Cats: a gold mine for ophthalmology.

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Cats: A Gold Mine for Ophthalmology Kristina Narfström,1,2 Koren Holland Deckman,3 and Marilyn Menotti-Raymond4 1

Department of Veterinary Medicine and Surgery, College of Veterinary Medicine, University of Missouri, Columbia, Missouri 65201; email: [email protected]

2

RetVet KB, 18593 Vaxholm, Sweden

3

Department of Chemistry, Gettysburg College, Gettysburg, Pennsylvania 17325; email: [email protected]

4

Laboratory of Genomic Diversity, Frederick National Laboratory for Cancer Research, Frederick, Maryland 21702; email: [email protected]

Annu. Rev. Anim. Biosci. 2013. 1:157–177

Keywords

The Annual Review of Animal Biosciences is online at animal.annualreviews.org

eye, hereditary disease, molecular genetics, animal model, therapy

This article’s doi: 10.1146/annurev-animal-031412-103629

Abstract

Copyright © 2013 by Annual Reviews. All rights reserved

Over 200 hereditary diseases have been identified and reported in the cat, several of which affect the eye, with homology to human hereditary disease. Compared with traditional murine models, the cat demonstrates more features in common with humans, including many anatomic and physiologic similarities, longer life span, increased size, and a genetically more heterogeneous background. The development of genomic resources in the cat has facilitated mapping and further characterization of feline models. During recent years, the wealth of knowledge in feline ophthalmology and neurophysiology has been extended to include new diseases of significant interest for comparative ophthalmology. This makes the cat an extremely valuable animal species to utilize for further research into disease processes affecting both cats and humans. This is especially true in the advancement and study of new treatment regimens and for extended therapeutic trials. Groups of feline eye diseases reviewed in the following are lysosomal storage disorders, congenital glaucoma, and neuroretinal degenerations. Each has important implications for human ophthalmic research.

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INTRODUCTION


The domestic cat is the most abundant of companion animals in the United States; currently approximately 90 million cats are maintained as pets (http://www.appma.org/press_industrytrends. asp). As valued members of a household, cats and dogs have experienced an extremely high level of medical surveillance by veterinarians and breeders over the years; consequently, over 200 hereditary diseases have been identified and reported in the cat (http://omia.angis.org.au/home/), many of which have established homology to human hereditary disease. Often these are rare disorders, as in the case of retinal cone-rod dystrophy (Rdy), which we discuss later. The abundance of spontaneously generated mutations associated with both disease phenotypes and phenotypically unique traits of morphology, coat color (X-linked orange), and pattern (variation for coat pattern) contribute to the cat’s attractiveness as a strong comparative medical model. Additionally, the cat has a long record as an important model in physiology, neurobiology, reproductive biology, and ophthalmology, which complements genomics discoveries as well as the value of the cat as an important comparative medical model (1–6). Long-lived, large animal models of homologous human disease are critical for testing the safety and efficacy of emergent medical modalities, such as induced pluripotent stem cell–tissue transplantation. Compared with traditional murine models, the cat demonstrates many features in common with humans, including a more similar physiology, a longer total life span, an increased size, and a genetically heterogeneous background. Rapid advances in feline genomics facilitate characterization of disease mechanisms on a molecular level. The domestic cat thus offers an increasingly strong comparative medical model for discovery, diagnostics, and development of therapeutic and treatment modalities, which will be of benefit to both cats and humans. This is especially true in the field of ophthalmology. Several hereditary ophthalmic diseases in the feline have been mapped or characterized on a molecular level within the past five years (7–9) (Table 1), and additional disease entities have been reported on a clinical level and are currently being investigated for molecular mechanisms (11). The development of genomics resources in the cat has facilitated the mapping and characterization of feline models of eye disease. Within the past several years, the cat genome has been sequenced at 23, 33, and 143 genome equivalents (12–14), with the 143 genome assembly currently nearing release. A recent high-resolution radiation hybrid map, including 2,662 markers, which provide an estimated average intermarker distance of 939 kilobases (15), and an autosomal genetic linkage map of the domestic cat (16) provided the physical framework for recent genome assemblies. Light resequencing of seven different cats of recognized breed characterized approximately 3 million single-nucleotide polymorphisms (SNPs) across the genome (14); this information has been used to generate a first-generation SNP array of 62,000 SNPs useful for association mapping in the cat. Sequence analysis of cDNA libraries generated from ten different feline tissues has been used to improve annotation of the 143 genome assembly. The availability of bacterial artificial chromosome clones positioned on the genome assembly (13) and an annotated browser (see http://lgd.abcc.ncifcrf.gov/cgi-bin/gbrowse/cat) (13) are additional genomic resources that facilitate mapping and association studies in the cat.

