Mikhail Karasikov, MSc

PhD Student

E-Mail
​mikhaika@get-your-addresses-elsewhere.inf.ethz.ch​
Phone
+41 43 254 0224
Address
Biomedical Informatics Group
Schmelzbergstrasse 26
8006 Zürich
Room
SHM 26 B 3

I am broadly interested in machine learning and bioinformatics. Currently, I am developing data structures for the genome assembly.

I graduated from the Moscow Institute of Physics and Technology studying data science at the Department of Control and Applied Mathematics.

Abstract High-throughput DNA sequencing data is accumulating in public repositories, and efficient approaches for storing and indexing such data are in high demand. In recent research, several graph data structures have been proposed to represent large sets of sequencing data and allow for efficient query of sequences. In particular, the concept of colored de Bruijn graphs has been explored by several groups. While there has been good progress towards representing the sequence graph in small space, methods for storing a set of labels on top of such graphs are still not sufficiently explored. It is also currently not clear how characteristics of the input data, such as the sparsity and correlations of labels, can help to inform the choice of method to compress the labels. In this work, we present a systematic analysis of five different state-of-the-art annotation compression schemes that evaluates key metrics on both artificial and real-world data and discusses how different data characteristics influence the compression performance. In addition, we present a new approach, Multi-BRWT, that shows an up to 50% improvement in compression performance over the current state-of-the-art and is adaptive to different kinds of input data. Using our comprehensive test datasets, we show that this improvement can be robustly reproduced for different representative real-world datasets.

Authors Mikhail Karasikov, Harun Mustafa, Amir Joudaki, Sara Javadzadeh, Gunnar Rätsch, Andre Kahles

Submitted bioRxiv

Link DOI

Abstract Motivation: Technological advancements in high-throughput DNA sequencing have led to an exponential growth of sequencing data being produced and stored as a byproduct of biomedical research. Despite its public availability, a majority of this data remains hard to query for the research community due to a lack of efficient data representation and indexing solutions. One of the available techniques to represent read data is a condensed form as an assembly graph. Such a representation contains all sequence information but does not store contextual information and metadata. Results: We present two new approaches for a compressed representation of a graph coloring: a lossless compression scheme based on a novel application of wavelet tries as well as a highly accurate lossy compression based on a set of Bloom filters. Both strategies retain a coloring even when adding to the underlying graph topology. We present construction and merge procedures for both methods and evaluate their performance on a wide range of different datasets. By dropping the requirement of a fully lossless compression and using the topological information of the underlying graph, we can reduce memory requirements by up to three orders of magnitude. Representing individual colors as independently stored modules, our approaches can be efficiently parallelized and provide strategies for dynamic use. These properties allow for an easy upscaling to the problem sizes common to the biomedical domain. Availability: We provide prototype implementations in C ++, summaries of our experiments as well as links to all datasets publicly at https://github.com/ratschlab/graph_annotation.

Authors Harun Mustafa, ingo Schilken, Mikhail Karasikov, Carsten Eickhoff, Gunnar Rätsch, Andre Kahles

Submitted Bioinformatics

Link DOI

Abstract Much of the DNA and RNA sequencing data available is in the form of high-throughput sequencing (HTS) reads and is currently unindexed by established sequence search databases. Recent succinct data structures for indexing both reference sequences and HTS data, along with associated metadata, have been based on either hashing or graph models, but many of these structures are static in nature, and thus, not well-suited as backends for dynamic databases. We propose a parallel construction method for and novel application of the wavelet trie as a dynamic data structure for compressing and indexing graph metadata. By developing an algorithm for merging wavelet tries, we are able to construct large tries in parallel by merging smaller tries constructed concurrently from batches of data. When compared against general compression algorithms and those developed specifically for graph colors (VARI and Rainbowfish), our method achieves compression ratios superior to gzip and VARI, converging to compression ratios of 6.5% to 2% on data sets constructed from over 600 virus genomes. While marginally worse than compression by bzip2 or Rainbowfish, this structure allows for both fast extension and query. We also found that additionally encoding graph topology metadata improved compression ratios, particularly on data sets consisting of several mutually-exclusive reference genomes. It was also observed that the compression ratio of wavelet tries grew sublinearly with the density of the annotation matrices. This work is a significant step towards implementing a dynamic data structure for indexing large annotated sequence data sets that supports fast query and update operations. At the time of writing, no established standard tool has filled this niche.

Authors Harun Mustafa, Andre Kahles, Mikhail Karasikov, Gunnar Raetsch

Submitted bioRxiv

Link DOI