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Amyotrophic lateral sclerosis
Genetics of amyotrophic lateral sclerosis
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    Genetics of amyotrophic lateral sclerosis

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    There are more than 25 genes known to be associated with amyotrophic lateral sclerosis (ALS) as of June 2018, which collectively account for about 70% of cases of familial ALS (fALS) and 10% of cases of sporadic ALS (sALS). About 5–10% of cases of ALS are directly inherited. Overall, first-degree relatives of an individual with ALS have a 1% risk of developing ALS. ALS has an oligogenic mode of inheritance, meaning that mutations in two or more genes are required to cause disease.

    C9orf72 is the most common gene associated with ALS, causing 40% of familial cases of ALS, as well as a small percentage of sporadic cases; it also causes about 25% of familial cases of frontotemporal dementia. The pathogenic mutation is a hexanucleotide repeat expansion (a series of six nucleotides repeated over and over); the more repeats in C9orf72, the more pathogenic the mutation. People without ALS tend to have fewer than 25 repeat units, while people with ALS due to a mutation in C9orf72 tend to have hundreds or thousands of repeat units. It is not clear exactly how many repeat units are needed to cause disease.

    SOD1, which codes for superoxide dismutase 1, is the second most common gene associated with ALS and causes about 12% of familial cases and about 2% of sporadic cases. More than 150 mutations in SOD1 have been described, almost all of which have an autosomal dominant mode of inheritance.

    TARDBP, which codes for TAR DNA-binding protein (TDP-43), is associated with 1–5% of familial ALS and less than 1% of sporadic ALS. While TARDBP mutations are somewhat rare in ALS, pathological aggregations of TDP-43 are seen in up to 97% of ALS patients and up to 50% of FTD patients.TDP-43 is involved in the repair of DNA double-strand breaks. It is recruited to DNA damage sites and interacts with proteins involved in the repair process of non-homologous end joining.

    FUS, which codes for "Fused in sarcoma" protein, is associated with 1–5% of familial ALS and less than 1% of sporadic ALS. FUS is an RNA-binding protein with a similar function to TDP-43.

    Some people have both ALS and frontotemporal dementia (FTD–ALS). The four main genes associated with FTD–ALS are C9orf72, CHCHD10, SQSTM1, and TBK1.C9orf72 repeat expansions explain about 40% of familial ALS and 25% of familial FTD; thus, C9orf72 provides a genetic explanation for most of the overlap between the two diseases. While about half of people with ALS have some degree of cognitive impairment, only 10-15% have cognitive impairment severe enough to meet the criteria for frontotemporal dementia (FTD). Additionally, about 15% of people with FTD have symptoms of motor neuron dysfunction that resemble ALS. Mutations in TARDBP, FUS, C9orf72 and other genes can cause ALS as well as related forms of frontotemporal dementia (FTD–ALS). Proteins made by these genes appear to have prion-like activity and form inclusion bodies in some instances of ALS.

    Genes

    As of May 2017 more than 20 genes have been associated with various types of ALS. As of 2016 these genes explained about 70% of familial ALS (fALS) and 15% of sporadic ALS (sALS). These associations include:

