The genetics of bone loss: challenges and prospects

Mechanistic advances in osteoporosis and anti-osteoporosis therapies

Osteoporosis is a type of bone loss disease characterized by a reduction in bone mass and microarchitectural deterioration of bone tissue. With the intensification of global aging, this disease is now regarded as one of the major public health problems that often leads to unbearable pain, risk of bone fractures, and even death, causing an enormous burden at both the human and socioeconomic layers. Classic anti-osteoporosis pharmacological options include anti-resorptive and anabolic agents, whose ability to improve bone mineral density and resist bone fracture is being gradually confirmed. However, long-term or high-frequency use of these drugs may bring some side effects and adverse reactions. Therefore, an increasing number of studies are devoted to finding new pathogenesis or potential therapeutic targets of osteoporosis, and it is of great importance to comprehensively recognize osteoporosis and develop viable and efficient therapeutic approaches. In this study, we systematically reviewed literatures and clinical evidences to both mechanistically and clinically demonstrate the state-of-art advances in osteoporosis. This work will endow readers with the mechanistical advances and clinical knowledge of osteoporosis and furthermore present the most updated anti-osteoporosis therapies.

Bone Health in Children with Rheumatic Disorders: Focus on Molecular Mechanisms, Diagnosis, and Management

Bone is an extremely dynamic and adaptive tissue, whose metabolism and homeostasis is influenced by many different hormonal, mechanical, nutritional, immunological and pharmacological stimuli. Genetic factors significantly affect bone health, through their influence on bone cells function, cartilage quality, calcium and vitamin D homeostasis, sex hormone metabolism and pubertal timing. In addition, optimal nutrition and physical activity contribute to bone mass acquisition in the growing age. All these factors influence the attainment of peak bone mass, a critical determinant of bone health and fracture risk in adulthood. Secondary osteoporosis is an important issue of clinical care in children with acute and chronic diseases. Systemic autoimmune disorders, like juvenile idiopathic arthritis, can affect the skeletal system, causing reduced bone mineral density and high risk of fragility fractures during childhood. In these patients, multiple factors contribute to reduce bone strength, including systemic inflammation with elevated cytokines, reduced physical activity, malabsorption and nutritional deficiency, inadequate daily calcium and vitamin D intake, use of glucocorticoids, poor growth and pubertal delay. In juvenile arthritis, osteoporosis is more prominent at the femoral neck and radius compared to the lumbar spine. Nevertheless, vertebral fractures are an important, often asymptomatic manifestation, especially in glucocorticoid-treated patients. A standardized diagnostic approach to the musculoskeletal system, including prophylaxis, therapy and follow up, is therefore mandatory in at risk children. Here we discuss the molecular mechanisms involved in skeletal homeostasis and the influence of inflammation and chronic disease on bone metabolism.

Genetic variation in NOD1/CARD4 and NOD2/CARD15 immune sensors and risk of osteoporosis

The present study was aimed to investigate the relationship between NOD1/CARD4 and NOD2/CARD15 gene polymorphisms and osteoporosis in the Turkish population. The first time we thought that the functional polymorphisms in NOD1/CARD4 and NOD2/CARD15 genes might have triggered the development of osteoporosis. The objective of our study was to determine the relationship between NOD1/CARD4 and NOD2/CARD15 SNPs and osteoporosis. The NOD1/CARD4 (rs5743336) and NOD2/CARD15 (rs2066847) SNPs were analyzed by PCR restriction fragment length polymorphism (PCR-RFLP) in 94 healthy controls and 164 subjects with osteoporosis. PCR products were digested with restriction enzymes AvaI for NOD1/CARD4 and ApaI for NOD2/CARD15. We found that NOD1/CARD4 genotype distribution of AA, GA and GG were 15, 44 and 41% for patients and 17, 46 and 37% for controls, respectively. NOD2/CARD15 mutation was found only in three patients (1.8%) as heterozygote. The results did not show any statistical difference between NOD1/CARD4 and NOD2/CARD15 genotype distribution of patients and healthy groups (χ² = 1.740, P=0.187; χ² = 1.311, P=0.519). However, the most frequent AG genotype (46%) of NOD1/CARD4 was observed in healthy controls, GG genotype (44%) of NOD1/CARD4 was observed as the most frequent in osteoporotic patients. NOD2/CARD15 WT/WT genotype, the most frequent genotype, was observed in both groups. Statistical analysis revealed that NOD1/CARD4 and NOD2/CARD15 polymorphisms are not associated with osteoporosis. However, a definite judgement is difficult to be made due to restricted number of patients and small size of control group. Further research is sorely warranted in this direction.

