jclub: Bloom Filter Trie - a data structure for pan-genome storage

Note: this is a blog post from the DIB Lab journal club.

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The paper:

http://www.techfak.uni-bielefeld.de/~stoye/dropbox/wabi2015final.pdf

"Bloom Filter Trie: a data structure for pan-genome storage."

by Guillaume Holley, Roland Wittler, and Jens Stoye.

Background

  • Pan Genome: Represents genes in a clade that comprises hundreds/thousands of strains that share large sequence parts but differ by individual mutations from one another.
  • Colored De Bruijn Graph (C-DBG) - A directed graph where each vertex represents a kmer and is associated with a color that represents the genome where the kmer occurs. An edge between vertex x and vertex y exists if x[2..k] = y[1..k-1]

Existing pan-genome Data structures

  • Burrows-Wheeler Transform (BWT) that rearranges the input data to enable better compression by aggregating characters with similar context.
  • FM-Index count and locate the occurrence of substring in BWT.
  • Bloom Filter BF - A bit array of n elements initialized with zeros, and it uses a set of hash functions for look-up and insertion in a constant time.
  • Sequence Bloom Filter (SBT) is a binary tree with BFs as vertices. A query sequence is broken into a set of kmers then checked if they exist in the vertex bloom filter. If yes, search is repeated on children if not existing the proceeds on other vertices.

Paper Scope: New pan-genome Data Structure

Proposes a new data structure for indexing and compressing a pan-genome as a C-DBG, the Bloom Filter Trie (BFT). The trie stores kmers and their colors. The new representation allows for compressing and indexing shared substrings. It also allows quick checking for the existence of a given substring. It is incremental, alignment and reference free, and allows for any format.

How BFT Works?

Colors Representation Before Compression

Colors are represented by a bit array initialized with 0s. Each color has an index i_color: color_x [i_color] =1 means kmer x has color i_color.

Colors Representation After Compression

If we can put each color set in a specific position in an array such that this position encodes into a number less than the color set size, then we can store the color set in the BFT using less space. Color set size is the number of kmers sharing a color multiplied by its size. So basically, they sort colors in a list based on the color set size in a decreasing order then they add each color set to an external array incrementally (if : integer encoding the position of the color in the array < size of the color). Finally, each color is replaced in the BFT by its position in the external array

Uncompressed Container

A set of tuples of capacity c <s, color_ps> where: x = ps and x is the path from root to v.

When number of suffixes exceeds c the container is burst.

In bursting process, each suffix s is split into a suffix s_suff and prefix s_pref

Prefixes are stored in a new compressed container.

Suffixes and their colors are stored in a new uncompressed containers.

Compressed Container and Bursting Procedure

An uncompressed container is replaced by a compressed one that stores q prefixes of suffix with links to children containing the suffix.

  • quer is a BF represented as m bit array and has f hash functions to record and filter the presence of q prefixes of suffix.
  • The q prefixes of suffix are stored in a bloom filter BF. q suffixes are stored in an array suf in lexicographic order of the prefixes of those suffixes.
  • pref[𝛂] =1 if a prefix a exists with 𝛂 as its binary representation.
  • clust is an array of q bits one per suffix of array suf. A cluster is a list of consecutive suffixes in array suf that has the same prefix.

BFT Operations

Look up

To check if a Spref ab with π›‚πœ· representation exist in a compressed container cc, the BF quer is checked and the prefix a existence is verified in the array pref. Remember that suffix prefixes are stored in quer during bursting process.

if a exist, then its hamming weight is computed which is the index of the cluster in which suffix b is likely located where i is the start position of the cluster. Remember that a cluster is a list of consecutive suffixes that has the same prefix, so b is compared to suffixes in that cluster.

To check of a kmer x exists in a BFT t, the look-up process traverses t and for each vertex v it checks its containers one after one. Remember that suffix and their colors are stored in uncompressed container during burst process, hence a vertex now either represent a suffix from an uncompressed container or a suffix rooted from its compressed container.

  • For the first case, and as the uncompressed container has no childs, a match indicates the presence of the kmer.
  • For the second case, the quer is checked for the x_suff[1..l]. If it is found, traverse is continued recursively to the corresponding children. The absence of of x_suff[1..l] means the absence of the kmer as it can’t be located in another container of vertex v. Remember that k is a multiple of l so kmer =k/l equal substrings.

