SHORTS
☘☘☘ Letters by Vivek Rai
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Diving Deeper Into BLAST - I

Understanding BLAST output

In my previous post, I mentioned about a project I was trying to work on: SequenceServer. Also, in the end I said that I would be writing about the BLAST algorithm (which is the backbone of this project) and how does it works so efficiently in producing alignments even with very long sequences. However, In this post I would like to talk about BLAST program and it's output before we go into the algorithm some time later.

After talking to Priyam (one of the project collaborators), I decided to work towards solving the issue #21, which in turn would help us to solve a large number of other dependent issues as well. This was supposed to be a major improvement and I really liked the idea of it. Especially since it was concerned with understanding the BLAST program and its output format.

Background 

Issue 21 deals with displaying an overview graphic for the output results obtained from blasting. This is a nice feature as it adds more towards visualizing and understanding the data easily (than going over a large number of hits in many cases). The problem however is that because of the hacky way in which the current implementation worked for displaying the final output results, it was not very easy to work with generation of visuals. The data generated from BLAST output had to be parsed first into a data layer and then interfaced with other features like displaying overview graphic or other rich information. This required three things from my side - Ruby, BLAST, and Patience.

Diving into XML 

The blast output format can be reconfigured using the outfmt option which is described in the NCBI’s BLAST Manual. The sample XML output (standard settings) has the following structure (for a real example blast.xml):

<?xml version="1.0"?>
<!DOCTYPE BlastOutput PUBLIC "-//NCBI//NCBI BlastOutput/EN" "http://www.ncbi.nlm.nih.gov/dtd/NCBI_BlastOutput.dtd">
<BlastOutput>
  <BlastOutput_program>
  <BlastOutput_version>
  <BlastOutput_reference>
  <BlastOutput_db>
  <BlastOutput_query-ID>
  <BlastOutput_query-def>
  <BlastOutput_query-len>
  <BlastOutput_param>
    <Parameters>
      <Parameters_matrix>
      <Parameters_expect>
      <Parameters_gap-open>
      <Parameters_gap-extend>
      <Parameters_filter>
    </Parameters>
  </BlastOutput_param>
<BlastOutput_iterations>
<Iteration>
  -- Iteration details --
<Iteration_hits>
<Hit>
  <Hit_num>
  <Hit_id>
  <Hit_def>
  <Hit_accession>
  <Hit_len>
  <Hit_hsps>
    <Hsp>
  -- HSP Details --
    </Hsp>
  </Hit_hsps>
</Hit>
<Hit> </Hit>
</Iteration_hits>
<Iteration_stat>
  -- Statistics about Query --
</Iteration_stat>
</Iteration>
</BlastOutput_iterations>
</BlastOutput>

The XML output is as detailed as it could be including all the information about Hit’s High Scoring Segment Pair or HSP, the query-database, alignment-lengths, the query & aligned-sequences along with the matches between them, and query-statistics. This is as useful as it could be and helps to generate very detailed statistics and overview too. I would recommend one to look at the given example of blast.xml to understand what kind of data is generated. To parse this generated XML output easily, I used the Ox rubygem, which is a simple and faster alternative of other XML parsers available in Ruby. In the beginning, I generated a simple Hash out of this parsed data using recursive traversal along the elements.

def report! 
  # Generates BLAST report which one or more multiple Query objects
  # based on the blast query string.

  parsed_out = Ox.parse(@result)
  hashed_out = node_to_dict(parsed_out.root)
  @program = hashed_out["BlastOutput_program"]
  @querydb = hashed_out["BlastOutput_db"]

  hashed_out["BlastOutput_iterations"].each do |n|
    @queries ||= {}
    @queries[n[2]] = Query.new(n[1], n[2], n[3], {}, n[5]["Statistics"])

    # Ensure a hit object is received. No hits, returns a newline.
    # Note that checking to "\n" doesn't work since n[4] = ["\n"]

    if n[4]==["\n"]
      @queries[n[2]][:hits] = "No hits found."
    else
      n[4].each do |hits|
        @queries[n[2]][:hits][hits[1]] = Hit.new(hits[0], hits[1], hits[2],
                                                 hits[3], hits[4], {})
        @queries[n[2]][:hits][hits[1]][:hsp] = HSP.new(*hits[5]["Hsp"].values)
      end
    end
  end
end

This Hash is then used for easy templating (using erb) and display of results in a more elegant and modular way. This keeps the data at one place which can then be used for overview graphic display or integration of other features and bug fixes in near future. All my work related to this can be found at @issue21 branch. The code written till now, is very premature and just works. I plan to refactor it majorily and incorporate some very nice features of Ruby language (which I am desperately trying to understand).

Here is a snapshot of what current output looks like, when SequenceServer is run.

What’s next? 

The second part of this task deals with actually displaying the overview graphic for the hits obtained using SequenceServer. To give an idea of what it looks like, see here and here. The target is to display similar graphic (but simple and easy to load/configure) for each hit in the Seqserv’s output. I am yet to start working on and possibly the next post could be derived out of that work.

I would be very glad if you drop by to review my code and send in any suggestions. Feel free to reach me by whatever means.