Identification of a single-nucleotide polymorphism within CDH2 gene associated with bone morbidity in childhood acute lymphoblastic leukemia survivors

Aim: To identify genetic markers associated with late treatment-related skeletal morbidity in survivors of childhood acute lymphoblastic leukemia (ALL). Patients & methods: To this end, we measured the association between reduction in bone mineral density or vertebral fractures prevalence and variants from 1039 genes derived through whole exome sequencing in 242 childhood ALL survivors. Top-ranking variants were confirmed through genotyping, and further explored with stratified analyses and multivariable models. Results: The minor allele of rs1944294 in CDH2 gene was associated with bone geometrical parameter, trabecular cross-sectional area (p = 0.001). The association was modulated by radiation therapy (p = 0.001) and post-treatment time (p = 0.0002). Conclusion: The variant in CDH2 gene is a potential novel risk factor of bone morbidity in survivors of childhood ALL.

Advanced Genetic Approaches in Discovery and Characterization of Genes Involved With Osteoporosis in Mouse and Human

Osteoporosis is a complex condition with contributions from, and interactions between, multiple genetic loci and environmental factors. This review summarizes key advances in the application of genetic approaches for the identification of osteoporosis susceptibility genes. Genome-wide linkage analysis (GWLA) is the classical approach for identification of genes that cause monogenic diseases; however, it has shown limited success for complex diseases like osteoporosis. In contrast, genome-wide association studies (GWAS) have successfully identified over 200 osteoporosis susceptibility loci with genome-wide significance, and have provided most of the candidate genes identified to date. Phenome-wide association studies (PheWAS) apply a phenotype-to-genotype approach which can be used to complement GWAS. PheWAS is capable of characterizing the association between osteoporosis and uncommon and rare genetic variants. Another alternative approach, whole genome sequencing (WGS), will enable the discovery of uncommon and rare genetic variants in osteoporosis. Meta-analysis with increasing statistical power can offer greater confidence in gene searching through the analysis of combined results across genetic studies. Recently, new approaches to gene discovery include animal phenotype based models such as the Collaborative Cross and ENU mutagenesis. Site-directed mutagenesis and genome editing tools such as CRISPR/Cas9, TALENs and ZNFs have been used in functional analysis of candidate genes in vitro and in vivo. These resources are revolutionizing the identification of osteoporosis susceptibility genes through the use of genetically defined inbred mouse libraries, which are screened for bone phenotypes that are then correlated with known genetic variation. Identification of osteoporosis-related susceptibility genes by genetic approaches enables further characterization of gene function in animal models, with the ultimate aim being the identification of novel therapeutic targets for osteoporosis.

LL-37 inhibits LPS-induced inflammation and stimulates the osteogenic differentiation of BMSCs via P2X7 receptor and MAPK signaling pathway

