Causal influence of muscle weakness on cardiometabolic diseases and osteoporosis

The causal roles of muscle weakness in cardiometabolic diseases and osteoporosis remain elusive. This two-sample Mendelian randomization (MR) study aims to explore the causal roles of muscle weakness in the risk of cardiometabolic diseases and osteoporosis. 15 single nucleotide polymorphisms (SNPs, P < 5 × 10−8) associated with muscle weakness were used as instrumental variables. Genetic predisposition to muscle weakness led to increased risk of coronary artery disease (inverse variance weighted [IVW] analysis, beta-estimate: 0.095, 95% confidence interval [CI]: 0.023 to 0.166, standard error [SE]:0.036, P-value = 0.009) and reduced risk of heart failure (weight median analysis, beta-estimate: − 0.137, 95% CI − 0.264 to − 0.009, SE:0.065, P-value = 0.036). In addition, muscle weakness may reduce the estimated bone mineral density (eBMD, weight median analysis, beta-estimate: − 0.059, 95% CI − 0.110 to − 0.008, SE:0.026, P-value = 0.023). We found no MR associations between muscle weakness and atrial fibrillation, type 2 diabetes or fracture. This study provides robust evidence that muscle weakness is causally associated with the incidence of coronary artery disease and heart failure, which may provide new insight to prevent and treat these two cardiometabolic diseases.


Statistical analyses
To determine causal influence of muscle weakness on each outcome, we conducted the inverse variance weighted (IVW) analysis because more than 2 SNPs were available.IVW method used a meta-analysis approach to combine Wald estimates for each SNP in order to get the overall estimates of the effect of muscle weakness on each outcome 23 .The weighted median and MR-Egger regression methods were also applied to estimate the effects.Cochrane's Q-statistic was used to assess the heterogeneity of SNP effects and P < 0.05 indicated significant heterogeneity 38 .MR pleiotropy residual sum and outlier test (MR-PRESSO) aimed to assess the presence of pleiotropy and the effect estimates were recalculated after outlying SNPs were excluded 39 .
All methods were carried out in accordance with relevant guidelines and regulations.All experimental protocols were approved and the ethical approval for each study can be found in the original publications (including informed consent from each participant).P < 0.05 indicated statistical difference.All of these analyses were conducted in R V.4.0.4 by using the R packages of 'MendelianRandomization' 40 , 'TwoSampleMR' 41 and 'MR-PRESSO' 42 .

Ethical approval
The ethical approval for each study included in this investigation can be found in the original publications.

Cardiometabolic diseases
We evaluated the causal effect of muscle weakness on coronary artery disease, heart failure, atrial fibrillation and type 2 diabetes in this MR analysis (Table 2).IVW analysis demonstrated that genetically muscle weakness played a significant causal role in the increased risk of coronary artery disease (beta-estimate: 0.095, 95% CI 0.023 to 0.166, SE:0.036,P-value = 0.009), but it was not supported by the weighted-median analysis (beta-estimate: 0.069, 95% CI − 0.023 to 0.161, SE:0.047,P-value = 0.141, Fig. 1).
After excluding these outlying SNP variants, these remarkable MR associations were confirmed between muscle weakness and increased risk of coronary artery disease (Fig. 1 and Table 3).In addition, muscle weakness was confirmed to have a causal effect on low risk of heart failure (beta-estimate: − 0.149, 95% CI − 0.241 to − 0.056, SE:0.047,P-value = 0.002, Fig. 1 and Table 3).The MR association between muscle weakness with other outcomes were not changed after excluding the outlying SNP variants (Table 3).