    Type OMIM (see references at OMIM link) Gene Locus Inheritance Year Identified Remarks
    ALS1 105400 SOD1 21q22.1 autosomal dominant, autosomal recessive 1993 The first gene associated with ALS, SOD1 accounts for about 12% of fALS and 1-2% of sALS.
    ALS2 205100 ALS2 2q33.1 autosomal recessive 2001 Juvenile-onset
    ALS3 606640 Un­known 18q21 autosomal dominant —
    ALS4 602433 SETX 9q34.13 autosomal dominant 1998
    ALS5 602099 SPG11 15q21.1 autosomal recessive 2010 Juvenile onset
    ALS6 608030 FUS 16p11.2 autosomal dominant/recessive 2009 Impaired DNA damage response. Occurs in about 5% of familial and 1% of sporadic ALS cases.
    ALS7 608031 Un­known 20p13 autosomal dominant —
    ALS8 608627 VAPB 20q13.3 autosomal dominant 2004
    ALS9 611895 ANG 14q11.2 autosomal dominant 2006
    ALS10 612069 TARDBP 1p36.2 autosomal dominant 2008 ALS with or without frontotemporal dementia. Impaired repair of DNA damage.
    ALS11 612577 FIG4 6q21 autosomal dominant 2009
    ALS12 613435 OPTN 10p13 autosomal dominant/recessive 2010
    ALS13 183090 ATXN2 12q24.12 autosomal dominant 2010 Preliminary research indicates that intermediate-length CAG trinucleotide repeats in the ATXN2 gene may be associated with increased risk of ALS, whereas longer repeats cause spinocerebellar ataxia type 2
    ALS14 613954 VCP 9p13.3 autosomal dominant 2010 Preliminary research indicates a possible link in ALS mechanism
    ALS15 300857 UBQLN2 Xp11.21 X-linked dominant 2011 Described in one family
    ALS16 614373 SIGMAR1 9p13.3 autosomal recessive 2011 Juvenile onset, very rare, described only in one family
    ALS17 614696 CHMP2B 3p11.2 autosomal dominant 2006 Very rare, reported only in a handful of people
    ALS18 614808 PFN1 17p13.2 autosomal dominant 2012 Very rare, described only in a handful of Chinese families
    ALS19 615515 ERBB4 2q34 autosomal dominant 2013 Very rare, as of late 2013 described only in four people
    ALS20 615426 HNRNPA1 12q13.13 autosomal dominant 2013 Very rare, as of late 2013 described only in two people
    ALS21 606070 MATR3 5q31.2 autosomal dominant 2014 Associated with 0.5-2.0% of ALS cases.
    ALS22 616208 TUBA4A 2q35 autosomal dominant 2014 Associated with 1% of fALS cases and 0.4% of sALS cases; not enough evidence to conclude it causes ALS or FTD as of 2018.
    ALS23 617839 ANXA11 10q22.3 autosomal dominant 2017 Associated with 1% of fALS and 1.7% sALS cases; considered a causal gene.
    ALS24 617892 NEK1 4q33 Un­known 2016 Associated with 3-5% of ALS cases; considered an ALS risk gene rather than a causative gene as of 2018.
    ALS25 617921 KIF5A 12q13.3 autosomal dominant 2018
    FTD-ALS1 105550 C9orf72 9p21.2 autosomal dominant 2011 The gene most commonly associated with ALS, C9orf72 accounts for 40% of fALS cases and 7% of sALS cases.
    FTD-ALS2 615911 CHCHD10 22q11.23 autosomal dominant 2014 Associated with less than 1% of ALS-FTD cases and about 2% of fALS cases.
    FTD-ALS3 616437 SQSTM1 5q35.3 autosomal dominant 2011
    FTD-ALS4 616439 TBK1 12q14.2 autosomal dominant 2015 Associated with 1.3% of ALS cases and 3-4% of ALS-FTD cases.
    IBMPFD2 615422 HNRNPA2B1 7p15.2 autosomal dominant 2013 Proposed names: Inclusion body myopathy with early-onset Paget disease with or without frontotemporal dementia 2 (IBMPFD2); multisystem proteinopathy 2 (MSP2). Very rare, as of late 2013 described only in two people

    Other genes

    The following genes associated with ALS have been discussed in a June 2018 literature review, but have not yet been added to the Online Mendelian Inheritance in Man database.

    Type OMIM Gene Locus Inheritance Year Identified Remarks
    — — C21orf2 21q22.3 Un­known 2016 Associated with less than 1% of ALS cases.
    — — CCNF 16p13.3 autosomal dominant 2016 Associated with 0.6%-3.3% of fALS-FTD cases.
    — — TIA1 2p13.3 autosomal dominant 2017 Associated with 2% of fALS cases and less than 0.5% of sALS cases.

    SOD1

    In 1993, scientists discovered that mutations in the gene (SOD1) that produces the Cu-Zn superoxide dismutase (SOD1) enzyme were associated with around 20% of familial ALS and 5% of sporadic ALS. This enzyme is a powerful antioxidant that protects the body from damage caused by superoxide, a toxic free radical generated in the mitochondria. Free radicals are highly reactive molecules produced by cells during normal metabolism. Free radicals can cause damage to DNA and proteins within cells. To date, over 110 different mutations in SOD1 have been linked with the disorder, some of which (such as H46R) have a very long clinical course, while others, such as A4V, are exceptionally aggressive. When the defenses against oxidative stress fail, programmed cell death (apoptosis) is upregulated. To date, 180 different mutations in SOD1 gene are known to cause familial ALS.

    A defect in SOD1 could be a loss or gain of function. A loss of SOD1 function could lead to an accumulation of DNA damage. A gain of SOD1 function could be toxic in other ways.

    Aggregate accumulation of mutant SOD1 is suspected to play a role in disrupting cellular functions by damaging mitochondria, proteasomes, protein folding chaperones, or other proteins. Hypotheses proposed in explaining structural instability causing the misfold in the mutant SOD1 include, (1) glutamate excitotoxicity caused by reduced astroglial glutamate transporter EAAT2; (2) abnormalities of mitochondria in which increased misfolded SOD1 are deposited in the spinal cord mitochondria leading to defects in mitochondrial transport causing energy depletion, disruption in Ca2+ buffering, activating synaptic dysfunction, and loss of neurons; (3) impaired axonal structure or transport defects, in which neurotrophic signaling is lost, with defective anterograde and retrograde axonal transport observed in early pathogenesis, and (4) free radical-mediated oxidative stress causing cytotoxicity.

    A 2016 paper proposed that SOD1 maturation and proteins regulating intracellular copper levels are potential therapeutic targets of SOD1-ALS.

    The DNA oxidation product 8-oxoG is a well-established marker of oxidative DNA damage. 8-oxoG accumulates in the mitochondria of spinal motor neurons of persons with ALS. In transgenic ALS mice harboring a mutant SOD1 gene, 8-oxoG accumulates in mitochondrial DNA of spinal motor neurons. Thus oxidative damage to mitochondrial DNA of motor neurons due to altered SOD1 may be a significant factor in the etiology of ALS.

