Titolo della tesi: Deciphering the role of the transcription factor SALL4A in the regulation of the Hedgehog pathway and in the pathogenesis of medulloblastoma
The Hedgehog (Hh) pathway is essential in embryonic development and its aberrant activation is responsible for the onset of several tumors. Hh signalling activation is promoted by the binding of Hh extracellular ligands to the Patched inhibitor receptor (PTCH1). This interaction determines the release of the inhibitory effect exerted by PTCH1 on the activator co-receptor Smoothened (SMO), thus triggering the signal transduction cascade that culminates with the activation of zinc-finger transcription factors of the GLI family, responsible for the expression of target genes involved in the most important biological processes (i.e., proliferation, survival, differentiation, and stemness).
Given the relevance of Hh signalling in tumorigenesis, the study of the molecular mechanisms as well as the identification of new key players that control its activity represent an area of great interest in tumor biology. To address this aim, my research has been focused on the characterization of novel interactors of the known Hh negative regulator and tumor suppressor RENKCTD11. Starting from the mass spectrometry analysis, we identified different potential RENKCTD11 interactors and among them we focused our attention on SALL4A.
SALL4A belongs to the 4-member Spalt-Like protein family (SALL1-4), first identified in Drosophila Melanogaster. SALL4A is a zinc-finger transcription factor that plays a fundamental role in maintaining pluripotency and in the self-renewal of embryonic stem cells (ESCs). SALL4A regulates many genes, thus making this transcription factor essential for the regulation of numerous signalling pathways. SALL4A is highly expressed in the earliest stages of development, and its expression progressively decreases until it is almost completely inhibited in adults. The over expression of SALL4A has been found in several tumor such as brain, breast, lung, and colon-rectal cancers.
We found that SALL4A is a substrate of RENKCTD11, which promotes its poly-ubiquitylation and proteasome-dependent degradation. We demonstrated that SALL4A acts as a positive regulator of the Hh pathway enhancing GLI1 transcriptional activity. Specifically, we show that SALL4A and HDAC1 form a complex with GLI1 increasing the binding affinity of the complex and favoring the recruitment of HDAC1 to GLI1. This event leads to GLI1 deacetylation and its activation (Canettieri et al. 2010). Of note, we observed that genetic depletion of SALL4A strongly reduces Hh-dependent MB cells growth in vitro and impairs tumor formation in vivo.
In conclusion, results obtained by my Ph.D. project unveil a novel molecular mechanism involved in Hh signaling control and contribute to elucidate the biology of Hh-dependent MB through the identification of SALL4A as a new player of Hh signalling. In this scenario, SALL4A stands as a potential target for the development of new and more effective therapies for Hh-dependent tumors care.