Densidad

A missense mutation in the UCH-Ll gene, He93Met, which caused a partial loss of the catalytic activity of this thiol protease was identified in a German family with familial

PD (Leroy et aL, 1998), see Section 1.14.4. An affected German sibling pair were found

to have an Ile93Met mutation. Affected members of this kindred became symptomatic with a resting tremor at the age of 51 and 49 years respectively, progressing to rigidity, bradykinesia, and postural instability. Both individuals showed a beneficial response to L-dopa replacement therapy. A paternal uncle and the paternal grandmother were also

affected, although, with the exception of the 2 sibs, all other affected individuals in this

pedigree were deceased. The lack of the affected phenotype in the father of the 2 patients showed that the mutation had incomplete penetrance in the family. Analysis of 500 chromosomes from control individuals of different ethnic backgrounds showed no example of the Ile93Metmutation. The high cross-species conservation of isoleucine93 in human, rat, mouse and yeast indicates its importance in UCH-Ll structure and function.

4.10.1 Collaborative GSPD study on UCH-Ll

To examine the importance of the Ile93Met mutation in UCH-Ll, sequencing of the entire coding region of UCH-Ll was performed in 11 GSPD autosomal dominant families (MF, Jacksonville) as well as 29 smaller families with at least 2 affected sib pairs. The families were all collected as part of GSPD (UK families by myself) and the sequencing results are included here for discussion purposes (see appendix 4). No mutations were detected in the UCH-Ll gene, although mutations in the regulatory or intronic regions of the gene were not sequenced. It was concluded that the Ile93Met variant must either be a rare cause of disease or a harmless substitution whose occurrence in the original family described reflects a chance co-occurrence.

A recent American study investigated the association of PD with this mutation and with a common polymorphism of the same gene (S18Y). The Ile93Met mutation was not identified in either cases or controls but those with the S 18Y polymorphism had a

significantly lower risk of PD (Maraganore et aL, 1999). Analysis of the S18Y

polymorphism in a large German sample of sporadic and familial PD cases also suggested this variant had a protective effect on the pathogenesis of sporadic PD

(Wintermeyer et aL, 2000). However, a further study failed to confirm the influence of

the S18Y polymorphism on the risk of developing PD (Mellick and Silbum, 2000). Overall, the evidence for or against UCH-Ll in familial PD is very weak and requires further work. The fact that other studies have also failed to detect the Ile93Met mutation as a cause of familial PD allow two possible conclusions to be drawn: the first is that the Ile93Met is indeed a rare cause of PD, the second is that the mutation has no effect on one’s risk of developing PD and its occurrence in two affected sibs is just an

accidental occurrence (Leroy et aL, 1998^ Harhangi et aL, 1999; Lincoln et aL, 1999).

These two possibilities can only be discriminated by the identification of the same or other UCH-Ll polymorphisms/mutations that segregate with familial PD or through the observation of this polymorphism in unaffected individuals.

The work reported here in section 4.7 has shown that the Ile93Met mutation is not a major cause of familial PD in Europe. Mutations in the regulatory or intronic regions were not excluded. Other studies have not reproduced the original findings. The frequency of the S18Y polymorphism was reported in 313 patients with sporadic PD

and 302 control subjects (Japanese and C aucasians). The frequency o f the m utant allele (Y) was significantly higher in Japanese control subjects (51.2% ) than in Japanese PD patients (43.4% ) (chi^=3.917, p=0.048<0.05). It appears that this polym orphism has a weak protective factor against PD in at least the Japanese population. T he frequencies o f Y allele and S/Y and Y /Y genotypes in the PD patients and the controls were significantly higher in Japanese than in Caucasian population (p<0.0001). The authors speculated that the role o f this polym orphism in PD m ay be different betw een

C aucasian and Japanese populations. (Zhang et a l , 2000). A further study in G erm an Parkinson’s disease patients showed that although sequence variants in the coding region o f U C H -L l are a rare event a protective effect o f the S18Y polym orphism m ay exist (W interm eyer et a l , 2000). H ow ever, a further A ustralian case-control study using patients with idiopathic PD found that the S18Y polym orphism did not confer

protection against developing idiopathic PD (M ellick and Silburn, 2000).

N evertheless, U C H -L l is a thiol protease and there is a plausible m echanism for its involvem ent in neurodegeneration. In particular, the Ile93M et m utation is considered to cause a partial loss of the catalytic activity o f the enzym e leading to aberrations in the proteolytic pathw ay and aggregation o f proteins (Leroy et a l , 1998). Enzym e kinetic studies o f the m utant and w ild-type proteins show ed that the lle93M et U C H -L l had nearly a 50% reduction in activity than the w ild-type protein. The natural substrate for U C H -L l is unknow n, so although this result m ust therefore be treated w ith caution, two m odels have been suggested which m ay explain the toxic effect o f this m issense

m utation on the neuron. T he first m odel proposes that the m utant protein is less soluble, therefore it cannot be degraded by the norm al processes and sim ply accum ulates. In the second m odel, the catalytic enzym e activity o f U C H -L l is altered by the am ino acid substitution, leading to altered turnover o f the unknow n substrate, w hich by its accum ulation acts as a “seed” for other proteins (Leroy et a l , 1998). Lew y bodies are strongly ubiquinated and im m unoreactive with neurospecific U C H -L l and some subunits o f the 26S proteosom e. A lthough these results do not confirm the involvem ent o f the U C H -L l gene in the pathogenesis o f PD, several reports have exam ined the form ation o f intraneuronal inclusion bodies and proposed that dysfunction o f the ubiquitin-dependent proteolytic pathw ay may be responsible (Lowe et a l , 1990).