Summation: Principles of Bioinformatics

Transcription

Summation: Principles of Bioinformatics
Summation: Principles of
Bioinformatics
• Review the key ingredients of the
Recipe for Bioinformatics.
• Use the Human Genome results as
examples for understanding the
importance of these ingredients in future
genomics and bioinformatics problems.
• Integrate these principles with all the
specifics you’ve learned this quarter:
these principles are present in
everything we’ve done in this class.
What is Genomics?
G=(MB)n
Genomics is any (molecular) biology
experiment taken to the whole genome
scale.
• Ideally in a single experiment.
• E.g. genome sequencing.
• E.g. DNA microarray analysis of gene
expression.
• E.g. mass spectrometry protein mixture
analyses: quantity, phosphorylation, etc.
Genomics Foundation: High
Throughput Technology
• Automation: any human step is a
bottleneck.
• Multiplexing & parallelization.
• Miniaturization.
• Read-out speed, sensitivity.
• “GMP” Q/A, reproducibility, “production
line” mindset.
In Genomics every question is
really an information
problem
• In molecular biology, experiments are
small and designed to test a specific
hypothesis clearly and directly.
• In genomics, experiments are massive
and not designed for a single
hypothesis.
• Every biology question about genomics
data corresponds to a computer science
problem: how to find the desired pattern
in a dataset.
Human Genome Sequencing
• The experimental part (the actual
sequencing) was easy. It was the
information problem that was hard.
• Assembly: the high frequency of repeats
in the human genome can fool you into
joining the wrong fragments
Purely Sequence-Based
Assembly
Celera believed they could assemble the whole human
genome from shotgun sequence fragments in this way.
But this approach failed. They had to use the public
domain map data to resolve problems in their assembly.
Genome Annotation
• Genes are what biologists really want, not just
the genome sequence.
• Unfortunately, most of the 32,000 gene
annotation is based on gene prediction, not
measured experimental evidence.
• It is likely that 50% of the reported genes are
wrong in details (individual exons,
boundaries) or entirely.
• The Drosophila annotation has recently been
shown to be deeply flawed.
• An information problem that is still not solved.
Definition of Bioinformatics
Bioinformatics is the study of the inherent
structure of biological information.
• Data-driven: let the data speak for
themselves.
• non-random patterns in the data.
• Measure significance of patterns as
evidence for competing hypotheses.
Computational Challenges
• Cluster genes by expression pattern
over the course of the cell cycle.
• Identify groups of genes that are coexpressed, co-regulated.
• Identify regulatory elements in common
to the promoters of these genes, that
make them be expressed at the same
time.
Solving the Information
Problem
• Modeling the problem: choosing what to
include, and how to describe them.
• Relating this to known information
problems.
• Algorithms for solution.
• Complexity: amount of time & memory
the algorithm requires.
Completeness Changes
Everything
• In molecular biology cleverness is
finding a way to answer a question
definitively by ignoring 99.99% of
genes. You can’t see them, so the
experiment must exclude them.
• In genomics cleverness is discovering
what becomes when possible when you
can see everything.
Have to switch our deepest assumptions.
What specifically can you
learn from Everything?
E.g. Protein function prediction:
• genomic neighbors method
• phylogenetic profiles
• domain fusion (Rosetta Stone method)
Microarray gene expression analysis
• meaningful signal not in just a few
genes
Example: Ortholog
Prediction
• Orthologs: two genes related by
speciation events alone. “the same
gene in two species”, typically, same
function.
• Paralogs: two genes related by at least
one gene-duplication & divergence
event.
Homology: an ortholog or a paralog?
• Experimentally very hard to answer.
Genomics Requires
Statistical Measures of
Evidence
• Evaluate competing hypotheses under
uncertainty--automatically?
• based on statistical tendencies, not
“proofs”
• false positives, false negatives
• the need for cross-validation
• the need for experimental validation
• best role: experiment interpretation and
planning
Measures of Evidence
• SNP identification from sequence data
• Genome annotation: Gene evidence?
Keys
• explicit, realistic likelihood models,
priors measured from tons of real data.
• Explicit evaluation of alternative models.
• Real posteriors, w/ measures of
uncertainty.
Integrating Independent
Evidence
• Typically, a calculation works with one
kind of data; hard to integrate very
different data.
