The Genetic and Functional Role of ABCA12 in Harlequin Ichthyosis

Abstract

Harlequin Ichthyosis (HI) is the most severe disorder in the family of autosomal recessive congenital ichthyosis (ARCI). Recessive mutations in the ABC transporter ABCA12 were found to be causative in HI. Loss of ABCA12 leads to defective transport of ceramides, impaired transportation of proteases, premature terminal differentiation, transepidermal water loss (TEWL) and the retention of squames, which leads to the characteristic clinical phenotype of thickened hyperkeratotic plates overlying the skin. This thesis focuses on the genetic and functional role of ABCA12 in the skin and how loss of ABCA12 leads to the HI epidermis. Several molecular genetic techniques were utilised to ascertain and evaluate possible causative mutations for HI in the gene ABCA12. The identification of novel and known, point and complex mutations give molecular diagnostics a greater reservoir to help identify possible causative mutations in future cases. The spectrum of ABCA12 mutations, which have been attributed to different diseases within ARCI, was discussed. A HI immortalised cell line was derived from a patient with a compound heterozygote mutation in ABCA12, with a nonsense p.Glu2264X and c.5382-2a/g, a complex splice site mutation of the putative acceptor site (AG) preceding exon 35. Further investigations assessed the use of low affinity acceptor sites by the spliceosome. Immunohistochemistry of the patient biopsy showed an absence of ABCA12 and a reduction of ABCA1 proteins. Knockdown of ABCA12 in control K17 immortalised keratinocytes decreased expression levels of ABCA1 and LXRβ proteins. Functional studies of the HI patient derived cell line validated a HI cellular phenotype in monolayer. The expression levels of involucrin, TGM1 and K10 proteins showed how the patient derived HI cell line under calcium shift initiated premature terminal differentiation in vitro, compared to controls. The activation of nuclear hormone receptors PPARβ/δ and LXRβ by the application of their agonists on the HI cell line increased the transcript and protein levels of the reduced ABCA1. The same agonists reduced specific markers of differentiation, which were activated prematurely in the HI cell line under calcium shift, suggesting a potential therapeutic strategy to explore.