FUS/TLS was initially identified as a fusion protein caused by chromosomal translocations in human cancers, especially liposarcomas. In these instances, the promoter and N-terminal part of FUS/TLS is translocated to the C-terminal domain of various DNA-binding transcription factors conferring a strong transcriptional activation domain to the fusion proteins. FUS/TLS was independently identified as the hnRNP P2 protein, a subunit of a complex involved in maturation of pre-mRNA.
Structure
FUS/TLS is a member of the TET protein family that also includes the EWS protein, the TATA-binding protein -associated factor and the Drosophila cabeza/SARF protein. FUS/TLS, EWS and TAFII68/TAF15 have a similar structure characterised by an N-terminal QGSY-rich region, a highly conservedRNA recognition motif, multiple RGG repeats, which are extensively dimethylated at arginine residues and a C-terminal zinc finger motif.
Function
The N-terminal end of FUS appears to be involved in transcriptional activation, while the C-terminal end is involved in protein and RNA binding. In addition recognition sites for the transcription factors AP2, GCF, Sp1 have been identified in FUS. Consistently, in vitro studies have shown that FUS/TLS binds RNA, single-stranded DNA and double-stranded DNA. The sequence specificity of FUS/TLS binding to RNA or DNA has not been well established; however, using in vitro selection, a common GGUG motif has been identified in approximately half of the RNA sequences bound by FUS/TLS. A later proposal was that the GGUG motif is recognised by the zinc finger domain and not the RRM. Additionally, FUS/TLS has been found to bind a relatively long region in the 3′ untranslated region of the actin-stabilising protein Nd1-L mRNA, suggesting that rather than recognising specific short sequences, FUS/TLS interacts with multiple RNA-binding motifs or recognises secondary conformations. FUS/TLS has also been proposed to bind human telomeric RNA 4 and single-stranded human telomeric DNA in vitro. Beyond nucleic acid binding, FUS/TLS was also found to associate with both general and more specialized protein factors to influence the initiation of transcription. Indeed, FUS/TLS interacts with several nuclear receptors. and with gene-specific transcription factors such as Spi-1/PU.1. or NF-κB. It also associates with the general transcriptional machinery and may influence transcription initiation and promoter selection by interacting with RNA polymerase II and the TFIID complex. Recently, FUS/TLS was also shown to repress the transcription of RNAP III genes and to co-immunoprecipitate with TBP and the TFIIIB complex.
FUS appears at sites of DNA damage very rapidly, which suggests that FUS is orchestrating the DNA repair response. The function of FUS in the DNA damage response in neurons involves a direct interaction with histone deacetylase 1. The recruitment of FUS to double-strand break sites is important for DNA damage response signaling and for repair of DNA damage. FUS loss-of-function results in increased DNA damage in neurons. Mutations in the FUS nuclear localization sequence impairs the poly polymerase -dependent DNA damage response. This impairment leads to neurodegeneration and FUS aggregate formation. Such FUS aggregates are a pathological hallmark of the neurodegenerative diseaseamyotrophic lateral sclerosis.
Clinical significance
FUS gene rearrangement has been implicated in the pathogenesis of both myxoid liposarcoma and low-grade fibromyxoid sarcoma. In 2009 two separate research groups analysed 26 unrelated families who presented with a type6 ALS phenotype, and found 14 mutations in the FUS gene. Subsequently, FUS has also emerged as a significant disease protein in a subgroup of frontotemporal lobar dementias, previously characterized by immunoreactivity of the neuronal inclusions for ubiquitin, but not for TDP-43 or tau with a proportion of the inclusions also containing a-internexin in a further subgroup known as neuronal intermediate filament inclusion disease. The disease entities which are now considered subtypes of FTLD-FUS are atypical frontotemporal lobar degeneration with ubiquitinated inclusions, NIFID and basophilic inclusion body disease, which together with ALS-FUS comprise the FUS-opathies. FTLD is the pathological term for the clinical syndrome of frontotemporal dementia. FTD differs from the more common Alzheimer's dementia in that memory is relatively well preserved; instead, the disease presents with a more temporal-lobe phenotype. Behavioural variant frontotemporal dementia, progressive non-fluent aphasia and semantic dementia are the three best-characterised clinical presentations. FUS positive FTLD tends to present clinically as a bvFTD but the correlation between underlying pathology and clinical presentation is not perfect.
Toxic mechanism in ALS
The toxic mechanism by which mutant FUS causes ALS is currently unclear. It is known that many of the ALS-linked mutations are located in its C-terminal nuclear localisation signal, resulting in it being located in the cytoplasm rather than the nucleus. This suggests either a loss of nuclear function, or a toxic gain of cytoplasmic function, is responsible for the development of ALS in these patients. Many researchers believe the toxic gain of cytoplasmic function model to be more likely as mouse models that do not express FUS, and therefore have a complete loss of nuclear FUS function, do not develop clear ALS-like symptoms.