• Likelihood Models provide easy way to
integrate many different types of data: if
they are really different, just multiply
them
• Independence; Factorization!
Statistical Problems
• Microarray analysis: hierarchical
clustering?
• Genome annotation: gene prediction?
• COGs: no statistics at all…
• Protein function prediction: distance
metrics instead of probabilistic modeling,
no posteriors.
From Reductionism to
Systems Analysis
• Mol. Biol.: dissect a complex
phenomenon into its smallest pieces;
characterize each.
• Very hard to put the pieces back together
again: Given AB, AC: A+B+C = ?
• Genomics: The cell as test-tube. Able to
see A+B+C (+D+E+…) working together.
• Study how all the components work
together as a system. Study system
behavior.
Cell-Cycle Regulated Genes
by whole genome mArray
Automated Discovery of Cell
Cycle Regulatory Elements
Rosetta Stone Assumption:
Fusion of functionally-linked domains
In organism 1:
A
A
In organism 2:
A'
B
B'
Implies proteins A and B may be functionally linked
PHYLOGENETIC PROFILE METHOD
From Hypothesis-Driven
to Data-Driven Science
• Mol. Biol.: can’t see 99.99% of genes, so
use black-box logic based on controls:
keep everything the same except for one
small change. Isolate a specific causeeffect.
• In reality you rarely have the perfect
control.
• Hypothesis driven: can only see what
you look for: a few genes, a few controls.
• Interpretable: ask a YES-NO question.
From Hypothesis-driven
to Data-driven Science
• Genomics: measure all genes at once.
• Don’t have to assume a hypothesis as
basis for designing the experiment.
• Objective: let the data speak for
themselves.
• Reality: vast amounts of data, very
complex, hard to interpret.
“System Science” or just “Stupid
Science”?
Stupid Science: Data-driven
Science Done Wrong
• No hypothesis.
• Assumptions: alternative models not explicitly
enumerated, weighed.
• Statistical basis of model either neglected or
only implicit (and therefore poor).
• No cross-validation: just one form of evidence.
• Greedy algorithms, sensitive to noise.
• Measures of significance weak or absent, both
computationally and experimentally.
Data-driven Science Done
Right
• Multiple competing hypotheses.
• Alternative models explicitly included,
computed, to eliminate assumptions.
• Statistical models clear, well-justified.
• Multiple, independent types of evidence.
• Robust algorithms w/ well demonstrated
convergence to global optimum.
• Rigorous posterior probability calculated for
all possible models of the data. Priors
derived from data. False +/- measured.
Implications of Data-driven
Science
• To get strong posteriors that can distinguish
multiple models, you need LOTS of data.
• Genomics is creating an unprecedented
avalanche of data, opportunities.
• A change in the nature of data: “lost” data in old
notebooks, journals, heads; vs. electronic
databases that can be queried, analyzed.
• The end of (purely) human analysis.
• Don’t confuse observations & interpretations.
Bioinformatics as Prediction
• Given a protein sequence,
bioinformatics would seek to predict its
fold.
• Given a genome sequence,
bioinformatics would seek to predict the
locations and exon-intron structures of
its genes.
• The ultimate test: make a blind
prediction (when no experimental data
A new kind of Bioinformatics
• The massive experimental data
produced by genomics projects has
created a demand for a fundamentally
different kind of bioinformatics, which
we can characterize (with some
exaggeration) as a mix of three
principles:
CHEAT
• Don’t even try to predict anything.
• Just say, “Give us all your experimental
data that contain the answer to this
question, and THEN we’ll tell you what
we think the answer is!”
• Focus is on statistically accurate
measurement of the strength of the
evidence for different interpretations of
the experimental data.
Steal Other People’s Data
• The massive amount of data being
produced in the public domain is an
opportunity for heavy duty data-mining,
using statistics to expose patterns that
would otherwise be missed in this huge
dataset.
Trust No One
What kind of data do we want?
• RAW EXPERIMENTAL DATA, ideally
straight from the sequencing machines.
• INTERPRETED DATA is untrustworthy.
• Actually, bioinformatics PREDICTIONS
are contaminating the experimental
databases!
