Epigenetic dysregulation is common in cancer, including in many central nervous system tumors. Mutations in genes encoding epigenetic readers, writers, and erasers, as well as mutations in histone proteins, have been identified as drivers in several types of brain tumors, and tumors can use epigenetic mechanisms to evade death following therapy. In addition to their critical role in cancer, epigenetic modifiers also play a key role in sexual differentiation. A number of epigenetic modifiers are found on the sex chromosomes resulting in potential differences in expression or activity between males and females. Additionally, masculinization of the body and brain is primarily driven by “organizational” effects of early testosterone exposure. These long lasting organizational effects are established through the epigenetic patterning of chromatin, and blocking the action of specific epigenetic writers and erasers can disrupt normal sexual differentiation. Despite the importance of epigenetic changes to both cancer and sexual differentiation, there are currently no studies examining how sex differences in the epigenetic landscape modify epigenetic changes in cancer or the efficacy of epigenetic therapeutics.
In order to tackle these critical questions our lab is taking two distinct approaches – studying how epigenetic readers and cell specific enhancers encode cellular sex identity, and investigating how early developmental patterning of sex differences determines cellular susceptibility to transformation.
Epigenetic Readers and Cellular Sex Identity
Bromodomain and extra-terminal domain (BET) family proteins are epigenetic readers that regulate gene expression through recruiting transcriptional complexes to target genes. One critical BET protein, Brd4, is commonly dysregulated in cancer, including in glioblastoma (GBM), the most common and aggressive form of brain tumor in adults. In GBM, Brd4 has been shown to act as a co-activator of oncogenic transcriptional programs. Brd4 also plays critical roles in the function of normal cells. The bulk of a cell’s complement of Brd4 is located at a small number of enhancer regions known as super enhancers (SEs). SEs are large clusters of enhancers that are associated with transcriptional control of cell identity genes. We are currently investigating whether cellular sex identity, differences in tumor phenotype and the sex-dependent response to treatment are mediated through differential Brd4-bound enhancer usage in male and female GBM cells.
Developmental Patterning of Sex Differences
When considering the biologic origin of a cell intrinsic sexually dimorphic phenotype, there are two potential causes: 1) organizational effects of gonadal hormone exposure in utero, and 2) differences in the expression levels of genes on the X and Y chromosomes. To tease apart these two potential causes, we are utilizing a transgenic mouse model known as the Four Core Genotypes (FCG) model. In mammals, masculine development is initiated by a specific gene on the Y chromosome, the Sry gene. This gene directs development of the testes, which then produce the surge of testosterone in males primarily responsible for masculinization. In the FCG model, the Sry gene is deleted from the Y chromosome and inserted onto an autosome, allowing for the separation of gonadal and chromosomal sex. Mice that inherit the Sry gene, regardless of whether they are XX or XY, develop testes and display behaviors indicative of brain masculinization, while mice that lack the Sry gene develop ovaries and display phenotypes associated with a feminized brain. This model can thus be used to determine if a sex difference in phenotype is the result of sex chromosomes or organizational effects of gonadal hormones. We are currently building a model of glioblastoma using FCG mice. This model will allow us to answer important questions about the mechanisms driving sex differences in brain tumor incidence and survival, and may lead to the identification of new targets or new approaches to treatment in men and women with GBM.