COMPARATIVE ASPECTS OF THE FELINE EYE The cat eye has many similarities to the human eye relative to overall structure and function. There are some important differences, however. The cat eye is smaller (meridional anteriorposterior axis of the bulb: 21.3 mm; equatorial axis of the bulb: 20.6) (17), and it has 158

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Table 1 Hereditary ophthalmic diseases that have been characterized in the domestic cat Disease/phenotype

Human pathology/phenotype

Gene

Reference

a-Mannosidosis

a-Mannosidosis

MAN2B1

1749_1752delCCAG leads to premature stop

(49)

Primary congenital glaucoma

Primary congenital glaucoma

LMBP2

Unpublished

(9)

Cone-rod dystrophy (Rdy)

Cone-rod dystrophy

CRX

n.546delC

(10, 8)

GLB1

R482P

(53)

HEXB

39delC leads to premature stop or 1467_1491 inv; del exon12

(57, 58)

GM2A

Del4bp in 30 region leads to frame shift

(56)

Gangliosidosis GM1 Gangliosidosis GM2


Feline mutation

Sandhoff disease

Gangliosidosis GM2 Mucopolysaccharidosis Type I Mucopolysaccharidosis Type VI

MPS I

IDUA

107_109 delCGA

(37)

Maroteaux-Lamy syndrome

ARSB

L476P (severe); D520N (mild phenotype)

(42, 43)

Mucopolysaccharidosis Type VII

MPS VII

GUSB

E351K

(47)

Mucolipidosis II

I-cell disease

GNPTAB

unknown

(59)

Retinal degeneration in the Abyssinian cat (rdAc)

Leber congenital amaurosis, Joubert, Senior-Loken, MeckelGruber, and Bardet-Biedl syndromes

CEP290

IVS50 þ9T > G

(7)

a considerably larger lens, which encompasses approximately 10% of the feline eyeball. The cornea and the anterior chamber of the cat eye are also larger on a relative scale than these structures in human eyes, because the cat has a more spherical cornea. Another difference between the cat eye and that of humans is the feline slit pupil, in which the sphincter muscles interlace, creating a scissors action during pupillary closure (17). In normal neonatal kittens, aqueous humor production and outflow conditions are much different compared with those observed in the adult cat. In the neonate, the anterior chamber is flattened, and there is apposition between the iris and the cornea. Then, over the first few months of age there is a process of expansion, and the anterior chamber becomes deeper, with widely spaced trabecular beams and well-defined uveal and corneoscleral meshworks. In contrast to that in the human eye, the trabecular meshwork of the adult cat is located within a long and wide ciliary cleft. The anterior chamber is deeper than in the human eye, and the opening of the iridocorneal angle in cats is much wider (18). Owing to the deep anterior chamber and the curvature of the cornea, it is actually possible to evaluate a significant part of the feline drainage angle by direct observation using focal illumination and magnification (19), although the details of the iridocorneal angle and opening of the ciliary cleft are more thoroughly evaluated by gonioscopy, using a goniolens and a handheld biomicroscope (18). Just as in the human eye, the maintenance of normal intraocular pressure (IOP) in cats depends on the equilibrium between aqueous outflow and the production of aqueous humor. In cats, more than 97% of the aqueous humor outflow occurs by the conventional route, exiting www.annualreviews.org

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the eye via the trabecular meshwork and angular aqueous plexus and moving into the intrascleral venous plexus and then ultimately into the general circulation (20). Thus, in comparison with the human eye, the cat eye does not have a structure similar to Schlemm’s canal. The uveoscleral route in the feline eye accounts for a very small part (less than 3%) of the aqueous humor outflow (18). Similar to the human eye, the cat eye has a dual retina with rod photoreceptors that greatly outnumber cones. In contrast to the human retina, which has a macula, an area with only cones that accounts for maximum visual acuity, the feline retina has a cone-rich area called the area centralis. Here, the cone distribution is characterized by a steep central increase in cone density. There is also an extended and elongated region along the central nasotemporal axis (21) of the fundus, called the visual streak. This area coincides approximately with the central increase in ganglion cell density (22). The rod-to-cone ratio reaches a low value of approximately 10 in the area centralis but rises steeply on either side of this region. In the near periphery, the ratio is 65 and rises to 100 at the ora serrata. The cones are interspersed throughout the rod mosaic and are arranged less regularly than the rods. The layer of the visual cell nuclei in the cat varies in thickness depending on retinal location, from 9 to 13 rows centrally to 7–9 rows peripherally (23). Further, most cats have a reflective cell layer behind the neuroretina and the retinal pigment epithelium (RPE). This layer, the tapetal or the tapetum lucidum area, is a roughly triangular, usually brightly colored region observed in the upper half of the fundus (24), and it enhances the effects of light stimulation on the photoreceptors (the rods and cones). The tapetal area varies in coloration from a deep blue in young kittens to green or yellow in adult cats (see Figure 1a for normal cat fundus appearance). This area is responsible for the reflectivity observed in the cat eye when light falls on the eye in the dark. Unlike the human eye, which most often has a rather evenly pigmented RPE, the RPE of the feline species is usually nonpigmented in the tapetal fundus, whereas in most cats, the nontapetal parts of the fundus have an abundance of melanin granules in the RPE.

