Inactivation of PNKP by mutant ATXN3 triggers apoptosis by activating the DNA damage-response pathway in SCA3.

Spinocerebellar ataxia type 3 (SCA3), also known as Machado-Joseph disease (MJD), is an untreatable autosomal dominant neurodegenerative disease, and the most common such inherited ataxia worldwide. The mutation in SCA3 is the expansion of a polymorphic CAG tri-nucleotide repeat sequence in the C-terminal coding region of the ATXN3 gene at chromosomal locus 14q32.1. The mutant ATXN3 protein encoding expanded glutamine (polyQ) sequences interacts with multiple proteins in vivo, and is deposited as aggregates in the SCA3 brain. A large body of literature suggests that the loss of function of the native ATNX3-interacting proteins that are deposited in the polyQ aggregates contributes to cellular toxicity, systemic neurodegeneration and the pathogenic mechanism in SCA3. Nonetheless, a significant understanding of the disease etiology of SCA3, the molecular mechanism by which the polyQ expansions in the mutant ATXN3 induce neurodegeneration in SCA3 has remained elusive. In the present study, we show that the essential DNA strand break repair enzyme PNKP (polynucleotide kinase 3'-phosphatase) interacts with, and is inactivated by, the mutant ATXN3, resulting in inefficient DNA repair, persistent accumulation of DNA damage/strand breaks, and subsequent chronic activation of the DNA damage-response ataxia telangiectasia-mutated (ATM) signaling pathway in SCA3. We report that persistent accumulation of DNA damage/strand breaks and chronic activation of the serine/threonine kinase ATM and the downstream p53 and protein kinase C-d pro-apoptotic pathways trigger neuronal dysfunction and eventually neuronal death in SCA3. Either PNKP overexpression or pharmacological inhibition of ATM dramatically blocked mutant ATXN3-mediated cell death. Discovery of the mechanism by which mutant ATXN3 induces DNA damage and amplifies the pro-death signaling pathways provides a molecular basis for neurodegeneration due to PNKP inactivation in SCA3, and for the first time offers a possible approach to treatment.This study was funded by NIH grant NS073976 to TKH and a John Sealy Grant to PSS

Histone deacetylases suppress cgg repeat-induced neurodegeneration via transcriptional silencing in models of Fragile X Tremor Ataxia Syndrome

Fragile X Tremor Ataxia Syndrome (FXTAS) is a common inherited neurodegenerative disorder caused by expansion of a CGG trinucleotide repeat in the 59UTR of the fragile X syndrome (FXS) gene, FMR1. The expanded CGG repeat is thought to induce toxicity as RNA, and in FXTAS patients mRNA levels for FMR1 are markedly increased. Despite the critical role of FMR1 mRNA in disease pathogenesis, the basis for the increase in FMR1 mRNA expression is unknown. Here we show that overexpressing any of three histone deacetylases (HDACs 3, 6, or 11) suppresses CGG repeat-induced neurodegeneration in a Drosophila model of FXTAS. This suppression results from selective transcriptional repression of the CGG repeat-containing transgene. These findings led us to evaluate the acetylation state of histones at the human FMR1 locus. In patient-derived lymphoblasts and fibroblasts, we determined by chromatin immunoprecipitation that there is increased acetylation of histones at the FMR1 locus in pre-mutation carriers compared to control or FXS derived cell lines. These epigenetic changes correlate with elevated FMR1 mRNA expression in pre-mutation cell lines. Consistent with this finding, histone acetyltransferase (HAT) inhibitors repress FMR1 mRNA expression to control levels in pre-mutation carrier cell lines and extend lifespan in CGG repeat-expressing Drosophila. These findings support a disease model whereby the CGG repeat expansion in FXTAS promotes chromatin remodeling in cis, which in turn increases expression of the toxic FMR1 mRNA. Moreover, these results provide proof of principle that HAT inhibitors or HDAC activators might be used to selectively repress transcription at the FMR1 locus.open293

The Reflective Debrief: Using Students’ Placement Experiences to Enrich Understandings of Distinct Kinds of Nutrition and Dietetic Practice

The practicum is one of the most important components in health professional education. It is also one of the most challenging. The hospital setting offers situations that may be physically, mentally and emotionally demanding on students. Under the pressure of being assessed, students may be reluctant to reflect on their challenges with placement supervisors, which may leave difficult experiences unprocessed. A university-based reflection and debrief following hospital placement has the advantage of avoiding this student-supervisor dynamic and embedding the evaluation of placement clearly in the university curriculum. This format was chosen for a formal reflection and debrief session that formed part of a week-long post-placement workshop held upon return to the university for nutrition and dietetic students at Griffith University at the end of their credentialing programme. The aim of the reflective debrief was to provide a staff-facilitated opportunity for students to reflect formally and collectively on their hospital placement experiences and to explore the implications for future dietetic work. The development, implementation and evaluation of the reflective debrief session following hospital placement highlight the contributions of the critical incident approach to practicum-based learning. The reflective debrief was underpinned by principles of effective reflection for health and social care students. It was person-oriented, delivered in face-to-face mode, used verbal reflection and was facilitated by trained dietitians. The reflective debrief used a two-stage process. In the first stage, critical incidents were reflected on by participants and discussed in small groups with trained facilitators. Following this, a large, facilitated group discussion synthesised themes arising from the small group discussions and linked them to future practice implications. The process and impact of the first implementation of the reflection and debrief were evaluated by an independent observer using a mixed methods approach. The critical incidents described by students fell into four distinct themes: self-management, professional identity formation, the challenge of learning within a new environment and key features of performing dietetic work in the hospital environment. Underlying each theme was the notion that the hospital placement was a transformational experience for these students. The critical incident approach used appeared to be useful as a basis for a reflective debrief that assisted students in enhancing their placement-based learning, processing emotional experiences and providing a lens through which they can view their suitability for a hospital-based career