Iris hypoplasia in mice that lack the alternatively spliced Pax6(5a) isoform

Abstract

PAX6 is an evolutionarily conserved transcription factor that plays a critical role in vertebrate and invertebrate eye formation. Heterozygous null mutations in the PAX6 gene result in aniridia in humans and a distinct small eye syndrome in rodents. Vertebrates primarily express two alternatively spliced isoforms of Pax6 that differ by the presence or absence of exon 5a (e5A) that encodes an additional 14 aa residues within the paired domain. The e5a-containing isoform, PAX6(5a), is specific to and conserved in vertebrates. To determine the role of PAX6(5a), we have generated mice that lack e5a of the Pax6 gene. Unlike Pax6 null mice that exhibit anopthalmia with central nervous system defects and lethality, 5a isoform-null mice have iris hypoplasia and defects in the cornea, lens, and retina. Although invertebrates have structures that respond to light intensity and act to restrict light exposure of the eyes, a significant and distinct feature of the vertebrate eye is its ability to regulate the amount of incoming light through contractile pupils. This feature of the eye not only allows vertebrates to see in various light conditions but also enhances image resolution. The requirement of the 5a isoform in iris formation suggests that the evolution of this isoform contributed to advanced features of the vertebrate eye. P AX6 is an essential and evolutionarily conserved transcription factor for eye formation (1-3). Heterozygous null mutations in the PAX6 gene result in distinct phenotypes in humans (aniridia) and rodents (small eye) (4, 5). Aniridia is a severe panocular disease that is characterized by lack of proper iris development, associated with cataracts, optic nerve hypoplasia, and glaucoma. Small eye syndrome is distinguished from aniridia by reduced eye size, cataracts, and poor lens development. Homozygous mutations in the PAX6 gene result in severe brain abnormalities, microencephaly, early postnatal death, and the absence of the eyes and nose in both humans (4) and rodents (5, 6).The Pax6 locus encodes two products caused by alternative splicing of exon 5a (e5a) that adds an additional 14 aa residues within the paired domain (7,8). The e5a-containing isoform, PAX6(5a), is specific to and conserved in vertebrates (9). Pax6 is expressed in the developing eye, nose, pancreas, and central nervous system (7, 10-13). PAX6 is a protein of 422 aa residues, and PAX6(5a) is 14 aa longer and exhibits unique DNA-binding properties (14). Structural analysis of PAX6 identified two DNA-binding domains (a paired domain at the amino terminus and a paired-like homeodomain located centrally), a glycine-rich region that links the two DNA-binding domains, and a transactivation domain at the carboxyl terminus. The paired domain of PAX6 is a bipartite DNA-binding motif and is believed to have two independent amino-terminal and carboxyl-terminal subdomains (15, 16). The paired domain can bind DNA alone or cooperatively by interacting with other DNA-binding domains, such as the homeodomain (17). Unlike other paired domains th...

An Allelic Series at the Paired Box Gene 6 (Pax6) Locus Reveals the Functional Specificity of Pax Genes

“…Pax6(5a), an alternatively spliced isoform of Pax6, contains a 14-amino acid insert within the PD that considerably changes its DNA binding specificity: the canonical PD binds to DNA via its N-terminal PAI domain, whereas the Pax6(5a) PD targets DNA with the C-terminal RED domain (24). Although the Pax6(5a) isoform only constitutes up to 20% of total Pax6 transcripts, loss of Pax6(5a) in human and mouse results in distinct ocular phenotypes (24,26,27). In addition, overexpression of Pax6(5a) in cerebral cortex progenitors was observed to inhibit cell proliferation without affecting cell fate, whereas overexpression of the canonical Pax6 inhibited cell proliferation but also potently increased neurogenesis (26,28).…”

PAX6, Paired Domain Influences Sequence Recognition by the Homeodomain

“…An alternatively spliced form of PAX6, known as PAX6-5a, contains an additional 14 amino acids in the PD (2), which disrupts the N subdomain and alters DNA binding properties (13,32). Loss-of-function of Pax6-5a in mouse leads to iris-hypoplasia and also defects in the retina, lens, and cornea (33). The PDs of other genes of the Pax family, the Pax2, Pax3, and the binding sequence of Pax-5 (BSAP), and Pax6 proteins can recognize similar DNA sequences (12, 34 -39).…”

“…Since patients with aniridia also have the PAX6-5a isoform, and Pax6-5a was found to be important in maintaining the normal structure of eyes in adult mice (33), these mutations on either end of the 5a-polypeptide insert have been used to examine the role of the PAX6-5a isoform. The results indicate that the functional properties of PAX6 and the PAX6-5a isoform are unique, and PD of PAX6 is required for the function of HD.…”

PAX6 Expression and Retinal Cell Death in a Transgenic Mouse Model for Acute Angle-Closure Glaucoma

“…Normal development of the inner retina is directly influenced by a well-elaborated spatial and temporal pattern of PAX6 expression. 1,[6][7][8][9][19][20][21] However, little is known about PAX6 expression and function in the adult mammalian retina. De Melo et al 22 showed that several homeobox genes including Pax6 are expressed in the young adult mouse retina and we recently reported continuous expression of PAX6 in the young, adult, and old normal human retina.…”

“…4 Moreover, gain-offunction studies demonstrated that targeted expression of Drosophila and PAX6 induces ectopic eyes in Drosophila. 5 Mutations in PAX6 lead to a variety of eye abnormalities including hereditary foveal hypoplasia, optic nerve malformation, and aniridia in mice [6][7][8][9] and humans. [10][11][12][13][14][15][16][17][18] PAX6 has been studied extensively in eye and retinal development.…”