Monday, May 6, 2024
5/6/2024
The majority of Alzheimer’s disease (AD) risk is genetic and genome wide association studies (GWAS) have identified 75 AD-associated SNPs. In other fields, pharmacologic agents targeting proteins identified through genetics have produced striking success. However, the AD field has lagged, partly because we have struggled to identify the molecular mechanisms whereby genetic variants impact AD. This proposal focuses on the phospholipase PLCG2, which was initially associated with AD through a missense SNP, rs72824905 (P522R), that strikingly reduces AD risk. PLCG2 modulates microglial function by mediating signaling from TREM2 and related receptors. P522R appears a hypermorphic mutation that increases PLCG2 function through an unclear mechanism. However, homozygosity for P522R is apparently deleterious, suggesting that PLCG2 activity must be tightly controlled for optimal brain aging. In considering PLCG2 genetics and AD, we recognized that PLCG2 expression and splicing relative to AD genetics have not been scrutinized. Indeed, a SNP upstream of PLCG2, rs12446759, was recently associated with AD risk in a GWAS; the actions of this GWAS SNP are not clear although eQTL compendia suggest that this SNP is associated with expression of the lncRNA RP11-960L18, some isoforms of which include PLCG2 beginning at exon 2. To address the gap in our knowledge regarding PLCG2 expression and AD genetics, we propose to elucidate the extent that genetics modulate PLCG2 expression and splicing, to identify the underlying mechanisms, and to identify proof-of-concept approaches to improve PLCG2 expression to reduce AD risk. In preliminary work, we identified a novel PLCG2 splice variant, D65-PLCG2, in human brain, quantified this isoform in AD and non-AD brain samples, and identified a SNP, rs1071644, within the variant exon that correlates with expression of this novel isoform. Remarkably, rs1071644 is associated with increased AD risk (p=9x10-8). Rs1071644 appears to be a novel AD risk factor because this SNP is not co-inherited with either the GWAS SNP, rs12446759 or P522R according to LDpair. In summary, our global hypothesis is that genetic variants impact PLCG2 splicing and/or expression to alter PLCG2 function and thereby modulate AD risk. To explore this hypothesis, we propose these Specific Aims: 1: Identify and quantify PLCG2 isoforms expressed in human brain as a function of AD neuropathology and genetics. 2: Compare PLCG2 and D65-PLCG2 protein expression and function. 3: Translate PLCG2 splicing genetics to identify modulators predicted to reduce AD risk. Successful completion of these studies will reveal rs1071644 as a novel AD risk factor that acts via PLCG2 splicing, show that the AD GWAS SNP rs12446759 alters expression of a novel RP11-960L18_PLCG2 isoform, test whether the D65-PLCG2 represents a gain or loss of function, and begin to translate these findings to pharmacologic modulation. By defining the extent that genetics regulate PLCG2 expression to alter AD risk, these results will inform drug development targeting this pathway.
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