The effects of vitamin D receptor polymorphism on secondary hyperparathyroidism and bone density after renal transplantation

Inflammation and Vitamin D Receptor Polymorphism: Impact on All-Cause and Cardiovascular Mortality in Mexican Women on Dialysis

Mineral bone disease (MBD) is common in dialysis patients. Genetics and the hormonal environment influence the clinical picture and outcomes of women. This study aimed to determine how these factors affect mortality. In 234 female dialysis patients on Continuous Ambulatory (48%) or Automated (29%) Peritoneal Dialysis or Hemodialysis (23%), MBD biochemical variables, as well as bone density and genetic Bsm1 polymorphism of vitamin D receptor (VDR) were performed at baseline. The cohort was followed-up by 17 (IQ range 15-31) months. According to VDR polymorphism, the distribution of patients was bb: 64% and BB+Bb: 36%. Fifty-five patients died from all-cause mortality; the hs-C-reactive protein level was the most significant risk in multivariate Cox analysis. Nineteen died from cardiovascular mortality. None of the variables were significant for cardiovascular mortality. Patients with bb plus inflammation had the highest risk in the analysis; the significance persisted after adjustment for age, diabetes, and parathyroid hormone levels HR 2.33 (95% CI, 1.01-8.33) and after further adjustment for time on dialysis, albumin, and Osteoprotegerin levels HR 3.49 (95% CI, 1.20-10.9). The presence of the bb genotype from VDR and inflammation had the highest risk of death from all-cause mortality in females on CAPD, APD, and HD patient.

Vitamin D Receptor (VDR) Gene Polymorphisms and High-Turnover Renal Osteodystrophy or Secondary Hyperparathyroidism in End-Stage Renal Disease: A Systematic Review

Chronic kidney disease (CKD) and end-stage renal disease (ESRD) are often complicated by high-turnover renal osteodystrophy (HTRO) and secondary hyperparathyroidism (SHPT), characterized by disturbances in mineral metabolism and skeletal abnormalities. Genetic variations within the vitamin D receptor (VDR) gene, known as VDR gene polymorphisms, have been implicated in modulating the susceptibility to HTRO and SHPT. This systematic review aims to evaluate the existing literature on the association between VDR gene polymorphisms and the development of these complications in ESRD and hemodialysis patients. A comprehensive literature search across multiple databases was conducted, and studies investigating VDR gene polymorphisms and HTRO or SHPT in ESRD or hemodialysis patients were included. The included studies examined various VDR gene polymorphisms, such as BsmI, ApaI, TaqI, and FokI, and their associations with clinical outcomes like parathyroid hormone (PTH) levels, bone mineral density, and the development of SHPT or HTRO. The findings suggest that certain VDR gene polymorphisms, notably the ApaI "aa" genotype, BsmI "bb" genotype, TaqI "tt" genotype, and FokI variant, may contribute to the pathogenesis of SHPT and HTRO by affecting PTH levels, bone turnover markers, and vitamin D sensitivity. However, the studies had relatively small sample sizes and were conducted in different populations, limiting generalizability. Further larger-scale studies, functional investigations, and exploration of gene-environment interactions are warranted to elucidate the underlying mechanisms and facilitate personalized treatment approaches for CKD and ESRD patients with mineral and bone disorders.

Chronic Kidney Disease-Mineral Bone Disease biomarkers in kidney transplant patients

Background:

Chronic Kidney Disease associated with Mineral Bone Disease (CKD-MBD) is frequent in kidney transplant patients. Post-transplantation bone disease is complex, especially in patients with pre-existing metabolic bone disorders that are further affected by immunosuppressive medications and changes in renal allograft function. Main biochemical abnormalities of mineral metabolism in kidney transplantation (KTx) include hypophosphatemia, hyperparathyroidism (HPTH), insufficiency or deficiency of vitamin D, and hypercalcemia.

Objective:

This review aimed to summarize the pathophysiology and main biomarkers of CKD-MBD in KTx.

