Widespread, exceptionally high levels of histone H3 lysine 4 trimethylation largely mediate "privileged" gene expression.

We examined the molecular determinants for sustained high-level expression of "privileged" genes, defined as the 0.03% most highly expressed genes within any specific cell. We identified histone modifications by chromatin immunoprecipitation analyses on Keratin 8, the most highly expressed gene in the human breast cancer cell line, MCF-7, based on serial analysis of gene expression. Quantitative comparisons to the "normal" counterpart cell line, MCF-10A, expressing 350-fold lower levels of Keratin 8 and other breast cancer cell lines expressing higher levels were performed using real-time PCR. Extraordinarily high levels of trimethyl histone H3 lysine 4 (H3K4) were found primarily in the first intron of the Keratin 8 gene stretching from 400 to 2000 bp downstream from the promoter in all breast cancer cells lines but not in MCF-10A cells. The highest levels of histone H3K4 trimethylation in MCF-7 cells ranged from 70% to 80% over input within 1200 bp of this region. Knockdown of mixed-lineage leukemia (MLL), the specific methyltransferase for histone H3K4, with MLL-specific siRNA decreased histone H3K4 trimethylation on the Keratin 8 gene and decreased Keratin 8 mRNA levels. Histone H3K4 trimethylation mediates approximately 86% of the elevated, sustained expression of the Keratin 8 gene in MCF-7 cells.