Oral diseases, such as periapical periodontitis and periodontitis, are characterized by inflammation-induced bone loss. LL-37, a human antimicrobial peptide (AMP), has multiple biological functions and the potential to promote osteogenesis. Therefore, this study aimed to investigate the regulatory effects of LL-37 within normal and inflammatory microenvironments. The roles of P2X7 receptor (P2X7R) and mitogen-activated protein kinase (MAPK) signaling pathway were also demonstrated. The results showed that LL-37 promoted bone marrow stromal cell (BMSC) proliferation, migration and osteogenic differentiation. LL-37 inhibited the expression of the inflammatory cytokines interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α) and receptor activator of nuclear factor kappa-B ligand (RANKL) at both protein and gene levels, and attenuated the lipopolysaccharide (LPS)-induced inhibition of osteogenesis. Immunofluorescence (IF) confirmed P2X7R expression in BMSCs. BBG, a P2X7R antagonist, significantly attenuated LL-37-promoted osteogenesis. The phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) and c-Jun NH2-terminal kinase (JNK) increased after LL-37 stimulation, which did not affect p38 phosphorylation. The effects of LL-37 on osteogenesis-related gene expression were markedly attenuated by selective inhibitors of ERK1/2 and JNK. Furthermore, a mouse model of LPS-stimulated calvarial osteolysis was established, and results showed that LL-37 markedly inhibited osteoclastic bone resorption. In conclusion, we speculate that LL-37 inhibits inflammation and promotes BMSC osteogenesis via P2X7R and MAPK signaling pathway.

Greater yogurt consumption is associated with increased bone mineral density and physical function in older adults

In this cohort of community dwelling older adults (>60 years), we observed significant positive associations between the frequencies of yogurt intake with measures of bone density, bone biomarkers, and indicators of physical function. Improving yogurt intakes could be a valuable health strategy for maintaining bone health in older adults.

INTRODUCTION

The associations of yogurt intakes with bone health and frailty in older adults are not well documented. The aim was to investigate the association of yogurt intakes with bone mineral density (BMD), bone biomarkers, and physical function in 4310 Irish adults from the Trinity, Ulster, Department of Agriculture aging cohort study (TUDA).

METHODS

Bone measures included total hip, femoral neck, and vertebral BMD with bone biochemical markers. Physical function measures included Timed Up and Go (TUG), Instrumental Activities of Daily Living Scale, and Physical Self-Maintenance Scale.

RESULTS

Total hip and femoral neck BMD in females were 3.1-3.9% higher among those with the highest yogurt intakes (n = 970) compared to the lowest (n = 1109; P < 0.05) as were the TUG scores (-6.7%; P = 0.013). In males, tartrate-resistant acid phosphatase (TRAP 5b) concentrations were significantly lower in those with the highest yogurt intakes (-9.5%; P < 0.0001). In females, yogurt intake was a significant positive predictor of BMD at all regions. Each unit increase in yogurt intake in females was associated with a 31% lower risk of osteopenia (OR 0.69; 95% CI 0.49-0.96; P = 0.032) and a 39% lower risk of osteoporosis (OR 0.61; 95% CI 0.42-0.89; P = 0.012) and in males, a 52% lower risk of osteoporosis (OR 0.48; 95% CI 0.24-0.96; P = 0.038).

CONCLUSION

In this cohort, higher yogurt intake was associated with increased BMD and physical function scores. These results suggest that improving yogurt intakes could be a valuable public health strategy for maintaining bone health in older adults.

Bone turnover markers: Emerging tool in the management of osteoporosis

Bone is a dynamic tissue which undergoes constant remodeling throughout the life span. Bone turnover is balanced with coupling of bone formation and resorption at various rates leading to continuous remodeling of bone. A study of bone turnover markers (BTMs) provides an insight of the dynamics of bone turnover in many metabolic bone disorders. An increase in bone turnover seen with aging and pathological states such as osteoporosis leads to deterioration of bone microarchitecture and thus contributes to an increase in the risk of fracture independent of low bone mineral density (BMD). These microarchitectural alterations affecting the bone quality can be assessed by BTMs and thus may serve as a complementary tool to BMD in the assessment of fracture risk. A systematic search of literature regarding BTMs was carried out using the PubMed database for the purpose of this review. Various reliable, rapid, and cost-effective automated assays of BTMs with good sensitivity are available for the management of osteoporosis. However, BTMs are subjected to various preanalytical and analytical variations necessitating strict sample collection and assays methods along with utilizing ethnicity-based reference standards for different populations. Estimation of fracture risk and monitoring the adherence and response to therapy, which is a challenge in a chronic, asymptomatic disease such as osteoporosis, are the most important applications of measuring BTMs. This review describes the physiology of bone remodeling, various conventional and novel BTMs, and BTM assays and their role in the assessment of fracture risk and monitoring response to treatment with antiresorptive or anabolic agents.

