Mutation or duplication of the protein coding sequence of the α-synuclein gene (SNCA) can cause autosomal dominant inherited Parkinson disease, but the genetic risk factors for the far more common sporadic form of the disease remain murky. Now, a large collaborative analysis has identified polymorphisms in the SNCA gene promoter that increase the risk of sporadic Parkinson disease. The new findings support the proposition that elevated expression of α-synuclein contributes to PD pathogenesis in the general population, and links pathology of sporadic PD to inherited forms of the disease, both of which feature synuclein-containing inclusions. The results, which appear in the August 9 issue of JAMA, come from the Genetic Epidemiology of Parkinson’s Disease (GEO-PD) Consortium.

On the tau front, three independent studies just out in Nature Genetics report the first examples of deletions of the tau gene in humans. Microdeletions spanning the tau gene and several other genes in the immediate area cause mild mental retardation with characteristic facial features and delayed development. While the studies do not unequivocally identify tau as the cause of the deletion phenotype, the results are consistent with an important role for the protein in normal human development.

Some studies have already pointed to SNCA promoter variants, and in particular, the length of a dinucleotide repeat sequence (REP1), as risk factors for sporadic PD, but the results have been inconsistent. To get a clearer picture, corresponding author Demetrius Maraganore of the Mayo Clinic in Rochester, Minnesota, and the consortium members performed a combined genetic analysis of PD patients from 18 participating sites around the world. Each site reported on genotyping of patients, and provided DNA for a central analysis of a subset of patients. All together, 2,692 patients and 2,652 controls were analyzed—data on about half had been previously published.

The authors identified three common REP1 alleles, consisting of 259, 261, and 263 base pairs. The 263-base-pair allele was significantly associated with PD in the cases versus controls (Odds ratio, 1.43; 95 percent confidence interval, 1.22-1.69; p <.001 for trend). There was also a trend toward reduced disease risk with the shortest allele, the 259-base-pair repeat. The results are consistent with a previous meta-analysis of published studies, except that the strongest effect detected in that analysis was protection by the shorter alleles, with a trend toward increased risk with longer alleles (Mellick et al., 2005).

To more broadly correlate promoter variability and disease, the investigators also analyzed two different single nucleotide polymorphisms. Neither showed any disease association, and only haplotypes that included the longer REP1 variant were associated with PD. Biologically, the results are consistent with increases in synuclein expression conferring risk for PD: Rep1 is major promoter for SNCA expression, and longer REP1 repeats are associated with increased promoter activity in vitro (Chiba-Falek and Nussbaum, 2001).

“Our study demonstrates that the SNCA gene is not only a rare cause of autosomal dominant Parkinson disease in some families, but also a susceptibility gene for Parkinson disease at the population level,” the authors conclude. They estimate that REP1 variability could explain about 3 percent of risk for PD in the general population. While the combination of REP1 with other variants could account for a large part of disease risk, they suggest, the small effect sizes of individual variants illustrates the need for large-scale, carefully standardized studies like this to uncover associations. The results are reminiscent of recent data on the LRRK2 protein: Mutations in the LRRK2 gene cause inherited forms of Parkinson disease but are also found in sporadic cases as well (see ARF related news story).

Tracking down tau, three groups—a British team led by Charles Shaw-Smith, Alan M. Pittman and Lionel Willatt; a group under Bert de Vries in Nijmegen, The Netherlands; and researchers working with Evan Eichler in Seattle, Washington—all took different paths to 17q21.31, the region containing the tau gene. But what they found there was entirely concordant, that a recurrent deletion of the region causes a syndrome of mild mental retardation, distinct facial features, and poor muscle tone. The deletion was present in up to 1 percent of mentally retarded patients, and appeared to result from homologous recombination within a common inversion polymorphism in the area. While gain-of-function mutations in tau are well known for causing frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17) and other neurodegenerative conditions, this is the first report of humans with tau deficiency. Although the deletion contained at least three other genes, tau represents a good candidate gene for generating the observed phenotype. Tau knockout mice show some muscle weakness and learning problems, and some subtle histological abnormalities in some axons, consistent with a role for human tau in normal development. The findings tentatively suggest that gain and loss of tau functions can lead to very different phenotypes.—Pat McCaffrey

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  1. This work confirms the association between synuclein and Parkinson disease (PD), which has been reported many times and complements a recent and excellent paper from the Gasser group. As with all meta-analyses, however, one has to worry that publication bias has prevented the publication of negative studies. The synuclein association for sporadic PD resembles the tau association for sporadic tangle disorders and seems to reflect a general principle that genetic variability at the loci underlying autosomal dominant disease contribute to the risk of their sporadic counterparts (Singleton et al., 2004) through a mechanism which involves increased expression of the risk allele.

