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Page 100 Perkins. J Transl Genet Genom 2022;6:95-110 https://dx.doi.org/10.20517/jtgg.2021.47

by 4 mpf, while green and blue cones were only partially reduced by 8 mpf. It is unclear why zebrafish with
mutations in exon 47 would exhibit more rapid degeneration of cones and the selective loss of red and UV
[94]
cones, when compared to zebrafish with mutations in exon 2. More recently, Messchaert et al. used
CRISPR/Cas9 technology to generate a 5-bp deletion in exon 20 of eys. Immunohistochemistry with anti-
EYS antibodies on retinas from the eys rmc101 mutant showed a complete loss of protein, indicating this was a
null allele. In 5 dpf larvae, the eys rmc101 mutant was reported to have shorter and more disorganized
photoreceptor outer segments than wild-type siblings, but no quantification was provided. By 2 mpf a
significant thinning of the INL and ONL was observed. Rhodopsin and the alpha subunit of cone
transducin were mislocalized in rods and cones, respectively by 5 mpf. The authors also explored Müller glia
morphology and did not observed evidence of gliosis or Müller glia proliferation. Of note, however, was that
eys rmc101 mutants had normal OKR behavior but a decrease in the ERG b-wave, which strongly suggested an
early defect in visual function as early as 5 dpf. All three groups reported that eys mutants exhibited
photoreceptor degeneration that starts by 2 mpf and progresses with age. Additional investigation with
these mutants will help reveal the cellular function of EYS in photoreceptors and explain the mechanisms
leading to photoreceptor death.

Photoreceptor cilium actin regulator
Two groups independently identified mutations in the centrosome 2 open reading frame 71 (C2ORF71) gene
that result in non-syndromic autosomal recessive RP in families of various national origins [95,96] . C2ORF71 is
[95]
exclusively expressed in the retina . Recent work demonstrated that the protein encoded by C2ORF71
interacts with several proteins associated with centrioles, microtubules, and factors that regulate actin
filament assembly . Based on these findings, Corral-Serrano et al. proposed that C2ORF71 be renamed
[97]
[97]
PCARE for photoreceptor cilium actin regulator. The zebrafish genome contains two pcare paralogs, pcare1
and pcare2 . By searching for synteny between the chromosomal region of the human PCARE gene and
[98]
the zebrafish genomes, the pcare1 gene was considered to be the ortholog of the human gene. A zebrafish
pcare1 mutant line was generated by CRISPR/Cas9 mutagenesis and the 29 bp deletion was predicted to
result in a truncating nonsense allele at amino acid 16 . Mutants exhibited disrupted actin assembly and
[98]
disorganized photoreceptor structure as early as 5 dpf. The pcare1 mutants were viable and at 6 mpf, the rod
outer segments were highly disorganized or missing and cone outer segments were significantly shorter
than wild-type siblings . This is consistent with the RP phenotype in humans. The rapid degeneration of
[98]
rods seen in pcare1 mutants would suggest that proliferation of Müller glia or rod precursors may occur,
although this was not investigated. Future studies of the pcare1 mutant could explore whether a regenerative
response follows the degeneration of photoreceptors, while functional studies of pcare2 may uncover a
similar role in photoreceptor maintenance.

Phosphodiesterase 6
Photoreceptors respond to light by activating a G-protein signaling pathway known as the
phototransduction cascade [99,100] . Upon absorption of a photon of light, rhodopsin or cone opsins activate
many molecules of the heterotrimeric G-protein transducin. The GTP-bound α-subunit of transducin (Gα)
t
activates its effector enzyme, cGMP phosphodiesterase 6 (PDE6). PDE6 hydrolyzes cGMP, resulting in a
decrease in cGMP concentration and the closure of cGMP-gated channels in the outer segment plasma
membrane. PDE6 is a multimeric enzyme. The rod-specific PDE6 is a heterotetramer, consisting of a α-
subunit, β-subunit, and two γ-subunits. The cone PDE6 consists of two cone-specific α’-subunits and two
[101]
cone-specific γ-subunits. The cone-specific α’-subunits are encoded by the PDE6C gene , Mutations in the
rod-specific PDE6A gene cause RP while mutations in PDE6C result in cone dystrophy and
[102]
achromatopsia [103,104] . Nishiwaki et al. conducted a genetic screen for zebrafish with defects in visual
[105]
behavior and identified several mutants, including eclipse (els). The els mutant exhibited irregular