Non-melanomatous skin cancers (NMSCs), which include basal and squamous cell carcinoma (BCC and SCC respectively), represent a significant burden on the population as well as an economic load to the health care system, yet treatments of these preventable cancers remain ineffective. Although primary prevention is possible through minimising sunlight exposure, the World Health Organisation estimates that between 2 and 3 million new cases of NMSCs are diagnosed each year, accounting for 1 in 3 of all newly diagnosed cancers. Furthermore, studies have estimated there has been a 2-fold increase in the incidence of NMSCs between 1960s and 1980s. The increase in cases of NMSCs as well as the lack of effective treatments makes the need for novel therapeutic approaches all the more necessary. To rationally develop more targeted treatments for NMSCs, a better understanding of the cell of origin, in addition to the underlying pathophysiological mechanisms that govern the development of these cancers, is urgent. NMSCs are generally thought to arise from specific types of stem cells that become the source of clonal expansion of tumourigenic cells. Previous research on SCC has alluded to these stem cells being localised in the basal compartment of the skin, which ordinarily houses the progenitor cells that contribute towards wound healing and normal cell turnover of overlying epidermal skin layers. More recent research has suggested that commitment to differentiation, which requires exiting the basal, progenitor-cell compartment, is a reversible mechanism. Genetic modifications engage differentiated cells into dedifferentiation, converting them into cancer-initiating cells (CICs) and thereby promoting a tumourigenic environment. Here we explore the most recent developments in the understanding of skin SCC cell of origin, and discuss a case study illustrating the loss of the Grainy-head like 3 (GRHL3) transcription factor in suprabasal layers, which confers a tendency towards tumour development and thereby challenging the “stem cell” theory of tumourigenesis.

Pemphigus vulgaris is an autoimmune blistering disease of the epidermis, caused by autoantibodies against desmosomal proteins, mainly desmogleins 1 and 3, which induce an impairment of desmosomal adhesion and blister formation. Recent findings have shown that inhibition of immunoglobulin G binding on the neonatal Fc receptor, FcRn, results in reduced autoantibody recycling and shortens their half-life, providing a valid treatment option for PV. We have here analyzed the role of FcRn in human keratinocytes treated with novel, recombinant anti-desmoglein-3 antibodies that induce pathogenic changes in desmosomes, such as loss of monolayer integrity, aberrant desmoglein-3 localization and degradation of desmoglein-3. We show that blocking IgG binding on FcRn by efgartigimod, a recombinant Fc fragment that is undergoing clinical studies for pemphigus, stabilizes the keratinocyte monolayer, whereas the loss of desmoglein-3 is not prevented by efgartigimod. Our data show for the first time that FcRn may play a direct role in the pathogenesis of pemphigus at the level of the autoantibody target cells, the epidermal keratinocytes. Our data also imply that in keratinocytes, FcRn may have functions different from its known function in IgG recycling. Therefore, stabilization of keratinocyte adhesion by FcRn blocking entities may provide a novel treatment paradigm for pemphigus.

The epidermis is the outermost layer of skin and provides a protective barrier against environmental insults. It is a rapidly renewing tissue undergoing constant regeneration, maintained by several types of stem cells. Hedgehog (HH) ligands activate one of the fundamental signaling pathways that contribute to epidermal development, homeostasis and repair. The HH pathway interacts with other signal transduction pathways such as those activated by Wnt and bone morphogenetic protein. Furthermore, aberrant activation of HH signaling is associated with various tumors, including basal cell carcinoma. Therefore, an understanding of the regulatory mechanisms of the HH signaling pathway is important to elucidate fundamental mechanisms underlying both organogenesis and carcinogenesis. In this review, we discuss the role of the HH signaling pathway in skin development, homeostasis and basal cell carcinoma formation, providing an update of current knowledge in this field.