a

b

c

d

250 μV 50 ms

WT S2 S4

Figure 1 Fundus appearances and electroretinograms (ERGs) of a normal Abyssinian cat and of rdAc individuals with the CEP290 mutation. Fundus photographs demonstrate (a) a one-year-old unaffected Abyssinian cat (wild type, WT), (b) a two-year-old affected Abyssinian cat with an early disease stage (S2) (23), and (c) a six-year-old Abyssinian with an advanced disease stage (S4) (23). Arrows in (b) and (c) indicate retinal vasculature that is attenuated, more so in the advanced stage (c) than in the early stage (b) of disease. For the same three cats, the waveforms of the dark-adapted full-field flash ERG recordings are shown, using 4 cd.s/m2 of white light stimulation for each of the recordings. The ERG response from the cat with the fundus appearance shown in (a) is normal [upper ERG recording shown in (d)]. It illustrates a quick and sharp negative deflection (downward part of the curve), which is the a-wave, followed by a positive part (upward) of the ERG response, which is the b-wave. The ERG response in regard to the cat fundus shown in (b) is reduced. A barely recordable ERG [almost a flat line: lower recording in (d)] is illustrated from the individual with the fundus appearance shown in (c). Amplitude and implicit time calibrations are indicated vertically and horizontally, respectively. Reproduced with permission from Thompson et al. (2010) Vet. Ophthalmol. 13(3):151–157.

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In primates, the entire global microcirculation and most of the orbital circulation are supplied via the internal carotid artery, which gives rise to the internal ophthalmic artery. In domestic animals, however, the external ophthalmic artery provides most of the ophthalmic circulation. In the adult cat, a single central retinal artery, such as that in the human eye, does not exist. Instead, in the cat, small arteries enter the eye from around the optic disc. In the posterior part of the feline globe, on either side of the optic nerve, the posterior ciliary arteries are observed. These are derived from the external ciliary arteries, which provide most of the blood supply to the various structures of the eye, except the optic nerve. For the latter, nutrition is provided by the internal ophthalmic artery, a rather small artery in the cat. Just as in humans, the retinal vasculature of cats is classified as holangiotic. It consists of arterioles and venules located on the surface of the retina, and the diameter of vessels decreases with distance from the optic nerve head (17) (see Figure 1a for normal cat retinal vasculature). The postnatal structural development of the cat retina is rather slow (25, 26). The photoreceptor outer segments grow continually in length until day 43 postpartum, whereas inner segments continue to elongate until at least 5 months postpartum (27). Just as in humans, the feline eye is not functionally or structurally mature at birth (for details, see Reference 28). Not until age 5–20 months postpartum are all parts of the retina of the cat completely adultlike in structure. Functionally, however, the feline retina matures somewhat earlier. Using electrophysiological functional testing procedures, it has been shown that the electroretinogram (ERG) is not completely adultlike until 2–3 months of age (29, 30). The cat has a binocular field of vision with an approximately 10-degree angle of vision on either side from the optic axis (17). The cat has been an important animal model in retinal electrophysiology and has been used for in-depth functional studies by using ERG. (See Figure 1d, upper recording, labeled wild type, for normal full-field flash ERG response in the cat.) Basic parameters have been worked out; thus, it was demonstrated that the a-wave reflects activity of the photoreceptors (31). The a-wave of the ERG results from the relative increase in sodium ions in the extracellular matrix as the photoreceptors hyperpolarize in response to light (32). As the visual signal is transmitted toward the inner retina to the bipolar cells, they depolarize, releasing potassium ions. The b-wave activity involves this movement of potassium between the bipolar and the Müller cells (32, 33). Actually, it was shown recently that ON-center bipolar cells contribute mainly to the b-wave of the ERG (34). Finally, the c-wave (not shown in Figure 1d) is generated mainly by hyperpolarization of the apical membrane of the RPE (35) and is the summed response of a slow positive wave (derived from the RPE) and a slow negative wave owing to responses generated by Müller cells in the inner retina. Owing to the rather impressive similarities between the cat and the human eye, the cat species has been used extensively in basic research on retinal and visual function and structure as well as on the entire optical system, including the visual tracts and visual cortex. During recent years, this wealth of basic knowledge has been extended to include specific disease processes shown to affect the cat eye, many of which have a counterpart in human ophthalmology. This makes the cat an extremely valuable species for further ophthalmic disease characterization, especially to advance new treatment regimens, particularly allowing for chronic therapeutic trials.

