Bone geometry and microarchitecture deficits in children with Alagille syndrome

Alagille syndrome (ALGS) is an autosomal dominant disorder attributed to mutations in the Notch signaling pathway. Children with ALGS are at increased risk for fragility fracture of unknown etiology. Our objective was to characterize bone mass, geometry, and microarchitecture in children with ALGS. This was a cross-sectional study of 10 children (9 females), ages 8-18 years, with a clinical diagnosis of ALGS. Bone density was assessed via DXA (Hologic Discovery A) at several skeletal regions. Tibia trabecular and cortical bone was assessed via pQCT (Stratec XCT 2000) at the distal 3% and 38% sites, respectively. Tibia bone microarchitecture was assessed via HR-pQCT (Scanco XtremeCT II) at an ultradistal site located at 4% of tibia length and a cortical site at 30% of tibia length. Z-scores were calculated for DXA and pQCT measures. In the absence of XtremeCT II HR-pQCT reference data, these outcome measures were descriptively compared to a sample of healthy children ages 5-20 years (n = 247). Anthropometrics and labs were also collected. Based on one-sample t-tests, mean Z-scores for height and weight (both p < .05), were significantly less than zero. DXA bone Z-scores were not significantly different from zero, but were highly variable. For pQCT bone measures, Z-scores for total bone mineral content at the distal 3% site and cortical bone mineral content, cortical area, and cortical thickness at the distal 38% site were significantly less than zero (all p < .05). There was good correspondence between pQCT measures of cortical thickness Z-scores and DXA Z-scores for aBMD at the whole body less head, 1/3 radius, and femoral neck (all p < .05). Compared to healthy children, those with ALGS generally had lower trabecular number and greater trabecular separation despite having greater trabecular thickness (measured via HR-pQCT). Bilirubin and bile acids, markers of hepatic cholestasis, were associated with poorer bone measures. For example, greater bilirubin was associated with lower trabecular number (Spearman's rho [ρ] = -0.82, p = .023) and greater trabecular separation (ρ = 0.82, p = .023) measured via HR-pQCT, and greater bile acids were associated with lower cortical area measured via pQCT (ρ = -0.78, p = .041) and lower serum insulin-like growth factor-1 (ρ = -0.86, p = .002). In summary, deficits in cortical bone size and trabecular bone microarchitecture were evident in children with ALGS. Further investigation is needed to understand the factors contributing to these skeletal inadequacies, and the manner in which these deficits contribute to increased fracture risk.