FUNCTIONAL ABNORMALITIES IN TRANSGENIC MICE EXPRESSING A MUTANT RHODOPSIN GENE

Purpose, To evaluate the consequences of the expression of a mutant mo use opsin gene on rod- and cone-mediated function. Experimental condit ions were chosen to provide a basis of comparison to the results repor ted for patients with autosomal dominant retinitis pigmentosa (ADRP) i n whom the proline at position 23 has been replaced by a histidine (P2 3H). Methods. The mutated gene product resulted in three substitutions in the rhodopsin molecule: P23H, glycine for valine at position 20 (V 20G), and leucine for proline at position 27 (P27L). Mice positive for the transgene were differentiated from normal littermates by the poly merase chain reaction. Electroretinograms (ERGs) were obtained from an esthetized mice between 1 and 9 months of age. After photically bleach ing approximately 18 % of the available rhodopsin, the time course of rod dark adaptation was examined by monitoring rod ERG amplitude recov ery. Rhodopsin densitometry was used to determine the relative amounts of rhodopsin in the retinae of normal and transgenic mice. Results. E RGs obtained from transgenic mice showed a significant reduction in ro d-mediated response amplitude at 1 month of age and a relatively dow p rogressive decrease thereafter. Cone-mediated ERGs, on the other hand, were nearly normal in amplitude for approximately the first 5 months after birth, but at later ages response amplitudes also underwent a pr ogressive decline. In the normal retina, rod ERG amplitudes returned t o prebleach levels within 30 minutes, whereas in transgenic mice respo nse amplitudes did not recover within a 2-hour test period. The age-re lated decline in rod-mediated electroretinal potentials seen in transg enic mice was paralleled by a concomitant fall in rhodopsin density, a nd the sensitivity losses obtained electroretinographically could be a ccounted for solely on the basis of reduced quantal absorption. Conclu sions, The pattern of functional changes seen in the transgenic mice a re in good agreement with those reported in patients with ADRP with th e P23H mutation in the rhodopsin gene. Particularly noteworthy is the fact that the changes in rhodopsin density and visual sensitivity are associated with a progressive shortening of the rod outer segments; th e histologic changes induced by the disease process in patients with A DRP have yet to be determined.