Rapid phenotyping of knockout mice to identify genetic determinants of bone strength

The genetic determinants of osteoporosis remain poorly understood, and there is a large unmet need for new treatments in our ageing society. Thus, new approaches for gene discovery in skeletal disease are required to complement the current genome-wide association studies in human populations. The International Knockout Mouse Consortium (IKMC) and the International Mouse Phenotyping Consortium (IMPC) provide such an opportunity. The IKMC generates knockout mice representing each of the known protein-coding genes in C57BL/6 mice and, as part of the IMPC initiative, the Origins of Bone and Cartilage Disease project identifies mutants with significant outlier skeletal phenotypes. This initiative will add value to data from large human cohorts and provide a new understanding of bone and cartilage pathophysiology, ultimately leading to the identification of novel drug targets for the treatment of skeletal disease.

Network Analysis Implicates Alpha-Synuclein (Snca) in the Regulation of Ovariectomy-Induced Bone Loss

The postmenopausal period in women is associated with decreased circulating estrogen levels, which accelerate bone loss and increase the risk of fracture. Here, we gained novel insight into the molecular mechanisms mediating bone loss in ovariectomized (OVX) mice, a model of human menopause, using co-expression network analysis. Specifically, we generated a co-expression network consisting of 53 gene modules using expression profiles from intact and OVX mice from a panel of inbred strains. The expression of four modules was altered by OVX, including module 23 whose expression was decreased by OVX across all strains. Module 23 was enriched for genes involved in the response to oxidative stress, a process known to be involved in OVX-induced bone loss. Additionally, module 23 homologs were co-expressed in human bone marrow. Alpha synuclein (Snca) was one of the most highly connected “hub” genes in module 23. We characterized mice deficient in Snca and observed a 40% reduction in OVX-induced bone loss. Furthermore, protection was associated with the altered expression of specific network modules, including module 23. In summary, the results of this study suggest that Snca regulates bone network homeostasis and ovariectomy-induced bone loss.

Calcium plus vitamin D supplementation during pregnancy interacts with polymorphisms in the promoter region of the VDR gene to affect postpartum bone mass of Brazilian adolescent mothers: A randomized controlled trial

OBJECTIVE

We investigated whether calcium plus vitamin D supplementation interacts with polymorphisms in the VDR gene promoter region to affect changes on maternal bone mass from 5 to 20 wk postpartum in Brazilian adolescent mothers.

METHODS

Pregnant adolescents (14-19 y) randomly received calcium plus cholecalciferol (600 mg/d + 200 IU/d, n = 30) or placebo (n = 26) from 26 wk of pregnancy until parturition. Bone mineral content (BMC), bone area (BA), and bone mineral density (BMD) at total body, lumbar spine, total hip, and femoral neck were evaluated at 5 and 20 wk postpartum. Serum 25-hydroxyvitamin D (25[OH]D) and parathyroid hormone concentrations were measured. Real-time polymerase chain reaction was used for genotyping rs7139166 (1521 pb G > C) and rs4516035 (1012 pb A > G). Interactions between supplementation and polymorphisms were adjusted for significant covariates.

RESULTS

Changes in serum 25(OH)D from pregnancy to postpartum differed between supplemented and placebo groups for mothers carrying 1521 GG/1012 AA genotypes (P = 0.004). Only in the placebo group, mothers carrying 1521 GG/1012 AA had greater reduction in total BMD z score, femoral neck BMC, and BMD from 5 to 20 wk postpartum compared with those with 1521 GC/1012 AG (P < 0.05). In the placebo group, total hip BA decreased from 5 to 20 wk postpartum in adolescents with 1521 GG/1012 AA, but increased in those with 1521 GC/1012 AG (P < 0.05), in contrast to the supplemented group.

