README.Rmd

---
title: "crisprBwa: alignment of gRNA spacer sequences using BWA"
output: 
  github_document:
    toc: true
bibliography: vignettes/references.bib
---

Authors: Jean-Philippe Fortin

Date: July 13, 2022



# Overview of crisprBwa

`crisprBwa` provides two main functions to align short DNA sequences to
a reference genome using the short read aligner BWA-backtrack [@bwa]
and return the alignments as R objects: `runBwa` and `runCrisprBwa`.
It utilizes the Bioconductor package `Rbwa` to access the BWA program
in a platform-independent manner. This means that users do not need to install
BWA prior to using `crisprBwa`. 


The latter function (`runCrisprBwa`) is specifically designed
to map and annotate CRISPR guide RNA (gRNA) spacer sequences using
CRISPR nuclease objects and CRISPR genomic arithmetics defined in
the Bioconductor package [crisprBase](https://github.com/crisprVerse/crisprBase).
This enables a fast and accurate on-target and off-target search of 
gRNA spacer sequences for virtually any type of CRISPR nucleases. 
It also provides an off-target search engine for our main gRNA design package [crisprDesign](https://github.com/crisprVerse/crisprDesign) of the 
[crisprVerse](https://github.com/crisprVerse) ecosystem. See the 
`addSpacerAlignments` function in `crisprDesign` for more details. 



# Installation and getting started

## Software requirements

### OS Requirements

This package is supported for macOS and Linux only.
Package was developed and tested on R version 4.2.1.


## Installation from Bioconductor

`crisprBwa` can be installed from from the Bioconductor devel branch
using the following commands in a fresh R session:

```{r, eval=FALSE}
if (!require("BiocManager", quietly = TRUE))
    install.packages("BiocManager")

BiocManager::install(version="devel")
BiocManager::install("crisprBwa")
```




# Building a bwa index

To use `runBwa` or `runCrisprBwa`, users need to first build a BWA
genome index. For a given genome, this step has to be done only once. 
The `Rbwa` package conveniently provides the function `bwa_build_index`
to build a BWA index from any custom genome from a FASTA file.

As an example, we build a BWA index for a small portion of the human
chromosome 12 (`chr12.fa` file provided in the `crisprBwa` package) and
save the index file as `myIndex` to a temporary directory:

```{r}
library(Rbwa)
fasta <- system.file(package="crisprBwa", "example/chr12.fa")
outdir <- tempdir()
index <- file.path(outdir, "chr12")
Rbwa::bwa_build_index(fasta,
                      index_prefix=index)
```

To learn how to create a BWA index for a complete genome or transcriptome,
please visit our [tutorial page](https://github.com/crisprVerse/Tutorials/tree/master/Building_Genome_Indices).



# Alignment using `runCrisprBwa`

As an example, we align 5 spacer sequences (of length 20bp) to the
custom genome built above, allowing a maximum of 3 mismatches between the 
spacer and protospacer sequences. 

We specify that the search is for the wildtype Cas9 (SpCas9) nuclease
by providing the `CrisprNuclease` object `SpCas9` available through the 
`crisprBase` package. The argument `canonical=FALSE` specifies that 
non-canonical PAM sequences are also considered (NAG and NGA for SpCas9).
The function `getAvailableCrisprNucleases` in `crisprBase` returns a character
vector of available `crisprNuclease` objects found in `crisprBase`.

We also need to provide a `BSgenome` object corresponding to the reference
genome used for alignment to extract protospacer and PAM sequences of the 
target sequences. 

```{r}
library(crisprBwa)
library(BSgenome.Hsapiens.UCSC.hg38)
data(SpCas9, package="crisprBase")
crisprNuclease <- SpCas9
bsgenome <- BSgenome.Hsapiens.UCSC.hg38
spacers <- c("AGCTGTCCGTGGGGGTCCGC",
             "CCCCTGCTGCTGTGCCAGGC",
             "ACGAACTGTAAAAGGCTTGG",
             "ACGAACTGTAACAGGCTTGG",
             "AAGGCCCTCAGAGTAATTAC")
runCrisprBwa(spacers,
             bsgenome=bsgenome,
             crisprNuclease=crisprNuclease,
             n_mismatches=3,
             canonical=FALSE,
             bwa_index=index)
```



# Applications beyond CRISPR

The function `runBwa` is similar to `runCrisprBwa`,
but does not impose constraints on PAM sequences.
It can be used to search for any short read sequence in a genome.

## Example using RNAi (siRNA design)

Seed-related off-targets caused by mismatch tolerance outside of the
seed region is a well-studied and characterized problem observed in RNA
interference (RNAi) experiments. `runBWa` can be used to map shRNA/siRNA seed
sequences to reference genomes to predict putative off-targets:

```{r, eval=TRUE}
seeds <- c("GTAAGCGGAGTGT", "AACGGGGAGATTG")
runBwa(seeds,
       n_mismatches=2,
       bwa_index=index)
```



# Reproducibility

```{r}
sessionInfo()
```


# References