FELINE DISEASES OF IMPORTANCE FOR COMPARATIVE OPHTHALMIC STUDIES Cats have in general been considered a rather healthy species when it comes to ophthalmic diseases. However, with the recent increase of interest in purebred animals and the use of inbreeding in strains of cats, hereditary diseases affecting the eye have increased significantly in the feline population. Some hereditary ophthalmic diseases of domestic felines that have been rather well www.annualreviews.org


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characterized during recent years are the lysosomal storage disorders, congenital glaucoma, and neuroretinal degenerative diseases. Some of the most important feline models for these diseases are hereby further summarized.

Lysosomal Storage Disorders


Lysosomal storage disorders are recessively inherited, inborn errors of metabolism in which a deficiency of one or more enzymes causes accumulation of a substrate—lipids, glycoproteins, or mucopolysaccharides—within the intracellular organelles of the lysosome. Some specific diseases that affect cats are mucopolysaccharidosis (MPS) types I, VI, and VII, GM1 and GM2 gangliosidosis, a-mannosidosis, and mucolipidosis II. These disorders were first discovered in humans and only later noted in cats. They usually develop during the cat’s first year of life and ultimately lead to severe visual problems and often also mental changes. The severe clinical signs of these disorders cause significantly reduced quality of life for the affected individuals (36). MPS type I results from a deficiency of a-L-iduronidase (IDUA) and has been observed in domestic shorthaired cats. Genetic characterization of IDUA cDNA identified a three–base pair (bp) deletion in the coding region of the gene, which generates the loss of one aspartic acid residue (37). Transient expression assays demonstrated a lack of IDUA activity from peptide generated from the aberrant message (37). The disorder leads to mental retardation, growth abnormalities, and a shortened life span in affected cats (38). Marked corneal cloudiness is typical for the disorder owing to intracytoplasmic vacuoles present in fibroblasts in the cornea, but also in sclera, choroid, conjunctiva, trabecular meshwork, iris stroma, and ciliary processes (39). In one study including affected cats (39), inclusions occupied most of the cell’s cytoplasm but did not result in hypertrophy. The cardiovascular system, bone marrow, cartilage, and bone as well as smooth and skeletal muscle were all affected by the disorder. Psychomotor testing in cats with MPS I is difficult owing to the corneal clouding and the severe skeletal lesions, such as fusion between the atlantooccipital joint and cervical intervertebrate articulations. Cats with the MPS I disorder were observed for 2.5 years before they also demonstrated neurologic abnormalities. MPS VI is characterized by a deficiency of the enzyme aryl-sulfatase B (ARSB). The disorder causes growth retardation, coarse facial features, corneal opacity, and skeletal deformities in affected individuals (40, 41). Affected individuals exhibit abnormal lysosomal storage in neurons and in glia cells distributed throughout the cerebral cortex. Affected structures in the eye are cornea-sclera, conjunctiva, uvea, and the RPE. Two mutations have been characterized in the cat (42, 43). Homozygosity for the L476P mutation generates a severe phenotype, including dwarfism, facial dysmorphia, and corneal clouding. A second mutation, D520N, in either a homozygous or compound heterozygous state (L476P/D520N), results in less severe symptoms (43). The L476P mutation is widespread in the Siamese breed (44). ARSB-adeno-associated gene therapy has demonstrated correction of corneal clouding of MPS VI–affected felines (45). MPS VII has been described to affect domestic shorthaired cats and results from a b-glucuronidase deficiency. Ophthalmic structures with lysosomal inclusions are observed in the cornea and the RPE (46). Sequence analysis of cDNA from affected individuals identified a polymorphism predicting an E351K substitution (47). a-Mannosidosis is characterized by a deficiency of lysosomal a-mannosidase. It has been observed in Persian and domestic shorthaired cats (48). Berg et al. (49) characterized a 4-bp deletion (1749_1752delCCAG), which leads to the introduction of a premature stop codon as the causative mutation in felines. The disorder leads to mental retardation, recurrent infections, skeletal changes, and hearing impairment (48). Affected ophthalmic structures are the cornea, lens, retina, and choroid (50). 162

