Altered keratinocyte differentiation is an early driver of keratin mutation-based palmoplantar keratoderma

Abstract

The type I intermediate filament keratin 16 (KRT16 gene; K16 protein) is constitutively expressed in ectoderm-derived appendages and in palmar/plantar epidermis and is robustly induced when the epidermis experiences chemical, mechanical or environmental stress. Missense mutations at the KRT16 locus can cause pachyonychia congenita (PC, OMIM:167200) or focal non-epidermolytic palmoplantar keratoderma (FNEPPK, OMIM:613000), which each entail painful calluses on palmar and plantar skin. Krt16-null mice develop footpad lesions that mimic PC-associated PPK, providing an opportunity to decipher its pathophysiology, and develop therapies. We report on insight gained from a genome-wide analysis of gene expression in PPK-like lesions of Krt16-null mice. Comparison of this data set with publicly available microarray data of PPK lesions from individuals with PC revealed significant synergies in gene expression profiles. Keratin 9 (Krt9/K9), the most robustly expressed gene in differentiating volar keratinocytes, is markedly downregulated in Krt16-null paw skin, well-ahead of lesion onset, and is paralleled by pleiotropic defects in terminal differentiation. Effective prevention of PPK-like lesions in Krt16-null paw skin (via topical delivery of the Nrf2 inducer sulforaphane) involves the stimulation of Krt9 expression. These findings highlight a role for defective terminal differentiation and loss of Krt9/K9 expression as additional drivers of PC-associated PPK and highlight restoration of KRT9 expression as a worthy target for therapy. Further, we report on the novel observation that keratin 16 can localize to the nucleus of epithelial cells, implying a potential nuclear function that may be relevant to PC and FNEPPK.

Pathophysiology of pachyonychia congenita‐associated palmoplantar keratoderma: new insights into skin epithelial homeostasis and avenues for treatment

“…Krt9 occurs exclusively in differentiating keratinocytes of volar skin and represents a predominant marker gene in this setting . In contrast to Krt9 , several differentiation markers appear to be upregulated in Krt16 null footpad skin, potentially as a compensatory mechanism . While this partial defect in terminal differentiation is currently unexplained, it occurs independent and ahead of the oxidative stress phenotype observed at a later stage of progression of PPK‐like lesions in this mouse model …”

“…In 2‐week‐old Krt16 null mice, corresponding to the ‘pre‐PPK stage’, footpad skin keratinocytes exhibit defects in selective aspects of terminal differentiation. At this early time point there are minimal alterations to the skin tissue histology but, already, a dramatic loss of K9 ( Krt9 /K9) expression has occurred, which then persists throughout lesion progression . Krt9 occurs exclusively in differentiating keratinocytes of volar skin and represents a predominant marker gene in this setting .…”

“…The availability of genome‐wide surveys of gene expression from both Krt16 null footpad skin lesions and PPK lesions from individuals with PC has provided an excellent opportunity to further test the strengths and limitations of the Krt16 null mouse as a valid model for PC‐based PPK. Merging the human PPK datasets with the murine Krt16 null footpad lesions dataset, based on human–mouse orthologous gene pairings, enabled multiple computational analyses . Pairwise comparisons of global transcriptional changes in Krt16 null footpad lesions and individual PC cases (three KRT6 cases, three KRT16 cases) generated statistically significant positive correlation values in all cases (Fig.…”

“…Correlation of transcriptional changes between Krt16 null footpad lesions and human PC ‐based PPK lesions. (a,b) Correlation coefficients ( R ) calculated from pairwise comparisons of microarray data from Krt16 null footpad lesions and human PC ‐based PPK lesions resulting from mutations in KRT 6 (a) or KRT 16 (b). (c) Correlation coefficients calculated from pairwise comparisons between individual cases of PC ‐based PPK with either KRT 6 or KRT 16 mutations.…”

A role for keratins in supporting mitochondrial organization and function in skin keratinocytes

“…Keratin proteins play a multifaceted role in keratinocyte homeostasis, and mutations in keratin genes lead to a diverse range of phenotypic outcomes. The K6/K16 keratin pairing, which is robustly wound-inducible, supports and promotes a number of cellular functions including structural integrity [17,24,39], cell migration [8,40], keratinocyte differentiation [41], regulation of innate immunity [7] and redox homeostasis [12,42]. Disruption of many of these cellular functions is poised to play a role in the pathophysiology of PC, in particular, in oral and palmoplantar keratoderma lesions [43].…”