23 research outputs found
High throughput short read alignment via bi-directional BWT
The advancement of sequencing technologies has made it feasible for researchers to consider many highthroughput biological applications. A core step of these applications is to align an enormous amount of short reads to a reference genome. For example, to resequence a human genome, billions of reads of 35 bp are produced in 1-2 weeks, putting a lot of pressure of faster software for alignment. Based on existing indexing and pattern matching technologies, several short read alignment software have been developed recently. Yet this is still strong need to further improve the speed. In this paper, we show a new indexing data structure called bi-directional BWT, which allows us to build the fastest software for aligning short reads. When compared with existing software (Bowtie is the best), our software is at least 3 times faster for finding unique best alignments, and 25 times faster for finding all possible alignments. We believe that bi-directional BWT is an interesting data structure on its own and could be applied to other pattern matching problems. Availability: http://www.bio5.cs.hku.hk:8080/P2BWT, where two human genomes are indexed for alignment. © 2009 IEEE.published_or_final_versionThe IEEE International Conference on Bioinformatics and Biomedicine (BIBM 2009), Washington, DC., 1-4 November 2009. In Proceedings of BIBM, 2009, p. 31-3
Exploring single-sample SNP and INDEL calling with whole-genome de novo assembly
Motivation: Eugene Myers in his string graph paper (Myers, 2005) suggested
that in a string graph or equivalently a unitig graph, any path spells a valid
assembly. As a string/unitig graph also encodes every valid assembly of reads,
such a graph, provided that it can be constructed correctly, is in fact a
lossless representation of reads. In principle, every analysis based on
whole-genome shotgun sequencing (WGS) data, such as SNP and insertion/deletion
(INDEL) calling, can also be achieved with unitigs.
Results: To explore the feasibility of using de novo assembly in the context
of resequencing, we developed a de novo assembler, fermi, that assembles
Illumina short reads into unitigs while preserving most of information of the
input reads. SNPs and INDELs can be called by mapping the unitigs against a
reference genome. By applying the method on 35-fold human resequencing data, we
showed that in comparison to the standard pipeline, our approach yields similar
accuracy for SNP calling and better results for INDEL calling. It has higher
sensitivity than other de novo assembly based methods for variant calling. Our
work suggests that variant calling with de novo assembly be a beneficial
complement to the standard variant calling pipeline for whole-genome
resequencing. In the methodological aspects, we proposed FMD-index for
forward-backward extension of DNA sequences, a fast algorithm for finding all
super-maximal exact matches and one-pass construction of unitigs from an
FMD-index.
Availability: http://github.com/lh3/fermi
Contact: [email protected]: Rev2: submitted version with minor improvements; 7 page
Optimum Search Schemes for Approximate String Matching Using Bidirectional FM-Index
Finding approximate occurrences of a pattern in a text using a full-text
index is a central problem in bioinformatics and has been extensively
researched. Bidirectional indices have opened new possibilities in this regard
allowing the search to start from anywhere within the pattern and extend in
both directions. In particular, use of search schemes (partitioning the pattern
and searching the pieces in certain orders with given bounds on errors) can
yield significant speed-ups. However, finding optimal search schemes is a
difficult combinatorial optimization problem.
Here for the first time, we propose a mixed integer program (MIP) capable to
solve this optimization problem for Hamming distance with given number of
pieces. Our experiments show that the optimal search schemes found by our MIP
significantly improve the performance of search in bidirectional FM-index upon
previous ad-hoc solutions. For example, approximate matching of 101-bp Illumina
reads (with two errors) becomes 35 times faster than standard backtracking.
