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\author{Lihua Julie Zhu, Michael Lawrence, Ankit Gupta, \\ Alper Kucukural, Manuel Garber, Scot Wolfe}
\begin{document}
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\title{GUIDEseq user's guide}
\maketitle
\tableofcontents
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\section{Introduction}
The most recently developed genome editing system, CRISPR-Cas9 has greater 
inherent flexibility than prior programmable nuclease platforms because 
sequence-specific recognition resides primarily within the associated sgRNA,
which permits a simple alteration of its recognition sequence. The short 
Protospacer Adjacent Motif (PAM), which is recognized by Cas9, is the chief 
constraint on the target site design density. Because of its simplicity and 
efficacy, this technology is revolutionizing biological studies and holds 
tremendous promise for therapeutic applications(Ledford, 2015; 
Cox et al., 2015).  

However, imperfect cleavage specificity of CRISPR/Cas9 nuclease within the 
genome is a cause for concern for its therapeutic application. S. pyogenes Cas9 
(SpyCas9)-based nucleases can cleave an imperfect heteroduplex formed between 
the guide sequence and a DNA sequence containing a functional PAM where the 
number, position and type of base mismatches can impact the level of activity
(Hsu et al., 2013; Mali et al., 2013; Fu et al., 2013). This degree of 
promiscuity is problematic for therapeutic applications, since 
the generation of DNA breaks at unintended (off-target) sites has the potential
to alter gene expression and function through direct mutagenesis or the 
generation of genomic rearrangements. Experimentally defining the number and 
activity of off-target sites for a given Cas9/sgRNA complex genome-wide is 
critical to assess and improve nuclease precision.

A new suite of genome-wide off-target detection methods have recently been 
described that can identify sites with low cleavage activity within a 
population of nuclease-treated cells. One of the most sensitive and 
straightforward methods to employ is GUIDE-seq (Tsai et al., 2015).
This method relies on erroneous NHEJ-mediated DNA repair to capture 
co-introduced blunt-ended double stranded oligonucleotides (dsODNs) at 
Cas9-induced breakpoints within the genome. The GUIDE-seq dsODNs display high 
insertion frequency (up to 50\% of the measured indel rate (Tsai et al., 2015)) 
at Cas9-induced DSBs, thereby tagging these loci for selective amplification and 
subsequent deep sequencing. The method is quite sensitive as off-target sites 
with >0.1\% indel frequency can be detected, and the frequency of dsODN 
insertion appears to be correlated with the frequency of Cas9-induced lesions 
at each site (Tsai et al., 2015). This method has been used successfully to 
evaluate the precision of Cas9 and its variants (tru-sgRNAs (Tsai et al., 2015) 
or PAM variants (Kleinstiver et al., 2015)). Given its favorable properties, 
GUIDE-seq could become a standard in the nuclease field for off-target analysis.

While the GUIDE-seq method is straightforward to employ, to date no 
bioinformatic tools have been released to the community to support the analysis
of this data. We developed \Biocpkg{GUIDEseq} package to faciliate the analysis
of GUIDE-seq dataset, including retaining one read per unique molecular 
identifier (UMI), filtering reads lacking integration oligo sequence (dsODNs), 
identifying peak locations (cleavage sites) and heights, merging cleavage sites
from plus strand and those from minus strand, and performing target and off
target search of the input gRNA. This analysis leverages our 
\Biocpkg{ChIPpeakAnno} package (Zhu et al., 2010) for merging cleavage sites 
from plus strand and minus strand, and \Biocpkg{CRISPRseek} package 
(Zhu et al., 2014) for defining the homology of any identified off-target site
to the guide sequence and Cas9 PAM specificity.  

\section{Workflow of GUIDE-seq data analysis}
Here is the workflow of GUIDE-seq data analysis with human sequence. First 
load \Biocpkg{GUIDEseq} and \Biocannopkg{BSgenome.Hsapiens.UCSC.hg19}.

