Droplet-compatible single-cell DNA methylation sequencing with PreTIC

The paper introduces PreTIC, a novel method that enables high-throughput, cost-effective single-cell DNA methylation profiling on commercial droplet platforms using off-the-shelf reagents, successfully generating over 13,000 methylomes from fixed frozen cells to resolve cell-specific epigenetic variation.

The Gist

Imagine your body is a massive library containing billions of books (your cells). Each book holds the same instruction manual (DNA), but different books have different pages highlighted or crossed out. These highlights are called DNA methylation, and they act like sticky notes telling the cell: "Read this page," or "Ignore this page." These notes determine if a cell becomes a skin cell, a blood cell, or a brain cell.

For a long time, scientists could only read these sticky notes by smashing all the books together into a giant pile (bulk sequencing). This gave them an average of the highlights, but they couldn't see the unique notes in individual books.

Recently, scientists learned how to read individual books one by one (single-cell sequencing), but doing this for DNA methylation was like trying to read a book while it's on fire. The chemicals needed to reveal the sticky notes (bisulfite conversion) are so harsh that they usually destroy the book's structure, making it impossible to use the fancy, high-speed machines (droplet microfluidics) that scientists love to use for other types of data.

The New Solution: PreTIC (The "Hydrogel Bubble" Trick)

The authors of this paper invented a clever new method called PreTIC (Pre-tagmentation In Situ Conversion). Here is how they solved the problem, using a simple analogy:

1. The Problem: The Book vs. The Fire
Normally, to read the methylation notes, you have to melt the book down to single pages (single-stranded DNA), burn the notes to reveal them, and then try to glue the pages back together. But the glue (enzymes) doesn't work well on the burnt pages, and the high-speed machines (like the 10x Genomics platform) need the book to stay in its original, double-sided form to work.

2. The Innovation: The "Hydrogel Bubble"
The researchers came up with a brilliant workaround. Imagine putting each book inside a tiny, invisible, waterproof bubble made of gelatin (a hydrogel).

• The Bubble: They encased the cell nuclei in this gel. This gel acts like a shield.
• The Fire: They then poured the harsh "fire" chemicals (bisulfite) over the bubbles. The gel protects the physical structure of the nucleus, keeping the book intact even while the chemicals do their work inside.
• The Repair: Once the notes are revealed, they use a special "glue" (second-strand synthesis) to fix the pages back into a double-sided book while it's still inside the bubble.

3. The Result: Ready for the Fast Lane
Now, the books are fixed, the notes are revealed, and the books are still in their original shape. Because they are in this perfect state, they can be fed directly into the 10x Genomics machine—the same high-speed, automated machine that scientists use for RNA and other tests.

Why This Matters

Before this, reading the methylation notes of single cells was slow, expensive, and required custom-built, complicated equipment. It was like trying to read a library one book at a time using a quill and parchment.

With PreTIC:

• Speed: They can process over 13,000 cells in just two days. That's like reading a whole wing of the library in the time it used to take to read one shelf.
• Simplicity: They use "off-the-shelf" reagents and standard machines. No custom parts or PhD-level engineering required to set up the machine.
• Real-World Application: They tested this on human blood cells (PBMCs). They successfully identified different types of immune cells (like T-cells, B-cells, and Monocytes) just by looking at their unique methylation "sticky notes."

The Big Picture

Think of PreTIC as a new pair of glasses that allows scientists to see the unique "highlighting" in every single cell of your body, quickly and cheaply. This opens the door to understanding diseases like cancer or autoimmune disorders at a much deeper level, revealing how individual cells go wrong, rather than just looking at the average of the whole group.

In short: They built a protective bubble that lets them use the "fire" needed to read DNA methylation without destroying the book, allowing them to use the world's fastest book-reading machines to map the epigenetic landscape of life.

Technical Summary

1. The Problem

Single-cell DNA methylation sequencing (scDNAm-seq) is crucial for understanding epigenetic heterogeneity but remains technically challenging and less accessible than single-cell RNA-seq or ATAC-seq.

• Workflow Limitations: Existing methods rely on low-throughput plate-based workflows or complex combinatorial indexing strategies requiring custom reagents and long timelines.
• Droplet Incompatibility: While droplet-based platforms (e.g., 10x Genomics) have revolutionized scRNA-seq and scATAC-seq via in situ tagmentation, they are incompatible with standard methylation conversion chemistries.
  • The Core Conflict: In situ tagmentation (used in droplet platforms) requires double-stranded DNA (dsDNA) and intact nuclei. Conversely, bisulfite conversion (the gold standard for methylation detection) requires single-stranded DNA (ssDNA) and destroys dsDNA, generating ssDNA products. This fundamental mismatch has prevented the adaptation of high-throughput droplet systems for scDNAm-seq.

