Modification of hydroxyapatite involves carbonate ion substitution in both patients with XLH and HYP mice

X-linked hypophosphatemia (XLH) is the most common form of the familial phosphate-wasting disorders, arising as a consequence of an inactivating mutation in the phosphate-regulating neutral endopeptidase, X-linked (PHEX) gene. While persistent osteomalacia characterizes this disorder, the impact of insufficient phosphate on hydroxyapatite composition, the major inorganic component of bone, is unknown in patients with XLH. We previously showed that the ratio of phosphate to carbonate in cortical bone mineral matrix is increased in HYP mice, a murine model of XLH, compared to wild type mice (Tommasini et al., ASBMR 2014). To determine if this phenomenon is reproduced in human mineralized tissues, tooth dentin was studied as a human proxy for bone. Dentin is deposited by odondoblasts, a cell of neural crest origin that appears in the 17-18th week of gestation and exists throughout the lifespan. We hypothesized that there would be an increase in carbonate ion substitution in permanent teeth as a consequence of the hypophosphatemia associated with the osteomalacia of XLH. We measured and compared the phosphate to carbonate ion ratio in both primary and permanent teeth from patients with XLH to those who are not affected using Raman spectroscopy. The carbonate ion substitution levels were higher in permanent teeth (0.39±0.11 vs. 0.19±0.01 p=.5E-9), but not statistically significant between the primary teeth (0.28±0.11 vs. 0.24 ±0.01 p=.05). These preliminary data suggest that, during early development, the fetus has sufficient phosphate mineral available for normal dentin mineralization to occur. However, in permanent tooth dentin deposition during the 2nd through 5th year of life, accelerated carbonate ion substitution occurs due to the phosphate-poor environment. Consistent with our previous data that showed bone mineral carbonate content increased during pregnancy and lactation in both wild type and HYP mice, the substitution of carbonate ions into the hydroxyapatite may be a means of compensating for increased mineral demand and hypophosphatemia. However, although carbonate ion substitution appears to provide adequate mineral for permanent tooth formation, calcium carbonate is more easily resorbable than hydroxyapatite. Thus, the increase in carbonate content may contribute to the clinical sequela of multiple dental caries and abscesses that XLH patients experience throughout their lifetime.