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The gangliosidoses, GM1 and GM2, are lysosomal storage enzyme diseases that demonstrate neurodegenerative and ocular pathology in domestic shorthaired, Siamese, and Korat cats (51, 52). GM1 is characterized as a deficiency of lysosomal b-galactosidase, which results in accumulation of the GM1 ganglioside in neurons (52). The causative mutation in felines was characterized by Martin et al. (53) as a missense substitution resulting in an arginine-to-proline substitution (R482P) at a position where amino acid substitution was demonstrated to cause GM1 in humans. Clinical signs of the disease are observed early, at two to three months of age. Ocular structures that are affected in the disorder are mainly the cornea, lens, and retina. The prognosis is serious for cats with the disorder, because the disorder is progressively and systemically debilitating (54). GM2 gangliosidosis results from aberrant degradation of the GM2 ganglioside, which in a healthy individual is brought about by the coordinated action of three gene products, the a and b subunits of b-N-acetylhexosaminidase and the GM2 activator (GM2A) protein (55–58). Mutations in any of these enzymes result in accumulation of the GM2 gangliosides in the lysosomes of affected neurons, which results in progressive deterioration of the central nervous system. Diffuse clouding of the cornea is reported in GM2-affected felines. Mucolipidosis II (I-cell disease) is caused by deficient activity of the enzyme N-acetylglucosamine1-phosphotransferase, which leads to a failure to internalize enzymes into lysosomes. Mucolipidosis II has been reported in a domestic shorthaired cat and involves mainly the cellular structures of the eyelids and retina (59). The cat is the only known animal model for this defect.

Glaucoma Glaucoma is considered to represent a large, diverse group of ocular abnormalities (18). In all species, this large group of serious disorders leads, through a common pathway, to optic nerve and retinal pathology, which results in vision loss or blindness. The most important risk factor for development of the disease entity in both humans and animals is an increase in IOP. However, elevated IOP alone, in the absence of changes in the optic nerve or in the retina, is termed ocular hypertension, to distinguish the disorder from manifest glaucoma. Glaucoma cannot be excluded based on a single low or a normal IOP, because IOP may fluctuate considerably both within and between days. There are circadian fluctuations in the IOP of normal cats on the order of 4 mm Hg (60) [normal IOP in cats tested using Tonopen, applanation tonometry: 19.7 6 5.6 mm Hg; using Tonovet, rebound tonometry: 20.74 6 0.5 mm Hg (18)]. Some difficulties in the evaluation of IOP in cats stem from the facts that: (a) the disease often has an insidious onset and a gradual and very slow progression; (b) glaucoma is often secondary to other ophthalmic disease processes, such as uveitis; (c) a single measurement of IOP during an eye examination may not accurately reflect the cumulative IOP to which the cat eye has been exposed; (d) degeneration of the ciliary body may limit aqueous production, ultimately lowering IOP; and (e) a reduced IOP is often obtained when the bulb has become distended owing to longterm increase in IOP, which results in a low reading of the IOP measurement. Primary glaucoma in cats may be congenital. Onset and clinical signs of disease appear shortly after birth but also may be first recognized later in life, especially in middle-aged and older cats. Feline open-angle glaucoma has been reported in cat breeds (e.g., in the Siamese and Burmese) but also in domestic shorthaired cats (61–63). Primary, narrow- to closed-angle glaucoma has also been reported in a small group of Burmese cats (64). Feline congenital glaucoma was reported in the literature as solitary cases of various ocular malformations, such as microphakia, ectopia lentis, pectinate ligament dysplasia and iridoschisis, multiple iridociliary cysts, and persistent pupillary membranes (63, 65). Cats were www.annualreviews.org


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obtained for breeding, and a colony was later established in the United States. Affected cats have bilaterally rather symmetric, slowly progressive glaucoma with elongated ciliary processes, globe enlargement, and spherophakia (18). Postnatal development of the ciliary structures is arrested in affected cats, and these cats maintain the immature development of the aqueous outflow pathways (described earlier).

