Methods for MicroArray Analysis Data Mining

1
Methods for Micro-Array Analysis Data Mining
Machine Learning Approaches
2
What is Micro-Array Analysis?
3
Analysis of Micro-Array Data

• Challenges posed
• Typical characteristic of micro array data is the
  large number of variables relative to the number
  of observations.
• Hidden knowledge in these data has to be
  discovered
• Eg.Gene expression data from 72 leukemia patients
  (samples) with 7,070 genes (variables)
• The study of the variability of gene expression
  patterns
• Problems
• How to analyze micro-array data with the
  following requirements met simultaneously ?
• Efficiency
• Accuracy
• Automation

4
Typical Micro-Array data set

• Suppose that the identical micro-array experiment
  is repeted p times (e.g. colon cancer cells from
  p patients compired with p wild tipes). Then we
  obtain a data set (mij i1,,G, j1,,p), in
  which mij is the expression ratio in gene I in
  jth experiment.
• Usually generate large data sets with expression
  values for thousands of genes (200020000) but
  not more than a few dozens of samples
• For example

5
Main objectives in micro-array data analyses

• 1. To find the genes that are differently
  expressed (DF) in the two samples (e.g. the given
  colon cancer sample / the wild type cells that
  submited a given treatment / no treatment ).
  Although biologists can discover DF genes even
  with p1, it has been realized lately that making
  independent replications is a good practise.
• Questions that could be asked
• - Which genes expression is modified by the
  condition? (it has been reported that many
  diseases, especially tumors, have never been
  caused by a single gene mutation but are the
  result of a series of gene changes)
• - Has the treatment changed the expression
  level of specific (target) genes / gene sequences
  to noticeably different levels? If so is it
  important (i.e. is the patients condition
  improved due to this change in expression levels)
  
• 2. To find genes that behave similarly in
  different conditions (i.e. clustering the row
  vectors) and to find subgroups of samples (or
  patients tissues), that are similar to each
  other (i.e. clustering the column vectors).
• - novel discovery of genes in related
  biological pathway or having related functions
• - clinically important subgroups of
  patients
• 3. Classification
• - For example Golub et al. (1999) 2
  types of leukemia / based on gene expression
  profile of each sample
• 4. Validation of the models, assessment of
  robustness/ predicting power of the classifiers
  (models)

6
Main objectives in micro-array data analyses
7
1.Finding differently expressed
genes.Parametrical methods t-test

• Standard t test
• H0 - no difference between the treatment
  and the controlled samples
• H1 - treatment has an influence.
• Knowing the probability distribution of the T
  variable under H0 (Student law of p-1 ddl), the
  actual T is computed and compared to this
  distribution.
• At a smaller p-value it is less likely to see
  extreme differences by chance.

8
1.Finding differently expressed genes. t-test

• Advantages simple and implemented in all
  comercial microarray analysis packages
• Disadvantages distributional assumptions and
  the problem of multiple testing (due to the small
  number of samples, we can not assume normality of
  the mean of the samples) . -gt what is the false
  descovery rate ?
• Alternatives empirical Bayes and parametric
  Bayes

9
1.Finding differently expressed genes.Fold
approach

• If the average expression level of the genes is
  examined
• If it changed by a certain number of folds, the
  gene is declared changed (on or off)
• Disadvantage does not reveal the desired
  correlation between the gene and its function.
  Does not find related genes.

10
Data Mining

11
Gene Expression grouping and classification.
Overview of existing approaches

• Micro-Array analysis employs machine learning
  algorithms and techniques to mine useful data.
• Unsupervised data analysis
• Principal Component Analysis (PCA)
• Hierarchical Clustering
• Non-Hierarchical Clustering
• K means
• Self organizing maps (a type of neural networks)
  
• Supervised data analysis
• Decision Trees - C5.0 implementation
• Artificial Neural Networks Back-propagation
  algorithm
• Two complementary techniques
• Cross-validation
• Multi-model approaches (boosting, bagging,
  stacking)

12
Principal Component Analysis (PCA)

• This is a technique for finding major
  combinations of data (I.e. genes that are
  regularly up- and down- regulated together)
• Objectives
• Graphically resume a large rectangular table of
  numbers, R, simplify its comprehension, find
  pertinent features.
• Reduce the dimensionality of the data set, (e.g.
  co-regulated genes)
• Graphically resume
• - The correlations between the
  variables.
• - Find new meaningful underlying
  variables (dimensions), resuming the initial
  variables in this way.
• MAXIMIZE THE INTRA-CLASS VARIANCE
• MINIMIZE THE INTER-CLASS VARI12ANCE
• - The proximities and the
  principal oppositions between the individuals
  
• Simple example
• Imagine a micro-array data set consisting of
  only 2 experiments (2 samples)
• Graphically represent the data.

13
Principal Component Analysis(PCA)

• Principal component analysis of a
  two-dimensional data cloud. The line shown is the
  direction of the first principal component, which
  gives an optimal (in the mean-square sense)
  linear reduction of dimension from 2 to 1
  dimensions.

14
Principal Component Analysis(PCA)

• Principal component analysis (PCA) involves a
  mathematical procedure that transforms a number
  of (possibly) correlated variables into a
  (smaller) number of uncorrelated variables called
  principal components.
• Illustration for the case of 2 samples
• The variance of the sample x is given by
• The variance of the sample y is given by
• The covariance of between x and y
• Then we can write
• This matrix is square and symmetric, admits a
  characteristic polynomial and is diagonalizable.
  Also admits a basis of orthogonal eigen vectors

15
Principal Component Analysis(PCA)

• Then, it exists a matrix U so that
• The 2 eigen vectors orthogonal. Represent a
  new system of INDEPENDENT coordinates. The
  quantities u11 and u12 are actually the
  coordinates of the new axis expressed in a
  vectorial format. Same for u21 and u22 .
• Each coefficient indicates the weight of a
  particular experiment within this component !
  (how much participates this experiment at the
  generation of this pattern)
• A translation and a rotation of the coordinate
  system.

