title: Stability of single copy transgene expression in CHOK1 cells is affected by histone modifications but not by DNA methylation.
abstract: Intraclonal heterogeneity of genetically modified mammalian cells has been observed as a phenomenon that has a strong impact on overall transgene expression levels and that limits the predictability of transgene expression in genetically modified cells, thereby hampering single cell based screening approaches. The underlying mechanism(s) leading to this variance are poorly understood. To study the dynamics and mechanisms of heterogeneity of early stage silencing we analyzed the expression in more than 100 independent clones of CHOK1 cells that harbour genetically stable integrates of single copy reporter cassettes driven by EF1α and CMV promoters. Single cell analysis showed intraclonal variability with heterogeneity in expression in genetically uniform populations. DNA methylation is a well known mechanism responsible for silencing of gene expression. Interestingly, loss of expression was not associated with DNA methylation of the CMV promoter. However, in most of the clonal populations expression could be increased by inhibitors of the histone deacetylases (HDACi) suggesting that heterogeneity of transgene expression is crucially governed by histone modifications. Further, to determine if the epigenetic status of transgene expression is governed by the chromosomal integration locus we targeted heterologous expression cassettes into two chromosomal sites using recombinase mediated cassette exchange (RMCE). The expression status of a particular clone was faithfully re-established when the same promoter used. In this way the problem of early stage cell clone instability can be bypassed. However, upon introduction of an unrelated promoter methylation-independent silencing was observed. Together, these results suggest that histone modifications are the relevant mechanisms by which epigenetic modulation of transgene expression cassettes is governed in the early phase of clone generation.
authors.affiliation: Model Systems for Infection and Immunity, Helmholtz Centre for Infection Research, 38124 Braunschweig, Germany.
authors.forename: Shawal
authors.initials: S
authors.lastname: Spencer
authors.affiliation: Model Systems for Infection and Immunity, Helmholtz Centre for Infection Research, 38124 Braunschweig, Germany.
authors.forename: Agustina
authors.initials: A
authors.lastname: Gugliotta
authors.affiliation: Boehringer Ingelheim Pharma GmbH & Co. KG, Dep. Process Science Germany, Birkendorfer Str. 65, 88397 Biberach/Riß, Germany.
authors.forename: Jennifer
authors.initials: J
authors.lastname: Koenitzer
authors.affiliation: Model Systems for Infection and Immunity, Helmholtz Centre for Infection Research, 38124 Braunschweig, Germany.
authors.forename: Hansjörg
authors.initials: H
authors.lastname: Hauser
authors.affiliation: Model Systems for Infection and Immunity, Helmholtz Centre for Infection Research, 38124 Braunschweig, Germany; Department of Experimental Hematology, Hannover Medical School, Carl-Neuberg-Str. 1, D-30625 Hannover, Germany. Electronic address: dagmar.wirth@helmholtz-hzi.de.
authors.forename: Dagmar
authors.initials: D
authors.lastname: Wirth
mesh_terms: D000818:Animals; D016466:CHO Cells; D002843:Chromatin; D006224:Cricetinae; D003412:Cricetulus; D003587:Cytomegalovirus / Q000235:genetics; D019175:DNA Methylation / Q000235:genetics*; D020868:Gene Silencing; D057809:HEK293 Cells; D006657:Histones / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D011401:Promoter Regions, Genetic / Q000235:genetics; D019076:Transgenes / Q000235:genetics*
chemical_list: D002843:Chromatin; D006657:Histones
keywords: DNA methylation; Epigenetic stability; HDACi; Heterogeneity; Silencing of recombinant gene expression