Hippocampal protein aggregation signatures fully distinguish pathogenic and wildtypeUBQLN2in amyotrophic lateral sclerosis

AbstractMutations in theUBQLN2gene cause X-linked dominant amyotrophic lateral sclerosis (ALS) and/or frontotemporal dementia (FTD) characterised by ubiquilin 2 aggregates in neurons of the motor cortex, hippocampus, cerebellum, and spinal cord. However, ubiquilin 2 neuropathology is also seen in sporadic and familial ALS or FTD cases not caused byUBQLN2mutations, particularlyC9ORF72-linked cases. This makes the mechanistic role of ubiquilin 2 mutations and the value of ubiquilin 2 pathology for predicting genotype unclear. Here we examine a cohort of 31 genotypically diverse ALS cases with or without FTD, including four cases withUBQLN2mutations (resulting in p.P497H, p.P506S, and two cases with p.T487I). Using double-, triple-, and five-label fluorescent immunohistochemistry, we mapped the co-localisation of ubiquilin 2 with phosphorylated TDP-43 (pTDP-43), dipeptide repeat aggregates, and p62, in the hippocampus of controls (n=5), or ALS with or without FTD in sporadic (n=19), unknown familial (n=3),SOD1-linked (n=1),C9ORF72-linked (n=4), andUBQLN2-linked (n=4) cases. We differentiate between i) ubiquilin 2 aggregation together with, or driven by, pTDP-43 or dipeptide repeat proteins, and ii) ubiquilin 2 self-aggregation driven byUBQLN2gene mutations. Together we describe a hippocampal protein aggregation signature that fully distinguishes mutant from wildtype ubiquilin 2 in ALS with or without FTD, whereby mutant ubiquilin 2 is more prone than wildtype to aggregate independently of driving factors. This neuropathological signature can be used to assess the pathogenicity ofUBQLN2gene mutations and to understand the mechanisms ofUBQLN2-linked disease.