Serum response factor

Transcription factor

Biology

Gene

GLI2

Smoothened

Sonic hedgehog

GLI1

Hedgehog

Signal transduction

Hedgehog signaling pathway

Cell biology

Cancer research

Genetics

Coactivator

Nature Medicine

A large proportion of basal cell carcinomas develop resistance independently of the canonical mutations in genes encoding hedgehog pathway components. An unbiased analysis investigating alternative pathways of resistance uncovers the role of cytoskeletal signaling in driving noncanonical activation of hedgehog signaling through nuclear translocation of SRF and MKL1. These results advance understanding of the mechanisms underlying drug resistance and provide new actionable insights for clinical translation. Hedgehog pathway–dependent cancers can escape Smoothened (SMO) inhibition through mutations in genes encoding canonical hedgehog pathway components; however, around 50% of drug-resistant basal cell carcinomas (BCCs) lack additional variants of these genes. Here we use multidimensional genomics analysis of human and mouse drug-resistant BCCs to identify a noncanonical hedgehog activation pathway driven by the transcription factor serum response factor (SRF). Active SRF along with its coactivator megakaryoblastic leukemia 1 (MKL1) binds DNA near hedgehog target genes and forms a previously unknown protein complex with the hedgehog transcription factor glioma-associated oncogene family zinc finger-1 (GLI1), causing amplification of GLI1 transcriptional activity. We show that cytoskeletal activation through Rho and the formin family member Diaphanous (mDia) is required for SRF–MKL-driven GLI1 activation and for tumor cell viability. Remarkably, nuclear MKL1 staining served as a biomarker in tumors from mice and human subjects to predict tumor responsiveness to MKL inhibitors, highlighting the therapeutic potential of targeting this pathway. Thus, our study illuminates, for the first time, cytoskeletal-activation-driven transcription as a personalized therapeutic target for combatting drug-resistant malignancies.

Noncanonical hedgehog pathway activation through SRF–MKL1 promotes drug resistance in basal cell carcinomas