Bone mineral density (BMD) is a key measurement of bone mass and an essential indicator of osteoporosis, which is prevalent in the aging society. In 1994, the World Health Organization gave the diagnosis standard of osteoporosis as 2.5 SD or more below the young adult average value (Kanis, 1994). The main characteristics of osteoporosis include loss of bone mass, deterioration of the bone microarchitecture, decrement of bone strength and increased risk of fractures, which lead to a systemic skeletal disorder with negative consequences on general health and quality of life in post menopause and in old age (Lane, 2006; Vidal et al., 2019; Capozzi et al., 2020). Fractures due to osteoporosis more likely occur on the hip, vertebral body and distal forearm, therefore, the BMD measurements of forearm (FA), lumbar spine (LS) and femoral neck (FN) are always taken by dual-energy X-ray absorptiometry (DXA) in patients to estimate the general risk of osteoporosis. With the continued ageing of the population worldwide, osteoporotic fractures could present an increasing prevalence and thus lead to higher rates of chronic pain, disability and even death in patients, as well as impose a major economic burden on healthcare systems (Sambrook and Cooper, 2006; Catalano et al., 2017). Current studies have found several risk factors that may decrease BMD (Kenny and Prestwood, 2000; Raisz, 2005; Li and Wang, 2018), but overall, the cause of osteoporosis still remains unclear, which brings difficulty in seeking for effective therapy for this disease.

Matrix metalloproteinases (MMPs) are a family of zinc-dependent neutral endopeptidases capable of degrading extracellular matrix components (Johansson et al., 2000). Previous studies have found that MMPs are expressed in bone tissue as key players in the digestion of bone matrix by osteoblasts, and are involved in bone-destructive lesions (Wahlgren et al., 2001; Azevedo et al., 2018; Fatemi et al., 2020), which indicates that MMPs may play a role in the pathogenesis of osteoporosis. It has been reported that the gene polymorphism of MMP-1 was associated with osteoporosis (Liang et al., 2019), and MMP-3 was negatively related to the osteoblast function markers of serum bone-specific alkaline phosphatase and osteocalcin while positively related to the resorptive function marker of serum cross-linked N- telopeptides of type I collagen (Momohara et al., 2005). Increased levels of MMP-7 and 9 in osteoclasts were reported to be associated with rheumatic osteoporosis (Yang et al., 2013), while MMP-8 participated in the healing process as well as embryonic bone development, and may play an important role in the remodeling of extracellular matrix molecules during bone and cartilage formation (Sasano et al., 2002). MMP-10 was found strongly expressed in osteoclasts and most mononuclear cells within the marrow and produced in an active form with associated degradation (Bord et al., 1998). Meanwhile, recombinant MMP-12 cleaved the putative functional domains of osteopontin and bone sialoprotein, two bone matrix proteins that strongly influence osteoclast activities, such as attachment, spreading and resorption (Hou et al., 2004). These studies strongly suggested the possibility that MMPs are related to osteoporosis. However, due to current randomized controlled trials which were based on either small samples or observational epidemiological studies, whether changes in MMP levels are correlated with BMD remains controversial.

Genome-wide association studies (GWAS) provide a new perspective for understanding genetic determinants that underlie complex disease. The technique of Mendelian randomization (MR), which employs single nucleotide polymorphism (SNPs) as instrumental variables, has been developed to identify causations between a wide range of risk factors and complex diseases. Unlike traditional observational studies, this analytical tool was less susceptible to confounding and reverse causation (Davey Smith and Hemani, 2014). MR has also been widely used these years to explore the causes of osteoporosis (Larsson et al., 2019; Zheng et al., 2019). Given that MMPs were hypothesized to participate in the development of osteoporosis, here we carried out an MR study to identify whether there existed causal associations between MMPs and BMD.

The MR schematic was shown in Figure 1. There were three underlying assumptions: 1) relevance assumption, genetic instrumental variables are associated with the risk factor of interest; 2) independence assumption, genetic variants are not associated with confounders; and 3) exclusion-restriction assumption, instrumental SNPs influence the outcome concerned only through the risk factor (Burgess et al., 2019). This study utilized publicly accessible datasets from published studies wherein formal consent from participants and ethical approval by committees had been obtained.

Osteoporosis is a common cause of morbidity and mortality worldwide especially in people aged over 60 years. Studies have shown that for decrease per 10 percent in bone mineral density, the risk of fracture increases 2–3 folds (Nguyen et al., 1993), and the mortality rate of patients caused by hip and spine fractures increases to 10–20% (Ioannidis et al., 2009). The causes of decrease in BMD have always been discussed in order to benefit for seeking effective therapy, and more and more risk factors are being identified to better predict the occurrence of osteoporosis and therefore avoid the severe complications of fracture.

The family of matrix metalloproteinases have been considered involved in basic pathological processes of osteoporosis for acting as key roles in the digestion of bone matrix by osteoblasts (Azevedo et al., 2018). However, different studies showed conflict results. For example, Zuo et al. (Zuo et al., 2020) found that MMP-8 was involved in the 17β-Estradiol replacement therapy for postmenopausal osteoporosis, while Viljakainen et al. (Viljakainen et al., 2017) found that there was no significant correlation between MMP-8 levels and low BMD. MR is an effective tool for identifying the causal association between certain exposure and disease while circumventing confounders, which might be the main cause of these inconsistent results. In the recent 3 years, a lot of factors that had been reported related to osteoporosis before have been re-evaluated by MR. Some were further confirmed to be associated with BMD, for instance, serum calcium (Sun et al., 2021), sex hormone-binding globulin (Qu et al., 2021) and age at menarche (Magnus et al., 2020), while others such as vascular endothelial growth factor, uric acid and serum vitamin D got no evidence for their correlations with osteoporosis (Lee and Song, 2019; Sun et al., 2019; Keller-Baruch et al., 2020). In a previous MR study of heel-ultrasound estimated BMD (Folkersen et al., 2020), there were no causal effects of MMP-1, 3, 7, 10, 12 in the European population. In this study, we found no evidence for the causal relationship between MMPs and DXA measured BMD at three common sites.

There are some limitations in present study. First, we could not identify the non-linear relationship between MMPs and BMD. Second, we only evaluated the effect of a small set of MMPs on BMD, but missed such types as MMP-2, -9 and -13, which might be in relation to osteoporosis according to previous studies (Bolton et al., 2009; Zheng et al., 2018). Further MR studies were warranted when relevant datasets are available. Third, it is noteworthy that our study was limited to the effect of circulating MMP levels on BMD, but the intracellular function of MMPs cannot be denied. Forth, both association data of MMPs and BMD were obtained from Europeans in this study. We should be cautious when generalizing the conclusion to other populations.

In this study, we found no evidence for causal relationships between MMPs (MMP-1, 3, 7, 8, 10, 12) and BMD in the European population.