Neuroretinal Degenerative Diseases


The rdAc cat model for retinal rod-cone degeneration. Narfström (66) first discovered retinal degeneration in the Abyssinian cat, designated rdAc, in a male cat used for breeding and show purposes that was brought to a small animal ophthalmology clinic in Sweden in 1981. The author later observed an extremely high prevalence for the disorder among the Swedish Abyssinian cat population, with 45% of individuals affected among the examined animals. A cooperative effort among the breeders, including reports of affected individuals and development of a population database, was instrumental in decreasing the prevalence of the disease mutation to its current frequency of 7% (67). With the causative mutation identified, breeders currently use genetic testing to make informed decisions relative to mating strategies. Affected cats are born with normal vision, and their retinas appear normal upon ophthalmoscopy (23). Clinical signs of disease are not observed until they are 1.5–3.5 years old and include color changes, such as a dark discoloration in the peripheral parts of the tapetal fundus. These abnormalities become more marked and generalized with time (Figure 1a,b). Later, the midperipheral and the peripheral parts of the fundus become hyperreflective, an indication of severe retinal degeneration, with changes leading to complete retinal atrophy. The retinal vasculature is also involved in the disease; retinal arterioles and venules become attenuated (Figure 1c). The end stage is a completely atrophic fundus with generalized hyperreflectivity, a pale optic nerve head, and a lack of visible vasculature. The disease is always bilaterally symmetrical and leads to blindness. Functional studies using ERGs were performed in affected, carrier, and normal age-matched cats to characterize the disease (68). Results showed that the a- and b-waves of the ERG were affected early in the disease process, which indicated that the photoreceptors were primarily involved in the disorder. It was also observed that an affected group of kittens could not be differentiated functionally by ERG from a group of normal control cats until they were approximately eight months old (69). Further, heterozygous cats, although they appeared ophthalmoscopically normal throughout life, had slightly reduced, dark-adapted b-wave amplitudes at high–light intensity stimulation in comparison to unrelated normal cats (70). Changes in the cone-mediated off-responses of the ERG were also described (71). Morphologically, a significant reduction in photoreceptor numbers, as observed by light microscopy, was usually not observed until the moderately advanced stage of disease, in 2–3year-old affected cats (72). Using electron microscopy, however, researchers observed early changes already at the age of five weeks: Individual photoreceptor outer segment lamellar discs were severely disorganized. The affected rods progressively became disrupted, and at the age of five to eight months (73) there was degeneration of entire rods. Cones appeared to be spared in eight-month-old affected animals, but degenerative changes in cones were observed approximately six months later (74). Using immunocytochemical studies in young cats homozygous for the defect and in adult cats at different stages of the disease, researchers found that interphotoreceptor retinoid-binding protein (IRBP) was reduced at an early stage, well before there were any signs of disease (71, 75). From the standpoint of its function both as a retinoid-binding and as a fatty-acid binding 164

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protein, an early reduction in IRBP could have highly deleterious effects on the photoreceptor cells, promoting the degeneration seen in affected Abyssinian cats. With the aim of comparing rdAc with human RP electrophysiologically, studies were initiated with analysis of a series of intensity-amplitude ERG results from young affected cats aged 8–104 weeks and compared with results from age-matched normal cats (76). The results were fitted to the Naka-Rushton equation by means of a mathematical package on the University of London mainframe computer system. The analysis showed that the amplitude of the maximum dark-adapted b-wave was reduced significantly in the older affected cats but that the value of k, a variable inversely equivalent to retinal sensitivity, was only slightly reduced by the retinal degenerative process. Further, ERG recordings from 55 age-matched heterozygous and homozygous cats were compared using a graphical representation of results, and maximum darkadapted ERG b-wave:a-wave ratios were calculated. These methods were helpful in the diagnosis of affected versus nonaffected cats at an early age, long before funduscopic changes could be observed in homozygous cats (77). Simultaneously, through analysis of component factors and studies of testing scores (78), a single parameter was found that was diagnostic for the disease: the amplitude of the a-wave, 10 s after maximum flash stimulus. A cut-off point for normal/affected cats could be described that was diagnostic in the rdAc cat disease but speculated to be useful also in other forms of hereditary retinal degenerations, such as in human RP (79). It was also of interest to study the regional variation of rhodopsin distribution in the retina and to compare rhodopsin levels with rod-mediated function, previously assessed by full-field ERGs. Studies were performed using imaging fundus reflectometry (80). Rhodopsin kinetics at different stages of disease were similar to those of normal cats, but there was a 20% reduction of rhodopsin levels in young cats homozygous for the defect. This occurred well before there were any funduscopic signs of disease, findings that were similar to those observed in certain types of RP patients (81). Another type of imaging and simultaneous functional testing was performed in groups of normal and rdAc-affected cats by using scanning laser ophthalmoscopy (SLO) and multifocal ERG (82). Just as in human RP, the central fundus was spared functionally until a comparably late stage in the disease. It was also possible to map the disease spatially very efficiently using the multifocal ERG, although there were some inherent problems with light scatter, owing to the reflectile feline tapetal cells that cover much of the fundus (83). It is well known that the generalized vascular attenuation that can be observed in the fundus is a common phenomenon in retinal degenerative disease processes. Whether the retinal choroidal circulation is also involved has long been an open question. To specifically study whether the retinal blood flow decreases with progression of disease and how retinal metabolism in the disease process is affected, in vivo studies of local blood flow in different parts of the eye were performed using radioactively labeled microspheres (84). Retinal formation of lactate and uptake of glucose were also determined. It was shown that retinal blood flow was severely decreased, whereas the choroidal microcirculation was not significantly affected by the retinal degeneration (85). Retinal and choroidal vascular integrity were further analyzed using indocyanine green (ICG) and fluorescein (FL) angiography as well as SLO visualization of the passage of ICG, mainly in choroidal structures, and of FL in retinal circulation (86). Further clarification was obtained with studies of retinal metabolism, blood flow, and oxygenation in a group of normal and rdAc-affected cats (87). Intraocular ERG recordings were performed with simultaneous measurements of intraretinal oxygen tension at various retinal depths using microelectrodes that penetrated the eye. Cats were also given a high or low concentration of oxygen to breathe, during which retinal vessels were photographed and later www.annualreviews.org