Methods:

A comprehensive and non-systematic search in PubMed was independently made with an emphasis on biomarkers in mineral bone disease in KTx.

Results:

CKD-MBD can be associated with numerous factors including secondary HPTH, metabolic dysregulations before KTx, and glucocorticoids therapy in post-transplant subjects. Fibroblast growth factor 23 (FGF23) reaches normal levels after KTx with good allograft function, while calcium, vitamin D and phosphorus, ultimately, result in hypercalcemia, persistent vitamin D insufficiency, and hypophosphatemia respectively. As for PTH levels, there is an initial tendency of a significant decrease, followed by a raise due to secondary or tertiary HPTH. In regard to sclerostin levels, there is no consensus in the literature.

Conclusion:

KTx patients should be continuously evaluated for mineral homeostasis and bone status, both cases with successful kidney transplantation and those with reduced functionality. Additional research on CKD-MBD pathophysiology, diagnosis, and management is essential to guarantee long-term graft function, better prognosis, good quality of life, and reduced mortality for KTx patients.

Lack of secondary hyperparathyroidism in sub-group of vitamin D deficient postmenopausal women: Is VDR gene polymorphism behind this mystery?

BACKGROUND AND AIMS

We aimed to determine the cause of non-secondary hyperparathyroidism (Non-SHPT) in Indian postmenopausal women.

MATERIALS & METHODS

334 apparently healthy postmenopausal women were assessed for bone mineral homeostaisis including Vitamin D, PTH and VDR polymorphism.

RESULTS

83% of the subjects had vitamin D deficiency further associated with VDR gene polymorphism (P 0.000). A sizable number of subjects (N = 83) did evoke SHPT despite low vitamin D levels. We observe that VDR gene polymorphism was strongly associated in the sub-group of non-SHPT.

CONCLUSION

lack of SHPT warrants researchers to study the pathophysiology of non-SHPT in detail to substantiate our findings.

Efficacy of weekly administration of cholecalciferol on parathyroid hormone in stable kidney-transplanted patients with CKD stage 1-3

Objectives

Kidney transplant (KTx) recipients frequently have deficient or insufficient levels of serum vitamin D. Few studies have investigated the effect of cholecalciferol in these patients. We evaluated the efficacy of weekly cholecalciferol administration on parathyroid hormone (PTH) levels in stable KTx patients with chronic kidney disease stage 1-3.

Methods

In this retrospective cohort study, 48 stable KTx recipients (37 males, 11 females, aged 52 ± 11 years and 26 months post-transplantation) were treated weekly with oral cholecalciferol (7500-8750 IU) for 12 months and compared to 44 untreated age- and gender-matched recipients. Changes in levels of PTH, 25(OH) vitamin D (25[OH]D), serum calcium, phosphate, creatinine and estimated glomerular filtration rate (eGFR) were measured at baseline, 6 and 12 months.

Results

At baseline, clinical characteristics were similar between treated and untreated patients. Considering the entire cohort, 87 (94.6%) were deficient in vitamin D and 64 (69.6%) had PTH ≥130 pg/mL. Serum calcium, phosphate, creatinine and eGFR did not differ between groups over the follow-up period. However, 25(OH)D levels were significantly higher at both 6 (63.5 vs. 30.3 nmol/L, p < 0.001) and 12 months (69.4 vs. 30 nmol/L, p < 0.001) in treated vs. untreated patients, corresponding with a significant reduction in PTH at both 6 (112 vs. 161 pg/mL) and 12 months (109 vs. 154 pg/mL) in treated vs. untreated patients, respectively (p < 0.001 for both).

Conclusions

Weekly administration of cholecalciferol can significantly and stably reduce PTH levels, without any adverse effects on serum calcium and renal function.

