Meta-analysis of the association between CD33 and Alzheimer's disease

Potential Biomarkers in Cerebrospinal Fluid and Plasma for Dementia

Background

The identification of biomarkers for different dementias in plasma and cerebrospinal fluid (CSF) has made substantial progress. However, they are observational studies, and there remains a lack of research on dementias with low incidence rates.

Objective

We performed a comprehensive Mendelian randomization to identify potential biomarkers for different dementia type.

Methods

The summary-level datasets encompassed 734 plasma and 154 cerebrospinal fluid proteins sourced from recently published genome-wide association studies (GWAS). Summary statistics for different dementias, including any dementia (refering to any type of dementia symptoms, 218,792 samples), Alzheimer's disease (AD, 63,926 samples), vascular dementia (212,389 samples), frontotemporal dementia (3,024 samples), dementia with Lewy bodies (DLB, 6,618 samples), and dementia in Parkinson's disease (216,895 samples), were collected from large GWAS. The primary method is inverse variance weighting, with additional sensitivity analyses conducted to ensure the robustness of the findings.

Results

The molecules released into CSF, namely APOE2 for any dementia, APOE2 and Siglec-3 for AD, APOE2 for vascular dementia, and APOE2 for DLB, might be potential biomarkers. CD33 for AD and SNCA for DLB in plasma could be promising biomarkers.

Conclusions

This is the first study to integrate plasma and CSF proteins to identify potential biomarkers for different dementias.

CD33 polymorphisms and Parkinson's disease Parkinson's disease in northern Chinese Han population: A case-control study

OBJECTIVE

Parkinson's disease (PD) represents the multisystem illness involving immunological and neuroinflammatory dysfunction. The present work focused on evaluating link of CD33 single nucleotide polymorphisms (SNPs) with PD vulnerability of the northern Chinese Han people, considering CD33's role as a critical immunoregulatory receptor in neuroinflammatory responses.

METHODS

The present case-control study included 475 PD cases together with 475 normal controls. A further division of PD patients into two categories was made: 74 patients with early-onset PD (EOPD; onset age ≤ 50 years) and 401 patients with late-onset PD (LOPD; onset age > 50 years). DNA extraction was conducted, followed by genotyping for 2SNPs of CD33 polymorphisms with polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP).

RESULTS

Alleles (G vs. A, P = 0.028) and AA genotypes (P = 0.042) of rs12985029 were significantly different between the groups. Distinctions were observed between the two groups in the recessive, co-dominant, and additive models (nominal P = 0.030, nominal P = 0.045, and P = 0.032). AA genotype frequency among male PD was higher compared to corresponding male controls (P = 0.034), and in the male group allele A was a factor causing the disease (P = 0.026). The rs12985029 genotypes and allele frequency were different in EOPD compared with LOPD (P = 0.002, P = 0.002, respectively), and in LOPD group relative to healthy control group (P = 0.020 and P = 0.004, separately). Regarding the rs3826656 polymorphism, the frequency of GA genotype was higher in the control group than in the case group (nominal P = 0.036). Overdominance and co-dominant models were different between these groups (P = 0.026, nominal P = 0.030). Subgroup analysis revealed genotype frequency differences between rs3826656 LOPD group and control group (P = 0.018). Furthermore, relationship between rs3826656 and rs12985029 (D' = 0.162, r2 = 0.021) did not reach a complete level of linkage disequilibrium (LD) of northern Chinese Han people.

CONCLUSION

This study establishes an association between CD33 rs12985029 and rs3826656 polymorphisms and PD risk among the selected northern Chinese Han people. The GA genotype, rs3826656, may act as a protective factor against PD, while the A allele, rs12985029,could be genetic risk factor related to PD. Future research should include larger sample sizes and other human populations to further investigate how CD33 polymorphisms contribute to PD.