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analyzed as to vascular width in relation to oxygenation. Through this research it was found that retinal vessels continue to autoregulate throughout the disease process and provide oxygen to the photoreceptor cells. Furthermore, retinal oxygen tension was reduced in relation to disease stage and was found to correlate very precisely to ERG a-wave amplitudes. This latter finding was observed long before there was any generalized retinal degeneration and abolishment of photoreceptor cells (88). It was thus apparent that early functional changes precede morphological alterations in the rdAc disease. The whole genome sequence of the cat was performed on an affected member of a cat pedigree segregating for the rdAc retinal degeneration. The first partial genome assembly of the cat, named Cinnamon, did not identify the causative SNP for rdAc, but it provided the genomic resources for mapping and sequence analysis of candidate genes. Sequence traces in the growing database of the 1.93 genome assembly were searched using human or dog genes plus flanking sequences to identify additional feline short tandem repeat loci in mapped regions of interest for fine-scale mapping (7). The causative mutation for rdAc was characterized in the centrosomal protein 290 (CEP290) gene, as a SNP in intron 50 that generates a strong canonical splice-donor site, used to the exclusion of the wild-type site in affected individuals. The use of this new splice site results in a 4-bp insertion to the transcript, which introduces a frame shift and premature stop codon, truncating the putative peptide by 159 amino acid residues (7). The CEP290-identified disease is a member in a class of related pathologies known as ciliopathies, which often affect multiple organ systems. They abrogate the function of primary cilia, which are specialized sensory organelles in mammals that respond to sensory stimuli and initiate signaling pathways (for a review, see Reference 89). The rod photoreceptor, a modified primary cilium, is dependent on a specialized method of transport, intraflagellar transport, which delivers phototransduction proteins and lipids synthesized in the inner segment to the outer segment of the cell, where they are utilized in assembly of photoreceptor discs. The CEP290 peptide is critical as one of the intraflagellar transport proteins. It is thus not surprising that functional alterations as shown for the rdAc disease preceded structural changes. A population genetic survey of cat breeds demonstrated that the rdAc mutation is relatively widespread and is present in 16 of 43 breeds examined, including an alarmingly high frequency (35%) of the risk allele in Siamese and Siamese-related breeds (Oriental shorthair, Colorpoint shorthair, Balinese, Javanese) (90). The CEP290 mutation also displayed a worldwide distribution and was present in individuals observed on three continents. Clinical evaluations demonstrated complete concordance between rdAc pathology and homozygosity of affected individuals with the CEP290 risk allele (90). Mutations in the homologous human CEP290 gene are a common cause of human blindness and appear in approximately 30% of patients with Leber congenital amaurosis, a defect observed in the newborn causing severe visual impairment or blindness (91). Additionally, mutations in CEP290 are causative of several rare, severe, early onset syndromic diseases in humans, including Joubert, Senior-Loken, Meckel-Gruber, and Bardet-Biedl syndromes, which cause blindness, mental retardation, and kidney failure, among other severe clinical symptoms (92–96). rdAc-affected cats do not suffer from these additional severe pathologies (23) and thus provide a stable model for studying therapeutic interventions of the progressive retinal degeneration. The Rdy cat model for retinal cone-rod dysplasia. A second hereditary retinal dystrophy was observed in a single Abyssinian male cat in England. A colony was generated from this single individual. The retinal dystrophy was studied extensively on a phenotypic level for many years (97). As photoreceptors never develop normal morphology, the disease was termed a rod-cone dysplasia (Rdy). An autosomal dominant mode of inheritance was shown for the defect. 166