Raising awareness on the therapeutic role of cholecalciferol in CKD: a multidisciplinary-based opinion

Vitamin D is recognized to play an essential role in health and disease. In kidney disease, vitamin D analogs have gained recognition for their involvement and potential therapeutic importance. Nephrologists are aware of the use of oral native vitamin D supplementation, however, uncertainty still exists with regard to the use of this treatment option in chronic kidney disease as well as clinical settings related to chronic kidney disease, where vitamin D supplementation may be an appropriate therapeutic choice. Two consecutive meetings were held in Florence in July and November 2016 comprising six experts in kidney disease (N = 3) and bone mineral metabolism (N = 3) to discuss a range of unresolved issues related to the use of cholecalciferol in chronic kidney disease. The panel focused on the following six key areas where issues relating to the use of oral vitamin D remain controversial: (1) vitamin D and parathyroid hormone levels in the general population, (2) cholecalciferol in chronic kidney disease, (3) vitamin D in cardiovascular disease, (4) vitamin D and renal bone disease, (5) vitamin D in rheumatological diseases affecting the kidney, (6) vitamin D and kidney transplantation.

Current evidence on vitamin D deficiency and kidney transplant: What's new?

Kidney transplant is the treatment of choice for end-stage chronic kidney disease. Kidneys generate 1,25-dihydroxyvitamin D (calcitriol) from 25-hydroxyvitamin D (calcidiol) for circulation in the blood to regulate calcium levels. Transplant patients with low calcidiol levels have an increased risk of metabolic and endocrine problems, cardiovascular disease, type 2 diabetes mellitus, poor graft survival, bone disorders, cancer, and mortality rate. The recommended calcidiol level after transplant is at least 30 ng/mL (75 nmol/L), which could require 1000-3000 IU/d vitamin D3 to achieve. Vitamin D3 supplementation studies have found improved endothelial function and acute rejection episodes. However, since kidney function may still be impaired, raising calcidiol levels may not lead to normal calcitriol levels. Thus, supplementation with calcitriol or an analog, alfacalcidiol, is often employed. Some beneficial effects found include possible improved bone health and reduced risk of chronic allograft nephropathy and cancer.

Clinical impact of hypercalcemia in kidney transplant

Hypercalcemia (HC) has been variably reported in kidney transplanted (KTx) recipients (5–15%). Calcium levels peak around the 3rd month after KTx and thereafter slightly reduce and stabilize. Though many factors have been claimed to induce HC after KTx, the persistence of posttransplant hyperparathyroidism (PT-HPT) of moderate-severe degree is universally considered the first causal factor. Though not proven, there are experimental and clinical suggestions that HC can adversely affect either the graft (nephrocalcinosis) and other organs or systems (vascular calcifications, erythrocytosis, pancreatitis, etc.). However, there is no conclusive evidence that correction of serum calcium levels might avoid the occurrence of these claimed clinical effects of HC. The best way to reduce the occurrence of HC after KTx is to treat as best we can the secondary hyperparathyroidism (SHP) during the uraemic stages. The indication to Parathyroidectomy (PTX), either before or after KTx, in order to prevent or to treat, respectively, HC after KTx, is still a matter of debate which has been revived by the availability of the calcimimetic cinacalcet for the treatment of PT-HPT. However, we still need to better clarify many points as regards the potential adverse effects related to either PTX or cinacalcet use in this clinical set, and we are waiting for the results of future randomized controlled trials to achieve some more definite conclusions on this topic.

Association of vitamin D receptor polymorphisms with the outcome of allogeneic haematopoietic stem cell transplantation

