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 Lamc2jeb 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 Lamc2jeb 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 Lamc2jeb 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.

大疱性表皮松解症（Epidermolysis Bullosa, EB）是一类由罕见突变引发的机械性大疱性疾病，其病理特征为皮肤与黏膜结构脆弱。尽管突变基因与现有突变类型可在较大程度上预测疾病的预期表现范围，但除病情最为严重的病例外，其余病例仍存在大量无法解释的表型异质性。这种难以解释的表型差异提示存在遗传修饰效应的可能性。
我们借助一种小鼠模型验证了这一假说：该模型可重现因层粘连蛋白γ2（Laminin gamma 2, Lamc2）功能减退型jeb等位基因所引发的非赫利茨型交界性大疱性表皮松解症（Junctional EB, JEB）。通过调控原本呈无症状状态的遗传背景，我们证实了遗传修饰因子对Lamc2jeb突变背景下真皮-表皮黏附强度以及JEB临床严重程度的显著影响。
我们采用结合数量性状位点（QTL）定位与位置克隆的无偏遗传研究策略，证实胶原XVIIα1链（Col17a1）是Lamc2jeb小鼠中非赫利茨型JEB的强效遗传修饰因子。该修饰因子由胶原XVII蛋白非胶原4结构域（non-collagenous 4 domain）中1~3个相邻氨基酸的变异所决定。这些等位基因变异可在Lamc2jeb突变的背景下改变真皮-表皮黏附强度，进而广泛影响JEB的临床严重程度。
综上，本研究结果阐明了原本无害的等位基因变异如何通过与致病突变发生上位性互作，进而影响罕见遗传性机械性大疱性疾病的严重程度。