Download Neutral DNA - Penn State University

Comparative Genomics I: Tools for
comparative genomics
Ross Hardison, Penn State University
James Taylor: Courant Institute, New York University
Major collaborators: Webb Miller, Francesca Chiaromonte, Laura
Elnitski, David King, et al., PSU
David Haussler, Jim Kent, Univ. California at Santa Cruz
Ivan Ovcharenko, Lawrence Livermore National Lab
CSH Nov. 11, 2006
Major goals of comparative genomics
• Identify all DNA sequences in a genome that are
functional
– Selection to preserve function
– Adaptive selection
• Determine the biological role of each functional sequence
• Elucidate the evolutionary history of each type of
sequence
• Provide bioinformatic tools so that anyone can easily
incorporate insights from comparative genomics into their
research
Three major classes of evolution
• Neutral evolution
– Acts on DNA with no function
– Genetic drift allows some random mutations to become fixed in a
population
• Purifying (negative) selection
– Acts on DNA with a conserved function
– Signature: Rate of change is significantly slower than that of neutral DNA
– Sequences with a common function in the species examined are under
purifying (negative) selection
• Darwinian (positive) selection
– Acts on DNA in which changes benefit an organism
– Signature: Rate of change is significantly faster than that of neutral DNA
Ideal case for interpretation
Negative selection
(purifying)
Similarity
Neutral DNA
Positive selection
(adaptive)
Position along chromosome
Exonic segments coding for regions
of a polypeptide with common
function in two species.
Exonic segments coding for regions
of a polypeptide in which change is
beneficial to one of the two species.
DCODE.org Comparative Genomics: Align your
own sequences
blastZ
multiZ and TBA
zPicture interface for aligning sequences
Automated extraction of sequence and annotation
Pre-computed alignment of genomes
• blastZ for pairwise alignments
• multiZ for multiple alignment
– Human, chimp, mouse, rat, chicken, dog
– Also multiple fly, worm, yeast genomes
– Organize local alignments: chains and nets
Webb Miller
• All against all comparisons
– High sensitivity and specificity
• Computer cluster at UC Santa Cruz
– 1024 cpus Pentium III
– Job takes about half a day
Jim Kent
• Results available at
– UCSC Genome Browser http://genome.ucsc.edu
– Galaxy server: http://www.bx.psu.edu
Schwartz et al., 2003, blastZ, Genome Research
Blanchette et al., 2004, TBA and multiZ, Genome Research
David Haussler
Genome-wide local alignment chains
Human: 2.9 Gb assembly. Mask interspersed repeats, break into 300 segments of 10 Mb.
Human
Mouse
blastZ: Each segment of human is given the opportunity to align with all mouse sequences.
Run blastZ in parallel for all human segments. Collect all local alignments above threshold.
Organize local alignments into a set of chains based on position in assembly and orientation.
Level 1 chain
Level 2 chain
Net
Comparative genomics to find functional sequences
Genome
size
2,900
Find
common
sequences
blastZ,
multiZ
2,400
Human
Identify
functional
sequences: ~ 145
Mbp
All mammals
1000 Mbp
2,500
Mouse
Rat
1,200
million base pairs
(Mbp)
Also birds: 72Mb
Papers in Nature from mouse and rat and chicken genome consortia, 2002, 2004
Use measures of alignment quality to discriminate
functional from nonfunctional DNA
• Compute a conservation score adjusted for the local
neutral rate
• Score S for a 50 bp region R is the normalized fraction of
aligned bases that are identical
– Subtract mean for aligned ancestral repeats in the
surrounding region
– Divide by standard deviation
p = fraction of aligned sites in R that are
identical between human and mouse
m = average fraction of aligned sites that
are identical in aligned ancestral repeats in
the surrounding region
Waterston et al., Nature
n = number of aligned sites in R
Decomposition of conservation score into
neutral and likely-selected portions
Neutral DNA (ARs)
All DNA
Likely selected DNA
At least 5-6%
S is the conservation score adjusted for variation in the local substitution rate.
The frequency of the S score for all 50bp windows in the human genome is shown.
From the distribution of S scores in ancestral repeats (mostly neutral DNA), can
compute a probability that a given alignment could result from locally adjusted
neutral rate.
Waterston et al., Nature
DNA sequences of mammalian genomes
• Human: 2.9 billion bp, “finished”
– High quality, comprehensive sequence, very few gaps
• Mouse, rat, dog, oppossum, chicken, frog etc. etc etc.
• About 40% of the human genome aligns with mouse
– This is conserved, but not all is under selection.
