Molecular evaluation of human Ubiquilin 2 gene PXX domain in familial frontotemporal dementia patients

Abstract

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) phenotypes appear in the same pedigrees [1] and can occur in the same individuals [2] (FTDALS, OMIM 105550). In fact about 2–3 % of ALS patients develop dementia [3]. Consequently, in some individuals, there appears to be a common genetic etiology for both diseases. Genetic variation identified in GRN, FUS, TDP43, and more recently, in C9ORF72 and UBQLN2 genes has been associated to familial ALS (FALS, OMIN 104105), familial FTD (OMIM 600274) and/or FTDALS consistent with the notion that both disorders may share genetic components and, probably, functional pathogenic mechanisms [4–8]. UBQLN2 gene was recently identified by Deng et al. [7] as a novel dominant but not fully penetrant X-linked FTDALS gene. Specifically, five segregating mutations in four different proline residues within the PXX repeat domain of UBQLN2 gene were identified in families afflicted with ALS/dementia. Importantly, the authors also found a correlation of hippocampal UBQLN2 pathology with dementia in ALS cases with or without UBQLN2 mutations, suggesting that UBQLN2 gene could be involved in ALS-related dementia, even without UBQLN2 mutations [7]. UBQLN2 gene encodes an ubiquitin-like protein (ubiquilin) that shares a high degree of similarity with related proteins in multiple eukaryotic organisms. Specifically, ubiquilins contain an N-terminal ubiquitin-like domain and a C-terminal ubiquitin-associated domain; therefore, UBQLN2 could be functionally linked to ubiquitination enzymatic processes. Functional analysis showed that mutations in UBQLN2 lead to an impairment of protein degradation [7]. Of note, UBQLN2 protein has also been shown to bind the Stch protein which is also involved in aggresome malfunction [9]. The role of ubiquitination in neurodegeneration is well established in different human neurodegenerative disorders such Parkinson’s disease or the Marinesco-Sjogren syndrome [10, 11]. Furthermore, it has been suggested that there is a link between Alzheimer’s disease and ubiquitins [12]. The purpose of the present study is to further explore the role of UBQLN2 mutations in familial FTD [13]. We examined 77 FTD index patients with a positive family history of dementia who had complete clinical evaluations. Importantly, we maintained FTD patients with observed male to male transmission (which makes a dominant X-linked transmission impossible) as negative controls of this study (n = 12). For the purpose of this study, we examined subjects with FTD and its subtypes (behavioral variant FTD (bvFTD), semantic dementia, progressive nonfluent aphasia), according to Neary et al. [14], as well as patients within the FTD spectrum, such as with those with combined phenotype of FTD and ALS, corticobasal syndrome or progressive supranuclear palsy (Table 1). All individuals agreed to participate in the study and blood samples were obtained after informed consent from Electronic supplementary material The online version of this article (doi:10.1007/s00415-012-6568-5) contains supplementary material, which is available to authorized users.