Identification of cellular origin and molecular mechanism in basal and squamous cell carcinomas

BCCs result from aberrant activation of HH signaling and several mouse models carrying mutations in HH signaling genes are capable to form tumors resembling to human BCCs.

To identify the cell lineage at the origin of BCC and to investigate the role of stem cells in tumor initiation, we followed a genetic approach where we conditionally expressed SmoM2 oncogene (a constitutively active Smoothened mutant) in distinct skin epidermal compartments including SCs. Targeting basal epidermis cells, showed that only SmoM2-clones in the inter follicular epidermis (IFE) and the infundibulum can progress into BCC, whereas SmoM2 expression in Bulge SCs or in matrix transit amplifying progenitor cells never leads to BCC formation. Progressively after SmoM2 expression, tumor-initiating cells lose their normal differentiation to adopt a hair placode-like shape and markers, demonstrating that biochemical and morphological tumour features can be misleading in extrapolating their cellular origin.

The molecular changes occurring in tumor initiating cells and the mechanisms regulating the early steps of cancer development are poorly characterized for the majority of tumors. To address these questions in BCC, we took advantage of our ability to isolate SmoM2 expressing cells at different stages of tumor initiation and progression. Transcriptional profiling of SmoM2-basal IFE cells isolated one week (normal histology) and 4 weeks (dysplastic lesion), suggests that adult IFE cells undergo a reprogramming into embryonic hair follicle (EHFP) like fate. In addition, we showed that Wnt/β-catenin signaling is essential for BCC initiating cell reprogramming into EHFP like fate and for tumor initiation in a cell autonomous manner. Finally, we show that EHFP reprogramming occurs also in human BCCs in addition to the presence of a similar canonical Wnt activation signature to the one revealed in the SmoM2-BCC mouse model.

SCC is the second most frequent skin cancers after BCC and mutations in p53 and Ras genes has been suggested to be potentially the primary events in this tumour. SCCs present signs of squamous differentiation, suggesting that SCCs may originate from the inter follicular epidermis (IFE). To identify the cell lineage at the origin of SCC and the role of the hair follicle SCs in tumor initiation, we use a genetic tools driving oncogenic KRas (KRasG12D) expression at physiological levels in different epidermal compartments.

Targeting KRasG12D expression in bulge SCs and their progeny or in IFE results in benign tumor development with no sign of malignant transformation. In contrast, KRasG12D expression in HF Transit amplifying (TA) matrix cells do not promotes any macroscopic tumors or microscopic defects in the epidermis. Interestingly, papillomas arising from the IFE express follicular markers such as CD34 and K17, indicating that the expression of HF markers by tumor cells does not necessarily reflect their cellular origin. Using a combination of deletion of both p53 alleles together with KRasG12D expression, we showed that bulge SCs and/or their progeny but not HF matrix TA cells, promote SCC formation, suggesting that additional genetic hits such as p53 are required to promote full-blown invasive skin SCC.

In summary, our work demonstrated the non-follicular origin of BCC resulting from Smo mutation, as well as the implication of the IFE progenitors in tumor initiation. We also revealed the progressive reprogramming of BCC initiating cells towards an EHFP-like fate and the key role of Wnt/β-catenin pathway in this process. In contrast, we showed the competence of several epidermal lineages to initiate benign tumors upon expression of KRasG12D oncogene at physiological levels. We also demonstrated that lineage -specific markers expression within tumor cells does not necessarily reflect their cellular origin. Finally, we demonstrated the requirement of additional hits, such as P53 loss, to promote malignant progression in the context of oncogenic Ras.