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Perkins. J Transl Genet Genom 2022;6:95-110 https://dx.doi.org/10.20517/jtgg.2021.47 Page 99

[82]
f378
after amino acid 99 . No retinal phenotype was described for cep290 . The cep290 fb208 mutant allele is a
[83]
10 bp deletion in exon 16 generated by CRISPR/Cas9 mutagenesis and results in a premature stop codon .
Cardenas-Rodriquez and colleagues generated maternal-zygotic (MZ) cep290 fb208 mutants, which lack
maternally-derived mRNA and protein in the yolk of zebrafish embryos and larvae, to exclude the
possibility that the presence of maternal Cep290 protein could mask phenotypes during larval stages .
[83]
MZcep290 fb208 mutants also exhibited normal vision as assessed by electroretinograms but did show mild
disorganization of outer segments and an accumulation of vesicular material at the base of the outer
[83]
segments . In adults, both cep290 fh297 and cep290 fb203 mutants exhibited progressive cone photoreceptor
degeneration, with the cep290 fh297 mutant losing approximately 40% of cones by 6 mpf and 80% of cones by
12 mpf. Rhodopsin mislocalized to the inner segments of rod photoreceptors. The proliferation of rod
precursors in the ONL suggested that rods degenerated in the cep290 fh297 mutant . Similar to what was
[81]
fh297
observed in the bbs2 mutant, Müller glia did not proliferate in the cep290 mutants. Analysis of Müller cell
proliferation or rod degeneration was not performed for the MZcep290 mutants.
fb208
Ceramide kinase-like
The ceramide kinase-like (CERKL) gene encodes a 532 amino acid polypeptide that contains a pleckstrin
homology (PH) and a diacylglycerol kinase (DAGK) domain . CERKL is a member of the ceramide kinase
[84]
protein family, which converts ceramide to ceramide-1-phosphate, but CERKL has not yet been shown to
possess enzymatic activity [85,86] . CERKL is expressed throughout the retina, as well as brain, lung, liver,
kidney, and pancreas . Despite this widespread expression pattern, mutations in CERKL have only been
[84]
hzu3
[87]
[84]
associated with nonsyndromic RP and with cone-rod dystrophy . A cerkl knockout zebrafish (cerkl )
was generated by TALEN technology and resulted in a 7 bp deletion that caused a premature nonsense
[88]
mutation. Visual impairment was detected in 7 dpf cerkl larvae by ERG analysis . Thinning of the ONL
was observed as early as 2 mpf, with continued thinning at 4 mpf and a significant loss of photoreceptors by
12 mpf. Immunohistochemical and western blotting analyses indicated that rod photoreceptor degeneration
preceded the degeneration of cones, suggesting that the cerkl mutant represents an ideal model for RP. Cell
proliferation was not assessed for the cerkl mutant, so it remains unclear if the rod precursors or Müller glia
attempt to regenerate photoreceptors. The mechanisms underlying photoreceptor degeneration due to loss
of CERKL function remain unclear and future studies of the cerkl zebrafish mutant may provide unique
insight.

Eyes shut
The eyes shut (EYS) gene encodes a large extracellular matrix protein of 3165 amino acids that is the
homolog of the Drosophila eyes shut gene. Mutations in EYS are a major cause of autosomal recessive RP in
humans across the world and account for the most prevalent form of RP in Japanese populations [89,90] . EYS is
highly expressed in the human retina but expression is absent in the retinas of mouse, rat, and cattle , thus
[91]
limiting the study of EYS function in the retina. To address this limitation three groups independently
[92]
generated mutations in the zebrafish eys gene. Yu et al. used CRISPR/Cas9 to generate multiple mutants
with truncating nonsense alleles in exon 2 of the eys gene. Three alleles, eys sny10 , eys sny13 , and eys sny14 , had
identical phenotypes of cone degeneration beginning between 4-6 mpf and slower rod degeneration that
was apparent by 14 mpf. Using antibodies specific for EYS, the group also reported that EYS protein
concentration near the connecting cilium and transition zone of the photoreceptors in both zebrafish and
primate retinas. Using TALEN technology, Lu et al. independently generated several truncating nonsense
[93]
alleles in exon 47 of the zebrafish eys gene. ERG analysis at 10 dpf revealed a significant decrease in b-wave
amplitude in eys mutants, indicating visual impairment. In these mutants, a decrease in ONL thickness was
noticed as early as 2 mpf, with more than 60% reduction by 16 mpf. Interestingly, cone subtypes differed in
the rate of degeneration. Lu et al. reported that the number of red and UV cones decreased significantly
[93]