A meta-analysis of genetic variant pathogenicity and sex differences in UBQLN2-linked amyotrophic lateral sclerosis and frontotemporal dementia

Ubiquilin 2, encoded by the X-linked UBQLN2 gene, is a ubiquitin-binding quality control protein. Pathogenic UBQLN2 genetic variants cause X-linked dominant amyotrophic lateral sclerosis and/or frontotemporal dementia (ALS/FTD), however, clinical phenotypes from these variants show striking inter- and intra-familial heterogeneity. Further, there are many UBQLN2 variants whose significance to disease is uncertain. Here, we examine the pathogenic potential of UBQLN2 variants reported in individuals with ALS/FTD and their non-symptomatic relatives. Meta-analysis from 27 published studies identified 186 affected individuals and 51 asymptomatic carriers, each harbouring one of 43 unique UBQLN2 coding variants. Features of identified variants, including evolutionary conservation, minor allele frequencies, localisation to protein domains, and in silico predictions of pathogenicity were compiled. Per biological sex, clinical features were compared between UBQLN2 variants segregated by pathogenicity. Pathogenic UBQLN2 variant carriers, most of whom are familial ALS cases, showed a sex-specific difference in age at onset wherein males developed disease on average 18.15 years prior to females (29.54 ± 11.9 versus 47.69 ± 13.4 years, p < 0.0001), with no change in disease duration (p = 0.2091). UBQLN2 variants of uncertain significance showed a bimodal distribution of onset age per sex suggesting a mixture of true benign and true pathogenic variants. In human brain tissue, two male UBQLN2 p.Thr487Ile (ALS-FTD and ALS) cases showed a greater burden of ubiquilin 2 aggregates than a related female case (ALS-FTD). These robust sex-specific differences in ALS/FTD presentation in carriers of pathogenic UBQLN2 variants may improve predictions of ALS/FTD risk in carriers, aiding in diagnosis and disease management.