Insertion

If the kmer already exist, its set of colors is only modified. Otherwise, a lookup process is continued till:

  • The prefix of the searched suffix does not exist
  • The kmer suffix does not exist

Then the kmer is inserted. Insertion is simple if the container is uncompressed. If the container is compressed, the insertion of of s_pref =ab is pretty complicated:

Remember in the look up process, the β€˜a’ prefix existence is verified by checking pref array. If it does not exist: it is a FP, and we can insert now by setting pref[𝛂] to 1. So, in next look up, the verification will lead to a TP index and start position of cluster pos are recomputed and updated. How? if it does exist: Then the suffix b is to be inserted into suf[pos]

Evaluation

Experiments presented in the paper show that BFT is faster than SBT while utilizing less memory.

  • KmerGenie was used to get optimal k size and mininal kmer count
  • Data insertion (loading) and kmer query was compared between SBT and BFT. Traversal time is also evaluated on BFT.
  • BFT was multiple times faster than the SBT on the building time while using about 1.5 times less memory. The BFT was about 30 times faster than the SBT for querying a k-mer.

Questions and Comments:

  • Essentially, a nice fast data structure for querying for k-mers and retrieving their colors. I guess this is for pangenomes, among other things.

  • They essentially use compressed nodes in the tree to efficiently store prefixes for large sections of the tree.

  • We worry about the peak mem usage diagram. It seems like a fair amount of memory is used in the making. How does this compare to the SBT? Do they compare peak memory usage or merely compressed memory usage?

  • It seems like one advantage that the SBT has is that with the BFT you cannot store/query for presence in individual data sets. So, for example, if you wanted to build indices for data sets spread across many different machines, you would have to do it by gathering all of the data sets in one place.

  • Both SBT and BFT get the compression mainly from bloom filter. The author did not discuss about why there is difference in compression ratio. Bloom filter size? The FP rate of bloom filters in used in SBT was mentioned as 7.2%, but FP rate of bloom filters in BFT were not mentioned in paper.

  • Another catch in the evaluation is that 1) loading cpu time difference in Table 1 of SBT and BFT may be from kmer counting (Jellyfish vs. CMK2); 2) When comparing the unique kmer query time, unique kmer were divided into subsets due to memory limit. Not sure whether this was a fair comparison.

  • How does false positive rate of all bloom filters (on all nodes) affect overall error rate, e.g. If BFT is converted back to k-mers, how many sequence error are there? (None, we think)

  • PanCake (alignment based) and RCSI (Reference based) were mentioned but not included in evaluation, which gave us the impression that they are not as efficient. Do they have any advantage?

  • BFT or SBT vs. khmer? (mentioned in intro but not discussed)

  • Pan genome (and transcriptome, proteome!) storage is super cool. (might not be relevant question here, but I am wondering:) How are genomes defined as "highly similar", as the authors restricted their test data sets to. At what point do species diverge to become too distant to analyze in this manner? e.g. how close is close, and what is too far?

    (CTB answer: it has something to do with how many k-mers they share, but I don't know that this has been really quantified. Kostas Konstantinidis et al's latest work on species defn might be good reading (http://nar.oxfordjournals.org/content/early/2015/07/06/nar.gkv657.full) as well as his Average Nucleotide Identity metric.)

  • Wondering how might BFT scale? Authors only tested prokaryotic sequences, 473 clinical isolates of Pseudomonas aeruginosa from 34 patients = 844.37 GB. Simulated data were 6 million reads of 100 b length for 31 GB. In comparison, MMETSP data are transcriptomic data from 678 cultured samples of 306 marine eukaryotic species representing more than 40 phyla (see Figure 2, Keeling et al. 2014) Not sure how large the entire MMETSP data set is, but probably on order of TB? http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001889 http://www.ncbi.nlm.nih.gov/nuccore?term=231566%5BBioProject%5D

  • Although they discussed SBT as existing data structure, and graphalign in khmer, it wasn't clear until end that one of the main goals of the paper, besides describing BFT, was to compare their BFT to SBT (Soloman and Kingsford 2015 http://biorxiv.org/content/biorxiv/early/2015/03/26/017087.full.pdf) I feel this should have been noted in the Abstract.

  • speed can partly come from being able to abort searches for k-mers partway through.

  • BFT is really specialized for the pangenome situation, where many k-mers are in common. The cluster approach will break down if the genomes aren't mostly the same?

  • we would have liked a more visual representation of the data structure to help build intuition.

Points for clarification or discussion:

  • c is defined as capacity, but this is not well-described. What is capacity?
  • BFT to khmer graphalign comparison?

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