Alx4 and Msx2 play phenotypically similar and additive roles in skull vault differentiation

Alx4 and Msx2 encode homeodomain-containing transcription factors that show a clear functional overlap. In both mice and humans, loss of function of either gene is associated with ossification defects of the skull vault, although the major effect is on the frontal bones in mice and the parietal bones in humans. This study was undertaken to discover whether Alx4 and Msx2 show a genetic interaction in skull vault ossification, and to test the hypothesis that they interact with the pathway that includes the Fgfr genes, Twist1 and Runx2. We generated Alx4(+/-)/Msx2(+/-) double heterozygous mutant mice, interbred them to produce compound genotypes and analysed the genotype-phenotype relationships. Loss of an increasing number of alleles correlated with an incremental exacerbation of the skull vault defect; loss of Alx4 function had a marginally greater effect than loss of Msx2 and also affected skull thickness. In situ hybridization showed that Alx4 and Msx2 are expressed in the cranial skeletogenic mesenchyme and in the growing calvarial bones. Studies of the coronal suture region at embryonic day (E)16.5 revealed that Alx4 expression was decreased, but not abolished, in Msx2(-/-) mutants, and vice versa; expression of Fgfr2 and Fgfr1, but not Twist1, was reduced in both mutants at the same stage. Runx2 expression was unaffected in the coronal suture; in contrast, expression of the downstream ossification marker Spp1 was delayed. Double homozygous pups showed substantial reduction of alkaline phosphatase expression throughout the mineralized skull vault; they died at birth due to defects of the heart, lungs and diaphragm not previously associated with Alx4 or Msx2. Our observations suggest that Alx4 and Msx2 are partially functionally redundant, acting within a network of transcription factors and signalling events that regulate the rate of osteogenic proliferation and differentiation at a stage after the commitment of mesenchymal stem cells to osteogenesis.