Restoring mismatch repair does not stop the formation of reciprocal translocations in the colon cancer cell line HCA7 but further destabilizes chromosome number

An important anticarcinogenic function of the mismatch repair (MMR) system is its role in preventing recombination between similar, but nonidentical (homeologous) sequences, thus preventing

chromosomal rearrangements. We recently identified a novel chromosomal instability (CIN) phenotype in an MMR defective colon cancer cell line (HCA7) characterized by an ongoing tendency to

multiple reciprocal chromosomal translocations. To analyse the relation between MMR and chromosomal changes more closely, the HCA7 stem clone was divided into three stocks. The first was

stably transfected with MLH1 expression plasmid, the second was regularly exposed to the demethylating agent 5-azacytidin to re-express the hypermethylated MLH1 gene, and the third was an

unmanipulated control stock. All stocks were propagated in vitro for 55–80 passages and, furthermore, some of the early passages were irradiated to induce DNA double-strand breaks.

Multiplex-fluorescent in situ hybridization (M-FISH) analysis showed that all three stocks acquired varying numbers of reciprocal translocations and other structural changes at some point.

Interestingly, the control stock, which is MMR defective, maintained its numerical chromosomal stability, while some of the MMR-proficient clones showed additional numerical instability.

Although the control stock was less sensitive to irradiation, its surviving clones showed marked stability of chromosome structure and number compared to the MMR-competent stocks. These

results show that restoring MMR does not prevent the development of reciprocal translocations but rather predisposes cells to numerical CIN after irradiation. Thus, the accumulating data

suggest that MMR defect may not be necessary for the development of reciprocal chromosomal translocations but might be permissive.

We thank Paul Edwards for valuable advice, Lauri Aaltonen for access to mycoplasma-free tissue culture facility, Pekka Ellonen for computer support, and Tuija Lundan, Amber Alsop, Reetta

Kariola and Saila Saarinen for expert technical advice/assistance. This work is funded by the Academy of Finland, the Sigrid Juselius Foundation, the Finnish Cancer Foundation, the Helsinki

University Central Hospital and the NIH Grant CA82282.

Department of Medical Genetics, Helsinki University, Helsinki, Finland

Department of Pathology, Faculty of Medicine, Zagazig University, Egypt

Departments of Pathology and Medical Genetics, Laboratory of Cytomolecular Genetics, Haartman Institute and HUSLAB, University of Helsinki and Helsinki University Central Hospital, Helsinki,

Finland

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