A humanized mouse model for UBQLN2-associated ALS-dementia

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Abstract The objective of this R21 proposal is to develop a humanized mouse model for UBQLN2-associated amyotrophic sclerosis (ALS) and frontotemporal dementia (FTD), devastating neurodegenerative disorders that lie on either end of a clinical disease spectrum. Among more than 20 different genes implicated in ALS/FTD, X-linked mutations in the ubiquitin (Ub) chaperone UBQLN2 cause diverse neurologic phenotypes, ranging from pure ALS with or without FTD, to primary lateral sclerosis and spastic paraplegia. ALS/FTD-associated mutations in UBQLN2 disrupt its folding and promote its aggregation; however, the mechanisms whereby UBQLN2 mutations instigate neurodegeneration in ALS/FTD are unclear. UBQLN2 mutant transgenic rodents recapitulated UBQLN2 histopathology but manifested highly discrepant neurological phenotypes ranging from patent motor neuron degeneration to no observable phenotype. A caveat to these studies—and transgenic rodent models in general—is that overexpression of wild-type UBQLN2 also elicits toxicity, likely through disruption of Ub homeostasis. On the other hand, while there is a strong need for models in which culprit ALS mutations are expressed at endogenous levels, mice harboring ALS-associated UBQLN2 knockin mutations exhibit weak phenotypes, precluding mechanistic studies. This has been a general problem for mouse models of human neurodegenerative diseases. In an attempt to augment UBQLN2 toxicity at endogenous expression levels, our group developed a combinatorial UBQLN2 mutant (UBQLN24XALS) harboring four different clinical mutations that reduce its solubility in a semi-additive manner. UBQLN24XALS caused enhanced toxicity relative to wild-type UBQLN2 or UBQLN2 clinical mutants when overexpressed in Drosophila or when expressed from a UBQLN2 knockin allele in iPS-derived motor neurons (iMNs). Genetic suppressors of UBQLN24XALS toxicity in Drosophila also suppressed its toxicity in iMNs suggesting that UBQLN24XALS toxicity mechanism is at least partially conserved in flies and mammals. Here, we propose to generate a conditional, humanized UBQLN24XALS mouse model to allow spatially and temporally controlled UBQLN24XALS in the mouse nervous system. After a full evaluation of neurodegenerative phenotypes, UBQLN24XALS mice will be used as a tool to decipher ALS disease pathways using ‘omics approaches and information gleaned from orthologous studies in Drosophila and cellular models of ALS-UBQLN2. Information learned should also inform ALS/FTD arising sporadically and from other genetic causes. The objectives of the proposal are to: 1) Generate conditional, humanized UBQLN2WT to UBQLN24XALS gene switch mice; 2) Histopathologic and behavioral assessment of Nestin (Nes)-Cre, UBQLN24XALS mice.