445 Does Tyrosine and Serine Phosphorylation of STAT3 Drive Cutaneous Squamous Cell Carcinoma in Recessive Dystrophic Epidermolysis Bullosa?

The early onset of aggressive cutaneous squamous cell carcinoma (cSCC) and its rapid progression in recessive dystrophic epidermolysis bullosa (RDEB) leads to premature mortality. Elevated levels of STAT3Tyr705 (pY-STAT3) phosphorylation have been demonstrated in RDEB-derived cSCC cells, showing that constitutive activation and dysregulation of the pathway play a role in RDEB-cSCC pathogenesis. Application of Ruxolitinib (JAK1/2 inhibitor) to an RDEB-cSCC xenograph mouse model, reduces the size of small tumours but is insufficient to completely reduce bigger tumours. We hypothesized that STAT3 phosphorylation at Serine 727 (pS-STAT3) might play a role in RDEB-cSCC tumour progression. Recently, pS-STAT3 has been shown to be relevant for the evolution of some cancers but it has not yet been studied in RDEB-cSCC. To test our hypothesis, we studied RDEB-cSCC patient samples with different degrees of de-differentiation (RDEB skin, edge and center of the RDEB-cSCC). pY-STAT3 was preferentially upregulated in the normal RDEB skin, followed by the tumour edge, with lowest expression at the tumour centre. In contrast, pS-STAT3 presented the opposite pattern: more in the tumour and less in the normal RDEB skin. We looked further into the JAK/STAT pathway using a Taqman array and Reactome. Three pathways were differentially expressed: MAPK, RAF and antigen B cell receptor activation, been upregulated in RDEB-SCC and less on the edge and normal RDEB skin. To further investigate and analyse cancer progression, we generated an RDEB-cSCC spheroid in vitro model which is: easy to generate; requires minimal cell; is suitable for drug testing; can recapitulate the different tumour stages. We have been able to control their size and use them for drug testing: size decreased 20% after 6 days of Ruxolitinib treatment. This new 3D RDEB-cSCC model along with the analysis of human samples allow us to better understand JAK/STAT signalling and RDEB-cSCC progression.