2. Methodology: PreTIC

The authors introduce Pre-tagmentation In Situ Conversion (PreTIC), a workflow that reconciles bisulfite conversion with droplet-based in situ tagmentation. The process is performed entirely in bulk prior to droplet generation.

Key Technical Steps:

1. Hydrogel Embedding: To preserve nuclear integrity during harsh chemical treatments, cells are fixed and embedded in a hydrogel matrix (adapted from expansion microscopy). This prevents nuclei from lysing during denaturation and bisulfite treatment.
2. In Situ Denaturation and Conversion: Inside the hydrogel-embedded nuclei, genomic DNA is denatured and subjected to bisulfite conversion. This converts unmethylated cytosines to uracils while leaving methylated cytosines intact.
3. Second-Strand Synthesis: A critical innovation is the in situ synthesis of the second DNA strand using random hexamers and Klenow Fragment. This regenerates double-stranded DNA containing uracils, making the substrate compatible with standard tagmentation enzymes.
4. Standard Droplet Workflow: The converted, intact nuclei are processed using the commercial 10x Genomics Single Cell ATACv2 workflow.
   • Modification: The standard Barcoding Enzyme in the 10x kit is replaced with Q5U® Hot Start High-Fidelity DNA Polymerase, which is capable of tolerating uracil residues generated during bisulfite conversion.
5. Library Preparation: Standard tagmentation, barcoding, and library construction follow the 10x protocol, requiring no custom oligonucleotides or dedicated microfluidics.

3. Key Contributions

• First Droplet-Compatible scDNAm-seq: PreTIC is the first method to enable whole-methylome profiling on a commercial droplet platform using off-the-shelf reagents.
• High Throughput: The method enables the profiling of >10,000 single-cell methylomes per sample (demonstrated with ~13,700 cells) in approximately two days.
• Simplicity and Scalability: It eliminates the need for custom reagents or complex combinatorial indexing, leveraging existing 10x infrastructure. The throughput can be scaled by utilizing additional channels.
• Compatibility with Fixed Samples: The workflow is optimized for fixed and frozen cells, facilitating the use of archived clinical samples.

4. Results

The authors validated PreTIC using three distinct experimental setups:

A. Species Mixing (Human-Mouse):

• Recovered 1,484 single-cell methylomes from a mixture of human (GM12878) and mouse (A20) cells.
• Doublet Rate: Estimated at 5.84%, indicating high purity.
• Coverage: Cells showed uniform genome-wide coverage without transcription start site (TSS) enrichment bias, comparable to single-cell whole-genome sequencing standards.

B. Human Cell Line Mixture:

• Profiled a mix of GM12878 (B cells), Jurkat (T cells), and THP-1 (monocytes).
• Clustering: UMAP analysis clearly separated the three cell lineages based on methylation patterns, even at low sequencing depths (~96k reads/cell).
• Reproducibility: Cells from independent runs co-clustered.
• Validation: Pseudobulk methylation profiles showed a strong correlation (r = 0.904) with ENCODE bulk whole-genome bisulfite sequencing (WGBS) data.

C. Primary Human PBMCs:

• Applied to fixed frozen Peripheral Blood Mononuclear Cells (PBMCs), recovering 13,701 single-cell methylomes.
• Data Quality: 99.63% of reads were retained; 86.40% aligned with minimal strand bias.
• Cell Type Resolution: Identified five major clusters corresponding to CD8+ T, CD4+ T, NK, Monocyte, and B cells.
• Biological Insights:
  • Global methylation (mCG) levels varied by lineage (e.g., monocytes and NK cells showed higher methylation).
  • Identified 117,047 cluster-specific Differentially Methylated Regions (DMRs).
  • 78.11% of these DMRs overlapped with candidate cis-regulatory elements (SCREEN annotations).
  • Gene set enrichment analysis confirmed functional relevance, highlighting immune processes like B cell activation and T cell differentiation.

5. Significance

• Democratization of scDNAm-seq: By utilizing commercial droplet platforms and standard reagents, PreTIC removes the technical barriers that have historically limited single-cell methylation studies to specialized labs.
• Clinical Applicability: The ability to process fixed, frozen samples with high throughput makes this method highly suitable for large-scale clinical studies and biobank analysis.
• Epigenetic Discovery: The method provides a scalable route to map epigenetic heterogeneity in complex tissues (like the immune system) at single-cell resolution, bridging the gap between bulk epigenomics and single-cell transcriptomics.
• Technical Innovation: The strategy of performing bisulfite conversion in situ within hydrogel-embedded nuclei followed by second-strand synthesis offers a novel solution to the "dsDNA vs. ssDNA" conflict in multi-omics workflows.