OPN1MW

The OPN1MW gene provides instructions for making a protein that is essential for normal color vision. This gene is active in the retina, a light-sensitive tissue at the back of the eye. The retina contains two types of light receptor cells called rods and cones. Rods are responsible for vision in low light. Cones provide vision in bright light, including color vision. Three types of cones each contain a special pigment (a photopigment) that is most sensitive to a particular wavelength of light.

The OPN1MW gene produces a photopigment that is more sensitive to light at the middle of the visible spectrum (yellow/green light). Cones with this pigment are usually called middle-wavelength-sensitive or M cones. In response to light at middle wavelengths, the photopigment triggers a series of chemical reactions within an M cone cell. These reactions ultimately alter the cell's electrical charge, generating a signal that is transmitted to the brain. The brain combines input from all three types of cones to produce normal color vision.

The middle-wavelength-sensitive pigment gene (OPN1MW) and long-wavelength-sensitive pigment gene (OPN1LW) are very similar and are located close together on the X chromosome. Most people have one copy of the OPN1LW gene and one or more copies of the OPN1MW gene. A nearby region of DNA, known as the locus control region (LCR), regulates the activity of these genes. Only the two pigment genes nearest the LCR are active in the retina.

color vision deficiency - caused by mutations in the OPN1MW gene

Genetic changes involving the OPN1MW gene cause a form of color vision deficiency that makes it difficult or impossible to distinguish between shades of red and green. Most red-green color vision defects result from structural rearrangements of the long- and middle-wavelength-sensitive pigment genes. In some cases, these rearrangements delete one or more copies of the OPN1MW gene. Other affected individuals have a hybrid pigment gene on the X chromosome instead of, or in addition to, separate copies of the OPN1MW and OPN1LW genes. This hybrid gene contains part of the OPN1MW gene and part of the OPN1LW gene. It typically has abnormal visual properties that affect red-green color vision.

Less commonly, red-green color vision defects can result from an OPN1MW mutation that changes a single protein building block (an amino acid) in the middle-wave-sensitive photopigment. This mutation, which inactivates the pigment, replaces the amino acid cysteine with the amino acid arginine at position 203 (written as Cys203Arg or C203R).

When M cones are completely nonfunctional, the specific type of red-green color vision deficiency that results is called deuteranopia. A less severe red-green color vision defect called deuteranomaly occurs when a hybrid pigment gene replaces the normal OPN1MW gene.

A condition called blue cone monochromacy occurs when genetic changes prevent both long- and middle-wavelength-sensitive photopigments from functioning normally. People with this condition have only cones with short-wave-sensitive photopigment (S cones). Because the brain must compare input from at least two types of cones to detect color, people who have only functional S cones have very poor color vision. Blue cone monochromacy can be caused by a deletion of the LCR, which normally controls the activity of the OPN1MW and OPN1LW genes. A loss of the LCR prevents these genes from producing any photopigments. This condition can also result from mutations in pigment genes that inactivate both the long- and middle-wavelength-sensitive photopigments.

A common variation (polymorphism) in either the OPN1MW or OPN1LW gene accounts for subtle differences in normal color vision. This change replaces the amino acid serine with the amino acid alanine at position 180 (written as Ser180Ala) in the resulting photopigment. Researchers suggest that the Ser180Ala polymorphism also plays a role in determining the severity of color vision loss in people with red-green color vision defects.