Clinical and functional characterization of a novel STUB1 frameshift mutation in autosomal dominant spinocerebellar ataxia type 48 (SCA48).

BACKGROUND: Heterozygous pathogenic variants in STUB1 are implicated in autosomal dominant spinocerebellar ataxia type 48 (SCA48), which is a rare familial ataxia disorder. We investigated the clinical, genetic and functional characteristics of STUB1 mutations identified from a Taiwanese ataxia cohort. METHODS: We performed whole genome sequencing in a genetically undiagnosed family with an autosomal dominant ataxia syndrome. Further Sanger sequencing of all exons and intron-exon boundary junctions of STUB1 in 249 unrelated patients with cerebellar ataxia was performed. The pathogenicity of the identified novel STUB1 variant was investigated. RESULTS: We identified a novel heterozygous frameshift variant, c.832del (p.Glu278fs), in STUB1 in two patients from the same family. This rare mutation is located in the U-box of the carboxyl terminus of the Hsc70-interacting protein (CHIP) protein, which is encoded by STUB1. Further in vitro experiments demonstrated that this novel heterozygous STUB1 frameshift variant impairs the CHIP protein's activity and its interaction with the E2 ubiquitin ligase, UbE2D1, leading to neuronal accumulation of tau and α-synuclein, caspase-3 activation, and promoting cellular apoptosis through a dominant-negative pathogenic effect. The in vivo study revealed the influence of the CHIP expression level on the differentiation and migration of cerebellar granule neuron progenitors during cerebellar development. CONCLUSIONS: Our findings provide clinical, genetic, and a mechanistic insight linking the novel heterozygous STUB1 frameshift mutation at the highly conserved U-box domain of CHIP as the cause of autosomal dominant SCA48. Our results further stress the importance of CHIP activity in neuronal protein homeostasis and cerebellar functions.

Amyloid ß-42 induces neuronal apoptosis by targeting mitochondria.

Alzheimer's disease (AD), with a typical pathological hallmark of amyloid­beta (Aß)­containing plaques and neurofibrillary tangles, is one of the most common types of chronic neurodegenerative diseases. Aß oligomers serve a crucial role in the pathogenesis of AD, and lead to neuronal loss. However, the precise mechanism of Aß oligomers in AD remains to be elucidated. The present study demonstrated that 10 µM Aß­42 activated the caspase signaling pathway, and induced significant apoptosis in primary cultured mouse cerebral cortical neurons. The results of reverse transcription­quantitative polymerase chain reaction and western blotting demonstrated that Aß­42 (10 µM) also significantly upregulated the transcription and expression of the mitochondrial fission protein dynamin­related protein 1 (Drp1), and downregulated the transcription and expression of mitochondrial fusion proteins, including mitofusin 1/2 (Mfn1/2) and mitochondrial dynamin like GTPase (OPA­1). Neurons were transfected with pDsRed2­Mito for mitochondrial imaging, which revealed that 10 µM Aß­42 induced mitochondrial fission in cortical neurons. In addition, 2',7'­dichlorodihydrofluorescein diacetate and tetramethylrhodamine ethyl ester staining indicated that Aß­42 increased the reactive oxygen species (ROS) level and reduced mitochondrial membrane potential in neurons. Inhibition of Drp1 activity by Mdivi­1 efficiently prevented Aß­42­induced ROS production and disruption of mitochondrial membrane potential. Loss of mitochondrial membrane potential may activate PTEN­induced putative kinase 1 (Pink1), the prominent sensor for mitochondrial damage, and trigger the process of mitophagy to remove the damaged mitochondria. In the present study, western blotting revealed that the levels of autophagy marker microtubule­associated proteins 1A/1B light chain 3B (LC3B) and Pink1 were upregulated after Aß­42 stimulation. In conclusion, these data indicated that Aß­42 induces neuronal apoptosis by targeting mitochondria, including promotion of mitochondrial fission, disruption of mitochondrial membrane potential, increasing intracellular ROS level and activation of the process of mitophagy. Therefore, mitochondria may represent a potential therapeutic target for AD in the future.