Molecular Identification of Collagen 17a1 as a Major Genetic Modifier of Laminin Gamma 2 Mutation-Induced Junctional Epidermolysis Bullosa in Mice

Epidermolysis Bullosa (EB) encompasses a spectrum of mechanobullous disorders caused by rare mutations that result in structural weakening of the skin and mucous membranes. While gene mutated and types of mutations present are broadly predictive of the range of disease to be expected, a remarkable amount of phenotypic variability remains unaccounted for in all but the most deleterious cases. This unexplained variance raises the possibility of genetic modifier effects. We tested this hypothesis using a mouse model that recapitulates a non-Herlitz form of junctional EB (JEB) owing to the hypomorphic jeb allele of laminin gamma 2 (Lamc2). By varying normally asymptomatic background genetics, we document the potent impact of genetic modifiers on the strength of dermal-epidermal adhesion and on the clinical severity of JEB in the context of the Lamc2(jeb) mutation. Through an unbiased genetic approach involving a combination of QTL mapping and positional cloning, we demonstrate that Col17a1 is a strong genetic modifier of the non-Herlitz JEB that develops in Lamc2(jeb) mice. This modifier is defined by variations in 1-3 neighboring amino acids in the non-collagenous 4 domain of the collagen XVII protein. These allelic variants alter the strength of dermal-epidermal adhesion in the context of the Lamc2(jeb) mutation and, consequentially, broadly impact the clinical severity of JEB. Overall the results provide an explanation for how normally innocuous allelic variants can act epistatically with a disease causing mutation to impact the severity of a rare, heritable mechanobullous disorder. Author Summary Epidermolysis bullosa (EB) is a group of rare genetic Mendelian disorders that result in mechanical fragility of the skin and mucosal membranes. Junctional EB is a subset caused by mutations that result in cleavage of the dermal-epidermal junction. All forms of EB demonstrate substantial variability in their clinical phenotype that is not readily explained. The possibility of genetic modifiers as the cause of this variability has been difficult to address in humans. We apply a mouse model carrying a hypomorphic allele of the laminin gamma 2 (Lamc2) gene to address the possibility of genetic modifiers of JEB. We document the potent impact of differing genetic backgrounds on multiple facets of the JEB syndrome expressed in these mice and show that three neighboring amino acid changes within the non-collagenous domain 4 of the collagen XVII protein strongly modify their disease. The study provides a molecular explanation of how a primary mutation that weakens one component of the cutaneous basement membrane is influenced by normally innocuous allelic variants of another component to affect strength of dermal-epidermal adhesion and consequently, the severity of JEB. This approach may guide the genetic prognosis and diagnosis of human EB disorders.