    UBQLN2, TARDBP

    The UBQLN2 gene encodes production of the protein ubiquilin 2 in the cell, which is a member of the ubiquilin family and controls the degradation of ubiquitinated proteins. Mutations in UBQLN2 interfere with protein degradation, leading to neurodegeneration and causing dominantly inherited, chromosome X-linked ALS and ALS/dementia.

    The TDP-43 protein, coded for by the TARDBP gene, is responsible for regulation of RNA expression. The discovery of mutations in the TARDBP gene, in relation to ALS, was the first proof that RNA processing defects lead to protein inclusions typical in RNA, and contribute to the pathogenesis of the disease. Other mutations that have been shown to be associated with ALS from GWAS include ATXN2,Nek1 and TBK1.

    TBK1, SQSTM1, OPTN

    The TBK1,SQSTM1, and OPTN genes are involved in producing a maturing autophagosome during autophagy. In 2016, it was observed that mutations in the TBK1 protein contributed to formation of the disease. Since the TBK1 protein is haploinsufficient, meaning mutations in the gene result in no protein production. This results in no phosphorylation of the p62 and optineurin proteins. As a result, motor neurons can no longer produce a functional autophagosome leading to the inhibition of autophagy.

    C9orf72

    C9orf72 gene produces a protein that is involved in the trafficking of an autophagosome during autophagy. C9orf72 protein will associate with proteins SMCR8 and WDR41 and this behaves as the Rab GDP-GTP exchange factor in vesicular transport during autophagy. Mutations in the C9orf72 gene lead to inhibition of the formation of the C9orf72 protein preventing the active transport of the autophagsome leading to inhibition of autophagy.

    Mitochondria

    Mitochondrial abnormalities, such as increased free radical production and impaired ATP production, have been observed but these mechanisms are unproven causes of ALS. SOD1 and TDP-43 mutations may play a role in causing mitochondria dysfunction.

    Increased markers of oxidative stress have been observed in sporadic cases of ALS, including 8-Oxo-2'-deoxyguanosine and 4-Hydroxynonenal. This hypothesis is further supported by various risk factors observed for ALS, such as trauma and exposure to certain chemicals that may play a role in increasing oxidative stress. However, failed trials with anti-oxidants and methodological limitation limit the hypothesis. One proposed mechanism of ALS incorporating both the genetic mutations of RNA binding proteins and oxidative stress, suggests that with age cells lose their ability to buffer against the genetic changes due to increasing oxidative stress resulting in the death of sensitive cells. A possible mechanism for dysregulation of glutaminergic neurotransmission may be through excessive oxidative stress of astrocytes.

    Given the co-occurrence and symptomatic overlap with frontotemporal dementia, they may share an underlying pathophysiology, such as dysregulated microRNA activity (possibly originating in a TDP-43 mutation.) However authors cautioned against assuming a causal role of microRNA dysregulation.

    History

    The first gene to be associated with ALS was SOD1, which was identified in 1993. It was the first time that linkage analysis was successful in identifying the genetic cause of a rare neurodegenerative disorder.SOD1 is one of the most common genes associated with ALS, accounting for about 12% of fALS and 1-2% of sALS. The second gene, NEFH, was identified in 1994, followed by SETX in 1998, ALS2 in 2001, DCTN1 in 2003, and CHMP2B in 2006. All of these genes are fairly rare; the next major ALS gene, TARDBP, was identified in 2008 and accounts for 4% of fALS and 1% of sALS. FUS was identified in 2009 and is seen in 4% of fALS and 1% of sALS. VCP was identified in 2010 and accounts for 1% of fALS and 1% of sALS; ATXN2, OPTN, and UBQLN2 were associated with ALS that same year.

    Another major milestone was the discovery of C9orf72 in 2011, which is the most common gene associated with ALS, accounting for about 40% of fALS cases and 7% of sALS cases. C9orf72 was also found to contribute significantly to frontotemporal dementia (FTD). SQSTM1 was also identified in 2011, but accounts for 1% of fALS and less than 1% of sALS. PFN1 was identified in 2012, HNRNPA1 and HNRNPA2B1 in 2013, CHCHD10, MATR3, and TUBA4A in 2014, and TBK1 in 2015. C21orf2, CCNF, and NEK1 were associated with ALS in 2016.

    The first genome-wide association study (GWAS) of ALS was published in 2007, and 14 GWASs total had been published through 2013. They have contributed significantly to our understanding of ALS genetics; for example, a 2010 GWAS studying ALS in Finland led to discovery of the role of mutations at the C9orf72 locus in ALS. However, a gene identified by a single GWAS may not actually be associated with ALS, especially if the cohort size is small. In outbred populations, thousands of cases (people with ALS) and controls (people without ALS) are required for a GWAS to have sufficient statistical power to confidently identify a gene's association with ALS.



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