New proteins involved in Spinocerebellar Ataxia provide insight into selective neuronal vulnerability

Neurodegenerative disorders like Alzheimer’s and Parkinson’s disease result from the loss of specific types of neurons due to abnormal accumulation of mutant proteins. Although specific brain regions have been found to be particularly vulnerable in each of these disorders, intriguingly, the disease-driving genes and the proteins they encode are usually broadly expressed and are known to function throughout the brain. The causes and exact mechanisms underlying this differential vulnerability of brain cells and regions to toxic mutant proteins are not well understood. A recent study from the lab of Dr. Huda Zoghbi, distinguished service professor at Baylor College of Medicine and founding director of the Jan and Dan Duncan Neurological Research Institute (Duncan NRI) at Texas Children’s Hospital addressed this question in the context of spinocerebellar ataxia type 1 (SCA 1) and uncovered the diversity of molecular players and pathways that contribute to the pathogenesis of this neurodegenerative disorder.

The discovery, published in Neuron, charts an investigative path for a better understanding of regional vulnerability in other neurodegenerative disorders.

SCA1 is a progressive neurodegenerative disorder that affects about one or two in 100,000 people worldwide and is primarily characterized by loss of motor coordination and balance. It is a monogenic disorder caused by the presence of expanded stretches of three DNA bases (CAG) in the ATXN1 gene. This results in the mutant ATXN1 protein having long uninterrupted stretches of polyglutamine (polyQ) repeats. In healthy neurons, ATXN1 interacts with Capicua (CIC), a protein that represses the expression of several genes. When polyQ-expanded mutant ATXN1 binds to CIC, the activity resulting from this interaction becomes enhanced leading to hyper-repression of CIC target genes which is eventually toxic to neurons. Prior research has demonstrated that all changes in the cerebellar Purkinje cells in SCA1 mouse models result from the enhanced activity of the ATXN1-CIC complex.

However, ATXN1 protein is widely expressed and functions in various regions of the brain. Notably, the presence of several non-cerebellar symptoms such as learning and memory deficits in SCA1 patients and animal models suggests that other brain regions and neurons besides cerebellar Purkinje cells are likely affected by the polyQ expansion of ATXN1.

“A key question that has surprisingly remained unexplored is whether CIC drives ATXN1’s toxicity in other brain regions or whether other molecular players drive toxicity in other affected neurons,” Dr. Huda Zoghbi, who is also a Howard Hughes Medical Institute investigator, said.

To address this question, Stephanie Coffin, a graduate student in the Zoghbi lab generated a new SCA1 mouse model in which the endogenous Atxn1 gene has an expanded polyglutamine stretch (to mimic the human mutation and its effect throughout the brain) but she also mutated the two amino acids that are critical for the ATXN1-CIC interaction.

The Zoghbi team found that these two mutations fully ablated the ATXN1-CIC interaction in all brain regions. “However, disruption of the ATXN1-CIC complex led to partial improvements in SCA1 neurological phenotypes like motor incoordination, respiration, and short lifespan and only normalized a subset of the gene expression changes,” said Dr. Stephanie Coffin. “Together, these findings gave us a clue that CIC is likely not the sole ATXN1 interactor that is driving SCA1 pathogenesis and that additional molecular players may be contributing to the development and progression of the variety of neurological symptoms seen in SCA1 patients and animal models.”

To identify additional interactors of ATXN1, they performed an unbiased proteomics screen using immunoprecipitation and mass spectrometry.

“Given that the transcriptional changes are a hallmark of SCA1 pathogenesis, we focused on three transcriptional factors that are expressed in various regions of the human and mouse brains – Zinc finger with KRAB and SCAN domains 1 (ZKSCAN1), Zinc finger and BTB domain containing 5 (ZBTB5) and Regulatory factor X1 (RFX1),” added Coffin, who is currently a Neuroscience Program Manager at Pelagos Pharma. “We validated the interaction between Atxn1 and these newly identified partners and discovered that the expression of RFX1 and ZKSCAN1 target genes was altered in SCA1 mice and human iNeurons. Moreover, these two newly identified partners together with CIC are predicted to regulate about 33% of the genes whose expression is altered in SCA1 mouse models, highlighting the important role they play in the pathogenesis of this disorder.”

“We were quite surprised to discover that for this single gene disorder, the mutant protein uses distinct partners to drive toxicity in different brain cells,” Zoghbi said. “In fact, this study underscores the importance of investigating partners of other disease-driving proteins not only for other polyglutamine diseases but broadly, for all neurodegenerative disorders.” It is only through such systematic studies that we can understand the mechanisms driving disease pathogenesis and be in a better position to explore therapeutic interventions.”

Others involved in the study are Mark A. Durham, Larissa Nitschke, Eder Xhako, Amanda M. Brown, Jean-Pierre Revelli, Esmeralda Villavicencio Gonzalez, Tao Lin, Hillary P. Handler, Yanwan Dai, Alexander J. Trostle, Ying-Wooi Wan, Zhandong Liu, Roy V. Sillitoe, and Harry T. Orr. They are affiliated with one or the following institutions: Baylor College of Medicine, the Jan and Dan Duncan Neurological Research Institute at Texas Children’s Hospital, the University of Minnesota, and the Howard Hughes Medical Institute. The study was funded by several grants from the National Institutes of Health and IDDRC, the JPB Foundation, the Howard Hughes Medical Institute, and the Cancer Prevention and Research Institute of Texas.