Tissue- and cell-specific casein gene expression. II. Relationship to site-specific DNA methylation.
The relationship between DNA methylation and the expression of the gamma- and beta-casein genes was investigated in both expressing and nonexpressing tissues and in isolated tumor cell subpopulations displaying differential casein gene expression. MspI/HpaII digestions of DNA isolated from liver, a totally nonexpressing tissue, indicated that specific sites of hypermethylation existed in these genes as compared to the DNA isolated from casein-producing lactating mammary gland. The positions of these sites were mapped in the gamma-casein gene by comparing total genomic DNA Southern blots to the restriction digests of several overlapping phage clones constituting the gamma-casein gene. In contrast, the methylation status of the HhaI sites in the gamma-casein gene was found to be invariant regardless of the expression status of the gene. The inverse correlation between the hypermethylation of certain MspI/HpaII restriction sites in the casein genes and their potential expressibility was further substantiated by studies in 7,12-dimethylbenz(a)anthracene- and N-nitrosomethylurea-induced mammary carcinomas, which have an attenuated casein gene expression, and in cell subpopulations isolated from the 7,12-dimethylbenz(a)-anthracene tumor which were either depleted or enriched in casein-producing cells. Analysis of total tumor DNAs indicated that the casein genes were hypermethylated at the same sites observed in liver. However, a very faint hybridization signal was observed in the HpaII digests, suggesting cell-specific methylation differences. We have confirmed the hypomethylation of at least two of these MspI/HpaII sites within the subpopulation containing the casein-producing cells at a level consistent with the relative enrichment in that fraction. These results demonstrate differential site-specific casein gene methylation not only between tissues but also between cell subpopulations within a single tissue.
D015127:9,10-Dimethyl-1,2-benzanthracene; D000818:Animals; D001483:Base Sequence; D002364:Caseins / Q000235:genetics*; D002469:Cell Separation; D004247:DNA / Q000378:metabolism*; D004248:DNA (Cytosine-5-)-Methyltransferases / Q000378:metabolism; D005260:Female; D005786:Gene Expression Regulation; D008325:Mammary Neoplasms, Experimental / Q000235:genetics / Q000473:pathology; D051381:Rats; D011919:Rats, Inbred Strains; D013379:Substrate Specificity
D002364:Caseins; D015127:9,10-Dimethyl-1,2-benzanthracene; D004247:DNA; D004248:DNA (Cytosine-5-)-Methyltransferases