Hip Geometry Variation in Homozygous Ectonucleotide Pyrophosphatase/ Phosphodiesterase 1 (ENPP1) Deficient Patients

ENPP1 is an important regulator of skeletal and soft tissue mineralization. Mouse models have demonstrated a link between defective ENPP1 and altered bone mineralization, including osteopenia and mild osteomalacia. In humans, rare homozygous loss of ENPP1 leads to generalized arterial calcification of infancy (GACI), characterized by vascular, joint, and organ calcification, hypophosphatemic rickets/osteomalacia, and skin and retinal findings. Heterozygous ENPP1 deficiency has been described in early-onset osteoporosis and an ENPP1 polymorphism was found to be strongly associated with variation in hip geometry, as measured by DXA hip structural analysis (HSA). However, the overall skeletal phenotype of ENPP1-deficiency is not well characterized in affected individuals. In addition to bone density, DXA provides surrogate information about mechanical strength, resistance to compression, tension, and buckling, offering a non-invasive and clinically accessible tool to assess hip geometry. Parameters include cross-sectional area (CSA), cross-sectional moment of inertia (CSMI), subperiosteal width (SPW), endocortical width (ECW), cortical thickness (CT), section modulus (Z), and buckling ratio (BR)—each measured in the narrow neck (NN), intertrochanteric (IT), and femoral shaft (FS) regions. SPW and ECW increase with age and are correlated with bone fragility. In mice, CSA and CSMI are inversely correlated with bone fragility when assessed by biomechanical tests. However, the opposite correlation has been observed in patients with fractures. HSA by DXA was assessed in 7 homozygous ENPP1-deficient patients (age range 5–56 years) and in 73 sex- and age-matched controls. All ENPP1-deficient patients were hypophosphatemic, of which, 2 were receiving treatment. Given the small sample size, standardized test statistics (t-scores) were derived for 21 hip geometry parameters; comparisons between groups were performed using the Mann-Whitney test. The Benjamini-Hochberg test was used to correct for multiple comparisons with a false discovery rate (FDR) q-value threshold set at 0.15. Compared with controls, ENPP1-deficient patients had significant changes in structural and derived measures of strength. Increases were seen in NN SPW, NN ECW, and FS SPW, which are correlated with bone fragility. In contrast, the increases seen in FS CSA, FS Z, FS CSMI, and NN CSMI are associated with improved bone strength. Although a q-value threshold of 0.15 indicates that 15% (~1/7) of all significant discoveries will represent a false positive, most findings represented true discoveries. These findings suggest that ENPP1 influences bone structure and strength, supporting findings from the aforementioned ENPP1 polymorphism study describing increased SPW in the NN and FS regions. This work contributes to the nascent body of literature studying the impact of ENPP1 on skeletal homeostasis.