Exome sequencing data analysis

Overview

question Questions
  • How to identify the genetic variation with the use of exome sequencing?
  • What is the pipeline of the process of finding genetic variation which caused the disease?
objectives Objectives
  • Identification of the genetic variation using the exome sequencing
  • Using FreeBayes calls for variants generating
  • Variant analysis and GEMINI queries
requirements Requirements

time Time estimation: 2 hours

Supporting Materials

Introduction

Exome sequencing means that all protein-coding genes in a genome are sequenced.

In Humans, there are ~180,000 exons that makes up 1% of the human genome which contain ~30 million base pairs. Mutations in the exome have usually a higher impact and more severe consequences, than in the remaining 99% of the genome.

With exome sequencing, one can identify genetic variation that is responsible for both Mendelian and common diseases without the high costs associated with whole-genome sequencing. Indeed, exome sequencing is the most efficient way to identify the genetic variants in all of an individual’s genes. Exome sequencing is cheaper also than whole-genome sequencing. With a high coverage rate of 100+ DP, 98% of all exons are covered.

Items where exome sequencing can’t identify genetic variation in:

  • All genes

Not all genes are in your exon data, especially those buried in stretches of repeats out towards the chromosome tips, aren’t part of exome sequencing chips

  • The handful of genes that reside in mitochondria, rather than in the nucleus
  • “Structural variants” such as translocations and inversions, that move or flip DNA but don’t alter the base sequence.
  • Triplet repeat disorders, such as Huntington’s disease and fragile X syndrome can’t be detected.
  • Other copy number variants will remain beneath the radar, for they too don’t change the sequence, but can increase disease risk.
  • Introns

A mutation that jettisons a base in an intron can have dire consequences: inserting intron sequences into the protein, or obliterating the careful stitching together of exons, dropping gene sections. For example, a mutation in the apoE4 gene, associated with Alzheimer’s disease risk, puts part of an intron into the protein.

  • “Uniparental disomy”

Two mutations from one parent, rather than one from each, appear the same in an exome screen: the kid has two mutations. But whether mutations come from only mom, only dad, or one from each has different consequences for risk to future siblings. In fact, a case of UPD reported in 1988 led to discovery of the cystic fibrosis gene.

  • Control sequences

Much of the human genome tells the exome what to do, like a gigantic instruction manual for a tiny but vital device. For example, mutations in microRNAs cause cancer by silencing various genes, but the DNA that encodes about half of the 1,000 or so microRNAs is intronic – and therefore not on exome chips.

  • Epigenetic changes

Environmental factors can place shielding methyl groups directly onto DNA, blocking expression of certain genes. Starvation during the “Dutch Hunger Winter” of 1945, for example, is associated with schizophrenia in those who were fetuses at the time, due to methylation of certain genes. Exome sequencing picks up DNA sequences – not gene expression

  • Gene-gene (epistatic) interactions

One gene affecting the expression of another can explain why siblings with the same single-gene disease suffer to a different extent. For example, a child with severe spinal muscular atrophy, in which an abnormal protein shortens axons of motor neurons, may have a brother who also inherits SMA but has a milder case thanks to a variant of a second gene that extends axons. Computational tools will need to sort out networks of interacting genes revealed in exome sequencing.

We will follow the pipeline:

Exome sequencing pipeline in Galaxy

details More detailed tutorial

For a more detailed tutorial, have a look at the tutorial on diploid variant calling. It follows a similar pipeline using genome in the bottle data, but with more details particularly on the theory behind.

Agenda

In Exome-Seq tutorial, we will deal with:

  1. Data-preprocessing
  2. Variant calling
  3. Post-processing
  4. Variant analysis

Data-preprocessing

In this tutorial, a child has a yet unknown disease. His parents are healthy. We will try to identify the genetic variation that is responsible for the disease using the exome sequencing data from both parents and the child.

Most of the data pre-processing have already be done on the raw exome sequencing. The raw exome sequences were mapped on hg19 version of the human genome. So, for each family member, we will start with one BAM file with mapping results.

hands_on Hands-on: Data pre-processing

  1. Import all 3 BAM’s into a new history:
  2. Specify the used genome for mapping (for each dataset)
    1. Click on Edit attributes (pencil icon on right panel)
    2. Select Human Feb 2009 on Database/Build
    3. Save it
  3. Import the reference genome tool:
    1. Go on Data Libraries in Shared data (top panel on Galaxy’s interface)
    2. Click on Training Data
    3. Select hg19
    4. Click on Import selected datasets into history (just below the top panel)
    5. Import it
    6. Convert it from 2bit to fasta with twoBitToFa from Convert Formats
  4. Follow the next steps for father data and then apply the generated workflow on other datasets

Variant calling

Generating FreeBayes calls

To call our variants, we will use FreeBayes. FreeBayes is a Bayesian genetic variant detector designed to find small polymorphisms, specifically SNPs (single-nucleotide polymorphisms), indels (insertions and deletions), MNPs (multi-nucleotide polymorphisms), and complex events (composite insertion and substitution events) smaller than the length of a short-read sequencing alignment.

hands_on Hands-on: Generating FreeBayes calls

  1. Select FreeBayes in the tool menu (left panel of Galaxy’s interface)
  2. Run FreeBayestool:
    1. Load reference genome from local cache
    2. Select the father data as the BAM dataset
    3. Use Human (Homo sapiens): hg19 as reference genome
    4. Choose other default settings
    5. Execute

trophy Congratulations! You have created you first VCF file, one of most complicated file formats in bioinformatics. In such a file your called variants are stored with one variant per line (+header lines).

