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Journal of Virology, February 1999, p. 1010-1022, Vol. 73, No. 2
Institute of Genetics, University of Cologne,
D-50931 Cologne, Germany
Received 22 December 1997/Accepted 20 October 1998
The insertion of adenovirus type 12 (Ad12) DNA into the hamster
genome and the transformation of these cells by Ad12 can lead to marked
alterations in the levels of DNA methylation in several cellular genes
and DNA segments. Since such alterations in DNA methylation patterns
are likely to affect the transcription patterns of cellular genes, it
is conceivable that these changes have played a role in the generation
or the maintenance of the Ad12-transformed phenotype. We have now
isolated clonal BHK21 hamster cell lines that carry in their genomes
bacteriophage
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Insertion of Foreign DNA into an Established
Mammalian Genome Can Alter the Methylation of Cellular DNA
Sequences

and plasmid pSV2neo DNAs in an integrated state. Most
of these cell lines contain one or multiple copies of integrated
DNA, which often colocalize with the pSV2neo DNA, usually in a single
chromosomal site as determined by the fluorescent in situ hybridization
technique. In different cell lines, the loci of foreign DNA insertion
are different. The inserted bacteriophage
DNA frequently becomes de
novo methylated. In some of the thus-generated hamster cell lines, the
levels of DNA methylation in the retrotransposon genomes of the
endogenous intracisternal A particles (IAP) are increased in comparison
to those in the non-
-DNA-transgenic BHK21 cell lines. These changes
in the methylation patterns of the IAP subclone I (IAPI) segment have
been documented by restriction analyses with methylation-sensitive
restriction endonucleases followed by Southern transfer hybridization
and phosphorimager quantitation. The results of genomic sequencing
experiments using the bisulfite protocol yielded additional evidence
for alterations in the patterns of DNA methylation in selected segments
of the IAPI sequences. In these experiments, the nucleotide sequences
in >330 PCR-generated cloned DNA molecules were determined. Upon
prolonged cultivation of cell lines with altered cellular methylation
patterns, these differences became less apparent, perhaps due to
counterselection of the transgenic cells. The possibility existed that
the hamster BHK21 cell genomes represent mosaics with respect to DNA
methylation in the IAPI segment. Hence, some of the cells with the
patterns observed after
DNA integration might have existed prior to
DNA integration and been selected by chance. A total of 66 individual BHK21 cell clones from the BHK21 cell stock have been
recloned up to three times, and the DNAs of these cell populations have been analyzed for differences in IAPI methylation patterns. None have
been found. These patterns are identical among the individual BHK21
cell clones and identical to the patterns of the originally used BHK21
cell line. Similar results have been obtained with nine clones isolated
from BHK21 cells mock transfected by the Ca2+-phosphate
precipitation procedure with DNA omitted from the transfection mixture.
In four clonal sublines of nontransgenic control BHK21 cells, genomic
sequencing of 335 PCR-generated clones by the bisulfite protocol
revealed 5'-CG-3' methylation levels in the IAPI segment that were
comparable to those in the uncloned BHK21 cell line. We conclude that
the observed changes in the DNA methylation patterns in BHK21 cells
with integrated
DNA are unlikely to preexist or to be caused by the
transfection procedure. Our data support the interpretation that the
insertion of foreign DNA into a preexisting mammalian genome can alter
the cellular patterns of DNA methylation, perhaps via changes in
chromatin structure. The cellular sites affected by and the extent of
these changes could depend on the site and size of foreign DNA insertion.
*
Corresponding author. Mailing address: Institute of
Genetics, University of Cologne, Weyertal 121, D-50931 Cologne,
Germany. Phone: 49-221-470-2386. Fax: 49-221-470-5163. E-mail:
doerfler{at}scan.genetik.uni-koeln.de.
This report is dedicated to R. Walter Schlesinger on the occasion
of his 85th birthday.
Present address: Institute of Virology, University of Cologne,
D-50935 Cologne, Germany.
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