Improved long transcript representation in Oxford Nanopore direct RNA sequencing with UltraMarathonRT

The authors introduce an improved Oxford Nanopore direct RNA sequencing protocol utilizing the ultraprocessive UltraMarathonRT enzyme at 30°C, which prevents RNA hydrolysis and significantly extends read lengths and isoform predictions compared to the standard 60°C Induro RT method.

The Gist

The Big Picture: Reading the "Original Manuscript"

Imagine you have a very long, fragile, and complex handwritten letter (your RNA) that contains the instructions for building a human. Scientists want to read this letter to understand how our bodies work.

For a long time, the only way to read these letters was to photocopy them first (making cDNA). But photocopying has a problem: the machine might skip pages, add typos, or miss the very long, winding sentences at the end.

Oxford Nanopore Direct RNA Sequencing (DRS) is a new technology that lets scientists read the original handwritten letter directly, without photocopying. This is amazing because it preserves special ink marks (chemical modifications) and captures the full length of the letter.

However, there's a catch: The original letters are incredibly fragile. If you try to feed them into the reading machine, they often tear or dissolve before the machine can finish reading them.

The Problem: The "Hot Water" Trap

To get the letter ready for the machine, scientists have to attach a special "handle" to the end of the paper so the machine can grab it. In the current standard method, this step requires heating the paper to 60°C (140°F).

Think of this like trying to attach a handle to a piece of wet tissue paper by dipping it in hot water.

• The Goal: Attach the handle.
• The Result: The hot water makes the tissue paper dissolve or tear apart (hydrolysis) before you can even use it.
• The Consequence: The machine ends up reading only the short, torn scraps of the letter, missing the long, important parts.

The Solution: UltraMarathonRT (The "Cool-Headed" Reader)

The authors of this paper, working with a company called RNAConnect, found a better way. They introduced a new enzyme called UltraMarathonRT (uMRT).

Here is how uMRT is different:

1. It works in the cold: Instead of needing 60°C, uMRT works perfectly at 30°C (room temperature). It's like attaching the handle to the tissue paper using cool water. The paper stays strong and doesn't dissolve.
2. It's a marathon runner: This enzyme is incredibly efficient. It can read through long, tangled, knotted sections of the letter without getting stuck or falling off.
3. It untangles knots: RNA often folds into complex 3D shapes (like a tangled ball of yarn). uMRT has a built-in "helicase" activity, which acts like a pair of scissors that gently cuts the knots so the enzyme can slide through smoothly.

The Experiment: Putting It to the Test

The scientists tested this new method against the old "hot water" method using two types of samples:

1. Universal Human Reference RNA: A standard "control" mix of RNA.
2. Human Brain RNA: This is the "hard mode." The brain is full of incredibly long, complex instructions (isoforms) that are very hard to read.

The Results:

• Less Breakage: With the new cool method, the RNA stayed intact. The old hot method destroyed about 30–50% of the RNA.
• Longer Reads: The new method produced much longer "readings." Instead of just seeing the first few paragraphs, they could read the whole book.
• More Discovery: Because they could read the long parts, they found more genes and more unique versions of genes (isoforms) than before. Specifically, in the brain samples, they found nearly double the number of very long transcripts (over 15,000 letters long) compared to the old method.

Why This Matters

Think of the old method as trying to read a novel by looking at a pile of torn-up pages. You get the gist, but you miss the plot twists and the ending.

The new UltraMarathonRT method is like having a gentle librarian who carefully unfolds the book and hands you the whole, intact novel.

The Impact:

• Better Medicine: By seeing the full, long versions of genetic instructions, scientists can better understand diseases like cancer or neurological disorders where long, complex genetic errors occur.
• New Biology: It allows researchers to discover "hidden" instructions in our DNA that were previously too long or fragile to be seen.
• Lower Cost: Because the method is so efficient, they might eventually be able to use smaller samples of RNA, making these tests cheaper and easier to run.

In a Nutshell

The paper says: "Stop cooking your RNA!" By switching from a hot, destructive process to a cool, gentle one using a super-efficient enzyme, scientists can now read the full, long, and complex stories hidden in our cells, leading to new discoveries in biology and medicine.

Technical Summary

1. Problem Statement

Oxford Nanopore Technologies (ONT) Direct RNA Sequencing (DRS) allows for the sequencing of native RNA molecules, preserving endogenous base modifications and enabling full-length isoform resolution without PCR amplification biases. However, the technology faces significant limitations regarding read length and transcript representation, particularly for long and highly structured RNAs.

The authors identified a critical bottleneck in the current ONT DRS library preparation workflow:

• RNA Hydrolysis: The standard protocol utilizes the reverse transcriptase Induro®, which requires incubation at 60°C in a magnesium-containing buffer. The authors hypothesized that these conditions accelerate the chemical hydrolysis (degradation) of the RNA strand before it reaches the nanopore.
• Consequence: This thermal degradation leads to the truncation of long transcripts, resulting in an underrepresentation of long isoforms (e.g., >10–15 kb) and a bias toward shorter reads, limiting the discovery of complex splicing events and full-length transcript structures.

