**2.13 ALS14 (VCP)**

Recently an exome sequencing study detected a mutation in the gene encoding valosincontaining protein (*VCP*) in an Italian family. Subsequent screening in 210 ALS cases from unrelated families identified four mutations in *VCP* in four different families from Italy and the USA (Johnson et al., 2010). Mutations in the gene for *VCP*, located on chromosome 9p13.3, are a known cause for the multi-system degenerative disease inclusion body myopathy with Paget's disease and frontotemporal dementia (IBMPFD) (Watts et al. 2004). IBMPFD, like ALS, is characterized pathologically by TDP-43 inclusions (Weihl et al., 2008).

VCP is an AAA+-ATPase that mediates ubiquitin-dependent extraction of substrates from multiprotein complexes for subsequent recycling or degradation by the proteasome. It plays a role in a variety of cellular functions including Golgi biogenesis, cell cycle regulation, DNA damage repair and protein homeostasis through the ubiquitin-proteasome system (Ju and Weihl, 2010). It is thought that *VCP* mutations result in the impairment of protein degradation trough both the ubiquitin-proteasome system and autophagy leading to the formation of inclusions. *VCP* mutations found in FTD and ALS have been shown to disrupt TDP-43 localization from the nucleus to the cytoplasm which could be caused by the disruption in protein homeostasis (Gitcho et al, 2009a; Ju and Weihl, 2010). In mice, a missense mutation in vacuolar sorting protein 54, the mouse homologue of VCP, causes motor neuron degeneration (Schmitt-John et al., 2005).

### **2.14 Other fALS associated genes**

In addition to the genes listed in the previous sections, several other genes have been implicated in fALS.

Dynactin 1 (*DCTN1*) was discovered as a candidate gene for ALS when a G59S mutation in this gene was identified in a family with a slowly progressive, autosomal dominant form of lower motor neuron disease without sensory symptoms (Puls et al., 2003; Puls et al., 2005). Subsequent sequencing of the *DCNT1* gene in 250 ALS patients revealed the presence of

Genetics of Amyotrophic Lateral Sclerosis 487

the risk factors identified to date have been consistently replicated. Furthermore, several of these associated genes have overlapping cellular functions such as in RNA metabolism, vesicle trafficking, and axonal transport. In this section, genes that have been associated with sALS will be discussed (Table 2). In addition to these genes, mutations in several fALS associated genes that were discussed in the previous section have been found in a portion of sALS cases.

> *Type of association found*

2 2 SNP association Protein has a role in oxidative

stress

increase risk

haemochromatosis

inclusions are a

number *SMN1* deletions cause SMA

affects SMA disease severity

interaction

results in an

Neurofilament-containing

pathological hallmark of ALS

*SMN2* copy number variation

Possible gene-environment

Peripherin-containing inclusions are a pathological hallmark of ALS. Possible involvement of abnormal splice forms.

Deletion of HRE in promoter

ALS phenotype in mice. Possible gender association.

*Additional information* 

Intermediate polyQ repeats

for sALS/interaction with TDP-43

Mutations are known to cause FTD. All patients have lower motor neuron signs consistent with PMA.

*Negative studies* 

2 0 Mutations

chromatosis 5 1 SNP association Mutations cause hereditary

SNP association/ mutations

2 6 SNP association

A study in 117 Scottish sALS patients showed association of a common SNP resulting in a D148E amino-acid change with ALS (Hayward et al., 1999). This finding was replicated in 169 Irish sALS patients (Greenway et al., 2004). In one study, DNA extracted from CNS tissue from 81 sALS patients was screened but the D184E SNP was not associated with ALS (Tomkins et al., 2000). A different study assessing 134 Italian sALS patients also failed to detect significant association between this SNP and ALS (Coppedè et al., 2010). These inconsistent association

**3.1 Apurinic endonuclease, multifunctional DNA repair enzyme (APEX1)** 

results might reflect a population-specific effect of the *APEX1* D184E allele.

