A methylated CpG island 3' in the apolipoprotein-E gene does not repress its transcription

CpG islands are always associated with the 5' end of housekeeping genes, covering their promoters and transcription start sites. CpG islands associated with genes of limited expression are less uniformly localized; the genes for apolipoprotein-E and -AI contain CpG islands corresponding to their last exons. As expected, the CpG island in the apo-AI gene is unmethylated in DNA from all tissues analyzed, expressing as well as non-expressing apolipoprotein-AI. In contrast, the apo-E CpG island is methylated in DNA from all tissues analyzed except sperm. The apo-E gene is transcribed in many tissues and is not repressed by this methylation