Initiation of DNA replication at the human beta-globin 3' enhancer.

The origin of DNA replication in the human beta-globin gene contains an initiation region (IR) and two flanking auxiliary elements. Two replicator modules are located within the upstream auxiliary sequence and the IR core, but the functional sequences in the downstream auxiliary element are unknown. Here, we use a combination of benzoylated-naphthoylated DEAE (BND) cellulose purification and nascent strand abundance assays to show that replication initiation occurs at the beta-globin 3' enhancer on human chromosome 11 in the Hu11 hybrid murine erythroleukemia (MEL) cell line. To examine replicator function, 3' enhancer fragments were inserted into an ectopic site in MEL cells via an optimized FRT/EGFP-FLP integration system. These experiments demonstrate that the 1.6 kb downstream auxiliary element is a third replicator module called bGRep-E in erythroid cells. The minimal 260 bp 3' enhancer is required but not sufficient to initiate efficient replication, suggesting cooperation with adjacent sequences. The minimal 3' enhancer also cooperates with elements in an expressing HS3beta/gamma-globin construct to initiate replication. These data indicate that the beta-globin replicator has multiple initiation sites in three closely spaced replicator modules. We conclude that a mammalian enhancer can cooperate with adjacent sequences to create an efficient replicator module.

Reproducible doxycycline-inducible transgene expression at specific loci generated by Cre-recombinase mediated cassette exchange.

Comparative analysis of mutants using transfection is complicated by clones exhibiting variable levels of gene expression due to copy number differences and genomic position effects. Recombinase-mediated cassette exchange (RMCE) can overcome these problems by introducing the target gene into pre-determined chromosomal loci, but recombination between the available recombinase targeting sites can reduce the efficiency of targeted integration. We developed a new LoxP site (designated L3), which when used with the original LoxP site (designated L2), allows highly efficient and directional replacement of chromosomal DNA with incoming DNA. A total of six independent LoxP integration sites introduced either by homologous recombination or retroviral delivery were analyzed; 70-80% of the clones analyzed in hamster and human cells were correct recombinants. We combined the RMCE strategy with a new, tightly regulated tetracycline induction system to produce a robust, highly reliable system for inducible transgene expression. We observed stable inducible expression for over 1 month, with uniform expression in the cell population and between clones derived from the same integration site. This system described should find significant applications for studies requiring high level and regulated transgene expression and for determining the effects of various stresses or oncogenic conditions in vivo and in vitro.

Evaluation of a549 as a new vaccine cell substrate: digging deeper with massively parallel sequencing.

In the past three decades, the use of tumorigenic cell substrates has been the topic of five Vaccine and Related Biological Products Advisory Committee (VRBPAC) meetings, including a review of the A549 cell line in September 2012. Over that period of time, major technological advances in biotechnology have improved our ability to assess the risk associated with using a tumorigenic cell line. As part of the September 2012 review, we assessed the history of A549 cells and evaluated the probable transforming event based on patterns of mutations to cancer genes. In addition, massively parallel sequencing was used to first screen then augment the characterization of A549 cells by searching for the presence of hidden viral threats using sequencing of the entire cellular transcriptome and comparing sequences to a curated viral sequence database. Based upon the combined results of next-generation sequencing technology along with standard cell characterization as outlined in published regulatory guidances, we believe that A549 cells pose no more risk than any other cell substrate for the manufacture of vaccines.