The Zebrafish fade out mutant: a novel genetic model for Hermansky-Pudlak syndrome.
OBJECTIVE
To characterize retinal morphology and visual system function in the zebrafish mutant fade out (fad) and to establish the mutant as a lower vertebrate model for Hermansky-Pudlak syndrome (HPS).


METHODS
Retinal morphology of fad larvae was examined between 3 and 9 days postfertilization (dpf) by standard histology, transmission electron microscopy, and immunohistochemistry examination. Apoptotic cells were visualized by TdT-mediated dUTP nick-end labeling (TUNEL) staining. Visual system function was probed by electroretinography and behavioral assessment by optokinetic response measurements. Blood clotting was evaluated by time to occlusion testing of blood vessels as an arterial thrombosis assay. The chromosomal location of fad was determined by simple sequence-length polymorphism mapping. Genomic fragments of candidate genes were cloned by standard molecular techniques and mapped to the zebrafish genome by radiation hybrid mapping.


RESULTS
Mutant fad larvae are hypopigmented and show structural defects in the outer retina. Melanosomes of these larvae in the retinal pigment epithelium are hypopigmented, generally smaller, and progressively reduced in number compared to nonmutant larvae. Progressive microvilli protrusions into the photoreceptor cell layer are not detectable, and photoreceptor outer segments get shorter and are misaligned. Photoreceptors subsequently undergo apoptosis, with a peak of cell death at 6 dpf. Electrical responses of the retina and visual performance are severely reduced. Blood clotting is prolonged in mutant fad larvae. Genomic mapping of fad reveals distinct genomic positions of the mutant gene from known human HPS genes.


CONCLUSIONS
The fad mutant shows syndromic defects in pigmentation, outer retinal structure and function, and blood clotting. This syndrome is characteristic of Hermansky-Pudlak syndrome (HPS), making fad a novel genetic model of HPS. The gene does not cosegregate with the known human HPS genes, suggesting a novel molecular cause of HPS.
Bahadori
Ronja
R
Swiss Federal Institute of Technology (ETH), Department of Biology, and Brain Research Institute, University of Zurich, Zurich, Switzerland.
Rinner
Oliver
O
Schonthaler
Helia Berrit
HB
Biehlmaier
Oliver
O
Makhankov
Yuri V
YV
Rao
Prashanth
P
Jagadeeswaran
Pudur
P
Neuhauss
Stephan C F
SC
D000818:Animals; D017209:Apoptosis; D004596:Electroretinography; D019084:Fluorescent Antibody Technique, Indirect; D005808:Genes, Recessive; D008040:Genetic Linkage; D005974:Glutamate-Ammonia Ligase / Q000378:metabolism; D022861:Hermanski-Pudlak Syndrome / Q000235:genetics* / Q000473:pathology; D017496:Hypopigmentation / Q000235:genetics*; D020287:In Situ Nick-End Labeling; D020460:Melanosomes / Q000235:genetics* / Q000473:pathology; D008957:Models, Genetic; D009154:Mutation; D017120:Nystagmus, Optokinetic / Q000502:physiology; D020419:Photoreceptor Cells, Vertebrate / Q000502:physiology / Q000648:ultrastructure; D010857:Pigment Epithelium of Eye / Q000473:pathology; D021101:Radiation Hybrid Mapping; D012164:Retinal Diseases / Q000235:genetics* / Q000473:pathology; D050825:Synaptosomal-Associated Protein 25 / Q000378:metabolism; D014914:Whole Blood Coagulation Time; D015027:Zebrafish / Q000235:genetics*
D050825:Synaptosomal-Associated Protein 25; D005974:Glutamate-Ammonia Ligase