CONCLUSION

Calcium plus vitamin D supplementation during pregnancy interacted with polymorphisms in the VDR gene promoter region affecting postpartum bone loss. The increased supply of calcium and vitamin D appeared to minimize postpartum bone loss particularly in adolescents with 1521 GG/1012 AA.

Replication study of three functional polymorphisms associated with bone mineral density in a cohort of Spanish women

Gene candidate and genome-wide association studies have revealed tens of loci of susceptibility for osteoporosis. Some limitations such as sample size, use of confounding variables, and control for multiple testing and for population stratification, however, represent common problems in these studies that make replication in independent cohorts desirable and even necessary. The main objective of the present study is to replicate previous data on three functional polymorphisms in a cohort of Spanish women. To that end, we performed an association study of three functional polymorphisms previously associated with bone phenotypes in the LRP5, TNFRSF11B, and FGFBP1 genes with low bone mineral density (BMD) in a cohort of 721 Spanish women, most of them postmenopausal. We detected a strong significant association, even when correcting for multiple comparisons, for polymorphism rs312009 in the LRP5 gene with low BMD at the lumbar-spine site. These were women with the CC genotype, which showed the worst bone parameters. Moreover, these women had a higher risk of osteoporosis (adjusted odds ratio 2.82, P = 0.001) than women with the TT/TC genotype. This association seems to be caused because the rs312009 single nucleotide polymorphism (SNP) is located at a binding site for the transcription factor RUNX2 at the 5' region of the LRP5 gene, and the T allele seems to be a better transcriber than the C allele. Regarding the other two SNPs, only the rs4876869 SNP in the TNFRSF11B gene showed a suggestive trend for both skeletal sites. These results underscore the significance of the LRP5 gene in bone metabolism and emphasize the significance of the replication of previous results in independent cohorts.

Relationship between CATSPERB, NR5A2 gene polymorphisms and Peak Bone Mineral Density in College Students in China

BACKGROUND

Peak bone mineral density (PBMD) is influenced by both genetic and environmental factors, genes explains most of variation. As the novel candidate genes CATSPERB and NR5A2 may have been associated with spinal PBMD in adult. This study was to investigate the relationship among these two genes^ PBMD and the life style factors in young female.

METHODS

The rs1298989 single nucleotide polymorphism (SNP) of the CATSPERB gene and the rs3762397 SNP of the NR5A2 gene were genotyped using SNaPshot® in 359 students from Xinjiang. The prospective study included 203 Han and 156 Uyghur subjects. PBMD was measured using quantitative computed tomography (QCT). Calcium, phosphate and alkaline phosphatase were measured by ELISA method. Physical activity, dietary calcium and life styles were assessed by questionnaire.

RESULTS

Both SNPs showed differences in genotype and allele frequencies (P < 0.05) between the Han and Uyghur subjects. Total calcium intake, energy intake, tea and milk intake were also significantly different between two groups (P < 0.05). Multiple regression analysis showed an association between PBMD and vitamin D intake (P = 0.000), milk (P = 0.000), exercise (P = 0.029), rs1298989 (P = 0.028), energy intake (P = 0.043).

CONCLUSION

This study demonstrated the polymorphisms of the rs1298989 and rs3762397 are associated with PBMD both in Han and Uyghur subjects. PBMD, in Xinjiang, appears to be associated with several known factors that are well described in the literature. While the genotypes of rs1298989 and rs3762397 do not appear have a strong effect on the PBMD.