To find BSgenome of other species, please refer to available.genomes in the
\Biocpkg{BSgenome} package. For example, 
\Biocannopkg{BSgenome.Hsapiens.UCSC.hg19} for hg19, 
\Biocannopkg{BSgenome.Mmusculus.UCSC.mm10} for mm10, 
\Biocannopkg{BSgenome.Celegans.UCSC.ce6} for ce6, 
\Biocannopkg{BSgenome.Rnorvegicus.UCSC.rn5} for rn5, \linebreak 
\Biocannopkg{BSgenome.Drerio.UCSC.danRer7} for Zv9, 
and \Biocannopkg{BSgenome.Dmelanogaster.UCSC.dm3} for dm3

Then specify the alignment file path as alignment.inputfile, and a umi file 
path as umi.inputfile containing unique molecularidentifier for each sequence.
\begin{scriptsize}
<<>>=
library(GUIDEseq)
umifile <- system.file("extdata", "UMI-HEK293_site4_chr13.txt", 
                       package = "GUIDEseq")
bedfile <- system.file("extdata","bowtie2.HEK293_site4_chr13.sort.bed",
                       package = "GUIDEseq")
bamfile <- system.file("extdata","bowtie2.HEK293_site4_chr13.sort.bam",
                       package = "GUIDEseq")
@
\end{scriptsize}

The alignment.inputfile is an alignment file in bed format containing CIGAR 
information. The alignment.inputfile contains chromosome, start, end, readID,
mapping quality, strand and CIGAR information as a tab delimited file. 
Here is an example line. \linebreak
chr13 27629253 27629403 HWI-M01326:156:1:113:4572:6938/1 44 + 150M \linebreak
Scripts for bin reads, remove adaptor, mapping to genome are available \linebreak
at http://mccb.umassmed.edu/GUIDE-seq/. 

The umi.inputfile is a tab delimited file containing at least two columns, 
read IDs and corresponding unique molecular identifiers (UMI). Script for 
creating umi.inputfile is available at \linebreak
http://mccb.umassmed.edu/GUIDE-seq/getUmi.pl. \linebreak 
An example input file is at \linebreak
http://mccb.umassmed.edu/GUIDE-seq/testGetUmi/. \linebreak
Please make sure to use R1 reads as input to getUmi.pl.

\subsection{Step 1: Remove PCR bias and obtain unique cleavage events }

PCR amplification often leads to biased representation of the starting sequence 
population. To track the sequence tags present in the initial sequence library,
unique molecular identifiers (UMI) are added to the 5 prime of each sequence 
in the staring library. The function \Rfunction{getUniqueCleavageEvents} uses 
the UMI sequence in the umi.inputfile (optionally contains umi plus the first 
few sequence from R1 reads) to obtain the starting sequence library. It also 
filters out reads that does not contain the integration oligo sequence, too 
short or not in the right paired configuration. 

For detailed parameter settings for function 
\Rfunction{getUniqueCleavageEvents}, \linebreak 
please type help(getUniqueCleavageEvents).

\begin{scriptsize}
<<>>=
uniqueCleavageEvents <- getUniqueCleavageEvents(bamfile, umifile, n.cores.max =1)
#uniqueCleavageEventsOld <- getUniqueCleavageEvents(bedfile, umifile)
uniqueCleavageEvents$cleavage.gr
@
\end{scriptsize}

\subsection{Step 2: Summarize cleavage events}
Calling the function \Rfunction{getPeaks} with the results from 
getUniqueCleavageEvents outputs summarized cleavage events for
each moving window with at least min.reads of cleavage events.

By default, window size is set to 20, step is set to 20, and min.reads is set
to 2. For detailed parameter settings using function 
\Rfunction{getPeaks}, please type help(getPeaks).

\begin{scriptsize}
<<>>=
peaks <- getPeaks(uniqueCleavageEvents$cleavage.gr, min.reads = 80)
peaks.gr <- peaks$peaks
peaks.gr$peaks
@
\end{scriptsize}

\subsection{Step 3: Merge peaks from plus and minus strand}
Calling the function \Rfunction{mergePlusMinusPeaks} with 
the output from getPeaks to merge peaks from plus strand and minus strand with 
specific orientation and within certain distance apart.

By default, plus.strand.start.gt.minus.strand.end is set to TRUE
and distance.threshold is set to 40, i.e., twice of the window size. 
For detailed parameter settings using function \linebreak
\Rfunction{mergePlusMinusPeaks}, please type \Rcode{help(mergePlusMinusPeaks)}.

\begin{scriptsize}
<<>>=
mergedPeaks <- mergePlusMinusPeaks(peaks.gr = peaks.gr,  
    output.bedfile = "mergedPeaks.bed")
mergedPeaks$mergedPeaks.gr
head(mergedPeaks$mergedPeaks.bed)
@
\end{scriptsize}

