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Ophthalmic and cone derived electrodiagnostic findings in outbred Miniature Long-haired Dachshunds homozygous for a RPGRIP1 mutation.

OBJECTIVE
To investigate ophthalmic and cone-derived electrodiagnostic findings in outbred Miniature Long-haired Dachshunds (MLHD) homozygous for a mutation in the RPGRIP1 gene previously associated with cone-rod dystrophy 1 (cord1).


ANIMALS
A total of 36 MLHD homozygous for the RPGRIP1 mutation and 23 dogs clear of the mutation (control group).


PROCEDURES
The dogs underwent ophthalmic examination and photopic electroretinogram (ERG) recordings.


RESULTS
None of the control dogs presented with clinical or ophthalmic signs consistent with cord1. Amongst the dogs homozygous for the mutation one presented with bilateral symmetrical total retinal atrophy. None of the other dogs in this group showed signs consistent with cord1. Photopic ERG recordings were available in 23 control dogs and 34 dogs homozygous for the mutation. Photopic a- and b-waves following four light stimuli (3 cdS/m(2) ) at a rate of 5.1 Hz were not significantly different between groups. The amplitudes of the 30 Hz flicker (128 flashes, 3 cdS/m(2) ) response were significantly reduced in the dogs homozygous for the PRGRIP1 mutation. The difference in age between the two groups did not significantly affect the difference.


CONCLUSION
Homozygosity of the RPGRIP1 mutation does not invariably result in early onset cord1. However, cone derived ERG recordings show evidence of a reduced cone or inner retinal function in homozygous but clinically normal MLHD. Modifying genes that have yet to be identified may influence an individual dog's risk of developing the blinding cord1 and also the age of onset and rate of progression.