Identification of Differentially Expressed and Spliced Genes with RNA Sequencing Analysis of CLL Specimens

Abstract 4582

High throughput sequencing of cellular mRNA provides a comprehensive analysis of the transcriptome. Besides identifying differentially expressed genes in different cell types, it also provides information of mRNA isoforms and splicing alterations. We have analyzed two CLL specimens and a normal peripheral blood B cells mRNA by this approach and performed data analysis to identify differentially expressed and spliced genes. The result showed CLLs specimens express approximately 40% more transcripts compared to normal B cells. The FPKM data (fragment per kilobase of exon per million) revealed a higher transcript expression on chromosome 12 in CLL#1 indicating the presence of trisomy 12, which was confirmed by fluorescent in-situ hybridization assay. With a two-fold change in FPKM as a cutoff and a p value cutoff of 0.05 as compared to the normal B cell control, 415 genes and 174 genes in CLL#1 and 676 and 235 genes in CLL#2 were up and downregulated or differentially expressed. In these two CLL specimens, 45% to 75% of differentially expressed genes are common to both the CLL specimens indicating that genetically disparate CLL specimens have a high percentage of a core set of genes that are potentially important for CLL biology.

Selected differentially expressed genes with increased expression (selectin P ligand, SELPLG, and adhesion molecule interacts with CXADR antigen 1, AMICA) and decreased (Fos, Jun, CD69 and Rhob) expression based on the FPKM from RNA-sequencing data were also analyzed in additional CLL specimens by real time PCR analysis. The expression data from RNA-seq closely matches the fold-change in expression as measured by RT-PCR analysis and confirms the validity of the RNA-seq analysis. Interestingly, Fos was identified as one of the most downregulated gene in CLL. Using the Cufflinks and Cuffdiff software, the splicing patterns of genes in CLL specimens and normal B cells were analyzed. Approximately, 1100 to 1250 genes in the two CLL specimens were significantly differentially spliced as compared to normal B cells. In this analysis as well, there is a core set of 800 common genes which are differentially spliced in the two CLL specimens. The RNA-sequencing analysis accurately identifies differentially expressed novel genes and splicing variations that will help us understand the biology of CLL.