Analysis of the rdd locus in chicken: a model for human retinitis pigmentosa.
OBJECTIVE
To identify the locus responsible for the blind mutation rdd (retinal dysplasia and degeneration) in chickens and to further characterise the rdd phenotype.


METHODS
The eyes of blind and sighted birds were subjected to ophthalmic, morphometric and histopathological examination to confirm and extend published observations. Electroretinography was used to determine age of onset. Birds were crossed to create pedigrees suitable for genetic mapping. DNA samples were obtained and subjected to a linkage search.


RESULTS
Measurement of IOP, axial length, corneal diameter, and eye weight revealed no gross morphological changes in the rdd eye. However, on ophthalmic examination, rdd homozygotes have a sluggish pupillary response, atrophic pecten, and widespread pigmentary disturbance that becomes more pronounced with age. Older birds also have posterior subcapsular cataracts. At three weeks of age, homozygotes have a flat ERG indicating severe loss of visual function. Pathological examination shows thinning of the RPE, ONL, photoreceptors and INL, and attenuation of the ganglion cell layer. From 77 classified backcross progeny, 39 birds were blind and 38 sighted. The rdd mutation was shown to be sex-linked and not autosomal as previously described. Linkage analysis mapped the rdd locus to a small region of the chicken Z chromosome with homologies to human chromosomes 5q and 9p.


CONCLUSIONS
Ophthalmic, histopathologic, and electrophysiological observations suggest rdd is similar to human recessive retinitis pigmentosa. Linkage mapping places rdd in a region homologous to human chromosomes 9p and 5q. Candidate disease genes or loci include PDE6A, WGN1, and USH2C. This is the first use of genetic mapping in a chicken model of human disease.
Division of Genomics and Bioinformatics, Roslin Institute, Edinburgh, Midlothian, UK.
David W
DW
Burt
David R
DR
Morrice
Douglas H
DH
Lester
Graeme W
GW
Robertson
Moin D
MD
Mohamed
Ian
I
Simmons
Louise M
LM
Downey
Caroline
C
Thaung
Leslie R
LR
Bridges
Ian R
IR
Paton
Mike
M
Gentle
Jacqueline
J
Smith
Paul M
PM
Hocking
Chris F
CF
Inglehearn
D000818:Animals; D001766:Blindness / Q000235:genetics* / Q000473:pathology; D002645:Chickens / Q000235:genetics*; D002874:Chromosome Mapping; D002875:Chromosomes / Q000235:genetics; D003433:Crosses, Genetic; D004252:DNA Mutational Analysis; D004195:Disease Models, Animal; D004596:Electroretinography; D005260:Female; D008040:Genetic Linkage; D005838:Genotype; D008297:Male; D009024:Morphogenesis; D009154:Mutation; D010641:Phenotype; D012162:Retinal Degeneration / Q000235:genetics* / Q000473:pathology; D015792:Retinal Dysplasia / Q000235:genetics* / Q000473:pathology; D012174:Retinitis Pigmentosa / Q000235:genetics / Q000473:pathology