Structural Variant Breakpoint

The molecular scar left at the breakpoint reveals the DNA repair mechanism responsible for the rearrangement:

• Non-homologous end joining (NHEJ): Blunt ends with 0-4 bp microhomology, often with small indels at the junction
• Microhomology-mediated break-induced replication (MMBIR): 2-15 bp shared sequence at breakpoints, characteristic of complex rearrangements
• Non-allelic homologous recombination (NAHR): Large stretches of high-identity sequence flanking the breakpoint, mediated by segmental duplications
• Fork stalling and template switching (FoSTeS): Microhomology-driven template jumps during replication, producing complex breakpoint patterns

Breakpoints are categorized by the precision of their mapping relative to the reference genome:

• Base-pair resolution: The exact nucleotide coordinate is known, typically from split reads or local reassembly spanning the junction
• Fragment-level resolution: The breakpoint is localized to a window defined by discordant read pair insert sizes, usually 200-500 bp
• Bin-level resolution: Only a broader genomic interval is identified, common in low-coverage whole-genome sequencing or array-based detection
• Breakpoint uncertainty directly impacts clinical reporting — variants of uncertain significance often require base-pair resolution for functional annotation

Breakpoints are non-randomly distributed across the genome, with strong enrichment in specific sequence contexts:

• Alu elements: The most common mediator of recurrent rearrangements, with Alu-Alu recombination driving deletions and duplications
• Segmental duplications: Blocks of >1 kb with >90% identity that serve as substrates for NAHR, enriched in pericentromeric and subtelomeric regions
• Low-copy repeats (LCRs): Mediate recurrent microdeletion/microduplication syndromes including 22q11.2 and 16p11.2
• Fragile sites: Chromosomal regions prone to breakage under replication stress, including FRA3B and FRA16D, frequently disrupted in cancer genomes

Two orthogonal signals from short-read sequencing provide complementary breakpoint evidence:

• Split reads: A single read where one segment maps to one side of the breakpoint and the remaining bases map to the other side. Provides base-pair resolution but requires the breakpoint to fall within a read
• Discordant read pairs: Mates mapping at unexpected distances or orientations relative to the insert size distribution. Provides fragment-level resolution and detects larger events
• Combined evidence: Modern callers like Manta and DELLY integrate both signals — discordant pairs define candidate regions, split reads refine exact coordinates
• Long reads (PacBio HiFi, Oxford Nanopore) often span entire breakpoints in a single read, unifying both signals

Some breakpoints exhibit intricate architectures beyond simple deletions or duplications:

• Chromothripsis: Tens to hundreds of clustered rearrangements on a single chromosome from a single catastrophic event, producing oscillating copy number profiles
• Chromoplexy: Chains of balanced interchromosomal translocations involving multiple chromosomes, common in prostate cancer
• Breakage-fusion-bridge cycles: Telomere loss leads to repeated fusion and breakage, generating inverted duplications and fold-back inversions
• These complex events require specialized detection algorithms that model the breakpoint graph rather than individual variant calls

The biological consequence of a breakpoint depends on its position relative to genomic features:

• Gene truncation: Breakpoint falls within a gene body, disrupting coding sequence and likely causing loss of function
• Gene fusion: Breakpoint joins two genes in-frame, creating a chimeric transcript — critical in oncogene activation (e.g., BCR-ABL1)
• Regulatory disruption: Breakpoint separates a gene from its cis-regulatory elements without altering coding sequence, causing position-effect variegation
• Topologically associating domain (TAD) disruption: Breakpoints that alter chromatin domain boundaries can cause enhancer adoption and gene misexpression, a mechanism implicated in congenital limb malformations

The use of sequencing technologies generating reads tens of thousands of bases long to span entire structural variants and their breakpoints in a single read.

• PacBio HiFi: Circular consensus sequencing providing high accuracy and length.
• Oxford Nanopore: Direct electronic sequencing with ultra-long reads exceeding 100kb.
• Advantage: Resolves breakpoints in repetitive regions where short reads fail to map uniquely.
• Mechanism: Reads directly align across deletion breakpoints or split at translocation junctions, providing single-molecule evidence.

A targeted computational method that performs de novo assembly of reads mapping to a specific region to resolve complex variants or haplotypes that cannot be determined by simple alignment.

• Process: Collects all reads with soft-clipped or discordant alignments at a candidate locus.
• Graph Construction: Builds a De Bruijn or overlap graph from collected reads.
• Path Finding: Traces paths through the graph to identify the novel sequence bridging the breakpoint.
• Utility: Essential for resolving complex events like chromothripsis or balanced translocations.

A mathematical representation of overlapping sequencing reads used to reconstruct a genome without a reference, where nodes represent sequences and edges represent overlaps.

• Application: Crucial for resolving complex structural variants where the derived allele is not present in the linear reference.
• Bubble Resolution: Heterozygous structural variants appear as bubbles in the graph; traversing both paths reveals the alternate haplotype.
• Breakpoint Identification: The exact coordinate where a path diverges from the reference backbone defines the breakpoint.

A compact representation within alignment files that summarizes the sequence of match, insertion, deletion, and clipping operations required to align a read to a reference genome.

• Soft Clipping (S): Bases at the read ends that do not align; a primary signal for breakpoints.
• Hard Clipping (H): Bases removed from the read that are not present in the alignment record.
• Split Reads: Reads with a large central deletion (D) or insertion (I) relative to the reference directly pinpoint the breakpoint.
• Example: 50M100S indicates 50 matching bases followed by 100 soft-clipped bases, suggesting a breakpoint.

The process of discarding sequencing reads with a low probability of being correctly aligned to the reference genome, reducing false positive variant calls caused by mismapped reads.

• MAPQ Score: Phred-scaled probability that the read is misaligned (e.g., MAPQ 0 = high chance of misalignment).
• Breakpoint Impact: Low MAPQ at breakpoints often indicates a true variant in a repetitive region; aggressive filtering removes real signals.
• Strategy: Use paired-end and split-read evidence to rescue low-MAPQ reads supporting a high-confidence structural variant call.

The proportion of sequencing reads supporting a variant allele relative to the total read depth at that locus, used to distinguish heterozygous germline variants from somatic mutations or artifacts.

• Germline Heterozygous: Expected VAF of ~50% for a balanced diploid genome.
• Somatic Mosaic: VAF significantly below 50% indicates a subclonal event or tissue-specific rearrangement.
• Breakpoint Application: Discordant read pairs and split reads supporting a structural variant breakpoint are counted to calculate the VAF of the rearrangement.
• Artifact Detection: Extremely low VAF (<5%) may indicate a sequencing or alignment artifact.