Tissue-specific methylation patterns and expression of the human apolipoprotein AI gene.
To better understand the tissue-specific expression of the human apolipoprotein (apo)AI gene, we performed a detailed analysis of the pattern of methylation of the gene in various human adult and embryonic tissues and in tissues of transgenic mice harboring the human apo-AI gene. In addition, the gene was analyzed also in liver and intestine-derived human cell lines (HepG2 and Caco2, respectively). Using methyl-sensitive restriction enzymes (HpaII, HhaI, and SmaI) and the appropriate radioactive probes, we were able to determine separately the status of methylation of the 5'-end, the body of the gene, and 3'-end flanking sequences. The apo-AI gene in tissues that express the gene was undermethylated at the 5'-end. However, the 5'-end of the gene in sperm and in all adult tissues that do not express the gene was heavily methylated. The body of the gene which contains a CpG island and the 3'-end flanking sequences were, in general, hypomethylated except for specific sites that showed partial methylation. In contrast, while the gene showed tissue-specific expression already in a 12-week-old embryo, the 5'-end was invariably hypomethylated in all tissues of the embryo. A human apo-AI transgene has recently been shown to be active exclusively in the liver, while the endogenous gene is expressed in both liver and intestine (6). We show here that the 5'-end of the apo-AI transgene was methylated in all tissues of the mouse (including intestine) except liver. The results presented here demonstrate a clear correlation between hypomethylation of the 5'-end and activity of the apo-AI gene. However, the observed methylation pattern of the gene in embryonic tissues suggests that tissue-specific expression precedes formation of the tissue-specific methylation pattern.
Shemer
R
R
Department of Cellular Biochemistry, Hebrew University, Hadassah Medical School, Jerusalem, Israel.
Walsh
A
A
Eisenberg
S
S
Breslow
J L
JL
Razin
A
A
D000818:Animals; D016632:Apolipoprotein A-I; D001054:Apolipoproteins A / Q000235:genetics* / Q000378:metabolism; D015152:Blotting, Northern; D015139:Blotting, Southern; D015870:Gene Expression; D006367:HeLa Cells; D006801:Humans; D008745:Methylation; D051379:Mice; D008822:Mice, Transgenic; D009693:Nucleic Acid Hybridization; D009928:Organ Specificity; D012313:RNA / Q000302:isolation & purification; D015183:Restriction Mapping
D016632:Apolipoprotein A-I; D001054:Apolipoproteins A; D012313:RNA