**Exposure of Human Skin Organoids to Low Genotoxic Stress Can Promote Epithelial-to-Mesenchymal Transition in Regenerating Keratinocyte Precursor Cells**

#### **Sophie Cavallero 1,2,3,4,**†**,**‡ **, Renata Neves Granito 1,2,3,4,**†**,§, Daniel Stockholm <sup>5</sup> , Peggy Azzolin 1,2,3,4, Michèle T. Martin 1,2,3,4,\* and Nicolas O. Fortunel 1,2,3,4,\***


Received: 18 July 2020; Accepted: 14 August 2020; Published: 18 August 2020

**Abstract:** For the general population, medical diagnosis is a major cause of exposure to low genotoxic stress, as various imaging techniques deliver low doses of ionizing radiation. Our study investigated the consequences of low genotoxic stress on a keratinocyte precursor fraction that includes stem and progenitor cells, which are at risk for carcinoma development. Human skin organoids were bioengineered according to a clinically-relevant model, exposed to a single 50 mGy dose of γ rays, and then xeno-transplanted in nude mice to follow full epidermis generation in an in vivo context. Twenty days post-xenografting, mature skin grafts were sampled and analyzed by semi-quantitative immuno-histochemical methods. Pre-transplantation exposure to 50 mGy of immature human skin organoids did not compromise engraftment, but half of xenografts generated from irradiated precursors exhibited areas displaying focal dysplasia, originating from the basal layer of the epidermis. Characteristics of epithelial-to-mesenchymal transition (EMT) were documented in these dysplastic areas, including loss of basal cell polarity and cohesiveness, epithelial marker decreases, ectopic expression of the mesenchymal marker α-SMA and expression of the EMT promoter ZEB1. Taken together, these data show that a very low level of radiative stress in regenerating keratinocyte stem and precursor cells can induce a micro-environment that may constitute a favorable context for long-term carcinogenesis.

**Keywords:** human epidermis; keratinocytes; stem cells; precursor cells; low-dose γ irradiation; regeneration; dysplasia; epithelial-to-mesenchymal transition (EMT); ZEB1
