Nanopore Sequencing for Detection and Characterization of Phosphorothioate Modifications in Native DNA Sequences

Investigating the origins of the mutational signatures in cancer

Most of the risk factors associated with chronic and complex diseases, such as cancer, stem from exogenous and endogenous exposures experienced throughout an individual's life, collectively known as the exposome. These exposures can modify DNA, which can subsequently lead to the somatic mutations found in all normal and tumor tissues. Understanding the precise origins of specific somatic mutations has been challenging due to multitude of DNA adducts (i.e. the DNA adductome) and their diverse positions within the genome. Thus far, this limitation has prevented researchers from precisely linking exposures to DNA adducts and DNA adducts to subsequent mutational outcomes. Indeed, many common mutations observed in human cancers appear to originate from error-prone endogenous processes. Consequently, it remains unclear whether these mutations result from exposure-induced DNA adducts, or arise indirectly from endogenous processes or are a combination of both. In this review, we summarize approaches that aim to bridge our understanding of the mechanism by which exposure leads to DNA damage and then to mutation and highlight some of the remaining challenges and shortcomings to fully supporting this paradigm. We emphasize the need to integrate cellular DNA adductomics, long read-based mapping, single-molecule duplex sequencing of native DNA molecules and advanced computational analysis. This proposed holistic approach aims to unveil the causal connections between key DNA modifications and the mutational landscape, whether they originate from external exposures, internal processes or a combination of both, thereby addressing key questions in cancer biology.

Modification mapping by nanopore sequencing

Next generation sequencing (NGS) has provided biologists with an unprecedented view into biological processes and their regulation over the past 2 decades, fueling a wave of development of high throughput methods based on short read DNA and RNA sequencing. For nucleic acid modifications, NGS has been coupled with immunoprecipitation, chemical treatment, enzymatic treatment, and/or the use of reverse transcriptase enzymes with fortuitous activities to enrich for and to identify covalent modifications of RNA and DNA. However, the majority of nucleic acid modifications lack commercial monoclonal antibodies, and mapping techniques that rely on chemical or enzymatic treatments to manipulate modification signatures add additional technical complexities to library preparation. Moreover, such approaches tend to be specific to a single class of RNA or DNA modification, and generate only indirect readouts of modification status. Third generation sequencing technologies such as the commercially available "long read" platforms from Pacific Biosciences and Oxford Nanopore Technologies are an attractive alternative for high throughput detection of nucleic acid modifications. While the former can indirectly sense modified nucleotides through changes in the kinetics of reverse transcription reactions, nanopore sequencing can in principle directly detect any nucleic acid modification that produces a signal distortion as the nucleic acid passes through a nanopore sensor embedded within a charged membrane. To date, more than a dozen endogenous DNA and RNA modifications have been interrogated by nanopore sequencing, as well as a number of synthetic nucleic acid modifications used in metabolic labeling, structure probing, and other emerging applications. This review is intended to introduce the reader to nanopore sequencing and key principles underlying its use in direct detection of nucleic acid modifications in unamplified DNA or RNA samples, and outline current approaches for detecting and quantifying nucleic acid modifications by nanopore sequencing. As this technology matures, we anticipate advances in both sequencing chemistry and analysis methods will lead to rapid improvements in the identification and quantification of these epigenetic marks.