First author Li and colleagues combined three different AD case-control series—from Washington University in St. Louis, Missouri; the University of California, San Diego; and a combined set from the University of Wales in Cardiff and King's College, London. This brought them to a total of 1,089 cases and 1,196 controls. In these samples, the researchers genotyped 282 SNPs under the linkage peak they had identified in previous work on the Washington University group.

The researchers report a strong late-onset AD (LOAD) association with SNPs in the gene for GAPD in this region. Since a number of GAPD isoforms are found elsewhere in the genome, the researchers looked for associations between LOAD and GAPD family genes on chromosomes with some evidence of AD risk loci. In the combined sample, Li and colleagues found associations with a GAPD isoform elsewhere on chromosome 12 and one on chromosome 19. However, not all of the GAPD isoforms were found to be significantly associated with LOAD in each of the three case-control series.

On the possibility that some of the different GAPD genes produce proteins that merge into common pathways, the researchers examined whether combinations of the genes were associated with LOAD. "A significant association between LOAD and a compound genotype of the three GAPD genes was observed in all three sample sets. Individually, these SNPs make differential contributions to disease risk in each of the case-control series, suggesting that variants in functionally similar genes may account for series-to-series heterogeneity of disease risk," the authors write.

So what do we know about GAPD? It's clearly an important "housekeeping" enzyme in glucose metabolism, but a critical role in apoptosis has also been proposed. There are intriguing reports on GAPD in Alzheimer brain, including a report of increased enzymatic activity (Soucek et al., 2003). However, a recent study found no difference in GAPD expression between AD and control brain (Gutala and Reddy, 2004).

Some questions for our readers: How strong do you find this claim for a GAPD association? How can we explain the apparent discrepancies between GAPD activity and expression in AD brain? And what about GAPD in apoptosis—does that need to be confirmed?—Hakon Heimer.

Reference:
Li Y, Nowotny P, Holmans P, Smemo S, Kauwe J, Hinrichs T, Tacey K, Doil L, van Luchene R, Garcia V, Rowland C, Schrodi S, Leong D, Gogic G, Chan J, Cravchik A, Ross D, Lau K, Kwok S, Chang S-Y, Catanese J, Sninsky J, White T, Hardy J, Powell J, Lovestone S, Thal L, Owen M, Williams J, Goate A, Grupe A. Association of late-onset Alzheimer’s disease with genetic variation in multiple members of the GAPD gene family. Proc Natl Acad Sci U S A. 2004 Oct 26 [Epub ahead of print] Abstract