NGS Leaders: A Community for Next-Generation Sequencing Pioneers

Editor’s Note: We are pleased to share an article submitted by Jeffrey Rosenfeld, PhD. Jeffrey is a Bioinformatics Scientist in the Division of High Performance and Research Computing at the University of Medicine and Dentistry of New Jersey (UMDNJ) and a Research Associate in the Division of Invertebrate Zoology at the American Museum of Natural History. 

May 15, 2012 : Jeffey Rosenfeld : In the past few years, the prices of sequencing have plummeted and now for a few thousand dollars, the complete sequence of an individual can be obtained. Even so, many scientists have opted to sequence just the exome (coding regions) of an individual and to ignore the rest of the genome. This focus on the exome has some justification, but I think that it is shortsighted and despite the higher cost, the sequencing of a complete genome is more valuable even if that means sequencing fewer samples.

The supporters of exome sequencing generally make the following points to justify their position:
A. The sequencing of an exome is much cheaper than the sequencing of a genome. It must be substantially cheaper to sequence 1% of the genome than the whole genome.
B. We don’t understand how to interpret non-coding variants and therefore we should limit our sequencing to genes that are well annotated.
C. Variants that are associated with a genetic disease are more likely to be found in a coding region since they directly alter the structure of a protein. 

I am not going to deny that there is some validity to these points, but I don’t think that they outweigh the shortcomings of exome sequencing and the benefits of whole genome sequencing that I will outline below. I understand that this is a contentious issue, and I welcome your comments whether you agree or disagree with my position.

1. Cost

The first reason the people generally look to exome sequencing is that of cost. Intuitively, the sequencing of 1% of the genome (the exome) should be cheaper than sequencing the entire genome. While this is true, the price differential is nowhere near 1:100 and is closer to 2:1 or 3:1 depending upon how the costs of the sequencing is calculated. Currently, a whole genome costs ~$4,000 and an exome costs ~$1,500. Why are these prices so close to each other? The answer is that the actual reagent cost of running the sequencer is not the only factor in the cost of a genome or an exome. For either type of experiment, library prep is required along with the costs associated with setting up a sequencing run of any size. For an exome, there is the additional cost of purchasing the selection kit which allow one to extract the coding sequences from raw DNA either using a microarray or in solution. This kit can cost several hundred dollars, and is therefore a substantial portion of the cost of exome sequencing.

Because of the lack of strong cost differential, the economic argument of favoring exome sequencing is not very strong. For the same amount of funding, a researcher would need to choose between say, 30 exomes and 10 genomes. While 30 samples are obviously better than 10, this is not a great differential. It is much less than the 1:100 differential that one would naively think of concerning the price of genome and exome sequences. An additional factor affecting the cost of exome sequencing is the time required to perform the hybridization. For the Nimblegen protocol, 72 hours of time are required for hybridization and 24 hours are required for the Agilent approach. These times add a delay into the time taken from sample to sequence which may be problematic for clinical applications