Abstract
Background. The Wnt/β-catenin signaling pathway plays a crucial role in bone development and metabolism. The low-density lipoprotein receptor-related protein 5 (LRP5), an important receptor in the Wnt signaling pathway, promotes the osteogenesis of osteoblasts and curbs bone resorption by osteoclasts.
Objectives. To determine the expression of LRP5 polymorphisms (rs556442 and rs638051) and their relationship with bone mineral density (BMD) and bone metabolism markers in postmenopausal patients with type 2 diabetes mellitus (T2DM) in Xinjiang, China.
Materials and methods. According to dual-energy X-ray (DEXA) and oral glucose tolerance test (OGTT) results, 226 postmenopausal women from Xinjiang were divided into the following groups: normal glucose tolerance (NGT) + normal bone mass group (group A), NGT + abnormal bone mass group (group B), T2DM + normal bone mass group (group C), and T2DM + abnormal bone mass group (group D).
Results. Femoral neck BMD was lower in group B women with the AG/GG genotype (mutant type) compared to women with the AA genotype (wild-type) at rs556442. Alkaline phosphatase (ALP) levels were lower in group D women with the AG/GG genotype (mutant type) compared to women with the AA genotype (wild-type) at rs556442 and rs638051. The factors influencing BMD (lumbar spine vertebrae 1–4 (L1–L4)) were triglyceride (TG) levels, body mass index (BMI), menopausal transition age, and age for rs556442 patients, and TG levels and menopausal transition age for rs638051 patients in group D. The factors affecting BMD (hip) were TG levels, BMI and age for rs556442 patients, and TG levels and age for rs638051 patients.
Conclusions. The LRP5 gene mutations are linked to bone metabolism disorders in postmenopausal women with T2DM and abnormal bone mass. High BMI and TG were positively associated with BMD, while increased age and menopausal transition age were negatively associated with BMD.
Key words: type 2 diabetes, bone metabolism, postmenopausal women, LRP5 polymorphism
Background
Increased blood glucose levels caused by insufficient insulin secretion or islet cell dysfunction in type 2 diabetes mellitus (T2DM) affect a number of metabolic processes in the human body. Genetic susceptibility is of great importance in T2DM.1 This condition not only affects the quality of life and survival status of patients but also causes considerable economic and psychological burden.2, 3
Osteoporosis (OP) is a common chronic complication in T2DM patients that is related to a decrease in estrogen levels in postmenopausal women. Goldshtein et al. showed that postmenopausal women with T2DM have an increased risk of osteoporotic fractures.4
The low-density lipoprotein receptor-associated protein 5 (LRP5) is a transmembrane receptor protein belonging to the low-density lipoprotein (LDL) receptor family. The LRP5 gene is located on chromosome 11q12–13 and encodes an accessory receptor for the Wnt ligand 5.5 The LRP5 promotes the production of insulin, which is conducive to islet signal transduction and bone formation of osteoblasts.6 It has been found that bone mineral density (BMD) is increased in mice with elevated LRP5 expression.7 In addition, LRP5 is involved in lipid metabolism.8 Recently, the relationship between the LRP5 rs41494349 gene polymorphism and OP in postmenopausal women with T2DM was reported.9 However, the relationship between LRP5 rs556442 and rs638051 gene polymorphisms and bone metabolism in postmenopausal women with T2DM in Xinjiang remains unknown. Therefore, we aim to fill this research gap and lay the foundation for the prevention and treatment of OP in postmenopausal women with T2DM in Xinjiang.
Objective
The study aimed to explore the expression of LRP5 polymorphisms rs556442 and rs638051, and their relationship with BMD and bone metabolism markers in postmenopausal patients with T2DM in Xinjiang.
Materials and methods
Research subjects
A total of 226 naturally postmenopausal women treated in the First Affiliated Hospital of Xinjiang Medicine School in Xinjiang, China, were enrolled into the study. Type 2 diabetes mellitus was diagnosed based on the 1999 World Health Organization (WHO) recommendations, and BMD was measured based on the 1994 WHO recommendations. Subjects were divided based on the oral glucose tolerance test (OGTT) and dual-energy X-ray (DEXA) results for BMD. Group A patients had a normal glucose tolerance and normal bone mass (50 patients), and group B consisted of patients with NGT and abnormal bone mass (49 patients). The T2DM patients with normal bone mass were constituted group C (47 patients), and T2DM patients with abnormal bone mass group D (80 patients).
This research complied with the Declaration of Helsinki. Approval from the Ethics Committee of the First Affiliated Hospital of Shihezi University School of Medicine, China, was obtained (approval No. 2015-125-01). We explained the risks, benefits and goals of the study to each participant. Those who agreed to participate signed a written informed consent form.
