Epigenetic targeting of Hedgehog pathway transcriptional output through BET bromodomain inhibition

Abstract

Hedgehog signaling drives oncogenesis in several cancers and strategies targeting this pathway have been developed, most notably through inhibition of Smoothened. However, resistance to Smoothened inhibitors occurs via genetic changes of Smoothened or other downstream Hedgehog components. Here, we overcome these resistance mechanisms by modulating GLI transcription via inhibition of BET bromodomain proteins. We show the BET bromodomain protein, BRD4, regulates GLI transcription downstream of SMO and SUFU and chromatin immunoprecipitation studies reveal BRD4 directly occupies GLI1 and GLI2 promoters, with a substantial decrease in engagement of these sites upon treatment with JQ1, a small molecule inhibitor targeting BRD4. Globally, genes associated with medulloblastoma-specific GLI1 binding sites are downregulated in response to JQ1 treatment, supporting direct regulation of GLI activity by BRD4. Notably, patient- and GEMM-derived Hedgehog-driven tumors (basal cell carcinoma, medulloblastoma and atypical teratoid/rhabdoid tumor) respond to JQ1 even when harboring genetic lesions rendering them resistant to Smoothened antagonists.

The BET Bromodomain Inhibitor I-BET151 Acts Downstream of Smoothened Protein to Abrogate the Growth of Hedgehog Protein-driven Cancers

“…Importantly, I-BET151-mediated inhibition of Brd4 activity also occurred in vivo, potently reducing the growth of a Ptch ϩ/Ϫ -driven medulloblastoma. These results, along with those of a similar recent study (35), provide evidence for a previously unappreciated role for the BET bromodomain proteins in HH signaling.…”

Epigenetic modification in chromatin machinery and its deregulation in pediatric brain tumors: Insight into epigenetic therapies

“…51 Interestingly, BRD4 inhibition has been reported to decrease cell viability in medulloblastoma cell lines and xenografts. 52,53 In addition to these epigenetic changes occurring in medulloblastoma, reprogramming of DNA methylation patterns in these tumors shows focal regions of low methylation linked to transcription-factor-binding sites, shedding light on differential transcriptional networks between subgroups. At the same time, increased methylation due to re-normalization of repressed chromatin in DNA methylation valleys was positively correlated with gene expression.…”

Efficacy of Hedgehog Pathway Inhibitors in Basal Cell Carcinoma

“…These may include the use of other SMO inhibitors when resistance mutations occur in the drug target itself, targeting components downstream of SMO, such as GLI, or combination therapy with drugs targeting other pathways such as PI3K. A promising future therapeutic option for downstream targeting of the Hh pathway is the use of bromodomain inhibitors, which were shown to inhibit GLI transcription and have recently entered the clinic for the treatment of other cancers (76).…”