Moreover, despite being performed purely in the index, the running time of
search using our optimal schemes (for up to two errors) is comparable to the
best state-of-the-art aligners, which benefit from combining search in index
with in-text verification using dynamic programming. As a result, we anticipate
a full-fledged aligner that employs an intelligent combination of search in the
bidirectional FM-index using our optimal search schemes and in-text
verification using dynamic programming outperforms today's best aligners. The
development of such an aligner, called FAMOUS (Fast Approximate string Matching
using OptimUm search Schemes), is ongoing as our future work
MICA: A fast short-read aligner that takes full advantage of Many Integrated Core Architecture (MIC)
Background: Short-read aligners have recently gained a lot of speed by exploiting the massive parallelism of GPU. An uprising alterative to GPU is Intel MIC; supercomputers like Tianhe-2, currently top of TOP500, is built with 48,000 MIC boards to offer ~55 PFLOPS. The CPU-like architecture of MIC allows CPU-based software to be parallelized easily; however, the performance is often inferior to GPU counterparts as an MIC card contains only ~60 cores (while a GPU card typically has over a thousand cores). Results: To better utilize MIC-enabled computers for NGS data analysis, we developed a new short-read aligner MICA that is optimized in view of MIC's limitation and the extra parallelism inside each MIC core. By utilizing the 512-bit vector units in the MIC and implementing a new seeding strategy, experiments on aligning 150 bp paired-end reads show that MICA using one MIC card is 4.9 times faster than BWA-MEM (using 6 cores of a top-end CPU), and slightly faster than SOAP3-dp (using a GPU). Furthermore, MICA's simplicity allows very efficient scale-up when multiple MIC cards are used in a node (3 cards give a 14.1-fold speedup over BWA-MEM). Summary: MICA can be readily used by MIC-enabled supercomputers for production purpose. We have tested MICA on Tianhe-2 with 90 WGS samples (17.47 Tera-bases), which can be aligned in an hour using 400 nodes. MICA has impressive performance even though MIC is only in its initial stage of development. Availability and implementation: MICA's source code is freely available at http://sourceforge.net/projects/mica-aligner under GPL v3. Supplementary information: Supplementary information is available as "Additional File 1". Datasets are available at www.bio8.cs.hku.hk/dataset/mica.published_or_final_versio
BASE: a practical de novo assembler for large genomes using long NGS reads
© 2016 The Author(s). Background: De novo genome assembly using NGS data remains a computation-intensive task especially for large genomes. In practice, efficiency is often a primary concern and favors using a more efficient assembler like SOAPdenovo2. Yet SOAPdenovo2, based on de Bruijn graph, fails to take full advantage of longer NGS reads (say, 150 bp to 250 bp from Illumina HiSeq and MiSeq). Assemblers that are based on string graphs (e.g., SGA), though less popular and also very slow, are more favorable for longer reads. Methods: This paper shows a new de novo assembler called BASE. It enhances the classic seed-extension approach by indexing the reads efficiently to generate adaptive seeds that have high probability to appear uniquely in the genome. Such seeds form the basis for BASE to build extension trees and then to use reverse validation to remove the branches based on read coverage and paired-end information, resulting in high-quality consensus sequences of reads sharing the seeds. Such consensus sequences are then extended to contigs. Results: Experiments on two bacteria and four human datasets shows the advantage of BASE in both contig quality and speed in dealing with longer reads. In the experiment on bacteria, two datasets with read length of 100 bp and 250 bp were used. Especially for the 250 bp dataset, BASE gives much better quality than SOAPdenovo2 and SGA and is simlilar to SPAdes. Regarding speed, BASE is consistently a few times faster than SPAdes and SGA, but still slower than SOAPdenovo2. BASE and Soapdenov2 are further compared using human datasets with read length 100 bp, 150 bp and 250 bp. BASE shows a higher N50 for all datasets, while the improvement becomes more significant when read length reaches 250 bp. Besides, BASE is more-meory efficent than SOAPdenovo2 when sequencing data with error rate. Conclusions: BASE is a practically efficient tool for constructing contig, with significant improvement in quality for long NGS reads. It is relatively easy to extend BASE to include scaffolding.published_or_final_versio
When a Dollar Makes a BWT
TheBurrows-Wheeler-Transform(BWT)isareversiblestring transformation which plays a central role in text compression and is fun- damental in many modern bioinformatics applications. The BWT is a permutation of the characters, which is in general better compressible and allows to answer several different query types more efficiently than the original string. It is easy to see that not every string is a BWT image, and exact charac- terizations of BWT images are known. We investigate a related combi- natorial question. In many applications, a sentinel character contains exactly one -character be inserted to turn w into the BWT image of a word ending with the sentinel character. We show that this depends only on the standard permutation of w and give a combinatorial characterization of such positions via this permutation. We then develop an O(n log n)-time algorithm for identifying all such positions, improving on the naive quadratic time algorithm