Data acquisition
General descriptive data of the subjects were collected, and body mass index (BMI) and waist-to-hip ratio (WHR) were calculated (Table 1). The subjects fasted for 8–10 h and antecubital blood was collected the next morning. Triglyceride (TG), low-density lipoprotein cholesterol (LDL-C), fasting plasma glucose (FPG) calcium (Ca), and alkaline phosphatase (ALP) levels, as well as other indices were measured using an automatic biochemical analyzer (bs-280; Mindray, Shenzen, China). Using high-performance liquid chromatography (Bio-Rad D10; Bio-Rad, Hercules, USA), the level of glycosylated hemoglobin (HbA1c) was determined. Bone mineral density was analyzed in the lumbar spine and femur using the DEXA method. The detection of the rs556442 and rs638051 polymorphisms of the LRP5 gene was performed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS).
Determination of single nucleotide polymorphism sites
According to the general principles for selecting single nucleotide polymorphisms (SNPs), we consulted the relevant literature and selected the SNPs related to the LRP5 gene. Then, we selected functionally related and important sites, such as missense mutations, which change the amino acid sequence and affect protein function. The best locus for the functional region of the LRP5 gene according to the National Center for Biotechnology Information (NCBI) website was chosen (https://www.ncbi.nlm.nih.gov/gene/?term=LRP5+and+human).10, 11 In this study, two SNPs of the LRP5 gene were selected – rs556442 and rs638051.
DNA extraction
Five milliliters of antecubital blood were collected from each subject in ethylenediaminetetraacetic acid (EDTA)-coated tubes. The DNA was extracted using a DNeasy Blood and Tissue Kit (Qiagen, Hilden, Germany) and stored at −80°C. The concentration and purity of DNA were assessed using a NanoDrop 2000 UV spectrophotometer (Thermo Fisher Scientific, Waltham, USA), and the absorption of nucleic acids at 260 nm was quantified.
Gene sequencing
Primers for amplification and single base extensions were designed using Sequenom assay designer software v. 3.0 (Sequenom, San Diego, USA). The primer sequences were as follows: rs556442, forward: 5’-GGGCAGCCAAGATCGAAC-3’ and reverse: 5’-CGTCCACCCAGAACAGCTT-3’; rs638051, forward: 5’-CTTTGGGCAGTGGGCTTAG-3’ and reverse: 5’-CACCCTCTGGACATAGCTCTGA-3’. Polymerase chain reaction (PCR) conditions involved 1) preheating the samples to 94°C for 4 min (1 cycle); 2) 94°C for 20 s (45 cycles); 3) 56°C for 30 s (45 cycles); 4) 72°C for 1 min (45 cycles); and 5) final extension at 72°C for 3 min (1 cycle). The PCR products were obtained using a 384-pad spectral chip (JL-PZY96BT; Yibaiju, Shanghai, China). Each PCR product was treated with shrimp alkaline phosphatase (SAP; Sequenom). The Iplex (Sequenom) single base extension reaction was performed. Resin (Xingruikebo, Shihezi, China) purification procedure was also performed using the mass array nanodispenser RS1000 spotter (Sequenom), and the extension product was moved to the 384-pad spectral chip.12, 13, 14 The MALDI-TOF MS (Sequenom) was performed to detect rs556442 and rs638051 genotypes under different conditions.15 After the above reaction steps, the chip was put into the mass spectrometer (Sequenom). The flight time of these ionized products depended on the quality of each allele, measured using mass spectrometer. The smaller the ion mass, the faster it arrives. Finally, the molecular weight and base type were determined according to the position of the simplicity peak to succeed in typing.16
Statistical analyses
The IBM SPSS v. 22.0 software (IBM Corp., Armonk, USA) was used to analyze the data. The data conforming to a normal distribution are presented as the mean ± standard deviation (M ±SD). General data and biochemical parameters were compared between the groups using independent sample t-tests. When the baseline data was homogeneous (age [years], menopausal transition age [years], BMI [kg/m2], and WHR), one-way analysis of variance (ANOVA) testing was used to conduct intergroup comparisons. Otherwise, analysis of covariance (ACNOVA) was used. The χ2 goodness-of-fit was determined using the Hardy–Weinberg equilibrium (HWE) test. Multiple linear regression was adopted to analyze the factors influencing BMD, and independent variables were selected using the best subset regression, which was based on adjusted R2 values. A p-value <0.05 was considered statistically significant.
The results of the statistical tests are available as Supplementary data.
Results
There were statistically significant differences in menopausal transition age, BMI, and WHR within groups A, B and D (p < 0.05, Table 1).
The ACNOVA demonstrated that FPG and HbA1c levels were statistically higher in groups C and D compared to group A (p < 0.01). Bone mineral density (femoral neck) and BMD (lumbar spine vertebrae 1–4 (L1–L4)) were statistically lower in groups B and D compared to group A (p < 0.01, Table 2).
The genotype frequencies of the 2 loci were consistent with the HWE (p > 0.05). There were no significant differences in genotype frequency and gene frequency between the rs556442 and rs638051 loci on the LRP5 gene (p > 0.05, Table 3).
At the rs556442 locus in the group B, femoral neck BMD of the AG/GG genotype was statistically lower than that of the AA genotype (p < 0.05). In group D, ALP level was statistically lower for the AG/GG genotype compared to the AA genotype (p < 0.05, Table 4). At the rs638051 locus in group D, ALP level was statistically lower for the AG/GG genotype compared to the AA genotype (p < 0.05, Table 5).