Recently, vitamin D receptor (VDR) polymorphism has been identified as an additional genetic factor associated with the outcome after allogeneic haematopoietic stem cell transplantation (HSCT) from HLA-matched sibling donors. In the present study, VDR ApaI, TaqI and FokI alleles were typed using single strand conformation polymorphism in 123 Polish recipients and their sibling or alternative donors to test the associations of VDR polymorphisms with HSCT outcome. Four VDR genotypes were identified as risk factors of acute graft-versus-host disease (aGVHD). Donor ApaI AA (OR = 7.245, P = 0.009), source of HSC (OR = 7.001, P = 0.007), transplantation from an alternative donor (OR = 6.630, P = 0.007) and donor FokI FF (OR = 4.473, P = 0.025) significantly contributed to the development of grades II-IV aGVHD, while recipient ApaI aa (OR = 3.233, P = 0.069), recipient FokI FF (OR = 2.558, P = 0.077) and female to male transplants (OR = 2.955, P = 0.099) were found to be less significant factors. In addition, the presence of ApaI aa genotype in the recipient was found to be associated with increased likelihood of death (P = 0.0228). The present study contributes to the studies demonstrating a role of VDR polymorphisms in HSCT outcome. In addition to previously described correlations of ApaI a allele and occurrence of severe grades III-IV aGVHD and (linked with ApaI aa) recipient TaqI TT genotype with aGVHD, the novel associations of recipient and donor FokI FF genotype and the increased aGVHD risk and recipient ApaI aa with survival were identified.

Failure of successful renal transplant to produce appropriate levels of 1,25-dihydroxyvitamin D

INTRODUCTION

Bone metabolism disturbances following renal transplantation (RT) are complex and multifactorial in origin. Abnormalities in 1,25-dihydroxyvitamin D levels in RT patients under treatment at our Bone Center prompted this retrospective study.

METHODS

Parameters of vitamin D metabolism were compared in RT patients and a cohort of patients with primary hyperparathyroidism (PHTP) who mimicked the hyperparathyroid state of the RT patients. Thirty-one RT recipients (from 300 reviewed) matched our inclusion criteria with a stable graft function for more than 1 year and a glomerular filtration rate (GFR) >50 mL/min per 1.73 m(2) (Group A); these were compared with 42 consecutive patients with PHTP who had been referred to the same Bone Center for treatment for over 1 month (Group B). Statistical analysis included the chi-square or Fisher's exact tests for categorical data and the Wilcoxon rank sum test for quantitative measures.

RESULTS

The mean (+/-SD) 1,25-dihydroxyvitamin D level was significantly lower (p < 0.001) in Group A patients (29.8 +/- 16.2) than in Group B patients (70.2 +/- 25.9) despite non-significant differences in the levels of parathyroid hormone (PTH) (mean: 184.0 vs.101.1;p < 0.29), phosphorus (mean: 3.2 vs. 3.1; p < 0.3) and 1,25-vitamin D (mean: 19.5 vs. 25.2; p < 0.06). Group A patients had lower levels (p < 0.05) of mean serum calcium and calculated GFR (9.3 mg/dL, 65.7 mL/min) than Group B patients (10.6 mg/dL, 97.6 mL/min). 1,25-Dihydroxyvitamin D significantly correlated with calcium (p < 0.001), 25-vitamin D (p < 0.005) and GFR (p < 0.001) in both groups, but there was a notable lack of association between 1,25-dihydroxyvitamin D and PTH (p < 0.64) or phosphorus (p < 0.26) in Group A patients. In this group, 1,25-dihydroxyvitamin D was not influenced by the type of immunosuppresion regimen (p < 0.06), use of biphosphonates (p < 0.73), presence of diabetes (p < 0.59), menopause in women (p < 0.08), season (p < 0.43) or race (p < 0.31). Our data indicate that 1,25-dihydroxyvitamin D metabolism remains disturbed for a considerable time after successful RT, with the result that the level of 1,25-dihydroxyvitamin D in RT patients is lower despite physiological signals that should stimulate its production. Our analysis of many clinical variables was unable to elucidate the underlying mechanism(s) for this disturbance.

CONCLUSION

Successful RT may not produce appropriate levels of 1,25-dihydroxyvitamin D commensurate to the elevated levels of PTH. This abnormality along with sustained hyperparathyroidism may contribute to bone loss following transplantation.

