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Genome Browsing and Visualization - IGV | Griffith Lab

Genomic Visualization and Interpretations

Genome Browsing and Visualization - IGV

It is often necessary to examine sequencing data aligned to specific regions of the genome in order to obtain a clearer picture of genomic events. One of the most popular tools for this is the Integrative Genomics Viewer. After this lab you should be able to perform the following tasks:

  1. Visualize a variety of genomic data
  2. Quickly navigate around the genome
  3. Visualize read alignments
  4. Validate SNP/SNV calls and structural re-arrangements by eye

Installing IGV

Java is necessary to run IGV, you can download the java runtime environment (JRE) for your operating system here. To determine if this step is necessary type java -version at a command prompt, if the program is not >= 1.7 you’ll need to upgrade it. IGV can be downloaded here. This tutorial will make use of IGV version 2.3, we strongly recommend that you upgrade IGV if you have an older version installed.

Data Set for IGV

We will be using publicly available Illumina sequence data from the HCC1143 cell line. The HCC1143 cell line was generated from a 52 year old caucasian woman with breast cancer. Additional information on this cell line can be found here: (tumor, TNM stage IIA, grade 3, primary ductal carcinoma) and HCC1143/BL (matched normal EBV transformed lymphoblast cell line). Reads within these cell lines have been filtered to Chromosome 21: 19,000,000-20,000,000 in order to reduce file sizes.

Visualization Part 1: Getting familiar with IGV

We will be visualizing read alignments using IGV, a popular visualization tool for HTS data.

First, lets familiarize ourselves with it.

Load a Genome and some Data Tracks

By default, IGV loads Human (hg19). If you work with another version of the human genome, or another organism altogether, you can change the genome by clicking the drop down menu in the upper-left. For this lab, we will be using Human (hg19).

We will also load additional tracks from the IGV Server using (File -> Load from Server...):

  • Ensembl genes (or your favourite source of gene annotations)
  • GC Percentage
  • dbSNP 1.3.1 or 1.3.7

Load hg19 genome and additional data tracks

You should see a listing of chromosomes for this reference genome. Choose 1, for chromosome 1.

Chromosome chooser

Navigate to chr1:10,000-11,000 by entering this into the location field (in the top-left corner of the interface) and clicking Go. This shows a window of chromosome 1 that is 1,000 base pairs wide and beginning at position 10,000.

Navigition using Location text field. Sequence displayed as thin coloured rectangles.

IGV displays the sequence of letters in a genome as a sequence of colours (e.g. A = green, C = blue, etc.). This makes repetitive sequences, like the ones found at the start of this region, easy to identify. Zoom in a bit more using the + button (top right) to see the individual bases of the reference genome sequence.

You can navigate to a gene of interest by typing it into the same box that the genomic coordinates are in and pressing Enter/Return. Try it for your favourite gene, or BRCA1 if you can not decide.

Gene model

Genes are represented as lines and boxes. Lines represent intronic regions, and boxes represent exonic regions. The arrows indicate the direction/strand of transcription for the gene. When an exon box become narrower in height, this indicates a UTR.

When loaded, tracks are stacked on top of each other. You can identify which track is which by consulting the label to the left of each track.

Region Lists

Sometimes, it is useful to save where you are, or to load regions of interest. For this purpose, there is a Region Navigator in IGV. To access it, click Regions > Region Navigator. While you browse around the genome, you can save some bookmarks by pressing the Add button at any time.

Bookmarks in IGV

Loading Read Alignments

We will be using the breast cancer cell line HCC1143 to visualize alignments. For speed, only a small portion of chr21 will be loaded (19M:20M).

HCC1143 Alignments to hg19:

Copy the files to your local drive, and in IGV choose File > Load from File..., select the bam file, and click OK. Note that the bam and index files must be in the same directory for IGV to load these properly. Alternatively, you can copy the link location and load File > Load from URL....

Load BAM track from File

Visualizing read alignments

Navigate to a narrow window on chromosome 21: chr21:19,480,041-19,480,386.

To start our exploration, right click on the track-name, and select the following options:

  • Sort alignments by start location
  • Group alignments by pair orientation

Experiment with the various settings by right clicking the read alignment track and toggling the options. Think about which would be best for specific tasks (e.g. quality control, SNP calling, CNV finding).

Changing how read alignments are sorted, grouped, and colored

You will see reads represented by grey or white bars stacked on top of each other, where they were aligned to the reference genome. The reads are pointed to indicate their orientation (i.e. the strand on which they are mapped). Mouse over any read and notice that a lot of information is available. To toggle read display from hover to click, select the yellow box and change the setting.

Changing how read information is shown (i.e. on hover, click, never)

Once you select a read, you will learn what many of these metrics mean, and how to use them to assess the quality of your datasets. At each base that the read sequence mismatches the reference, the colour of the base represents the letter that exists in the read (using the same colour legend used for displaying the reference).