Discussion
Our two-sample MR study found the robustly causal effect of muscle weakness on increased risk of coronary artery disease and decreased risk of heart failure, and these strong MR associations were confirmed by the sensitivity analyses.These positive findings indicated that the regulatory mechanisms of muscle weakness may provide new insight to prevent and treat coronary artery disease and heart failure.In addition, muscle weakness may have a causal role in reduced eBMD.We found no causal effect of muscle weakness on atrial fibrillation, type 2 diabetes or fracture.
Several observational studies and meta-analysis explored the association between muscle results and cardiometabolic diseases, but no conclusive results were found 9,13,43 .One meta-analysis revealed that handgrip strength was an independent predictor of cardiometabolic diseases in community-dwelling populations, but this this association was not significant after adjusting for baseline risk factors 9,43 .One recent MR analysis found no causality in the association between handgrip strength (European population) and coronary artery disease (mixed population).The large-scale genetic discovery analysis identified 16 loci associated with grip strength (P < 5 × 10 −8 ) among 195,180 individuals as instrumental variables 10 , and that MR study included GWAS summary data related to coronary heart disease among 184,305 individuals 44 .
Our large-scale MR study was performed in larger populations including 256,523 individuals of European descent for muscle weakness and 547,261 individuals of European descent for coronary artery disease.Totally, 15 loci associated with grip strength (P < 5 × 10 −8 ) were used as instrumental variables.The results provided the robust evidence for the causal association between muscle weakness and increased risk of coronary artery disease, which was confirmed by multiple sensitivity analyses.Muscle weakness and low muscle mass reduces total energy expenditure, which may result in high fat mass.Accumulated body fat mass triggers chronic inflammation, and is thought to be a risk factor for the development and progression of coronary artery disease [45][46][47] .
One leading cause of heart failure is coronary artery disease, but heart failure can be also caused by arrhythmias, hypertension, type 2 diabetes mellitus, obesity, and lifestyle factors (such as smoking).A large-scale observational study found that higher hand grip strength was independently associated with lower incidence of heart failure 48 .On the contrary, our MR study revealed that muscle weakness was causally associated with lower incidence of heart failure, which was confirm by the IVW analysis after excluding the outlying SNPs (beta-estimate: − 0.149, 95% CI − 0.241 to − 0.056, SE:0.047,P-value = 0.002, Fig. 1 and Table 3).This positive finding was very interesting, and may be attributed by the atrophy of the muscle fibers and reduced requirement of cardiac output due to low muscle mass 49 .
Patients with osteoporosis typically have the features of low bone mass, BMD and bone strength, which can increase the risk of fracture [50][51][52][53][54] .Several observational studies revealed the significant correlation between low grip strength and low BMD of the bones adjacent to the muscles related to grip [55][56][57] .In 1,168 menopausal women, Osei-Hyiaman et al. found the significant relationship between grip strength and BMD of metacarpal index 55 .Hasegawa et al. revealed that BMD of the distal radius was more associated with hand grip strength than with cross-sectional muscle area 57 .In contrast, Zimmermann et al. documented that hand grip strength in postmenopausal women showed no impact on vertebral BMD, but only affected femur BMD 58 , while Foley et al. documented no correlation between hand grip strength and femoral BMD 59 .
Considering these insistent results, our MR analyses revealed that muscle weakness may have a causal role in reduced eBMD.It is postulated that muscle contraction force provides a mechanical stress on the bones, which is accepted as an important osteogenic stimulus.There is bi-directional bone-muscle crosstalk, which is probably mediated by cytokines, osteokines, myokines, and other growth factors 60 .In addition, low BMD associated with muscle weakness may be associated with systemic inflammation and oxidative stress 61,62 .
Our results demonstrated that genetically muscle weakness was unlikely to be causally associated with atrial fibrillation, type 2 diabetes or fracture.The potential causal effect of muscle weakness to reduce eBMD was not translated to affect the risk of fracture.This two-sample MR study aims to investigate the causal effect of muscle weakness on the risk of cardiometabolic diseases and osteoporosis, and has the advantage of preventing reverse causation and confounding factors.The intercepts for the MR-Egger analysis suggest no directional pleiotropy for all outcomes.However, several limitations should be taken into consideration.Firstly, all the included participants are of predominantly European, and we can not directly apply our findings for other populations.Secondly, GWAS summary statistics can not be used to conduct MR analysis based on different age stratums.Thirdly, the contribution of muscle weakness to low eBMD is not translated to increased incidence of fracture, but the detail mechanisms are unclear.

Conclusion
This two-sample MR study provides strong evidence to confirm that muscle weakness is a significantly causal factor for increased risk of coronary artery disease and reduced risk of heart failure, and the related mechanisms may help prevent and treat these two diseases.

Figure 1 .
Figure 1.OR (95% CI) for causal association between muscle weakness and each outcome through multiple analyses.

Table 1 .
Details of studies and datasets used for analyses.

Table 2 .
Mendelian randomization estimates of muscle weakness on outcomes.

Table 3 .
Mendelian randomization estimates between muscle weakness and outcomes after excluding outliers detected by MR-PRESSO.