Gene polymorphism association studies in dialysis: bone and mineral metabolism

Abnormalities in bone and mineral metabolism have a significant impact on morbidity and mortality among patients with end-stage renal disease (ESRD). In addition to confounding environmental factors, genetic susceptibility factors may also influence the occurrence and severity of these abnormalities and account for interindividual variability among patients. Indeed, polymorphisms involving genes of the calcium/parathyroid hormone/calcitriol axis have been associated with bone and mineral metabolism abnormalities. This review summarizes studies involving polymorphisms of candidate genes and their effects on the development of complications related to bone and mineral metabolism abnormalities among patients with ESRD.

Vitamin D receptor polymorphisms and diseases

The vitamin D endocrine system is central to the control of bone and calcium homeostasis. Thus, alterations in the vitamin D pathway lead to disturbances in mineral metabolism. Furthermore, a role for vitamin D has been suggested in other diseases, like cancer, diabetes and cardiovascular disease. Expression and nuclear activation of the vitamin D receptor (VDR) are necessary for the effects of vitamin D. Several genetic variations have been identified in the VDR. DNA sequence variations, which occur frequently in the population, are referred to as "polymorphisms" and can have biological effects. To test whether there is a linkage between VDR polymorphisms and diseases, epidemiological studies are performed. In these studies, the presence of a variation of the gene is studied in a population of patients, and then compared to a control group. Thus, association studies are performed, and a link among gene polymorphisms and diseases can be established. Since the discovery of VDR polymorphisms a number of papers have been published studying its role in bone biology, renal diseases, diabetes, etc. The purpose of this review is to summarize the vast amount of information regarding vitamin D receptor polymorphisms and human diseases, and discuss its possible role as diagnostic tools.

Effect of kidney transplantation on bone

A broad range of different factors aggravates renal osteodystrophy, which is present in virtually all patients with chronic kidney disease and after successful kidney transplantation. Altered hormonal status, including sex hormones and parathyroid hormone (PTH), a deficit of 1,25(OH)(2) vitamin D(3) (calcitriol), immunosuppressive therapy and post-operative immobilization contribute to a progressive loss of bone density and structure. The decrease of bone mass is particularly prominent during the first 6 months after kidney transplantation and is associated with an increased number of fractures, both compared with the normal population as well as with dialysis patients. At particular risk are patients with a history of diabetes, long duration of haemodialysis and post-menopausal women. To prevent post-transplant bone loss prescription of steroids should be minimized and withdrawn as early as possible. Additional intake of alpha-calcidol [25(OH) vitamin D(3)] or calcitriol, despite normal serum levels, reduces persistent hyperparathyroidism after kidney transplantation, improves intestinal calcium absorption and activates osteoblasts. Inhibition of osteoclasts by biphosphonate therapy seems to effectively reverse bone loss during the early and late course of kidney transplantation. However, as the majority of transplant recipients have a low-turnover bone disease, inhibition of osteoclasts, through which bone turnover is impaired, might further reduce osteoblast activity and promote osteoid synthesis. Most investigations were small-scale studies with 10-100 participants and a follow up of only 12 months. This makes conclusions on the effect of any intervention on the fracture rate impossible. Larger, randomized multicentre studies investigating bone-sparing therapy on hard end points are therefore advocated.

Cinacalcet Hydrochloride (Sensipar) in Hemodialysis Patients on Active Vitamin D Derivatives with Controlled PTH and Elevated Calcium × Phosphate

