Epigenetics and methylation analysis
Epigenetics, the study of heritable phenotypic changes that do not involve alteration of the nucleotide sequence, plays a key role in gene expression and has been associated with many diseases, including cancer. As PCR removes base modifications, their detection via traditional sequencing technologies requires the use of special library preparation steps (e.g. bisulfite conversion) which damage nucleic acids, resulting in very short sequencing reads. With nanopore sequencing, PCR is not required, enabling DNA and RNA modifications to be preserved and directly sequenced — with no additional library prep steps. Long-range epigenetic changes, structural variants, and single nucleotide polymorphisms can be identified and phased in a single dataset.
Directly detect DNA and RNA methylation with high reproducibility and low bias
Using nanopore sequencing, researchers have directly identified DNA and RNA base modifications at single nucleotide resolution, including 5mC, 5hmC, 6mA, and BrdU in DNA, and m6A in RNA, with detection of further natural or synthetic epigenetic modifications possible through training basecalling algorithms.
One of the most widespread genomic modifications is 5-methylcytosine (5mC), which most frequently occurs at CpG dinucleotides in the human genome. Benchmarking genome-wide nanopore 5mC detection against whole-genome, short-read bisulfite sequencing, the traditional method of 5mC detection, has shown nanopore sequencing to enable gold-standard 5mC calling (Figure 1). Nanopore technology calls a higher number of CpG positions in the human genome, requires less sequencing data, and shows more even genomic coverage; analysis runtime is also significantly shorter.
Benefits of nanopore technology over traditional bisulfite sequencing for methylation analysis:
- More even genomic coverage with lower GC bias
- Higher number of CpG positions called at lower read depth
- Simplified haplotype phasing of methylated bases using long reads
- Greater experimental reproducibility
- Considerably faster data analysis
Target important genomic regions without PCR for cost-effective characterisation of methylation patterns
Reduced-representation bisulfite sequencing (RRBS) is frequently used to perform methylation analysis without the need for whole-genome sequencing; however, the method is expensive and time consuming. Reduced-Representation Methylation Sequencing (RRMS) with Oxford Nanopore enables cost-effective, genome-wide characterisation of methylation patterns across regions of interest. RRMS utilises adaptive sampling: a real-time, flexible, and precise method to enrich for regions of interest by depleting off-target regions during a sequencing run, without requiring special library preparation steps.
With adaptive sampling or Cas9 targeted sequencing, Oxford Nanopore offers scalable, flexible, PCR-free methods of targeted methylation detection - whether in a single target, a panel of genes, or across multiple, large regions of interest.
Detecting methylation in repetitive regions of the human genome
‘we are just scratching the surface about unveiling epigenetic control of these [repetitive] regions’
With the limited capacity of traditional short-read sequencing technologies to access repeat-rich regions, such areas of the human genome have remained underexplored. Ariel Gershman and colleagues at Johns Hopkins University, USA, have been using long nanopore reads to reveal the methylome of previously unexplored large repetitive arrays in the human genome. Analysis of higher order repeats (HORs) at chromosome centromeres, which provide binding sites for the centromere-associated histone variant CENPA, uncovered distinctive patterns of hypomethylation and hypermethylation. Her team demonstrated for the first time how long nanopore reads could reveal long-range, allele-specific DNA methylation patterns across HORs and other classes of satellite repeats. These analyses have the potential to advance our understanding of the role of methylation in chromosome segregation and its association with genetic disorders.
How do I detect base modifications using nanopore sequencing?
PromethION is suitable for high-throughput epigenetic studies of larger genomes, such as human and many plant genomes, whilst MinION and GridION devices are ideal for epigenetic analysis of microbial genomes or eukaryotic transcriptomes and targeted genomic regions. Amplification-free library preparation is required in order to maintain base modifications. For DNA sequencing, we recommend the Ligation Sequencing Kit. For RNA modification analysis, the Direct RNA Sequencing Kit is required. Nanopore sequencing is the only currently available technology that allows direct sequencing of native RNA, with no requirement for amplification or reverse transcription.
For gold-standard 5mC methylation calling in the human genome, we recommend the algorithm Remora, which is integrated into Guppy and MinKNOW – the software onboard nanopore sequencing devices. Remora models run alongside basecalling, and provide superior performance to that of existing modification calling tools. Remora models for the detection of both 5mC and 5hmC are available. For more advanced usage, such as if you wish to train your own models to detect further epigenetic modifications of interest, you can access the Remora repository on GitHub. Next, we recommend using modbamtobed: a tool integrated into EPI2ME that aggregates per-read modification calls to per-genomic position modification frequencies, outputting a bedMethyl file.