Module 9: IGV Curation for Long Reads: Split Reads, Haplotype Blocks & Technical Artifacts
Tested Stacks:
GATK 4.6.1
Samtools 1.21
Nextflow 24.10
Seurat v5.1
Python 3.12
Prerequisites: Basic Linux CLI & Sequencing Formats
📚 Chapter 9 of 12
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1. Configuring IGV for Long-Read Exploration
Integrative Genomics Viewer (IGV) defaults are optimized for $150\text{ bp}$ Illumina reads. Loading long reads ($20 - 50\text{ kb}$) under default settings will overwhelm system RAM and cause display freezing.
Optimal IGV configuration for long reads:
- Navigate to View → Preferences → Alignments.
- Set Visibility range threshold to $100\text{ kb}$.
- Set Maximum read depth to $100$.
- Enable Show soft-clipped bases.
- Right-click the BAM track and select Color alignments by → tag → enter
HP(to display maternal vs. paternal haplotype phasing).
2. The Anatomy of Structural Variant Signatures in IGV
Genuine genomic structural variants present distinct multi-read alignment signatures:
| Variant Type | Primary IGV Alignment Signature | Supplementary Alignment (SA) Behavior |
|---|---|---|
| Heterozygous Deletion | Sharp dip in read depth to $\sim 50\%$; multiple reads display consistent internal CIGAR deletions (D) or sharp soft-clipping at the same genomic coordinate. | Reads spanning the breakpoint continue alignment on the other side of the deleted interval. |
| Inversion | Reads spanning the inverted segment display split alignments with opposite strand orientations (colored blue/teal in IGV). | One segment aligns forward (+), the contiguous segment aligns reverse complement (-). |
| Translocation | Reads terminate abruptly at a defined breakpoint with extensive soft-clipping; mates or supplementary segments map to an entirely different chromosome. | SA tag points to distal chromosome coordinates. |
3. Filtering Out Technical Artifacts
To avoid false-positive variant reporting, clinical bioinformaticians must recognize common third-generation sequencing artifacts:
- Homopolymer Indel Noise: High frequency of single-base insertions or deletions in homopolymer runs (e.g.
AAAAAAA). If an indel has variable lengths across reads and lacks support in orthogonal chemistry, it is a technical artifact. - Chimeric Inverted Reads: A single continuous read containing inverted repeats joined by an adapter ligation artifact. Real inversions show multiple independent reads with identical breakpoints.
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Common Pitfalls & Troubleshooting Checklist
- Java Heap Exhaustion (GATK / Picard): If processes terminate with
java.lang.OutOfMemoryError, set your environment variable explicitly:export _JAVA_OPTIONS="-Xmx16g -Djava.io.tmpdir=/tmp". - Index Timestamp Mismatch: If tools complain that a BAM index is missing or out of date, regenerate with
samtools index -@ 4 sample.bam. - Conda Channel Conflicts: Avoid ABI incompatibilities between bioconda and conda-forge by setting strict priority:
conda config --set channel_priority strict. - Chromosome Naming Contamination: Always verify whether reference FASTA headers use
chr1or1; divergent naming causes silent alignment drops.
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