Application of the Nicking Loop™ targeted library preparation method to DNBSEQ™ sequencing

This study demonstrates that the PCR-free Nicking Loop™ targeted library preparation method is fully compatible with MGI's DNBSEQ™ sequencing platform, achieving performance metrics and variant detection accuracy comparable to its established use on Illumina systems.

The Gist

Imagine you are a detective trying to find a single, tiny clue hidden inside a massive library of books. In the world of genetics, that "clue" is a specific mutation in your DNA, and the "library" is your entire genome.

For years, scientists have had a very specific, well-oiled machine (Illumina) to read these books. But recently, a new, faster, and different kind of machine (MGI's DNBSEQ) has arrived. The problem? The old detective tools (library preparation kits) were designed specifically for the old machine. If you tried to use them on the new machine, they wouldn't fit, or you'd have to do a lot of extra, messy work to convert the clues.

This paper is about a new, super-smart detective tool called Nicking Loop™ that works perfectly with both the old and new machines without needing any conversion.

Here is the breakdown of how it works, using some everyday analogies:

1. The Problem: The "Square Peg in a Round Hole"

Most DNA testing involves cutting your DNA into small, straight pieces (linear) and gluing special tags on the ends so the machine can read them. This works great for the old machines. But the new machines (like DNBSEQ) prefer to read DNA that is shaped like a circle.

Usually, if you want to use the new machine, you have to take your straight DNA, cut it up, and then painfully glue it into a circle. It's like trying to force a square peg into a round hole, or trying to turn a straight piece of string into a bracelet just to fit it into a specific jewelry box. It's inefficient and can introduce errors.

2. The Solution: The "Magic Loop"

The Nicking Loop™ method is like a magic origami technique. Instead of cutting and pasting, it takes your straight DNA and folds it into a perfect circle right at the very beginning of the process.

• The Loop: Think of this as a special ring that snaps onto your DNA.
• The Early Tag: Usually, you wait until the very end of the process to write a name tag on a package so you know who it belongs to. With Nicking Loop™, you write the name tag (the "sample index") on the ring before you even start folding the DNA. This means every single piece of DNA is identified immediately, reducing the chance of mixing up samples.

3. The Process: Rolling the Dough

Once the DNA is in this circular ring, the scientists use a process called Rolling Circle Amplification (RCA).

• Analogy: Imagine you have a tiny ball of dough (your circular DNA). You put it on a conveyor belt that keeps rolling over it, copying the dough over and over again. This creates one very long, continuous string of dough (a "concatemer") that is just the same recipe repeated thousands of times.
• Why do this? It makes the signal loud and clear without using "PCR" (a common method that can sometimes accidentally create fake mutations, like a photocopier that starts smudging the image).

4. The Two Paths: One Tool, Two Machines

This is the cool part. Once you have that long string of copied DNA, you can send it down two different paths depending on which machine you want to use:

• Path A (The New Machine - DNBSEQ): You cut the long string into small, individual circles. These circles are then packed into tiny "nanoballs" (like packing marbles into a box) and fed into the new DNBSEQ machine. Because the DNA was already a circle, the machine loves it.
• Path B (The Old Machine - Illumina): You take the long string, cut it into straight pieces, and add the standard tags the old machine needs.

5. The Result: A Perfect Match

The authors tested this by using "fake" DNA samples with known errors (like a book with a known typo). They ran these samples through both the new machine (DNBSEQ) and the old machine (Illumina) using the same Nicking Loop™ method.

The findings were amazing:

• Accuracy: Both machines found the "typos" with almost identical accuracy.
• Consistency: The "noise" (random errors) was the same on both.
• Efficiency: The new method worked just as well on the new machine as the old method did on the old machine.

The Big Picture Takeaway

Think of the Nicking Loop™ as a universal adapter.

In the past, if you bought a new phone (the DNBSEQ machine), you might have had to throw away your old charger and buy a whole new set of accessories. This paper shows that with the Nicking Loop™, you can use the same high-quality "charger" (the library prep method) for both your old phone and your new phone.

It proves that this method is platform-agnostic. It doesn't matter if you use the old standard or the new, faster technology; the Nicking Loop™ ensures you get the same high-quality, accurate results. This is huge for doctors and researchers because it means they can switch to newer, cheaper, or faster machines without having to re-invent the wheel or worry about their test results changing.

Technical Summary

1. Problem Statement

Next-generation sequencing (NGS) has expanded beyond established linear platforms (e.g., Illumina) to include circular template-based platforms (e.g., MGI's DNBSEQ™, PacBio, Oxford Nanopore). However, a significant bottleneck exists in targeted sequencing workflows:

• Platform Incompatibility: Most library preparation protocols are optimized for linear, double-stranded DNA (dsDNA) libraries compatible with Illumina's bridge amplification.
• Conversion Complexity: Adapting these linear workflows for circular platforms often requires additional conversion steps, increasing complexity, material loss, and potential for error propagation.
• Clinical Needs: Targeted sequencing is critical in clinical settings (e.g., somatic variant detection in oncology, infectious disease) where high depth, cost-efficiency, and low error rates are paramount. There is a need for a platform-agnostic library preparation method that natively supports circular architectures without compromising performance.

