Haploinsufficiency of Pumilio1 leads to SCA1-like neurodegeneration by increasing wild-type Ataxin1 levels in a miRNA-independent manner. V. A. Gennarino1,2, R. Singh3, J. J. White2,3,4, K. Han1,2,5, A. De Maio2,6, P. Jafar-Nejad1,2, A. di Ronza1,2, H. Kang1,2,7, H. T. Orr8, R. V. Sillitoe2,3,4,6, H. Y. Zoghbi1,2,3,5,6,9 1) Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, Texas, 77030, USA; 2) Jan and Dan Duncan Neurological Research Institute at Texas Childrens Hospital, Houston, Texas, 77030, USA; 3) Department of Pathology and Immunology, Baylor College of Medicine, Houston, Texas, 77030, USA; 4) Department of Neuroscience, Baylor College of Medicine, Houston, Texas, 77030, USA; 5) Howard Hughes Medical Institute, Baylor College of Medicine, Houston, Texas, 77030, USA; 6) Program in Developmental Biology, Baylor College of Medicine, Houston, Texas, 77030, USA; 7) National Institute of Supercomputing and Networking, Korea Institute of Science and Technology Information, Daejeon, South Korea; 8) Institute for Translational Neuroscience, Department of Laboratory Medicine and Pathology, University of Minnesota, Minneapolis, Minnesota; 9) Department of Pediatrics, Baylor College of Medicine, Houston, Texas, 77030, USA.

   Accumulation of mutant disease-driving proteins in specific brain regions promotes different neurodegenerative disorders including Alzheimer, Parkinson, and spinocerebellar ataxias. Spinocerebellar ataxia type 1 (SCA1) is a fatal dominant neurodegenerative disease characterized by progressive loss of motor abilities primarily due to degeneration of Purkinje neurons. SCA1 is caused by expansion of an unstable CAG repeat in Ataxin1 (ATXN1). The resulting protein harbors an expanded polyQ tract rendering the protein toxic through a gain-of-function mechanism. A striking feature of ATXN1 is the extraordinarily long 3UTR (~7kb) that we hypothesized must harbor key post-transcriptional regulatory elements. We rationalized that deciphering the post-transcriptional mechanisms regulating ATXN1 levels in vivo will provide a better insight into factors that might contribute to SCA1 pathogenesis. We identified a conserved binding motif for the RNA-binding protein Pumilio1 (PUM1) in the 3UTR of ATXN1 mRNA. Using RNA-Clip we found that Pum1 physically interacts with the conserved binding site of the Atxn1-3UTR in mouse brain. It is known that PUM1 can regulate microRNA-dependent gene silencing by induction of a conformational switch in the 3UTR of its targets. However, by mutating miRNA targets sites in ATXN1 as well as knocking down AGO2, we found that PUM1 modulates ATXN1 independent of miRNA interaction. Importantly, Pum1 heterozygous (Pum1+/-) mice show an increase of both Atxn1 protein and mRNA levels by 30 to 50% in the brain and a wide range of neurological defects, such as impaired motor coordination and Purkinje cells degeneration, similar to those in SCA1 mouse model. Moreover, removing one allele of Pum1 greatly enhanced the disease progression in SCA1 mice. Interestingly, breeding Pum1+/- mice to mice lacking one wild-type allele of Atxn1 (Atxn1+/-) normalized Atxn1 levels and rescued most of the behavioral and pathological phenotypes observed in Pum1+/- mice. These data demonstrate that precise levels of Pum1 are critical for neuronal maintenance, that Atxn1 is a key target of Pum1 and mediates crucial neuropathological features of its haploinsufficiency, and that a mild increase in Atxn1 levels is detrimental to neurons. Lastly, these data underscore the potential for PUM1 and ATXN1 as candidate neurodegenerative disease genes either through haploinsufficiency or duplication, respectively, or through mutations in their regulatory regions.

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