Decoding genetic and epigenetic information embedded in cell free DNA with adapted SALP-seq

Cell free DNA (cfDNA) in human plasma carries abundant information, which has therefore been the key sample for noninvasive prenatal testing (NIPT) and liquid biopsy. Especially by using the rapidly developed next-generation sequencing (NGS) techniques, the genetic and epigenetic information embedded in cfDNA has been effectively and extensively decoded. In this process, a key step is to construct the NGS library. Due to its high degradation, the single strand-based method was reported to be more qualified to construct the NGS library of cfDNA. In order to develop a new simple method for this application, this study adapted our recently developed single strand adaptor library preparation (SALP) method for constructing an NGS library of cfDNA. In the improved method, cfDNA was firstly denatured into single strands and then ligated with a single strand adaptor (SSA) that had a 3 ' overhang of 3 random bases by using T4 DNA ligase. The SSA-ligated DNA was converted into double-stranded DNA with an additional adenine at the other end by polymerizing with Taq polymerase. Next, a barcode T adaptor (BTA) was ligated to this end. Finally, the cfDNA ligated with two adaptors was amplified with the Illumina-compatible primers for NGS. Using the method, this study successfully sequenced 20 cfDNA samples from 16 esophageal cancer patients and 4 healthy people. By bioinformatics analysis, this study found the genetic and epigenetic difference between patients and healthy people and identified 23 epigenetic and 28 genetic altered esophageal cancer-specific genes.