Impairment of the non-homologous end joining and homologous recombination pathways of DNA double strand break repair: Impact on spontaneous and radiation-induced mammary and intestinal tumour risk in Apc min/+ mice.

Female Apc(min/+) mice carrying the BALB/c variant of Prkdc or heterozygous knockout for Xrcc2, were sham- or 2 Gy X-irradiated as adults to compare the effect of mild impairments of double-strand break (DSB) repair pathways, non-homologous end joining (NHEJ) and homologous recombination (HR) respectively on spontaneous and radiation-induced mammary and intestinal tumorigenesis. Mice with impaired NHEJ showed no difference in incidence of spontaneous mammary tumours, compared with matched controls, (2.46 fold, P=0.121) and significantly less following irradiation (radiation-induced excess; 0.35 fold, P=0.008). In contrast mice with impaired HR presented with significantly less spontaneous mammary tumours than matched controls (0.33 fold, P=0.027) and significantly more following irradiation (radiation-induced excess; 3.3 fold, P=0.016). Spontaneous and radiation-induced intestinal adenoma multiplicity in the same groups were significantly greater than matched controls for mice with impaired NHEJ (sham; 1.29 fold, P<0.001, radiation-induced excess; 2.55 fold, P<0.001) and mice with impaired HR showed no significant differences (sham; 0.92 fold, P=0.166, radiation-induced excess; 1.16, P=0.274). Genetic insertion events were common in spontaneous tumours from NHEJ impaired mice compared with matched controls. γH2AX foci analysis suggests a significantly faster rate of DSB repair (MANOVA P<0.001) in intestinal than mammary tissue; apoptosis was also higher in irradiated intestine. To conclude, results suggest that pathway of choice for repair of spontaneous and radiation-induced DSBs is influenced by tissue type. NHEJ appears to play a greater role in DSB repair in intestinal tissue since impairment by functional change of Prkdc significantly increases the rate of mis-repair in intestinal but not mammary tissue. HR appears to play a greater role in DSB repair in adult mammary tissue since impaired HR results in significant changes in mammary but not in the intestinal tumorigenesis. This indicates that early DNA damage response and repair is important for cancer susceptibility and plays a role in determining tissue specificity of cancer risk.