Gender- and region-specific alterations in bone metabolism in Scarb1-null female mice

A positive correlation between plasma levels of HDL and bone mass has been reported by epidemiological studies. As scavenger receptor class B, type I (SR-BI), the gene product of Scarb1, is known to regulate HDL metabolism, we recently characterized bone metabolism in Scarb1-null mice. These mice display high femoral bone mass associated with enhanced bone formation. As gender differences have been reported in HDL metabolism and SR-BI function, we investigated gender-specific bone alterations in Scarb1-null mice by microtomography and histology. We found 16% greater relative bone volume and 39% higher bone formation rate in the vertebrae from 2-month-old Scarb1-null females. No such alteration was seen in males, indicating gender- and region-specific differences in skeletal phenotype. Total and HDL-associated cholesterol levels, as well as ACTH plasma levels, were increased in both Scarb1-null genders, the latter being concurrent to impaired corticosterone response to fasting. Plasma levels of estradiol did not differ between null and WT females, suggesting that the estrogen metabolism alteration is not relevant to the higher vertebral bone mass in female Scarb1-null mice. Constitutively, high plasma levels of leptin along with 2.5-fold increase in its expression in white adipose tissue were measured in female Scarb1-null mice only. In vitro exposure of bone marrow stromal cells to ACTH and leptin promoted osteoblast differentiation as evidenced by increased gene expression of osterix and collagen type I alpha. Our results suggest that hyperleptinemia may account for the gender-specific high bone mass seen in the vertebrae of female Scarb1-null mice.

Calcium and cholecalciferol supplementation provides no added benefit to nutritional counseling to improve bone mineral density in survivors of childhood acute lymphoblastic leukemia (ALL)

BACKGROUND

We sought to improve lumbar spine bone mineral density (LS-BMD) in long-term survivors of childhood acute lymphoblastic leukemia (ALL) using calcium and cholecalciferol supplementation.

PROCEDURE

This double-blind, placebo-controlled trial randomized 275 participants (median age, 17 [9-36.1] years) with age- and gender-specific LS-BMD Z-scores <0 to receive nutritional counseling with supplementation of 1,000 mg/day calcium and 800 International Unit cholecalciferol or placebo for 2 years. The primary outcome was change in LS-BMD assessed by quantitative computerized tomography (QCT) at 24 months. Linear regression models were employed to identify the baseline risk factors for low LS-BMD and to compare LS-BMD outcomes.

RESULTS

Pre-randomization LS-BMD below the mean was associated with male gender (P = 0.0024), White race (P = 0.0003), lower body mass index (P < 0.0001), and cumulative glucocorticoid doses of ≥ 5,000 mg (P = 0.0012). One hundred eighty-eight (68%) participants completed the study; 77% adhered to the intervention. Mean LS-BMD change did not differ between survivors randomized to supplements (0.33 ± 0.57) or placebo (0.28 ± 0.56). Participants aged 9-13 years and those 22-35 years had the greatest mean increases in LS-BMD (0.50 ± 0.66 and 0.37 ± 0.23, respectively). Vitamin D insufficiency (serum 25[OH]D <30 ng/ml) found in 296 (75%), was not associated with LS-BMD outcomes (P = 0.78).

CONCLUSION

Cholecalciferol and calcium supplementation provides no added benefit to nutritional counseling for improving LS-BMD among adolescent and young adult survivors of ALL (93% of whom had LS-BMD Z-scores above the mean at study entry).

Association between single nucleotide polymorphisms of the estrogen receptor 1 and receptor activator of nuclear factor kappa B ligand genes and bone mineral density in postmenopausal Taiwanese

OBJECTIVE

To investigate the relationship between single nucleotide polymorphisms (SNPs) of the genes encoding the estrogen receptor 1 (ESR1) and the receptor activator of nuclear factor kappa B ligand (RANKL) and bone mineral density (BMD) in postmenopausal Taiwanese.

MATERIALS AND METHODS

Five ESR1 SNPs and three RANKL SNPs in 467 women were genotyped. Results of genotyping were correlated with BMD that had been adjusted for body mass index (BMI), age, and years after menopause.

