Functions of FUS/TLS from DNA repair to stress response: implications for ALS

Abstract

Fused in sarcoma/translocated in liposarcoma (FUS/TLS or FUS) is a multifunctional DNA-/RNA-binding protein that is involved in a variety of cellular functions including transcription, protein translation, RNA splicing, and transport. FUS was initially identified as a fusion oncoprotein, and thus, the early literature focused on the role of FUS in cancer. With the recent discoveries revealing the role of FUS in neurodegenerative diseases, namely amyotrophic lateral sclerosis and frontotemporal lobar degeneration, there has been a renewed interest in elucidating the normal functions of FUS. It is not clear which, if any, endogenous functions of FUS are involved in disease pathogenesis. Here, we review what is currently known regarding the normal functions of FUS with an emphasis on DNA damage repair, RNA processing, and cellular stress response. Further, we discuss how ALS-causing mutations can potentially alter the role of FUS in these pathways, thereby contributing to disease pathogenesis.

Keywords: DNA damage repair; FUS/TLS; RNA processing; amyotrophic lateral sclerosis; stress granules; stress response.

Figure 1.

The functional domains within fused in sarcoma (FUS). FUS binds DNA, RNA, and proteins to perform a diverse array of functions. Summarized here are the known functions of FUS annotated onto the domain structure of the protein. Note. QGSY-rich = glutamine-glycine-serine-tyrosine-rich or prion-like domain; Gly-rich = glycine-rich; RGG = arginine-glycine-glycine-rich; RRM = RNA recognition motif; ZFD = zinc finger domain; NLS = nuclear localization signal; ALS = amyotrophic lateral sclerosis.

Figure 2.

FUS directly binds DNA. (a) FUS binds the promoters of >1,000 genes, indicative of a role in transcriptional regulation. (b) FUS binds both single- and double-stranded DNA and is important for two critical steps in homologous recombination: D-loop formation and homologous DNA pairing. When a double-strand break occurs in DNA, the 5′ end of the break is trimmed back to create a 3′ overhang of single-stranded DNA. This 3′ single-stranded DNA then binds a complementary sequence within duplex DNA of a homologous chromosome or sister chromatid, a process called strand invasion (reviewed in X. Li & Heyer, 2008). (c) FUS binds G-quadruplexes in telomeres. (d) Analogous to the role of FUS in D-loop formation, FUS may also be important for T-loop formation at the ends of telomeres. T-loops are formed when a single-stranded, G-rich DNA overhang at the end of a chromosome forms a loop and anneals to a complementary 5′ C-rich sequence (Griffith et al., 1999; reviewed in Greider, 1999).

Figure 3.

FUS is recruited to sites of DNA damage and contributes to DNA-damage repair. Under normal conditions, FUS (green oval) and common repair proteins (triangles) localize to sites of laser-induced DNA damage (yellow star). Under conditions of FUS knockdown, these repair proteins are not recruited to sites of DNA damage and the efficiency of both homologous recombination and nonhomologous end joining is reduced. Mutant FUS (red ovals) is still able to localize to sites of damage in the absence of endogenous FUS (**discrepancy in the literature for the degree of localization of variant R521G). Exogenous mutant FUS does not fully rescue DNA-damage repair when endogenous FUS is knocked-down (*exception, FUS H517Q), although mutant FUS is able to recover NHEJ to a greater extent than HR (*NHEJ is fully recovered by FUS H517Q). Note. FUS = fused in sarcoma; KD = knockdown; PARP = adenosine diphosphate [ADP] ribose polymerase; HR = homologous recombination; NHEJ = nonhomologous end joining; p-ATM = phosphorylated-ataxia telanogiectasia mutated; NBS1 = Nijmegen breakage syndrome-1; HDAC = histone deacetylase 1; 53BP1 = p53-binding protein 1.

Figure 4.

The differential response of FUS to cellular stress. Cells expressing exogenous WT or endogenous FUS (top panels) and ALS-linked mutant FUS (bottom panels) are shown under different cellular conditions. (a) Under normal conditions, WT/endogenous FUS is localized predominantly to the nucleus while ALS-FUS variants with mutations in the nuclear localization domain undergo varying degrees of cytoplasmic mislocalization. (b) Under conditions of oxidative stress, heat shock, or ER stress, WT/endogenous FUS remains nuclear while mutant FUS that is already mislocalized to the cytoplasm incorporates into stress granules. (c) Under conditions of hyperosmolar stress, WT/endogenous FUS translocates to the cytoplasm and incorporates into stress granules. Under these conditions, endogenous FUS is thought to play a prosurvival role. Mutant FUS proteins that are already mislocalized to the cytoplasm also associate with stress granules (unpublished data), although the implications of this interaction for ALS are unknown. Note. ER = endoplasmic reticulum; ALS = amyotrophic lateral sclerosis; WT = wild type; FUS = fused in sarcoma.