PAX6

PAX6 is the most researched of the Pax genes and appears throughout the literature as a "master control" gene for the development of eyes and sensory organs, certain neural and epidermal tissues as well as other homologous structures, usually derived from ectodermal tissues. This transcription factor is most famous for its use in the interspecifically induced expression of ectopic eyes and is of medical importance because heterozygous mutants produce a wide spectrum of ocular defects such as Aniridia in humans.

PAX6 protein is highly conserved across species, and for instance mouse PAX6 can trigger eye development in Drosophila melanogaster.

Genomic organisation of the PAX6 locus varies considerably between species, including the number and distribution of exons, cis-regulatory elements, and transcription start sites. The first work on genomic organisation was performed in quail, although the picture of the mouse locus is the most complete to date. This consists of 2 confirmed promoters (P0 and P1), 16 exons, and at least 6 enhancers. The 16 confirmed exons are numbered 0 through 13 with the additions of exon α located between exons 4 and 5, and the alternatively spliced exon 5a. Each promoter is associated with its own proximal exon (exon 0 for P0, exon 1 for P1) resulting in transcripts which are alternatively spliced in the 5’ un-translated region.

The vertebrate PAX6 locus encodes at least three different protein isoforms, these being the canonical PAX6, PAX6(5a), and PAX6(ΔPD). The canonical PAX6 protein contains an N-terminal paired domain, connected by a linker region to a paired-type homeodomain, and a prolein/serine/threonine (P/S/T)-rich C-terminal domain. The paired domain and paired-type homeodomain each have DNA binding activities, while the P/S/T-rich domain possesses a transactivation function. PAX6(5a) is a product of the alternatively spliced exon 5a resulting in a 14 residue insertion in the paired domain which alters the specificity of this DNA binding activity. The nucleotide sequence corresponding to the linker region encodes a set of three alternative translation start codons from which the third PAX6 isoform originates. Collectively known as the PAX6(ΔPD) or pairedless isoforms, these three gene products all lack a paired domain. The pairedless proteins possess molecular weights of 43, 33, or 32kDa, depending on the particular start codon used. PAX6 transactivation function is attributed to the variable length C-terminal P/S/T-rich domain which stretches to 153 residues in human and mouse proteins.

Of the four Drosophila Pax6 orthologues, it is thought that the eyeless (ey) and twin of eyeless (toy) gene products share functional homology with the vertebrate canonical Pax6 isoform, while the eyegone (eyg) and twin of eyegone (toe) gene products share functional homology with the vertebrate Pax6(5a) isoform. Eyeless and eyegone were named for their respective mutant phenotypes.