Chromatographic Evidence
G
G
T
G
Hs#S785496
zu42c08.r1
G
G
T
C
C
C
G
G
T
G
A
Hs#S1065649
oz03ho7.x1*
A
T
C
C
C
Science by Computer?
• No human scientist will ever look at all these
data.
• To make discoveries in these data, scientific
judgment of evidence must be formalized as a
computation.
• Computational Inference about hidden states H
from observable states
posteriorO:
likelihood
prior
Bayes’ Law
p(O | H ) p( H )
p ( H | O) 
 p(O | h) p(h)
h
Sum over all hidden states
Diversity Kills Bayes Law
• Posterior probability p(M|obs) assumes
all observations came from one model.
• e.g. gene prediction: predict “best gene
structure”. Completely ignores
possibility of alternative splicing.
• What if there are multiple, different
models in reality? Observations will
appear contradictory…
• Must treat world as a hidden mixture of
models: don’t know how many; don’t
weight
When data is Mixed Up…
No correlation?
height
weight
Real Results are Hidden
Baseball
players
Basketball
players
Good correlation
within each
group!
height
…or Completely Wrong
weight
Sumo
wrestlers
Overall
correlation line
height
Basketball
players
weight
Mixture Evidence must be
convincing!
height
Mixture Evidence must be
convincing!
weight
Or we could arbitrarily
split up the data any
way we like, to
generate any desired
(ridiculous)
conclusion!
height
Not just one Genome!
a Hidden Mixture
• Generalize linear model S to partial
order graph with hidden edge
r67
probabilities rij:
S1
S2
S3
S4
SNP
S5
S6
S7
S8
S9
S10
S11
r69
splice
Cf. Gene Prediction: assume only one model possible (no
alternative splicing), so treat rij as binary (0 or 1).
Evidence Confidence
1-r
C
A
G
G
T
C
r
T
A
G
G
C
G
1
 p(obs | r ) p( r )dr
•Odds ratio that a feature exists:
0
log
log p(r > 0 | obs)
p(obs | r  0) Pr( r  0)
•Does the probability of the observations drop catastrophically
when we eliminate a given model feature (ie. Set its r=0)?
•That means there are some observations that cannot be
explained well any other way! This is strong evidence.
t
1
-t
p(obs | t > 0) = 10-3
p(obs | t = 0) = 10-7.2
Evidence – Confidence
LOD VALUE of 4.2
Sorting out EST complexity
• Need to allow for real divergences within an
alignment e.g. chimeras, paralogs, alt.
splicing…
• Detect clustering errors by dividing alignment
into groups of divergent sequences (possible
paralogs).
• Apply graph theory (mathematics of
branching structures) to deal with this.
• Developed new multiple sequence alignment
method to do this: Partial Order Alignment
• Two cases: with genomic sequence, or
Linear Alignment: assumes NO
Structural Divergences
Cluster AA702884
C vs. T polymorphism
Novel SNP, not
previously identified.
Linear
MSA
• simple assembly
• substitutions
• simple indels
Major Divergences within an Alignment: “Partial Order”
Branching
Loops
• multiple domains
• chimeric sequences
• paralogous genes
• alternative splicing
or polyadenylation
• not simple indels
• alternative splicing
• paralogous genes
• multiple domains
Find Optimal Traversals
Use graph theory to find the minimum number of traversals needed
to completely encode the alignment.
Completely encoded by a
single traversal
Can only be encoded by
two distinct traversals
Assign each EST to the traversal that encodes it
Separation of Paralogous
Groups
#Unigene clusters
Partial Order structure in Unigene
4500
4000
3500
3000
2500
2000
1500
1000
500
0
Series1
1
2
3
4
5
6
7
8
9
10
#of distinct bundles
Most Unigene clusters contain mutually inconsistent sequences (e.g.
branching, correlated substitutions suggesting paralogs, etc.)
Pairwise Multiple Sequence
Alignment
O(N2) pairwise distances;
find minimum spanning tree;
iteratively align via shortest edges.
3
2
4
1
Multiple Domains
Proteins may share a domain, but differ elsewhere:
One domain may be
observed in many
proteins. One protein
may contain many
domains.
As a linear MSA: Should it be stored as...
In linear MSAs, indel
placement is often
arbitrary. This leads to
arbitrary differences in
Or as...
gap penalties charged
in later alignments...
