Introduction of fluorescence-based Real-Time PCR (RT-PCR) is increasingly used to detect multiple pathogens simultaneously and rapidly by gene expression analysis of PCR amplification data. PCR data is analyzed often by setting an arbitrary threshold that intersect the signal curve in its exponential phase if it exists. The point at which the curve crosses the threshold is called Threshold Cycle (CT) for positive samples. On the other, when such cross of threshold does not occur, the sample is identified as negative. This simple and arbitrary however not an elagant definition of CT value sometimes leads to conclusions that are either false positive or negative. Therefore, the purpose of this paper is to present a stable and consistent alternative approach that is based on machine learning for the definition and determination of CT values.

MOTIVATION: Microarray technology enables the study of gene expression in large scale. The application of methods for data analysis then allows for grouping genes that show a similar expression profile and that are thus likely to be co-regulated. A relationship among genes at the biological level often presents itself by locally similar and potentially time-shifted patterns in their expression profiles. RESULTS: Here, we propose a new method (CLARITY; Clustering with Local shApe-based similaRITY) for the analysis of microarray time course experiments that uses a local shape-based similarity measure based on Spearman rank correlation. This measure does not require a normalization of the expression data and is comparably robust towards noise. It is also able to detect similar and even time-shifted sub-profiles. To this end, we implemented an approach motivated by the BLAST algorithm for sequence alignment. We used CLARITY to cluster the times series of gene expression data during the mitotic cell cycle of the yeast Saccharomyces cerevisiae. The obtained clusters were related to the MIPS functional classification to assess their biological significance. We found that several clusters were significantly enriched with genes that share similar or related functions.