Illumina whole-genome complementary DNA–mediated annealing, selection, extension and ligation platform: assessing its performance in formalin-fixed, paraffin-embedded samples and identifying invasion pattern–related genes in oral squamous cell carcinoma

SummaryHigh-throughput gene expression profiling from formalin-fixed, paraffin-embedded tissues has become a reality, and several methods are now commercially available. The Illumina whole-genome complementary DNA–mediated annealing, selection, extension and ligation assay (Illumina, Inc) is a full-transcriptome version of the original 512-gene complementary DNA–mediated annealing, selection, extension and ligation assay, allowing high-throughput profiling of 24 526 annotated genes from degraded and formalin-fixed, paraffin-embedded RNA. This assay has the potential to allow identification of novel gene signatures associated with clinical outcome using banked archival pathology specimen resources. We tested the reproducibility of the whole-genome complementary DNA–mediated annealing, selection, extension and ligation assay and its sensitivity for detecting differentially expressed genes in RNA extracted from matched fresh and formalin-fixed, paraffin-embedded cells, after 1 and 13 months of storage, using the human breast cell lines MCF7 and MCF10A. Then, using tumor worst pattern of invasion as a classifier, 1 component of the “risk model,” we selected 12 formalin-fixed, paraffin-embedded oral squamous cell carcinomas for whole-genome complementary DNA–mediated annealing, selection, extension and ligation assay analysis. We profiled 5 tumors with nonaggressive, nondispersed pattern of invasion, and 7 tumors with aggressive dispersed pattern of invasion and satellites scattered at least 1 mm apart. To minimize variability, the formalin-fixed, paraffin-embedded specimens were prepared from snap-frozen tissues, and RNA was obtained within 24 hours of fixation. One hundred four down-regulated genes and 72 up-regulated genes in tumors with aggressive dispersed pattern of invasion were identified. We performed quantitative reverse transcriptase polymerase chain reaction validation of 4 genes using Taqman assays and in situ protein detection of 1 gene by immunohistochemistry. Functional cluster analysis of genes up-regulated in tumors with aggressive pattern of invasion suggests presence of genes involved in cellular cytoarchitecture, some of which already associated with tumor invasion. Identification of these genes provides biologic rationale for our histologic classification, with regard to tumor invasion, and demonstrates that the whole-genome complementary DNA–mediated annealing, selection, extension and ligation assay is a powerful assay for profiling degraded RNA from archived specimens when combined with quantitative reverse transcriptase polymerase chain reaction validation.