Autosomal dominant mutations in LITAF are responsible for the rare demyelinating peripheral neuropathy, Charcot–Marie–Tooth disease type 1C (CMT1C). The LITAF protein is expressed in many human cell types and we have investigated the consequences of two different LITAF mutations in primary fibroblasts from CMT1C patients using confocal and electron microscopy. We observed the appearance of vacuolation/enlargement of late endocytic compartments (late endosomes and lysosomes). This vacuolation was also observed after knocking out LITAF from either control human fibroblasts or from the CMT1C patient-derived cells, consistent with it being the result of loss-of-function mutations in the CMT1C fibroblasts. The vacuolation was similar to that previously observed in fibroblasts from CMT4J patients, which have autosomal recessive mutations in FIG4. The FIG4 protein is a component of a phosphoinositide kinase complex that synthesises phosphatidylinositol 3,5-bisphosphate on the limiting membrane of late endosomes. Phosphatidylinositol 3,5-bisphosphate activates the release of lysosomal Ca2+ through the cation channel TRPML1, which is required to maintain the homeostasis of endosomes and lysosomes in mammalian cells. We observed that a small molecule activator of TRPML1, ML-SA1, was able to rescue the vacuolation phenotype of LITAF knockout, FIG4 knockout and CMT1C patient fibroblasts. Our data describe the first cellular phenotype common to two different subtypes of demyelinating CMT and are consistent with LITAF and FIG4 functioning on a common endolysosomal pathway that is required to maintain the homeostasis of late endosomes and lysosomes. Although our experiments were on human fibroblasts, they have implications for our understanding of the molecular pathogenesis and approaches to therapy in two subtypes of demyelinating Charcot–Marie–Tooth disease.

中文翻译：

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脱髓鞘夏科-玛丽-图思病两种亚型常见的功能失调的内溶酶体途径


LITAF 的常染色体显性突变导致罕见的脱髓鞘性周围神经病，夏科-玛丽-图思病 1C 型 (CMT1C)。 LITAF 蛋白在许多人类细胞类型中表达，我们使用共聚焦和电子显微镜研究了 CMT1C 患者原代成纤维细胞中两种不同 LITAF 突变的后果。我们观察到晚期内吞区室（晚期内体和溶酶体）出现空泡/扩大。在从对照人成纤维细胞或 CMT1C 患者来源的细胞中敲除 LITAF 后，也观察到这种空泡形成，这与 CMT1C 成纤维细胞功能丧失突变的结果一致。空泡形成与之前在 CMT4J 患者的成纤维细胞中观察到的类似，这些患者在Fig4中具有常染色体隐性突变。 Fig4 蛋白是磷酸肌醇激酶复合物的组成部分，该复合物在晚期内体的限制膜上合成磷脂酰肌醇 3,5-二磷酸。磷脂酰肌醇 3,5-二磷酸通过阳离子通道 TRPML1 激活溶酶体 Ca2+ 的释放，这是维持哺乳动物细胞内体和溶酶体稳态所必需的。我们观察到 TRPML1 的小分子激活剂 ML-SA1 能够挽救 LITAF 敲除、FIG4 敲除和 CMT1C 患者成纤维细胞的空泡表型。我们的数据描述了脱髓鞘性 CMT 的两种不同亚型所共有的第一个细胞表型，并且与 LITAF 和 Fig4 在共同的内溶酶体途径上发挥作用一致，该途径是维持晚期内体和溶酶体稳态所需的。 尽管我们的实验是在人类成纤维细胞上进行的，但它们对我们理解脱髓鞘夏科-玛丽-图思病两种亚型的分子发病机制和治疗方法具有重要意义。