A fusion gene is a hybrid gene that combines parts of two or more original genes. They can form as a result of chromosomal rearrangements (such as translocation, interstitial deletion, or chromosomal inversion) or abnormal transcription and have been shown to act as drivers of malignant transformation or/and progression in various neoplasms (1). The discovery and characterization of fusion genes hav been greatly facilitated by the use of NGS (2) and several computational algorithms have been developed to detect them.
This chapter covers will illustrate how to detect fusion genes by:
TopHat-Fusion is a version of TopHat with the ability to align reads across fusion points and detect fusion genes resulting from breakage and re-joining of two different chromosomes or from rearrangements within a chromosome (3). It is independent of gene annotation and can discover fusion products from known genes, unannotated splice variants of known genes or completely unknown genes.
The reads are first aligned to the genome. The unaligned reads resulting from this initial alignment are split into multiple 25 bp sequences which are, in turn, aligned to the genome by Bowtie. The TopHat-Fusion algorithm identifies the cases where the first and the last 25 bp segments are aligned to either two different chromosomes or two locations on the same chromosome (spacing is defined by the user). The whole read is used to identify a fusion point. After the initial fusion candidates are defined, all the segments from the initially unaligned reads are realigned against the fusion points (as well as intron boundaries and indels). The resulting alignments are combined with the full read alignments.
The most up to date TopHat-Fusion version implemented in Partek® Flow® when the manual was written (2.1.0) focuses on fusions due to chromosomal rearrangements, while fusions resulting from read-through transcription or trans-splicing were not supported. For details as well as discussion of TopHat-Fusion options, see TopHat-Fusion home page (4).
TopHat-Fusion is integrated in the TopHat 2 task and is invoked by using the Fusion search check box in the Alignment options dialog (Figure 1).
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The output is generated as a new data node Fusion results (Figure 2) stemming as part of the if the TopHat 2 align reads task (in addition to Aligned reads node and, optionally, Unaligned reads node).
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Selecting the Fusion results data node opens the task menu, with four options (Figure 3): Data summary report, Fusion report, Fusion attribute report, and Download data.
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Clicking the Download data downloads a *.fusion file to the local computer. The file is human readable and can be opened in a text editor (example in Figure 4). For details refer to TopHat-Fusion documentation.
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A list of annotated fusion genes, in a form of Fusion report can be obtained by first selecting the Fusion report task node (Figure 2) and then the Task report link from the task menu (Figure 3). Since the task provides an annotated report, an annotation file needs to be specified first (Figure 5).
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The resulting Fusion report task node (Figure 6) can be double clicked to reveal the full table (Figure 7).
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Each row of the table in Figure 7 is a potential fusion event, with the columns providing the following information.
All the columns can be sorted by using the arrow buttons in column headers, while the type-in boxes can be used for searching. TopHat-Fusion does not report exact start and stop position for each side of the fusion event. It has a single location for the end of the upstream segment (Stop 1) and the beginning of the downstream segment (Start 2). Therefore, columns Start 1 and Stop 2 are added for (internal) consistency with other Partek Flow tools.
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The checkboxes Disrupted Genes and Gene/Gene fusions are filter tools. When selected, Disrupted Genes removes all the rows (fusion events) which have no genes assigned to it, i.e. those that merge two intergenic regions. However, if there is a fusion between a gene and an intergenic region, it will be kept in the table. The Gene/Gene fusions filters in only those fusion events which have an annotated gene on both sides of the breakpoint. In other words, only gene to gene fusions are kept in the table.
Another table which can be generated based on a Fusion results node is the Fusion attribute report (Figure 3). When the option is selected, it brings up the dialog shown in Figure 8. First, you need to specify one or more categorical attributes (Select attribute(s) to test), which have at least two categories (see Data tab). Second, you need to specify an annotation file, using the Assembly and Gene/feature annotation drop-down lists.
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A new data node, Fusion attribute report, is generated in the Analysis tab (Figure 9) and it provides access to the Task report link in the task menu.
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The output, Fusion report table (Figure 10) resembles the basic TopHat-Fusion output (Figure 7); each row of the table is a single fusion event while the information on the merged segments is on the columns.
The checkboxes Disrupted Genes and Gene/Gene fusions are filter tools. When selected, Disrupted Genes removes all the rows (fusion events) which have no genes assigned to it, i.e. those that merge two intergenic regions. However, if there is a fusion between a gene and an intergenic region, it will be kept in the table. The Gene/Gene fusions filters in only those fusion events which have an annotated gene on both sides of the breakpoint. In the other words, only gene to gene fusions are kept in the table.
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STAR aligner also has the ability to detect fusion genes (referred to as “chimeric alignments”) (5). During the first phase of alignment, STAR searches for maximal mappable prefixes (seeds) of sequencing reads. In the second phase, all the seeds that align within user-defined genomic windows are stitched together. If an alignment within one genomic window does not cover the entire read sequence, STAR will try to find two or more windows that cover the entire read. This essentially results in detection of fusion events, with different parts of reads aligning to distal genomic locations, or different chromosomes, or different strands.
STAR fusion detection algorithm is integrated with STAR aligner and fusion detection is activated by tick-marking Chimeric alignment option in the Advanced options of the aligner (the Advanced options dialog is reached via Configure link in the setup dialog). As soon as the Chimeric alignment is selected, additional options, specific to the fusion search algorithm, are shown (Figure 11). For discussion on the options details, see STAR documentation.
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The output is associated with the Chimeric results data node (Figure 12), which is a part of STAR results (in addition to Aligned reads node and, optionally, Unaligned reads node).
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Selecting the Chimeric results node opens the toolbox (Figure 13) with two options: Data summary report or Download data.
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Clicking on the Download data results in download of a .fusion file to the local computer. The file is human readible and can be opened in a text editor (example in Figure 14). For details refer to STAR's documentation.
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References
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