Comparative genomics suggests loss of keratin K24 in three evolutionary lineages of mammals

Abstract

Keratins are the main cytoskeletal proteins of epithelial cells and changes in the expression of keratins have contributed to the evolutionary adaptation of epithelia to different environments. Keratin K24 was proposed to be a differentiation marker of epidermal keratinocytes but the significance of K24 expression in the epidermis versus other tissues has remained elusive. Here, we show by RT-PCR, western blot, and immunofluorescence analyses that K24 is highly expressed in the epithelium of the cornea whereas its expression levels are significantly lower in other stratified epithelia including in the epidermis. To investigate the evolutionary history of K24, we screened the genome sequences of vertebrates for orthologs of the human KRT24 gene. The results of this comparative genomics study suggested that KRT24 originated in a common ancestor of amniotes and that it was lost independently in three clades of mammals, i.e. camels, cetaceans, and a subclade of pinnipeds comprising eared seals and the walrus. Together, the results of this study identify K24 as component of the cytoskeleton in the human corneal epithelium and reveal previously unknown differences of keratin gene content among mammalian species.

Update of the keratin gene family: evolution, tissue-specific expression patterns, and relevance to clinical disorders

“…The type II KRT80 protein in Cnidaria (jellyfish) is apparently lost and then does not reappear until the Testudines Order (turtle). These findings are consistent with the notion that keratin genes can be lost, gained and/or repurposed [ 45 , 46 ].…”

“…For example, Cetaceans (e.g., whale, dolphin, porpoise) lack expression of KRT24, but, in its absence, the putative partners of KRT24 (i.e., KRT3 and KRT5) interact with KRT14 and KRT12. This finding indicates that keratin proteins can become dispensable in some species, while being repurposed in others [ 45 , 46 ].…”

“…Another fascinating example of a missing keratin protein is the absence of the type I keratin KRT24 in whale and walrus—a feature that is thought to play a role in the evolutionary adaptation of these species. Comparative genomics studies have suggested that KRT24 originated in a common ancestor of Amniotes (a clade of tetrapod vertebrates), but then was lost independently in three clades of mammals (i.e., camels, cetaceans, and a subclade of pinnipeds including the eared-seal and walrus) [ 45 , 46 ]. At first glance, our data (Fig.…”

Losing Genes: The Evolutionary Remodeling of Cetacea Skin

“…Hair keratins were repurposed to form baleen or are expressed in the epithelium of the tongue (Ehrlich et al, 2019a). KRT24 is expressed in several epithelia including the cornea and the oesophagus (Ehrlich et al, 2019b). It originated in a common ancestor of amniotes and was lost in three mammalian clades: camels, a subclade of pinnipeds (eared seals and the walrus) and cetaceans (Figure 1; Ehrlich et al, 2019b).…”

“…KRT24 is expressed in several epithelia including the cornea and the oesophagus (Ehrlich et al, 2019b). It originated in a common ancestor of amniotes and was lost in three mammalian clades: camels, a subclade of pinnipeds (eared seals and the walrus) and cetaceans (Figure 1; Ehrlich et al, 2019b). Krt24 remnants were found in several cetaceans, such as the bottlenose dolphin (Tursiops truncatus).…”