16
Principal Component Analysis(PCA)

• The first principal component - as much of the
  variability in the data as possible,
• Each succeeding component - as much of the
  remaining variability as possible.
• Imagine cloud of data in many dimentions ?
  benefits !
• The projection of a point A (x, y) on a axis u
  (u1, u2) is obtained by performing the scalar
  product of the coordinates of this point and the
  vectorial coordinates of the axis projection
  xu1yu2.
• Now, our the points are the genes.
• It is intersting to plot the eigen values, which
  expresses the way that the variability of data is
  repartised in the new coordinate system. The
  relative sizes of the major and minor axes in the
  ellipse.

17
Principal Component Analysis(PCA)

• Application to sporulation time-series
  observations of differential expression for
  thousands of genes across multiple conditions
• Usually, the first component has all positive
  coefficients, indicating a weighted average of
  all experiments
• The second principal coefficient has negative
  values at early time points and positive values
  for the latter time points, indicating a measure
  of change in expression

18
Machine Learning for Micro-Array Analysis
clustering

• Cluster analysis
• Identification of new subgroups or classes of
  some biological entity (e.g. ,tumors)

19
Hierarchical Clustering

• Hierarchical cluster methos differ in
• the distance measure selected
• the manner in which the distances are computed
  between the growing clusters and the remaining
  members of the data set
• Single Linkage. Disadvantage - loose clusters
• Complete Linkage. Disadvantage to compact
  clusters of very similar size.
• Average Linkage
• Unweighted pair-group method average (UPGMA) To
  groups of the lowest average distance are joined
  to form a new cluster.

20
Hierarchical Clustering

• Euclidian and Manhattan sensitive to absolute
  expression levels. Reveal genes that have similar
  expression levels.
• A and B have aproximately the
  same expression levels
• Correlation coefficient with centering sensitive
  to expression profiles. Reveal genes that have
  similar expression profiles.
• D and E enhanced
• A and C repressed
• Absolute correlation coefficient
• A, C, D, E may be involved in
  the same biological pathway

21
K-means Clustering

• 1. Randomly assign data to the clusters.
• Suppose there are m genes per cluster.
• 2. Calculate an average expression vector for
  each cluster i.
• This Corresponds to the centroid of the
  cluster.
• 3. Calculate a mean interclass distance
  between each point and the centroid, for each
  cluster.
• 4. Move the data from one class to another.
• Aim of minimizing the averall interclass
  distance measure.
• ADVANTAGES easy to implement.
• DISADVANTAGES computationally intensive.
• outcome
  determined by such factors as distance metrics
  chose.

22
Non-parametric models

• Models that rely heavily on the empirical
  analysis of large data sets rather than on prior
  domain knowledge
• Non-parametric Approaches
• Decision trees, Neural networks, Genetic
  algorithms, and Nearest neighbor methods.
• Fundamental assumption
• Consistently observed relationships or
  patterns in large data sets will recur in future
  observations.
• Advantages
• Does not require a thorough understanding of the
  underlying system or problem
• Can be used to build arbitrarily complex models,
  that are highly non-linear and not restricted by
  human comprehension.

23
Decision Tree

• Strengths
• Clearly indicates which attributes are most
  important for prediction or classification.
• Weaknesses
• Limited ability to handle estimation or
  regression tasks where the goal is to predict the
  value of a continuous variable
• Error-prone when the number of training examples
  per class is small

24
Neural Networks

• Strengths
• Ability to handle a range of problem tasks
  including classification (discrete outputs) and
  estimation or regression tasks (continuous
  outputs)
• Provision of an indication (through sensitivity
  analysis) of which attributes are most important
  for prediction or classification

• Weaknesses
• The risk of premature convergence to an inferior
  solution (this is normally addressed by
  performing a sensible cross-validation procedure)

25
Multi-Model Approaches

• Problem with the regular models
• Instability of Prediction Method
• Sensitivity of the final model to small changes
  in the training set.
• Unstable machine learning methods
• Decision trees
• Stable methods
• k-nearest neighbor
• Neural models

Now, let us see an approach to address the
instability problem.
26
Machine Learning for Micro-Array Analysis

• Cross validation
• To test the robustness of the classifier
• Algorithm choice depends on
• Attributes
• Ratio of the training data
• TP,TNif TP is small- over-fitting occurs
• Combined approaches
• Limited amount of training data, the individual
  classifier may not represent the true hypotheses.
• Combined classifier may produce a good
  approximation to the true hypotheses.

27
Multi-Model Approaches

• Common methods for constructing multi-model
  systems
• Boosting, Bagging, and Stacking

• What they do?
• Creates and Combines multiple classifiers
• How are they different from each other?
• Differ in how the classifiers are trained and in
  how their outputs are combined.
• How they improve accuracy?
• They improve accuracy by focusing the learning
  process on examples in the data that are harder
  to model than others.

28
Boosting
29
Boosting Algorithm

• Step 1 Form the Learning set and validation set
  (with uniform and without replacement sampling).
• Step 2 N different training set replicas are
  sampled adaptively (with non-uniform sampling
  probabilities and with replacement)
• Step 3 Build each classifier, f'i (x), based on
  the training set.
• Step 4 Establish each classifiers performance
  by testing it against the learning set.
• Step 5 Calculate a weight for each classifier
  based on its performance
• Step 6 Combine model by means of a weighted
  voting scheme, where each individual prediction
  model carries a different weight.