Genetics of ALS

Although familial amyotrophic lateral sclerosis (ALS) cases are present at only 10% of all ALS cases, knowing the underlying genes of those cases and the mechanisms of the mutations that lead to ALS can be of great help in determining how the disease works, even outside of those familial cases.[1] The SOD1 gene has been known for a long time as a gene present in many cases of familial ALS, however, the mechanisms explaining how SOD1 mutations aggravate neuron degeneration are unknown. It is thought that SOD1 mutations can have a variety of effects that result in neuron degeneration such as protein aggregation and oxidative stress.[1] The ALS2 gene and alsin protein have a role in protecting motor neurons, and can even suppress the activities of SOD1 mutations. Mutations in ALS2, like SOD1 mutations do not lead to full blown ALS but rather result in predictable muscular atrophy, and as such are good models of ALS.[2] TBD-43 is another gene that when mutated leads to protein aggregates, that can be seen in both FALS as well as SALS (sporadic ALS) cases. Work in the genetically linked ALS cases could be crucial to the discovery of the mechanisms that cause protein aggregation, and could lead to the discovery of drugs that can prevent protein aggregation and in turn could remedy victims of ALS.

1. SOD1 mutations and their effects on neuron degeneration.

SOD1 Protein aggregation diagram
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This is a diagram showing the ways in which SOD1 aggregations can affect a neuron.[11]

The SOD1 gene is responsible for the super oxide dismutase 1 enzyme, and mutations in this gene are seen in 20% of all FALS cases. The enzyme normally functions as a homodimer bound to copper and zinc, that metabolizes superoxides in the body into oxygen and hydrogen peroxide.[1]

1.1. Loss, or gain of function

It has long been thought that the cause of neuron degeneration with the SOD1 gene was the result of the loss of protein function; mutations in the SOD1 gene would interfere with proper protein function and as a result the neurons would die.[1] However in SOD1 null mice, not much was seen in the way of aggressive neuron death like in ALS. SOD1 null mice were shown to have later onset motor deficits and weaker muscle. This does not correlate with the aggressive motor neuron death seen in ALS, and instead correlates more with the oxidative stress that comes with normal aging. With old age muscle weakness and slowness due to reactive oxygen species is common, but this process is accelerated in SOD1 null mice. SOD1 null mice show up an average of 50% lower muscle mass than normal mice and observable movement deficits.[2] This is consistent with the role of SOD1 protein as an enzyme that metabolizes reactive oxygen species, as without the enzyme the effects of oxidative stress would go unhindered. This does not explain the loss of motor neurons present in ALS. If SOD1 mutations are still present in 20% of FALS cases, then the possibility of a gain of function should be observed as well. When looking at mice overexpressing SOD1 as opposed to SOD1 null mice, mice were shown to have aggressive motor neuron death despite having functional SOD1 enzymes. So instead, a better model for SOD1 and ALS may be to observe the gain of function rather than the loss of function, with the gain of function being protein aggregation. However this is not to say that the loss of function is unimportant, as muscle weakness and decreased SOD1 dimutase activity due to the aggregation of those proteins are featured in those with FALS.[1]

1.2. Protein aggregation

Protein aggregates, or lewy bodies, are characteristic of many different neurodegenerative disorders and ALS is no exception. Underlying mechanisms for the actual aggregation of the proteins are not well understood but the aggregation of proteins is seen in to have a dramatic effect when induced in transgenic mice.[1] Protein aggregates can be observed by locating proteins of interest, say SOD1, and extracting them from the cerebrospinal fluid of transgenic mice and testing for detergent solubility. Detergent insoluble proteins imply protein aggregation, and the effects of the aggregation can be observed in these mice.[3] Overexpression of WT (wild type) SOD1 is not enough to catalyze aggregation of protein, but in conjunction with certain mutants of SOD1, protein aggregation and motor neuron death has been observed.[3] There have been as many as 150 observed mutations of the SOD1 gene, that may all behave differently in terms of aggregation. In some examples, transgenic mice heterozygous for WT and mutant SOD1-G73R do not display paralysis or other ALS like features, but mice heterozygous for WT and mutant SOD1-L126Z/L76WT display typical ALS like paralysis, as well as insoluble WT SOD1 protein, indicating that the mutant SOD1 proteins are aggregating with the WT SOD1.[3] Despite these showings, many things remain unclear. How protein aggregates cause cell death, and how they target specific groups of neurons such as motor neurons, leading to ALS.

