Loss-of-Function FERMT1 Mutations in Kindler Syndrome Implicate a Role for Fermitin Family Homolog-1 in Integrin Activation

Abstract

Kindler syndrome is an autosomal recessive disorder characterized by skin atrophy and blistering. It results from loss-of-function mutations in the FERMT1 gene encoding the focal adhesion protein, fermitin family homolog-1. How and why deficiency of fermitin family homolog-1 results in skin atrophy and blistering are unclear. In this study, we investigated the epidermal basement membrane and keratinocyte biology abnormalities in Kindler syndrome. We identified altered distribution of several basement membrane proteins, including types IV, VII, and XVII collagens and laminin-332 in Kindler syndrome skin. In addition, reduced immunolabeling intensity of epidermal cell markers such as ␤1 and ␣6 integrins and cytokeratin 15 was noted. At the cellular level, there was loss of ␤4 integrin immunolocalization and random distribution of laminin-332 in Kindler syndrome keratinocytes. Of note, active ␤1 integrin was reduced but overexpression of fermitin family homolog-1 restored integrin activation and partially rescued the Kindler syndrome cellular phenotype. This study provides evidence that fermitin family homolog-1 is implicated in integrin activation and demonstrates that lack of this protein leads to pathological changes be- Kindler syndrome (KS; OMIM 173650) is a rare autosomal recessive disorder characterized by trauma-induced skin blistering, skin atrophy, and poikiloderma.1 KS results from pathogenic mutations in the FERMT1 (formerly KIND1 or C20orf42) gene that encodes fermitin family homolog-1 (FFH1) (formerly kindlin-1 or kindlerin), an actin cytoskeleton and focal adhesion-associated molecule.2,3 FFH1 is mainly expressed in basal keratinocytes. [3][4][5][6] In KS, however, there is often a complete absence of FFH1 protein expression, although some variability may occur. 6 Thus far, 37 different loss-of-function FERMT1 mutations have been identified. 7-9 The mechanism by which these pathogenic FERMT1 mutations result in skin atrophy and blistering, however, remains unclear. Indeed, insight from studies on KS skin and FFH1 has been limited. Ultrastructural and immunohistochemical studies on KS skin have revealed a disrupted and reduplicated cutaneous basement membrane, 4,10 -16 and silencing of FFH1 in HaCaT cells results in reduced cell adhesion, proliferation, and spreading. 17 In addition, cultured KS keratinocytes display decreased cell adhesion and proliferation and exhibit multiple cell polarities

The kindlin family: functions, signaling properties and implications for human disease

“…The first symptoms of KS manifest during infancy and are characterized by trauma-induced skin blistering, which is due to the impaired integrin function in basal keratinocytes (Fassihi et al, 2005;Lai-Cheong et al, 2009;Penagos et al, 2004) (Fig. 2A).…”

“…Intriguingly, colony-forming efficiency assays of serially cultured primary keratinocytes that had been isolated from KS patients indicated accelerated depletion and premature senescence of SCs, suggesting that, similar to those mice that lack kindlin-1 in keratinocytes (Rognoni et al, 2014), the enhanced SC proliferation eventually leads to SC exhaustion, which is accompanied by a loss of SC-marker expression in the patient skin (Lai-Cheong et al, 2009;Piccinni et al, 2013). Two main defects have been identified in the KS mouse model that contribute to the hyperproliferation of Box 1.…”

Expression of exon-8-skipped kindlin-1 does not compensate for defects of Kindler syndrome

“…5a). Total and activated b1 integrin was assessed as described previously [9]. Total surface b1 integrin detected by the mouse monoclonal anti-b1 integrin Ab (4B7R) was similar in the cells transfected with KIND1wt (Fig.…”

“…The KIND1 gene is the human homolog of the Caenorhabditis elegans gene, unc-112, which encodes a membrane-associated structural/signaling protein that has been implicated in linking the actin cytoskeleton to the extracellular matrix (ECM) [7,8]. Kindlin-1 deficiency is associated with cutaneous basement membrane zone abnormalities and reduced integrin activation [9]. Also, kindlin-1 is necessary for lamellipodia formation in vitro, which is mediated by RhoGTPase signaling [10].…”

Kindlin-1 Enhances Axon Growth on Inhibitory Chondroitin Sulfate Proteoglycans and Promotes Sensory Axon Regeneration

“…Previous studies have shown that kindlin-1 overexpression in keratinocytes obtained from Kindler syndrome patients restores the normal cellular phenotype by enhancing integrin activation (Lai-Cheong et al, 2009). We therefore examined the effects of kindlin expression on integrin activation and signaling in cultured adult sensory neurons.…”