Loss of Iroquois homeobox transcription factors 3 and 5 in osteoblasts disrupts cranial mineralization

• Osteoblast-specific deletion of Irx3 and Irx5 causes mineralization defects in parietal, interparietal, and frontal bones of newborn mice.
• 3–4 week old Irx3flox/flox/Irx5−/−/Osx-Cre+ mice show similar bone fragility to Hamamy syndrome patients who have mutations in IRX5.
• Osx-Cre transgene alone causes an early bone mineralization phenotype that recovers later.

Cranial malformations are a significant cause of perinatal morbidity and mortality. Iroquois homeobox transcription factors (IRX) are expressed early in bone tissue formation and facilitate patterning and mineralization of the skeleton. Mice lacking Irx5 appear grossly normal, suggesting that redundancy within the Iroquois family. However, global loss of both Irx3 and Irx5 in mice leads to significant skeletal malformations and embryonic lethality from cardiac defects. Here, we study the bone-specific functions of Irx3 and Irx5 using Osx-Cre to drive osteoblast lineage–specific deletion of Irx3 in Irx5−/− mice. Although we found that the Osx-Cre transgene alone could also affect craniofacial mineralization, newborn Irx3flox/flox/Irx5−/−/Osx-Cre+ mice displayed additional mineralization defects in parietal, interparietal, and frontal bones with enlarged sutures and reduced calvarial expression of osteogenic genes. Newborn endochondral long bones were largely unaffected, but we observed marked reductions in 3–4-week old bone mineral content of Irx3flox/flox/Irx5−/−/Osx-Cre+ mice. Our findings indicate that IRX3 and IRX5 can work together to regulate mineralization of specific cranial bones. Our results also provide insight into the causes of the skeletal changes and mineralization defects seen in Hamamy syndrome patients carrying mutations in IRX5.