Alzheimer’s disease (AD), also known as senile dementia, is a common neurodegenerative disease [1]. Its primary clinical manifestations include neuropsychiatric symptoms such as progressive memory dysfunction, cognitive impairment, personality change, and language disorder, seriously affecting social, occupational, and life functions [2]. AD is more common in people over 60 years old [3]. The symptoms of AD are relatively insidious and easy to be ignored [4]. Genetic factors are widely recognized as risk factors for AD [5]. Epidemiological studies have underscored a substantial AD risk, 3.5 times higher than that of the general population, among first-degree relatives of AD patients relative to the general population [6]. Most AD cases occur sporadically and coincide with characteristic neuropathological lesions of amyloid plaques accumulation [7]. Given the continued lack of clarity surrounding the etiology and pathogenesis of AD, there is no specific method to reverse and prevent the progression of the disease. Active monitoring and early intervention of susceptible AD population are very important to delay the decline of quality of life [8].

Matrix metalloproteinases (MMP) hold significant importance as proteases that regulate growth and development [9]. Within the nervous system, MMPs are expressed in the nervous system and play a role in regulating myelin turnover, axon generation and angiogenesis [10]. As previously reported by Helbecque et al. [11], 6A/6A homozygous of the MMP-3 polymorphism carriers are at increased risk of dementia. However, ambiguous results are reported in the Japanese, in which no impact of this polymorphism on the risk of AD is determined [12], that might be attributed to racial diversity. Among the MMP family, MMP-2 is one of the most studied and most frequently expressed MMPs in the nervous system [13]. In AD patients, enhanced expression of MMP-2 has been detected in astrocytes surrounding A$\beta$ plaques and brain endothelial cells [14]. Besides, the accumulation of MMP-2 has been tested near the neurofibrillary tangles in the early stage of AD, which is related to the elimination of toxic truncated tau species in AD brains [15]. In AD mice models, MMP-2 is at high expression during the initial stage of the disease, and constitutively expressed with the disease progression [16]. In AD patients, plasma activity of MMP-2 shows positive correlation with mini-mental state examination (MMSE) score, reflecting its diagnostic potential for the earlier detection of AD [17, 18].

The MMP-2 gene is located on chromosome 16q13-21 [19], and it is known that single nucleotide polymorphisms (SNPs) can influence the transcriptional activity and expression of MMP-2 [20, 21]. MMP-2 –1575 G$\mathrm{>}$A (rs243866), MMP-2 –735 C$\mathrm{>}$T (rs2285053) and MMP-2 –1306 C$\mathrm{>}$T (rs243865) are common polymorphic loci in the promoter region of MMP-2 gene [22]. These SNPs could lead to the dysregulation of MMP-2 [20]. Existing studies have presented the genetic association of MMP-2 gene SNPs with various human diseases [23]. However, limited study has been conducted to assess the MMP-2 gene polymorphisms associated with AD so far. Therefore, a hospital-based case-control study was carried out to clarify the relationship of MMP-2 gene candidate SNPs with AD susceptibility in the Chinese Han population.

Currently, the incidence of AD exhibited a gradual upward trend [25]. The development of AD is influenced by multiple factors, including environmental and genetic factors [26]. One such genetic factor of interest is MMP-2, a brain-enriched protease, which can mediate myelin turnover, axon generation and angiogenesis [10]. Notably, increased MMP-2 has been detected in the neurofibrillary tangles during the early stages of AD, suggesting its involvement in the elimination of toxic truncated tau species [15]. In recent decades, the study potential of gene SNPs in molecular genetics research has gained significant attention [27]. In the current study, three loci including rs2285053, rs243865 and rs243864 were taken as the study objects to investigate their relationship with AD susceptibility, which were polymorphic in the Chinese Han population. The results revealed that MMP-2 gene rs243866 polymorphism was associated with AD susceptibility, and the AA genotype carriers may predispose the individuals to AD by acting as the risk factors. In addition, both rs243866 and rs243865 polymorphisms were closely related to the onset age of AD. Nonetheless, we did not find any relationship between MMP-2 gene rs2285053 SNP and AD emotivity. In addition, based on the detailed demographic information of the present study population, several established modifiable risk factors for AD were demonstrated, including less education and stroke history. Consistently, the results of a country study in the Slovak Republic have presented that low levels of education are related to an increased risk of AD and dementia [28]. Furthermore, a history of stroke is recognized as a contributing factor to the susceptibility of AD [29, 30]. Thus, the clinical indicators were adjusted to more accurately present the genetic value of MMP-2 gene polymorphic loci for AD, it was concluded that rs243866 AA genotype was still significantly correlated with AD risk.

