**3. TAR DNA-Binding Protein 43 (TARDBP gene)**

TAR DNA-binding protein 43 is homologous to the heterogeneous nuclear ribonucleoproteins (hnRNPs) [33], which are involved in RNA processing, and its abnormal cellular distribution is one of the key feature of ALS and frontotemporal lobar degeneration (FTLD) [34].

The protein is highly conserved, widely expressed and predominantly localized to the nucleus with a very small amount being present in the cytoplasm [34-35].

### **3.1. Genotype**

early age of onset, spinal onset with proximal distribution weakness, bulbar involvement and

Another mutation, L106P, has been found in a patient who presented similar clinical pattern with spinal onset with weakness mainly in proximal areas; however in this patient, 30 months after disease onset, weakness remained restricted to the upper limbs without pyramidal signs and it was consistent with brachial amyotrophic diplegia, a relatively slowly progressive

Corrado et al. [2] suggested that the nonsense mutation in exon 5 was present in SALS patients with severe and rapid clinical course, analogous to what found for most SOD1 mutations leading to a truncated protein. Conversely, N65S and A95T are both associated to a slowly progressive course of the disease, similarly to other mutations (H46R, D76V, H13T, L144P, G93V, I151T, D90A, A89T) detected in patients with a disease duration >10 years. In addition N65S seems to be strictly correlated to a prevalent involvement of the lower motor neurons

In 2011 in their article, Del Grande et al. [3] showed a similar phenotype in three unrelated patients with sporadic SOD1 mutation D11Y: slow progression, initial distal limbs muscles involvement and predominant lower motor neurons signs. The topographic distribution in distal muscles was a constant feature over many years, with only late impairment of proximal or bulbar muscles (respiratory muscles involvement after 7-10 years). All three patients had slight pyramidal signs (hyperactive reflexes, Babinski sign without increase of muscular tone).

In 2011 Luigetti et al. [24] described a strange case report of a sporadic patient with SOD1 G93D mutation disclosing a rapid progression of the disease. The beginning of symptoms was weakness in upper limbs, without involvement of lower limbs or bulbar functions. Over a 2 year-follow up the patient showed a rapidly progressive course with involvement of lower limbs, bulbar and respiratory muscles and the patient died after 30 months since the onset.

This case is in contrast with literature data [25-27]: other patients with SOD1 mutation (FALS) presented a slowly progressive diseas with a long-lasting paucisymptomatic phase. The authors discovered a novel heterozygous ANG missense mutation (c. 433 C>T, p.R145C), so

Penco et al. [28] described a family with same mutation of SOD1, in which there was wide variability of disease expression among family members. The ANG IVS1+27 variant in the heterozygous state was found in the proband that disclosed an aggressive clinical course. Though this variant occurred in noncoding region and no prediction of splicing alteration was

These findings support a possible pathogenetic role of ANG mutation with influence on clinical

Often bulbar onset is associated with older age of disease presentation without significant

made, the authors speculated that this variant contributed to the clinical phenotype.

they hypothesised a role in pathogenesis and clinical phenotype [24].

manifestations in patient with SOD1 mutation.

difference of distribution between FALS and SALS [21].

These data are confirmed by international literature [29-32].

a rapid disease course about two years.

80 Current Advances in Amyotrophic Lateral Sclerosis

variant of motor neuron disease [23].

and only at the spinal cord.

The human *TARDBP* gene (Entrez Gene ID 23435) is located on chromosome 1p36.22, and it codes for a protein of 414 amino acids.

Mutations in *TARDBP* gene associated with ALS disease have been discovered fro the first time in 2008 [34, 36].

The proposed mutational frequency is ~5% for FALS and 0.5-2% for SALS. To date, more than 30 different TARDBP mutations have been described, all of which are missense substitutions. With a single exception, all of them are clustered in the C-terminal glycine-rich region encoded by exon 6. The most common mutation is A382T.

Mutations in *TARDBP* gene associated with ALS disease have been discovered fro the first time in 2009 and in the same year a Italian screening has been performed [9]. The Italian results showed a higher frequency of *TARDBP* mutations in SALS Italian patients compared to individuals of mainly Northern European origin (2.7% vs. 1%).

The frequency of mutations in *TARDBP* gene in Italian patients (4.4%) are similar to other population studies (about 3 to 4% of FALS cases) [37, 38].

