Systems and methods for using paired-end data in directed acyclic structure
Methods of analyzing a transcriptome that involves obtaining at least one pair of paired-end reads from a transcriptome from an organism, finding an alignment with an optimal score between a first read of the pair and a node in a directed acyclic data structure (the data structure has nodes representing RNA sequences such as exons or transcripts and edges connecting pairs of nodes), identifying candidate paths that include the node connected to a downstream node by a path having a length substantially similar to an insert length of the pair of paired-end reads, and aligning the paired-end rends to the candidate paths to determine an optimal-scoring alignment.
1. A method of analyzing a transcriptome, the method comprising:
obtaining from an annotated transcriptome database—using a computer comprising a processor coupled to the memory—a plurality of exons and introns from a genome;
using the processor to transform the plurality of exons and introns into a directed acyclic data structure comprising nodes representing known RNA sequences and edges connecting the nodes;
obtaining a pair of paired-end reads generated by sequencing a transcriptome of an organism;
using the processor to transform the first read of the pair into an alignment with an optimal score between that first read of the pair and a node in the directed acyclic data structure;
identifying, using the processor, candidate paths within the directed acyclic data structure that include the node connected to a downstream node by a path having a length substantially similar to an insert length of the pair of paired-end reads;
excluding non-candidate paths from alignments involving the pair of paired-end reads;
aligning, using the processor, the paired-end reads to the candidate paths to determine an optimal-scoring alignment by:
calculating match scores between a second read of the pair and nodes in the candidate paths, and
looking backwards to predecessor nodes in the candidate paths while not considering any nodes in the non-candidate paths to identify a back-trace through the candidate paths that gives an optimal score,
wherein the back-trace that gives the optimal score corresponds to an optimal scoring alignment of the pair of paired-end reads to the candidate paths, and
wherein the directed acyclic data structure held in the memory prior to obtaining the pair of paired-end reads includes at least one path that has a node that the second read of the pair aligns to but that is not included during the aligning step due to being excluded as a non-candidate path; and
identifying an isoform of an RNA from the organism using the optimal scoring alignment of the paired-end reads.
2. The method of claim 1 , further comprising determining optimal-scoring alignments for each of the plurality of pairs of paired-end reads.
3. The method of claim 1 , wherein the identified isoform is a novel isoform.
4. The method of claim 3 , further comprising causing the processor to update the directed acyclic data structure to represent the novel isoform.
5. The method of claim 4 , wherein updating the directed acyclic data structure to represent the novel isoform comprises adding at least one new node.
6. The method of claim 5 , wherein the node and the downstream node represent a pair of exons from which the pair of paired-end reads were obtained.
7. The method of claim 1 , further comprising:
aligning a plurality of pairs of paired-end reads to the directed acyclic data structure;
determining distances between the aligned pairs of the plurality of pairs of paired-end reads and the frequencies of the distances between the aligned pairs; and
determining a set of isoform paths and isoform frequencies such that representing the isoform paths through the structure at the isoform frequencies results in pairs of features being included in the isoform paths at the frequencies of the distances between the aligned pairs.
8. The method of claim 7 , wherein the features include exons.
9. The method of claim 8 , wherein each path of the determined set of isoform paths represents a transcript isoform present in the organism.
10. The method of claim 7 , wherein the distances between the aligned pairs comprise path lengths from an upstream end of a first aligned member of each pair to a downstream end of a second aligned member of that pair.