Functional rescue of degenerating photoreceptors in mice homozygous for a hypomorphic cGMP phosphodiesterase 6 b allele (Pde6bH620Q).
OBJECTIVE
Approximately 8% of autosomal recessive retinitis pigmentosa (RP) cases worldwide are due to defects in rod-specific phosphodiesterase PDE6, a tetramer consisting of catalytic (PDE6alpha and PDE6beta) and two regulatory (PDE6gamma) subunits. In mice homozygous for a nonsense Pde6b(rd1) allele, absence of PDE6 activity is associated with retinal disease similar to humans. Although studied for 80 years, the rapid degeneration Pde6b(rd1) phenotype has limited analyses and therapeutic modeling. Moreover, this model does not represent human RP involving PDE6B missense mutations. In the current study the mouse missense allele, Pde6b(H620Q) was characterized further.


METHODS
Photoreceptor degeneration in Pde6b(H620Q) homozygotes was documented by histochemistry, whereas PDE6beta expression and activity were monitored by immunoblotting and cGMP assays. To measure changes in rod physiology, electroretinograms and intracellular Ca(2+) recording were performed. To test the effectiveness of gene therapy, Opsin::Pde6b lentivirus was subretinally injected into Pde6b(H620Q) homozygotes.


RESULTS
Within 3 weeks of birth, the Pde6b(H620Q) homozygotes displayed relatively normal photoreceptors, but by 7 weeks degeneration was largely complete. Before degeneration, PDE6beta expression and PDE6 activity were reduced. Although light-/dark-adapted total cGMP levels appeared normal, Pde6b(H620Q) homozygotes exhibited depressed rod function and elevated outer segment Ca(2+). Transduction with Opsin::Pde6b lentivirus resulted in histologic and functional rescue of photoreceptors.


CONCLUSIONS
Pde6b(H620Q) homozygous mice exhibit a hypomorphic phenotype with partial PDE6 activity that may result in an increased Ca(2+) to promote photoreceptor death. As degeneration in Pde6b(H620Q) mutants is slower than in Pde6b(rd1) mice and can be suppressed by Pde6b transduction, this Pde6b(H620Q) model may provide an alternate means to explore new treatments of RP.
Davis
Richard J
RJ
Department of Pathology and Cell Biology, Brown Glaucoma Laboratory, College of Physicians and Surgeons, Columbia University, New York 10032, USA.
Tosi
Joaquin
J
Janisch
Kerstin M
KM
Kasanuki
J Mie
JM
Wang
Nan-Kai
NK
Kong
Jian
J
Tsui
Ilene
I
Cilluffo
Marianne
M
Woodruff
Michael L
ML
Fain
Gordon L
GL
Lin
Chyuan-Sheng
CS
Tsang
Stephen H
SH
D000483:Alleles; D000818:Animals; D015153:Blotting, Western; D002118:Calcium / Q000378:metabolism; D016923:Cell Death; D002460:Cell Line; D054707:Cyclic Nucleotide Phosphodiesterases, Type 6 / Q000235:genetics*; D004247:DNA / Q000235:genetics*; D004591:Electrophoresis, Polyacrylamide Gel; D004596:Electroretinography; D015870:Gene Expression; D006720:Homozygote; D007424:Intracellular Fluid / Q000378:metabolism; D051379:Mice; D008809:Mice, Inbred C3H; D008810:Mice, Inbred C57BL; D008811:Mice, Inbred DBA; D018613:Microscopy, Confocal; D008854:Microscopy, Electron; D009154:Mutation; D012162:Retinal Degeneration / Q000235:genetics* / Q000378:metabolism / Q000473:pathology; D012374:Rod Cell Outer Segment / Q000378:metabolism* / Q000503:physiopathology / Q000648:ultrastructure; D015398:Signal Transduction
D004247:DNA; D054707:Cyclic Nucleotide Phosphodiesterases, Type 6; C517438:Pde6b protein, mouse; D002118:Calcium