With decreasing costs to sequence whole human genomes (currently $1,550 for 35X coverage), we frequently hear researchers ask, “Why should I only sequence protein coding genes” ?
First, WGS of entire populations is still quite expensive. These types of projects are currently only being performed by large centers or government entities, like Genomics England, a company owned by UK’s Department of Health, which announced that they would sequence 100,000 whole genomes by 2017. At Genohub’s rate of $1,550/genome, 100,000 genomes would cost $155 million USD. This $155 million figure only includes sequencing costs and does not take into account labor, data storage and analysis which is likely several fold greater.
Second, the exome, or all ~180,000 exons comprise less than 2% of all sequence in the human genome, but contain 85-90% of all known disease causing variants. A more focused dataset makes interpretation and analysis a lot easier.
Let’s assume you’ve decided to proceed with exome sequencing. The next step is to either find a service provider to perform your exome capture, sequencing and analysis or do it yourself. Genohub has made it easy to find and directly order sequencing services from providers around the world. Several of our providers offer exome library prep and sequencing services. If you’re only looking for someone to help with your data analysis, you can contact one of our providers offering exome bioinformatics services. Whether you decide to send your samples to a provider or make libraries yourself, you’ll need to decide on what capture technology to use, the number of reads you’ll need and what type of read length is most appropriate for your exome-seq project.
There are currently three main capture technologies available: Agilent SureSelect, Illumina Nextera Rapid Capture, Roche Nimblegen SeqCap EZ Exome. All three are in-solution based and utilize biotinylated DNA or RNA probes (baits) that are complementary to exons. These probes are added to genomic fragment libraries and after a period of hybridization, magnetic streptavidin beads are used to pull down and enrich for fragmented exons. Each of these three exome capture technologies is compared in a detailed table: https://genohub.com/exome-sequencing-library-preparation/. Each kit has a varying numbers of probes, probe length, target region, input DNA requirements and hybridization time. Researchers planning on exome sequencing should first determine whether the technology they’re considering covers their regions of interest. Only 26.2 Mb of total targeted bases are in common, and only small portions of the CCDS Exome are uniquely covered by each tech (Chilamakuri, 2014).
Our Exome Guide breaks down the steps you’ll need to determine how much sequencing and what read length is appropriate for your exome capture sequencing project.