Epigenetic Therapies.

Epigenetic therapies are emerging for pediatric cancers. Due to the relatively low mutational burden in pediatric tumors, epigenetic dysregulation and differentiation blockade is a hallmark of oncogenesis in some childhood cancers. By targeting epigenetic regulators that maintain tumor cells in a primitive developmental state, epigenetic therapies may induce differentiation. The most well-studied and clinically advanced epigenetic-targeted therapies include azacitidine and decitabine, which inhibit DNA methylation through competitive inhibition of the enzymatic activity of the DNA methyltransferase family enzymes. These DNA hypomethylating agents are Food and Drug Administration (FDA) approved for hematologic malignancies. The discovery that DNA hypermethylation occurs in patients with isocitrate dehydrogenase (IDH) mutations has led to the development and FDA approval of IDH inhibitors for hematologic and solid tumors. Epigenetic dysregulation in pediatric tumors is also driven by changes in the "histone code" that either promote oncogene expression or repress tumor suppressors. Cancers whose chromatin landscape is characterized by such aberrant histone posttranslational modifications may be amenable to targeted therapies that inhibit the chromatin-modifying enzymes that read, write, and erase these histone modifications. Small molecules that inhibit the enzymatic activity of histone deacetylases, acetyltransferases, and methyltransferases have been approved for the treatment of some adult cancers, and these agents are currently under investigation in various pediatric tumors. Chromatin regulatory complexes can be hijacked by oncogenic fusion proteins that are produced by chromosomal translocations, which are common drivers in pediatric cancer. Small molecules that disrupt oncogenic fusion protein activity and their associated chromatin complexes have demonstrated remarkable promise, and this approach has become the standard treatment for a subset of leukemias driven by the PML-RARA oncogenic fusion protein. A deeper understanding of the mechanisms that drive epigenetic dysregulation in pediatric cancer may hold the key to future success in this field, as the landscape of druggable epigenetic targets is also expanding.