Preconceptional carcinogenesis refers to environmental factors acting on the germ cells of parents before conception to increase the incidence of neoplasia and other pathologies in the progeny. Such carcinogenic effect has been observed in the offspring of male mice exposed to chromium(III), an environmental agent. Offspring lungs and livers are among the target tissues for this effect. DNA methylation analysis of sperm DNA has identified allele-specific hypomethylation in the spacer promoter region of the 45S ribosomal RNA (rRNA) gene. To determine if this paternal epigenetic change has any biological correlate in the offspring, male Swiss mice were treated with 1 mmol/kg chromium(III) chloride and mated 2 weeks later to untreated females. From their offspring, day-19 fetal livers (6 litters in treated group; 8 litters in control group) and 6-week young adult lungs (9 litters in treated and 8 litters in control group) were examined for the level of 45S rRNA transcript by reverse transcription-polymerase chain reaction (RT-PCR), with both real-time PCR and pyrosequencing-based quantification for method cross-validation. In comparison with vehicle controls, significantly higher levels of 45S rRNA were found in livers of fetuses (P <0.05) and in lungs of 6-week old adult offspring of treated males (P<0.05). The differences remained significant after adjusting for litter membership, gender and litter size. DNA methylation in the rRNA spacer-promoter region was further determined by a bisulfite-PCR-pyrosequencing method in a subset of the 6-week lung samples and showed negative correlation with the 45S rRNA level, consistent with the notion of activation of RNA expression by hypomethylation. It is known that ribosome levels can be rate-limiting for cancer cell growth. Detection of up-regulation of the 45S rRNA gene transcript in the offspring after paternal exposure to chromium(III) links epimutation in sperm with an RNA alteration in offspring target tissue relevant to carcinogenesis, and provides a possible epigenetic mechanism for father-mediated preconceptional carcinogenesis.
[Proc Amer Assoc Cancer Res, Volume 47, 2006]