Aligning bisulfite-converted DNA reads to a genome
==================================================

Bisulfite is used to detect methylated cytosines.  It converts
unmethylated Cs to Ts, but it leaves methylated Cs intact.  If we then
sequence the DNA and align it to a reference genome, we can infer
cytosine methylation.

To align the DNA accurately, we should take the C->T conversion into
account.  Here is how to do it with LAST.

Let's assume we have bisulfite-converted DNA reads in a file called
"reads.fastq" (in fastq-sanger format), and the genome is in
"mygenome.fa" (in fasta format).  We will also assume that all the
reads are from the converted strand, and not its reverse-complement
(i.e. they have C->T conversions and not G->A conversions).

First, we need to run lastdb twice, for forward-strand and
reverse-strand alignments::

  lastdb -uBISF my_f mygenome.fa
  lastdb -uBISR my_r mygenome.fa

Then, there are several steps:

1. Convert all Cs in the reads to Ts.  (Debatable: this slightly
   degrades LAST's ability to align the reads, but it avoids a bias,
   due to unconverted DNA being easier to align than converted DNA.)

2. Align the reads, one strand at a time.

3. Merge the alignments, and find a unique best alignment for each
   part of each read.

4. Undo the conversion from step 1.

The last-bisulfite script (in the examples directory) carries out
steps 1-4.  Its usage is::

  last-bisulfite.sh my_f my_r reads.fastq > results.maf

You can parallelize it like this::

  parallel-fastq "last-bisulfite.sh my_f my_r" < reads.fastq > results.maf

Tips
----

* To go faster, try gapless alignment (add -j1 to the lastal options).
  Often, this is only minusculely less accurate than gapped alignment.

* The .tis files created by lastdb (e.g. my_f.tis, my_r.tis) are
  identical.  So you can shave a few GB by linking them::

    ln -f my_f.tis my_r.tis

Paired-end DNA reads
--------------------

You can align paired-end reads by combining the preceding recipe with
the one in `<last-pair-probs.html>`_.  This gets a bit complicated, so
we provide a last-bisulfite-paired script in the examples directory.
Typical usage::

  lastdb -uBISF my_f mygenome.fa
  lastdb -uBISR my_r mygenome.fa

  last-bisulfite-paired.sh my_f my_r reads1.fastq reads2.fastq > results.maf

This assumes that reads1.fastq are all from the converted strand
(i.e. they have C->T conversions) and reads2.fastq are all from the
reverse-complement (i.e. they have G->A conversions).  It requires GNU
parallel to be installed.  You are encouraged to customize this
script.
