Alterations of bone material properties in adult patients with X-linked hypophosphatemia (XLH).

X-linked hypophosphatemia (XLH) caused by PHEX mutations results in elevated serum FGF23 levels, renal phosphate wasting and low 1,25-dihydroxyvitamin D. The glycophosphoprotein osteopontin, a potent inhibitor of mineralization normally degraded by PHEX, accumulates within the bone matrix. Conventional therapy consisting of supplementation with phosphate and vitamin D analogs is burdensome and the effects on bone material poorly characterized. We analyzed transiliac bone biopsies from four adult patients, two of them severely affected due to no diagnosis and no treatment until adulthood. We used light microscopy, qBEI and FTIRI to study histology, histomorphometry, bone mineralization density distribution, properties of the organic matrix and size of hypomineralized periosteocytic lesions. Non-treatment resulted in severe osteomalacia, twice the amount of mineralized trabecular volume, multiple osteon-like perforations, continuity of lamellae from mineralized to unmineralized areas and distinctive patches of woven bone. Periosteocytic lesions were larger than in treated patients. The latter had nearly normal osteoid thicknesses, although surface was still elevated. The median calcium content of the matrix was always within normal range, although the percentage of lowly mineralized bone areas was highly increased in non-treated patients, resulting in a marked heterogeneity in mineralization. Divalent collagen cross-links were evident independently of the mineral content of the matrix. Broad osteoid seams lacked measurable pyridinoline, a mature trivalent cross-link and exhibited considerable acidic lipid content, typically found in matrix vesicles. Based on our results, we propose a model that possibly integrates the relationship between the observed mineralization disturbances, FGF23 secretion and the known osteopontin accumulation in XLH.

Changes in bone matrix mineralization after growth hormone treatment in children and adolescents with chronic kidney failure treated by dialysis: a paired biopsy study.

BACKGROUND: Patients with chronic kidney disease (CKD) develop renal osteodystrophy with alterations in bone turnover, mineralization, and volume (TMV). A specific skeletal complication in children is growth impairment, which currently is treated by recombinant human growth hormone (rhGH). The effects on bone material properties are poorly understood. This study assesses the effects of rhGH treatment on bone matrix mineralization. STUDY DESIGN: Observational study. SETTING & PARTICIPANTS: 18 short children and adolescents (aged 3.6-16 years) with CKD on dialysis therapy. PREDICTOR: rhGH treatment for 1 year. OUTCOMES: Tetracycline-labeled bone biopsy classified according to the TMV system. MEASUREMENTS: Bone mineralization density distribution (BMDD) was evaluated by quantitative backscattered electron imaging in trabecular and cortical compartments. Additional data for patients' height and biochemical bone serum parameters were obtained. RESULTS: Prior to rhGH treatment, our cohort showed low bone turnover and high mineralization densities versus reference data: Ca(mean) (weighted mean calcium content) in cancellous bone, +3.3% (P = 0.04); Ca(mean) in cortical bone, +6.7% (P < 0.001); Ca(peak) (mode of the BMDD) in cancellous bone, +5.0% (P < 0.001); Ca(peak) in cortical bone, +8.2% (P < 0.001); Ca(width) (heterogeneity in mineralization), no significant difference for cancellous (P = 0.2) and cortical (P = 0.1) bone; Ca(high) (portion of fully mineralized bone) in cancellous bone, 5-fold greater (P < 0.001); Ca(high) in cortical bone, 14-fold greater (P < 0.001); Ca(low) (portion of low mineralized bone) in cancellous bone, +23.9% (P = 0.02); Ca(low) in cortical bone, -22.2% (P = 0.05). After rhGH treatment, height increased by 9.1 cm (P < 0.001) and bone turnover indices to normal values or beyond. Matrix mineralization was lesser and more heterogeneous compared to baseline: Ca(width) for cancellous bone, +15.3% (P < 0.001); Ca(width) for cortical bone, +34.1% (P < 0.001). Ca(mean), Ca(peak), and Ca(high) for cancellous bone and Ca(mean) and Ca(peak) for cortical bone were no longer significantly different from reference data. Ca(high) for cortical bone dramatically decreased after treatment but was still substantially greater than reference data. LIMITATIONS: Low case number per TMV subgroup, no measurements of fibroblast growth factor 23. CONCLUSIONS: Children and adolescents with CKD and growth deficiency are at risk of having low bone turnover. rhGH treatment improves height and concomitantly bone modeling/remodeling, which appears beneficial for bone matrix mineralization.

Fragility fractures in men with idiopathic osteoporosis are associated with undermineralization of the bone matrix without evidence of increased bone turnover.

