Practical applications of hmms chromhmm
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Practical applications of HMMs : ChromHMM. Sushmita Roy Nov 5th. Chromatin organization and gene expression. http:// =eYrQ0EhVCYA. ChIP-seq to measure histone data. Adapted from Dewey lecture and Peter Park Nature Genetics Review.

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Practical applications of HMMs : ChromHMM

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Practical applications of HMMs: ChromHMM

Sushmita Roy

Nov 5th

Chromatin organization and gene expression

ChIP-seq to measure histone data

Adapted from Dewey lecture and Peter Park Nature Genetics Review

ChIP-seq data for multiple marks

Chromatin state: A specific combinations of mark values.

Important because it can be used to segment the genome into biologically

meaningful units.

Problem definition

  • Given

    • A collection of genome-wide measurements of chromatin marks

  • Do

    • Segment the genome into N chromatin states

An HMM for segmenting genomes using chromatin marks

  • HMM

    • State: chromatin state

    • Emission->multiple chromatin marks

    • Need a multi-variate HMM

Binarizing the chromatin data

  • Each mark is represented by a binary variable vt,m:

    • 1: mark is present

    • 0: mark is absent




Genomic sequence








ChromHMM with 3 states





ChromHMM notation

  • pk,mdenotes the probability of mark mbeing ON in state k

  • Emission probability of M marks per state is a product of M bernoulli random variables.

  • bk,l denotes the probability of transitioning from state i to state j

  • ak: initial probability of state k

Learning the ChromHMM

  • Need to figure out the number of states

  • Learn HMMs for K=2 to 80 states with a penalty factor to penalize the number of parameters

  • State transitions: start with the fully connected HMM, and if set parameters to zero if <10-10

  • Final model had 51 states

Learned Emission parameters

Emission parameters for state 5


Example output around CAPZA2 gene from ChromHMM

Input chromatin marks

Inferred state sequences

Posterior probability distributions of all 51 states around CAPZA gene

Max posterior state

Posterior probability values of each state


  • HMMs are powerful models to capture sequential data

  • Very popular in computational biology

    • Gene annotation

    • Representation of a profile: protein domain finding

    • Genome segmentation

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