• About 5-6% of the human genome is under purifying selection since the
rodent-primate divergence
• About 1.2% codes for protein
• The 4 to 5% of the human genome that is under selection but does not
code for protein should have:
– Regulatory sequences
– Non-protein coding genes (UTRs and noncoding RNAs)
– Other important sequences
Leverage
many species
to improve
accuracy and
resolution of
signals for
constraint
ENCODE multi-species
alignment group
Margulies et al., 2007
Score multi-species alignments for features
associated with function
• Multiple alignment scores
– Margulies et al. (2003) Genome Research 13: 2505-2518
– Binomial, parsimony
• PhastCons
– Siepel et al. (2005) Genome Research 15:1034-1050
– Phylogenetic Hidden Markov Model
– Posterior probability that a site is among the most highly conserved
sites
• GERP
– Cooper et al. (2005) Genome Research 15:901-913
– Genomic Evolutionary Rate Profiling
– Measures constraint as rejected substitutions = nucleotide
substitution deficits
phastCons: Likelihood of being constrained
• Phylogenetic Hidden
Markov Model
• Posterior probability that
a site is among the
most highly conserved
sites
• Allows for variation in
rates along lineages
c is “conserved” (constrained)
n is “nonconserved” (aligns but
is not clearly subject to
purifying selection)
Siepel et al. (2005) Genome
Research 15:1034-1050
Larger genomes
have more of the
constrained DNA
in noncoding
regions
Siepel et al. 2005,
Genome Research
Some constrained introns are editing complementary
regions:GRIA2
Siepel et al. 2005,
Genome Research
3’UTRs can be highly constrained over large
distances
Siepel et al. 2005,
Genome Research
3’ UTRs contain RNA processing signals, miRNA targets,
other regions subject to constraints
Ultraconserved elements = UCEs
• At least 200 bp with no interspecies differences
–
–
–
–
Bejerano et al. (2004) Science 304:1321-1325
481 UCEs with no changes among human, mouse and rat
Also conserved between out to dog and chicken
More highly conserved than vast majority of coding regions
• Most do not code for protein
– Only 111 out of 481overlap with protein-coding exons
– Some are developmental enhancers.
– Nonexonic UCEs tend to cluster in introns or in vicinity of genes
encoding transcription factors regulating development
– 88 are more than 100 kb away from an annotated gene; may be
distal enhancers
GO category analysis of UCE-associated genes
• Genes in which a
coding exon overlaps a
UCE
– 91 Type I genes
– RNA binding and
modification
– Transcriptional
regulation
• Genes in the vicinity of
a UCE (no overlap of
coding exons)
– 211 Type II genes
– Transcriptional
regulation
– Developmental
regulators
Bejerano et al. (2004) Science
Intronic UCE in SOX6 enhances expression
in melanocytes in transgenic mice
UCEs
Tested UCEs
Pennacchio et al.,
http://enhancer.lbl.gov/
The most stringently conserved
sequences in eukaryotes are mysteries
• Yeast MATa2 locus
– Most conserved region in 4 species of yeast
– 100% identity over 357 bp
– Role is not clear
• Vertebrate UCEs
– More constrained than exons in vertebrates
– Noncoding UCEs are not detectable outside chordates, whereas coding
regions are
• Were they fast-evolving prior to vertebrate/invertebrate divergence?
• Are they chordate innovations? Where did they come from?
– Role of many is not clear; need for 100% identity over 200 bp is not
obvious for any
• What molecular process requires strict invariance for at least 200 nucleotides?
• One possibility: Multiple, overlapping functions
Finding and analyzing genome data
NCBI Entrez
Ensembl/BioMart
UCSC Table Browser
Galaxy
http://www.ncbi.nlm.nih.gov
http://www.ensembl.org
http://genome.ucsc.edu
http://www.bx.psu.edu
Browsers vs Data Retrieval
• Browsers are designed to show selected information on one locus or
region at a time.
– UCSC Genome Browser
– Ensembl
• Run on top of databases that record vast amounts of information.
• Sometimes need to retrieve one type of information for many
genomics intervals or genome-wide.
• Access this by querying on the tables in the databases or “data marts”
– UCSC Table Browser
– EnsMart or BioMart
– Entrez at NCBI
Retrieve all the protein-coding exons in humans
Galaxy: Data retrieval and analysis
• Data can be retrieved from multiple
external sources, or uploaded from
user’s computer
• Hundreds of computational tools
– Data editing
– File conversion
– Operations: union, intersection,
complement …
– Compute functions on data
– Statistics
– EMBOSS tools for sequence
analysis
– PHYLIP tools for molecular
evolutionary analysis
– PAML to compute substitutions per
site
• Add your own tools
Galaxy via Table Browser: coding exons
Retrieve human mutations
Find exons with human mutations: Intersection
Compute length using “expression”
Statistics on exon lengths
Plot a histogram of exon lengths
Distribution of (human mutation) exon lengths
What is that really long exon? Sort by length
SACS has an 11kb exon