Viewing read information for a single aligned read

Visualization Part 2: Inspecting SNPs, SNVs, and SVs

In this section we will be looking in detail at 8 positions in the genome, and determining whether they represent real events or artifacts.

Two neighbouring SNPs

  • Navigate to region chr21:19,479,237-19,479,814
  • Note two heterozygous variants, one corresponds to a known dbSNP (G/T on the right) the other does not (C/T on the left)
  • Zoom in and center on the C/T SNV on the left (chr21:19,479,321 is the SNV position)
  • Sort alignments by base
  • Color alignments by read strand

Example1. Good quality SNVs/SNPs

Notes:

  • High base qualities in all reads except one (where the alt allele is the last base of the read)
  • Good mapping quality of reads, no strand bias, allele frequency consistent with heterozygous mutation

What does Shade base by quality do?

This will change the opacity of the base in IGV based on how confident the sequencer was in calling that base using the phred score. This is beneficial in determining if a called variant is real or artifactual.

How does Color by read strand help?

Coloring by read strand will indicate if the DNA fragment sequenced was on the positive or negative strand. A variant occurring on only one strand could indicate an artifact.

Homopolymer region with indel

Navigate to position chr21:19,518,412-19,518,497

Example 2a

  • Group alignments by read strand
  • Center on the A within the homopolymer run (chr21:19,518,470), and Sort alignments by -> base

Example 2b

  • Center on the one base deletion (chr21:19,518,452), and Sort alignments by -> base

Notes:

  • The alt allele is either a deletion or insertion of one or two Ts
  • The remaining bases are mismatched, because the alignment is now out of sync
  • The dpSNP entry at this location (rs74604068) is an A->T, and in all likelihood an artifact
  • i.e. the common variants from dbSNP include some cases that are actually common misalignments caused by repeats

Coverage by GC

Navigate to position chr21:19,611,925-19,631,555. Note that the range contains areas where coverage drops to zero in a few places.

Example 3

  • Use Collapsed view
  • Use Color alignments by -> insert size and pair orientation
  • Load GC track
  • See concordance of coverage with GC content

Why are there blue and red reads throughout the alignments?

The reads are colored by insert size, in paired data a blue read indicates the insert size is smaller than expected indicating a deletion. Conversely a red read indicates the insert size is larger than expected indicating an insertion.

Heterozygous SNPs on different alleles

Navigate to region chr21:19,666,833-19,667,007

Example 4

  • Sort by base (at position chr21:19,666,901)

Note:

  • There is no linkage between alleles for these two SNPs because reads covering both only contain one or the other

Low mapping quality

Navigate to region chr21:19,800,320-19,818,162

  • Load repeat track (File -> Load from server...)

Load repeats

Example 5

Notes:

  • Mapping quality plunges in all reads (white instead of grey). Once we load repeat elements, we see that there are two LINE elements that cause this.

Homozygous deletion

Navigate to region chr21:19,324,469-19,331,468

Example 6

  • Turn on View as Pairs and Expanded view
  • Use Color alignments by -> insert size and pair orientation
  • Sort reads by insert size
  • Click on a red read pair to pull up information on alignments

Notes:

  • Typical insert size of read pair in the vicinity: 350bp
  • Insert size of red read pairs: 2,875bp
  • This corresponds to a homozygous deletion of 2.5kb

Mis-alignment

Navigate to region chr21:19,102,154-19,103,108

Example 7

Notes:

  • This is a position where AluY element causes mis-alignment.
  • Misaligned reads have mismatches to the reference and well-aligned reads have partners on other chromosomes where additional ALuY elements are encoded.
  • Zoom out until you can clearly see the contrast between the difficult alignment region (corresponding to an AluY) and regions with clean alignments on either side

Translocation

Navigate to region chr21:19,089,694-19,095,362

Example 8

  • Expanded view
  • Group alignments by -> pair orientation
  • Color alignments by -> pair orientation

Notes:

  • Many reads with mismatches to reference
  • Read pairs in RL pattern (instead of LR pattern)
  • Region is flanked by reads with poor mapping quality (white instead of grey)
  • Presence of reads with pairs on other chromosomes (coloured reads at the bottom when scrolling down)

Visualization Part 3: Automating Tasks in IGV

We can use the Tools menu to invoke running a batch script. Batch scripts are described on the IGV website:

Download the batch script and the attribute file for our dataset:

Now run the file from the Tools menu:

Automation

Notes:

  • This script will navigate automatically to each location in the lab
  • A screenshot will be taken and saved to the screenshots directory specified
Genome Browsing and Visualization - UCSC | Griffith Lab

Genomic Visualization and Interpretations

Genome Browsing and Visualization - UCSC

The UCSC genome browser is a powerful web application for exploring the genomes of a variety of organisms in the context of annotation tracks. Let’s start by navigating to the UCSC genome browser homepage at http://genome.ucsc.edu and clicking on Genome Browser. Over the course of this tutorial we will be highlighting features with a transparent textbox, please pay attention to these as they are relevant to the discussion.

