Loss of viral genomes from hamster tumor cells and nonrandom alterations in patterns of methylation of integrated adenovirus type 12 DNA

Abstract

The insertion stability and DNA methylation patterns of integrated adenovirus type 12 (Ad12) genomes were investigated in Ad12-induced tumors and in tumor cell lines established from them as a function of time of passage under culture conditions. Upon subcultivation of cells from some of the tumors, the viral genomes were eliminated, apparently in a stepwise process with segments of the left termini of Ad12 DNAs persisting the longest. Morphological variants of these tumor cells lost all viral DNA and yet retained the oncogenic phenotype. All 13 independently isolated clones from one revertant line were devoid of Ad12 DNA. It could not be ruled out that very short sequence elements of viral DNA, such as promoters or enhancing sequences, could have persisted in these variants. The extent of viral DNA methylation was minimal in Ad12-induced tumors, although the viral genome was not extensively expressed, if at all. Upon passage in culture, the levels of viral DNA methylation increased. It was interesting that establishment of the final methylation pattern of integrated Ad12 DNAs required many cell generations after the fixation of foreign DNA in the host genome. The shift in methylation was nonrandom. The late parts of the inserted viral genomes became methylated more extensively than did the early gene segments.

Insertion of adenovirus type 12 DNA in the vicinity of an intracisternal A particle genome in Syrian hamster tumor cells

“…Origin of the T1111(2) tumor and of cell line H1111(2). The origin of the Ad12-induced hamster tumor T1111(2) and some of its characteristics were described elsewhere (27,29). The cell line established from this tumor, H1111(2) (27), and BHK21(B3) hamster cells (51) were propagated in Dulbecco modified Eagle medium (3) supplemented with 10% fetal calf serum.…”

“…Methylation of 5'-CCGG-3' and 5'-GCGC-3' sequences close to the site of Adl2 DNA integration. In previous work, the patterns of methylation of integrated Adl2 and Ad2 DNA sequences were investigated extensively (10,24,28,29,53,57). The degree of methylation of integrated Adl2 DNA sequences might be influenced by methylation patterns preexisting at the sites of integrative recombination.…”

Mutagenesis induced by integration processes and evolution of nuclear genome

“…Operons E1A and E1B, com pris ing the trans for ma tion re gions, re spon si ble for ma lig nant trans for ma tion of cells, are local ised on the left end of ge nome, the right end lo cates the early E4 re gion, which con trols the pro cess of trans for ma tion proper. The anal y sis of the in te gra tion of this one and some other DNA-con tain ing vi ruses into the chro mo somal DNA re vealed sev eral stages of es tab lish ing of the in tegrated state and ma lig nant trans for ma tion of cells [20][21][22][23][24]. The main stages can be pre sented as fol lows:…”

“…3). It is com monly known that cells elim i nate vi ral nu cle otide se quences with the pe riod of time -it starts with the elim i na tion of the frag ment of the older se quence and then spreads to the early re gions of oncoviral ge nome [23]. As a re sult of some ma lig nantly trans formed cells there are al most no vi ral nu cle o tide se quences at all.…”

“…This process may ac quire the pat tern of ho mol o gous re com bina tion with sub se quent in ser tion of the exo gen ic nu cleo tide se quence. In ad eno vi rus es, the in te gra tion may be per formed us ing short ho mol o gous re gions be tween viral and cell se quences sim i larly to the site-spe cific recom bi na tion [21][22][23][24], how ever, the de tails of this process re main un veiled. The in cor po ra tion may take place us ing the non-ho mol o gous re com bi na tion, provid ing the in te gra tion of MGE (this is the ba sic mech anism for retro virus es) [8].…”