NGLess: NGS Processing with Less Work

NGLess is a domain-specific language for NGS (next-generation sequencing data) processing.

Note: This is pre-release software, currently available as a preview only. Please get in touch if you want to use it in your work.For questions, you can also use the ngless mailing list.

NGLess is best illustrated by an example:

Example

ngless "0.0"
input = fastq(['ctrl1.fq','ctrl2.fq','stim1.fq','stim2.fq'])
preprocess(input) using |read|:
    read = read[5:]
    read = substrim(read, min_quality=26)
    if len(read) < 31:
        discard

mapped = map(input,
                reference='hg19')
write(count(mapped, features=['gene']),
        ofile='gene_counts.csv',
        format={csv})

Basic features

  • preprocessing and quality control of FastQ files
  • mapping to a reference genome (implemented through bwa)
  • annotation and summarization of the alignments using reference gene annotations

Ngless has builtin support for some model organisms:

  1. Homo sapiens (hg19)
  2. Mus Muscullus (mm10)
  3. Rattus norvegicus (rn4)
  4. Bos taurus (bosTau4)
  5. Canis familiaris (canFam2)
  6. Drosophila melanogaster (dm3)
  7. Caenorhabditis elegans (ce10)
  8. Saccharomyces cerevisiae (sacCer3)

and the standard library includes support for mOTUs.

Traditional Unix command line usage

ngless can be used as a traditional command line transformer utility, using the -e argument to pass an inline script on the command line.

The -p (or --print-last) argument tells ngless to output the value of the last expression to stdout.

Converting a SAM file to a FASTQ file

Extract file reads from a SAM (or BAM) file:

$ ngless -pe 'as_reads(samfile("file.sam"))' > file.fq

This is equivalent to the full script:

ngless "0.0" # <- version declaration, optional on the command line
samcontents = samfile("file.sam") # <- load a SAM/BAM file
reads = as_reads(samcontents) # <- just get the reads (w quality scores)
write(reads, ofname=STDOUT) # <- write them to STDOUT (default format: FASTQ)

This only works if the data in the samfile is single ended as we pipe out a single FQ file. Otherwise, you can always do:

ngless "0.0"
write(as_read(samfile("file.sam")),
        ofile="output.fq")

which will write 3 files: output.1.fq, output.2.fq, and output.singles.fq (the first two for the paired-end reads and the last one for reads without a mate).

Getting aligned reads from a SAM file as FASTQ file

Building on the previous example. We can add a select() call to only output unmapped reads:

$ ngless -pe 'as_reads(select(samfile("file.sam"), keep_if=[{mapped}]))' > file.fq

This is equivalent to the full script:

ngless "0.0" # <- version declaration, optional on the command line
samcontents = samfile("file.sam") # <- load a SAM/BAM file
samcontents = select(samcontents, keep_if=[{mapped}]) # <- select only *mapped* reads
reads = as_reads(samcontents) # <- just get the reads (w quality scores)
write(reads, ofname=STDOUT) # <- write them to STDOUT (default format: FASTQ)

Reading from STDIN

For a true Unix-like utility, the input should be read from standard input. This can be achieved with the special file STDIN. So the previous example now reads

$ cat file.sam | ngless -pe 'as_reads(select(samfile(STDIN), keep_if=[{mapped}]))' > file.fq

Obviously, this example would more interesting if the input were to come from another programme (not just cat).

Full documentation

Frequently Asked Questions (FAQ)

Building and installing

Again, please note that this is pre-release software. Thus, we do not provide any easy to install (pre-built) packages at the moment, but they will be provided once the software is released. However, any comments (including bug and build reports), are more than welcome.

stack is highly recommended. Install it and running stack build should (1) download all dependencies with the correct versions and (2) build ngless. It will perform this task in its own sandbox so it will not interfere with any other work.

Authors