Selecting an organism and assembly

This will take us to the browser gateway where we can select the organism we wish to view as well as the assembly for that organism. For this tutorial we will be using the Febuary 2009 assembly of the human genome (GRCh37/hg19), let’s go ahead and select that from the Human Assembly dropdown menu and click on “GO”.

We are now in the genome browser for our chosen reference assembly, there is alot of information here, but let’s start with the basics, navigating around. We can jump to a position or genome by entering them in the highlighted text box below, let’s jump to PIK3CA which has coordinates chr3:178,866,311-178,952,497. After jumping to PIK3CA we can see in the image below that PIK3CA is 86,187 base pairs long and resides on the q arm of chromosome 3 (green text boxes). We can shift our viewing window with the arrow keys on the top left and we can zoom in and out using the zoom buttions on the right (red text boxes). In addition clicking and dragging inside the coordinate track within the browser will zoom to the window highlighted by such an action. Performing this action within any of the other tracks will shift the viewing window left or right. These are analogous to using the buttons mentioned above but allow for more user control.

Genome browser tracks

One of the features that makes the genome browser so usefull are the tracks displayed in the context of the genome assembly. Users can add and remove tracks by using the buttons below the the genome browser, clicking on the name of a track will allow for more fine grain control of that track and give additional information regarding that track such as version numbers. A number of tracks are turned on by default, but let’s go ahead and hide all tracks to get a better sense of what’s going on, click on the “hide all” button below the genome browser.

The browser should now be empty of tracks, let’s go ahead and add the ensembl gene track, find Ensembl Genes under “Genes and Gene Predictions” and click on the link. You should see a page similar to the one below. Go ahead and set the Display mode to “full” and turn “Color track by codons” on. This will turn on ensembl gene annotations using ensembl version 75.

As is exhibited below we can now see the full list of ensembl transcipts of PIK3CA based on the ensembl version 75 annotations of which there are 5. You can add as many tracks onto the viewer as you want but note that the available tracks will differ between species and even between assemblies.

Turn on the GTEx track, which tissue has the highest expression of PIK3CA?

Get a hint!

Try clicking on the track in the genome browser once it's enabled

Answer

Cells - EBV-transformed lymphocytes have the highest expression at 11.3 RPKM.

BLAST-like alignment tool

Our discussion of the genome browser would not be complete without mentioning the BLAST-like alignment tool commonly refered to as BLAT. As the name would suggest BLAT works similar to BLAST, however it works in conjunction with the UCSC genome browser and aligns only to the specified genome assembly. Let’s use BLAT to look at which section of the hg19 genome assembly the first primer pair in Supplemental Table S2 is amplifying, from the paper: “Non-exomic and synonymous variants in ABCA4 are an important cause of Stargardt disease.”. The forward primer used in this experiment is TTTCTGAAATTGGGATGCAG and the reverse primer is GTTTTCCCAGGCAGAACAGA. Go ahead and input these primers into BLAT in fasta format. Make sure to select the human hg19 reference assembly, then go ahead and click on submit.

BLAT will search the genome and output a table of possible matches between the genome assembly and the primer sequences. Unsurprisingly there are only two entries, one for each primer and each with 100% identity. Go ahead and click on “browser” to view one of these on the UCSC genome browser. The second primer should be close by, try and zoom out to find it.

Which gene do these primers attempt to amplify?

Answer

ABCA4

UCSC Table Browser

BLAT allowed us to get a region based on a sequence, but what if we want to do the reverse of that, to get a sequence based on a region. Fortunately the UCSC genome browser has a tool for that as well called the UCSC Table Browser. Let’s get the DNA sequence for the gene PRLR using this tool. First navigate to the UCSC Table Browser, there are alot of options here but we only need to concern ourselves with a few. First make sure that the proper assembly is specified, in our case this should be hg19 (red boxes). Next because we want to return the sequence of the PRLR gene “Genes and Gene Precitions” should be selected under group, and let’s go ahead and look for this gene in the “UCSC Genes” track (green boxes). Further the table should be set to “knownGene” (blue box).

Next go ahead and click on “paste list” under “identifiers (names/accessions)” (see arrow in the figure above) and specify that PRLR is the gene we want the sequence for.

From there change the “output format” to “sequence” and click on “get output”. The table browser will ask what type of sequence we want for PRLR, let’s go ahead and get the protein sequence.

How many transcript are there for PRLR in the UCSC known genes track? Use the results from the table browser.

Answer

9

Try using BLAT on the protein sequence for entry “uc021xxl.1”, which ensembl transcript does this entry correspond to?

Answer

ENST00000310101