Response of the ENPP1-Deficient Skeletal Phenotype to Oral Phosphate Supplementation and/or Enzyme Replacement Therapy: Comparative Studies in Humans and Mice.

Inactivating mutations in human ecto-nucleotide pyrophosphatase/phosphodiesterase-1 (ENPP1) may result in early-onset osteoporosis (EOOP) in haploinsufficiency and autosomal recessive hypophosphatemic rickets (ARHR2) in homozygous deficiency. ARHR2 patients are frequently treated with phosphate supplementation to ameliorate the rachitic phenotype, but elevating plasma phosphorus concentrations in ARHR2 patients may increase the risk of ectopic calcification without increasing bone mass. To assess the risks and efficacy of conventional ARHR2 therapy, we performed comprehensive evaluations of ARHR2 patients at two academic medical centers and compared their skeletal and renal phenotypes with ENPP1-deficient Enpp1asj/asj mice on an acceleration diet containing high phosphate treated with recombinant murine Enpp1-Fc. ARHR2 patients treated with conventional therapy demonstrated improvements in rickets, but all adults and one adolescent analyzed continued to exhibit low bone mineral density (BMD). In addition, conventional therapy was associated with the development of medullary nephrocalcinosis in half of the treated patients. Similar to Enpp1asj/asj mice on normal chow and to patients with mono- and biallelic ENPP1 mutations, 5-week-old Enpp1asj/asj mice on the high-phosphate diet exhibited lower trabecular bone mass, reduced cortical bone mass, and greater bone fragility. Treating the Enpp1asj/asj mice with recombinant Enpp1-Fc protein between weeks 2 and 5 normalized trabecular bone mass, normalized or improved bone biomechanical properties, and prevented the development of nephrocalcinosis and renal failure. The data suggest that conventional ARHR2 therapy does not address low BMD inherent in ENPP1 deficiency, and that ENPP1 enzyme replacement may be effective for correcting low bone mass in ARHR2 patients without increasing the risk of nephrocalcinosis. © 2021 American Society for Bone and Mineral Research (ASBMR).

High dose vitamin D supplementation does not rescue bone loss following Roux-en-Y gastric bypass in female rats.

Postoperative bone loss and increased fracture risk associated with Roux-en-Y gastric bypass (RYGB) have been attributed to vitamin D/calcium malabsorption and resultant secondary hyperparathyroidism (HPT). Adequate vitamin D supplementation (VDS), particularly in an older female population, reduces incidence of secondary HPT but the effect on bone loss and fracture risk remains unclear. To investigate whether VDS corrects the RYGB bone phenotype, 41 obese adult female rats were randomized to RYGB with 1000 IU (R1000) or 5000 IU (R5000) vitamin D/kg food or a sham surgical procedure with either paired (PF) or ad libitum (AL) feeding. Bone turnover markers, urinary calcium/creatinine ratio (CCR), and serum calciotropic and gut hormones were assessed throughout a 14-week postoperative period. Femurs were analyzed by micro-computed tomography (µCT), three-point bending test, and histomorphometry. 1000 IU animals had low 25­hydroxyvitamin D (25(OH)D), high serum parathyroid hormone (PTH), and very low urine CCR levels. 5000 IU corrected the 25(OH)D and secondary HPT but did not increase urine CCR or serum levels of 1,25­dihydroxyvitamin D (1,25(OH)D) significantly between RYGB groups. Compared to sham animals at 14 weeks, RYGB animals had significantly higher serum osteocalcin (OCN) and C-terminal telopeptide (CTX) levels. The gut hormone peptide tyrosine tyrosine hormone (PYY) was higher in the RYGB groups, and leptin was lower. µCT and biomechanical testing revealed RYGB females had decreased cortical and trabecular bone volume and weaker, stiffer bone than controls. Histomorphometry showed decreased bone volume and increased osteoid volume with increased mineral apposition rate in RYGB compared to controls. No differences in bone phenotype were identified between 1000 IU and 5000 IU groups, and osteoclast numbers were comparable across all four groups. Thus, in our model, 5000 IU VDS corrected vitamin D deficiency and secondary HPT but did not rescue RYGB mineralization rate nor the osteomalacia phenotype. Longer studies in this model are required to evaluate durability of these detrimental effects. Our findings not only underscore the importance of lifelong repletion of both calcium and vitamin D but also suggest that additional factors affect skeletal health in this population.