ABRF2010 Tutorial

Tutorial: Viewing sequencing data in the UCSC Genome Browser

Presented March 20, 2010 as part of the workshop Tools to Facilitate Management, Analysis and Visualization of 2nd Generation Sequencing Data at ABRF 2010.

This page serves as a step-by-step guide to the demo of UCSC Genome Browser tools presented at the workshop.

Genome Graphs with .sgr data

Some of the result files from the ChIPSeq toolset that Charlie Nicolet showed us earlier are in the .sgr format, i.e. tab-separated text that specifies chromosome, position and a numeric signal value. This format can be uploaded to UCSC's Genome Graphs tool for genome-at-a-glance view of the data.

1. In a new web browser window or tab, go to http://genome.ucsc.edu/.
2. On the left side of the page, there is a blue sidebar. Click on "Genome Graphs", the 7th link from the top of the sidebar.
3. Now you should see a layout of human chromosomes.
4. In the assembly menu, change the selection to "Mar. 2006 (NCBI36/hg18)".
5. Click the upload button. Paste this URL: http://where?/chr21_extended_redbin.sgr into the form, and click submit.
6. You should see confirmation screen; click OK to return to the Genome Graphs display.
7. Genome Graphs doesn't show the data until you select it. Select "redbin 1" from the menu on the left. (session link)
8. Now the data appear -- in this case, for chr21 only. Click on a peak.
9. Click on a peak to jump to the Genome Browser, displaying a 1,000,000-base region of the genome corresponding to the location you clicked. (session link)
10. Note the track at the top labeled "redbin 1" -- that is the data shown in Genome Graphs. Peaks correspond to upstream regions of genes, which makes sense for RNA polymerase II ChIP-seq.

Transforming .sgr into bedGraph for Genome Browser viewing

The Genome Browser can draw a somewhat nicer display of .sgr data, if we do a simple transform of the .sgr data into a similar format called bedGraph. bedGraph has both start and end genomic coordinates, so scores can be drawn over regions as opposed to single bases. Here is a command that translates .sgr to bedGraph, expanding each data point to cover a 30-base region:

   perl -wpe '@w=split("\t"); $_ = "$w[0]\t" . ($w[1]-1) . "\t" . ($w[1]+29) . "\t$w[2]";' \
     chr21_extended.txt_redbin.sgr > redbinGt10__.bed

If you would like to show only datapoints above a certain threshold, say 5, you can make the command slightly more complicated:

   perl -wpe '@w=split("\t"); $_ = "$w[0]\t" . ($w[1]-1) . "\t" . ($w[1]+29) . "\t$w[2]"; \
              if ($w[2] < 5) {$_ = "";}' \
     chr21_extended.txt_redbin.sgr > redbinGt5.bed

This line can be added to the beginning of redbinGt5.bed, in order to add labels to the track display (without the line break used for readability here):

   track name=redbinGt5 description="redbin items with score >= 5, expanded to 30 bases" \
      type=bedGraph autoScale=off
   add settings: height 16, vertical range 0-100

You can download the file here. Right-click the link and choose "Copy link location".

1. In the Genome Browser, click the "manage custom tracks" button below the image.
2. paste the link that you just copied (plaintext url) into the first box on the form and click submit.
3. You should see a confirmation page like this (photo) (session link)
4. Click on "genome browser" in the top blue bar (does "Go to Genome Browser" do the right thing?)
5. Now the new custom track shows grayscale representations of the score. You can click on the track title ("redbin items with score > 5") to expand the display to bar graph.

zoom in?

Comparing with UCSC-hosted data

Now we will take a look at some ChIP-seq data from the ENCODE project.

1. In the Genome Browser, scroll down below the image to the "Regulation" section. Click "+" to expand.
2. Click on the link titled "Yale TFBS". This will take you to the track controls.
3. In the large table under "Select subtracks by cell line and factor",
4. Hit the "-" button in upper left corner to deselect all subtracks. Note that HeLa-S3 cell line is in 3rd column of checkboxes.
5. Scroll down to factor Pol2. select the 3rd check box. Only HeLa/Pol2 is selected for display.
6. Scroll back up to the top of the page. Change visibility to "pack" and click submit.
7. In the Genome Browser, HeLa/Pol2 peaks and signal appear below "Spliced ESTs". session link

Viewing alignments in BAM format

• samtools conversion -- provide link to bam

   export2sam.pl --read1=chr21_export.txt \
   | sed -re 's/(chr.*)\.fa/\1/' \
     > chr21.sam

Add these header lines to the beginning of the file (note: the words must be separated by tab characters, not spaces):

@HD	VN:1.0	GO:none	SO:coordinate
@SQ	SN:chr21	LN:46944323

Use samtools to convert SAM to BAM, sort the BAM, and index the sorted BAM:

   samtools view -bSo chr21.unsorted.bam chr21.sam
   samtools sort chr21.unsorted.bam chr21
   samtools index chr21.bam

Now the sorted chr21.bam and its index file chr21.bam.bai must be put on a web server (ftp, http, or https). For this example, we will use http://where?/chr21.bam

1. In the Genome Browser, click the "manage custom tracks" buttom below the image.
2. Click the "add custom tracks" button.
3. Paste this text into the box:

track name=redbin type=bam bigDataUrl=http://where?/chr21.bam visibility=pack

1. Click submit. Click "Go to Genome Browser".

• view ~1kB of bam with alignment qualities in grayscale
• base-level view of bam, w/base qualities in grayscale

Where to learn more

• Help, FAQ
• genome@soe.ucsc.edu and list archives
• genomewiki (this site)
• OpenHelix