Abstract: Xeroderma pigmentosum group C (XP-C) is a rare autosomal recessive genodermatosis stemming from mutations in XPC, the damage-recognition factor of global genome nucleotide excision repair. Patients exhibit extreme photosensitivity and a drastically increased susceptibility to skin malignancies. Pigmentary abnormalities, such as lentigines and hyperpigmentation of sun-exposed skin, remain largely underexplained, although they are the earliest clinical signs of the disease. Existing XP-C models have focused on DNA repair and carcinogenesis, while the pigmentary aspect of the disease has received little attention. This project was designed to characterize that aspect, from the melanocyte itself to its epidermal microenvironment. Our work examined the molecular consequences of XPC loss in melanocytes through quantitative proteomics (LC-MS/MS) of wild-type and previously established XPC-knockout (XPC-KO) human melanocytes, in the presence and the absence of UVB exposure. Unsupervised clustering, functional enrichment and protein interaction network analyses converged on a constitutive type I interferon signal in XPC-KO melanocytes and confirmed by immunoblotting. While wild-type melanocytes reached comparable levels only after UVB exposure, XPC-deficient melanocytes carry, in the absence of any stress, an interferon response that normal cells mount only under genotoxic stress. Pharmacological JAK inhibition modified neither the UVB sensitivity nor the photoproduct repair defect of XPC-KO melanocytes, suggesting that this inflammatory phenotype accompanies rather than causes the DNA repair phenotype. To study pigmentation in a physiologically relevant context, we then developed a pigmented full-thickness reconstructed human skin model in which melanocytes synthesize melanin and transfer it to the epidermal layers without UV. We then implemented the pigmented epidermal model to explore the impact of XPC-deficient microenvironment on the overall pigmentary phenotype. Combining XPC-KO or wild-type keratinocytes with the same pigmented melanocytes made it possible to detect the contribution of the XP-C epidermal microenvironment. XP-C keratinocytes were sufficient to induce a marked hyperpigmentation of wild-type melanocytes, with increased melanin content and tyrosinase activity. This suggests that the distinct pigmentary feature of XP-C is not exclusively melanocyte-autonomous but is driven, at least in part, by the keratinocyte microenvironment. Treatment of the epidermal equivalents with JAK inhibitors or with N-acetylcysteine partially attenuated this hyperpigmentation, pointing to an inflammatory component that contributes to, without fully accounting for, the phenotype.
Together, these results identify a constitutive type I interferon signature as a hallmark of XPC deficiency in melanocytes and reveal the XP-C keratinocyte microenvironment as a trigger of hyperpigmentation with an inflammatory arm. Beyond XP-C, the naturally pigmented skin model developed during this work provides a platform for studying keratinocyte–melanocyte crosstalk in pigmentary disorders. This work extends the understanding of XP-C beyond DNA repair and identifies the epidermal microenvironment as a potential target to modulate the pigmentary manifestations of the disease. ​

​​Direction
Walid Rachidi, Eric Sulpice​