- Open Access
TopHat-Recondition: a post-processor for TopHat unmapped reads
© Brueffer and Saal. 2016
Received: 13 January 2016
Accepted: 20 April 2016
Published: 4 May 2016
TopHat is a popular spliced junction mapper for RNA sequencing data, and writes files in the BAM format – the binary version of the Sequence Alignment/Map (SAM) format. BAM is the standard exchange format for aligned sequencing reads, thus correct format implementation is paramount for software interoperability and correct analysis. However, TopHat writes its unmapped reads in a way that is not compatible with other software that implements the SAM/BAM format.
We have developed TopHat-Recondition, a post-processor for TopHat unmapped reads that restores read information in the proper format. TopHat-Recondition thus enables downstream software to process the plethora of BAM files written by TopHat.
TopHat-Recondition can repair unmapped read files written by TopHat and is freely available under a 2-clause BSD license on GitHub: https://github.com/cbrueffer/tophat-recondition.
RNA sequencing (RNA-seq) has become as a cornerstone of genomics research. TopHat and TopHat2 [1, 2] (jointly referred to as TopHat from here on) is a highly-cited spliced read mapper for RNA-seq data that is used in many large-scale studies around the world, for example in breast cancer . A search for the term “TopHat” in the NCBI Gene Expression Omnibus (GEO) and the European Nucleotide Archive (ENA) yields 288 and 197 datasets using TopHat, respectively, with the true number being likely much higher.
TopHat writes read data in the BAM format – the binary version of the Sequence Alignment/Map (SAM) format , but unlike other read mappers, it writes separate files for reads it could map to the reference genome (accepted_hits.bam) and reads it could not map (unmapped.bam). Although many analyses focus on mapped reads alone, often it is necessary to consider unmapped reads, for example to perform quality assurance, to deposit the data in online archives, or to analyze the unmapped reads themselves.
However, all released versions of TopHat to date (version ≤ 2.1.1) generate unmapped.bam files that are incompatible with common downstream software, e.g., the Picard suite (http://broadinstitute.github.io/picard), SAMtools , or the Genome Analysis Toolkit (GATK) . Even if the problems leading to the incompatibility are corrected in future versions of TopHat, an immense amount of data has already been aligned with affected versions and would need to be realigned, and potentially reanalyzed. TopHat-Recondition is a post-processor for TopHat unmapped reads that corrects the compatibility problems, and restores the ability to process BAM files containing unmapped reads.
Results and discussion
TopHat’s unmapped.bam incompatibility with other tools has three origins: software bugs resulting in violations of the SAM/BAM specification (https://samtools.github.io/hts-specs/SAMv1.pdf), divergences from the specification’s recommended practices, and different interpretation of acceptable values for some of the file format’s fields between software.
Two TopHat issues impair compatibility: First, all unmapped read-pairs lack the 0x8 bit (next segment in the template unmapped) in their FLAGS field. This leads to downstream software incorrectly assuming the reads to be mapped. Second, for unmapped reads where the FLAGS field declares the paired read to be mapped, this mapped paired read may be missing from the sequence files. This makes the unmapped read’s fields invalid and can lead to software searching for, and failing to find the paired read.
The SAM/BAM specification contains a section on recommended practices for implementing the format. For read-pairs with one mapped and one unmapped read, TopHat does not follow the recommendations that RNAME and POS of the unmapped read should have the same field values as the mapped read. Additionally we found that setting RNEXT to the mapped read’s RNEXT value, and PNEXT to 0 improves compatibility.
Lastly, there are differing interpretations of which field values are acceptable in certain conditions between software packages. For example, the valid range of values for the BAM mapping quality (MAPQ) is 0-255. For unmapped reads, TopHat always sets the MAPQ value of unmapped reads to 255, and BWA  sets the value to greater than 0 in certain conditions, while the Picard suite asserts that this value be 0 and returns an error when encountering such a read, which can confuse users.
Some BAM-processing software, e.g., Picard and GATK can be configured to accept reads that do not conform to its expectations by ignoring errors, thus allowing processing to succeed. However, the resulting BAM files remain non-compliant to the specification which can lead to issues in later analysis steps that are difficult to debug.
The occurrence of these problems is dependent on both the sequencing depth and the percentage of unmapped reads in the dataset; a higher value in either category can result in a higher rate of errors.
TopHat-Recondition either repairs or works around these problems, which allows processing to complete with all SAM/BAM-compliant software without relying on reducing strictness requirements.
Usage information and a walk-through example can be found in Additional file 1.
TopHat-Recondition enables easy and fast post-processing for TopHat unmapped reads. The tool can be used to process TopHat-written unmapped reads to make them compatible with downstream tools such as samtools, the Picard suite and GATK, which is currently not possible with the stock unmapped reads. This will increase the utility of the immense amount of RNA-seq data that has been analyzed by TopHat.
Availability and requirements
Project name: TopHat-ReconditionProject home page: https://github.com/cbrueffer/tophat-recondition Operating system(s): Platform independentProgramming language: PythonOther requirements: PysamLicense: 2-clause BSDAny restrictions to use by non-academics: none
The authors would like to thank the TopHat developers for their software and permission to use the name TopHat as part of TopHat-Recondition, and Christof Winter for fruitful discussions about the SAM/BAM file format.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
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