(14c) Chromatin Association of Transgenes Regulates the Transcription of Monoclonal Antibodies in Dhfr-Amplified Cho Cells

Chinese hamster ovary (CHO) cells are the dominant expression system for therapeutic monoclonal antibodies (mAb). Dihydrofolate reductase (DHFR) ?mediated gene amplification is a popular method for development of cell lines with high mAb productivities. Previously, we performed a comparative study on the gene copy number, mRNA level, and specific productivity of CHO cells with differential productivities and determined that enhanced productivity in amplified cell lines is a result of both gene amplification and increased transcription per gene copy. To understand the epigenetic causes for differential transcriptional rates among cell lines, we compared recombinant gene organizational patterns, chromosomal localization, and chromatin association in the previously studied CHO cells. By fluorescence in situ hybridization experiments, we determined that the immunoglobulin HC and LC genes are incorporated into a single site in the chromosome in each cell line. Our DNAseI footprinting analysis showed that a larger portion of HC and LC gene copies are located in the euchromatic region in amplified cells than in the unamplified parental cell lines. Our results suggest that chromatin association of transgenes may regulate transcription of transgenes by altering the gene copy ratio between heterochromatin and euchromatin during DHFR-mediated gene amplification.