Exams of retinal function [i.e., electroretinograms (ERGs)], mutant rhodopsin localization, and ultrastructural features were not described. anti-rhodopsin antibodies. The outer segments, although shortened, contained well-packed discs. Proliferation of the endoplasmic reticulum as reported in expressing dominant rhodopsin mutations UPA was not observed. The accumulation of rhodopsin-laden vesicles likely represents aberrant transport of rhodopsin from the inner segments to the nascent disc membranes of the outer segments. It is possible that photoreceptor degeneration occurs because of a failure to renew outer segments at a normal rate, thereby leading to a progressive shortening of outer segments, or because of the loss of cellular contents to the extracellular space, or because of both. Mutations in the rhodopsin gene are a cause of dominant and recessive retinitis pigmentosa (RP), accounting for about 10% of all cases. Over 70 dominant alleles have been identified (1), most of which are missense mutations. They do not cause photoreceptor degeneration because of haploinsufficiency, since at least one recessive null allele does not cause disease in heterozygous carriers (2). Dominant rhodopsin mutants are therefore gain-of-function or dominant-negative mutants. A number of studies have been carried out to understand the pathogenic mechanism of rhodopsin mutants. On the basis of analyses of rhodopsin mutants expressed in cultured embryonic kidney cell lines, several classes were defined (3, 4, 5). A small number of mutants, designated class I, are like wild type (wt) opsin in their ability to localize to the cytoplasmic membrane and to bind 11-(6). These observations form the basis for a number of hypotheses regarding the pathogenic mechanism of dominant rhodopsin mutations (see HG6-64-1 studies, yet they tend to cause a more severe form of RP than that found in patients with dominant RP due to other mutations (7). This would indicate that expression of the mutant phenotype requires a photoreceptor cell environment and that the pathogenic mechanism of such mutants is best studied in an setting. Another group has developed a line of transgenic mice expressing a pig rhodopsin with the proline-347 to serine (P347S) mutation. It was HG6-64-1 found to develop photoreceptor degeneration and to exhibit an increased level of cAMP in the retina, a HG6-64-1 frequent finding associated with cell death in a variety of tissues (8). Tests of retinal function [i.e., electroretinograms (ERGs)], mutant rhodopsin localization, and ultrastructural features were not described. In the present study, we generated transgenic mice carrying a human P347S allele to investigate the early pathogenic events by which this rhodopsin mutant leads to photoreceptor cell death. MATERIALS AND METHODS Generation of Transgenic Mice. Genomic DNA was purified from the peripheral blood of a patient with dominant RP due to the P347S mutation in the rhodopsin gene. The DNA was partially digested with have not supported this hypothesis (11, 13, 17, 18, 19). In the current study, the normal ERG a-wave amplitudes seen in the predegenerate P347S mice are inconsistent with constitutive activation of the phototransduction cascade, but they are consistent with the apparently normal ability of P347S rhodopsin to activate transducin, to be phosphorylated by rhodopsin kinase, and to bind arrestin as shown by Weiss (8). An alternative hypothesis, the cellular congestion hypothesis, suggests that mutant rhodopsins accumulate in and congest the intracellular machinery for protein synthesis, processing, and/or transport. This hypothesis is supported by studies of mutant rhodopsins that cause photoreceptor degeneration in (20, 21). Massive proliferation of rough endoplasmic reticulum is observed, presumably due to the accumulation of mutant and wt opsins (20). Such a mechanism has not been established in mammalian photoreceptors. Class II mutants would be expected to display a similar transport defect, given their intracellular accumulation in cultured cells (3, 4, 5). However, in transgenic mice the bulk of P23H (class II) mutant rhodopsin localizes to the outer segments (10). One study of transgenic mice expressing a mutant rhodopsin missing the last five amino acids, Q344ter (class I), found partial retention of the mutant rhodopsin in the inner segments and the perinuclear regions, which led to the suggestion that the carboxyl terminus of rhodopsin might have a role in rhodopsin transport to the outer segments (13). In the current study of another class I mutant involving the carboxyl terminus, P347S, there was no accumulation of mutant rhodopsin in the.