Cluster validation Integration
ICES Bioinformatics
Overview
INTRODUCTION
MICROARRAY ANALYSIS
VALIDATION OF THE RESULTS
• Statistical validation
• Biological validation INTEGRATION
Cluster validation
Preprocessing 1
Clustering Algorithm 1
Preprocessing 2
Clustering Algorithm 2
Clustering Algorithm 3
Parameter
Setting 1 Parameter
Setting 2 Parameter Setting 3
Validation
Why cluster validation?
• Different algorithms, parameters
• Intrinsic properties of the dataset (sensitivity to noise, to outliers)
STATISTICAL VALIDATION
• Sensitivity analysis
– Leaf one out cross validation (FOM) – Sensitivity analysis
• Gaussian noise
• ANOVA
• Cluster coherence testing
– Euclidean distance score – Gap statistics
Statistical validation
Validation
Figure of Merit (sensitivity towards an experiment)
• Tested cluster algorithm is applied to all experimental conditions except the left out condition
• Hypothesis: if the cluster algorithm is robust it can predict the measured values of the left out condition
• To estimate the predictive power of the algorithm FOM is calculated
FOM is the root mean square deviation in the left-out condition e of the individual gene expression levels relative to their
cluster means
This is repeated for all conditions and the average FOM is calculated
Statistical validation
Validation
Yeung et al., 2001
Sensitivity analysis towards the signal to noise ratio
Sensitivity analysis = A way of assigning confidence to the cluster membership
– create new in silico replica's of the dataset of interest by adding a small amount of noise on the original data – treat new datasets as the original one and cluster
– Genes consistently clustered together over all in silico replicas are considered as robust towards adding noise
How to determine the noise?
Statistical validation
Validation
• Gaussian noise with 0 and standard deviation
estimated as the median standard deviation for the log-ratios for all genes across the different experiment Bittner et al. 2000
How to determine the noise? How to generate simulated datasets?
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*• noise based on the appropriate ANOVA model
describes the noise term
•The values are the estimates from the original fit
•The are drawn with replacement from the studentized residuals of the original fit
Clustering is repeated on the simulated datasets
Statistical validation
Validation
Comparing cluster results
• cluster label known:
determine the stability of a gene: the percent of bootstrap cluster experiments in which the gene matches to the same cluster
• cluster label unknown:
• Identify pairs of genes that cluster together in C^ and count the frequency with which such pairs cluster together in the bootstrapped clusters C^*. When each pair of genes clusters together reliably stable clusters will emerge
• RAND INDEX (Yeung et al. 2001)
• Jaccard coefficient (Ben-Hur et al. 2002)
Approximate the confidence in the clustering output of a gene
Statistical validation
Validation
Cluster exp 1 C1
Cluster exp 2 C1
Cluster exp 3 C1
Cluster exp 4 C1
Cluster exp 1 C1
C2 C3
…
Cluster exp 2 C1
C2 C3
…
Cluster exp 3 C1
C2 C3
…
Cluster exp 4 C1
C2 C3
…
RAND index
• statistic designed to assess the degree of agreement between two partitions
• Usually an unknown partition against an external standard Adjusted RAND index
• adjusted so that the expected value of the RAND index between two random partions is zero
Statistical validation
Validation
d c
b a
d RAND a
a: the number of object pairs that are clustered together in data set 1 and in dataset 2 b: the number of object pairs that are clustered together in data set 1 but not in dataset 2 c: the number of object pairs that are clustered together in data set 2 but not in dataset 1 d: the number of object pairs that are put in different clusters in both datasets
a, d: agreement between cluster results b, c: disagreement between cluster results
The rand index is defined as the fraction of agreement that is the number of pairs of objects that are either in same groups in both partitions (a) or in different groups in both partitions (b) divided by the total number of pairs of objects (a + b + c +d).
The rand index lies between 0 and 1.
• Jaccard coefficient
Statistical validation
...
0 0 1
0 0 1
...
0 1 0
Matrix A:
cluster results of dataset 1
# genes
# genes
...
0 0 1
0 0 1
...
1 0 0
Matrix B:
cluster results of dataset 2
# genes
# genes
Cij=1: if xiand xjbelong to the same cluster; 0 otherwise
...
0 0 1
0 0 1
...
0 1 0
Matrix A:
cluster results of dataset 1
# genes
# genes
...
0 0 1
0 0 1
...
