The field of Parkinson disease (PD) genetics has just grown. A paper in the September 10 Nature Genetics online reveals that PARK9, located on chromosome 1 and responsible for Kufor-Rakeb syndrome, which features dementia in addition to typical symptoms of Parkinson disease, codes for a neuronal-type lysosomal ATPase with no known function. The finding adds a new twist to the study of PD and supports growing evidence linking the disease to malfunctioning lysosomes and neurodegeneration (see ARF related news story). In other PD news this week, two papers report on the links between disease symptoms and a specific mutation in the LRRK2 gene, identified at the PARK8 locus almost 2 years ago. The studies suggest that the relationship between the severity of disease and the G2019S mutation may be quite complex.

Kufor-Rakeb syndrome (KRS) was originally identified in a Jordanian kindred, but Christian Kubisch, University of Cologne, leading an international team of collaborators from Germany, England, Chile, and Jordan, focused on a large Chilean family to identify the gene: several members of the family are afflicted with a disease that closely resembles KRS. Using mutation screening and linkage analysis, first author Alfredo Ramirez and colleagues eliminated some of the usual PD suspects, including loci coding for parkin, DJ-1, and PINK1, but found that PARK9 was linked to a region between the last two loci. After narrowing down this segment to 3.2 Mb of DNA containing about 40 genes, none of which were obvious functional candidates for KRS, Ramirez and colleagues sequenced all the exons of the heretofore uncharacterized ATP13A2, a gene predicted by expressed-sequence tag and homology data. The authors found two mutations in the ATPase, one each inherited from the father and the mother, neither of whom is affected by the disease.

ATP13A2 codes for a large ATPase with 10 transmembrane domains. The first mutation, a single nucleotide deletion, introduces a premature stop codon that truncates the protein just before the last three transmembrane domains. The second mutation, a guanine-to-adenine transition at a highly conserved splice site, is predicted to reduce splicing efficiency by about 90 percent. In fact, when Ramirez and colleagues analyzed RNA from affected family members, they found that exon 13 was completely skipped, removing a large segment of the third transmembrane domain. “The remaining hydrophobic residues of this domain will not be able to span the membrane, which should lead to distortion of transmembrane topology and also to loss of function,” write the authors. In collaboration with Amir Al-Din at King Hussein Medical Center in Amman, Kubisch and colleagues then analyzed DNA from the original Jordanian kindred and found a 22 base pair duplication in ATP13A2 in all affected family members. This duplication also leads to a frame shift, introducing a stop codon that eliminates the six C-terminal transmembrane domains.

How this ATPase, or lack thereof, brings about KRS is unclear. Ramirez and colleagues found that the protein is normally found in the brain, and by laser microdissection analysis they confirmed that it is expressed in individual dopaminergic neurons of the substantia nigra (SN)—which degenerate in PD—and the associated ventral tegmental area. Quantitative RNA amplification experiments also showed that its expression in the brain is strongest in the SN and weakest in the cerebellum.

Because there are no antibodies available for ATP13A2, the authors studied its cellular localization by expressing epitope-tagged proteins in COS7 cells. The native ATPase turned up in the lysosome, while the truncated mutant proteins seemed to get stuck in the endoplasmic reticulum. But very little of the truncated proteins could be detected, indicating that they are actively degraded. The proteasome machinery most likely takes care of these misfolded proteins because Ramirez found that the proteasome inhibitor MG-132 stabilized the truncated variants. “This may at least partially explain the neurodegeneration in KRS—for example, by proteasomal dysfunction owing to overload with mutant ATP13A2, which in turn might cause toxic aggregation,” write the authors.

But Ramirez and colleagues also accept that lysosomal dysfunction following loss of the ATPase may contribute to the disease, perhaps by compromising lysosomal protein degradation. On this point it is worth noting that lysosomal degradation of α-synuclein, which also causes PD if overexpressed or mutated, may be important in PD pathology (see Webb et al., 2003). Also, mutations in other lysosomal proteins have been found in patients with sporadic PD (Aharon-Peretz et al., 2004; Goker-Alpan et al., 2004). And this leads to another fascinating facet of ATP13A2—when Ramirez and colleagues analyzed SN neurons taken postmortem from sporadic PD patients, they found 10-fold higher levels of the ATPase mRNA, indicating, perhaps, a compensatory upregulation of the protein in surviving neurons.

