A USP28-∆Np63 axis is a druggable vulnerability of squamous tumour formation and maintenance

The oncoprotein ∆Np63 is an essential transcriptional master- and cell identity regulator in squamous cell carcinoma (SCC) of various origins, encompassing lung, head and neck, oesophagus, cervix and skin. While in non-transformed cells ∆Np63 protein abundance is tightly regulated by the ubiquitin proteasome system (UPS), in tumours E3-ligases ubiquitylating ∆Np63, such as FBXW7, are commonly mutated or lost, resulting in a hyper-stabilisation of the oncogenic driver. Targeting ∆Np63 protein abundance in SCC could present a possible therapeutic avenue. Here, we report that the deubiquitylase USP28 regulates ∆Np63 protein stability and abundance in SCC by counteracting the degradative UPS system. Interference with USP28 activity by pharmacological inhibition specifically affected human SCC cell lines and, finally, we were able to demonstrate in vivo using CRISPR/Cas9 mouse models that Usp28 is required for SCC induction and maintenance. Hence, targeting USP28 is a viable option to tackle ∆Np63 protein abundance in SCC tumours. RNA-seq analysis after knockdown or inhibition of USP28 and deltaNp63 in A431 cells (human squamous cell carcinoma cell line).