Deletions/ insertions/ mutations

*Associated* 

*APEX* 

*CHMP2B* 

*VEGF* 

*HFE* Haemo-

*NEFH* Neurofilament-

*SMN1* Survival motor

*SMN2* Survival motor

Vascularendothelial growth factor

*Gene Protein Positive* 

Apurinic endonuclease, DNA repair enzyme

Chromatin modifying protein 2B

*studies* 

*ATXN2* Ataxin-2 6 0 PolyQ repeats

heavy 5 3

*PON1, 2, 3* Paraoxonase 7 3

Table 2. Genes associated with sporadic ALS

neuron 1 3 1 Abnormal copy

neuron 2 1 5 Deletions

*PRPH* Peripherin 3 0 Mutations

three heterozygous missense mutations in one sALS and three fALS cases with typical ALS (Münch et al., 2004). An additional mutation was detected in a patient with ALS and his brother who had FTD (Münch et al., 2005). The pathogenicity of these variants has however not been established. Screening for *DCTN1* mutations in a cohort of ALS, FTD or ALS-FTD patients did not result in the identification of disease segregating variants (Vilariño-Güell et al., 2009). One of the missense variants identified in a sALS case was also found in controls in the same study (Vilariño-Güell et al., 2009). Interestingly, five mutations in *DCTN1* were found in eight families with Perry syndrome, a disease that is characterized by Parkinsonism and TDP-43- and ubiquitin- positive inclusions (Farrer et al., 2009).

In a 3-generation family with typical ALS, a mutation in the D-amino acid oxidase (*DAO*) gene was identified (Mitchell et al., 2010). However, screening of an additional 322 unrelated fALS cases did not reveal any other causal mutation in this gene (Mitchell et al., 2010). Additional screening will be needed but *DAO* mutations seem to be very rare in ALS. Because of their structural and functional similarities to FUS, the genes encoding TAF15 RNA polymerase II, TATA box binding protein associated factor (*TAF15*) and Ewing sarcoma breakpoint region 1 (*EWS*) were screened in fALS cases (Ticozzi et al., 2010). Two

missense mutations in *TAF15* (A31T and R395Q) were identified in three fALS cases and not in 1159 controls. However, one of the fALS cases with an R395Q mutation also carried a mutation in *TARDBP*. Moreover, the R395Q is in close proximity to two non-pathogenic variants, suggesting it is a benign polymorphism (Ticozzi et al., 2010).

Recently, a mutation in the sigma non-opioid intracellular receptor 1 (*SIGMAR1*) gene was identified in an autosomal recessive family with juvenile ALS (Al-Saif et al., 2011). Interestingly, variants in the 3'UTR of *SIGMAR1* were described in three ALS-FTD families (Luty et al., 2010).

An X-linked dominant ALS locus has been reported but has not been further described (Siddique et al., 1998). Recently, mutations in the gene encoding ubiquitin-like protein ubiquilin 2 (*UBQLN2*) were identified as the cause of dominantly inherited X-linked ALS and ALS/dementia (Deng et al., 2011).

Several family pedigrees contain individuals affected by ALS, FTD or both. The first linkage study performed in 16 of these ALS-FTD families found linkage to chromosome **9q21-q22**, designated as ALS-FTD1 (Hosler et al., 2000). This association has thus far not been replicated in other ALS-FTD families. Linkage to chromosome 9p in ALS-FTD families (ALS-FTD2) has also been reported. A hexanucleotide repeat expansion in the chromosome 9 open reading frame 72 (*C9ORF72*) gene was recently identified as the causal genetic defect of ALS-FTD2 and will be discussed in a next section (Dejesus-Hernandez et al., 2011; Renton et al., 2011). Mutations in the gene encoding microtubule-associated protein tau (*MAPT*) have been reported in patients with ALS or FTD (Hutton et al., 1998).

Finally, mutations in the neurofilament heavy (*NEFH*) gene and the paraoxonase genes (*PON1, 2, 3*) have been identified in fALS cases and these genes will be discussed in more detail in the following section.
