Molecular Bases of Disease
- Notch2tm1.1Ecan mice, which harbor a mutation replicating that found in Hajdu–Cheney syndrome, exhibit marked osteopenia because of increased osteoclast number and bone resorption. Hairy and enhancer of split 1 (HES1) is a Notch target gene and a transcriptional modulator that determines osteoclast cell fate decisions. Transcript levels of Hes1 increase in Notch2tm1.1Ecan bone marrow–derived macrophages (BMMs) as they mature into osteoclasts, suggesting a role in osteoclastogenesis. To determine whether HES1 is responsible for the phenotype of Notch2tm1.1Ecan mice and the skeletal manifestations of Hajdu–Cheney syndrome, Hes1 was inactivated in Ctsk-expressing cells from Notch2tm1.1Ecan mice.
- Notch receptors play critical roles in cell-fate decisions and in the regulation of skeletal development and bone remodeling. Gain–of–function NOTCH2 mutations can cause Hajdu-Cheney syndrome, an untreatable disease characterized by osteoporosis and fractures, craniofacial developmental abnormalities, and acro-osteolysis. We have previously created a mouse model harboring a point 6955C→T mutation in the Notch2 locus upstream of the PEST domain, and we termed this model Notch2tm1.1Ecan. Heterozygous Notch2tm1.1Ecan mutant mice exhibit severe cancellous and cortical bone osteopenia due to increased bone resorption.
- Hajdu Cheney syndrome (HCS) is characterized by craniofacial developmental abnormalities, acro-osteolysis, and osteoporosis and is associated with gain–of–NOTCH2 function mutations. A mouse model of HCS termed Notch2tm1.1Ecan harboring a mutation in exon 34 of Notch2 replicating the one found in HCS was used to determine whether the HCS mutation sensitizes the skeleton to the osteolytic effects of tumor necrosis factor α (TNFα). TNFα injected over the calvarial vault caused a greater increase in osteoclast number, osteoclast surface, and eroded surface in Notch2tm1.1Ecan mice compared with littermate WT controls.