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Perkins. J Transl Genet Genom 2022;6:95-110 https://dx.doi.org/10.20517/jtgg.2021.47 Page 101

photoreceptor structure and a complete absence of the ERG b-wave. Subsequent mapping and cloning of
the eclipse locus identified a missense mutation in the pde6a’ (pde6c) gene that resulted in a Met175Arg
amino acid substitution. They found that pde6c was expressed in cones and that cones degenerated rapidly.
Despite the lack of functional cone vision, the mutants survived into adulthood. Few cones remained at
3 wpf and although rod morphology was compromised at 3 wpf, the rods remained functional. By 6 mpf,
[106]
rods appeared normal but cones were absent from the central retina . As early at 5 wpf, the number of
proliferating cells (BrdU+) was increased in both the INL and ONL of the pde6c mutants. At 5 wpf,
however, the retina continues to rapidly expand and add new rods into the cone mosaic as the fish grows. It
is unclear if the proliferation observed in the pde6c mutants reflects an attempt to regenerate dying rods, the
expansion of rod numbers as the fish grows, or a combination of both. The significant increase in
proliferating cells within the INL, however, may suggest a regenerative response due to the rapid
[107]
degeneration of cones. Recently, a mutation in the rod-specific pde6a gene was reported . This mutant
had been generated by ENU mutagenesis and identified at the European Zebrafish Resource Centre. The
mutation introduced a stop codon that produced a truncated protein at glutamine 70 (pde6a Q70X ).
[29]
Disruption of pde6a did not result in OKR defects at 5 dpf, as the OKR is a cone-driven response . The
visual motor response (VMR) assay measures the locomotor activity of larvae to rapid light-dark
[108]
changes . In VMR assays, the pde6a mutant had reduced VMR responses, suggesting impaired ability to
detect light. Mutation of pde6a resulted in a reduction in rod size at 5 dpf and an almost complete absence
[107]
of rods by 21 dpf. Cone numbers were reduced slightly at 21 dpf, but morphology appeared normal . No
results were reported for later ages. Future studies will be needed to determine the long-term effect of pde6a
loss on cones and whether the proliferation of cells in the INL of pde6c mutants can be enhanced to
potentially regenerate cones.

RP2
X-linked retinitis pigmentosa (XLRP) can present as an aggressive and early form of RP that can occur
within the first 4 years of life . Mutations in the RP2 gene were found to cause XLRP and these account
[109]
[110]
for approximately 16% of all XLRP cases . RP2 encodes a 350-amino acid protein that stimulates the
GTPase activity of tubulin and also interacts with ADP-ribosylation factor-like-3 (ARL3) to stimulate its
GTPase activity [111,112] . RP2 binds membranes and localizes to the cilium and photoreceptor membrane [111,113] .
Based on these observations, RP2 has been proposed to facilitate vesicular trafficking of proteins from the
Golgi to the connecting cilium . The zebrafish rp2 knockout line was generated using TALEN genome
[113]
editing . The rp2 mutants completely lack Rp2 protein and show mild visual impairment at 7 dpf with a
[114]
30% reduction in the scotopic ERG b-wave compared to wild-type controls. Although mutants exhibited
reduced visual function, no morphological differences appeared until 4 mpf, when rod outer segments were
approximately 20% shorter. By 7 mpf, both rods and cones were shorter, consistent with a RP-like
phenotype. Ultrastructural analysis of 10 mpf rp2 mutants revealed that the rod outer segments were almost
completely missing, while the remaining outer segments were disorganized. While the phenotype of the rp2
mutant follows a progressive rod-cone dystrophy consistent with RP, it is worth noting that the rate of
degeneration is quite slow compared to humans with RP2 mutations. The rp2 mutant may serve as a useful
model to explore the mechanisms of photoreceptor degeneration and regeneration.

Retinitis pigmentosa GTPase regulator interacting protein 1
The retinitis pigmentosa GTPase regulator interacting protein 1 (RPGRIP1) gene encodes a 1259 amino acid
protein with several alternatively spliced isoforms and was first identified as a binding partner to
RPGR [115,116] . Mutations in RPGR are responsible for the majority of cases of XLRP, while mutations in
RPGRIP1 result in LCA [117,118] and cone-rod dystrophy . RPGRIP1 is highly expressed in the retina, with
[119]
weaker expression only found in the testis . RPGRIP1 protein colocalizes with RPGR at the photoreceptor
[116]