The role of pax genes during embryogenesis

The paired box was first found in Drosophila segmentation genes. At the present nine murine and human paired box (fax) genes have been isolated and they are classified into four groups by their sequence homology and genomic organization. Pax proteins are considered to bind to specific DNA sequences and modulate transcriptional activity. In addition, both murine mutants and human congenital diseases such as undulated, Splotch (Waadenburg syndrome I in human) and Small eye (aniridia in human) have been assigned to some of the Pax family members, i.e., Pax-1, Pax-3 and Pax-6, respectively. Recently, we found a new mutant rat (rSey) with the lack of eyes and nose in the homozygote. The phenotype and genetical trait of rSey is similar to that of the mouse Sey strain with mutations in the Pax-6 gene. At the genomic level, rSey posses a single base (G) insertion in an exon of Pax-6 gene, generating an abnormal 5' donor splice site, thereby producing the truncated mRNA. Anterior midbrain crest cells in the homozygous rSey embryos reached the eye rudiments but did not migrate any further to the nasal rudiments. Pax-6 gene is intensely expressed in the frontonasal ectoderm which serves as scaffold for migratiaon of anterior midbrain crest cells, suggesting that the Pax-6 gene is involved in conducting migration of these crest cells toward the frontonasal region. Recombination study revealed that the failure of lens differentiation in homozygous rSey embryos is assumed to be the result also from defects in the head ectoderm that could not respond to inductive signals from the presumptive retina. Pax-6 gene is also expressed in the basal plate of the hindbrain and spinal cord. We found that the abducent and hypoglossal nerves are missing in rSey homozygotes, suggesting ûiat Pax-6 is involved in development of motor neurons. Taking together, Pax-6 has multiple functions in several developmental events where cell-cell interaction/recognition is important.