1 0 0
Matrix B:
cluster results of dataset 2
# genes
# genes
Cij=1: if xiand xjbelong to the same cluster; 0 otherwise
C1,C2 ijCij1Cij2
1 1 2 2 1 2
2 1 2
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( C C C C C C
C L C
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Jaccard coefficient J
Based on the clusters of one dataset, binary pair vectors are calculated, where each element corresponds to a unique pair of genes and had a value one if both genes were clustered into the same cluster and zero otherwise.
From two such pairvectors, where one was derived from the first dataset and the other from the second dataset, the jaccard coefficient is computed. This coefficient compares the correlation between both obtained binary matrices.
Cluster coherence testing
k points (genes) in cluster p experiments (dimensions)
average profile of cluster
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k V p
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Vw:
• Variance of the genes about the the cluster average averaged over all experiments
•Maximizes coherence of the genes within a cluster
x
Statistical validation
Euclidian distance
Validation
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p experiments Cluster
average profile
x
jVB:
•Describes how the average at each experimental point oscillates around the average of the average cluster profile
•Maximizes variance across experiments
x
average profile of clusterStatistical validation
Gap score
Validation
Score function:
•R2 select clusters containing tightly co-expressed genes (minimal Vw) showing a high variable profile (high VB) across the experiments (ie affected by the
signal studied).
•Score is compared to a similar score calculated based on a randomly generated cluster (bootstrapping)
•The difference between the score of the randomly generated cluster and the cluster of interest is calculated. (gapstatistics)
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w B
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S Gapscore
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Gap statistics
Statistical validation
Validation
Overview
INTRODUCTION
MICROARRAY ANALYSIS
VALIDATION OF THE RESULTS
• Statistical validation
• Biological validation INTEGRATION
Biological validation
dataset
small clusters
• contain genes with highly similar profile (+)
• some information given up in first step (-)
validate “core” clusters
Motif finding DNA level
literature/
knowledge
extend clusters
big clusters
• contain all real positives (+)
• increasing number of false positives (-)
Validation
Microarrays and TextMining
Rationale:
Clustering Accession Nrs
AC0020
D11428 SRS,
Medline, GeneCards,..
Manual Query : huge task
data Literature/
knowledge
Validation
Biological validation
Controlled vocabularies
Cluster number Graphical representatio n of cluster Number of ORFs MIPS functional category (top-level) ORFs within functional category P-value (- log10)
1 426 energy
transport facilitation 47
40 10 5 3 196 cell growth, cell
division and DNA synthesis
48 5
4 149 protein synthesis cellular organisation
71 107
50 19 5 159 cell rescue, defense,
cell death and ageing 20 4 6 171 cell growth, cell
division and DNA synthesis
76 24
9 78 cell growth, cell division and DNA synthesis
23 4
37 11 metabolism 9 6
Cumulative
hypergeometric distribution
Biological validation
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p-value that this degree of enrichment could have occurred by chance (implemented in Ontoexpress)
2 test or Fisher exact test (as implemented in FATIGO software)
Biological validation
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N1: number of genes on the chip
N2: number of differentially expressed genes
Microarrays and Motif Finding
cDNA arrays
Motif finding
Clustering Preprocessing of the data
EMBL BLAST Upstream regions
Gibbs sampling
Validation
Biological validation
Overview
INTRODUCTION
MICROARRAY ANALYSIS
VALIDATION OF THE RESULTS
• Statistical validation
• Biological validation INTEGRATION
• IT level
• Algorithmic level
Integration
Integration
Need for integrated tool Validation
Overview
INTRODUCTION
MICROARRAY ANALYSIS
VALIDATION OF THE RESULTS
• Statistical validation
• Biological validation INTEGRATION
• IT level
• Algorithmic level
Integration
Need for integrated algorithms Validation
• Retain high sensitivity (minimize number of false negatives)
• Reduce level of noise (minimize number of false positives)
• In corporate a priori information
• Combine data from different sources that can mutually confirm each other
• Example: sequence information and expression profiles
• Server rMotif (Lapidot and Pilpel, 2003)
• Selects genes from a microarray if
– Contain a motif
– Have a highly correlated expression profile
Integration
Validation
• Motif diagnosis tool
• measures the extent to which a set of genes that contain a
given motif in their promoter) display expression profiles similar to each other at a given set of conditions (analyzed by
microarrays)
• score (EC expression coherence) of a set of N genes is defined as the number of p pairs of genes in the set for which the Euclidean distance between the mean and variance normalized profiles falls below a threshold D, divided by the total number of pairs in the set
• EC= p/[(0.5(N)(N-1)]
Integration
Validation
Integration
Validation