    References:

    . The law of mass action applied to neurodegenerative disease: a hypothesis concerning the etiology and pathogenesis of complex diseases. Hum Mol Genet. 2004 Apr 1;13 Spec No 1:R123-6. PubMed.

  2. This study builds upon existing genotyping and combines studies to provide a compelling confirmation of the well-reported association between allele length of the promoter polymorphism REP1 in α-synuclein (SNCA) and risk for Parkinson disease (1-5). These data, along with previous functional data (6) and the observation that triplication of the SNCA locus causes disease (7) are absolutely consistent with the idea that increased production of α-synuclein is a risk factor for Parkinson disease and other Lewy body disorders. While this is unlikely to be predictive of disease risk, it reaffirms the predicted role of expression of the deposited species in diseases of protein deposition (8), consistent with SNCA multiplication in PD and trisomy 21/APP duplication in Alzheimer disease (9).

    References:

    . Increased susceptibility to sporadic Parkinson's disease by a certain combined alpha-synuclein/apolipoprotein E genotype. Ann Neurol. 1999 May;45(5):611-7. PubMed.

    . Alpha synuclein promoter and risk of Parkinson's disease: microsatellite and allelic size variability. Neurosci Lett. 2003 Jan 9;336(1):70-2. PubMed.

    . Alpha-synuclein haplotypes implicated in risk of Parkinson's disease. Neurology. 2004 Jan 13;62(1):128-31. PubMed.

    . Australian data and meta-analysis lend support for alpha-synuclein (NACP-Rep1) as a risk factor for Parkinson's disease. Neurosci Lett. 2005 Feb 28;375(2):112-6. PubMed.

    . Association of alpha-synuclein Rep1 polymorphism and Parkinson's disease: influence of Rep1 on age at onset. Mov Disord. 2006 Apr;21(4):534-9. PubMed.

    . Functional analysis of intra-allelic variation at NACP-Rep1 in the alpha-synuclein gene. Hum Genet. 2003 Oct;113(5):426-31. PubMed.

    . alpha-Synuclein locus triplication causes Parkinson's disease. Science. 2003 Oct 31;302(5646):841. PubMed.

    . The law of mass action applied to neurodegenerative disease: a hypothesis concerning the etiology and pathogenesis of complex diseases. Hum Mol Genet. 2004 Apr 1;13 Spec No 1:R123-6. PubMed.

    . APP locus duplication causes autosomal dominant early-onset Alzheimer disease with cerebral amyloid angiopathy. Nat Genet. 2006 Jan;38(1):24-6. PubMed.

References

News Citations

  1. Not Just a Family Affair: Dardarin Mutations Predict Sporadic PD

Paper Citations

  1. . Australian data and meta-analysis lend support for alpha-synuclein (NACP-Rep1) as a risk factor for Parkinson's disease. Neurosci Lett. 2005 Feb 28;375(2):112-6. PubMed.
  2. . Effect of allelic variation at the NACP-Rep1 repeat upstream of the alpha-synuclein gene (SNCA) on transcription in a cell culture luciferase reporter system. Hum Mol Genet. 2001 Dec 15;10(26):3101-9. PubMed.

Further Reading

Primary Papers

  1. . Collaborative analysis of alpha-synuclein gene promoter variability and Parkinson disease. JAMA. 2006 Aug 9;296(6):661-70. PubMed.
  2. . A new chromosome 17q21.31 microdeletion syndrome associated with a common inversion polymorphism. Nat Genet. 2006 Sep;38(9):999-1001. PubMed.
  3. . Discovery of previously unidentified genomic disorders from the duplication architecture of the human genome. Nat Genet. 2006 Sep;38(9):1038-42. PubMed.
  4. . Microdeletion encompassing MAPT at chromosome 17q21.3 is associated with developmental delay and learning disability. Nat Genet. 2006 Sep;38(9):1032-7. PubMed.