NGS Sequence Analysis for Regulation and Epigenomics Timothy Bailey - - PowerPoint PPT Presentation

NGS Sequence Analysis for Regulation and Epigenomics

Timothy Bailey Winter School in Mathematical and Computational Biology July 3, 2012

NGS Analysis and Transcriptional Regulation

• RNA-seq

– Measuring transcription levels (gene expression) – Detecting RNA regulators (e.g., miRNA)

• ChIP-seq (and ChIP-exo)

– Chromatin modifications – Binding of transcription factor proteins

Talk Overview

• I. Basic Transcriptional Regulation
• II. ChIP-seq and ChIP-exo
• III. Analyzing ChIP-seq & ChIP-exo data

a) Mapping b) Peak calling c) Motif discovery & Enrichment Analysis d) Location analysis

Part I: Basic Transcriptional Regulation

Source: ¡Steven ¡Chu ¡

Transcription Factors

• Mammalian transcription is controlled

(in part) by about 1400 transcription factor (TF) proteins.

• These proteins control transcription in

two main ways:

– Directly, by promoting (or preventing) the assembly of the pre-initiation complex. – Indirectly, by modifying the chromatin.

BASAL TRANSCRIPTION: ¡

• The pre-initiation complex assembles at

the core promoter.

• This results in only low levels of

transcription because the interaction is unstable.

DNA ¡

+ ¡

Core ¡Promoter ¡ TATA ¡ ¡ ¡ ¡INR ¡

DNA ¡ Proximal ¡Promoter ¡ TATA ¡ ¡ ¡ ¡INR ¡

PROXIMAL PROMOTER:

• The proximal promoter extends upstream
• f the promoter.
• It contains binding sites for repressor and

activator transcription factors.

• This stabilizes the transcriptional
• machinery. ¡
• Some transcription factors (“activators”)

bind to sites in the proximal promoter. ACTIVATORS:

• This increases transcription. ¡

DNA ¡

+ ¡ + ¡

Proximal ¡Promoter ¡ TATA ¡ ¡ ¡ ¡INR ¡

• This reduces transcription.
• Their binding can block binding by co-

factors and activators.

• Some factors do not stabilize the

transcriptional machinery. REPRESSORS:

+++ ¡

DNA ¡

+ ¡ + ¡

Proximal ¡Promoter ¡ TATA ¡ ¡ ¡ ¡INR ¡

ENHANCER REGIONS:

DNA ¡

+ ¡ + ¡

Proximal ¡Promoter ¡ TATA ¡ ¡ ¡ ¡INR ¡ Enhancer ¡Region ¡

1-­‑-­‑100Kb ¡

• Often very distant—1000s of base pairs. ¡
• Groups of binding sites located upstream
• r downstream of a promoter. ¡

ENHANCER REGIONS:

• DNA looping brings factors into contact

with transcriptional machinery. ¡

• Bound activators increase transcription. ¡
• Both activator and repressor transcription

factors can occupy enhancer regions. ¡

+++ ¡

DNA ¡

+ ¡ + ¡

Proximal ¡Promoter ¡ TATA ¡ ¡ ¡ ¡INR ¡ Enhancer ¡Region ¡

+++ ¡

Chromatin modification by TFs:

DNA ¡

+ ¡ + ¡

Proximal ¡Promoter ¡ TATA ¡ ¡ ¡ ¡INR ¡ Enhancer ¡Region ¡

• Tissue-specific transcription factors can

bind to HATs, causing chromatin to open. ¡

• This can increase transcription.
• Histone Acetyltransferases (HATs)

acetylate histones. ¡

Specific ¡ General ¡

HAT ¡

Part II: ChIP-seq & ChIP-exo

Source: ¡Steven ¡Chu ¡

ChIP-seq

ChIP-Exo

Rhee ¡and ¡Pugh, ¡Cell ¡201. ¡

Rhee ¡and ¡Pugh, ¡Cell ¡2011. ¡

ChIP-seq & ChIP-exo

Part III: Analyzing ChIP-seq Data

Source: ¡Steven ¡Chu ¡

Analyzing TF ChIP-seq Data

• Key messages of this talk:

– Use controls! – Validate your data at each step. – But this is Science! What could possibly go wrong…?