2. Methodology

The study developed and optimized a new DRS workflow centered around UltraMarathonRT® (uMRT), a group II intron reverse transcriptase with intrinsic helicase activity and high processivity.

Key Methodological Changes:

• Enzyme Substitution: Replaced Induro® (60°C) with uMRT, which operates optimally at 30°C. This lower temperature significantly reduces the rate of RNA hydrolysis.
• Workflow Optimization: The authors introduced several modifications to the library preparation steps to maximize RNA integrity and library yield:
  1. RNA Conditioning: A step to reduce intramolecular RNA interactions.
  2. Adapter Annealing: A heat-cool-anneal step (55°C for 1 min, snap chill to 4°C) to improve RT adapter (RTA) binding to the poly(A) tail.
  3. Ligase Swap: Utilization of a different DNA ligase.
  4. Bead Purification: Added a purification step between ligation and reverse transcription to remove inhibitors.
  5. Quench Buffer: Addition of a quench buffer prior to heat inactivation of the RT enzyme.

Experimental Design:

• Stability Assay: Incubated a 7.6 kb in vitro transcribed HCV RNA fragment in various RT buffers (uMRT, Induro, SuperScript IV, Maxima) at their respective optimal temperatures for 30 minutes to measure degradation.
• Benchmarking: Compared the optimized uMRT workflow against the standard ONT Induro workflow using two human total RNA samples:
  1. Universal Human Reference RNA (UHRR): A standard benchmark sample.
  2. Human Brain RNA: Chosen for its high complexity and abundance of long, alternatively spliced isoforms.
• Sequencing: Libraries were sequenced on ONT MinION flow cells using SQK-RNA004 chemistry.
• Analysis: Data was analyzed using ONT's EPI2ME workflow, IsoQuant (for strict isoform calling), and SQANTI3 (for isoform classification and splicing event characterization).

3. Key Results

A. RNA Integrity and Stability

• Degradation Rates: In stability assays, RNA incubated in uMRT conditions (30°C) remained 98% intact after 30 minutes. In contrast, Induro (60°C) and other high-temperature RT conditions resulted in 30–50% degradation.
• Processivity: uMRT successfully synthesized cDNA from poly-dT priming for human genes ranging from 1.2 kb to 20 kb, confirming its ability to handle long templates.

B. Sequencing Performance

• Library Yield: The optimized uMRT workflow produced ~25% higher RNA-cDNA hybrid yields compared to the Induro protocol.
• Read Lengths: uMRT libraries showed modest but consistent improvements in mean, median, and N50 read lengths.
• Quality: Mapping rates and per-read quality scores (Q-scores) remained comparable between the two methods, indicating that the new protocol did not compromise data quality.

C. Transcriptome Discovery (Isoform Detection)

• Long Transcript Enrichment: The most significant gain was observed in longer transcripts. While both protocols performed similarly for transcripts <2 kb, uMRT showed a progressive enrichment in counts for bins ≥5 kb, ≥10 kb, and ≥15 kb.
• Human Brain Specifics: The effect was more pronounced in Human Brain RNA (rich in long isoforms) than in UHRR.
  • uMRT detected nearly double the percentage of transcripts ≥15 kb compared to Induro.
  • The 10th longest transcript identified by uMRT was longer than the longest transcript identified by Induro.
• Novel Isoforms: Using SQANTI3, uMRT libraries identified:
  • ~17% more "Novel in Catalog" (NIC) isoforms.
  • 15–20% more "Novel Not in Catalog" (NNIC) isoforms.
  • 24–28% more transcripts with intron retention (a feature often lost in truncated reads).
  • 15–20% more novel canonical splice sites.

4. Key Contributions

1. Identification of a Thermal Bottleneck: The study definitively demonstrates that the high-temperature requirement (60°C) of current DRS protocols causes significant RNA hydrolysis, artificially truncating long reads.
2. Development of a Low-Temperature DRS Protocol: The authors successfully adapted the DRS workflow for a 30°C reverse transcription step using uMRT, preserving RNA integrity.
3. Optimized Workflow Kit: The paper details a comprehensive, optimized protocol (including RNA conditioning, specific annealing steps, and purification) that increases library yield and enables sequencing with lower input amounts (down to 100 ng of poly(A)+ RNA).
4. Enhanced Discovery Power: The new method significantly improves the detection of long, complex isoforms and novel splicing events, particularly in tissues with high transcript complexity like the brain.

5. Significance

This work represents a major advancement for native RNA sequencing. By mitigating RNA degradation during library preparation, the UltraMarathonRT-based DRS workflow unlocks the full potential of Oxford Nanopore technology for:

• Full-Length Isoform Characterization: Enabling accurate reconstruction of transcripts that were previously too long to be fully captured.
• Complex Biology: Providing deeper insights into neurobiology, developmental biology, and cancer transcriptomics where long isoforms and complex splicing (including intron retention) play critical roles.
• Standardization: Offering a commercially available, reproducible kit that improves the sensitivity of DRS without sacrificing data quality, making the technology more accessible for discovering novel proteoforms and RNA modifications.

The authors conclude that while RNA fragility remains a challenge, shifting the reverse transcription step to a lower temperature is a critical step toward preserving the native state of long RNA molecules for sequencing.