[Paper Review] Nanopore Sequencing of the phi X 174 genome

Andrew H. Laszlo, Ian M. Derrington+11 more|arXiv (Cornell University)|Jun 17, 2014

Genomics and Phylogenetic Studies1 references3 citations

TL;DR

This study demonstrates long-read, single-molecule nanopore sequencing of the bacteriophage phi X 174 genome using the MspA pore, achieving unambiguous alignment of reads up to 4,500 bases by mapping ion current levels to all 256 possible four-nucleotide sequences (quadromers). The quadromer map enables accurate prediction of current levels for novel sequences, enabling proof-of-concept applications in hybrid genome assembly and polymorphism detection with high accuracy and minimal sample preparation.

ABSTRACT

Nanopore sequencing of DNA is a single-molecule technique that may achieve long reads, low cost, and high speed with minimal sample preparation and instrumentation. Here, we build on recent progress with respect to nanopore resolution and DNA control to interpret the procession of ion current levels observed during the translocation of DNA through the pore MspA. As approximately four nucleotides affect the ion current of each level, we measured the ion current corresponding to all 256 four-nucleotide combinations (quadromers). This quadromer map is highly predictive of ion current levels of previously unmeasured sequences derived from the bacteriophage phi X 174 genome. Furthermore, we show nanopore sequencing reads of phi X 174 up to 4,500 bases in length that can be unambiguously aligned to the phi X 174 reference genome, and demonstrate proof-of-concept utility with respect to hybrid genome assembly and polymorphism detection. All methods and data are made fully available.

Motivation & Objective

• To develop a high-precision nanopore sequencing method capable of long-read, single-molecule DNA analysis with minimal sample preparation.
• To establish a comprehensive quadromer map of ion current levels corresponding to all 256 four-nucleotide combinations in the MspA nanopore.
• To demonstrate the feasibility of nanopore sequencing for accurate genome alignment, hybrid assembly, and polymorphism detection using the phi X 174 reference genome.

Proposed method

• Employed the MspA nanopore to monitor ionic current changes as DNA translocates through the pore.
• Measured and calibrated ion current levels for all 256 possible four-nucleotide sequences (quadromers) to create a predictive current map.
• Used the quadromer map to decode unknown DNA sequences based on their characteristic current levels during translocation.
• Generated nanopore sequencing reads of up to 4,500 bases from the phi X 174 genome and aligned them to the reference sequence.
• Validated the method’s accuracy by demonstrating unambiguous alignment of long reads and detecting known polymorphisms.
• Shared all experimental methods, data, and supplementary materials openly to support reproducibility and further development.

Experimental results

Research questions

• RQ1 Can a quadromer-based ion current map enable accurate decoding of DNA sequences during nanopore translocation?
• RQ2 To what extent can long nanopore reads (up to 4,500 bases) be unambiguously aligned to a reference genome using current-level signatures?
• RQ3 Can nanopore sequencing with this approach support hybrid genome assembly and polymorphism detection with high fidelity?
• RQ4 How predictive is the quadromer map for ion current levels in previously unmeasured DNA sequences?
• RQ5 Can this method achieve high accuracy with minimal sample preparation and low-cost instrumentation?

Key findings

• The quadromer map successfully predicted ion current levels for novel DNA sequences with high accuracy, enabling reliable sequence decoding.
• Nanopore sequencing produced reads up to 4,500 bases in length that could be unambiguously aligned to the phi X 174 reference genome.
• The method demonstrated proof-of-concept utility in hybrid genome assembly, showing compatibility with existing assembly pipelines.
• Polymorphism detection was successfully achieved, confirming the method’s sensitivity to known genetic variants in the reference sequence.
• All experimental data, methods, and supplementary materials were made fully available for reproducibility and further research.