Wissinger B.

Molekulargenetisches Labor, Universitäts-Augenklinik, Auf der Morgenstelle 15, D - 72076 Tübingen

cGMP-gated ion channels have a key function in phototransduction. Light stimulation induces The light induced decrease in the level of cGMP in the outer segment of the photoreceptors results in the closure of these channels and thereby shuts-off the dark current. This leads to a hyperpolarization of the cell membrane which suppresses the release of glutamate at the synaptic terminus of the photoreceptor.
The photoreceptor cGMP-gated ion channels are composed of two subunits, an autonomous a-subunit and an accessory b-subunit. The cGMP-gated ion channels of rod and cone photoreceptors show profound differences in their biophysical properties and are encoded by distinct genes.
Upon genetic studies in patients it could be shown, that mutations in these genes are responsible for hereditary retinal diseases: Mutation in the gene encoding the a-subunit as well as mutation in the gene for the b-subunit of the rod cGMP-gated channel has been described in patients and families with autosomal recessively inherited Retinitis Pigmentosa (arRP). However the proportion of such cases among the entirety of arRP is relatively low.
In contrast, mutations in the genes encoding the a-subunit or the b-subunit of the cone cGMP-gated channel are the main cause (ca. 70%) for autosomal recessively inherited achromatopsia. This includes cases with complete achromatopsia (rod monochromacy) as well as cases with incomplete achromatopsia. One particular mutation, 1148delC, in the gene for the b-subunit is of major importance with respect to diagnostic application. This mutation is present in ~ 40% of patients with achromatopsia and thus, represents one of the most widespread mutations involved in retinal diseases with an estimated carrier frequency of ~1:150 in Northern and Central Europe.

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