Fractures are Highly Correlated with Bone Density and Inversely Correlated with Bone Turnover Markers in Autosomal Dominant Osteopetrosis.

Autosomal Dominant Osteopetrosis (ADO) is a rare, osteosclerotic disorder usually caused by missense variants in the CLCN7 gene, resulting in impaired osteoclastic bone resorption. Penetrance is incomplete and disease severity varies widely, even among relatives within the same family. Although ADO can cause visual loss, osteonecrosis, osteomyelitis, and bone marrow failure, the most common complication of ADO is fracture. We are conducting a natural history study to characterize disease progression and determinants of disease severity. We hypothesized that baseline bone mineral density and bone turnover markers would correlate with self-reported fracture history. We report cross-sectional analysis of baseline data from the natural history study in 54 individuals (42 adults, 12 children). In adults, Z scores for both volumetric (r = 0.87, p < .001) and areal bone mineral density (aBMD) of the lumbar spine, and Z scores for femoral neck, and total hip aBMD (r = 0.77 to 0.78; p < .001) were correlated with lifetime fracture number. Tartrate resistant acid phosphatase, a marker of osteoclast number, correlated positively with fracture (r = 0.52, p = .004) consistent with an adaptive response of higher numbers of osteoclasts among more severely affected individuals. However, fracture number correlated inversely with the bone resorption markers serum C-telopeptide (r = -0.60, p < .001) and urine N-telopeptide/creatinine ratio (r = -0.35, p = .047), suggesting that ADO subjects who have the most reduced osteoclast activity have a greater tendency to fracture. Correlation coefficients between fractures, BMD and bone turnover markers were similar when limited to the 37 adults with disease causing CLCN7 variants. There were no statistically significant differences between subjects with the most common CLCN7 variant (G215R), the most common variant in our cohort, compared to other CLCN7 variants with respect to fracture, bone density measures or biochemical markers of bone turnover. These data demonstrate that bone density and biochemical bone turnover markers are indicators of ADO severity as defined by fracture number.