mSigSpectra

Build mutational-spectrum catalogs from VCF files.

mSigSpectra is the lean, variant-caller-agnostic successor to ICAMS — it keeps the numerical core (VCF reading, annotation, indel justification, and SBS / DBS / ID catalog construction) and drops everything else. Plotting is now handled by the separate mSigPlot package. Shiny, PDF / zip reporting, and per-caller VAF extraction are intentionally out of scope.

What it does

read_vcf()  →  split_vcf()  →  annotate_vcf()  →  vcf_to_catalog()

read_vcf() — reads any VCF file body. Variant-caller-agnostic: it does not parse FORMAT / sample columns or extract VAF.
split_vcf() — partitions a VCF into SBS / DBS / ID sub-tables by REF/ALT length alone.
annotate_vcf() — adds flanking sequence context (BSgenome::getSeq), transcript strand (GenomicRanges::findOverlaps against shipped GENCODE tables), and — for indels — left-justifies and categorizes them (COSMIC-83 / Koh-89 / Koh-476).
vcf_to_catalog() — produces a single catalog of any of these types: SBS96, SBS192, SBS1536, DBS78, DBS136, DBS144, ID83, ID89, ID166, ID476.

The output is a plain numeric matrix with attributes (type, counts_or_density, ref_genome, region, abundance). No S3 classes. Functions key off attr(x, "type") via explicit checks.

Installation

# install.packages("remotes")
remotes::install_github("steverozen/mSigSpectra")

You also need the BSgenome package(s) for your reference genome(s):

BiocManager::install(c(
  "BSgenome.Hsapiens.1000genomes.hs37d5",   # GRCh37 / hg19
  "BSgenome.Hsapiens.UCSC.hg38",            # GRCh38 / hg38
  "BSgenome.Mmusculus.UCSC.mm10"            # GRCm38 / mm10
))

For plotting, install mSigPlot:

remotes::install_github("steverozen/mSigPlot")

Quick example

library(mSigSpectra)
library(mSigPlot)

vcf <- read_vcf("my_sample.vcf", filter = "PASS")
parts <- split_vcf(vcf)

ann_sbs <- annotate_vcf(parts$SBS, ref_genome = "GRCh38",
                        variant_type = "SBS")
sbs96 <- vcf_to_catalog(ann_sbs, type = "SBS96",
                        ref_genome = "GRCh38", region = "genome",
                        sample_name = "my_sample")

plot_SBS96(sbs96)

See vignettes/mSigSpectra.Rmd for a full walk-through that reads a real VCF, generates SBS / DBS / ID catalogs, writes them to disk, and plots each one with mSigPlot.

Migrating from ICAMS

ICAMS	mSigSpectra
`ReadVCFs()` / `SimpleReadVCF()`	`read_vcf()`, `read_vcfs()`
`SplitListOfVCFs()`	`split_vcf()`
`AnnotateSBSVCF()` / `AnnotateDBSVCF()` / `AnnotateIDVCF()`	`annotate_vcf(variant_type = ...)`
`VCFsToCatalogs()`	`vcfs_to_catalogs()`
`as.catalog()`	`as_catalog()`
`TransformCatalog()`	`transform_catalog()`
`ReadCatalog()` / `WriteCatalog()`	`read_catalog()` / `write_catalog()`
`PlotCatalog()`	(now in `mSigPlot::plot_SBS96()` etc.)

UpperCamelCase is gone. S3 catalog classes (SBS96Catalog, IndelCatalog, …) are gone — catalogs are plain matrices with attributes. The variant.caller argument and the per-caller VAF extractors (GetStrelkaVAF, GetMutectVAF, etc.) are gone.

Gotchas you have to handle yourself

mSigSpectra deliberately stays out of several judgment calls that ICAMS made for you. The reader returns the rows the VCF actually contains. You are responsible for sanitizing them before downstream analysis if the caller / VCF source has any of the following quirks:

1. Adjacent SBSs are not merged into DBSs

Some callers (notably Strelka) emit a real DBS as two adjacent SBSs at positions p and p+1. ICAMS’s Strelka-specific code path tried to detect these by VAF similarity and merged them into a single DBS row. mSigSpectra never does this — the result is one fewer DBS and two extra SBSs per pair. If your VCFs come from such a caller and you need DBSs, merge adjacent SBSs yourself (using your own VAF column) before calling split_vcf().

