**5.2 Molecular aspects**

232 Novel Aspects on Epilepsy

prosaposin that can be cleaved to saposins A, B, C and D (Gopalakrishnan et al., 2004). Most

Only four disease-causing mutations have been described to date (http://www.ucl.ac.uk/ncl).

Sialidosis (MIM#256550) is a LSD caused by the inherited deficiency of the lysosomal enzyme alpha-N-acetyl-neuraminidase-1 (NEU1), which cleaves the terminal sialic acid

Therefore, the deficiency of NEU1 leads to the accumulation of sialic acid (Nacetylneuraminic acid) covalently linked to oligosaccharides and/or glycoproteins. This aspect distinguishes sialidoses from sialurias, in which the neuraminidase activity is normal or elevated with a storage and excretion of 'free' sialic acid, rather than 'bound' forms.

A systematic classification of Sialidosis has been provided by Lowden and O'Brien in 1979, who divided them in two main clinical variants: Type I, the milder form of the disease, which lacks the physical changes (normosomatic) and Type II, a more severe form with an earlier onset, which can be subdivided in 2 different phenotypes: congenital/neonatal and

Patients affected with type I sialidosis, (normomorphic or 'cherry-red-spot, myoclonus syndrome'), generally manifest first clinical signs during school-age period or early adulthood. Progressive reduction of visual acuity, red-green and night blindness, bilateral cherry-red spots, punctate corneal opacity and nistagmus, are prominent symptoms. Ocular involvement is accompanied or followed by the appearance of motor impairment, with walking difficulties and myoclonus. Same cases may present seizures. In contrast with type II forms, these patients generally do not present dysmorphisms or bone dysplasia and they

Type II congenital sialidosis may manifest in utero with foetal hydrops or foetal ascites while the neonatal form is characterised by diffused edema, hepatosplenomegaly, ascites and Hurler's like clinical signs: facial dysmorphisms, umbilical and inguinal hernias, short trunk with a prominent sternum, kyphosis, and dysostosis multiplex (Froissart et al., 2005). Severe dysmrphisms (coarse facies, pectus carinatum, short trunk, exaggerated toracic kyphosis, and wadding gait) as well as growth delay characterize also infantile phenotypes, cherry-red spot, corneal opacity, hearing loss, progressive neurodegeneration and cognitive deterioration with myoclonic seizures. Skeletal imaging shows dysostosis multiplex with vertebral abnormalities and generalized osteoporosis. Renal involvement, nephrosialidosis, may be present in some patients with proteinuria evolving to nephrotic syndrome (Okada et

Juvenile onset is charactrized by less pronounced dysmorphic signs with muscular hypotonia and hypotrophy, ataxia, and myoclonic seizures. Cherry-red spots and corneal opacities are constantly present, as well as hearing loss. Pyramidal syndrome with cerebellar anomalies and peripheral neuropaty have been described. Mental retardation is costant. Survival rarely exceed the second, third decade of life (Winter et al., 1980; Caciotti et al.,

patients accumulate autofluorescent lysosomal deposits with GRODs.

residues of several oliogosaccharides and polypeptides.

have a normal intelligence. Survival is usually long.

**5. Sialidosis** 

**5.1 Clinical aspects** 

juvenile forms.

al., 1983).

2009; Canafoglia et al., 2011).

The human *NEU1* gene (Gen Bank AF040958) has been located to chromosome 6p21.3 within the region of the major histocompatibility complex (Bonten et al., 1996; Pshezhetsky et al., 1997). It contains 6 exons and spans approximately 3.5 kb of genomic DNA (Milner et al., 1997).

The *NEU1* gene encodes a protein of 415 aminoacids including a signal sequence, a central hydrophobic core and a more polar c-terminal domain (Bonten et al., 1996). After the removal of the signal peptide and glycosilation the protein would have a molecular mass of 45 kD. In fact, western blot studies have demonstrated the presence of two major bands of 44 -45 kD which yielded a 40 kD protein after de-glycoslation (Bonten et al., 1996). NEU1 exists as a multienzyme complex with at least two other proteins,-galactosidase and the protective protein/cathepsin A (PPCA) (d'Azzo et al., 2001). The association with PPCA is necessary for its enzymatic activity. The association with PPCA stabilizes the active conformation of NEU1 in lysosomes. Moreover, since NEU1 is poorly mannose 6-phosphorylated, it depends on PPCA for its correct compartmentalization and catalytic activation in lysosomes (van der Spoel et al., 1998; van der Spoel et al., 2000; Yamamoto et al., 1987).

About 45 different mutations in *NEU1* gene have been reported to date (http://www.hgmd.org/). Almost all of them have been found in single families and most of them are missense mutations. Bonten et al. have studied the impact of some missense mutations on NEU1 protein distribution and catalytic activity and they classified these mutant proteins in 3 groups: 1-catalytically inactive and not lysosomal; 2-catalytically inactive and lysosomal and 3-catalytically active and lysosomal. A good correlation between the residual activity of mutant proteins and the severity of the disease has been found. In fact, patients with the severe type II infantile form presented mutations from group 1 while those with a mild form of type I disease had at least one mutation from group 3. Mutations from group 2 were found mainly in patients with the juvenile form of type II sialidosis with an intermediate phenotype (Bonten et al., 2000).
