Friday, April 19, 2013

Common ptifalls of FDR estimation part one

Today’s most widely used method for FDR estimation is the target-decoy strategy. This is a well-established method in statistics and started to be used in proteomics around 2007.

In this approach, a decoy database that contains the same number of proteins as the target database are searched together by the database search engine to identify peptides. As illustrated in the figure, the blue colors indicate the target hits and the orange colors indicate the decoy hits, the squares are the false hits, and circles are true hits. 

The decoy proteins are randomly generated so that any decoy hit is supposedly a false hit. Since the search engine doesn’t know which sequences are from target and which are from decoy, when it makes a mistake, the mistake falls in the target and decoy databases with equal probability. Thus, the total number of false target hits can be approximated by the number of decoy hits in the final result. And the FDR can be estimated by the ratio between the numbers of decoy hits and the number of target hits. 

The target-decoy strategy is a powerful method for FDR estimation. However, as we will discover in the next little while, such a powerful method must be used with caution to avoid FDR underestimation. 

The first pitfall in the use of target-decoy approach for FDR estimation is due to the so-called multiple round search strategy in today’s database search software. 

This multi-round search was popularized by the X!Tandem program published in 2004, in order to speed up the computation. The first round uses a fast but less sensitive search method to quickly identify a shortlist of proteins from the large database. Then, the second round uses a more sensitive but slower search method to identify peptides, but only from the short list of proteins. This effectively speeds up the search without sacrificing too much sensitivity. Indeed, X!Tandem is one of the fastest search algorithm used today.

However, as pointed out by a paper published in JPR in 2010, this multiple-round search strategy screws up the target-decoy estimation of the FDR. The reason is that after the first round, there will be more target proteins than the decoy in the short list. Thus, if the second round search makes a mistake, the mistake will be more likely in the target proteins. So, we will end up with fewer decoy hits than the actual false target hits. This causes the FDR underestimation.

The JPR paper in 2010 provided a fix to this problem. But a year later, in another JPR paper, Bern and Kil pointed out that the fix was wrong, and proposed a different fix that required the change of the search engine’s algorithm. This shows that the FDR estimation is very tricky, even the experts can sometimes get it wrong. 

In PEAKS, we used a new approach, called decoy fusion to solve this problem. 

Instead of mixing the target and decoy databases, we append a decoy sequence to each target protein.

So, after the fast search round, the protein shortlist will still contain the same length of target and decoy sequences. And the false hits of the second round will have the equal chance to be from the target and decoy sequences. This recreates the balance and can accurately estimate the FDR in the multiple-round search setting.

*The content of this post is extracted from "Practical Guide to Significantly Improve Peptide Identification Sensitivity and Accuracy" by Dr. Bin Ma, CTO of Bioinformatics Solutions Inc. You can find the link to the guide on this page.

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