CD33 isoforms in microglia and Alzheimer's disease: Friend and foe

Alzheimer's disease (AD) is the most common form of neurodegenerative disease and is considered the main cause of dementia worldwide. Genome-wide association studies combined with integrated analysis of functional datasets support a critical role for microglia in AD pathogenesis, identifying them as important potential therapeutic targets. The ability of immunomodulatory receptors on microglia to control the response to pathogenic amyloid-β aggregates has gained significant interest. Siglec-3, also known as CD33, is one of these immunomodulatory receptors expressed on microglia that has been identified as an AD susceptibility factor. Here, we review recent advances made in understanding the multifaceted roles that CD33 plays in microglia with emphasis on two human-specific CD33 isoforms that differentially correlate with AD susceptibility. We also describe several different therapeutic approaches for targeting CD33 that have been advanced for the purpose of skewing microglial cell responses.

Peripheral level of CD33 and Alzheimer's disease: a bidirectional two-sample Mendelian randomization study

Increased expression of CD33 in the brain has been suggested to be associated with increased amyloid plaque burden, while the peripheral level of CD33 in Alzheimer's disease (AD) patients and its role in AD remain unclear. The current study aimed to systematically explore the bidirectional relationship between peripheral CD33 and AD. Genome-wide association study (GWAS) datasets of AD (N_cases: 21982; N_controls: 41944), blood CD33 mRNA level, the plasma CD33 protein level, and CD33 expression on immune-cell subtypes were obtained from GWASs conducted in the European population. Eligible IVs were extracted from the GWASs. MR estimates were calculated by inverse-variance weighting (IVW) and other sensitivity analyses. The main statistical analyses were conducted using TwoSampleMR (v.0.5.5) in R package (V.4.1.2).In the forward MR analysis (CD33 as exposure and AD as outcome), the IVW results indicated that elevated blood CD33 mRNA level (OR [95% CI] = 1.156[1.080, 1.238], p = 3.25e-05), elevated serum CD33 protein level (OR [95% CI] = 1.08 [1.031, 1.139], p = 1.6e-03) and increased CD33's expression on immune cell subtypes (p < 0.05) were all leading to a higher risk of AD. And sensitivity analyses supported these findings. While the reverse MR analysis (AD as exposure and CD33 as outcome) indicated that AD was not leading to the elevation of CD33's protein level in the blood (p > 0.05). In conclusion, our results indicated that elevated peripheral expression of CD33 was causal to the development of AD. Future studies are needed to work on developing CD33 as a biomarker and therapeutic target in AD.

Cellular and molecular influencers of neuroinflammation in Alzheimer's disease: Recent concepts & roles

Alzheimer's disease (AD), an extremely common neurodegenerative disorder of the older generation, is one of the leading causes of death globally. Besides the conventional hallmarks i.e. Amyloid-β (Aβ) plaques and neurofibrillary tangles (NFTs), neuroinflammation also serves as a major contributing factor in the pathogenesis of AD. There are mounting evidences to support the fundamental role of cellular (microglia, astrocytes, mast cells, and T-cells) and molecular (cytokines, chemokines, caspases, and complement proteins) influencers of neuroinflammation in producing/promoting neurodegeneration and dementia in AD. Genome-wide association studies (GWAS) have revealed the involvement of various single nucleotide polymorphisms (SNPs) of genes related to neuroinflammation with the risk of developing AD. Modulating the release of the neuroinflammatory molecules and targeting their relevant mechanisms may have beneficial effects on the onset, progress and severity of the disease. Here, we review the distinct role of various mediators and modulators of neuroinflammation that impact the pathogenesis and progression of AD as well as incite further research efforts for the treatment of AD through a neuroinflammatory approach.

