Carcinogenesis Advance Access originally published online on December 5, 2006
Carcinogenesis 2007 28(6):1341-1346; doi:10.1093/carcin/bgl235
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DHFR and MSH3 co-amplification in childhood acute lymphoblastic leukaemia, in vitro and in vivo
Northern Institute for Cancer Research, Medical School, Newcastle University, Newcastle upon Tyne NE2 4HH, UK
* To whom correspondence should be addressed. Tel: +44 191 246 4411; Fax: +44 191 246 4301; Email: a.g.hall{at}ncl.ac.uk
The MSH3 and dihydrofolate reductase (DHFR) genes, located on chromosome 5, share a common promoter but are divergently transcribed. Dysregulation of the mismatch repair (MMR) pathway has been found to occur in cell line models due to co-amplification of MSH3 as a coincident effect of DHFR amplification, acquired as a mechanism generating resistance to methotrexate (MTX). The increased levels of MSH3 perturbed MutS
function resulting in hypermutability and increased resistance to thiopurines, drugs whose cytotoxic effects are triggered by MutS. The relevance of this phenomenon in clinical samples is unknown but is extremely pertinent in childhood acute lymphoblastic leukaemia (ALL) in which children are exposed for prolonged periods to both MTX and thiopurines such that a single amplification event involving both the DHFR and the MSH3 genes may cause chemotherapeutic resistance to both agents. Thus, we have generated a leukaemic cell line (PreB697) and a normal human lymphoblastoid cell line (TK6) that are resistant to a pharmacologically relevant dose of MTX and show that while increased DHFR levels result in MTX resistance, the associated increased levels of MSH3 are insufficient to perturb MutS functionality, in terms of MMR capacity or 6-thioguanine sensitivity. In addition, we show that although low-level DHFR amplification occurs alone in a significant number of samples, both at disease onset and relapse, co-amplification of both MSH3 and DHFR is rarely found in primary ALL samples, even after prolonged MTX therapy and is not at a sufficiently high level to perturb MMR function.
Abbreviations: ALL, acute lymphoblastic leukaemia; DHFR, dihydrofolate reductase; FBS, fetal bovine serum; IDL, insertion/deletion loop; MMR, mismatch repair; mRNA, messenger RNA; MTX, methotrexate; 6-TG, 6-thioguanine; TBP, TATA box binding protein
Received September 22, 2006; revised November 16, 2006; accepted November 19, 2006.