title: Statistical model comparison applied to common network motifs.
abstract: BACKGROUND
  Network motifs are small modules that show interesting functional and dynamic properties, and are believed to be the building blocks of complex cellular processes. However, the mechanistic details of such modules are often unknown: there is uncertainty about the motif architecture as well as the functional form and parameter values when converted to ordinary differential equations (ODEs). This translates into a number of candidate models being compatible with the system under study. A variety of statistical methods exist for ranking models including maximum likelihood-based and Bayesian methods. Our objective is to show how such methods can be applied in a typical systems biology setting.
  
  
  RESULTS
  We focus on four commonly occurring network motif structures and show that it is possible to differentiate between them using simulated data and any of the model comparison methods tested. We expand one of the motifs, the feed forward (FF) motif, for several possible parameterizations and apply model selection on simulated data. We then use experimental data on three biosynthetic pathways in Escherichia coli to formally assess how current knowledge matches the time series available. Our analysis confirms two of them as FF motifs. Only an expanded set of FF motif parameterizations using time delays is able to fit the third pathway, indicating that the true mechanism might be more complex in this case.
  
  
  CONCLUSIONS
  Maximum likelihood as well as Bayesian model comparison methods are suitable for selecting a plausible motif model among a set of candidate models. Our work shows that it is practical to apply model comparison to test ideas about underlying mechanisms of biological pathways in a formal and quantitative way.
authors.affiliation: Departament de Física i Enginyeria Nuclear, Universitat Politècnica de Catalunya, Edifici GAIA, Rambla de Sant Nebridi s/n 08222, Terrassa, Barcelona, Spain. nuria.domedel@upc.edu
authors.forename: Núria
authors.initials: N
authors.lastname: Domedel-Puig
authors.forename: Iosifina
authors.initials: I
authors.lastname: Pournara
authors.forename: Lorenz
authors.initials: L
authors.lastname: Wernisch
mesh_terms: D000465:Algorithms; D020816:Amino Acid Motifs; D001089:Arabinose; D001499:Bayes Theorem; D019295:Computational Biology; D003198:Computer Simulation; D004926:Escherichia coli / Q000235:genetics; D005407:Flagella / Q000378:metabolism; D005690:Galactose / Q000378:metabolism; D016013:Likelihood Functions; D008957:Models, Genetic; D015233:Models, Statistical; D011336:Probability; D049490:Systems Biology; D013997:Time Factors
chemical_list: D001089:Arabinose; D005690:Galactose