Paradoxical combination of osteosclerosis and osteopenia in an adult woman with biallelic loss-of-function variants escaping nonsense-mediated decay.

Osteoclasts are essential for bone resorption, playing a crucial role in skeletal development, homeostasis, and remodeling. Their differentiation depends on the RANK receptor encoded by the gene, with defects in this gene linked to osteoclast-poor sclerosing skeletal dysplasias. This report presents a 37-yr-old woman with normal height, valgus deformities that were treated surgically, frequent fractures, scoliosis, mildly elevated BMD, sclerotic diaphyseal bone, and metaphyseal widening. Initially suspected of having dysosteosclerosis, her diagnosis shifted toward Pyle disease due to the valgus deformity and prominent metaphyseal widening and translucency. Genetic analysis identified 2 pathogenic variants: a nonsense mutation c.1093G>T, p.(Glu365) and a frameshift mutation c.1266_1268delinsCC, p.(Leu422Phefs104). Thus, genetic and clinical assessment converged on the diagnosis of a mild form of dysosteosclerosis. Both mutations introduced premature stop codons but escaped complete nonsense-mediated decay, potentially permitting residual protein function. Analysis of patient-derived osteoclasts cultured on glass surfaces showed partial differentiation. However, in vitro resorptive function was strongly impaired, which was clinically reflected by reduced serum concentration of the bone resorption marker CTx. Despite this impairment, the retained residual resorptive function likely explains the patient's relatively mild clinical presentation. These findings underscore the complex genetic interactions that affect osteoclast function, leading to a spectrum of phenotypes in osteoclast-related bone disorders.