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Page 96 Perkins. J Transl Genet Genom 2022;6:95-110 https://dx.doi.org/10.20517/jtgg.2021.47

cone-rod dystrophy, Leber congenital amaurosis (LCA), and choroideremia affect the eye alone. RP is also
associated with several syndromic and systemic conditions, where the disease impacts one or more other
organs. These diseases can include Usher syndrome, in which RP is associated with hearing loss, Joubert
syndrome (JBTS) and Bardet-Biedl syndrome (BBS), which are two ciliopathies with systemic effects, and
Senior-Loken syndrome, which presents with RP and kidney disease. It was recently estimated that
[2]
approximately 5.5 million people worldwide (~1 in 1380 people) are afflicted by IRDs and this has a
significant cost on both individuals and society. The financial impact of IRDs associated with economic and
wellbeing costs was estimated to range between $13-31 billion in the United States and between CAN$669-
1638 billion in Canada for 2019 .
[3]

Until recently, no therapeutic options existed that could slow or reverse the disease progression of IRDs.
The ongoing success in identify the genetic causes of IRDs has allowed strategies such as gene therapy and
genome editing to show promise and early success . While encouraging, gene augmentation and gene
[4-6]
correction strategies still require that most therapies be specifically tailored to each genetic mutation or
mutated exon, of which there are thousands. To circumvent these limitations, there has been considerable
[7,8]
interest in strategies that promote tissue regeneration either through stem cell transplantation or
regeneration from endogenous stem cell populations [9,10] . Although adult mammals show limited potential
for endogenous retinal regeneration [11,12] , zebrafish readily regenerate their retina after injury [13-15] and restore
visual function . In zebrafish, the Müller glia respond to injury by reprogramming and re-entering the cell
[16]
cycle to produce multipotent neural progenitors [15,17,18] . There is now considerable interest in exploiting this
regenerative capacity for possible therapies for the treatment of IRDs. As no single therapeutic strategy will
be a “magic bullet” for IRDs, including regenerative medicine, it remains essential to test any approaches in
animal models that recapitulate human disease.

Since being established as an animal model for developmental genetics in the 1970s [19,20] , zebrafish have
become increasingly popular as a disease model, including as a model for IRDs [21-28] . Investigators initially
used genetic screens of fish mutagenized by ethylnitrosourea (ENU) to identify dozens of mutants with
defects in retinal structure and function [29-33] . Simple histological methods [31,34,35] and visual behaviors, such as
the optokinetic response (OKR) [29,30] , were highly successful at identifying novel mutant phenotypes in larval
z e b r a f i s h a n d u l t i m a t e l y d i s c o v e r i n g f a c t o r s i m p o r t a n t f o r p r o t e i n trafficking , o n e
c
[36]
phototransduction [37,38] , retinal development [39,40] , and many others. Interestingly, almost all of these
mutations affects cones or cones and rods and the large majority were lethal within the first two weeks of
age. Why would impaired vision result in lethality? Insight comes from the study of the no optokinetic
response f (nof) mutant, which was identified in ongoing screens for mutants that lacked an OKR
behavior . The nof mutation introduced a stop codon in the alpha subunit of cone transducin, leading to
[38]
complete loss of protein. The nof mutants were morphologically indistinguishable from their wild-type
siblings and no evidence of retinal degeneration was observed. Nevertheless, the nof larvae did not survive
beyond 12 dpf under standard rearing conditions. If the mutants were raised in a “soup” containing a 10-
fold higher concentration of paramecium, a large number survived to adulthood. The conclusion was that
loss of cone function impaired the ability of larvae to effectively hunt paramecia using visual cues and feed
during the first few weeks of life. In other words, severe visual impairment leads to starvation under
standard laboratory rearing conditions. A notable exception was the early search for dominant forms of
retinal degeneration in zebrafish by Li and Dowling . In this study, the investigators used the “escape
[33]
response” behavior assay to screen for mutants. Zebrafish swim in circles when placed in a small, circular
container but will rapidly turn and swim in the opposite direction when challenged. F1 animals (i.e.,
heterozygotes) derived from ENU mutagenized founders were dark adapted and screened for elevated visual
thresholds to identify mutants. The night blindness a (nba) mutant exhibited elevated visual thresholds and