PO Alignment of Multi
Domain Sequences
MATK
ABL1
GRB2
CRKL
M
A
KINASE
M
SH3
A
SH2
G
G
C
SH3
SH3
C
Data Convergence: the
Genome is the Glue
• Biology has become highly specialized,
fragmented: natural result of reductionism.
• But ultimately most activities attach to a gene
or group of genes.
• Because of evolution, genes connect to each
other through orthology (across species) and
paralogy (duplication events).
• Discovery is connecting previously unrelated
facts about phenotypes and causes
Examples
• Human proteins work in yeast. Use
yeast to figure out protein-protein
interactions…
• Drosophila get high on cocaine. Use
them to study addiction, therapies.
• C. elegans on Prozac...
• Spiders build crazy webs on caffeine...
Human
Mouse
Expansion of Gene Families
Star Topologies:
O(N2) power in O(N) links
mutants
Regulatory sites
phenotypes
Alternative
splicing
Model
organisms
activities
ligands
expression
genes
domains
proteins
polymorphisms
screens
development
mapping
Disease associations
folds
motifs
Genomics & Bioinformatics
• Incredible increase in experimental data
production is making possible entirely
new analyses.
• Bioinformatics required for interpretation
of the meaning of the raw data.
• A lot of discovery is possible, but…
Evidence Matters!
• The genome is a very complex place.
• Every possible trap for naïve
bioinformatics analysis is actually there:
repeats, paralogs, polymorphisms
• Rigorous statistical measurement of our
confidence is the only thing that will
keep us from making silly mistakes.
Sources of Uncertainty
Experimental Factors
Type of errors caused: (+) false positive, (-) false negative
Chimeric ESTs
(+) in methods that simply compare ESTs.
Genomic contamination
EST fragmentation
(+) in methods that don’t screen for pairs of mutually exclusive
splices.
(+) in methods that don’t screen for fully valid splice sites (which
requires genomic mapping, intronic sequence.)
(+/-) in methods that don’t correct misreported orientation, or don’t
distinguish overlapping genes on opposite strands
(+/-). Single pass EST sequencing error can be very high locally
(e.g. >10% at the ends). Need chromatograms.
Where ESTs end cannot be treated as significant.
EST coverage limitations,
bias
Genomic coverage,
assembly errors
(-). Most genes have very few ESTs, from even fewer tissues.
The main barrier to alternative splice detection.
(-) in methods that map ESTs on the genome. Short contigs may
cause >25% false negatives.
RT / PCR artifacts
EST orientation error,
uncertainty
Sequencing error
Modrek & Lee, Nature Genetics (in press).
Bioinformatics Factors
Alignment size limitations
(-) in methods that can’t align >102, >103 sequences
Alignment degeneracy
(+/-). Alignment of ESTs to genomic is frequently degenerate around
splice sites.
“Pathological” assemblies
(+/-). What should assembly programs do when the assembled reads
disagree in regions (e.g. alt. splicing)? Programs vary.
Non-standard splice sites?
(+) in methods that don’t fully check splice sites; (-) in methods that do
restrict to standard splice sites.
Arbitrary cutoff thresholds
(+/-) in methods that use cutoffs (e.g. “99% identity”).
Rigorous measures of
evidence
(+/-). How can the strength of experimental evidence for a specific
splice form be measured rigorously?
Mapping ESTs to the
genome
(-) in methods that map genomic location for each EST.
Paralogous genes
(+) in all current methods, but mostly in those that don’t map genomic
location or don’t check all possible locations.
Biological Interpretation
Factors
Defining the coding region
Predicting ORF in novel genes; splicing may change ORF.
Predicting impact in UTR
Relatively little has been proved for UTR effects.
Predicting impact in protein
Motif, signal, domain prediction, and functional effects.
Assessing and correcting
for bias
Our genome-wide view of function is under construction. Until then,
we have unknown selection bias.
Spliceosome errors?
Is splicing perfect? I.e. does it only make correct forms?
What’s truly functional?
Just because a splice form is real (i.e. present in the cell) doesn’t
mean it’s biologically functional. Conversely, even an mRNA isoform
that makes a truncated, inactive protein might be a biologically valid
form of functional regulation.