularly those involving the low-temperature dynamics of probe–primer hybrids.
The dipstick pre-screen: detecting oligo interactions before they cause failures
One of Dr. Adhikary’s most practical contributions is a deceptively simple pre-screening test: mix primers and probe with running buffer, dip a lateral-flow strip, and watch what happens.
The power of this step doesn’t come from extraordinary strip sensitivity but from the deliberately high concentration of primers and probes. At these levels, any unintended interaction between labeled oligos becomes obvious, long before an amplification reaction would betray the same issue.
If a visible test line appears at this stage, the oligos are forming unwanted complexes, and the design is rejected immediately.
For multiplex assays, or any project under development pressure, this quick check can spare weeks of troubleshooting and sidestep entire families of downstream artifacts.
Synthetic DNA controls: establishing rigorous performance baselines
Synthetic double-stranded DNA fragments, matching the assay’s target region, allow controlled assessment of: analytical sensitivity, reproducibility across reagent lots, tolerance to matrix inhibitors, primer/probe mismatch behavior.
In Dr. Adhikary’s SARS-CoV-2 work, synthetic fragments enabled him to establish detection at ~5 fg (~2×10⁵ copies), providing a quantitative anchor independent of sample variability.
This step also prevents premature conclusions based on inconsistent biological samples, a common pitfall when validating isothermal assays.
An end-to-end workflow for decentralized testing
Dr. Adhikary’s final assay uses:
- Direct lysis of saliva or gargle fluid
- Reverse transcription using freeze-dried reagents
- RPA/RAA amplification at 37 °C
- Lateral-flow readout with no instruments required
In cell culture–derived SARS-CoV-2 cDNA received from the Institute of Virology (University of Marburg) the assay reliably distinguished infected from uninfected samples.
The time-to-result remained in the 8–15 minute window, a major advantage for field deployment.
Practical Workflow Notes: A Protocol-Oriented Summary of Dr. Adhikary’s Method
1. Oligo design
- 30–35 nt primers
- 46–52 nt probe with THF + 3′ block
- Amplicon: 100–200 bp
- Check interactions with MFEprimer
2. Pre-amplification dipstick test
- Mix primers + probe + running buffer
- Insert dipstick
- Test line = reject oligo set
3. Sensitivity calibration
- Titrate synthetic DNA from 108 to 10⁰ copies
- Identify LOD
4. Reaction setup
Typical 50 µL RPA reaction:
- Primers 0.4–0.5 µM
- Probe 0.12–0.2 µM
- 37 °C, 8–15 min
- Mix midway to improve homogeneity
5. Lateral flow readout
- Dilute reaction 1:20
- Dipstick read within 5 minutes
This structured workflow translates Till’s approach into a reproducible template for assay development.
Looking for setting up your isothermal workflow?
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