**3.6.3 ALS3**

One large European kindred affected by an adult onset, autosomal dominant form of ALS has been linked to chr18q21 (Hand et al 2002). Patients in this family present with classical ALS involving progressive weakness in the limbs and bulbar regions with both upper and lower motor neurone signs. A candidate region of 7.5cM was identified on chr18, however, the pathogenic mutation is not yet known (Hand et al 2002).

### **3.6.4 ALSX**

Linkage analysis of a 5-generation pedigree identified an adult onset, dominantly inherited locus on Xp11-q12. The causative gene has very recently been found to be ubiquilin 2 (*UBQLN2*), which encodes a cytosolic ubiquitin-like protein (Deng et al 2011). Mutation screening of additional cohorts of patients found a further 4 missense mutations in unrelated FALS cases, with all mutations affecting proline amino acids in the proline-x-x repeat region near the carboxyl end of the protein. Clinically, age of onset was variable (16- 71 years) in the affected individuals, and although males were more likely to have an earlier age of onset, disease duration was similar. Some patients also showed symptoms of dementia. Post-mortem material from two unrelated FALS cases showed the classical skein like inclusions were positive for UBQLN2. The identified missense mutations lead to impairment of the protein degradation pathway in a cell model of *UBQLN2*-related ALS.

### **3.6.5 ALS-FTD1: 9p21-q22**

**3.6 Genetic loci linked to ALS 3.6.1 ALS5: Spatacsin (***SPG11***)** 

disturbance (Hehr et al 2007).

**3.6.2 ALS7** 

(Sapp et al 2003).

**3.6.3 ALS3** 

**3.6.4 ALSX** 

2010).

526 Amyotrophic Lateral Sclerosis

The study of three consanguineous Tunisian pedigrees originally established linkage of chr15q15-q21 to an autosomal recessive form of ALS (Hentati et al 1998). A more recent study of 25 unrelated FALS families revealed 10 pedigrees with linkage to the same region and disease associated mutations in the spatacsin (*SPG11)* gene (Orlacchio et al 2010). Clinically, FALS patients with linkage to this region experience a juvenile onset, slowly progressive motor neuropathy associated with both upper and lower motor neurone signs. Disease duration is typically over 10-40 years without sensory symptoms and an absence of the feature of thin corpus callosum (Hentati et al 1998; Orlacchio et al

Mutations in this gene have been previously found to be the most common cause of autosomal recessive hereditary spastic paraplegia with thin corpus callosum (HSP-TCC), a condition characterised by progressive spasticity of lower limbs, mild cognitive impairment and a thin, but otherwise normally structured, corpus callosum (Abdel Aleem et al 2011). All but one of the mutations identified in FALS are also present in HSP-TCC pedigrees and the majority of these are truncating which may suggest a loss of function and a common pathological mechanism between the two conditions (Salinas et al 2008). The *SPG11* gene has 40 exons and encodes the highly conserved Spatacsin protein, which is ubiquitously expressed in the central nervous system (Salinas et al 2008). Although the function of Spatacsin remains unknown, neuropathological studies of HSP-TCC patients with *SPG11* mutations have revealed accumulations of membranous material in non-myelinated axons which are suggestive of axonal transport

To date, only one pedigree with ALS7 and linkage to chr20ptel-p13 has been identified (Sapp et al 2003). The family included 15 siblings, two of which were affected by an autosomal dominantly inherited form of ALS with mid-life onset and a rapid disease course of less than 2 years. The authors found probable linkage to a 6.25cM region of chr20 though more individuals from this pedigree are needed to confirm the findings

One large European kindred affected by an adult onset, autosomal dominant form of ALS has been linked to chr18q21 (Hand et al 2002). Patients in this family present with classical ALS involving progressive weakness in the limbs and bulbar regions with both upper and lower motor neurone signs. A candidate region of 7.5cM was identified on chr18, however,

Linkage analysis of a 5-generation pedigree identified an adult onset, dominantly inherited locus on Xp11-q12. The causative gene has very recently been found to be ubiquilin 2 (*UBQLN2*), which encodes a cytosolic ubiquitin-like protein (Deng et al 2011). Mutation screening of additional cohorts of patients found a further 4 missense mutations in

the pathogenic mutation is not yet known (Hand et al 2002).

A locus for FALS that arises in conjunction with FTD has been identified in 5 American families at chr9p21-q22 (Hosler et al 2000). Affected patients had adult onset of either: ALS and FTD, ALS alone or ALS with dementia. Disease duration was typically less than 4 years although one individual had a slow progression and survived for 15 years. No pathogenic mutations have been identified for this region to date (Hosler et al 2000).

### **3.6.6 ALS-FTD2: 9p13.2-p21.3**

Linkage of autosomal dominant FALS and FTD to chr9p13.2-p21.3 has been established in two pedigrees, one large Dutch kindred and a Scandinavian family (Morita et al 2006; Vance et al 2006). Clinically, all members with ALS had definite or probable ALS by the El-Escorial Criteria with mid-life onset and a typical disease course of around 3 years. In the Scandinavian family ALS and FTD occurred separately, in contrast, affected individuals in the Dutch kindred all had features of both conditions. Linkage has been narrowed down to a 12cM (11Mb) region of chr9, however the pathogenic gene mutations have yet to be identified (Morita et al 2006; Vance et al 2006).

### **4. Conclusion**

FALS accounts for 5% of ALS; an underlying mutation has been identified in approximately a third of these cases (Kiernan et al 2011). FALS causing mutations are used as a window into familial and the clinically indistinguishable sporadic disease; generating genetic models of ALS allows investigations into the mechanisms of motor neuronal degeneration, the identification of therapeutic targets and screening for candidate therapeutic agents (Van Damme & Robberecht 2009). However, the discovery of pathogenic mutations in ALS by linkage analysis is difficult because a relatively low prevalence and rapid disease course make large pedigrees difficult to obtain, therefore novel strategies to identify pathogenic mutations are essential (Hand & Rouleau 2002).

With the evolution of next generation sequencing technology, exhaustive sequencing of exonic regions of the genome has been used to identify pathogenic mutations in the *VCP* gene in ALS, and genetic mutations responsible for other diseases have also been identified from relatively few related or unrelated patients (Bowne et al 2011; Hoischen et al 2010; Johnson et al 2010a; Ng et al 2010; Ng et al 2009; Nikopoulos et al 2010; Simpson et al 2011). Exome sequencing, unlike a linkage analysis and positional cloning approach, is not targeted at a candidate region. Therefore it is likely that a large number of potential genetic variations will be discovered; the difficulty then is to determine which, if any, are pathogenic. However, next generation sequencing offers the potential for identifying at least some of the genes responsible for the remaining uncharacterised causes of FALS.

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An expanded GGGGCC hexanucleotide repeat in *C9ORF72* has just been published as the cause of 9p-linked ALS-FTD, following next generation sequencing of the disease associated region (Renton et al 2011, DeJesus-Hernandez et al 2011). Expansions have been identified not only in ALS-FTD pedigrees, but also in familial FTD, familial ALS and sporadic ALS. Estimated frequencies vary from 23.5% to 46.4% for familial ALS and 4.1% to 21% for sporadic ALS. The expansion, which is non-coding, is therefore the most common genetic cause of ALS identified to date.
