Bone densitometry versus bone histomorphometry in renal transplanted patients: a cross-sectional study

The Relationship between Sclerostin and Kidney Transplantation Mineral Bone Disorders: A Molecule of Controversies

Kidney transplantation is the most effective treatment option for most patients with end-stage kidney disease due to reduced mortality, decreased cardiovascular events and increased quality of life compared to patients treated with dialysis. However, kidney transplantation is not devoid of both acute and chronic complications including mineral bone disorders (MBD) which are already present in patients with chronic kidney disease (CKD) before kidney transplantation. The natural history of MBD after kidney transplantation is variable and new markers are needed to define MBD after kidney transplantation. One of these promising molecules is sclerostin. The main action of sclerostin is to inhibit bone formation and mineralization by blocking osteoblast differentiation and function. In kidney transplant recipients (KTRs), various studies have shown that sclerostin is associated with graft function, bone parameters, vascular calcification, and arterial stiffness although non-uniformly. Furthermore, data for inhibition of sclerostin with monoclonal antibody romosozumab for treatment of osteoporosis is available for general population but not in KTRs which osteoporosis is highly prevalent. In this narrative review, we have summarized the studies investigating the change of sclerostin before and after kidney transplantation, the relationship between sclerostin and laboratory parameters, bone metabolism and vascular calcification in the context of kidney transplantation. We also pointed out the uncertainties, explained the causes of divergent findings and suggest further potential study topics regarding sclerostin in kidney transplantation.

Soluble Klotho, a Potential Biomarker of Chronic Kidney Disease-Mineral Bone Disorders Involved in Healthy Ageing: Lights and Shadows

Shortly after the discovery of Klotho, interest grew in its potential role in chronic kidney disease (CKD). There are three isoforms of the Klotho protein: αKlotho, βKlotho and γKlotho. This review will focus on αKlotho due to its relevance as a biomarker in CKD. αKlotho is synthesized mainly in the kidneys, but it can be released into the bloodstream and urine as soluble Klotho (sKlotho), which undertakes systemic actions, independently or in combination with FGF23. It is usually accepted that sKlotho levels are reduced early in CKD and that lower levels of sKlotho might be associated with the main chronic kidney disease-mineral bone disorders (CKD-MBDs): cardiovascular and bone disease. However, as results are inconsistent, the applicability of sKlotho as a CKD-MBD biomarker is still a matter of controversy. Much of the inconsistency can be explained due to low sample numbers, the low quality of clinical studies, the lack of standardized assays to assess sKlotho and a lack of consensus on sample processing, especially in urine. In recent decades, because of our longer life expectancies, the prevalence of accelerated-ageing diseases, such as CKD, has increased. Exercise, social interaction and caloric restriction are considered key factors for healthy ageing. While exercise and social interaction seem to be related to higher serum sKlotho levels, it is not clear whether serum sKlotho might be influenced by caloric restriction. This review focuses on the possible role of sKlotho as a biomarker in CKD-MBD, highlighting the difference between solid knowledge and areas requiring further research, including the role of sKlotho in healthy ageing.

Fractal characteristics of the trabecular pattern of the mandible in patients with renal transplantation

OBJECTIVE

In this study, we examined the mandibular trabecular bone structures by performing fractal dimension (FD) analysis in patients who underwent renal transplantation (RTx).

METHODS

Our study is an observational study with 69 RTx patients and 35 control group patients. The mean FD values of the patient and control groups were calculated and compared. In addition, biochemical parathyroid hormone (PTH), serum calcium, phosphorus, alkaline phosphatase (ALP), and vitamin-D parameters and FD values of both groups were analyzed.

RESULTS

FD values were significantly lower in the patient group than in the healthy group (p < .05). In the RTx group compared to the control group, ALP (90.71 ± 34.25-66.54 ± 16.8, respectively) (p < .001) and PTH (75.76 ± 38.01-38.17 ± 12.39, respectively) (p < .001) values were higher. There was a positive correlation between the FD values and ALP (rspearman  = .305, p = .011) and a negative correlation between FD values and vitamin-D (rspearman  = .287, p = .017) of patients with RTx.

CONCLUSION

FD values were found to be lower in patients who underwent RTx compared to the control group. It should be considered that FD analysis can be a method that can be used to evaluate trabecular bone structure in patients undergoing RTx.

Improvement of Mineral and Bone Disorders After Renal Transplantation

BACKGROUND

Posttransplant mineral and bone diseases are causes of fractures, and their association with cardiovascular events is being studied.

METHODS

We analyzed the evolution of biochemical, histological, and imaging parameters pre- and 1 y post-renal transplantation in 69 patients and correlated mineral and bone findings with coronary calcifications. At inclusion and after 12 mo, clinical data and echocardiographic findings were recorded, and laboratory evaluations, radiography of the pelvis and hands, and bone biopsy were performed. Noncontrast cardiac computed tomography was performed during the second evaluation.

RESULTS

Serum levels of fibroblast growth factor 23 and sclerostin decreased in all patients, parathyroid hormone levels decreased in 89.8% of patients, bone alkaline phosphatase levels decreased in 68.1% of patients, and alpha-Klotho levels increased in 65.2% of patients. More than half of the patients presented with renal osteodystrophy at both biopsies, but histological findings improved: a significant transition from high to normal or low turnover and no significant differences in volume, mineralization defect, or cortical porosity at the 2 evaluations. Alpha-Klotho, sclerostin, and bone alkaline phosphatase shifts affect bone changes. Neither echocardiographic findings nor vascular calcification scores differed between the 2 points. Both the pretransplant period (dialysis vintage, sclerostin, and low bone volume at baseline) and the maintenance of abnormalities in the posttransplant period (high turnover posttransplant) were the most reliable predictors of the severity of the coronary calcification percentile.

CONCLUSIONS

Renal transplantation improved bone and mineral abnormalities. The pretransplant period determines the severity of calcification.

Patterns of renal osteodystrophy one year after kidney transplantation

BACKGROUND

Renal osteodystrophy is considered common, but is not well characterized, in contemporary kidney transplant recipients. This study reports extensively on bone phenotype by bone histomorphometry, bone densitometry, and novel bone biomarkers 1 year after kidney transplantation.

METHODS

A transiliac bone biopsy and dual energy x-ray absorptiometry were performed in 141 unselected kidney transplant recipients in this observational cohort study. Blood and 24 hr urine samples were collected simultaneously.

RESULTS

Median age was 57 ± 11 years, 71% were men, and all were of Caucasian ethnicity. Bone turnover was normal in 71% of patients, low in 26%, and high in just four cases (3%). Hyperparathyroidism with hypercalcemia was present in 13% of patients, of which one had high bone turnover. Delayed bone mineralization was detected in 16% of patients, who were characterized by hyperparathyroidism (137 vs. 53 ρg/mL), a higher fractional excretion of phosphate (40 vs. 32%), and lower levels of phosphate (2.68 vs 3.18 mg/dL) and calcidiol (29 vs. 37 ng/mL) compared to patients with normal bone mineralization. Osteoporosis was present in 15-46% of patients, with the highest prevalence at the distal skeleton. The proportion of osteoporotic patients was comparable across categories of bone turnover and mineralization.

CONCLUSION

The majority of kidney transplant recipients, including patients with osteoporosis, have a normal bone turnover at 1-year post-transplant. Low bone turnover is seen in a substantial subset, while high bone turnover is rare. Vitamin D deficiency and hypophosphatemia represent potential interventional targets to improve bone health post-transplant.