A Missense Variant Affecting the N‐Terminal Domain of the Laminin‐332 β3 Chain Results in a Distinct Form of Junctional Epidermolysis Bullosa With Altered Granulation Tissue Response and No New Blistering: A Second Family Report

ABSTRACT

Junctional epidermolysis bullosa (JEB) is a rare genodermatosis characterized by fragility of the skin and mucous membranes due to alterations in the dermal epidermal junction. This condition manifests as mechanically induced bullous lesions that heal with hypertrophic granulation tissue and/or atrophic scars. Here, we report two brothers carrying a homozygous LAMB3 missense variant, p.Gly254Asp, which affects the N‐terminal end of the laminin‐332 (LM332) β3 chain, previously described in another JEB family sharing a common ethnic origin and LAMB3 haplotype with the siblings reported here. Moreover, all affected patients with p.Gly254Asp mutation from both families exhibits a distinct phenotype consisting of a few localized long‐standing skin lesions characterized by excessive granulation tissue formation or keloid scars, without new blistering, and associated with amelogenesis imperfecta. Our patients also showed nail dystrophy, expanding the phenotypic spectrum and confirming the peculiar role of the N‐terminal end of the β3 chain in regulating the granulation tissue response associated with the wound healing process.

To the editor:

Junctional epidermolysis bullosa (JEB) is a genetically heterogeneous skin disorder characterized by alterations of the dermal‐epidermal junction [1]. Laminin‐332 (LM332) is a component of this junction, linking the ventral surface of basal keratinocytes to the basement membrane (BM) through specific interactions with integrin α6β4 in the hemidesmosomes and α3β1 in the focal adhesions [1]. LM332 also modulates cell behavior by transmitting information to keratinocytes. Variants of LAMA3, LAMB3, and LAMC2 genes, encoding, respectively, the three constituting chains (α3, β3, and γ2) of LM332, generally result in mechanically induced skin and mucosal blistering. The phenotypic spectrum of JEB ranges from the most severe forms, typically caused by the complete absence of LM332 expression to the mildest ones attributable to missense variants, allowing the expression of an altered but functional protein product (Table S1). As an example of this genotype–phenotype correlation, we report the case of two Tunisian brothers (Figure 1A), carrying the homozygous missense variant in LAMB3 (OMIM*150310), chr1[GRCh37]:209805989 NM_000228:exon8:c.761G>A:p.Gly254Asp (Figure 1B, identified by whole‐exome sequencing). Both exhibit a mild JEB phenotype with partial tooth agenesis, enamel dysplasia, acrocyanosis, nail dystrophy, and paronychia affecting multiple fingers, dystrophy of toenails and absence of nails of big toes. The patients present several chronic ulcerative lesions with prominent granulation tissue in the absence of new blister formation (Figure 2C).

FIGURE 1.

(A) Family pedigree, with the probands IV.6 and IV.8, born from consanguineous parents; (B) localization of the Gly254Asp variant in LAMB3 gene and protein. Schematic representation of the LN (laminin N terminal), LE1 (laminin EGF‐like motif 1), LCC (laminin coiled coil) domains. The homozygous nucleotide variant was detected by WES (SureSelect Focused Exome Panel; Agilent Technologies, Waldbrron, Germany); (C) location of the Gly254Asp variant, highlighted in red, in the model of LAMB3 protein structure. The thermodynamic stability of the protein carrying this variant was investigated through FoldX, a protein design algorithm that uses an empirical force field (https://foldxsuite.crg.eu/), as described by Biagini et al. [2]. A model of the wild‐type LAMB3 structure was obtained through the automated protein homology‐modeling server SwissModel (https://swissmodel.expasy.org/). The difference in free‐energy (ΔΔG) between the mutant and wild‐type protein (ΔG _mut, ΔG _wt) was assessed and resulted equal to 10.8873, indicative of a highly destabilizing effect of the variant on the protein structure; (D) structure of LM332: A trimeric molecule shaped like a cross, with the long arm formed by the twisted C‐terminal ends of the α, β, and γ chains, and three short arms consisting of their N‐terminal ends.

FIGURE 2.

Clinical features of the patients showing: (A) absence of several permanent teeth, tooth hypoplasia, periodontitis, and severe enamel hypo/dysplasia; (B) onychodystrophy with paronychia; (C) prominent granulation tissue in a chronic ulcerative lesion of groin; (D) toenail dystrophy and bilateral absence of the great toenails.

The variant was inherited from both parents, who are consanguineous and healthy, apart from dental anomalies (Figure S1). This is the second Tunisian family reported with this variant. The pairwise proportions of Identity By Descent (IBD) between the two brothers and the previous reported patient [3] calculated with PLINK (www.cog‐genomics.org/plink/1.9/) revealed a PI_HAT (proportion IBD) of 0.9809 and 0.9723, respectively, suggesting a common ancestor.

In the first report, El‐Hachem et al. [3] hypothesized that this variant causes misfolding of the protein structure with the loss of a disulfide bridge. This hypothesis is supported by our protein thermodynamic assay, which predicted an extremely unstable mutated polypeptide (Figure 1C).

The N‐terminal end of β3 chain interacts with type VII collagen (COL7), which plays a critical role in wound healing, required for both re‐epithelialization and dermal granulation tissue development [4]. Interestingly, autoantibodies against COL7 are considered causative of epidermolysis bullosa acquisita (EBA) [5]. Moreover, variants of LAMA3A mapping to the 3′N‐terminal end of LM332 α‐chain cause laryngo‐onycho‐cutaneous syndrome, characterized by localized wounds mainly affecting the eyes, larynx, and nails, with excessive production of granulation tissue, in the absence of blistering (OMIM #245660).

In conclusion, the newly reported family confirmed the association of the homozygous p.Gly254Asp variant with a distinct form of JEB, characterized by the absence of new blistering and skin lesions mainly localized in the perianal region, delayed wound healing and dental anomalies. It also expands the phenotypic spectrum to include nail involvement. Moreover, it corroborates that variants in the N‐terminal ends of the α3 and β3 subunits of LM332 are causative for the wound healing response, producing similar but differently located skin lesions.

Ethics Statement

The family herein described gave a written consent to participate in this study. All procedures were performed as part of the diagnostic evaluation of the patients. The analyses were carried out in accordance with the ethical standards of our Institution and with the 1964 Helsinki declaration and its later amendments.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Data S1.

Figure S1.

Figure S2.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.