The pathogenesis of primary osteoporosis in younger individuals is still elusive. An important determinant of the biomechanical competence of bone is its material quality. In this retrospective study we evaluated bone material quality based on quantitative backscattered electron imaging to assess bone mineralization density distribution (BMDD) in bone biopsies of 25 male patients (aged 18-61 years) who sustained fragility fractures but were otherwise healthy. BMDD of cancellous bone was compared with previously established adult reference data. Complementary information was obtained by bone histomorphometry. The histomorphometric results showed a paucity of osteoblasts and osteoclasts on the bone surface in the majority of patients. BMDD revealed a significant shift to lower mineralization densities for cancellous bone values: CaMean (weighted mean Ca content, -5.9%), CaPeak (mode of the BMDD, -5.6%), and CaHigh (portion of fully mineralized bone, -76.8%) were decreased compared to normative reference; CaWidth (heterogeneity in mineralization, +18.5%) and CaLow (portion of low mineralized bone, +68.8; all P < 0.001) were significantly increased. The shift toward lower mineral content in the bone matrix in combination with reduced indices of bone formation and bone resorption suggests an inherent mineralization defect leading to undermineralized bone matrix, which might contribute to the susceptibility to fragility fractures of the patients. The alteration in bone material might be related to osteoblastic dysfunction and seems fundamentally different from that in high bone turnover osteoporosis with a negative bone balance.

Novel PHEX mutation associated with hypophosphatemic rickets.

BACKGROUND: X-linked hypophosphatemia (XLH) is the most prevalent heritable form of rickets. It is a dominantly inherited disorder, characterized by renal phosphate wasting, abnormal vitamin D and PTH metabolism, and defective bone mineralization. Inactivating mutations in the gene encoding PHEX (phosphate-regulating gene with homologies to endopeptidases on the X chromosome) have been found to be associated with XLH. METHODS: We report about a 54-year-old male patient who exhibited the typical features of XLH, and in whom mutational analysis using PCR and sequencing was performed. Additionally, extensive laboratory and radiological investigations were carried out. RESULTS: A 1-bp deletion in exon 2 of the PHEX gene was detected (177delC), which, to the best of our knowledge, has not been reported yet. This deletion results in a premature stop codon (C59X), suggesting a truncation of the PHEX protein. Furthermore, elevated FGF23 and PTH levels as well as an increased axial bone mineral density score were measured. CONCLUSIONS: We present a male patient with XLH, who harbors a novel mutation in the PHEX gene, which might be the cause for his disease. Our data support previous findings and therefore contribute to the decipherment of the pathogenetic pathways of XLH.

Mineralization pattern of vertebral bone material following fragility fracture of the spine.

Little is known whether trabecular bone matrix mineralization is altered at the site of osteoporotic vertebral fractures. Bone mineralization density distribution (BMDD) was assessed in trabecular bone of acute, single-level compression fractures of the spine at various stages of fracture repair using quantitative backscattered electron imaging (qBEI). The grading of the repair stage was performed by histological methods. From 20 patients, who underwent either kyphoplasty (n=18) or vertebroplasty (n=2), a vertebral bone biopsy was taken prior to cement augmentation. Six patients took bisphosphonates (BP) prior to fracture. Three study groups were formed: N1=early-, N2=late-healing and B=BP treatment at late healing stage. In general, all groups had an altered BMDD when compared to historical normative reference data. Mean matrix mineralization (CaMean) was significantly (p<0.001) lower in all groups (N1: -5%, N2: -16%, and B2: -16%). In N2, CaMean was -13.1% (p<0.001) lower than N1. At this stage, deposition of new bone matrix and/or formation of woven bone are seen, which also explains the more heterogeneous matrix mineralization (CaWidth). Moreover, BP treatment (B2) led to a significant reduction in CaWidth (-28.5%, p<0.001), when compared to N2. Bone tissue from vertebrae with acute compression fractures reveals a large variation in matrix mineralization depending on the stage of repair. Bisphosphonate treatment does affect the mineralization pattern of tissue repair. The low mineralization values found in early stage of repair suggest that altered bone material properties may play a role in the occurrence of fragility fractures of the spine.

Altered bone matrix mineralization in a patient with Rett syndrome.

Rett syndrome (RTT) is a common X-linked neurodevelopmental disorder caused by mutations in the coding region of methyl-CpG-binding 2 (MECP2) gene. Patients with RTT have a low bone mineral density and increased risk of fracture. However, very little is known if bone matrix mineralization is altered in RTT. A 17-year-old girl with a classical form of RTT with a heterozygous nonsense mutation in exon 3 in the MECP2-gene was treated in our hospital. Her femoral neck BMD is 43.3% below the 3rd percentile when compared to age and sex-matched controls. She underwent surgery for correction of her scoliosis, which provided a unique opportunity to obtain bone tissue to study bone matrix mineralization (Bone Mineralization Density Distribution-BMDD) using quantitative backscattered electron imaging (qBEI) and histomorphometry. BMDD outcomes were compared to recently published normative reference data for young individuals. qBEI analysis showed a significant shift to lower matrix mineralization despite histomorphometric indices indicate a low bone turnover. There was a reduction in CaMean (-7.92%) and CaPeak (-3.97%), which describe the degree of mineralization. Furthermore the fraction of low mineralized matrix (CaLow: +261.84%) was dramatically increased, which was accompanied with an increase in the heterogeneity of mineralization (CaWidth: +86.34%). Our findings show a significantly altered bone matrix mineralization of a typical patient with RTT. This may partly explain the low bone density seen in these patients. These results also warrant further studies on the molecular role of MECP2 in bone matrix mineralization.