Active vitamin D derivatives attenuate the severity of secondary hyperparathyroidism but often increase serum calcium (Ca) and phosphorus (P) as a result of enhanced intestinal absorption. The calcimimetic cinacalcet HCl lowers parathyroid hormone (PTH) and tends to decrease Ca x P. A 16-wk, open-label clinical trial was conducted in adult hemodialysis patients who had controlled PTH (biointact PTH [biPTH] 80 to 160 pg/ml) and elevated Ca x P (> 55 mg2/dl2) and were receiving paricalcitol > 6 microg/wk (or an equipotent dose of an alternative active vitamin D derivative). At the start of the study, active vitamin D derivatives were decreased to a mean equivalent dose of paricalcitol 6 microg/wk, and cinacalcet was titrated from 30 mg/d to a maximum possible dose of 180 mg/d. Of the 72 study patients, 53 (74%) completed 8 wk of dose titration with cinacalcet. In response to cinacalcet, the following mean percentage changes were observed: biPTH, -1.8%; Ca, -9.7% (P < 0.0001), phosphorus, -11.1% (P < 0.0001), and Ca x P, -20.1% (P < 0.0001). At the end of the study, approximate Kidney Disease Outcomes Quality Initiative targets for biPTH (< or = 160 pg/ml) were achieved in 85% (45 of 53) of patients and for Ca x P (< or = 55 mg2/dl2) in 72% (38 of 53) of patients. Concurrent achievement of both targets occurred in 47% (25 of 53) of patients. In this open-label clinical trial, hemodialysis patients who had controlled PTH but elevated Ca x P and were taking moderate- to high-dose active vitamin D derivatives achieved improved control of mineral metabolism with a combination of low-dose active vitamin D derivatives and cinacalcet. The long-term effects of this treatment regimen on clinical outcomes should be tested prospectively.

Genetic determinants of bone mass

PURPOSE OF REVIEW

This review examines recent advances in the analysis of genetic determinants of bone mass. It addresses both human and animal linkage studies as well as genetic manipulations in animals, inbred mouse models, and candidate gene analyses.

RECENT FINDINGS

Recent studies have implicated novel regulatory pathways in bone biology including both the neuroendocrine system and metabolic pathways linked to lipid metabolism. Variations in the lipoprotein receptor-related protein 5 (LRP5), part of the Wnt-frizzled pathway, were independently identified by linkage in high and low bone mass families. Subsequently, other high bone mass syndromes have been shown to have mutations in this gene. Neural studies have shown the skeletal regulatory activity of leptin and neuropeptide Y receptors via the hypothalamus. Subsequently, the beta-adrenergic pathway has been implicated, with important changes in bone mass. The lipoxygenase 12/15 pathway, identified through inbred mouse models and through pharmacologic studies with specific inhibitors, has also been shown to have important effects on bone mass. These studies exemplify the value of genetic models both to identify and then confirm pathways by mutational study and pharmacologic interventions. Continuing candidate gene studies often performed with multiple loci complement such discoveries. However, these studies have not focused on the clinical endpoint of fracture and few have included large enough groups to engender confidence in the associations reported, as such studies may require thousands of individuals. Interestingly, results often differ by ethnicity, age, or gender. A small proportion have examined whether relevant genes influence response to treatment.

SUMMARY

The combinations of human and animal genetic linkage studies have advanced understanding of the regulation of bone mass. Studies ranging from linkage to pharmacology provide optimism for new targets and treatments for osteoporosis.

Osseous complications of pediatric transplantation

Adult stature and peak bone mass are achieved through childhood growth and development. Multiple factors impair this process in children undergoing solid organ transplantation, including chronic illness, pretransplant osteodystrophy, use of medications with negative impact on bone, and post-transplant renal dysfunction. While growth delay and short stature remain common, the most severe forms of transplant-related bone disease, fracture and avascular necrosis, appear to have become less common in the pediatric age group. Osteopenia is very prevalent in adult transplant recipients and probably also in pediatrics, but its occurrence and sequelae are difficult to study in these groups due to methodological shortfalls of planar densitometry related to short stature and altered patterns of growth and development. Although the effect on lifetime peak bone mass is not clear, data from adult populations suggest an elevated long-term risk of bone disease in children receiving transplants. Optimal management of pretransplantation osteodystrophy, attention to post-transplant renal insufficiency among both renal and non-renal transplant patients, reduction of steroid dose in select patients, and supplementation with calcium plus vitamin D during expected periods of maximal bone loss may improve bone health. Careful research is required to determine the role of bisphosphonate therapy in pediatric transplantation.