Novel enzyme-based reduced representation method for DNA methylation profiling with low inputs

DNA methylation at cytosine-phospho-guanine (CpG) residues is a vital biological process that regulates cell identity and function. Although widely used, bisulfite-based cytosine conversion procedures for DNA methylation sequencing require high temperature and extreme pH, which lead to DNA degradation, especially among unmethylated cytosines. Enzymatic methylation sequencing (EM-seq), an enzyme-based cytosine conversion method, has been proposed as a less biased alternative for methylation profiling. Compared to bisulfite-based methods, EM-seq boasts greater genome coverage with less GC bias and has the potential to cover more CpGs with the same number of reads (i.e., higher signal-to-noise ratio). Reduced representation approaches enrich samples for CpG-rich genomic regions, thereby enhancing throughput and cost effectiveness. We hypothesized that enzyme-based technology could be adapted for reduced representation methylation sequencing to enable high-resolution DNA methylation profiling on low-input samples, including those obtained from clinical specimens. We leveraged the well-established differences in methylation profile between mouse CD4+ T cell populations to compare the performance of a novel reduced representation EM-seq (RREM-seq) procedure against an established reduced representation bisulfite sequencing (RRBS) protocol. While the RRBS method failed to generate reliable DNA libraries when using <2 ng of DNA (equivalent to DNA from around 350 cells), the RREM-seq method successfully generated reliable DNA libraries from 1–25 ng of mouse and human DNA. Ultra-low-input (<2-ng) RREM-seq libraries’ final concentration, regulatory genomic element coverage, and methylation status within lineage-defining Treg cell-specific super-enhancers were comparable to RRBS libraries with more than 10-fold higher DNA input. RREM-seq also successfully detected lineage-defining methylation differences between alveolar Tconv and Treg cells obtained from mechanically ventilated patients with severe SARS-CoV-2 pneumonia. Our RREM-seq method enables single-nucleotide resolution methylation profiling using low-input samples, including from clinical sources. ### Competing Interest Statement NM is currently an employee and owns stock in Vertex Pharmaceuticals. BDS holds United States Patent No. US 10,905,706 B2, Compositions and Methods to Accelerate Resolution of Acute Lung Inflammation, and serves on the Scientific Advisory Board of Zoe Biosciences. The other authors have no competing interests to declare.