Comparison of effect of tumor promoter treatments on DNA methylation status and gene expression in B6C3F1 and C57BL/6 mouse liver and in B6C3F1 mouse liver tumors

Abstract

The effects of different liver tumor-promoting treatments (i.e., a choline-devoid, methionine-deficient (CMD) diet, phenobarbital (PB), or both) on Ha-ras and raf methylation status and expression were determined in mouse strains with different susceptibilities to liver tumor formation: the relatively sensitive B6C3F1 and the relatively resistant C57BL/6. Additionally, B6C3F1 mouse liver tumors, spontaneous or PB induced, were assessed for alterations in global DNA methylation status and expression of Ha-ras and raf. The CMD diet led to hypomethylation of Ha-ras and raf after 12 wk of administration in B6C3F1 and C57BL/6 mice. At this early phase of tumor promotion, the frequency of increased expression of both Ha-ras and raf mRNAs was higher in the B6C3F1 but not the C57BL/6 mice. This is a mechanism that may, in part, underlie the heightened sensitivity of the B6C3F1 mouse to liver tumorigenesis. Subpopulations of B6C3F1 mouse liver tumors displayed altered global methylation status, with both hypomethylation and hypermethylation evident. Carcinomas were significantly more hypomethylated than adenomas. The level of raf mRNA was not changed in spontaneous or PB-induced B6C3F1 mouse liver tumors. Increased expression of Ha-ras was evident in some spontaneous B6C3F1 liver tumors and in most of the PB-induced liver tumors. These experiments support the concept that altered DNA methylation plays a key role in tumorigenesis and indicate that the high propensity of the B6C3F1 mice to liver tumorigenesis may be due, in part, to a decreased ability to maintain normal methylation status.

Aberrant DNA methylation and gene expression in livers of newborn mice transplacentally exposed to a hepatocarcinogenic dose of inorganic arsenic

“…In particular, it is thought that aberrant DNA methylation is important in the development of liver cancers (Goodman and Watson, 2002), and is a significant epigenetic mechanism that underlines the aberrant expression of genes involved in mouse liver carcinogenesis (Counts et al, 1997). In the present study, arsenic-exposed newborn mouse liver showed various gene expression changes, accounting for approximately 5% of genes on the array.…”

Multiple Genes Exhibit Phenobarbital-Induced Constitutive Active/Androstane Receptor–Mediated DNA Methylation Changes during Liver Tumorigenesis and in Liver Tumors

“…A limited number of genes have been associated with mouse liver tumorigenesis, that is, CAR (Huang et al, 2005;Yamamoto et al, 2004), b-catenin (Aydinlik et al, 2001;Strathmann et al, 2006), c-Myc (Vorce and Goodman, 1991), Ha-ras (Counts et al, 1997;Vorce and Goodman, 1991;Wiseman et al, 1986), Ki-ras (Vorce and Goodman, 1991), and Raf (Ray et al, 1994). Previously, 146 total unique RAMs were observed in liver tumor-susceptible PB-treated CAR WT mice (precancerous and tumor tissue), as compared with their resistant KO counterpart and we suggested that, at least some of, these changes might be playing critical roles in the carcinogenesis process (Phillips et al, 2007).…”

Ancestral bias in the Hras1 gene and distal Chromosome 7 among inbred mice

“…To identify any sequence variants in the Hras1 gene that could account for strain differences in the frequency of Hras1 -initiated liver tumors, we sequenced the gene from 14 inbred strains of mice: 12 classic laboratory inbred strains (A/J, AKR/J, C57BL/6J, C57BR/cdJ, BALB/cByJ, C3H/HeJ, CBA/J, DBA/2J, SM/J, SWR/J, 129X1/SvJ, and 129P3/J) and 2 distantly related inbred strains, M. m. castaneous (CAST/EiJ) and M. m. spretus (SPRET/EiJ). The 4.475-kb sequenced region of the Hras1 gene spans all of the introns and exons and 0.9 kb of 5′- and 0.6 kb of 3′-flanking sequence based on the Hras1 transcript expressed in the mouse liver (Counts et al 1997 ). In this approximately 4.5 kb of sequence, 11 of the 12 classic inbred strains have exactly the same sequence.…”

“…The genomic Hras1 structure is shown with blocks indicating exons and the shaded portions representing the translated regions. This structure corresponds to an approximately 1-kb transcript expressed in the mouse liver (Counts et al 1997 ). The horizontal lines next to the strain names indicate the genomic sequence coverage of that particular strain(s).…”