Hajdu-Cheney Syndrome in a Two-Generation Family: Longitudinal Skeletal Progression and Differential Therapeutic Responses in a Mother and Her Son.

Hajdu-Cheney syndrome (HCS) is a rare genetic skeletal disorder caused by truncating variants of NOTCH2, characterized by progressive bone resorption and marked phenotypic heterogeneity. Despite advances in understanding Notch signaling in skeletal biology, longitudinal clinical data tracking disease evolution from early childhood through adolescence are lacking. Here, we report a rare longitudinal intrafamilial observation of HCS in a mother and her son carrying the same NOTCH2 pathogenic variant, providing novel insights into disease evolution and therapeutic response. Over extended follow-up, the son exhibited early vertebral fragility despite preserved or supranormal bone mineral density (BMD), whereas the mother developed severe osteoporosis, progressive acro-osteolysis, and multiple vertebral fractures. Longitudinal analysis revealed a dissociation between vertebral fragility and densitometric decline, challenging the paradigm that low BMD is the primary driver of skeletal morbidity in HCS. Treatment responses differed between the two patients, with bisphosphonate therapy in the son associated with stabilized BMD without altering vertebral structural progression, and denosumab in the mother associated with increased BMD, but not preventing progression of acro-osteolysis. Additionally, the emergence of extra-skeletal features during adolescence expands the phenotypic spectrum of HCS and suggests previously unrecognized systemic involvement. These data highlight intrinsic limitations of current therapeutic strategies and emphasize the need for targeted interventions addressing sustained Notch2 activation. Our findings contribute to the understanding of the natural history and therapeutic challenges of HCS, providing the framework for future mechanistic and translational research.