Enriched Methylomes of Low-input and Fragmented DNA Using Fragment Ligation EXclusive Methylation Sequencing (FLEXseq).

Methylome profiling is an emerging clinical tool for tumor classification and liquid biopsies. Here, we developed FLEXseq, a genome-wide methylation profiler that enriches and sequences the fragments of DNA flanking the CCGG motif. FLEXseq strongly correlates (Pearson's r = 0.97) with whole genome bisulfite sequencing (WGBS) while enriching 18-fold. To demonstrate the broad applicability of FLEXseq, we verified its usage across cells, body fluids, and formalin-fixed paraffin-embedded (FFPE) tissues. DNA dilutions down to 250 pg decreased CpG coverage, but bias in methylation remained low (Pearson's r ≥ 0.90) compared to a 10 ng input. FLEXseq offers a cost-efficient, base-pair resolution methylome with potential as a diagnostic tool for tissue and liquid biopsies.