title: Tiling array data analysis: a multiscale approach using wavelets.
abstract: BACKGROUND
  Tiling array data is hard to interpret due to noise. The wavelet transformation is a widely used technique in signal processing for elucidating the true signal from noisy data. Consequently, we attempted to denoise representative tiling array datasets for ChIP-chip experiments using wavelets. In doing this, we used specific wavelet basis functions, Coiflets, since their triangular shape closely resembles the expected profiles of true ChIP-chip peaks.
  
  
  RESULTS
  In our wavelet-transformed data, we observed that noise tends to be confined to small scales while the useful signal-of-interest spans multiple large scales. We were also able to show that wavelet coefficients due to non-specific cross-hybridization follow a log-normal distribution, and we used this fact in developing a thresholding procedure. In particular, wavelets allow one to set an unambiguous, absolute threshold, which has been hard to define in ChIP-chip experiments. One can set this threshold by requiring a similar confidence level at different length-scales of the transformed signal. We applied our algorithm to a number of representative ChIP-chip data sets, including those of Pol II and histone modifications, which have a diverse distribution of length-scales of biochemical activity, including some broad peaks.
  
  
  CONCLUSIONS
  Finally, we benchmarked our method in comparison to other approaches for scoring ChIP-chip data using spike-ins on the ENCODE Nimblegen tiling array. This comparison demonstrated excellent performance, with wavelets getting the best overall score.
authors.affiliation: Diagnostic Radiology, Yale University, New Haven, CT, USA. alexander.karpikov@yale.edu
authors.forename: Alexander
authors.initials: A
authors.lastname: Karpikov
authors.forename: Joel
authors.initials: J
authors.lastname: Rozowsky
authors.forename: Mark
authors.initials: M
authors.lastname: Gerstein
mesh_terms: D000465:Algorithms; D047369:Chromatin Immunoprecipitation; D020411:Oligonucleotide Array Sequence Analysis / Q000379:methods*; D058067:Wavelet Analysis