MMP-2 has been recognized for its potential to degrade Tau fibrils, activated MMP-2 and phosphorylated tau has been colocalized in neurofibrillary tangles of AD brain [31]. During the early stage of AD, activated MMP-2 has been detected in the entorhinal cortex [15], suggesting a potential early self-protective response in AD patients. Based on our present findings, cases carrying rs243866 AA genotype were more susceptible to AD after adjusting other clinical indicators. This susceptibility might be related to the reduced MMP-2 levels and the resulting accumulation of Tau [32]. However, it is worth noting that a recent study reported no significant differences in the genotype and allele distributions between AD patients and health controls from Slovakia, which was inconsistent with our current findings [33]. This inconsistency might be attributed to the differences in the race and age of the included patients. Our study focused on Han Chinese AD patients including those younger than 65 years of age, which differed from the late-onset AD patients from Slovakia in Ocenasova et al.’s study [33]. In addition, the significant association between AA genotype and AD susceptibility was still obtained in this study after taking into account clinical confounding factors and excluding confounding interference by regression analysis. Thus, the results of the present case-control study presented the genetic association of MMP-2 gene rs243866 polymorphism with the susceptibility of AD in the Chinese Han population. Moreover, the stratification analysis results demonstrated that MMP-2 gene rs243866 was related to the onset age of AD, and early-onset AD risk in rs243866 AA genotype carriers was 6.528 times higher than those in GG genotype carriers. It is known that the anti-aging gene Sirtuin 1 (SIRT1) is linked to AD and various chronic diseases [34]. It is identified to be a critical regulator of MMP-2 expression, stability, and activity [35, 36]. MMP-2 expression and activity are regulated by SIRT1 at posttranslational level [37]. In light of the regulatory role of SIRT1 in MMP-2 expression, the participation of its expression changes in the association between MMP-2 gene SNPs and AD susceptibility is interesting. It will be meaningful for further exploration in future studies.

Rs243865 is another common SNP in MMP-2 gene that can influence MMP-2 activity. It causes the C to T transition at position –1306 in the promoter region of MMP-2 gene, resulting in decreased MMP-2 activity [38]. Previous reports have indicated that individuals carrying rs243865 TT genotype exhibited low activity of MMP-2 relative to those carrying CC genotype [39]. In our current study, no significant difference was detected for MMP-2 gene rs243865 genotypes between the case and control groups. However, an examination of the relationship of rs243865 polymorphism with the onset age of AD demonstrated that carriers of the rs243865 CC genotype have a later age of onset of AD than those with the TT genotype, suggesting a potential protective effect of the rs243865 CC against the development of AD. Nevertheless, this difference did not reach a significant level. Consistently, the subtle relationship between MMP-2 gene rs243865 polymorphism and AD emotivity has also been presented by Durmanova et al. [40] in patients from Slovakia. These findings highlight the potential role of MMP-2 gene rs243865 polymorphism in AD occurrence, although conclusive evidence remains elusive. Furthermore, when considering stratification analysis results by age, our results confirm a significant correlation between MMP-2 gene polymorphism and the onset age of AD. Notably, rs243865 TT genotype carriers exhibited a 4.048-fold higher risk of early onset of AD than CC genotype carriers. Another SNP in MMP-2 gene, rs2285053, is also analyzed in the present study population, but no clear correlation was identified in both the development and onset age of AD. In the current study, only polymorphic loci in the promoter region of MMP-2 gene were included, variants in the gene coding region were not included. It is the limitation of the study, which is interesting for further study.

In conclusion, the present findings underscore a significant association between MMP-2 gene rs243866 polymorphism and AD onset age and risk. The presence of the rs243866 AA genotype emerges as a significant predictor of AD risk, offering valuable insights for the early detection of AD. furthermore, our findings emphasize a distinct correlation between MMP-2 gene rs243865 polymorphisms and the onset age of AD.

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

LNL and LRL contributed to the study conception and design. Material preparation, data collection and analysis were performed by LNL, LRL, YTL, TYZ and WTZ. The first draft of the manuscript was written by LNL and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

Each subject signed a written informed consent, and the study was performed under the approval of the Ethnic Committee of Science and Technology Innovation Park of the Fourth Affiliated Hospital of Harbin Medical University (approval number: 2020-068).

Not applicable.

This research received no external funding.

The authors declare no conflict of interest.