2. Methodology

The study evaluated the compatibility of the Nicking Loop™ method—a PCR-free, targeted library preparation technique—with MGI's DNBSEQ™-G99 platform. The workflow was compared against a benchmark linear Nicking Loop™ workflow sequenced on an Illumina MiSeq.

Key Technical Workflow:

1. Target Conversion: Linear synthetic DNA targets (mimicking Variant Allele Frequencies [VAF] of 0%, 1%, 5%, 10%, and 20%) were hybridized with a Nicking Loop™ construct (bridge oligo, left/right probes, and a sample-specific "Loop" oligo).
2. Circularization & Amplification: The hybridized construct was circularized to form Circular Single-Stranded DNA (CssDNA). This was followed by Rolling Circle Amplification (RCA) using Phi29 polymerase to generate long concatemers.
3. Platform-Specific Processing:
   • For DNBSEQ™ (Circular): RCA concatemers were folded into hairpin structures, cleaved by nicking enzymes (Nb.BsrDI or Nb.BbvCI) to release monomers, and religated to form circular libraries with a nick. These were converted into DNA Nanoballs (DNBs) via RCA and sequenced on the DNBSEQ™-G99.
   • For Illumina (Linear): RCA concatemers underwent limited-cycle PCR to introduce flowcell binding sequences, creating linear libraries for bridge amplification on the MiSeq.
4. Early Indexing: A unique feature of Nicking Loop™ is the incorporation of a sample-specific index into the Loop oligo during the initial hybridization step. This allows for early sample multiplexing, which is particularly advantageous for the DNBSEQ™ workflow where the entire circular structure is sequenced.
5. Data Analysis: Reads were merged, subsampled (to match Illumina depth for direct comparison), and analyzed for UMI distribution, error profiles, and VAF concordance using proprietary pipelines.

3. Key Contributions

• First Demonstration on DNBSEQ™: This study validates the Nicking Loop™ method on the MGI DNBSEQ™ platform, extending its applicability beyond PacBio and Oxford Nanopore.
• Native Circular Workflow: It demonstrates a workflow that converts linear target DNA directly into circular templates suitable for circular sequencing, eliminating the need for intermediate linear-to-circular conversion steps common in other protocols.
• PCR-Free Amplification: The method utilizes RCA for amplification rather than PCR, minimizing amplification bias and error propagation, which is crucial for detecting low-frequency variants.
• Platform-Agnostic Indexing: It confirms that early sample indexing (introduced at the probe hybridization stage) is robust and effective for demultiplexing on DNBSEQ™, a capability not always present in standard linear workflows.

4. Key Results

The study compared circular libraries (DNBSEQ™) against linear libraries (Illumina MiSeq) using synthetic reference samples.

• UMI Distribution & Diversity:

  • Both platforms showed highly uniform template sampling.
  • Singleton UMIs: DNBSEQ™ yielded 97.36% singleton UMIs, while Illumina yielded 97.03%. This indicates that both methods effectively capture unique molecules with minimal PCR duplicates.
  • Read counts per UMI were comparable, with negligible differences (<0.1%) favoring the circular format in some cases.

• Error Profiles:

  • DNBSEQ™ (Circular): 99.74% of reads were error-free (0 mismatches). Single-base mismatches occurred in 0.25% of reads.
  • Illumina (Linear): 99.29% of reads were error-free. Single-base mismatches occurred in 0.70% of reads.
  • Conclusion: The circular workflow on DNBSEQ™ exhibited a slightly lower error rate, though both were highly accurate.

• Variant Allele Frequency (VAF) Concordance:

  • VAF measurements across the tested range (0–20%) were strongly concordant between the two platforms.
  • Correlation: Spearman's $\rho$ = 0.939.
  • This confirms that the platform-specific downstream processing (circularization vs. linear PCR) does not distort quantitative variant detection.

5. Significance

• Clinical Applicability: The high concordance in VAF detection and low error rates make Nicking Loop™ on DNBSEQ™ a viable solution for clinical applications requiring high sensitivity, such as detecting somatic mutations in circulating cell-free DNA (cfDNA) or infectious disease monitoring.
• Workflow Efficiency: By supporting native circular library preparation, the method reduces the number of enzymatic steps and reagents required compared to converting linear libraries for circular sequencing.
• Scalability: The successful integration with DNBSEQ™ (a high-throughput platform) suggests that this method can scale for large-scale targeted sequencing projects while maintaining the benefits of early indexing and PCR-free amplification.
• Future-Proofing: As the NGS landscape shifts toward diverse chemistries, Nicking Loop™ offers a flexible, platform-independent strategy that decouples library preparation from specific sequencing hardware constraints.

In summary, this paper establishes that the Nicking Loop™ method is a robust, high-fidelity, and platform-agnostic solution for targeted sequencing, successfully bridging the gap between linear target DNA and circular NGS platforms like DNBSEQ™ without sacrificing accuracy or quantitative precision.