2. Multi-allelic sites: `ALT = "G,T"` style

Tri-allelic and tetra-allelic sites can be encoded as a single VCF row with multiple comma-separated alleles in ALT (A → G,T), or as multiple rows at the same (CHROM, POS), or with the bcftools norm -m - “split” representation. mSigSpectra:

Multi-allelic-on-one-row rows (ALT contains ,) — read_vcf() passes them through. split_vcf() will not classify them as SBS / DBS / ID since nchar(ALT) is 3+. They land silently in split_vcf()$discarded. If you want to include them, split each alternate allele into its own row first (e.g. with bcftools norm -m -any).
Same-position multi-row rows are kept by read_vcf(). The optional check_and_remove_discarded_variants() will discard them with reason “Variant with same CHROM, POS, REF but different ALT” — call it explicitly if you want this behavior.

3. “Complex” indels and equivalent-length substitutions

A complex change like REF = "AG", ALT = "TC" (two-base substitution where neither base matches) can equally well be represented as two adjacent SBSs (A → T and G → C), or as a single DBS, or as one deletion plus one insertion. There is no community standard. mSigSpectra:

REF[1] != ALT[1] and lengths differ — discarded by check_and_remove_discarded_variants() as “Complex indel”; left in place by the bare read_vcf() + split_vcf() path (where nchar(REF) != nchar(ALT) flags it as ID even though it does not conform to the ICAMS / COSMIC indel convention of a shared first base).
3-bp+ same-length substitutions like ACT → TGA — discarded by check_and_remove_discarded_variants() as “Variant involves three or more nucleotides”; classified into split_vcf()$discarded by the split_vcf() path. If you have a curated convention for these, apply it before catalog construction.

Some callers emit REF = "TA", ALT = "TT" (the T at position 1 is unchanged). These are degenerate DBSs — really single-base substitutions in disguise. check_and_remove_discarded_variants() discards them as “Wrong DBS variant”; split_vcf() will classify them as DBS unless you filter first.

5. Ambiguous reference bases

Variants with non-{A,C,G,T} REF bases (e.g. N) cannot be canonicalized. check_and_remove_discarded_variants() removes them; vcf_to_catalog() ultimately drops them when the pentanucleotide context contains N and warns.

6. FILTER conventions vary by caller

The default read_vcf(filter = TRUE) keeps rows where FILTER is in {"PASS", ".", ""} — the union of common caller defaults. This is not bit-exact compatible with any single caller — Strelka and Mutect keep only "PASS", Freebayes uses ".". If you need parity with one specific caller, pass filter = "PASS" (or your caller’s exact status string) explicitly.

Suggested defensive pipeline

vcf <- read_vcf(file, filter = "PASS")
clean <- check_and_remove_discarded_variants(vcf, name_of_vcf = file)
parts <- split_vcf(clean$df, name_of_vcf = file)
# Inspect clean$discarded.variants and parts$discarded for anything you
# want to recover or re-encode before the annotate / catalog step.

Status

Version 0.0.1.
R CMD check: 0 errors, 0 warnings.
235 testthat assertions across 13 test files.
SBS96 catalog construction is bit-exact against ICAMS for Mutect VCFs; diverges from ICAMS’s Strelka path by the documented no-adjacency-merge design choice.
DBS catalog construction passes structural / shape tests; full numeric parity vs ICAMS not yet verified (planned).
ID83 / ID89 / ID476 catalogs are wired through end-to-end on real Strelka ID VCFs.
transform_catalog() (counts ↔︎ density) and collapse_catalog() (SBS1536 → SBS96, SBS192 → SBS96, DBS144 → DBS78) implemented.
read_catalog() / write_catalog() round-trip ICAMS-native CSV losslessly; SigProfiler input formats parse for SBS96, SBS1536, ID83.

License

GPL-3.

Citation

If you use mSigSpectra in published work, please cite the original ICAMS papers:

Boot et al., Genome Research 2018, doi:10.1101/gr.230219.117.
Boot et al., Genome Research 2020, doi:10.1101/gr.255620.119.

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