Things that can go wrong in ChIP-seq…

• 1. Low affinity antibody
• 2. Non-specific antibody
• 3. Contamination
• 4. Poor choice of peak calling algorithm (or

parameters) … etc.

Steps in ChIP-seq Data Analysis

• 1. Mapping: where do the sequence “tags”

map to the genome?

• 2. Peak Calling: where are the regions of

significant tag concentration?

• 3. Motif Discovery: what is the binding

motif?

• 4. Location Analysis: where are the peaks w/

respect to genes, promoters, introns etc?

1) Mapping ChIP-seq Tags

• Tags: ChIP-seq produces a pool of

“tags” (~100bp)

• Tag Count: measure of enrichment of region
• Negative Control: “input DNA” tag count

Tallack ¡et ¡al, ¡Genome ¡Res., ¡2019 ¡

2) ChIP-seq Peak Calling

• ChIP-seq produces

a pool of “tags”.

• Tags are currently

about 100 bp long.

• Tag is the 5’ end of

a DNA fragment.

• But DNA is double-

stranded so…

Wilbanks ¡and ¡FaccioM, ¡PLoS ¡One, ¡2010 ¡

ChIP-seq Peak Calling

• Peak callers combine overlapping tags to

get the “peak height”.

• Sometimes strand information is used

to combine tags on opposite strands.

• Fold-enrichment (tag count / control tag

count) is usually used as the criterion for declaring a peak.

…ChIP-seq Peak Callers

Wilbanks ¡and ¡FaccioM, ¡PLoS ¡One, ¡2010 ¡

Sanity check: are your peaks reasonable

• Width: TF ChIP-seq peaks should be relatively

short (< 300bp) compared to histone modification peaks.

– Are your peaks too wide?

• Number: Is the number of TF ChIP-seq peaks

reasonable?

– Some key TFs bind ~30,000 sites but your TF probably only binds far fewer (~1000?)

• Location: Do your peaks co-occur with histone

marks and genes your TF regulates?

• The next analysis steps will help you answer

these questions!

3) Motif Discovery & Enrichment Analysis

• If your TF binds DNA directly (and

sequence-specifically), Motif Discovery should find its binding motif.

• The DNA-binding motif of your TF

should be centrally enriched in the peaks, and hould be Central Motif Enrichment Analysis (CMEA) should find it.

Caveats in ChIP-seq Motif Analysis

• Peak regions may

contain other TF motifs due to looping.

• The binding of the

ChIP-ed factor “X” may be indirect.

• ChIP-ed motif might

be weak due to assisted binding.

Farnham, ¡Nature ¡Reviews ¡GeneMcs, ¡2009 ¡

TF Binding Motif Discovery

• ChIP-seq provides

extremely rich data for inferring the DNA-binding affinity

• f the ChIP-ed

transcription factor.

• In principle,

discovering the motif is simple. ààà

• ChIP-seq peaks tend

to be within +/- 50bp

• f the bound factor.
• So we just examine

the peak regions for enriched patterns.

MEME Suite tools for ChIP-seq motif discovery and enrichment

• The MEME Suite (http://meme.nbcr.net) contains

several motif discovery and enrichment algorithms appropriate for ChIP-seq data analysis.

– Discovery & Enrichment: MEME-ChIP – Discovery: MEME, DREME, GLAM2 – Enrichment: CentriMo, AME

Example: Motif discovery in NFIC ChIP-seq data

• Pjanic et al. predicted 39,807 ChIP-seq

peaks in NFIC ChIP-seq data.

• They do not report a using motif discovery
• n these peaks.
• We used MEME-ChIP which runs both

MEME and DREME to perform motif discovery on the 100-bp NFIC ChIP-seq peak regions.

Machanick ¡& ¡Bailey, ¡BioinformaMcs, ¡2011 ¡

Motif discovery fails in the (original) NFIC dataset

• An NFIC motif is know from in vitro data,

based on only 16 sites.

• MEME and DREME fail to find this motif in

the NFIC data.

• But so do the other algorithms we tried:

Amadeus, peak-motifs, Trawler and Weeder.

The problem: poor peak calling!

• We applied a

different ChIP-seq peak calling algorithm (ChIP-peak) which predicts only 700 peaks (rather than 40,000).

