In the May 21 PNAS online, a multi-institutional collaboration of researchers report that single nucleotide polymorphisms (SNPs) in the gene for low-density lipoprotein receptor-related protein 6 (LRP6) increase risk for late-onset Alzheimer disease (LOAD). LRP6 is a co-receptor for Wnt, which plays a crucial role in embryogenesis and development and perhaps also adult neurogenesis (see ARF related news story). The finding suggests that Wnt signaling might be related to AD pathology, perhaps even a future therapeutic target.
That Wnt may play a role in AD is not a new idea. Senior authors Randall Moon and Giancarlo De Ferrari, from the University of Washington, Seattle, and others, have proposed that Wnt/β-catenin signaling might be perturbed in AD. Given that the LRP6 gene lies in a region of chromosome 12 that prior genome-wide association studies have linked to AD, the researchers decided to search for LRP6 variants that might associate with the disease. De Ferrari, Andreas Papassotiropoulos from the University of Basel, Switzerland, and Travis Biechele, University of Washington, joint first authors on the paper, together with a host of international collaborators, examined both case-control and family-based datasets. They found one haplotype containing a single non-synonymous variation that associates with AD. They found that this LRP6 variant attenuates β-catenin signaling. Interestingly, the polymorphism only appears to increase risk for the disease in people who are ApoE4 negative.
For the case-control study, the researchers genotyped samples from Zurich, Newcastle, U.K., and centers throughout the U.S., for a total of 398 cases and 339 controls. One synonymous coding SNP (18e, a C to T variation) significantly associated with AD, while one non-synonymous SNP (14e, a T to C variation that resulted in a valine-for-isoleucine switch at position 1062) trended toward association. On further analysis, the authors found that the 18e association was mainly due to the contribution of the Zurich samples. However, when stratifying the data, the authors found that the T allele of the 18e SNP was strongly associated with AD in ApoE4-negative individuals in the combined Zurich and U.K. datasets, suggesting this allele may be a risk factor for late-onset AD.
As the 18e variant does not alter the LRP protein sequence it is hard to see how it may have a functional impact; it might be in linkage disequilibrium with a different variant. To test this possibility the researchers examined a panel of seven haplotype tags. They found one five-SNP haplotype, containing both the 14e and 18e SNP, that strongly associated with AD. From this the researchers determined that the association of the 14e/18e pair with AD was highly significant in ApoE4-negative samples.
To corroborate these findings, De Ferrari and colleagues turned to family-based analysis. When they genotyped affected sibling pair samples from the NIMH/National Cell Repository for AD databank, they found that the 14e SNP significantly associated with AD in the ApoE4-negative subgroup. The 18e SNP also associated with AD, but limited statistical power in this analysis restricts this finding to being a trend.
The data suggest that the 14e and 18e SNPs are in linkage disequilibrium and somehow contribute to AD progression. Exactly how they might do so is unclear. If additional studies confirm this association, the role of LRP6 will come in for close scrutiny. De Ferrari and colleagues have begun addressing some of the pertinent questions in the present paper. For example, isoleucine-1062 is highly conserved between LRP6 and its homologs. Therefore, even a fairly conservative substitution, such as to valine, might compromise protein activity. To test this, the authors expressed both isoleucine and valine variants in neuron-derived HEK293T cells harboring a Wnt/β-catenin-driven luciferase reporter gene. They found that luciferase activity was fivefold lower in cells expressing the valine LRP6 variant even though both variants appeared to be equally expressed on the cell surface. The valine variant similarly reduced β-catenin-driven transcription in the HT22 neuronal cell line, though this effect was not statistically significant. The authors also report that LRP6 is expressed in the adult hippocampus, a major site of AD pathology.
It is unclear why the LRP6 variants only associate with AD in people who carry ApoE2 and 3, not 4. However, there is evidence that ApoE4 has an inhibitory effect on Wnt signaling (Caruso et al., 2006). “Given that the Wnt/β-catenin signaling pathway can be modulated pharmacologically, it is anticipated that further research in this area may contribute to therapeutic approaches to prevent or to treat AD,” write the authors.—Tom Fagan
- Caruso A, Motolese M, Iacovelli L, Caraci F, Copani A, Nicoletti F, Terstappen GC, Gaviraghi G, Caricasole A. Inhibition of the canonical Wnt signaling pathway by apolipoprotein E4 in PC12 cells. J Neurochem. 2006 Jul;98(2):364-71. PubMed.
- De Ferrari GV, Papassotiropoulos A, Biechele T, Wavrant De-Vrieze F, Avila ME, Major MB, Myers A, Sáez K, Henríquez JP, Zhao A, Wollmer MA, Nitsch RM, Hock C, Morris CM, Hardy J, Moon RT. Common genetic variation within the low-density lipoprotein receptor-related protein 6 and late-onset Alzheimer's disease. Proc Natl Acad Sci U S A. 2007 May 29;104(22):9434-9. PubMed.