Epidermolysis bullosa (EB) is a genotypically heterogeneous group of disorders characterized by cutaneous blistering and erosions with a tremendous spectrum of severity. One of the distinct forms of EB, Kindler EB (KEB), manifests with blistering and poikiloderma; this subtype of EB is caused by mutations in the FERMT1 gene encoding kindlin-1. In this study, we investigated a patient clinically diagnosed as KEB with reduced FERMT1 gene expression and intensity of immunostaining for kindlin-1. Transmission electron microscopy showed lamina densa reduplication, frequently observed in KEB. However, no mutations were identified in FERMT1 in this patient with consanguineous parents, and this gene resided outside of genomic regions of homozygosity (ROH). Instead, whole-exome sequencing and homozygosity mapping identified a homozygous sequence variant at the +4 position of intron 2 in the USB1 gene, encoding an exoribonuclease required for processing of U6 snRNA, a critical component of spliceosomes. Examination of the patient's RNA by RNA-Seq confirmed the pathogenicity of this variant, causing aberrant splicing predicted to result in loss of function of USB1. Mutations in this gene have been reported in patients with poikiloderma and neutropenia, with a few reported cases in association with skin fragility, a condition distinct from the KEB phenotype. Transcriptome analysis revealed that several genes, expressed in the cutaneous basement membrane zone and previously associated with different subtypes of EB, were differentially downregulated at the mRNA level. EB-associated mRNA downregulation was confirmed at protein levels by skin immunofluorescence. These observations provide a novel mechanism for blistering and erosions in the skin as a result reduced presence of adhesion complexes critical for stable association of epidermis and dermis at the level of cutaneous basement membrane zone.

大疱性表皮松解症 (EB) 是一组基因型异质性疾病，其特征是皮肤起泡和糜烂，严重程度非常严重。EB 的一种不同形式，金德勒 EB (KEB)，表现为起泡和皮肤异色病；这种EB亚型是由编码kindlin-1的FERMT1基因突变引起的。在这项研究中，我们调查了一名临床诊断为 KEB 的患者，其FERMT1基因表达和 kindlin-1 免疫染色强度降低。透射电子显微镜显示致密层重复，在 KEB 中经常观察到。然而，在该具有近亲父母的患者中，在FERMT1中未发现突变，并且该基因位于纯合子基因组区域 (ROH) 之外. 相反，全外显子组测序和纯合性作图在USB1基因内含子 2 的 +4 位置鉴定出纯合序列变体，编码处理 U6 snRNA（剪接体的关键组成部分）所需的外切核糖核酸酶。通过 RNA-Seq 检查患者的 RNA 证实了这种变异的致病性，导致异常剪接，预计会导致USB1 功能丧失。在皮肤异色症和中性粒细胞减少症患者中已报告了该基因的突变，少数报道的病例与皮肤脆弱有关，这种情况与 KEB 表型不同。转录组分析显示，在皮肤基底膜区表达并先前与不同 EB 亚型相关的几个基因在 mRNA 水平上存在差异性下调。通过皮肤免疫荧光在蛋白质水平上证实了 EB 相关的 mRNA 下调。这些观察结果为皮肤起泡和糜烂提供了一种新的机制，因为粘附复合物的存在减少了对于皮肤基底膜区域水平的表皮和真皮的稳定结合至关重要。