RESULTS

Those with the ESR1 Crs1884054 allele were found to have a lower BMD at LS2-4/Lateral view (p = 0.005 and permutated p = 0.046), and those with the ESR1 haplotype Trs2234693-Ars922996 had a higher risk for low BMD also at LS2-4/Lat (OR = 1.8, 95% CI = 1.1-2.9). In addition, women without the RANKL haplotype Grs2148072-Crs2200287-Grs922996 had a higher risk for low BMD at LS1-4/AP (OR = 2.09, 95% CI = 1.21 ∼ 3.64). Stratification analyses revealed that those with ESR1 AArs1884054 and RANKL Ars2148072 (p = 0.032) or RANKL Trs2200287 (p = 0.007) had a lower BMD at LS1-4/AP.

CONCLUSION

Genotypes of these SNPs of ESR1 and RANKL may help us predict the osteoporosis risk in menopausal women.

Understanding the Basis of Genetic Studies: Adolescent Idiopathic Scoliosis as an Example

STUDY DESIGN

A review of the general concepts of genetics studies with specific reference to adolescent idiopathic scoliosis (AIS).

OBJECTIVES

To equip the average spine surgeon with the vocabulary and understanding needed to understand the genetics of scoliosis and the approaches used to identify risk genes.

SUMMARY OF BACKGROUND DATA

Adolescent idiopathic scoliosis is a multifactorial disease. Increasing evidence from families and monozygotic twins suggests the involvement of genetic factors. An estimation of heritability also indicates a strong influence of genetics on the disease. Increasing focus has been placed on identifying genes and genetic variants associated with AIS.

REVIEW

This is a review of genes and genetic variations, the phenotype definition of AIS in genetics studies, concepts and approaches to identifying associated genes, and the evaluation of results. Different types of genetic variations are present in the genome. These variations may modulate the expression or function of protein products, which in turn alter individuals' susceptibility to disease. Identifying the variants related to AIS requires an objective and clearly defined phenotype, among which the Cobb angle is commonly used. The phenotype helps classify subjects into cases and controls. By selecting candidate genes of growth factors and hormonal receptors, which are speculated to be involved in the mechanism of disease, the variants within these genes were compared between cases and controls to identify any differences. Another approach was to use large families and inspect the co-segregation of variants and phenotypes. Recently, arrays covering the variants of the whole genome were developed and assist in high-throughput screening for associated genes.

CONCLUSIONS

Genetic factors have an important role in AIS. Deciphering the genes and genetic variants associated with AIS can improve our understanding of the mechanisms of the disease, as well as assist in designing treatment methods and preventive measures.

Clinical impact of recent genetic discoveries in osteoporosis

Osteoporotic fracture carries an enormous public health burden in terms of mortality and morbidity. Current approaches to identify individuals at high risk for fracture are based on assessment of bone mineral density and presence of other osteoporosis risk factors. Bone mineral density and susceptibility to osteoporotic fractures are highly heritable, and over 60 loci have been robustly associated with one or both traits through genome-wide association studies carried out over the past 7 years. In this review, we discuss opportunities and challenges for incorporating these genetic discoveries into strategies to prevent osteoporotic fracture and translating new insights obtained from these discoveries into development of new therapeutic targets.

Genome-wide approaches for identifying genetic risk factors for osteoporosis

Osteoporosis, the most common type of bone disease worldwide, is clinically characterized by low bone mineral density (BMD) and increased susceptibility to fracture. Multiple genetic and environmental factors and gene-environment interactions have been implicated in its pathogenesis. Osteoporosis has strong genetic determination, with the heritability of BMD estimated to be as high as 60%. More than 80 genes or genetic variants have been implicated in risk of osteoporosis by hypothesis-free genome-wide studies. However, these genes or genetic variants can only explain a small portion of BMD variation, suggesting that many other genes or genetic variants underlying osteoporosis risk await discovery. Here, we review recent progress in genome-wide studies of osteoporosis and discuss their implications for medicine and the major challenges in the field.

