Expression of Base Excision, Mismatch, and Recombination Repair Genes in the Organogenesis-Stage Rat Conceptus and Effects of Exposure to a Genotoxic Teratogen, 4-Hydroperoxycyclophosphamide ROBERT K. VINSON AND BARBARA F. HALES * Department of Pharmacology and Therapeutics, McGill University, Montre ´al, Que ´bec, Canada, H3G 1Y6 ABSTRACT Background: DNA repair capability may influence the outcome of genotoxic teratogen exposure. The goals of this study were to assess the expression of base excision repair (BER), mismatch repair (MMR), and re- combination repair (RCR) genes in the mid-organogen- esis rat conceptus and to determine the effects on expression of exposure to the genotoxic teratogen, 4-hydroperoxycyclophosphamide (4-OOHCPA). Methods: The expression of 17 BER, MMR, and RCR genes was examined in gestational day (GD) 10 –12 rat conceptuses using the antisense RNA (aRNA) tech- nique. Embryos were cultured with 10 M 4-OOHCPA to examine effects on gene expression. Results: Yolk sacs and embryos had similar gene expression patterns for all three DNA repair pathways from GD10-12. Transcripts for APNG, PMS1, and RAD54 were present at high concentrations in both tissues. The remainder of the genes were expressed at low levels in yolk sac, with a few not detected on GD10 and 11. In the embryo, transcripts for most genes were low on GD10 and 11; several increased by GD12. After exposure to 4-OOHCPA for 24 hr, XRCC1 and RAD57 expression decreased in yolk sac, whereas only RAD51 transcripts decreased in the embryo. By 44 hr, transcripts for all BER genes decreased in yolk sac; in the embryo, most BER, MMR, and RCR genes decreased, many below the level of detection. Conclusions: The expression of DNA repair genes in the mid-organogenesis rat conceptus is varied and subject to down-regulation by 4-OOHCPA. DNA repair gene expression may determine the consequences of genotoxicant exposure during development. Teratology 64:283–291, 2001. © 2001 Wiley-Liss, Inc. INTRODUCTION The integrity of genetic material in the cell is crucial for proper cell growth and function. To maintain this integrity, genotoxic damage must be repaired before cell division occurs as it has the ability to modify gene or chromosome structure, leading to gene expression changes, altered cell functioning, and abnormal cell differentiation and growth in nascent daughter cells. Repairing these alterations is particularly important during the periods of cellular proliferation that occur during development, as aberrant cell growth or death can lead to malformations (Sulik et al., ’88; Wolpert, ’99). Birth defects observed after teratogen exposure depend on the nature of the insult and the stage of development of the embryo. Many known teratogens are genotoxic (Bishop et al., ’97), including thalidomide (Parman et al., ’99), dioxin (Shertzer et al., ’98), py- rimethamine (Tsuda et al., ’98), and phenytoin (Liu and Wells, ’95). Although the link between genetic in- stability and teratogenesis is not fully established (for review, see Ferguson and Ford, ’97), the role of DNA damage in abnormal cell growth, death, and function- ing points to a causal role in the etiology of malforma- tions (Snow, ’97; Wells et al., ’97). Abbreviations used: 4-OOHCPA, 4-hydroperoxycyclophosphamide; AP, apurinic/apyrimidinic; APE/Ref-1, apurinic/apyrimidinic endonuclease/ redox factor-1; APNG, alkylpurine-DNA-N-glycosylase; BCNU, N,N'- bis(2-chloroethyl)-N-nitrosourea; BER, base excision repair; CP, cyclo- phosphamide; DSB, double strand DNA break; GD, gestational day; ICL, interstrand cross-link; LigIII, DNA ligase III; MGMT, methylguanine- DNA methyltransferase; MLH1, MutL homolog 1; MMR, mismatch re- pair; MMS, methylmethane sulfonate; MNNG, N-methyl-N'-nitro-N-ni- trosoguanidine; MRE11, meiotic recombination gene 11; MSH2, MutS homolog 2; NER, nucleotide excision repair; PolB, DNA polymerase ; PMS1, postmeiotic segregation increased 1; PMS2, postmeiotic segrega- tion increased 2; RAD50, RAD51, RAD52, RAD54, RAD57, radiation sensitivity genes 50, 51, 52, 54, and 57; RCR, recombination repair; SDS, sodium dodecyl sulphate; SSB, single strand DNA break; SSC, sodium chloride citrate; XRCC1, X-ray repair cross-complementing gene 1. Grant sponsor: Canadian Institutes of Health Research; Grant spon- sor: FCAR Que ´bec. *Correspondence to: Barbara F. Hales, Department of Pharmacology and Therapeutics, McGill University, 3655 Promenade Sir-William- Osler, Room 110, Montre ´al, Que ´bec, Canada, H3G 1Y6. E-mail: bhales@pharma.mcgill.ca Received 9 May 2001; Accepted 18 August 2001 TERATOLOGY 64:283–291 (2001) © 2001 WILEY-LISS, INC.