In group D (T2DM + abnormal bone mass), the best subset regression analysis was performed with BMD (L1–L4) and BMD (femoral neck) as the response variables and age (X1), menopausal transition age (X2), BMI (X3), WHR (X4), FPG (X5), HbA1c (X6), TG (X7), high-density lipoprotein (HDL) (X8), LDL (X9), Ca (X10), phosphorus (P) (X11), ALP (X12), and genotype (X13) as independent variables.
The best subset regression analysis demonstrated that lower TG level and BMI, older age and higher menopausal transition age at the rs556442 locus were risk factors for a decreased BMD (L1–L4). Lower TG level and higher menopausal transition age at the rs638051 locus were risk factors for decreased BMD (L1–L4). Except for higher menopausal transition age, all variables were also risk factors for a decreased BMD (femoral neck, Table 6).
Discussion
In an aging population, the incidence rate of OP is increasing with each calendar year, and high disability rate among OP patients places a heavy financial burden on the society.17 As confirmed in other studies, postmenopausal women with T2DM have a decreased bone mass due to a rapid decline in estrogen levels, putting them at high risk for OP.18, 19 Therefore, at the gene level, further study of the pathogenesis of OP can provide a theoretical basis for explaining the occurrence of T2DM with OP. Currently, it is unclear whether T2DM can lead to the reduction of BMD. However, an increasing number of studies have shown that T2DM can increase bone fragility,20, 21 and such fragility leads to an increase in T2DM with OP.22, 23 The pathogenesis of T2DM complicated by OP is multifactorial and influenced by genetic and environmental factors. The Wnt signaling pathway is crucial in the axis differentiation of multicellular organisms,24 where LRP5 is a transmembrane receptor of the Wnt protein.25 Regarding the Wnt signaling pathway, studies on LRP5 gene polymorphisms and T2DM or OP have been published both in China and in other countries, but there are few reports on the relationship between LRP5 gene polymorphisms and OP in postmenopausal women with T2DM.
This study found that the genotype and allele frequency distribution of the rs556442 and rs638051 loci of LRP5 comply with the Hardy–Weinberg law of genetic balance. This indicates that the population selected for this study has relatively stable heritability and is a representative of the target population. The genotype distribution was dominated by the AA genotype, which was 64.6% and 57.1% for rs556442 and rs638051, respectively (Table 3). The wild-type homozygous genotype was the most common, the mutant heterozygous genotype was the 2nd most common, and the mutant homozygous genotype was the least common. Astiazar et al.26 and Koay et al.27 found that the LRP5 gene mutation can reduce BMD and increase the incidence of OP. This study found that for rs556442 in group B, the BMD (femoral neck) of the AG/GG genotype (mutant) was lower compared to the AA genotype (wild-type, 0.68 ±0.08 compared to 0.75 ±0.10 g/cm2, p = 0.013). At rs556442, ALP level for the AG/GG genotype (mutant-type) was lower in group D than in the AA genotype (wild-type, 66.36 ±21.02 compared to 77.46 ±20.66 U/L, p = 0.025; Table 4). In group D, at rs638051, ALP level for the AG/GG genotype (mutant-type) was lower compared to the AA genotype (wild-type, 66.54 ±25.38 compared to 80.00 ±16.83 U/L, p = 0.026). These findings suggest that mutations at rs556442 and rs638051 loci may be related to BMD and bone metabolism (Table 5). Wang et al. showed that LRP5 gene polymorphisms are genetically linked to increases in blood lipid levels, BMI and obesity.28 Through the best subset regression analysis, decreased BMI and TGs and higher menopausal transition age and age were found to be risk factors for decreased BMD, suggesting that higher BMI and TG levels are associated with a lower risk of abnormal BMD among diabetic patients, similar to the findings of Li et al.29 Thus, postmenopausal women with dyslipidemia should receive screening to prevent the occurrence of OP.
Limitations of the study
The adjusted R2 in the best subset regression analysis model established for rs556442 and rs638051 was relatively low, which was considered to be related to the quantity and quality of the included independent variables.
In the future, our research group will consider investigating more important factors that may affect the occurrence and development of diseases, such as FINS, FCP, ISI, PINP, CTX, NTX, etc., which include many invasive independent variables with high diagnostic value, so as to further improve the value of the model.
Conclusions
To summarize, mutations at rs556442 and rs638051 loci of the LRP5 gene are related to bone metabolism in postmenopausal women in Xinjiang. Therefore, postmenopausal women with T2DM should undergo screening and early intervention strategies for the prevention of OP.
Supplementary data
The results of the statistical tests are available as Supplementary data at https://doi.org/10.5281/zenodo.7196566. The package contains the following files:
Supplementary Table 1. Normality test of the variables presented in Table 1,2.
Supplementary Table 2. Homogeneity of variances test of the variables presented in Table 1,2.
Supplementary Table 3. Welch ANOVA of the statistical values presented in Table 1.
Supplementary Table 4. ANCOVA of the statistical values presented in Table 2.
Supplementary Table 5. Post hoc tests of the statistical values presented in Table 1.
Supplementary Table 6. Post hoc tests of statistical values presented in Table 2.