\subsection{Step 4: Off target analysis of extended regions around the 
identified cleavage sites}
Calling the function \Rfunction{offTargetAnalysisOfPeakRegions} with
input gRNA, peaks and genome of interest, to annotate identified cleavage sites 
with sequence homolgy to input gRNA.
For detailed parameter settings using function \linebreak
\Rfunction{offTargetAnalysisOfPeakRegions}, \linebreak
please type help(offTargetAnalysisOfPeakRegions)

\begin{scriptsize}
<<>>=
library(BSgenome.Hsapiens.UCSC.hg19)
peaks <- system.file("extdata", "T2plus100OffTargets.bed",
    package = "CRISPRseek")
gRNAs <- system.file("extdata", "T2.fa",
    package = "CRISPRseek")
outputDir <- getwd() 
offTargets <- offTargetAnalysisOfPeakRegions(gRNA = gRNAs, peaks = peaks,
    format=c("fasta", "bed"),
    peaks.withHeader = TRUE, BSgenomeName = Hsapiens,
    upstream = 50, downstream =50, PAM.size = 3, gRNA.size = 20,
    PAM = "NGG", PAM.pattern = "(NAG|NGG|NGA)$", max.mismatch = 2,
    outputDir = outputDir,
    orderOfftargetsBy = "predicted_cleavage_score",
    allowed.mismatch.PAM = 2, overwrite = TRUE
   )
@
\end{scriptsize}

\subsection{Run all steps in one workflow function}
The function \Rfunction{GUIDEseqAnalysis} is a wrapper function that uses the 
UMI sequence or plus the first few bases of each sequence from R1 reads to 
estimate the starting sequence library, piles up reads with a user defined 
window and step size, identify the cleavage sites, merge cleavage sites from 
plus strand and minus strand, followed by off target analysis of extended 
regions around the identified cleavage sites. 
For detailed parameter settings using function \Rfunction{GUIDEseqAnalysis}, \linebreak
please type help(GUIDEseqAnalysis)

\begin{scriptsize}
<<>>=
gRNA.file <- system.file("extdata","gRNA.fa", package = "GUIDEseq")
system.time(guideSeqRes <- GUIDEseqAnalysis(
    alignment.inputfile = bamfile, 
    umi.inputfile = umifile, gRNA.file = gRNA.file, 
    orderOfftargetsBy = "peak_score",
    descending = TRUE, n.cores.max = 1,
    BSgenomeName = Hsapiens, min.reads = 1))
names(guideSeqRes)
@
\end{scriptsize}

\section{References}
\begin{thebibliography}{1}
\bibitem{Cox,D.B.T. et al., 2015}Cox, D.B.T. et al. 
Therapeutic genome editing: prospects and challenges. Nat Med, 21, 121-131.
\bibitem{Fu,Y. et al., 2013}Fu, Y. et al. High-frequency off-target 
mutagenesis induced by CRISPR-Cas nucleases in human cells. Nature 
biotechnology, 31, 822-826.
\bibitem{Hsu et al., 2013}Hsu et al. DNA targeting specificity 
of rNA-guided Cas9 nucleases. Nat Biotechnol. 2013. 31:827-834.
\bibitem{Kleinstiver,B.P. et al., 2015} Kleinstiver, B.P. et al. Engineered 
CRISPR-Cas9 nucleases with altered PAM specificities. Nature. 2015
\bibitem{Ledford, H. 2015}Ledford, H. CRISPR, the disruptor. 
Nature 2015. 522, 20-24.
\bibitem{Mali et al., 2013}Mali P. et al. CAS9 transcriptional activators for 
target specificity screening and paired nickases for cooperative genome 
engineering. Nat Biotechnol. 2013. 31(9):833-8
\bibitem{Tsai,S.Q. et al., 2015}Tsai,S.Q. et al. GUIDE-seq 
enables genome-wide profiling of off-target cleavage by CRISPR-Cas nucleases. 
Nature biotechnology 2015. 33, 187-197.
\bibitem{Zhu L.J. et al., 2010}Zhu L.J. et al., 2010. 
ChIPpeakAnno: a Bioconductor package to annotate ChIP-seq and ChIP-chip data. 
BMC Bioinformatics 2010, 11:237doi:10.1186/1471-2105-11-237.
\bibitem{Zhu et al., 2014}Lihua Julie Zhu, Benjamin R. Holmes, Neil Aronin 
and Michael Brodsky. CRISPRseek: a Bioconductor package to identify 
target-specific guide RNAs for CRISPR-Cas9 genome-editing systems. Plos One 
Sept 23rd 2014
\end{thebibliography}

\section{Session Info}
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sessionInfo()
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\end{document}
