A linkage study of candidate loci in familial Parkinson's Disease

Parkinson's disease (PD) is considered to be a multifactorial disease. Seventy to ninety percent of cases are estimated to be sporadic, while ten to thirty percent are familial [1]. Several rare PD families have been reported displaying different modes of inheritance. In the majority of families there is no clear inheritance pattern; however, in some cases is the disease clearly inherited in an autosomal dominant or an autosomal recessive mode. Clinically and neuropathologically, familial and sporadic PD seem to be similar [1], although in some familial cases atypical features accompany parkinsonism. In frontotemporal dementia linked to chromosome 17 (FTDP-17), patients display frontal lobe dementia and mutations have been identified in the Tau gene on chromosome 17 [2]. Dystonia is a prominent feature in dopa-responsive dystonia, a disease in which mutations have been found in the GTP cyclohydrolase 1 gene on chromosome 4 [3].

To date, ten genes have been implicated in PD. The first PD mutation was found in the α-synuclein gene on chromosome 4 in a large Greek-American kindred with autosomal dominant inheritance [4]. Clinical and neuropathological features were similar to sporadic PD, although with relatively early disease onset and rapid course. In 1998, mutations in the Parkin gene were reported to be segregating with juvenile autosomal recessive parkinsonism [5]. Subsequently, a large number of Parkin mutations were reported both in familial early-onset PD and sporadic early-onset cases [6]. A mutation was also found in the Ubiquitin carboxy-terminal hydrolase L1 (UCH-L1) gene in one German sib pair with PD [7]. Recently, mutations in the DJ-1 gene were associated with autosomal recessive parkinsonism in two European families [8]. In addition to the genes described above, studies have linked six previously uncharacterized loci to PD. The PARK3 locus on chromosome 2 was linked to the disease in several autosomal dominant PD families and the penetrance of the putative gene was estimated at about forty percent [9]. In 1999, a chromosome 4p haplotype was reported to be segregating with the disease in a large pedigree with autosomal dominant PD [10]. In this pedigree individuals who did not have PD but postural tremor also shared the haplotype, suggesting a common mutation for the two phenotypes. In a large Sicilian family with autosomal recessive early-onset parkinsonism a locus on chromosome 1p (PARK6) was reported to be linked to the disease [11]. Hicks and coworkers reported linkage of another locus on chromosome 1p (PARK10) to late-onset PD in 51 Icelandic families [12]. Further, the PARK8 locus on chromosome 12 was linked to PD in a Japanese family with autosomal dominant parkinsonism [13]; and recently, a PD susceptibility locus on chromosome 2 was reported in 150 PD families [14].

In these 12 PD families there was no evidence for linkage to any of the loci tested using parametric linkage analysis under a model of autosomal dominant inheritance with reduced penetrance, as well as non-parametric linkage analysis. When performing parametric linkage analysis it is important to specify both the disease-locus and marker-locus parameters correctly. Generally, multipoint analysis is more sensitive to various types of errors than two-point analysis [35, 36]. In two-point analysis misspecification of inheritance mode may result in overestimation of recombination fraction and reduced power to detect linkage, while in multipoint analysis false exclusion of linkage may occur [36]. In addition, genotyping errors may cause false exclusion in multipoint analysis [37] and incorrect allele frequencies and recombination fractions may result in reduced power and/or false positive findings [38]. In the present study we performed both two-point and multipoint analysis, the former being the most robust type of analysis and the latter using the most information. Both when narrow and broad PD were analyzed there were only small differences between two-point parametric lod scores at theta = 0 and multipoint parametric lod scores. At theta = 0.1, differences between two-point and multipoint lod scores were larger, although the scores were still negative for all markers except three. Nevertheless, this fact may indicate possible presence of bias in the parametric multipoint analysis due to errors in specification of the model parameters.

One problem related to the specification of inheritance model is low penetrance. Given the complex phenotype, including only affected individuals in the analysis may be preferable. To compare this approach with the results from the autosomal dominant model with reduced penetrance including all individuals, we performed affecteds-only analysis for the PARK3 locus, which also initially was identified using this approach [9]. Compared to the autosomal dominant model with incomplete penetrance, multipoint parametric lod scores generated by the affecteds-only model were higher (less negative), indicating a loss of information. Accordingly, information content as given by Genehunter was markedly reduced. Thus, despite the fact that affecteds-only analysis had been preferable, the loss of information content precluded further affecteds-only analyses for the other loci. Since inheritance mode in the families appeared autosomal dominant, it seemed appropriate to perform the analyses under such a model. Using data from a select number of markers we evaluated age-dependent penetrance versus low penetrance for all age groups. The latter model generated somewhat higher (less negative) lod scores, indicating more conservative values, however there were no large differences. Since low penetrance for all age groups imposes fewer restrictions to the model, we subsequently chose to model penetrance in this way.

The parametric linkage analysis was complemented with a non-parametric linkage approach, which has the advantage of not making assumptions about inheritance mode although it is less powerful than parametric linkage analysis. This fact is reflected by the data in the sense that the non-parametric multipoint lod scores often approached zero. Generally, the non-parametric analysis generated higher lod scores than the parametric, a fact that was most apparent when comparing multipoint non-parametric lod scores to two-point lod scores at theta = 0 and multipoint parametric lod scores, respectively. When comparing multipoint non-parametric lod scores to two-point lod scores at theta = 0.1, differences were smaller. The reason for the discrepancy is most likely that in parametric analysis assumptions are made about mode of inheritance, imposing restrictions to the data structure. Again, multipoint parametric analysis is more sensitive to modeling errors than two-point analysis, with the possible effect of inducing false exclusions [35, 36]. In summary, although the multipoint parametric results indicated exclusion of linkage, our sample contained too few affected individuals in order to draw definite conclusions from the non-parametric analysis. Thus, we are less inclined to say that linkage is excluded based on the non-parametric results.

One fact that might reduce our possibility to demonstrate linkage is heterogeneity. At present there is substantial evidence that several genes contribute to PD. Unfortunately, we were unable to investigate heterogeneity in this study due to the small sample size. However, some families demonstrated positive lod scores for a few loci, indicating that heterogeneity might exist despite our selection of Caucasian families of European descent from a limited geographical area.

The etiology of PD is considered to be complex, involving several genetic factors as well as potential interactions between genetic and environmental factors [1]. We conclude that using both parametric and non-parametric linkage analyses there was no evidence for linkage to PD in the families included in this study. Although linkage was excluded for the majority of loci under an autosomal dominant model with incomplete penetrance, we could not exclude linkage using non-parametric methods. The fact that none of the loci tested showed negative lod scores in all families demonstrates the possibility that some of them are of relevance to individual families. Alternatively, other loci, known or yet unknown, may be associated with PD in these families.