One issue not addressed in the paper is how closely KRS resembles Parkinson disease. KRS, for example, is characterized by additional symptoms, such as dementia, that are not found in typical PD. This issue is discussed in depth in the following Alzforum comment by Mark Cookson from the National Institute on Aging, Bethesda, Maryland.

As for LRRK2, aka “dardarin,” found at the PARK8 locus (see ARF related news story), Matthew Farrer and colleagues at the Mayo Clinic in Jacksonville and the University of Miami, Florida, report that the most common LRRK2, and PD, mutation, a glycine-to-serine substitution at amino acid 2019 of the kinase, results in remarkably varied symptoms. Writing in the September Archives of Neurology, first author Spiridou Papapetropoulos and colleagues evaluated five patients with the mutation. One had familial PD, one had died at age 68 with no pathological signs of neurodegeneration, and the other three had sporadic PD with varying age of onset (41-79) and pathological features. The findings support earlier data suggesting that this mutation is either not fully penetrant or that the age of onset can be sufficiently late that some carriers fail to develop symptoms in their lifetimes.

In a similar vein, and in the same journal, Lianna Ishihara from the University of Cambridge, England, and international collaborators from Europe, Africa, Japan, and the U.S., report that among 26 patients carrying two copies of the G2019S mutation, symptoms of Parkinson disease are no worse than seen in those carrying only one copy of the mutant gene. Again, the data seem to suggest that there is more to the etiology of PD than simply the right titer of mutant LRRK2.—Tom Fagan

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  1. The important question when thinking about the exciting identification of ATP13A2 mutations by Ramirez and others is: is this Parkinson disease? The Kufor-Rakeb syndrome is reported to have many of the features of parkinsonian disorders, including bradykinesia and a response of the extrapyramidal signs to L-DOPA. However, the disease also has several other components that distinguish it from typical PD, such as spasticity, vertical gaze defects, as well as dementia. Some of these signs are so unusual that it has been suggested that the disease is regarded as nosologically distinct from PD, and put into a category with other rare inherited diseases where parkinsonism is part of the clinical spectrum (Williams et al., 2005). In the same article, Williams points out that there is some overlap with the lysosomal storage disease Niemann-Pick disease type C (NPC). This observation, with the appropriate caveat that the L-DOPA responsiveness in Kufor-Rakeb differs from NPC, seems prescient now that the Kufor-Rakeb gene is cloned as a lysosomal ATPase, ATP13A2, by Ramirez and colleagues.

    Mutations turn out, perhaps unsurprisingly given the recessive inheritance of Kufor-Rakeb, to be fairly simple loss-of-function mutations. Although the wild-type protein is found in lysosomes, mutants are associated with ER and other compartments. Most likely, the mutant forms are degraded by ERAD (ER-associated degradation) or a similar process. Given that the mutations remove whole helixes, the protein will probably not fold correctly and will be degraded rather than mature for lysosomal function. This leads to the logical question: what is the function of wild-type ATP13A2 that is so critical for a number of neurons? As Ramirez et al. point out, the substrate specificity of ATP13A2 (or any other type 5 P-type ATPase) is unknown, but this is clearly the next piece of information required for understanding this disease.

