Molecular Biology, Pathobiology, and Genetics Embryonic Lethality after Combined Inactivation of Fancd2 and Mlh1 in Mice

DNA repair defects are frequently encountered in human cancers. These defects are utilized by traditional therapeutics but also offer novel cancer treatment strategies based on synthetic lethality. To determine the consequences of combined Fanconi anemia (FA) and mismatch repair pathway inactivation, defects in Fancd2 and Mlh1 were combined in one mouse model. Fancd2/Mlh1 double-mutant embryos displayed growth retardation resulting in embryonic lethality and significant underrepresentation among progeny. Additional inactivation of Trp53 failed to improve the survival of Fancd2/Mlh1–deficient embryos. Mouse fibroblasts were obtained and challenged with cross-linking agents. Fancd2-deficient cells displayed the FA-characteristic growth inhibition after mitomycin C (MMC) exposure. In primary fibroblasts, the absence of Mlh1 did not greatly affect the MMC sensitivity of Fancd2-deficient and Fancd2-proficient cells. However, in Trp53mutant immortalized fibroblasts, Mlh1 deficiency reduced the growthinhibiting effect of MMC in Fancd2mutant and complemented cells. Similar data were obtained using psoralen/UVA, signifying that MLH1 influences the cellular sensitivity to DNA interstrand cross-links. Next, the effect of MLH1 deficiency on the formation of chromosomal aberrations in response to crosslinking agents was determined. Surprisingly, Mlh1 mutant fibroblasts displayed a modest but noticeable decrease in induced chromosomal breakage and interchange frequencies, suggesting that MLH1 promotes interstrand cross-link repair catastrophe. In conclusion, the combined inactivation of Fancd2 and Mlh1 did not result in synthetic lethality at the cellular level. Although the absence of Fancd2 sensitized Mlh1/Trp53mutant fibroblasts to MMC, the differential survival of primary and immortalized fibroblasts advocates against systemic inactivation of FANCD2 to enhance treatment of MLH1-deficient tumors. [Cancer Res 2009;69(24):OF1–8]