Most *TARDBP* mutations are missense changes in exon 6, encoding for Gly-rich C-terminal region that allows to bind single-stranded DNA, RNA and proteins [39, 40] (Figure 3).

**Figure 3.** Distribution of TDP-43 mutations detected in sporadic ALS patients [41].

#### **3.2. Phenotype**

Many individuals who present with a pure ALS phenotype also develop pathological features of FTD and vice versa. Recently TDP-43 is identified as the major protein of inclusions in FTD and ALS brain tissues, suggesting that both degenerative diseases belong to a clinio-patho‐ logical spectrum of overlapping central nervous system disorders. Dominant mutations in the gene encoding the deposited protein account for at least some cases of these diseases.

**4.1. Genotype**

45], as *TARDBP*.

**4.2. Phenotype**

and ~1% in SALS [46, 47, 48].

codes for a protein of 525 amino acids.

The human *FUS/TLS* gene (Entrez Gene ID 2521) is located on chromosome 16p11.2, and it

Genetics of ALS and Correlations Between Genotype and Phenotype in ALS — A Focus on Italian Population

http://dx.doi.org/10.5772/56547

83

Mutations in *FUS/TLS* gene in ALS patients have been discovered for the first time in 2009 [37,

Following the original reports [37, 45], several other groups identified additional variants in ALS cohorts of different ethnicities, proposing an overall mutational frequency of ~4% in FALS

To date more than 30 different mutations have been described, the vast majority of which are

In the next year a Italian screening has been performed [10]. The results of the Italian screening are in accord with the interanation screening. The Italian data showed that the percentage of

In 2010 Corrado et al. [10] identified 9 SALS or FALS patients carrying FUS missense mutations. Site of onset was both in upper and lower limbs, age at onset was lower (median 50 years). One of these patients with sporadic ALS and FUS mutation presented, at the age of 34, bilateral scapular girdle muscle weakness with unusual neck flexor/extensor muscle weakness with cramps and fasciculations, no weakness in the lower limbs has been demonstrated. Another patient with FALS and FUS mutation, at the age of 54, slowly developed weakness of the neck flexor/extensor muscles and bilateral scapular girdle and proximal upper limb muscle weakness, with subsequent impairment of the pelvic girdle, no bulbar involvement has been shown but slight proximal weakness in the lower limbs; one year after onset, the symptoms extended to the bulbar region. So these two patients developed an unusual proximal sym‐

Ticozzi N et al. in 2009 [49] described two patients with FALS and mutation of FUS that

missense substitutions and the rest are frameshift or nonsense mutations (Figure 3).

*FUS* missense mutations is 0.7% of Italian SALS cases, 4.4% in FALS.

**Figure 4.** Distribution of FUS mutations detected in sporadic ALS patients [41].

metrical upper limbs onset and axial involvement.

presented the same clinical phenotype.

Corrado et al. [9] described 12 different missense mutations in TARDBP, all located in exon 6, in 18 patients with ALS, both FALS and SALS. Patients don't share a homogeneous clinical phenotype: the average age at onset is 53,2 +/- 14,5 years, the site of onset is mainly spinal (88%), disease duration varies from 17 to 87 months. But in contrast to what is expected from the similarity of TDP-43 pathological deposits in ALS and FTD, none of patients tested worldwide with FTD carried TARDBP mutations. On the contrary, a TARDBP mutation (p.G294V) is discovered in patients with ALS and dementia of Alzheimer type. He developed dementia 3 years before the onset of MND. It is possible that the concurrence of the two diseases is only by chance.

Piaceri et al. [42] described clinical heterogeneity in patients with ALS and mutations in TARDBP. Age at onset is between 49 and 62 years, site of onset is both spinal and bulbar with different involvement of upper or lower motor neuron, disease duration varies from 9 to 85 years. Nobody has FTD. One patient has p.ALA382Thr mutation in exon 6.

In literature also [9, 36] this mutation is associated with some differences in phenotype, that are in site of onset (bulbar-spinal in France and spinal in Italy), disease duration (28-73 months in France, 17-60 months in Italy) and age at onset (50 years in France, 32-69 years in Italy). Italian and French patients shared a common haplotype with allele D1S2667 and D1S489, so there was a common founder for the mutation.

Literature datas [43] suggested that in TARDBP patients site of onset is in the upper limbs, with both upper and lower motor neuron signs but with disease progression lower signs became predominant. Age at onset is mean of 54 years, disease duration mean of 58 months. Some patients presented cognitive impairment that met criteria for FTD.
