The role of calcium homeostasis in endometriosis: a comprehensive study of multiple types of Mendelian randomization
Authors: Deng Zhi-Min , Dai Fang-Fang , Wang Rui-Qi , Yang Xiao , Cheng Yan-Xiang 
Published: August 2023

Abstract
Background: Endometriosis is considered to be a complex gynecological disorder that affects the health and quality of life of affected women. The etiology and pathogenesis of endometriosis remain unclear, and few modifiable risk factors have been identified. It is generally believed that endometriosis is a chronic inflammatory disease, which can cause local immune disorders. And calcium homeostasis of the body is closely related to inflammation and immunity. Although observational studies have assessed the association between calcium homeostasis regulatory factor levels and endometriosis risk, the conclusions have been inconsistent. Therefore, the aim of this study was to explore the causal relationship between calcium homeostasis regulators and endometriosis risk using publicly available genome-wide association studies (GWAS) aggregated statistics.
Methods: The Mendelian randomization (MR) analysis was performed using GWAS data, including calcium (N = 315,153), serum 25-Hydroxyvitamin D (25(OH)D) (N = 496,946), parathyroid hormone (PTH) (N = 3,301), endometriosis (N = 77,257), endometriosis of ovary (N = 72,200), endometriosis of rectovaginal septum and vagina (N = 70,329), endometriosis of intestine (N = 69,146), endometriosis of fallopian tube (N = 69,085), endometriosis of pelvic peritoneum (N = 71,922), endometriosis of uterus (N = 71,341), and Unspecified/other endometriosis (N = 70,404). Four levels of MR analysis were undertaken, starting with single univariate MR and multivariate MR to test the correlation between calcium homeostasis regulatory factors and endometriosis, followed by inverse MR to explore the effect of endometriosis on body calcium homeostasis. And further two-sample MR to probe the relationship between calcium levels and endometriosis subtypes. Cochran's Q test, MR-Egger intercept test, leave-one-out analysis and funnel plot were utilized for sensitivity analysis.
Results: The two-sample MR analysis revealed a strong positive causal relationship between genetically predicted calcium levels and endometriosis risk (IVW: OR = 1.15, 95% CI: 1.02-1.29, p = 0.018). Notably, the results of MVMR analysis demonstrated that the positive correlation of calcium levels on endometriosis still held even after correction for 25(OH)D and PTH (OR = 1.14, 95% CI: 1.02-1.28, p = 0.026). After removing outliers using MR-PRESSO to ensure that horizontal pleiotropy was eliminated, MVMR analysis was performed again, and the causal association between calcium levels and endometriosis remained significant (OR = 1.13, 95% CI: 1.01-1.27, p = 0.033). The inverse MR analysis discovered a causal association between endometriosis and 25(OH)D (β = 0.01, 95% CI: 0.00-0.02, p = 0.007) and calcium (β = 0.02, 95% CI: 0.00-0.04, p = 0.035). The two-sample MR analysis we employed to further investigate that calcium levels were positively and causally associated only with endometriosis of uterus (IVW: OR = 1.23, 95% CI: 1.01-1.49, p= 0.038), with no suggestion of a causal relationship with the risk of other types of endometriosis subtypes.
Conclusion：The comprehensive study of multiple types of MR provides genetic evidence for a causal relationship between calcium homeostasis and endometriosis risk, demonstrating that calcium levels are a risk factor for endometriosis. It also emphasizes the importance of monitoring calcium levels in patients suffering from endometriosis, which may provide dietary guidance for patients with endometriosis.