The textbook definition of Next-Generation Sequencing(NGS) is a high-throughput DNA sequencing methodology that makes use of parallelization to process up to half a million sequences concurrently. It is being used to (re)sequence genomes, determine the DNA-binding sites of proteins (ChIP-seq), sequence transcriptomes (RNA-seq). Compared to traditional(Sanger) sequencing, NGS is faster and ~1000x less expensive.
There are many methods for performing an RNA-seq experiment. In fact, the techniques are evolving so rapidly it can be difficult to decide which one to use. A basic choice mainly includes several steps (Figure1):
Library Preparation—The sequencing library is prepared by random fragmentation of the DNA or cDNA sample, followed by 5’ and 3’ adaptor ligation.Adapter-ligated fragments are then PCR amplified and gel purified.
Cluster Generation—For cluster generation, the library is loaded on a lawn of surfaces bound with oligos complementary to the library adapters. Each fragment is then amplified into distinct,clonal clusters through bridge amplification.
Sequencing—DNA polymerase catalyzes the incorporation of fluorescently labeled deoxyribonucleotide triphosphates(dNTPs) into a DNA template strand during sequential cycles of DNA synthesis and nucleotide is identified by fluorophore excitation.
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RNA samples are typically sequenced with either single end(SE) or pair end(PE) sequencing.
Single-End Sequencing: Single-read sequencing involves sequencing DNA from only one end, and is the simplest way for sequencing (Figure 2).
Paired-End Sequencing: A major advance in NGS technology occurred with the development of pair-end (PE) sequencing (Figure 2). PE sequencing enables both ends of the DNA fragment to be sequenced.
There has been a lot of discussion about anti-sense transcription and its biological relevance. For example, in some cases transcription creates anti-sense RNA constructs that may play a role in regulating gene expression. In fact, long noncoding RNA (lncRNA) analysis depends on directional RNA sequencing. As a result, it is often desirable to create libraries that retain the strand orientation of the original RNA targets so that you can directly determine which strand was being transcribed in your sample.
(adapted from: https://chipster.csc.ﬁ/manual/library-type-summary.html)
It is often desirable to create libraries that retain the strand orientation of the original RNA targets. For example, in some cases transcription creates anti-sense RNA constructs that may play a role in regulating gene expression. In fact, long noncoding RNA (lncRNA) analysis depends on directional RNA sequencing. Methods for preparing directional RNA-seq libraries are now readily available and different tools and softwares have different options to take these into account (Figure 3).
In general, there are three types of library preps:
"second-strand" = directional, where the ﬁrst read of the read pair (or in case of single end reads, the only read) is from the transcript strand
"ﬁrst-strand" = directional, where the ﬁrst read (or the only read in case of SE) is from the opposite strand.
The reads on the left are from the same strand as the transcript, and their pairs on the right are from the opposing strand. The number above the read states which read it is, the ﬁrst (/1) or the second (/2). Thus, perhaps a bit unintuitively, the ﬁrst case, "fr-ﬁrststrand" is such that the ﬁrst read (/1) is actually from the opposing strand as the transcript, and second read (/2) is from the transcript strand.
Why is this so important? If you use wrong directionality parameter, in the read counting step the reads are considered to be from the wrong strand. This means that in the cases where there are no genes on that other strand, you won't get any hits, and if there are genes in the same location on the other strand, your reads are counted for that wrong gene.
How can I check I chose correctly? It's a good idea to check that!
Using the tool RNA-seq strandedness inference and inner distance estimation using RseQC: We added this tool under the Quality control category to help you. The tool aligns subsets of the input FASTQ ﬁles against the reference genome, and this alignment is then compared to the reference annotation to deduce the strandedness. Make sure you select the correct reference when running the tool. Check out the help page of this tool for more information!
In aligners like HISAT2 and Tophat you can also do a comparison and check the mapping rate. Take a small subset of your reads and run HISAT2/TopHat with the different parameters and compare the results, and check the log ﬁle.
In HTSeq you can also run the tool with different options and check the number of reads that are not counted for any gene (=the "no-feature reads"). (In Chipster, open ﬁle htseq-count-info.txt).
Be extra careful to assign the paired ﬁles correctly! Using these parameters assumes you are giving the ﬁles in speciﬁc order: read1, read2. In Chipster always check from the parameters window that your ﬁles are assigned correctly.
Below we list some common library preparation kits and their corresponding parameters in different tools. Is your kit missing from the list? If you have the data generated with that kit and ﬁgure out the library type, please let us know too, so we can add that kit to the list below.
Information regarding the strand is not conserved (it is lost during the ampliﬁcation of the mRNA fragments).
TruSeq RNA Sample Prep kit
HISAT2 / TopHat / Cufﬂinks / Cuffdiff: library-type fr-unstranded
HTSeq: stranded -- no
Directional, ﬁrst strand:
The second read (read 2) is from the original RNA strand/template, ﬁrst read (read 1) is from the opposite strand. The information of the strand is preserved as the original RNA strand is degradated due to the dUTPs incorporated in the second synthesis step.
All dUTP methods, NSR, NNSR
TruSeq Stranded Total RNA Sample Prep Kit
TruSeq Stranded mRNA Sample Prep Kit
NEB Ultra Directional RNA Library Prep Kit
Agilent SureSelect Strand-Speciﬁc
HISAT2 / TopHat / Cufﬂinks / Cuffdiff: library-type fr-ﬁrst (
strand HTSeq: stranded -- reverse (
featurecount -s 2)
Directional, second strand:
The ﬁrst read (read 1) is from the original RNA strand/template, second read (read 2) is from the opposite strand. The directionality is preserved, as different adapters are ligated to different ends of the fragment.
Directional Illumina (Ligation), Standard SOLiD
ScriptSeq v2 RNA-Seq Library Preparation Kit (ribo-seq)
SMARTer Stranded Total RNA
Encore Complete RNA-Seq Library Systems
HISAT2 / TopHat / Cufﬂinks / Cuffdiff: library-type fr-secondstrand (
HTSeq: stranded -- yes (
featurecount -s 1)
Note also that the --fr/--rf/--ff or "Order of mates to align" parameter in Bowtie has similar sounding parameter options: [--fr: "Forward/reverse", --rf: "Reverse/Forward", --ff: "Forward/forward"]. However, these parameters are a bit different story, as they explain how the paired end reads are oriented towards each other (-> <-, -> -> or <- ->). The default (--fr, -> <-) is appropriate for Illumina's paired-end reads: it means that read 1 appears upstream of the reverse complement of read 2, or vice versa. When running TopHat, the library-type parameter is delivered to Bowtie, so the user doesn't have to worry about that too much.