Association between G‐Protein Coupled Receptor 55 (GPR55) Single Nucleotide Polymorphisms and Alzheimer’s Disease

Abstract Background G‐protein‐coupled receptor 55 (GPR55) is a lysophosphatidylinositol and novel cannabinoid receptor. GPR55 signalling and single nucleotide polymorphisms (SNPs) have been implicated as a potential therapeutic target for neurodegenerative diseases such as Alzheimer’s Disease (AD); however, foundational knowledge on the role of GPR55 in AD progression remains lacking. We hypothesised GPR55 SNP minor allele carriers would have greater brain atrophy, cognitive decline, and AD‐related biomarker changes. Method This retrospective, longitudinal, observational study used data from the Alzheimer’s Disease Neuroimaging Initiative (ADNI). We selected participants with clinically normal cognition, mild cognitive impairment (MCI) or mild AD from ADNI2 and ADNIGO phases as a discovery sample. Exploratory screening of GPR55 SNPs identified rs2969126G/C associated with brain volume. Cerebrospinal fluid amyloid beta‐42 (CSF‐Aβ42), total and phosphorylated‐tau (t/p‐tau) were considered. Mini‐Mental State Exam and the Logical Memory Delayed Recall Test were also selected to evaluate overall cognitive status, executive function, and memory. ANCOVA models evaluated baseline differences between SNP carriers and major allele homozygotes (dominant model), controlling for sex, age, baseline cognitive status and APOE e4 across AD biomarkers, brain volumetrics and cognitive measures. Longitudinal associations were assessed as interactions between SNP and visit (baseline, 12 and 24 months) in mixed models. Result In ADNI2/GO (n = 755), minor allele carriers represented 15% of the sample. Significantly greater decline in CSF‐Aβ42 levels was observed in Aβ42‐positive clinically normal minor allele carriers (F (1,45.2) = 6.35, p = 0.015). Carriers with MCI had significantly higher t‐tau (F (1,126) = 4.303, p = 0.04) and p‐tau concentrations (F (1,129) = 6.98, p = 0.009) longitudinally. Significantly smaller hippocampal volumes over time were seen in minor allele carriers in the whole group (F (1,444) = 5.74, p = 0.017), with a trend in the AD/MCI subgroup (F (1,325) = 3.364, p = 0.06). Clinically normal minor allele carriers had significantly poorer baseline logical memory scores (F (1,285) = 6.92, p = 0.009). These results did not replicate in the ADNI3 cohort. Conclusion Carriers of the GPR55 SNP rs2969126G/C minor allele had greater Aβ42 and tau biomarker abnormalities, smaller brain volumes and poorer memory scores over time in different stages of AD. This preliminary study prompts replication studies in different stages of AD progression to implicate GPR55 as a potential therapeutic target for AD.