Exploiting formyl peptide receptor 2 to promote microglial resolution: a new approach to Alzheimer's disease treatment

Alzheimer's disease and dementia are among the most significant current healthcare challenges given the rapidly growing elderly population, and the almost total lack of effective therapeutic interventions. Alzheimer's disease pathology has long been considered in terms of accumulation of amyloid beta and hyperphosphorylated tau, but the importance of neuroinflammation in driving disease has taken greater precedence over the last 15-20 years. Inflammatory activation of the primary brain immune cells, the microglia, has been implicated in Alzheimer's pathogenesis through genetic, preclinical, imaging and postmortem human studies, and strategies to regulate microglial activity may hold great promise for disease modification. Neuroinflammation is necessary for defence of the brain against pathogen invasion or damage but is normally self-limiting due to the engagement of endogenous pro-resolving circuitry that terminates inflammatory activity, a process that appears to fail in Alzheimer's disease. Here, we discuss the potential for a major regulator and promoter of resolution, the receptor FPR2, to restrain pro-inflammatory microglial activity, and propose that it may serve as a valuable target for therapeutic investigation in Alzheimer's disease.

Association of CD33 rs3865444:C˃A polymorphism with a reduced risk of late-onset Alzheimer's disease in Slovaks is limited to subjects carrying the APOE ε4 allele

CD33 rs3865444:C>A single nucleotide polymorphism (SNP) has been previously associated with the risk of late-onset Alzheimer's disease (LOAD); however, the results have been inconsistent across different populations. CD33 is a transmembrane receptor that plays an important role in AD pathogenesis by inhibiting amyloid β42 uptake by microglial cells. In this study, we aimed to validate the association between rs3865444 and LOAD risk in the Slovak population and to evaluate whether it was affected by the carrier status of the major LOAD risk allele apolipoprotein (APOE) ε4. CD33 rs3865444 and APOE variants were genotyped in 206 LOAD patients and 487 control subjects using the polymerase chain reaction-restriction fragment length polymorphism method and direct sequencing, respectively. Logistic regression analysis revealed a significant association of rs3865444 A allele with a reduced LOAD risk that was only present in APOE ε4 allele carriers (AA + CA versus CC: p = .0085; OR = 0.45; 95% CI = 0.25-0.82). On the other hand, no such association was found in subjects without the APOE ε4 (p = .75; OR = 0.93; 95% CI = 0.61-1.42). Moreover, regression analysis detected a significant interaction between CD33 rs3865444 A and APOE ε4 alleles (p = .021 for APOE ε4 allele dosage and p = .051 for APOE ε4 carriage status), with synergy factor (SF) value of 0.49 indicating an antagonistic effect between the two alleles in LOAD risk. In conclusion, our results suggest that CD33 rs3865444:C˃A substitution may reduce the risk of LOAD in Slovaks by antagonizing the effect conferred by the major susceptibility allele APOE ε4.

Microglial Drug Targets in AD: Opportunities and Challenges in Drug Discovery and Development

Alzheimer’s disease (AD) is a large and increasing unmet medical need with no disease-modifying treatment currently available. Genetic evidence from genome-wide association studies (GWASs) and gene network analysis has clearly revealed a key role of the innate immune system in the brain, of which microglia are the most important element. Single-nucleotide polymorphisms (SNPs) in genes predominantly expressed in microglia have been associated with altered risk of developing AD. Furthermore, microglia-specific pathways are affected on the messenger RNA (mRNA) expression level in post-mortem AD tissue and in mouse models of AD. Together these findings have increased the interest in microglia biology, and numerous scientific reports have proposed microglial molecules and pathways as drug targets for AD. Target identification and validation are generally the first steps in drug discovery. Both target validation and drug lead identification for central nervous system (CNS) targets and diseases entail additional significant obstacles compared to peripheral targets and diseases. This makes CNS drug discovery, even with well-validated targets, challenging. In this article, we will illustrate the special challenges of AD drug discovery by discussing the viability/practicality of possible microglia drug targets including cluster of differentiation 33 (CD33), K_Ca3.1, kynurenines, ionotropic P2 receptor 7 (P2X7), programmed death-1 (PD-1), Toll-like receptors (TLRs), and triggering receptor expressed in myeloid cells 2 (TREM2).