Project 1: “Uncovering novel genetic variants linked to progression of tauopathy in PSP and CBD”
PSP and CBD are Parkinsonian disorders with predominant tau pathology at autopsy. A common non-recombining MAPT haplotype (MAPT H1) increases the risk of PSP and CBD, and recently our genome-wide association efforts identified variation in three additional genes/loci (STX6, EIF2AK3 and MOBP/Appoptosin). To date, it is unclear which variation at each locus is responsible for the risk and what is the underlying pathomechanism of disease. Also, other genes as yet undetected may contribute to susceptibility to PSP and CBD. Therefore, the studies proposed in Project 1 will resolve the disease-associated genetic variation within the exome and whole-genome sequence data from PSP and CBD patients to determine the pathological consequences and mechanisms, leading to the identification of new therapeutic targets and potential biomarkers. Research in Project 1 will make use of existing data available through the ADSP, the Mayo Clinic Biobank, and NHGRI sequencing efforts. Core C will provide postmortem tissue from PSP cases on which sequencing data are available, and will assist Project 1 with the quantitation of tau pathologic burden in Aim 3. As the studies progress, the data generated by Project 1 will inform the studies of Project 2, investigating genetic modifiers in a genetically-diverse AAV-based model of tauopathy. The discoveries of this project will also influence the direction of Core C, determining which of the most promising targets will be selected for antibody and assay development.
Project 2: "Utilization of genetic diversity to identify modifiers of tauopathy”
To uncover novel genetic variants that influence tau toxicity and recapitulate the genetic diversity characteristic of the human population, Project 2 aims to take advantage of the observation that severity of tauopathy in transgenic mouse models is significantly affected by altering the genetic background. While most transgenic lines have been created on a standard genetic background, a single inbred strain does not incorporate one of the major sources of phenotypic variation in human populations: genetic diversity. The lack of genetic diversity also limits the translational utility of standard mouse models because it grossly underestimates the variation of responses that will be seen in the human population. Therefore, a genetically diverse mouse model of tauopathy will be created in the current study, using AAV to drive mutant tau expression in a panel of Collaborative Cross (CC) recombinant inbred mice, as well as Diversity Outbred (DO) mice produced from random repeated outcrossing of CC strains. Collectively, the CC and DO mouse populations offer exceptional allelic diversity and high mapping resolution, carrying 45 million SNPs and structural variants and, thus, providing a unique opportunity to discover new genes that regulate tau toxicity in vivo. This is particularly powerful considering that a true genetic modifier of tauopathy may fail to reach statistical significance and remain undetected in human genetics studies as a consequence of the confounding impact of environmental variables. Importantly, as the CC and DO mice are housed in a controlled environment, the influence of environmental factors will be minimal in the current study. Although there are currently no genetically diverse models of tauopathy that are representative of the diversity within the human population, the development of such a model would enable researchers to test the efficacy and toxicity of new therapeutic strategies, improving translational relevance by incorporating the genetic variation missing from traditional models. Overall, the goals of Project 2 are to identify key genes that modulate disease severity in an AAV-based genetically diverse model of tauopathy. After the discovery of new potential modifiers of tau toxicity, human homologs of the murine genes identified will be scrutinized in Project 1 to determine if novel variants can be detected in PSP and/or CBD patients. To provide insight into how modifier genes might be altered in tauopathy, Core C will develop new antibodies and assays to monitor changes in protein expression, subcellular localization, and potentially function.
In Project 3/competitive supplement, Dr. Fitzpatrick will advance the discoveries of our CWOW by providing unique insight into the consequences of genetic factors on tau pathology at the atomic level. As proof-of-concept, Dr. Fitzpatrick recently demonstrated that cryo-electron microscopy (cryo-EM) can be utilized to determine the atomic structures of tau filaments in a patient with Alzheimer’s disease (AD). In fact, the structural model revealed by cryo-EM identified K321 as a key residue for filament formation, which is in perfect accord with our discovery that acetylation of K321 inhibits tau aggregation (Carlomagno et al., 2017). By incorporating cryo-EM methodology into our Center, we will now examine the role of genetic factors, including mutations in the tau gene (MAPT), in determining filament morphology and influencing phenotypic variation in neurodegenerative tauopathies.
Core A (Administration Core) will oversee administrative, fiscal and reporting activities of the Center and will convene regularly scheduled meetings of the Center Executive Steering Committee (biweekly meetings) and external advisors (biannual meetings of the External Advisory Committee). Core activities are fostered by an administrative assistant to help with grants management, reporting and compliance.
Core C (Human Validation Core) will generate new research tools to provide the Center with the means to uncover the underlying mechanistic basis for altered disease risk mediated by each novel genetic variant/modifier discovered, as well as identifying common pathways that encompass multiple genetic modifiers and would thus represent ideal pathways to target therapeutically. The Core will also assist all projects with neuropathologic characterization and quantitation of tau burden.