Germline Mutation in EXPH5 Implicates the Rab27B Effector Protein Slac2-b in Inherited Skin Fragility

Abstract

The Rab GTPase Rab27B and one of its effector proteins, Slac2-b (also known as EXPH5, exophilin-5), have putative roles in intracellular vesicle trafficking but their relevance to human disease is not known. By using whole-exome sequencing, we identified a homozygous frameshift mutation in EXPH5 in three siblings with inherited skin fragility born to consanguineous Iraqi parents. All three individuals harbor the mutation c.5786delC (p.Pro1929Leufs(∗)8) in EXPH5, which truncates the 1,989 amino acid Slac2-b protein by 52 residues. The clinical features comprised generalized scale-crusts and occasional blisters, mostly induced by trauma, as well as mild diffuse pigmentary mottling on the trunk and proximal limbs. There was no increased bleeding tendency, no neurologic abnormalities, and no increased incidence of infection. Analysis of an affected person's skin showed loss of Slac2-b immunostaining (C-terminal antibody), disruption of keratinocyte adhesion within the lower epidermis, and an increased number of perinuclear vesicles. A role for Slac2-b in keratinocyte biology was supported by findings of cytoskeletal disruption (mainly keratin intermediate filaments) and decreased keratinocyte adhesion in both keratinocytes from an affected subject and after shRNA knockdown of Slac2-b in normal keratinocytes. Slac2-b was also shown to colocalize with Rab27B and β4 integrin to early adhesion initiation sites in spreading normal keratinocytes. Collectively, our findings identify an unexpected role for Slac2-b in inherited skin fragility and expand the clinical spectrum of human disorders of GTPase effector proteins.

CALML5 is a ZNF750- and TINCR-induced protein that binds stratifin to regulate epidermal differentiation

“…Interestingly, some of these proteins displayed strongly altered spectral counts in the CALML5-depleted state, suggesting that their interactions with SFN may be modulated by CALML5. A number of these proteins have known roles in skin disease, such as EXPH5 (McGrath et al 2012). We verified these results by targeted BioID IP of EXPH5, which confirmed an interaction with SFN that was diminished by 44% in CALML5-depleted cells (Supplemental Fig.…”

Molecular Heterogeneity of Epidermolysis Bullosa Simplex: Contribution of EXPH5 Mutations

“…As a consequence of the deletions, frameshifts in codons 466 and 966 occurred and created a premature termination of translation after 44 and 11 amino acids, respectively. The mutations are located within the large exon 6 of EXPH5, upstream of the previously reported mutation, c.5768delC (Figure 2b; McGrath et al, 2012). To the best of our knowledge, the two variants have not been reported before (HGMD Professional 2013.1).…”

“…Well-defined subtypes of EBS are associated with typical clinical features and molecular pathology, but patients with mild skin fragility often exhibit less characteristic features rendering the determination of the candidate gene difficult (Groves et al, 2010;Liu et al, 2012;Pigors et al, 2012;Kiritsi et al, 2013). Recently, a homozygous frameshift mutation in the exophilin-5 gene (EXPH5) was identified in three individuals of a consanguineous Iraqi family with mild skin fragility (McGrath et al, 2012). EXPH5 encodes the Rab27 effector protein called synaptotagminlike protein lacking c2 domains b (Slac2b).…”

Rab27a GTPase modulates L-type Ca2+ channel function via interaction with the II–III linker of CaV1.3 subunit

“…Using homology domain analysis, we identified a putative binding domain for Rab27a in the Ca V 1.3 subunit: an amino acid sequence homologous to the Exophilin-coil domain of MyRIP (aka Exophilin8) in the II-III linker of the Ca V 1.3 subunit. Exophilin-coil domains are conserved coiled-coil sequences, which are considered to be a putative Rab-binding site of Rab effectors [56][57][58][59]. In order to prove that this MyRIP homology sequence possibly represents a binding site for Rab27a, we generated a mutant Ca V 1.3Δ2601-2673 that lacks this Exophilin-coil-homolog sequence.…”