Translating CD33 Genetic Mechanism into a Novel Alzheimers Therapeutic

Recent genome wide association studies have identified a set of single nucleotide polymorphisms (SNP)s that are associated with Alzheimer disease (AD) risk; these findings are essentially unequivocal given their statistical power. The overarching theme of this proposal is that elucidating the mechanism of action of these SNPs will identify “bottlenecks” in AD pathways. These bottlenecks biologically define rate limiting steps in AD pathways and as such, constitute robust pharmacologic targets. Hence, a SNP with modest actions may reduce AD risk by 10% but a drug that acts strongly at the same target may have a large effect on AD risk. This proposal seeks to elucidate the mechanism of action of rs3865444, an AD-associated SNP in CD33, and translate this mechanism into a therapeutic target. CD33 is a seven-exon gene that encodes a microglial sialic acid receptor. Upon sialic acid binding, CD33 inhibits microglial activation through its cytosolic domain. The ligand-binding domain of CD33 is encoded by exon 2. Our hypothesis is based upon novel and highly compelling preliminary results that the AD-protective allele of rs3865444 increases the proportion of CD33 lacking exon 2 (D2-CD33). Since D2-CD33 lacks the sialic acid binding domain, this isoform likely encodes a non-functional CD33 protein. Additionally, since CD33 acts as a dimer, the D2-CD33 protein may represent a dominant negative CD33. Interestingly, large pharma have developed humanized monoclonal antibodies against CD33 for acute myeloid leukemia (AML); these antibodies, without their chemotherapeutic cargo, have potential AD relevance as CD33 antagonists. Overall, these results lead to our global hypothesis: Reduced CD33 function decreases AD risk whether CD33 inhibition is due to genetics or pharmacologic agents. To test our hypothesis, we propose the following Specific Aims: (i) Elucidate the mechanisms underlying the CD33 AD SNP. Rs3865444 is not associated with CD33 expression but rather is very strongly associated with CD33 exon 2 splicing; the protective allele increases D2-CD33 from ~10% to ~40% of total CD33 expression; (ii) Compare CD33 and D2-CD33 function, especially relative to AD pathogenesis and (iii) Translate CD33 genetics into a novel therapeutic mimic. Overall, this proposal will develop our compelling mechanistic genetic results and translate these results into AD-preventive therapeutics.