Alain HOVNANIAN

The skin forms a mechanical and immune barrier that is essential to the body's survival. Our team is studying several serious genetic skin disorders affecting children and adults, in which these protective functions are severely impaired. These include rare, monogenic and orphan diseases, the genes responsible for which have been identified by our laboratory: dystrophic epidermolysis bullosa, Netherton syndrome, Darier's disease and Hailey-Hailey disease, as well as common, polygenic inflammatory skin conditions such as hidradenitis suppurativa and psoriasis. Our projects aim to gain a better understanding of the molecular mechanisms involved in these diseases and to identify the factors responsible for their phenotypic variability, with a view to developing new therapeutic strategies using gene and cell therapy, protein replacement, small molecules and/or pharmacological approaches.

We are conducting an initial Phase I/II ex vivo gene therapy trial for a recessive form of EBD, using autologous grafts genetically modified with a reliable, self-inactivating (SIN) retroviral vector expressing type VII collagen (an orphan drug). This first clinical trial in humans will be essential for assessing the safety, efficacy and potential of this therapeutic approach. Other approaches developed by the team include homologous recombination using CRISPR/Cas9 in iPS cells, and exon skipping, for which we have generated a humanised COL7A1 mouse, read-through of nonsense mutations using an aminoglycoside as a read-through inducer, genetically modified fibroblasts and mesenchymal stem cells (MSCs) derived from bone marrow in a murine xenograft model using human EBDR skin equivalents. A viable mouse model of EBDR carrying a homozygous hypomorphic Col7a1 mutation was generated and studied in the laboratory. This model has provided a better understanding of the development of fibrosis and serves as an in vivo model for therapeutic approaches. Transcriptomic and proteomic studies aimed at identifying modifier genes and biological pathways involved in disease severity and in the development of squamous cell carcinomas in EBD are currently underway.

We have characterised the biological cascade that leads to skin inflammation, allergy and abnormal desquamation in the SN. The cascade involves the unopposed activities of kallikreins (KLK) 5 (KLK5), KLK7 and KLK14 following defective inhibition by LEKTI, leading to the activation of the protease-activated receptor 2 (PAR2), thymic stromal lymphopoietin (TSLP) and the production of pro-inflammatory cytokines. KLK5 is a major therapeutic target, which activates pro-KLK7 and pro-KLK14. As part of industrial and academic collaborations, we are testing highly potent and specific inhibitors of KLK5 and KLK7 in in vivo models. Biotherapeutic approaches aimed at blocking pro-inflammatory cytokines overexpressed in the NS are also under investigation. The team has developed several mouse models, including a transgenic mouse overexpressing human KLK5, a viable conditional Spink5-/- mouse that allows for systemic and/or topical treatment, and a double double-knockout mouse model (Spink5-/-xKlk5-/-), which have confirmed the roles of klk5 and klk7 in the pathogenesis of SN and are useful models for testing drugs.

We had previously identified ATP2A2 (a gene encoding the SERCA2 protein, a calcium pump in the endoplasmic reticulum (ER)) as the defective gene in Darier's disease. We have recently shown that the absence of calcium transport leads to ER stress and abnormal trafficking of intercellular adhesion molecules. We have also shown that inhibition of ER stress by pharmacological agents leads to the relocalisation of E-cadherin and desmosome components to the plasma membrane, suggesting that these agents have therapeutic potential. We have also recently identified various biological pathways involved in abnormal differentiation and inflammation of the skin, which could guide new therapeutic approaches.

The team is developing exome sequencing approaches to identify new genes associated with severe monogenic or polygenic skin diseases. These include rare keratinisation and inflammatory disorders such as Olmsted syndrome – for which we have recently identified a new defective gene – severe ichthyosiform erythroderma, and common conditions such as hidradenitis suppurativa and psoriasis. Functional studies of the identified genes are currently underway to gain a better understanding of the pathogenesis of these diseases, to elucidate the biological pathways involved, and to identify therapeutic targets.