To date, four complete genomic screens of multiplex families (two or more sampled, affected family members) have been reported for PD (34-37). As with many other complex disorders, the results of these studies are not entirely consistent. However, there is substantial overlap of results in several regions, particularly those on chromosomes 5q (36-38), 6q (36,38), 9q (35,38), and 10q (35,36), suggesting that these regions may be the most promising for follow-up. The overlap on chromosome Xq is also very good in two studies (36,38). The relatively short distances between the peak markers across these independent studies provide evidence for linkage and support the existence of PD susceptibility loci on 5q, 6q, 9q, 10q, and Xq.
Follow-up studies from the Parkinson Study Group with increased sample sizes and/or more stringent definitions of PD (39) have strengthened their strongest initial findings on chromosomes 2q, 6q, and Xq (36). A meta-analysis of these linkages studies has not yet been performed mainly due to the difficulty of combining data from the different genetic markers that have been genotyped in different datasets.
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