2. ASL2

ALS2 Family Pedigree
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This is a pedigree from two highly inbred families containing the ALS2 gene, and it displays the autosomal recessive nature of the mutation.[10]

The ALS2 gene encodes for the protein named alsin. ALS2 mutations have been observed in FALS cases, being inherited in a recessive manner, and being present in cases of juvenile onset ALS. Alsin’s functions are not yet known, but it contains domains similar to that of other GEF’s (guanine nucleotide exchange factors).[5] Certain domains of alsin have also been shown to be related to cytoplasmic transport but it is not yet known how those domains work in conjunction with the other domains.[6]

2.1. ALS2 Loss of function

When ALS2 knockout mice were tested, the results produced mice with mild motor impairment and neuronal death, however, it was not comparable to that of full ALS. These findings do not completely agree with the findings of knockout zebra fish having severe motor impairments, and humans with homozygous mutations of ALS2 experiencing juvenile onset ALS.[4] The loss of function is due to prematurely truncated proteins, which are the result of deletions that cause frameshift mutations.[6] It is not completely clear what the result of the loss of function is, however, in post mortem studies of FALS patients with ALS2 mutations excessive reactive oxidative species were seen which can contribute to motor neuron death in a similar manner to loss of SOD1 function.[7]

2.2. ALS2 and SOD1

As mentioned in the previous section regarding SOD1, SOD1 mutants forming protein aggregates result in cell death. It has been shown that in neuronal cells with mutant SOD1 expression along with WT ALS2, have reduced rates of mortality. This is due to the RhoGEF domain, which was shown to bind to SOD1 mutants and reduce the ability of SOD1mutants to form neurotoxic aggregates.[6] Various mutations of ALS2 were also shown to be able to prevent SOD1 mutant aggregation, though not all of them were able to do so. It should be noted that alsin and alsin mutants were not able to bind to WT SOD1 protein.[6] Although the exact mechanisms are unclear the ability for alsin to bind and suppress SOD1 are not clear, the ability of alsin to prevent aggregation show the complexities in protein aggregation and the degeneration of neurons can be the result of many different mutations synchronized to produce the phenotypes seen in ALS.

3. TDP-43 and ALS

Fluoresced TBD-43 protein
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This image shows fluoresced protein aggregates in different forms of ALS.[8]

TDP-43, or TAR DNA binding protein 43, is a protein that normally helps regulate transcription by stabilizing and splicing mRNA.[8] Mutations are inherited in an autosomal dominant manner and all consist of mutations of the same exon at the C-terminal domain.[10] Poly-ubiquinated forms as part of Lewy bodies have been noted in many different neuronal disease cases, including ALS, dementia, Alzheimer's, and others.[8] Although its presence is notable in FALS, it is also noted in SALS, marking an important connection between the two variants of ALS.[9] Something that sets TDP-43 apart from other protein aggregates is that the half-life of the mutant TDP-43 protein is extended compared to that of the wild type, which could be an underlying factor contributing to Lewy bodies. The lengthening of the half-life also could lead to a greater chance of phosphorylation or ubiquination which are noted in pathological TDP-43 mutations.[9] Mutated TDP-43 and WT FUS proteins have been shown to aggregate together within the cytoplasm of neurons, which could be an additional effect of lengthened half-life. Loss of function of TDP-43 and FUS protein due to protein aggregation could also contribute to neuropathy, as both proteins play a role in RNA regulation, though the loss of function has not been proven to be involved in neuropathy.[10]

A quick demonstration on ALS genetics research
An example of how genes related to ALS, similar to SOD1 and ALS2, are discovered[12]
Bibliography
1. Saccon, R et al. Is SOD1 loss of function involved in amyotrophic lateral sclerosis? Brain. (2013); 138(8): 2342-2358.
2. Larkin, L.M. et al. Skeletal muscle weakness due to deficiency of CuZn-superoxide dismutase is associated with loss of functional innervation. Am J Physiol Regul Integr Comp Physiol. (2011); 301(5): 1400-1407.
3. Prudencio, M, Duranzo, A, Whitelege, J and Borchelt, D. An examination of wild-type SOD1 in modulating the toxicity and aggregation of ALS-associated mutant SOD1. Hum Mol Genet. (2010); 19(24): 4774-4789.
4. Gros Luis, F et al. Als2 mRNA splicing variants detected in KO mice rescue severe motor dysfunction phenotype in Als2 knock-down zebrafish. Human Molecular Genetics. (2008); 17(17): 2691-2702.
5. Pasinelli,P, Brown, R. Molecular biology of amyotrophic lateral sclerosis: insights from genetics. Nat Rev Neurosci. (2006); 7(9): 710-723.
6. Kanekura, K et al. Alsin, the product of the ALS2 gene, suppresses SOD1 mutant neurotoxicity through RhoGEF domain by interacting with SOD1 mutants. J Biol Chem. (2004); 279(18); 19247 – 19256.
7. Cai, H et al. ALS2/Alsin knockout mice and motor neuron diseases. Neurodegen Dis. (2008); 5(6): 359-366.
8. Mackenzie, I et al. Pathological TDP-43 Distinguishes sporadic amyotrophic lateral sclerosis from amyotrophic lateral sclerosis with SOD1 mutations. Ann Neurol. (2007); 61(5): 427-434.
9. Ling, S et al. ALS-associated mutations in TDP-43 increase its stability and promote TDP-43 complexes with FUS/TLS. (2010); 107(30): 13318-13323.
10. Lagier-Tourenne, C, Cleveland, D. Rethinking ALS: the FUS about TDP-43. Cell. (2009); 136(6): 1001-1004.
11. Hadano, S et al. A gene encoding a putative GTPase regulator is mutated in familial amyotrophic lateral sclerosis 2. Nat Genet. (2001); 29(2): 166-173.

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