Introduction to NGS data analysis¶
NGS technologies, such as WGS, RNA-Seq, WES, WGBS, ChIP-Seq, generate significant amounts of output data. Before we start talking about various applications available on Genestack and how to choose appropriate ones for your analysis, let’s take a moment to go through the basics of sequencing analysis. To help you better understand the processes involved, we will use the example of genetic variant analysis for WES (Whole Exome Sequencing) data. A typical WES data analysis pipeline includes raw reads quality control, preprocessing, mapping, post-alignment processing, variant calling, followed by variant annotation and prioritization (Bao et al., 2010).
The first thing you need to do with sequencing data is to assess the quality of raw sequencing data. For example, you will get a general view on number and length of reads, if there are any contaminating sequences in your sample or low-quality sequences.
After that, you can do some preprocessing procedures to improve the initial quality of your data. For example, if your sequencing data is contaminated due to the sequencing process, you may choose to trim adaptors and contaminants from your data. Quality control and preprocessing are essential steps because if you do not make sure your data is of good quality to begin with, you cannot fully rely on analysis results.
After you have checked the quality of your data and if necessary, preprocessed it, the next step is mapping, also called aligning, of your reads to a reference genome or reference transcriptome. It allows determining the nucleotide sequence of data being studied with no need of de novo assembly because obtained reads are compared with a reference already existed in a database. For example, in our case, aligning WES reads allows you to discover nucleotides that vary between a reference sequence and the one being tested. The accuracy of the further variant identification depends on the mapping accuracy (The 1000 Genomes Project Consortium, 2010).
After you have mapped your reads, it is a good idea to check the mapping quality, as some of the biases in the data only show up after the mapping step.
Similarly to what you have done before with raw sequencing reads, if you are unsatisfied with the mapping quality, you can process the mapped reads and, for instance, remove duplicated mapped reads (which could be PCR artifacts). Post-alignment processing is very important, as it can greatly improve the accuracy and quality of further variant analysis.
Once the sequence is aligned to a reference genome, the data needs to be analyzed in an experiment-specific fashion. Here we will use the WES reads mapped against the reference genome to perform variant analysis, including variant calling and predicting the effects found variants produce on known genes (e.g. amino acid changes or frame shifts). In this step you compare your sequence with the reference sequence, look at all the differences and try to establish how big of an influence do these changes have on the gene. For instance, if it is a synonymous variant, it will probably have low influence on the gene as such a change causes a codon that produces the same amino acid. However, if it is a large deletion, you can assume that it will have a large effect on the gene function.
When it comes to visualising your data: the standard tool for visualisation of mapped reads and identified variants is the Genome Browser. Since visualization is one of the concepts at the core of our platform, on Genestack you will find a range of other useful tools that will help you better understand your data considering their nature. For example, for WES or WGS data, we suggest using Variant Explorer which can be used to sieve through thousands of variants and allow users to focus on their most important findings.