pantas

This repository contains the codebase of pantas, a pangenomic approach for performing haplotype-aware differential AS events quantification across RNA-Seq conditions. pantas is based on the notion of annotated spliced pangenomes, which are spliced pangenomes augmented with additional information needed for AS events inference and quantification.

Alongside pantas, we provide a set of utilities to build and index a (annotated) spliced pangenome. We devised two alternative construction pipelines, one for full genome analysis and one for the analysis of a panel of genes of interest (reduced indexing).

Installation

git clone https://github.com/AlgoLab/pantas.git

# Install dependencies (all available from bioconda)
mamba create -c bioconda -c conda-forge -n pantas \
             python=3.10 biopython gffutils intervaltree bcftools samtools gffread vg=1.56 snakemake-minimal
mamba activate pantas

cd pantas
bash setup.sh
# if setup ends properly, you should see the message "--- Everything done! ---"

pantas pipeline

All steps of pantas can be easily run using the pantas script.

# Print running modes
./pantas -h

All modes print a help message if run with the -h argument. Here we provide the minimum command lines arguments that are required to run pantas:

# Augment the annotated spliced pangenome with alignment information (run this for each replicate)
./pantas augment [condition1-rep1.gaf] [spliced-pangenome.annotated.gfa] > [condition1-rep1.gfa]

# Call events from each **augmented** graph
./pantas call [sample.gfa] [annotation.gtf] > [sample-events.csv]

# Quantify events across conditions (provide the two conditions with comma-separated path to the events csv)
./pantas quant [condition1-rep1.csv,condition1-rep2.csv,condition1-rep3.csv] \
      [condition2-rep1.csv,condition2-rep2.csv,condition2-rep3.csv] > [quantification.csv]
             
# Remap quantification on the linear reference using the annotation
./pantas remap [quantification.csv] [annotation.gtf] > [quantification.remap.csv]

Input preparation

The input of pantas are: an annotated spliced pangenome and the replicates aligned to this graph.

To build and index an annotated spliced pangenome, we provide a snakemake pipeline (build/build.smk) that can be conveniently run via the pantas script:

./pantas build -t [threads] -o [/path/to/out/dir] [reference.fa] [annotation.gtf] [variants.vcf.gz]

The annotated spliced pangenome is stored in the working directory directory (-o argument):

# Annotated spliced pangenome in GFA format:
pantranscriptome-annotated.gfa
# Compressed graph:
pantranscriptome.xg

The graph can then be indexed using `vg index`:
``` sh
vg index --gcsa-out [pantranscriptome.gcsa] --dist-name [pantranscriptome.dist] [pantranscriptome.xg]

This will produce 3 files in the working directory:

# Index:
pantranscriptome.dist
pantranscriptome.gcsa
pantranscriptome.gcsa.lcp

To map each replicate to the annotated spliced pangenome, we suggest to use vg mpmap:

vg mpmap -x [pantranscriptome.xg] -g [pantranscriptome.gcsa] -d [pantranscriptome.dist] -f [sample_1.fq] -f [sample_2.fq] -F GAF > [sample.gaf]

Example

The example subdirectory contains example data that can be used to test pantas:

# Prepare the graph
./pantas build -t 4 -o example/pantas-index example/4.fa example/4.gtf example/4.vcf.gz

# Index the graph
vg index --progress --threads 4 --gcsa-out example/pantas-index/pantranscriptome.gcsa --dist-name example/pantas-index/pantranscriptome.dist example/pantas-index/pantranscriptome.xg

# Align the RNA-Seq sample to the graph
vg mpmap -x example/pantas-index/pantranscriptome.xg \
         -g example/pantas-index/pantranscriptome.gcsa \
         -d example/pantas-index/pantranscriptome.dist \
         -f example/reads_1.fq -f example/reads_2.fq -F GAF > example/reads.gaf

# Augment the annotated spliced pangenome with alignment information
./pantas augment example/reads.gaf example/pantas-index/pantranscriptome-annotated.gfa > example/pantranscriptome-annotated-wreads.gfa

# Call all annotated events with minimum support 0 (since example RNA-Seq sample is very small)
# Note that using -w 0 is equivalent to extract all events from the graph
./pantas call -w 0 example/pantranscriptome-annotated-wreads.gfa example/4.gtf > example/reads.events.csv

# Quantify the events across the two conditions (an an example here we are using the same file twice)
./pantas quant example/reads.events.csv example/reads.events.csv > example/quant.csv

# Remap step
./pantas remap example/quant.csv example/4.gtf > example/quant-remap.csv
# this should produce 205 events

Custom output format

Annotated and augmented spliced pangenome

The annotated spliced pangenome augmented with alignment information (output of augment mode of pantas) is stored in a GFA file where optional fields are used to store the annotation. We refer to the documentation.

Events

The events (output of call mode) are stored in a CSV file:

• event type (ES, A3, A5, IR)
• annotated/novel
• chromosome (e.g., 4)
• gene name (e.g., FBgn0004859)
• strand (e.g., +)
• junction1, based on annotation (e.g., FBtr0308074.4.5 or ? if novel)
• junction1, in graph space (e.g., 2057>2065, meaning the junction link segments 2057 and 2065)
• junction1, on linear reference (e.g., 4:50614-50744)
• support for junction1 (e.g., 3)
• junction2, junction2 based on annotation, junction2 in graph space, junction2 on linear reference, support for junction2
• junction3, junction3 based on annotation, junction3 in graph space, junction3 on linear reference, support for junction3

We note that in the case of an exon skipping (or a cassette exon), all three junctions will be reported. In the case of an alternative splice site events, only two junctions are reported (1 and 2). A point (.) indicates that the junction is not used in the event.

Quantification

The differential quantification across conditions (output of quant mode of pantas) is stored in a CSV file:

• event type
• annotated/novel
• chromosome
• gene name
• strand
• transcripts annotation for junctions 1, 2, and 3 (3 fields)
• edges for junctions 1, 2, and 3 (3 fields)
• support for canonical isoform involved in the event (one value per condition, separated by /)
• support for minor isoform involved in the event (one value per condition, separated by /)
• PSI value for condition 1
• PSI value for condition 2
• ΔPSI

Remapping

The differential quantification across conditions remapped on the linear reference (output of remap mode of pantas) is stored in a CSV file:

• event type
• annotated/novel
• reference/haplotype (telling if event involves reference transcripts or haplotype transcripts)
• chromosome
• gene name
• strand
• transcripts annotation for junctions 1, 2, and 3 (3 fields)
• edges for junctions 1, 2, and 3 (3 fields)
• reference coordinates for junctions 1, 2, and 3 (3 fields)
• support for canonical isoform involved in the event (one value per condition, separated by /)
• support for minor isoform involved in the event (one value per condition, separated by /)
• PSI value for condition 1
• PSI value for condition 2
• ΔPSI

Experiments

Experimental evaluation scripts can be found in the ./exps subdirectory of this repository. We provide three snakemake pipelines to replicate the experiments descriped in the paper.

• ./exps/1-dm-sim/ contains the instructions to replicate the evaluation on simulated data from Drosophila Melanogaster
• ./exps/2-dm-real/ contains the instructions to replicate the evaluationon real data from Drosophila Melanogaster
• ./exps/3-homo-real/ contains the instructions to replicate the evaluation on real data from human

Additional details can be found in the README files available in these subdirectories.

Authors

pantas is developed by Simone Ciccolella, Davide Cozzi, and Luca Denti.

For inquiries on this software please open an issue.