• MEME discovers the

NFI-family binding motif in this new set

• f peaks.

“site-­‑probability” ¡curve ¡ ¡

TGGC

AA

C

C

A

PosiMon ¡of ¡Best ¡Site ¡ Probability ¡

Central Motif Enrichment Analysis: CentriMo

• CentriMo searches

for known motifs whose sites are most centrally enriched in the ChIP-seq regions.

• Use 500bp regions

centered on each ChIP-seq peak.

500-­‑bp ¡ChIP-­‑seq ¡regions ¡ W=120 ¡ L=500 ¡ S ¡= ¡number ¡of ¡“successes” ¡= ¡4 ¡ T ¡= ¡number ¡of ¡“trials” ¡= ¡5 ¡

Bailey ¡et ¡al, ¡NAR ¡2012 ¡

0.0005 0.001 0.0015 0.002 0.0025 0.003

• 250 -200 -150 -100
• 50

50 100 150 200 250 probability position of best site in sequence MA0119.1 p=2.4e-031,w=295,n=5409 MA0244.1 p=4.6e-015,w=381,n=39398 MA0161.1 p=7.3e-015,w=329,n=39356 MA0099.1 p=5.5e-014,w=343,n=34267 MA0406.1 p=8.1e-012,w=323,n=31383

Central Motif Enrichment confirms the known NFIC motif—even in the original peaks

• NFIC motif is most centrally enriched of 862 JASPAR

+UniPROBE motifs (p = 10-31).

TGGC

AA

C

C

A

NFIC ¡

• However, standard motif enrichment algorithms (including AME)

do not show the NFIC as the most enriched motif.

1. Published successful KLF1 ChIP-seq.

• 2. Published failed KLF1 ChIP-seq.

Central Motif Enrichment Analysis shows when things go right (or wrong).

• 0.002
• 0.001

0.001 0.002 0.003 0.004 0.005 0.006 0.007

• 250 -200 -150 -100
• 50

50 100 150 200 250 probability position of best site in sequence MA0039.2 p=4.4e-066,w=111,n=712 Klf7_primary p=6.9e-056,w=103,n=676 MA0140.1 p=1.5e-048,w=177,n=693 MA0035.2 p=2.4e-040,w=194,n=756

• 1. ¡Successful ¡KLF1 ¡ChIP-­‑seq ¡

A

G

A

C

T

CA

CC

G

ACC

C

C

T A

KLF4 ¡

0.0005 0.001 0.0015 0.002 0.0025

• 250 -200 -150 -100
• 50

50 100 150 200 250 probability position of best site in sequence MA0039.2 p=7.2e-001,w=365,n=11404

MA0039.2 Position

A

G

A

C

T

CA

CC

G

ACC

C

C

T A

p ¡= ¡0.7 ¡

• 2. ¡Failed ¡KLF1 ¡ChIP-­‑seq ¡

KLF4 ¡

Motif Spacing Analysis finds co- factor motifs and TF complexes

4) Location Analysis

• Counts how often TF binding sites are in, say,

promoters, intergenic or intragenic regions.

Farnham, ¡Nature ¡Reviews ¡GeneMcs, ¡2009 ¡

Predicting Target Genes

• Location analysis allows identification of target

genes.

• TF binding sites in promoters probably are

regulatory.

• “Nearest TSS” rule is
• ften used to assign

binding sites to target genes, but distal sites may regulate some

• ther gene via

chromatin looping.

Farnham, ¡Nature ¡Reviews ¡GeneMcs, ¡2009 ¡

Example: Binding near TSSs

• Histogram of

distances from Klf1 ChIP-seq peak to the nearest TSS.

• KLF1 has a

population of binding sites in promoters (small hump on left), but most are distal.

Tallack ¡et ¡al, ¡Genome ¡Res., ¡2019 ¡

¡ ¡

Acknowledgements

The MEME Suite

• Tom Whitington
• Philip Machanick
• James Johnson
• Martin Frith
• William Noble
• Charles Grant
• Shobhit Gupta

KLF Project

• Michael Tallack
• Tom Whitington
• Andrew Perkins
• Sean Grimmond
• Brooke Gardiner
• Ehsan Nourbakhsh
• Nicole Cloonan
• Elanor Wainwright
• Janelle Keys
• Wai Shan Yuen