CNGB3

The CNGB3 gene provides instructions for making a protein that forms part of an ion channel. Ion channels are openings in the cell membrane that transport electrically charged atoms (ions) into and out of cells. Specifically, the CNGB3 protein is part of a family of proteins that form cyclic nucleotide-gated (CNG) channels. CNG channels are involved in transmitting information about vision and smell from sensory cells to the brain.

The CNGB3 protein forms one part (the beta subunit) of a CNG channel that is necessary for normal vision. These channels are present in light receptor cells called cones. As part of the light-sensitive tissue at the back of the eye (the retina), cones provide vision in bright light, including color vision. Other light receptor cells in the retina, called rods, are responsible for vision in low light.

In cones, CNG channels remain open under dark conditions. Positively charged ions can flow into the cell through these open channels. In response to light, these channels close to stop the inward flow of ions. This change in ion transport alters the cone cell's electrical charge, which generates a signal that the brain interprets as vision.

color vision deficiency - caused by mutations in the CNGB3 gene

More than 25 mutations in the CNGB3 gene have been identified in people with color vision deficiency. These mutations cause complete achromatopsia, which is a loss of all color vision. Worldwide, the most common mutation that causes this condition deletes a single DNA building block (a base pair) from the CNGB3 gene. This mutation can be written as 1148delC.

Complete achromatopsia occurs frequently in Pingelapese islanders, who live on one of the Eastern Caroline Islands of Micronesia. Among the Pingelapese, this condition results from a mutation that changes a single protein building block (an amino acid) in the CNGB3 protein. This mutation replaces the amino acid serine with the amino acid phenylalanine at position 435 in the protein (written as Ser435Phe or S435F).

Mutations in the CNGB3 gene probably lead to an abnormally small, nonfunctional version of the CNGB3 protein that cannot form working CNG channels. Without these channels, cones cannot regulate the inward flow of ions. As a result, these light receptor cells are unable to transmit visual signals to the brain. A loss of normal cone function causes a lack of color vision, reduced sharpness, and other vision problems associated with complete achromatopsia.