UNC13A in amyotrophic lateral sclerosis: from genetic association to therapeutic target

Abstract

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease with limited treatment options and an incompletely understood pathophysiology. Although genomewide association studies (GWAS) have advanced our understanding of the disease, the precise manner in which risk polymorphisms contribute to disease pathogenesis remains unclear. Of relevance, GWAS have shown that a polymorphism (rs12608932) in the UNC13A gene is associated with risk for both ALS and frontotemporal dementia (FTD). Homozygosity for the C-allele at rs12608932 modifies the ALS phenotype, as these patients are more likely to have bulbar-onset disease, cognitive impairment and FTD at baseline as well as shorter survival. UNC13A is expressed in neuronal tissue and is involved in maintaining synaptic active zones, by enabling the priming and docking of synaptic vesicles. In the absence of functional TDP-43, risk variants in UNC13A lead to the inclusion of a cryptic exon in UNC13A messenger RNA, subsequently leading to nonsense mediated decay, with loss of functional protein. Depletion of UNC13A leads to impaired neurotransmission. Recent discoveries have identified UNC13A as a potential target for therapy development in ALS, with a confirmatory trial with lithium carbonate in UNC13A cases now underway and future approaches with antisense oligonucleotides currently under consideration. Considering UNC13A is a potent phenotypic modifier, it may also impact clinical trial outcomes. This present review describes the path from the initial discovery of UNC13A as a risk gene in ALS to the current therapeutic options being explored and how knowledge of its distinct phenotype needs to be taken into account in future trials.

Keywords: ALS; frontotemporal dementia; neurobiology; neurogenetics; neuromuscular.

Figure 1

Manhattan plot and fine locus mapping describing the location of risk SNPs in the genetic structure of UNC13A. (A) shows a Manhattan plot of cross-ancestry data available through Project MinE, showing the relation of SNP hits at different chromosome locations (x-axis) with a corresponding −log p-value (y-axis). The red line (5.0x10⁻⁸) indicates the threshold for genome wide significance. Significant SNP hits on chromosome 19 are highlighted in green, with the top variant (rs12608930) annotated. (B) shows SNP locations on the UNC13A gene on chromosome 19 (x-axis) with their significance in relation to ALS (y-axis), derived from data available through project Mine. rs12608932 is indicated by a red dot, rs12973192 is indicated by a yellow dot. The genetic intronic/exonic structure of UNC13A is depicted, read from 5’ to 3’. The detailed graph depicts the location of risk SNPs in relation to the cryptic exon. Parts of the image were adapted from Locuszoom. (C) shows a schematic representation of the canonical UNC13A protein with associated domains. The CaM domain is the Calmodulin binding domain. (D) shows the evolutionary stability through different species. The core of the UNC13 protein is conserved both through different species as well as in different subforms. ALS, amyotrophic lateral sclerosis; SNP, single-nucleotide polymorphism.

Figure 2

Overall survival for rs12608932 polymorphism. The p-value indicates the difference in cumulative risk between homozygous C-allele carriers versus non-carriers under a recessive model. Based on data from Van Rheenen et al.

Figure 3

Overview of the clinical phenotype of ALS patients conveyed by rs12608932. The left frame depicts brain regions with increased pTDP-43 burden in patients with C/C genotype in rs2608932 when compared to A/A: middle frontal cortex (8.26 times higher), temporal cortex (4.40 times higher) and motor cortex (3.04 times higher). The right frame depicts regions with increased cortical thinning associated with homozygosity for the C-allele at rs2608932, found through magnetic resonance imaging: the right fusiform cortex and the left inferior temporal cortex. The right part depicts clinical characteristics more likely to be present in patients homozygosity for the C-allele at rs2608932. Percentages are compared to the AA/AC. Brain images were adapted using Brainpainter. ALS, amyotrophic lateral sclerosis; ECAS, Edinburgh Cognitive and Behavioural ALS Screen, FTD, frontotemporal dementia, FVC, forced vital capacity, pTDP-43, phosphorylated 43kDa TAR DNA-binding protein.

Figure 4

Pathological cryptic exon inclusion and the putative disease mechanisms in UNC13A. 1. Cryptic exon inclusion has been found to be the pathological basis in UNC13A associated ALS. As a result of mislocalisation of TDP-43 outside of the cell nucleus, transcripts of UNC13A (1a) are spliced incorrectly, leading to the inclusion of the cryptic exon in mature UNC13A transcripts. This leads to nonsense mediated decay of proteins produced by reading messenger RNA containing the cryptic exon and vastly lower functional protein levels (1c). 2. UNC13A has been found to be involved in several processes pertaining to synaptic transmission. UNC13A forms and maintains synaptic active zones due, which orchestrates the localisation of several proteins essential to neurotransmission, such as calcium channels, RIM-molecules, synaptobrevin and syntaxin-1 (2a). UNC13A maintains the readily releasable pool, a collection of synaptic vesicles able to fuse quickly in order to facilitate neurotransmission (2b). UNC13A induces a change in conformation of syntaxin-1 to an open state, which activates it and enables the assembly of SNARE-complexes. UNC13A orchestrates the docking of 6 SNARE-complexes to each synaptic vesicle in order for it to tether to the presynaptic membrane. RIM interacts with UNC13A and recruits calcium (Ca2+) channels to the docked vesicle (2d). UNC13A primes synaptic vesicles in order to lower the Calcium (Ca2+) threshold needed for fusion with the presynaptic membrane (2e). Due to an influx of Ca2+, the synaptic vesicles fuse fully and neurotransmitters are released. A decrease in function of UNC13A could lead to a decrease in several or all of the aforementioned processes.