Monoallelic Mutations in the Translation Initiation Codon of KLHL24 Cause Skin Fragility

Abstract

The genetic basis of epidermolysis bullosa, a group of genetic disorders characterized by the mechanically induced formation of skin blisters, is largely known, but a number of cases still remain genetically unsolved. Here, we used whole-exome and targeted sequencing to identify monoallelic mutations, c.1A>G and c.2T>C, in the translation initiation codon of the gene encoding kelch-like protein 24 (KLHL24) in 14 individuals with a distinct skin-fragility phenotype and skin cleavage within basal keratinocytes. Remarkably, mutation c.1A>G occurred de novo and was recurrent in families originating from different countries. The striking similarities of the clinical features of the affected individuals point to a unique and very specific pathomechanism. We showed that mutations in the translation initiation codon of KLHL24 lead to the usage of a downstream translation initiation site with the same reading frame and formation of a truncated polypeptide. The pathobiology was examined in keratinocytes and fibroblasts of the affected individuals and via expression of mutant KLHL24, and we found mutant KLHL24 to be associated with abnormalities of intermediate filaments in keratinocytes and fibroblasts. In particular, KLHL24 mutations were associated with irregular and fragmented keratin 14. Recombinant overexpression of normal KLHL24 promoted keratin 14 degradation, whereas mutant KLHL24 showed less activity than the normal molecule. These findings identify KLHL24 mutations as a cause of skin fragility and identify a role for KLHL24 in maintaining the balance between intermediate filament stability and degradation required for skin integrity.

Mutations in KLHL24 Add to the Molecular Heterogeneity of Epidermolysis Bullosa Simplex

“…5 and in contrast to the observations made by Lin et al (2016), the intensity of keratin 14 immunolabeling was not discernibly different from control skin ( Figure S3), with just some unevenness in keratin 14 labeling within keratinocytes, as noted by He et al (2016).…”

“…With increasing age, blistering severity tends to lessen. None of the individuals in our series manifested any hair abnormalities, including the only adult (case 7, Table 1), contrary to some other published cases where adults were found to have hair loss, particularly in the scalp (Lin et al, 2016;He et al, 2016).…”

“…Lin et al (2016) discovered heterozygous mutations in the methionine start codon of KLHL24 (Kelch-like family member 24) in five unrelated individuals with EBS (China, Israel). Almost simultaneously, He et al (2016) reported a further 14 cases from 10 families (Germany, Switzerland, Finland, Qatar, Italy). KLHL24 is part of the family of >40 genes with a Kelch-like motif that forms part of a ubiquitin-ligase complex (Dhanoa et al, 2013).…”

“…Based on this cohort of cases, and also those published (Lin et al, 2016;He et al, 2016), the phenotype associated with pathogenic mutations in KLHL24, although subject to variability, tends to present with quite marked birth trauma -especially on the lower legs, with additional early blistering often involving the trunk and upper limbs.…”

“…These observations invoked a new mechanism germane to inherited skin blistering, namely dysregulation of autoubiquitination. He et al (2016) also identified truncated KLHL24 resulting from the start codon mutations and use of a downstream methionine initiation codon. These authors observed abnormal intermediate filaments in keratinocytes and fibroblasts, with evidence for irregular and fragmented keratin 14, and data to support an altered balance in the stability and degradation of this keratin (He et al, 2016).…”