Relative expression analysis for identifying perturbed pathways.

The computational identification from global data sets of stable and predictive patterns of gene and protein relative expression reversals offers a simple, yet powerful approach to target therapies for personalized medicine and to identify pathways that are disease-perturbed. We previously utilized this approach to identify a molecular classifier with near 100% accuracy for differentiating gastrointestinal stromal tumor (GIST) and leiomyosarcoma (LMS), two cancers that have very similar histopathology, but require very different treatments. Differential Rank Conservation (DIRAC) is a novel approach for studying gene ordering within pathways and is based on the relative expression ranks of participating genes. DIRAC provides quantitative measures of how pathway rankings differ both within and between phenotypes. DIRAC between pathways in a selected phenotype contrasts the scenarios where either (i) pathways are ranked similarly in all samples; or (ii) the ordering of pathway genes is highly varied. We examined gene expression in GIST and LMS tumor profiles and identified pathways that appear to be tightly regulated based on high conservation of gene ordering. The second form of DIRAC manifests as a change in ranking (i.e., shuffling) between phenotypes for a selected pathway. These variably expressed pathways serve as signatures for molecular classification, and the ability to accurately classify microarray samples provided strong validation for the pathway-level expression differences identified by DIRAC.