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Calibration, error modeling and variance stabilization of microarray data

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Calibration, error modeling and variance stabilization ofmicroarray data

Anja von HeydebreckMartin Vingron

MPI Molecular Genetics Berlin

Wolfgang HuberHolger Sültmann Annemarie Poustka

Div.Molecular

Genome Analysis

DKFZ Heidelberg

OverviewIAM, Univ. HD, 13 Feb 2003

o What are microarrays good for?

- functional genomics

- cancer

o What do they measure?

(very schematically)

o The data and the challenges

o Model formulation

o Simplification

by variance stabilizing transformation

o Parameter estimation

o Results

simulated data

real data

15 Feb 2001:"The human genome is sequenced"

But what does the sequence do?

>gi|22046029|ref|NT_029998.5|Hs7_30253 Homo sapiens chromosome 7 reference genomic contig GATCTTATCTATCATGTTCACCTCCCAAGAGGTGAACATATCCCCCAAAGCCTGATAGAGAGAAGATGCTCATTAATATTTAATGCATGACCATGTGCAGACTTGGGAGGAAAAATATGCCTCAGCCTATCAATATTGGACCTTAATAAACAAGGATGTTTCTGCATCATTTCCCCACAACACCGAACAAGTGTGGCTCACTGTGGATGTTTAAGCAAATGCATTGTTTTTCCAGTTATATATCTGGTAGAGATGAGGCCATTGATAGGAATGGGAAGACGATCTCCTTTTATTTTGATGACCCAGCATGGCTGAACACTCAGTGACTACCACTGCACTTTGTTGTACTTTCAGCATTAGAGATGCCAGCCCTGTAGGATATAAAACAGGAACATCTAGTCCTCAATTATATTCAGAATTACTCAAGTCTTAGAAGCACCACTTGTCTTTTTTCAAGGGAGAGAAATGCTCAAGTGATGGGCTGAAGTGAAGGGAGGGAGTCACTCACTTGAACGGTTCCCTTAGGCTGTGTGGATGCAAACAGCATTAGACAATGACACTGACAGTGGGAAATGCACTGGAGACGATGACTGGCAAAGCCCTCCTTTTCTCCCCATCCACTATAGATACTGACAGCAAAGGGTTTGTCACAATGACAACTATACACTCCCAATATCACAGAAGAAGGAGGAATAAAAGGGTATATTATGAGTGACTGAAGTTTAGAATAAATTAATAAATATTATGTCCCTCATCCATAGAAACCACAAAGGTCTAGTAAGGCTAAGGATATAACAAGAAAATAATATGAATATTTGCTTCCCCTTCCTAGTGTAATAGAGTAAGTTACAAATGGCTTCAGGAAGGGGAGAGAGGAAGAAGAGTGGATGAGATACGTAAGAGTGCTTGAGGGCTAATTTTATGAAAGCTTTGGGAAGTTTTAAGAAAAAGAAAAGCTATTTTTCAAGGTACATGTGTGTATGCGTGTGTGTGTGTGTGTGTGTGTGTGTGTGTGTGTGTGTGTGTGAAAGACAGAAGAAAGAGGGAGACCTAAGAAGACTATGAGACACTAAGAGAAAAATTAAGGTAAAAAAGACACACACTTAGAAAAACACACATAGGGAGGAGGGAGGAGGTTAAGACATTTTACTATGTGCTGTGAATGGAAACTACAAACCATTTTTGATATATGCAATATATATACATATATACACACATATACATATGTATTTAAATATTTAAATTACATTTTCTCTTTTTTTAGAGATATGGTTTCACTATGTCACTCTGCCCAGGCTGCAGTACAGTGGTTGTTCACAGTCATGATCATAGCACATTATAGCCTTGAACTCCTGGGCTCAAGCAACCCTCCTGTATTAGTCTCCCCAGTAGTTGGGATTACTAGCATATGCCACCATGTCCACCTTTATGCTTTTTAAAGTGAAAAACCATACTAAGAATGAGGCAGCTCAACTTAATAATAAAAACATTTCAAATGTAAAGAAATTTACAAAAGAAAAACAATCAACCCCATTAAAATTGGGCAAAGGGAATGAACAGACACTTTTCAAAAGAATACATGCATGCAGCCAACAAACATACAAAAAAAAAGTTCAACATCACTGATCATTAGAGAAATGCAAATCAAAACCATAATGAGATACCATCTCACACCAGTCAGAATAGCTATCATTAAAAAGTCAAAAAATAACAGATGCTAGTGAGGCTATGGAGAAAAGGGAATGCTTATACACTGTTGTTGGGTGTGCAAATCAGTTCAATCATTGTGCAAGGAAAGTGATTCCTCAAAGAGCTAAAAGCAGAGCTACCATTCGACCCAGTAATCCCACTACTGGGTATATACCCAGATGAATATAAACCATTCTACCATAAAGACACATGCATACAAATGTTCATTGCAGCACTGTTCACAATAGCAAAAGTATGGGATCAACCTAATGCCCATCAATGACAGATTGGATAAAGAAAATGTGGTACATATACACCATGGAATACTATGCCGCCATTAAAAATGATATCATGTCTTTTGCTGGAATATGGATGGACCTTCTATTATCCTTAGCAAACTAATGCAGGAACAGAAAACCAAATATAGCATACTCTCAGTTATAAGTGGGAGCTAAA

translation

DNA

mRNA

Protein

Regulatory network

Organism

Transcription and translationFunctional Genomics

Goals:

o experimentally identify all transcripts

o characterize their function

o characterize their interactions (a.k.a. ‘systems biology’)

many levels of detail

contingent on what is experimentallly accessible

separate noise, technical artifacts from biologically relevant observations

Functional genomics and cancer

Cancer: somatic cells acquire mutations to become anti-social, proliferate excessively

