title: Pooling breast cancer datasets has a synergetic effect on classification performance and improves signature stability.
abstract: BACKGROUND
  Michiels et al. (Lancet 2005; 365: 488-92) employed a resampling strategy to show that the genes identified as predictors of prognosis from resamplings of a single gene expression dataset are highly variable. The genes most frequently identified in the separate resamplings were put forward as a 'gold standard'. On a higher level, breast cancer datasets collected by different institutions can be considered as resamplings from the underlying breast cancer population. The limited overlap between published prognostic signatures confirms the trend of signature instability identified by the resampling strategy. Six breast cancer datasets, totaling 947 samples, all measured on the Affymetrix platform, are currently available. This provides a unique opportunity to employ a substantial dataset to investigate the effects of pooling datasets on classifier accuracy, signature stability and enrichment of functional categories.
  
  
  RESULTS
  We show that the resampling strategy produces a suboptimal ranking of genes, which can not be considered to be a 'gold standard'. When pooling breast cancer datasets, we observed a synergetic effect on the classification performance in 73% of the cases. We also observe a significant positive correlation between the number of datasets that is pooled, the validation performance, the number of genes selected, and the enrichment of specific functional categories. In addition, we have evaluated the support for five explanations that have been postulated for the limited overlap of signatures.
  
  
  CONCLUSIONS
  The limited overlap of current signature genes can be attributed to small sample size. Pooling datasets results in more accurate classification and a convergence of signature genes. We therefore advocate the analysis of new data within the context of a compendium, rather than analysis in isolation.
authors.affiliation: Information and Communication Theory Group, Faculty of Electrical Engineering, Mathematics and Computer Science, Delft University of Technology, Mekelweg 4, 2628 CD Delft, The Netherlands. m.h.vanvliet@tudelft.nl
authors.forename: Martin H
authors.initials: MH
authors.lastname: van Vliet
authors.forename: Fabien
authors.initials: F
authors.lastname: Reyal
authors.forename: Hugo M
authors.initials: HM
authors.lastname: Horlings
authors.forename: Marc J
authors.initials: MJ
authors.lastname: van de Vijver
authors.forename: Marcel J T
authors.initials: MJ
authors.lastname: Reinders
authors.forename: Lodewyk F A
authors.initials: LF
authors.lastname: Wessels
mesh_terms: D001943:Breast Neoplasms / Q000235:genetics*; D019295:Computational Biology; D030541:Databases, Genetic; D020869:Gene Expression Profiling; D006801:Humans; D015201:Meta-Analysis as Topic; D011897:Random Allocation; D018401:Sample Size; D015983:Selection Bias; D012986:Software Validation