Retinits pigmentosa encompasses a group of genetic disorders that involves 1 in 3000 individuals worldwide and gradually leads to blindness. Premature death of photoreceptors leads to progressive night blindness, contaction of the field of vision and finally to the complete loss of vision. Some forms of this disease have a mitochondrial pattern of inheritance whereas other types of disorder are inherited in a sex-linked or autosomal recessive or autosomal dominant pattern.
Retinits pigmentosa results from degeneration of the photoreceptors. It has been shown that this denereation is associated with mutations of certain proteins that are implicated in the phototransduction. Visualization is a complex process which begins with absorption of light by the visual pigment, rhodopsin. This is followed by the following series of interactions:
The heterogimderic guanine nucleotide binding protein, tansducin has alpha, beta and gamma subunits. Photoactivated rhodopsin leads to stimulation of the alpha subunit of transducin to exchange its GDP to GTP.
The transducin alpha dissociates from transducin beta-gamma complex.
The transducin alpha binds to gamma subunit of cGMP-phosphodiesterase (PDE). This interaction relieves the inhibitory effect of the gamma subunit on the catalytic alpha and beta subunits of PDE and allows the hydrolytic activity of these subunits to be increased almost 300 fold.
The activated PDE leads to a decrease in the amount of c-GMP.
The decrease in the cGMP-gated cation channels on the plasma membrane of rods leads to a neural response.
The termination of the response to light requires reversal of these molecular interactions such as quenching of the photoactivated rhodopsin, transducin alpha and PDE and restoration of the concentration of the cGMP.
The dominant form of retinits pigmentosa is associated with certain types of mutations in the rhodopsin whereas other types involve mutations in peripherin/RDS. The function of peripherin which is found in the periphery of the outer segment discs is currently unknown. The autosomal recessive type of retinitis pigmentosa in mice (rd1/rd1) and Irish Setter dogs is associated with mutations in the beta subunit of PDE. Mutation in the alpha subunit of PDE is also known to be associated with retinits pigmentosa. In the May 17, 96 issue of Science, Tsang et al report that lack of the gamma subunit of the PDE also results in retinal degeneration. Thus, presence of rhodopsin, and the subunits of the PDE is vital to the integrity of the photoreceptors and aberrations in these proteins leads to the degeneration of these cells and blindness.
References:
Tsang SH, 1996 Retinal degeneration in mice lacking the gamma subunit of the rod cGMP phosphodiesterase. Science, 272, 1026-1029, 1996
Retinits pigmentosa encompasses a group of genetic disorders that involves 1 in 3000 individuals worldwide and gradually leads to blindness. Premature death of photoreceptors leads to progressive night blindness, contaction of the field of vision and finally to the complete loss of vision. Some forms of this disease have a mitochondrial pattern of inheritance whereas other types of disorder are inherited in a sex-linked or autosomal recessive or autosomal dominant pattern. 
The heterogimderic guanine nucleotide binding protein, tansducin has alpha, beta and gamma subunits. Photoactivated rhodopsin leads to stimulation of the alpha subunit of transducin to exchange its GDP to GTP. 
