Next Generation Sequencing

Learning Objectives

  • Describe the basis of the term “Next Generation Sequencing” with respect to Sanger sequencing.

  • Describe the core principles of the Illumina NGS process, including detailed knowledge of the four-step process

    • Library construction

    • Cluster generation

    • Sequencing by synthesis

    • Data Analysis

  • Describe in detail the principles of some applications of NGS, including Whole Exome Sequencing (WES) and RNA-seq

Basis of "Next Generation Sequencing" (NGS) Compared to Sanger Sequencing

NGS represents a massively parallel sequencing technology, offering higher throughput and speed than traditional Sanger sequencing. Here’s how they compare:

  1. Throughput:

    • Sanger sequencing reads one DNA sequence at a time (1 reaction = 1 sequence).

    • NGS can sequence millions of DNA fragments simultaneously.

  2. Scale and Cost:

    • Sanger: Costly for large-scale sequencing; suited for short sequences or specific regions (e.g., small genes).

    • NGS: Economical for large-scale projects, enabling whole-genome or transcriptome sequencing.

  3. Speed:

    • Sanger requires time-intensive electrophoresis for each reaction.

    • NGS processes multiple fragments in parallel, generating results within days.

  4. Applications:

    • Sanger: Gold standard for confirming specific mutations or small-scale sequencing.

    • NGS: Ideal for genomic research, clinical diagnostics, and large-scale studies like GWAS.

Core Principles of Illumina NGS

The Illumina sequencing platform relies on sequencing by synthesis (SBS). The process consists of four key steps:

1. Library Construction

  • Objective: Prepare DNA for sequencing by breaking it into small fragments and adding adapters for compatibility with the sequencing platform.

  • Steps:

    1. Fragmentation: Shear DNA into 200–300 bp fragments using sonication or enzymes.

    2. End Repair: Modify DNA ends to allow ligation of adapters.

    3. Adapter Ligation: Add adapter sequences to each fragment. Adapters include:

      • Sequencing primers binding sites.

      • Indices for sample identification in multiplex sequencing.

      • Flow cell anchors for attachment during cluster generation.

    4. Size Selection: Select and purify fragments of the desired size using beads or gel electrophoresis.

2. Cluster Generation

  • Objective: Amplify DNA fragments to detectable levels by creating clusters of identical sequences on a solid surface (flow cell).

  • Steps:

    1. Hybridization: Single-stranded DNA library fragments bind to complementary oligonucleotides on the flow cell.

    2. Bridge PCR: DNA bends to form a bridge, and amplification occurs directly on the flow cell surface, creating clusters.

    3. Denaturation: Double-stranded clusters are denatured, leaving single-stranded DNA templates for sequencing.

3. Sequencing by Synthesis (SBS)

  • Objective: Determine the nucleotide sequence of each DNA fragment by synthesizing complementary strands base-by-base.

  • Steps:

    1. Nucleotide Incorporation: A DNA polymerase adds one fluorescently labeled, reversible terminator nucleotide (A, T, G, or C) at a time.

    2. Imaging: After each nucleotide addition, a high-resolution camera captures fluorescence signals.

    3. Cleavage: The fluorescent tag and terminator group are removed, enabling the addition of the next nucleotide.

    4. Iteration: Steps are repeated for up to 250 cycles, yielding read lengths of 50–300 bp.

4. Data Analysis

  • Objective: Convert raw sequencing signals into biologically meaningful data.

  • Steps:

    1. Base Calling: Fluorescent signals are translated into nucleotide sequences.

    2. Alignment: Reads are aligned to a reference genome to identify variations.

    3. Quantification: For RNA-seq, quantify gene expression levels.

    4. Annotation: Classify variants (e.g., SNPs, insertions, deletions).

Applications of NGS

1. Whole Exome Sequencing (WES)

  • Objective: Sequence all protein-coding regions (exons) in the genome (~1-2% of the genome).

  • Steps:

    1. Target Enrichment: Use probes to capture exon sequences from fragmented DNA.

    2. Library Preparation: Construct libraries for sequencing.

    3. NGS Sequencing: Sequence the captured regions.

    4. Analysis:

      • Align reads to a reference genome.

      • Identify variants (e.g., pathogenic mutations in disease-related genes).

  • Advantages:

    • Focuses on regions where most disease-causing mutations occur.

    • Faster and cheaper than whole-genome sequencing.

2. RNA-seq

  • Objective: Analyze the transcriptome to measure gene expression levels and identify alternative splicing or fusion transcripts.

  • Steps:

    1. RNA Extraction: Extract total RNA or mRNA.

    2. cDNA Synthesis: Convert RNA into complementary DNA (cDNA) using reverse transcription.

    3. Library Construction: Prepare cDNA for sequencing by adding adapters.

    4. Sequencing: Perform NGS using SBS.

    5. Analysis:

      • Map reads to the reference transcriptome.

      • Quantify expression levels based on read counts.

      • Identify differentially expressed genes.

  • Applications:

    • Compare gene expression between conditions (e.g., healthy vs. diseased tissue).

    • Discover novel transcripts and isoforms.

Summary

NGS technologies like Illumina sequencing have revolutionized genomics, offering unparalleled speed, accuracy, and scale compared to traditional Sanger sequencing. Core applications include:

  • WES: Efficiently identifies disease-causing mutations in exons.

  • RNA-seq: Captures comprehensive transcriptome data for expression analysis.

  • Advantages: NGS is increasingly used in research and clinical diagnostics due to its scalability and cost-efficiency.

If you'd like further details on any specific aspect, feel free to ask!