Abstract SY40-04: Reprogramming of the epigenome by MLL links early life environmental exposures to adult cancer risk
Authors: Wang Quan , Trevino Lindsey , Prins Gail , Bosland Maarten , Ho Shuk-Mei , Medvedovic Mario , Walker Cheryl Lyn 
Published: August 2015

The mixed lineage leukemia (MLL) oncogene has long been known to be a target for translocations that drive development of several types of leukemia. Later, the discovery that MLL functions as a histone methyltransferase in the COMPASS complex revealed a key role for MLL in programming of the epigenome. MLL “writes” the histone H3, lysine 4 trimethyl (H3K4me3) “mark” on chromatin of active genes. The appreciation that alterations in MLL, as well as other epigenetic “readers, writers and erasers”, play a key role as cancer drivers has heralded a new and exciting era in epigenetic therapies targeting alterations of the epigenome that occur in cancers associated with defects in these genes.

Most recently, we have discovered that epigenetic “readers, writers and erasers” themselves, not just their cognate genes, are the targets for environmental carcinogens. Critical periods of tissue and organ development have been shown to be exquisitely sensitive to environmental exposures, which can disrupt the activity of epigenetic “readers, writers and erasers”, and in so doing, (re)program the epigenome in such as way as to increase susceptibility to tumors in adulthood. This has been best shown in the male and female reproductive tract, where this developmental (re)programming results in a heightened transcriptional response to steroid hormones in the (re)programmed tissues, promoting development of hormone-dependent tumors. In the case of hormone-dependent uterine tumors, for example, this can increase the penetrance of a germline tumor suppressor gene defect from ∼50% to 100% in exposed individuals.

We have focused on developmental (re)programming caused by early life exposures to endocrine disrupting chemicals (EDCs), which bind hormone receptors such as the estrogen receptor (ER). We find that EDCs activate non-genomic ER signaling in developing tissues, turning on kinases that inappropriately phosphorylate, and alter the activity of, epigenetic “readers, writers and erasers”. In the developing rat prostate, for example, even a brief EDC exposure (once a day on PND 1,3, and 5) activates several non-genomic signaling cascades to reprogram genes in the KEGG prostate cancer pathway. As a result of such early life EDC exposure, in adult animals both basal (e.g. secretoglobins, kallikreins) and androgen-induced (e.g. Grb2 and ErbB2) gene expression is elevated, promoting development of PIN lesions by 12 months and prostate cancer at 24 months of age.

In exploring mechanisms responsible for developmental (re)programming, we observed in the developing prostate that EDC exposure increased MLL cleavage (required to generate an N- and C-terminal active dimer) and H3K4me3 levels. Genetic, pharmacologic, and cell-based studies revealed that when PI3K/AKT is activated by non-genomic ER signaling, AKT binding and phosphorylation of MLL at S532 increased, inducing N- and C-terminal cleavage and formation of active MLL dimer. Studies in vivo in the developing prostate revealed EDC exposure on PND 1,3, and 5 similarly induced non-genomic PI3K/AKT signaling and increased levels of the active MLL dimer. Chromatin immunoprecipitation (ChIP) confirmed that activation of MLL by AKT increased MLL binding and H3K4me3 promoter methylation at genes that became reprogrammed including as Grb2, Scgb2a1/2 in the secretoglobin cluster and Klk1c10 in the kallikrein gene cluster. Importantly, elevated H3K4me3 at the promoter of these genes persisted into adulthood in the prostate of exposed rats in which expression of these genes was significantly increased.

These data reveal a new pathway for regulation of MLL activity by PI3K/AKT, which is vulnerable to disruption by environmental exposures during critical periods of development. As such it points to a new epigenetic mechanism by which early life environmental exposures can increase tumor susceptibility, not via mutations, but by inducing changes in the epigenome that persist into adulthood and increase risk across the life course.

Citation Format: Quan Wang, Lindsey Trevino, Gail Prins, Maarten Bosland, Shuk-Mei Ho, Mario Medvedovic, Cheryl Lyn Walker. Reprogramming of the epigenome by MLL links early life environmental exposures to adult cancer risk. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr SY40-04. doi:10.1158/1538-7445.AM2015-SY40-04