Current cancer classifications:

o affected organ

o cell type of origin

o apparent grade of de-differentiation

Goals:

o molecular taxonomy: more precise, causal

o molecular diagnosis: better estimation of risk and thus treatment strategy

o molecular therapy (new drugs: "silver bullets")

mRNA from

tissue biopsies,

cell lines

fluorescent detection of the amount of sample-probe binding

probes:

gene-specific DNA strands

tissue A

tissue B

tissue C

ErbB2

0.02

1.12

2.12

VIM

1.1

5.8

1.8

ALDH4

2.2

0.6

1.0

CASP4

0.01

0.72

0.12

LAMA4

1.32

1.67

0.67

MCAM

4.2

2.93

3.31

microarraysStochastic

o similar effect on many

measurements

o corrections can be estimated from data

o too random to be ex-plicitely accounted for

o remain as “noise”

Calibration

Error model

Sources of variationamount of RNA in the biopsy

efficiencies of

-RNA extraction

-reverse transcription

-labeling

-fluorescent detection

probe purity and length distribution

spotting efficiency, spot size

cross-/unspecific hybridization

stray signal

Fundamental challenges

Need to estimate from the data:

calibration

error bars

To consider

no. probes is large, no. arrays small

large dynamic range

parametric vs. non-parametric

power

outliers and heavy tails

variance-bias trade-off

bi per-sample

normalization factor

bk sequence-wise

probe efficiency

hik ~ N(0,s22)

“multiplicative noise”

ai per-sample offset

Lik local background provided by image analysis

eik ~ N(0, bi2s12)

“additive noise”

measured intensity = offset + gain true abundance

Parameters

data: yki e.g. (n=20000) (d=100)

- sample normalization and offset (ai, bi): 2 d

- scale of additive noise (s12): 1

- scale of multiplicative noise (s22): 1 (=asymptotic CV)

- probe efficiency (bk): n

variance stabilization

Xu a family of random variables with EXu=u, VarXu=v(u). Define

var f(Xu ) independent of u

derivation: linear approximation

1.) constant variance (‘additive’)

2.) constant CV (‘multiplicative’)

3.) offset

4.) additive and multiplicative

variance stabilizing transformationstransformed scale: model simplified

data: yki e.g. (n=20000) (d=100)

sample normalization and offset (ai, bi): 2 d

noise (c2): 1

true abundance of gene k (mk): n

profile log-likelihood

resistant regression

o profile maximum likelihood estimator: sensitive to deviations from normality

o model assumes mk = mkii - differentially transcribed genes act as outliers.

o robust variant of PML estimator, à la Least Trimmed Sum of Squares regression.

o works as long as <50% of genes are differentially transcribed

Least trimmed sum of squares regression

- least sum of squares

- least trimmed sum of squares

R

S

S

R

Resultso verification of the approximation

o sample size dependence

o outliers, heavy tails

o sensitivity and selectivity

Verification of the approximation

Ym = meh+nh ~ N(0, sh2)n ~ N(0,1)

h(y) =

arsinh(cy)

with

c2=exp(sh2)-1

evaluation: sensitivity / specificity in detecting differential abundance

o Data: paired tumor/normal tissue from 19 kidney cancers, in color flip duplicates on 38 cDNA slides à 4000 genes.

o 6 different strategies for normalization and quantification of differential abundance

o Calculate for each gene & each method: t-statistics, permutation-p

oAllowing the same number of false positives for each method, compare the number of genes they find.

evaluation: comparison of methods

one-sided test for „up“ one-sided test for „down“

more accurate quantification of differential expression higher sensitivity / specificity

application

Identification of differentially expressed genes k by F-(type)- statistic

o within one row: remove intensity dependence of the variance

o across rows: often d<10

- pool variance estimation („fold change criterion“)

- use regularized variances

i: samples

nd

n»d

k: genes

Summary

o estimate calibration and variance parametersfrom the data

o applicable to cDNA chips, filters, and oligo chips (e.g.Affy)

oavoid the problems of (log-)ratios at low intensities, in particular the sensitive dependence on small fluctuations

osoftware available: www.bioconductor.org

o related work: B. Durbin, D. Rocke (UC Davis)

Acknowledgements

Uni Heidelberg

Günther Sawitzki

MPI Molekulare Genetik

Tim Beißbarth

DFCI Harvard

Robert Gentleman

UMC Leiden

Judith Boer

RZPD

Anke Schroth

Bernd Korn

… and

many more!

DKFZ Heidelberg

Molecular Genome Analysis

Frank Bergmann

Andreas Buneß

Katharina Finis

Florian Haller

Yvonne Keßler

Jörg Schneider

Klaus Steiner

Stephanie Süß

Markus Vogt

Friederike Wilmer

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