The identification and characterisation of the causative gene mutation for keratolytic winter erythema (KWE) in South African families

Abstract

Keratolytic winter erythema (KWE) is a rare autosomal dominant skin disorder characterized by recurrent episodes of palmoplantar erythema and epidermal peeling, and symptoms worsen in winter. KWE is relatively common in South African (SA) Afrikaners and was mapped to 8p23.1-p22 through a common haplotype in SA families. The aim of this study was to identify and characterize the causal mutation for KWE in SA families. Targeted resequencing of 8p23.1-22 was performed in three families and seven unrelated controls. Reads were aligned to the reference genome using BWA. GATK and Pindel were used to call small and large structural variants, respectively. A 7.67 kb tandem duplication was identified upstream of the CTSB gene and encompassing an enhancer element that is active in a keratinocytes (based on H3K27ac data). The tandem duplication segregated completely with the KWE. The tandem duplication overlaps with a 15.93 kb tandem duplication identified in two Norwegian families at a 2.62 kb region encompassing the active enhancer suggesting that the duplication of the enhancer leads to the KWE phenotype. Existing chromatin structure, CTCF binding and chromatin interaction data from several cell lines, including keratinocytes were analysed and three potential topological subdomains were identified, all containing the enhancer and CTSB, or CTSB and FDFT1 or both genes and NEIL2. Additionally, we showed that the enhancer’s activity correlated with CTSB expression, but not with FDFT1 and NEIL2 expression in differentiating keratinocytes and other cell lines. RNA polymerase II ChIA-PET interaction data in cancer cell lines showed that the enhancer interacts with CTSB but not FDFT1 or NEIL2. These data suggest that the enhancer normally regulates CTSB expression. Relative gene expression and immunohistochemistry from palmar biopsies from South African and Norwegian participants (7 Affected and 7 Controls) showed a significantly higher expression of CTSB, but not FDFT1 and NEIL2, in affected individuals compared to the controls and that CTSB was significantly more abundant in the granular layer of affected individuals compared to controls. We conclude that the enhancer duplication causes KWE by upregulating CTSB expression and causing an overabundance of CTSB in the granular layer of the epidermis.