Retinal homeobox protein Rx also known as retina and anterior neural fold homeobox is a

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

that in humans is encoded by the RAX

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

. The RAX gene is located on chromosome 18 in humans, mice, and rats.

Function

This gene encodes a

homeobox A homeobox is a Nucleic acid sequence, DNA sequence, around 180 base pairs long, that regulates large-scale anatomical features in the early stages of embryonic development. Mutations in a homeobox may change large-scale anatomical features of ...

-containing

transcription factor In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription (genetics), transcription of genetics, genetic information from DNA to messenger RNA, by binding t ...

that functions in eye development. The gene is expressed early in the eye primordia, and is required for retinal cell fate determination and also regulates stem cell proliferation. Towards the end of late gastrulation a single eye field has formed and splits into bilateral fields via action by the signaling molecule, sonic hedgehog (Shh) secreted from the forebrain. Rax and Six-3 (also a transcription factor) maintain the forebrain's ability to secrete Shh by inhibiting activity of the signaling molecule Wnt. Rax (Retina and Anterior Neural Fold Homeobox) is a gene in the OAR (Otx, Arx,& Rax) subgroup of the paired-like homeodomain family of transcription factors. Discovered in 1997, the Rax gene is known to contribute to the development of the retina,

hypothalamus The hypothalamus (: hypothalami; ) is a small part of the vertebrate brain that contains a number of nucleus (neuroanatomy), nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrin ...

pineal gland The pineal gland (also known as the pineal body or epiphysis cerebri) is a small endocrine gland in the brain of most vertebrates. It produces melatonin, a serotonin-derived hormone, which modulates sleep, sleep patterns following the diurnal c ...

and

pituitary gland The pituitary gland or hypophysis is an endocrine gland in vertebrates. In humans, the pituitary gland is located at the base of the human brain, brain, protruding off the bottom of the hypothalamus. The pituitary gland and the hypothalamus contr ...

Clinical significance

Mutations in this gene have been reported in patients with defects in ocular development, including

microphthalmia Microphthalmia (Greek: , ), also referred as microphthalmos, is a developmental disorder of the eye in which one (unilateral microphthalmia) or both (bilateral microphthalmia) eyes are abnormally small and have anatomic malformations. Microphthalm ...

anophthalmia Anophthalmia (Greek: ἀνόφθαλμος, "without eye") is the medical term for the absence of one or both eyes. Both the globe and the ocular tissue are missing from the orbit. The absence of the eye will cause a small bony orbit, a constricte ...

, and

coloboma A coloboma (from the Greek , meaning "defect") is a hole in one of the structures of the eye, such as the iris, retina, choroid, or optic disc. The hole is present from birth and can be caused when a gap called the choroid fissure, which is ...

. Mutations to the Rax gene cause malformation of the retinal field, including anophthalmia and microphthalmia. Individuals who have a mutation in the RAX gene fail to develop ocular structures, referred to as anophthalmia. RAX mutant individuals can also have microphthalmia, where one or both of the eyes is smaller than normal.

Animal studies

Rax genes are conserved among vertebrates. RAX knockout mice have no eyes and abnormal forebrain formation. In the frog ''

Xenopus tropicalis The western clawed frog (''Xenopus tropicalis'') is a species of frog in the family Pipidae, also known as tropical clawed frog. It is the only species in the genus ''Xenopus'' to have a diploid genome. Its genome has been sequenced, making it a ...

'', Rax mutants are eyeless; the future retinal tissue instead has diencephalon and telencephalon features. Due to a genome duplication at the basis of the teleost fish lineage, fishes contain three Rax genes: Rx1, Rx2, and Rx3.

Zebrafish The zebrafish (''Danio rerio'') is a species of freshwater ray-finned fish belonging to the family Danionidae of the order Cypriniformes. Native to South Asia, it is a popular aquarium fish, frequently sold under the trade name zebra danio (an ...

and

medaka The Japanese rice fish (''Oryzias latipes''), also known as the medaka, is a member of genus ''Oryzias'' ( ricefish), the only genus in the subfamily Oryziinae. This small (up to about ) native of Japan is a denizen of rice paddies, marshes, po ...

mutants in Rx3 are eyeless.

References

Transcription factors {{gene-18-stub