Post-processing FreeBayes calls

Before we can continue, we need to post-process this dataset by breaking compound variants into multiple independent variants and filter the VCF file to simplify the variant representation.

hands_on Hands-on: Post-processing FreeBayes calls

  1. Split your allelic primitives (gaps or mismatches) into multiple VCF lines with VcfAllelicPrimitives from VCF Tools tool:
    1. Select the FreeBayes output file as VCF dataset
    2. Make sure Maintain site and allele-level annotations when decomposing and Maintain genotype-level annotations when decomposing are set to Yes
  2. Filter your VCF file with SnpSift Filter from Annotation to only conserve SNPs with a quality >= 30 and a coverage >= 10

comment Constructing the correct expression

Have a look at the examples that can help you construct the correct expression for the Filter criteria.

Annotate your variants

To annotate the variants, we use the dbSNP, the NCBI database of genetic variation and then hg19 database with SnpEff.

hands_on Hands-on: Annotating variants

  1. Import the dbSNP_138.hg19.vcf in your history (Build 138 data, available on the human assembly (GRCh37/hg19))
  2. Assign the known variant ID from dbSNP to your variants, using SnpSift Annotate from Annotation
  3. Annotate your variants with some functional information
    1. Download hg19 database with SnpEff Download from Annotation
    2. Launch annotation of your variants with SnpEff from Annotation, using the downloaded database (reference genome from your history)

Look at your “INFO” column again in the generated VCF file. You will get some gene names for your variants, but also a predicted impact and if your variant is located inside of a known gene.

comment HTML Report

You can also have a look at the HTML report. It contains a number of useful metrics such as distribution of variants across gene features.

Getting the final VCF

At this stage, you have your first annotated variants and in theory everything you need for your further studies is included in your VCF file.

hands_on Hands-on: Getting VCF file

  1. Extract your history to a workflow tool
  2. Apply this workflow to the other BAM files tool

    comment Comments

    You should now have 3 annotated variant files, from mother, father and the patient. It might be a good idea to rename them accordingly.

  3. Combine all 3 files into one with the tool VCFcombine from VCF Tools tool

Now that we have an annotated VCF file it is time to peek inside our variation data

Post-processing

hands_on Hands-on: Post-processing

  1. Create a pedigree file (PED) like this
#family_id	sample_id	paternal_id	maternal_id	sex	phenotype	ethnicity
family1	RS024M-MOTHER	-9	-9	2	1	CEU
family1	RS024V-FATHER	-9	-9	1	1	CEU
family1	RS024P-PATIENT	RS024V-FATHER	RS024M-MOTHER	1	2	CEU
  1. Use the tool GEMINI load in Gemini to create a database out of your combined VCF file and the PED file.

    comment Comments

    This creates a sqlite database. To see the content of the database use GEMINI_db_info

Either way you have now a database with all your variants, with pedigree relations, additional annotations and most importantly its fast to search.

comment GEMINI Tools

Have a look at all different Gemini tools and run as many tools as possible on your GEMINI databases. Try to get a feeling of what is possible with a variant database in GEMINI.

Variant analysis

GEMINI query is the most versatile of all the GEMINI tools. You can use it to ask ‘interesting’ questions in simple SQL (see the GEMINI handbook on its usage).

GEMINI query examples:

  • select chrom, start, end from variants will show you some information on all variants that were found in any of the three samples
  • select chrom, start, end, (gts).(*) from variants will also show you the genotype of each sample also with the help wildcards
  • select chrom, start, end, gene, impact, (gts).(*) from variants v where v.impact_severity='HIGH' will show you some more information and filter out only those variants that have a high impact
  • select chrom, vcf_if, start, end, ref, alt, gene, impact, (gts).(*) from variants v where v.impact_severity='HIGH' also shows you the reference allele and the alternative allele and the RSID for the SNP if it exists

comment Header

Switch on the --header parameter**

details Advanced GEMINI usage

To go further on Gemini, you can have a look at the following tutorials:

And for a more detailed tutorial on variant data generation in Galaxy, have a look at the tutorial on diploid variant calling.

keypoints Key points

  • With exome sequencing, one can identify genetic variation that is responsible for some diseases
  • Exome sequencing is the most efficient way to identify the genetic variants in all of an individual's genes
  • One can use GEMINI queries to access database with all the genetic variants
  • In order to generate such database one can use FreeBayes

Useful literature

Further information, including links to documentation and original publications, regarding the tools, analysis techniques and the interpretation of results described in this tutorial can be found here.

congratulations Congratulations on successfully completing this tutorial!