Relationship between plasminogen activator inhibitor type-1 (PAI-1) gene polymorphisms and osteoporosis in Turkish women

OBJECTIVE

The development of osteoporosis is associated with several risk factors, such as genetic structures that affect bone turnover and bone mass. The impact of genetic structures on osteoporosis is not known. Plasminogen activator inhibitor type-1 regulates the bone matrix and bone balance. This study assessed the correlation between plasminogen activator inhibitor type-1 gene 4G/5G polymorphisms and osteoporosis in a population of Turkish women.

METHODS

A total of 195 postmenopausal female patients who were diagnosed with osteoporosis (Group I) based on bone mineral density measurements via dual-energy x-ray absorptiometry and 90 females with no osteoporosis (Group II) were included in this study. Correlations between PAI-1 gene 4G/5G polymorphisms and osteoporosis were investigated through the identification of PAI-1 gene 4G/5G polymorphism genotypes using the polymerase chain reaction.

RESULTS

No significant differences in the genotype and allele frequency of 4G/5G plasminogen activator inhibitor type-1 polymorphisms were observed between the two groups, and both groups exhibited the most frequently observed 4G5G genotype.

CONCLUSION

No correlation between the development of osteoporosis in the female Turkish population and 4G/5G plasminogen activator inhibitor type-1 gene polymorphisms was observed.

The transcriptional profile of mesenchymal stem cell populations in primary osteoporosis is distinct and shows overexpression of osteogenic inhibitors

Primary osteoporosis is an age-related disease characterized by an imbalance in bone homeostasis. While the resorptive aspect of the disease has been studied intensely, less is known about the anabolic part of the syndrome or presumptive deficiencies in bone regeneration. Multipotent mesenchymal stem cells (MSC) are the primary source of osteogenic regeneration. In the present study we aimed to unravel whether MSC biology is directly involved in the pathophysiology of the disease and therefore performed microarray analyses of hMSC of elderly patients (79–94 years old) suffering from osteoporosis (hMSC-OP). In comparison to age-matched controls we detected profound changes in the transcriptome in hMSC-OP, e.g. enhanced mRNA expression of known osteoporosis-associated genes (LRP5, RUNX2, COL1A1) and of genes involved in osteoclastogenesis (CSF1, PTH1R), but most notably of genes coding for inhibitors of WNT and BMP signaling, such as Sclerostin and MAB21L2. These candidate genes indicate intrinsic deficiencies in self-renewal and differentiation potential in osteoporotic stem cells. We also compared both hMSC-OP and non-osteoporotic hMSC-old of elderly donors to hMSC of ∼30 years younger donors and found that the transcriptional changes acquired between the sixth and the ninth decade of life differed widely between osteoporotic and non-osteoporotic stem cells. In addition, we compared the osteoporotic transcriptome to long term-cultivated, senescent hMSC and detected some signs for pre-senescence in hMSC-OP.

Our results suggest that in primary osteoporosis the transcriptomes of hMSC populations show distinct signatures and little overlap with non-osteoporotic aging, although we detected some hints for senescence-associated changes. While there are remarkable inter-individual variations as expected for polygenetic diseases, we could identify many susceptibility genes for osteoporosis known from genetic studies. We also found new candidates, e.g. MAB21L2, a novel repressor of BMP-induced transcription. Such transcriptional changes may reflect epigenetic changes, which are part of a specific osteoporosis-associated aging process.

Fast linear mixed model computations for genome-wide association studies with longitudinal data

Genome-wide association studies are characterized by a huge number of statistical tests performed to discover new disease-related genetic variants [in the form of single-nucleotide polymorphisms (SNPs)] in human DNA. Many SNPs have been identified for cross-sectionally measured phenotypes. However, there is a growing interest in genetic determinants of the evolution of traits over time. Dealing with correlated observations from the same individual, we need to apply advanced statistical techniques. The linear mixed model is popular but also much more computationally demanding than fitting a linear regression model to independent observations. We propose a conditional two-step approach as an approximate method to explore the longitudinal relationship between the trait and the SNP. In a simulation study, we compare several fast methods with respect to their accuracy and speed. The conditional two-step approach is applied to relate SNPs to longitudinal bone mineral density responses collected in the Rotterdam Study.