    So, given the clinical and molecular evidence to date, should we regard Kufor-Rakeb syndrome as (a) one of the lysosomal diseases that has parkinsonism as part of its spectrum or (b) a disease etiologically related to ”true” Parkinson disease. As has been discussed in other forums, this central question is a common one to many forms of recessively inherited diseases and is critical to our understanding of how (or whether) to place these types of genetic disorders in the same pathway as PD. An obvious way to help our classification would be to have autopsy material available from people carrying ATP13A2 mutations. This would allow us to ask obvious questions such as: is the pathology a synucleinopathy and therefore PD-like or a tauopathy and therefore PSP-like? Is there a myelination deficit? Is there evidence of lysosomal dysfunction and, if there is, is this also seen in sporadic PD? One of the very interesting pieces of data in the Ramirez paper is the observation of increased ATP13A2 expression in sporadic PD, although it is not quite clear how this relates to the loss-of-function pathogenic mutations. Unfortunately, autopsies for these rare recessive diseases, often found in remote kindreds, are rarely performed, so such questions, sadly, may remain open for some time.

    References:

    . Kufor Rakeb disease: autosomal recessive, levodopa-responsive parkinsonism with pyramidal degeneration, supranuclear gaze palsy, and dementia. Mov Disord. 2005 Oct;20(10):1264-71. PubMed.

  2. Ramirez et al. report the identification of mutations in a novel gene, ATP13A2, in Kufor-Rakeb syndrome, an autosomal recessive condition associated with pyramidal tract degeneration, dementia, and parkinsonism. Because of the prominent parkinsonian signs, the causative gene for this condition has been labeled PARK9. Based on prior linkage and analysis of a new family from Chile with this condition, and using a candidate gene approach, the authors have identified three types of mutations in ATP13A2. Although the mechanisms for each mutation are different, they all lead to truncation of the protein and loss of its localization to the lysosomal membrane. Thus, at first glance this appears to be a loss of function condition: it is autosomal recessive, and the mutations appear to lead to loss of function of the normal protein.

    What could be the normal function of this protein? Based on its sequence homology and localization, this predominantly neuronal protein may function as a lysosomal ATPase, preserving the lysosomal pH gradient. Our previous studies had suggested that aberrant α-synuclein can impair lysosomal acidification (Stefanis et al., 2001), providing a common pathogenetic mechanism for parkinsonism based on loss of lysosomal function (Cuervo et al., 2004 and ARF related news story). However, somewhat at odds with this idea, the authors report that mRΝΑ expression of ATP13A2 is actually increased in sporadic Parkinson disease patients. It may be that this occurs as a compensatory response against some sort of lysosomal stressor. The authors argue also for the possibility of gain of function of the mutant forms, through aberrant localization, aggregation within the endoplasmic reticulum, and resultant proteasomal dysfunction. Although I believe that this interpretation is more speculative, it potentially links this new gene with the ubiquitin-proteasome pathway, which is extensively implicated in the context of Parkinson disease and neurodegeneration in general. Whatever the precise mechanism of the effects of ATP13A2 mutants on neuronal homeostasis, this novel discovery further implicates aberrant protein degradation in the pathogenesis of Parkinson disease and related disorders.

    References:

    . Expression of A53T mutant but not wild-type alpha-synuclein in PC12 cells induces alterations of the ubiquitin-dependent degradation system, loss of dopamine release, and autophagic cell death. J Neurosci. 2001 Dec 15;21(24):9549-60. PubMed.

    . Impaired degradation of mutant alpha-synuclein by chaperone-mediated autophagy. Science. 2004 Aug 27;305(5688):1292-5. PubMed.

References

News Citations

  1. Lysosomes and Proteasomes Compete for PD Researchers' Attention
  2. PARK8 is Cloned: Introducing…"Dardarin"

Paper Citations

  1. . Alpha-Synuclein is degraded by both autophagy and the proteasome. J Biol Chem. 2003 Jul 4;278(27):25009-13. PubMed.
  2. . Mutations in the glucocerebrosidase gene and Parkinson's disease in Ashkenazi Jews. N Engl J Med. 2004 Nov 4;351(19):1972-7. PubMed.
  3. . Glucocerebrosidase mutations are an important risk factor for Lewy body disorders. Neurology. 2006 Sep 12;67(5):908-10. PubMed.

Further Reading

Primary Papers

  1. . Hereditary parkinsonism with dementia is caused by mutations in ATP13A2, encoding a lysosomal type 5 P-type ATPase. Nat Genet. 2006 Oct;38(10):1184-91. PubMed.