Pathogenic Genome Signatures That Damage Motor Neurons in Amyotrophic Lateral Sclerosis

Abstract

Amyotrophic lateral sclerosis (ALS) is the most frequent motor neuron disease and a neurodegenerative disorder, affecting the upper and/or lower motor neurons. Notably, it invariably leads to death within a few years of onset. Although most ALS cases are sporadic, familial amyotrophic lateral sclerosis (fALS) forms 10% of the cases. In 1993, the first causative gene (SOD1) of fALS was identified. With rapid advances in genetics, over fifty potentially causative or disease-modifying genes have been found in ALS so far. Accordingly, routine diagnostic tests should encompass the oldest and most frequently mutated ALS genes as well as several new important genetic variants in ALS. Herein, we discuss current literatures on the four newly identified ALS-associated genes (CYLD, S1R, GLT8D1, and KIF5A) and the previously well-known ALS genes including SOD1, TARDBP, FUS, and C9orf72. Moreover, we review the pathogenic implications and disease mechanisms of these genes. Elucidation of the cellular and molecular functions of the mutated genes will bring substantial insights for the development of therapeutic approaches to treat ALS.

Keywords: amyotrophic lateral sclerosis; cell damage; genome signature; motor neuron; neurodegeneration.

Figure 1

Proportional contribution of mutated genes in ALS and timeline of ALS gene study. (A) The proportion of mutant genes in fALS cases. (B) The proportion of mutant genes in sALS cases. (C) Timeline of ALS gene discoveries and researches for SOD1, TARDBP, FUS, and C9orf72. The Y-axis shows the number of publications in PubMed that include the terms “ALS” and “the gene name” by year until November 2020.

Figure 2

A scheme for illustrating neuropathogenesis of CYLD, S1R, GLT8D1, and KIF5A genes in ALS. (A) CYLD: Overexpression of CYLD inhibits the nuclear transport of TDP-43 from the cytoplasm, leading to the mislocalization of TDP-43. Lower TDP-43 level in the nucleus decreases the expression of ATG7, which is responsible for the fusion of lysosomes with autophagosomes. This results in a loss of autolysosome formation, causing a loss of autophagy function. (B) S1R: Mutant S1R leads to ER calcium and ATP depletion and perturbs mitochondrial dynamics and respiration. Subsequently, depletion of ATP leads to TDP-43-induced toxicity. (C) GLT8D1: (1) Mutant GLT8D1 increases the ganglioside signaling, leading to the transit of mature gangliosides to the cell membrane where they disrupt cell signaling. (2) On the other hand, knock down of GLT8D1 impairs its glycosyltransferase activity from the Golgi and diminishes ganglioside signaling. (D) KIF5A: Lack of KIF5A expression disrupts axonal transport, accumulates phosphorylated neurofilaments and APP, and causes cytoskeletal defects in neurons. (Inlet) In contrast, the disruption of axonal transport reduces APP level in the synapse and triggers neurodegeneration.

Figure 3

A scheme for illustrating neuropathogenesis of mutant genes (SOD1, TARDBP, FUS, and C9orf72) with major contribution to ALS. (A) SOD1: Mutant SOD1 (mSOD1) leads to oxidative stress by elevating free radicals that damage protein folding and impair axonal transport in motor neurons. Additionally, misfolded mSOD1 protein is not able to transport across mitochondrial membranes and accumulates onto the outer mitochondrial membrane, triggering motor neuronal death. On the other hand, mSOD1-induced ER stress results in the activation of UPR and ERAD, causing motor neuronal damage. (B) TARDBP: (1) Mutant TDP-43 (mTDP-43) is aggregated in the cytosol and TDP-43 dysfunction abnormally accumulates mRNA in the nucleus. (2) The loss of nuclear TDP-43 function leads to inhibition of TDP-43 binding to STMN2 RNA and mis-splicing of STMN2 RNA, which subsequently inhibits axonal regeneration of motor neurons. (3) On the other hand, upregulation of cytoplasmic TDP-43 forms inclusion bodies and it further propagates to neighbor cells, causing motor neuronal degeneration. (C) FUS: Mutant FUS (mFUS) disrupts nucleocytoplasmic transport and leads to its depletion from the nucleus. Then, mFUS aggregates in the cytoplasm and induces neurotoxicity. mFUS may also disrupt translation of transcripts associated with mitochondrial function, which leads to mitochondrial shrinkage. (Inlet) Moreover, mFUS binds to mature mRNAs in the cytoplasm and affects mRNA translation. Suppressed protein translation impairs the dendrites and axon terminals. Inhibition of local intra-axonal protein translation, that are required for synaptic maintenance and function, causes synaptic defects and dysfunctions. (D) C9orf72: (1) The nuclear RNA foci are generated by the aggregation of repeat-containing C9orf72 RNAs in the nucleus and cause neurotoxicity. (2) Sequestration of RanGAP by G4C2 RNA disrupts the nucleocytoplasmic transport function. The loss of nuclear Ran depletes nuclear TDP-43 levels and elevates cytoplasmic TDP-43 levels. (3) Furthermore, the imported dipeptide repeats (DPRs) into the nucleus are associated with nucleolar proteins and cause nucleolar stress. (Inlet) The loss of C9orf72 function in endosomal trafficking regulation by interactions with nuclear pore complex proteins eventually increases cytoplasmic TDP-43 inclusions. Furthermore, reduction of C9orf72 expression inhibits Shiga toxin transportation from the plasma membrane to the Golgi apparatus, and alters the ratio of LC3, an autophagosome marker, leading to autophagy dysregulation.

Figure 4

A scheme for exhibiting variant sites in SOD1 protein. (Top) the SOD1 protein structure; (Bottom) 180 SOD1 variants shown in ALS cases. Data were taken from the ALS online database (ALSoD) [79]. The variations are only identified on the amino acid sequence of the protein, and not all are proven to be pathogenic for ALS.

Figure 5

A scheme for exhibiting variants sites in TDP-43 protein: (Top) the TDP-43 domain structure; and (Bottom) the 44 TDP-43 variants shown in patients with ALS. NLS, nuclear localization signal; NES, nuclear export signal; RRM, RNA recognition motif.

Figure 6

A scheme for exhibiting variants sites in FUS/TLS protein: (Top) the FUS domain structure; and (Bottom) the 72 FUS variants shown in patients with ALS. RGG, Arg-Gly-Gly.

Figure 7

C9orf72 gene structure, transcript variants, and protein isoforms. The C9orf72 gene consists of 11 coding exons (green) and non-coding exons (orange). The G4C2 or C4G2 HRE is located in the first intron of variants 1 and 3 and within the promoter region of variant 2. This figure was adapted from Balendra and Isaacs’s study [146].