Abstract
Four male probands presented with severe leg bowing after 1 yr of age. During 41 patient-yrs of observation, linear growth remained near the 3rd percentile (Tanner charts); bone X-rays show only coarse trabeculation and sclerosis in stress areas. Microradiography (R. Steendijk, Amsterdam) shows poor mineralization of forming osteones and perilacunar areas. Serum [Pi] is <1 mM, as it is in X-linked hypophosphatemia (XLH); TRPi is significantly higher (76±7.3 μmoles/100 ml GF) in HBD than in XLH (31±2.8 μmoles/100 ml GF, p<0.01) at the equivalent serum [Pi]. Tubular responses to bovine PTH and TmPi are abnormal (depressed) in HBD but different from XLH. Serum iPTH and 25-OHD are normal in HBD. 1α-DHD3 by mouth (3-4 μg/d × 8 wk) does not restore TRPi or serum [Pi] to normal. The equivalent hypophosphatemias are clearly the result of different Pi transport defects in HBD and XLH; the more severe, rachitic bone disease in XLH points to an additional defect involving Pi access to bone in that disease. Pedigree studies indicate that HBD is not X-linked; otherwise the inheritance of the condition remains unclear.
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