Disrupted keratinocyte differentiation and increased chemokine and antimicrobial peptide expression in palmoplantar keratoderma due to keratin 9 mutation

The clinical phenotype of palmoplantar keratoderma (PPK), due to keratin (KRT) 9 mutations, consists of debilitating diffuse hyperkeratosis on palms and soles. However, the hyperkeratoses also exhibit an erythematous periphery, indicating an accompanying hyperinflammatory response. We used single cell RNA (scRNA) sequencing of palmar skin biopsies combined with confirmatory PCR and immunofluorescence to characterize the repair response in PPK due to KRT9 mutations. After scRNA sequencing, we first analyzed keratinocyte (KC) differentiation in KRT9mut PPK compared to normal control skin. Basal KRT5 and KRT14 were upregulated in KRT9mut differentiated KCs. Suprabasal KRT1 and KRT10 were downregulated in KRT9mut in clusters of differentiated KC. KRT2 was strongly downregulated in KRT9mut cells, and KRT6 and KRT16 were strongly upregulated in KRT9mut skin. Interestingly, CALML5, encoding a calcium-binding protein, was downregulated both in KC and fibroblasts of KRT9mut palmar skin biopsies. Comparing chemokine expression, CCL2 and CXCL12 were increased in KRT9mut PPK in multiple cell types, including KC and fibroblast clusters, as compared to normal control. In contrast, CXCL1 and CCL19 were selectively decreased in fibroblasts, whereas CCR7 was decreased in KRT9mut dendritic cells as compared to control. Finally, dermcidin was upregulated in KRT9mut sweat glands and DEFB1, S100A8 and S100A9 were additionally increased in keratinocytes in KRT9mut palmar skin biopsies. Based on these data, we hypothesize that altered keratinocyte differentiation in KRT9mut skin initiates a previously unrecognized hyperinflammatory response in KRT9mut PPK. As KRT9 mutations put skin barrier function at risk, epidermal keratinocytes (KC) respond with an exaggerated, futile repair response, resulting in chronic inflammation.