Investigation of Pathogenesis of Myotonic Dystrophy Type 1 in Central Nervous System

• Main Authors: Pei-Ying Wang, 汪佩瑩
• Other Authors: Guey-Shin Wang
• Format: Others
• Language: en_US
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/82g8ad

博士 === 國立陽明大學 === 分子醫學博士學位學程 === 106 === Myotonic dystrophy (DM) is the most common cause of adult onset muscular dystrophy. Brain involvement in DM type 1 (DM1) includes mental retardation, psychiatric disorders and neurodegeneration. Cognitive impairment associated with structural change in the brain is prevalent in DM1. The occurrence of cognitive impairment in individuals with DM1 is high, however, the mechanism of causing the deficits remains unknown. The genetic basis of DM1 is caused by an expansion of CTG repeats in the 3’ untranslated region (UTR) of the Dystrophia Myotonica Protein Kinase (DMPK) gene. DMPK mRNA containing expanded CUG repeats accumulates in nuclear foci and affect nuclear and cytoplasmic activities of RNA binding protein muscleblind like (MBNL) family. Dysfunction of MBNL has been implicated in DM1 neural pathogenesis. How expanded CUG RNA and MBNL dysfunction affect cognitive function and brain structure remains elusive. To study the neural pathogenesis of DM1, we established a brain-specific DM1 mouse model, EpA960/ CaMKII-Cre, expressing the expanded CUG RNA in the postnatal brain. The EpA960/ CaMKII-Cre model recapitulates several features of DM1 brain including nuclear RNA and MBNL foci formation, misregulated alternative splicing, learning disability and neuro-degeneration. The pathological abnormality characterized by a time-course study showed that hippocampus-related learning and synaptic potentiation were impaired before structural changes, followed by progressive reduction of axon and dendrite integrity and aberrant MBNL2-regulated alternative splicing. In addition, cytoplasmic MBNL1 on dendrites decreased before dendrite degeneration suggesting reduced cytoplasmic MBNL1 as an early event response to the pathogenic RNA. We further investigated the causal mechanism of cytoplasmic MBNL1 reduction and the function of cytoplasmic MBNL1. We found that neurons expressing expanded CUG RNA and MBNL1-depleted neurons exhibited similar morphological impairment. MBNL1 cytoplasmic, but not nuclear, isoform promoted neurite outgrowth and reversed the morphological defects caused by expanded CUG RNA. The reduced cytoplasmic MBNL1 caused by expanded CUG RNA was due to MBNL1 cytoplasm-to-nucleus translocation. The cytoplasmic localization of MBNL1 was regulated by lysine 63 (K63)-linked polyubiquitination. Reduced cytoplasmic MBNL1 in the EpA960/CaMKII-Cre brain was consistent with the reduced extent of K63 ubiquitination. Expanded CUG RNA induced the deubiqutination of cytoplasmic MBNL1, which resulted in nuclear translocation andmorphological defects that could be ameliorated by inhibiting K63-linked polyubiquitin chain degradation. These results suggest that K63-linked ubiquitination of MBNL1 is required for its cytoplasmic localization and that deubiquitination of cytoplasmic MBNL1 is pathogenic in DM1 brain.