mutation and tumoral PD-L1 expression are associated with depth of invasion in desmoplastic melanomas.

Background: Desmoplastic melanoma (DM) is a rare subtype of spindle cell malignant melanoma characterized by frequent local recurrences and hematogenous spread, but without molecular classification. The aim of the study was to investigate in a DM series the incidence of relevant gene alterations in cancer, the programmed death-ligand 1 (PD-L1) expression status and the association with clinicopathological features and melanoma progression.

Methods: A total of 38 patients were included. Clinical follow-up and the histopathological features of all cases were retrospectively collected. PD-L1 expression by immunohistochemistry (IHC) and genomic alterations by real-time PCR were determined in 34 samples. Additionally, a molecular analysis by next-generation sequencing was performed in 25 DMs.

Results: Tumors occurred predominantly in men (76%) and in the head and neck region (50%). Most tumors were pure DMs (66%), containing less than 10% of conventional melanoma. Overall, 48% of our cohort harbored mutations, most of them showing a molecular signature associated with ultraviolet (UV)-oncogenesis, and 29%, mutations. A positive correlation between with depth of invasion (P=0.005) and presence of elastosis (P=0.002) was found. High-expression of PD-L1 in tumor cells was observed in 38% of cases and correlated with depth of tumoral infiltration (P=0.003), (P=0.016), PD-1 (P<0.001) and tumor-infiltrating lymphocytes (TILS) (P<0.001). PD-L1 expression in immune cells correlated with PD-1 (P=0.006), tumoral PD-L1 expression (P=0.029) and mutation (P=0.002). Survival correlated with depth of invasion (P=0.003), stage of tumors (P=0.015), positive sentinel lymph node (P=0.004), lymph node metastasis (P=0.024) and distant metastasis (P<0.001).

Conclusions: Our results suggest that progressed DMs with deep tumoral infiltration frequently harbor mutations, PD-L1 expression and present a high inflammatory response, probably related to adaptive immune resistance in this tumor-type.