Alterations in the methylation status and expression of the raf oncogene in phenobarbital‐induced and spontaneous B6C3F1 mouse liver tumors

Abstract

The liver tumor-prone B6C3F1 mouse (C57Bl/6 female x C3H/He male), in conjunction with the more susceptible C3H/He paternal strain and the resistant C57BL/6 maternal strain, is an excellent model for studying the mechanisms involved in carcinogenesis. The study reported here indicated that the B6C3F1 mouse inherited a maternal raf allele containing a methylated site not present in the paternal allele. Seven days after partial hepatectomy or after administration of a promoting dose of phenobarbital (PB) for 14 d; raf in B6C3F1 mouse liver was hypomethylated. The additional methylated site in the allele inherited from C57BL/6 was not maintained. The methylation status of raf in the liver of the C57BL/6 mouse was not affected by PB treatment. This indicates that the B6C3F1 mouse is less capable of maintaining methylation of raf than the C57BL/6 strain is. In both PB-induced and spontaneous B6C3F1 liver tumors, raf was hypomethylated in a nonrandom fashion. The level of raf mRNA increased in seven of 10 PB-induced tumors but in only one of five spontaneous tumors, whereas the level of Ha-ras mRNA increased in nine of 10 PB-induced tumors and in four of five spontaneous tumors. The results of our investigation (a) support the hypothesis that hypomethylation of DNA is a nongenotoxic mechanism involved in tumorigenesis, (b) support the notion that PB promotes liver tumors that develop along a pathway different from that leading to spontaneous tumors, and (c) indicate that differences in DNA methylation between C57BL/6 and B6C3F1 mice could, in part, account for the unusually high tendency of the latter strain to develop liver tumors.

Association of arsenic-induced malignant transformation with DNA hypomethylation and aberrant gene expression

“…Alterations in DNA methylation patterns can be fixed during DNA replication and evidence suggests that methylation status of a gene can participate, with other gene control mechanisms, to balance the transcriptional level of the gene (25). Hypomethylation is associated with activation of various oncogenes during cancer development, such as c-myc, c-fos, and H-ras in human and rat liver tumors (5,6) and raf (32) in mouse liver tumors. Additionally, c-myc overexpression is correlated with its specific hypomethylation in HL-60 cells (33) and in early rat liver tumors (34).…”

Alterations in DNA methylation are early, but not initial, events in ovarian tumorigenesis

“…Indeed, changes in genomic methylation, either focal or global, are one of the most consistent findings in human cancers (Gama-Sosa et al, 1983; Makos et al, 1993;Vertino et al, 1993) and an epigenetic mechanism was proposed whereby such changes may contribute to neoplastic transformation by changing the expression patterns of genes involved in growth control (Antequera et al, 1990;Jones et al, 1990). For example, hypomethylation of several proto-oncogenes including c-myc (Munzel et al, 1991;Sharrard et al, 1992), ras (Bhave et al, 1988), raf (Ray et al, 1994), bcl-2 (Hanada et al, 1993), erb-Al (Lipsanen et al, 1988), and c-fins (Felgner et al, 1991) was reported in various types of cancerous tissues. In addition, expression of several tumour-suppressor genes including p16/CDKN2 (Merlo et al, 1995;Gonzalez-Zulueta et al, 1995;Herman et al, 1994), retinoblastoma (Greger et al, 1989;Sakai et al, 1991) and von Hippel Lindau gene (Herman et al, 1994) can be turned off by methylation.…”

The traditional toxicologic paradigm is correct: dose influences mechanism.

“…The idea that altered DNA methylation plays a variety of roles in carcinogenesis, none of which are mutually exclusive, and involves genetic and epigenetic events (20,32) Tumorigenesis in mouse liver is used as our model system (36,37,(39)(40)(41) and phenobarbital serves as a nongenotoxic rodent tumor promoter (42). We use the B6C3F1 (C57BL/6 x C3H/He) mouse, which is highly sensitive to the development of spontaneous and chemical-induced liver tumors (43,44).…”

“…The results of our research indicate that the B6C3F1 mouse is deficient with regard to is ability to maintain normal DNA methylation. This appears to underlie in part its uniquely high susceptibility toward development of liver tumors (22,23,36,39,40). We believe that hypomethylation is relevant to tumorigenesis in both rodents and humans (23) and that humans may be less susceptible than rodents in part because of a better ability of human cells to maintain normal patterns of DNA methylation [reviewed in Counts and Goodman (23) and Counts et al (39)].…”