Parkinson Advances: Cloning Cure for Mouse Model, New Gene Identified

Scientists took a step closer to a stem cell treatment for neurodegenerative disease this week. In the March 23 Nature Medicine online, researchers led by Lorenz Studer at the Sloan-Kettering Institute, New York, reported using a therapeutic cloning strategy to produce stem cells that ameliorate Parkinson-like symptoms in a mouse model of the disease. The success of the strategy suggests that it may be feasible in humans, too, though there is much work to be done before that becomes a reality. In other Parkinson disease (PD) news this week, researchers report that they have narrowed down PARK11, one of 13 regions of DNA linked to familial PD, to the gene for a protein called GIGYF2. Though its exact function is unknown, there are indications this protein may interact with the product of another well-studied Parkinson gene, LRRK2. The PARK11 study was reported in the March 19 The American Journal of Human Genetics online.

Therapeutic cloning, the ability to generate de novo new tissue that is a genetic match to an adult, is of great interest to sufferers and scholars of neurodegenerative diseases alike, because it raises the possibility that neurons lost to PD, Alzheimer’s, ALS, and other diseases could be replaced. One strategy for therapeutic cloning is somatic cell nuclear transfer (SCNT), whereby a nucleus from a patient’s own cell, often a skin cell, is transferred into an enucleated egg. The transfer can kick start embryogenesis, yielding a rich source of pluripotent embryonic stem (ES) cells. ES cells can, in theory, be turned into any cell type for transplant. Put back into the donor, nuclear transfer-derived ES cells (ntES) avoid the risk of immune rejection because the cells are a precise genetic match to the host. SCNT has been successfully used to generate stem cells from mammals, most famously Dolly the sheep, but not yet from humans (see ARF related news story).

Studer and colleagues show the first example of autologous transplantation by this method, using SCNT to derive dopaminergic neurons for transfer into mice with Parkinson-like symptoms. First author Viviane Tabar and colleagues used skin cells from affected mice as a source of nuclear material for SCNT. From 24 parkinsonian mice, the researchers generated 187 ntES cell lines—at least one from each mouse. The researchers expanded the cell lines, then used them to generate dopaminergic neurons that were transplanted back into the same hosts. Over the next 11 weeks the researchers put the mice through a range of behavioral tests to check for improvement of Parkinson-like symptoms.

The researchers found that mice receiving the transplants (100,000 ntES derived cells with about 20 percent dopaminergic neurons) had significant improvement in all tests. For example, amphetamine-induced rotations dropped from about 6 per minute before transplantation, to zero 11 weeks after. Spontaneous rotations also fell to zero and the animals’ tendency to favor the unaffected paw (dopaminergic lesions were only induced on one side) also was abolished. The authors conclude that Although technically complex, with an average interval of >10 months from lesioning to transplantation endpoint, these data demonstrate the feasibility of treating individual parkinsonian mice via therapeutic cloning and suggest considerable therapeutic potential for the future.

Whether this technique will ever be used in humans remains to be seen. There are ethical objections to human cloning, even for therapeutic purposes, and there are technical obstacles. Getting SCNT to work with human tissue is a giant hurdle, but even if that is accomplished, achieving the correct transplant dose may also take some fine tuning. In this work, for example, one of the mice showed signs of neural overgrowth. The graft grew to >100,000 dopaminergic neurons as opposed to the ~19,000 seen in the other animals. In addition, stem cell transplantation has been going on in Parkinson’s patients for some time using allogenic grafts, and getting the right dose was also one of the early problems with this treatment. In some patients the grafts effectively overdosed recipients with dopamine, leading to unwelcome side effects (see ARF related news story). Those experiments have been curtailed because of insufficient stem cell lines, which is one reason why advances in SCNT technology is so welcome.

The second paper sheds some light on the biology behind some familial forms of PD. Researchers led by Robert Smith at Brown University, Providence, Rhode Island, have narrowed down the PARK11 locus to a single gene. PARK11 is located on a region of chromosome 2 that contains 73 potential gene candidates and several microsatellite markers. The genetic marker that has the highest statistical linkage lies within the gene for GIGYF2 (Grb10-Interacting GYF Protein-2), which has no clear function. Now, first author Corinne Lautier and colleagues have sequenced the entire GIGYF2 gene in families affected by PD, and controls. They conclude that GIGYF2 is a strong candidate for the PARK11 gene.

Lautier and colleagues genotyped 123 Italian and 126 French-Caucasian patients with inherited PD and also 131 Italian and 96 French controls. They found seven different GIGYF2 missense mutations among 12 unrelated PD patients. They also found DNA insertions in four other patients. None of these mutations turned up in controls. In four families with amino acid substitutions in more than one PD case, the mutations segregated with the disease, indicative of causality. However, the researchers also found that there are unaffected carriers, suggesting that the mutations are not 100 percent penetrant.

The study is clearly very exciting but needs independent replication in another laboratory, said Mark Cookson, Institutes of Health, Bethesda, Maryland, via e-mail. He added that although some segregation data is provided, which supports pathogenicity and is consistent with dominant inheritance at reduced penetrance, the gene is very polymorphic and it is important to show that the mutations are definitively causal in additional families. One approach would be to sequence GIGYF2 in the original samples that generated the linkage peak for PARK11, and thus show that the pathogenic mutations segregate with disease.

In the meantime, the finding might also spur some new avenues of research. For example, it is not clear what GIGYF2, or its homolog GIGYF1, does. Both bind to Grb10, a protein adaptor, and Smith and colleagues showed that this adaptor may recruit GIGYFs to activated insulin-like growth factor receptors and heighten signaling (see Giovannone et al., 2003). Altered insulin and insulin-like signaling have been linked to neurodegeneration (see ARF related news story) and to Alzheimer disease (see ARF related news story).

Grb10 has also been linked to protein tyrosine kinase activity (see Holt and Siddle, 2005), which is interesting in the context of another observation. Lautier and colleagues found that the age of onset (33) in one patient was much earlier than in her father (age of onset 66) from whom she inherited a GIGYF2 mutation. Interestingly, this daughter also inherited a LRRK2 PD mutation from her mother (age of onset 61). The earlier onset and severe clinical course in the index patient suggest additive effects of the GIGYF2 and LRRK2 mutations, write the authors. Given that LRRK2 is a tyrosine kinase, this finding raises the possibility that GIGYF2 and LRRK2 may interact or impinge on a single signaling pathway.

Tabar V, Tomishima M, Panagiotakos G, Wakayama S, Menon J, Chan B, Mizutani E, Al-Shamy G, Ohta H, Wakayama T, Studer L. Therapeutic cloning in individual parkinsonian mice. Nature Medicine 2008 Mar 23. Advanced online publication. Abstract

Lautier C, Goldwurm S, Durr A, Giovannone B, Tsiaras WG, Pezzoli G, Brice A, Smith RJ. Mutations in the GIGYF2 (TNRC15) gene at the PARK11 Locus in Familial Parkinson Disease. Am. J. Hum. Genet. 2008 Apr;82:1-12. Abstract

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