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Affected kittens can be differentiated from normal individuals by 4–5 weeks of age (8). The pupillary light reflexes are slower in affected individuals than in normal cats, and the pupils are slightly dilated in affected kittens. By 6 weeks of age, a pendular type of nystagmus (fast movement of the eyes, from side to side) can often be observed. By 7–8 weeks of age, there are ophthalmoscopic changes, mainly in the area centralis region. There are a mottled appearance and a grayish discoloration in this area, which extend peripherally within only a few weeks. By 12 weeks, there is generalized hyperreflectivity of the tapetal fundus, slight depigmentation of the nontapetal area, and generalized severe vascular attenuation. The disease leads rapidly to visual impairment, which is observed within the first 4 months of life. Electrophysiologically, the photoreceptors never reach functional maturation. At 7 weeks of age, the ERG demonstrates markedly reduced a- and b-waves of the dark-adapted ERG, whereas light-adapted ERGs are mainly nonrecordable (98). In the same studied individuals at 12 weeks of age, ERGs demonstrate nonrecordable light-adapted responses; dark-adapted responses show a deflection (negative-going wave), replacing the abnormal b-wave. The latter increases in negativity with increasing light-intensity stimulation (8). The ERG becomes nonrecordable thereafter. Functionally, it is thus apparent that the cone system is primarily affected in the disease. Morphological studies have shown that affected individuals already demonstrate abnormal and retarded photoreceptor development at 22 days as observed by electron microscopy. Neither rods nor cones develop normally. There is defective synaptogenesis, with degeneration beginning in the central retinal regions and progressing toward the periphery (98, 99). Recently it was clarified that the Rdy defect is a cone-rod dystrophy (8): The earliest ophthalmoscopic changes are observed in the area centralis region, an area of the feline retina in which the concentration of cones is high in comparison with that of rods. Further, as the electrophysiological studies described above have shown, the cone system is nonfunctional at a time when functional activity is still induced by the rod system (8), a fact previously also noted in the original electrophysiological studies performed in the Rdy cat (98). However, at the time of the initial studies, the disease was designated a rod-cone dysplasia, since changes were thought to affect the rod system earlier than the cone system, but with later affection of both types of photoreceptors. The causative mutation for Rdy was characterized as a single-base deletion in the last exon of the cone rod homeobox (CRX) gene (n.546delC), which introduces a frameshift and premature stop codon, resulting in the truncation of the C terminus by 38% relative to the normal wild-type CRX protein (Figure 2) (8). In humans, the CRX peptide has been characterized as a transcription

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3

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Figure 2 CRX protein structure in Felis catus. Comparison between (a) the wild-type feline CRX protein and (b) the putative truncated CRX protein. Abbreviations: , start codon; X, stop codons; Y, exon splice junctions. Shaded boxes: protein domains, defined as the homeobox, the WSP domain, the transcriptional transactivation domains 1 and 2 (TTD1 and TTD2), and the OTX tail (103, 128). Domains are drawn to scale. Reproduced from Reference 8.

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factor critical in activating genes involved in photoreceptor development and maintenance (100– 102). The putative truncated peptide retains domains previously characterized in humans as responsible for promoter binding and nuclear localization, but it lacks the two domains critical for transcriptional transactivation (103). cDNA analysis of Rdy individuals demonstrated that both CRX wild-type and mutant mRNA are transcribed and persist in vivo (8). In addition, Deckman et al. (104) recently demonstrated, by Western blot analysis, that retinal tissue from affected Rdy cats exhibits both wild-type and truncated CRX proteins. A CRX hemizygous (þ/null) mouse model reportedly develops normal photoreceptor cells (105), which suggests that haploinsufficiency of CRX peptide in affected individuals is not the causative mechanism of pathology. We thus hypothesized that the truncated CRX peptide possibly could compete with binding and activity of the wild-type CRX transcription factor in the Rdy cat. Electrophoretic mobility shift assays have since demonstrated binding of the truncated feline CRX peptide with two CRX-binding recognition sequences located in the rhodopsin promoter region (Ret-4 and BAT-1) (101, 106, 107). The development of an assay to measure the transcriptional transactivation activity of wild-type and Rdy-truncated CRX peptides is currently ongoing. These studies have improved our understanding of the molecular mechanisms underlying feline Rdy pathology. The Rdy model provides a valuable large mammal model to explore protein function and therapeutic interventions that have potential relevance not only to CRX-related disorders but for other autosomal dominant conditions in humans, as reviewed by Seyhan (108). To date, there are no large mammal models for developing therapies, gene-therapy based or nonviral based, for autosomal dominant vision disorders. Large mammals have a longer life span and ocular structures more similar to humans than rodent models (109). Therefore, the feline Rdy model is one of the first large mammal models that can be used to extend the initial mouse (impdh1, rho, and peripherin 2) and rat (rho) autosomal dominant therapy studies (110–115).

MOLECULAR MECHANISMS AND TREATMENT MODALITIES An understanding of the molecular mechanisms underlying pathology is the first step in developing therapeutic treatments to prevent, restore, or minimize the effects of genetic disease. Determining the biological mechanism of pathology in blinding diseases in animal models for Leber congenital amaurosis, RP, and cone-rod dysplasia has led to successful therapeutic genetherapy treatments (116), as witnessed by the recombinant adeno-associated virus (rAAV)based treatments for canine RPE65 autosomal recessive disorder (117, 118). Two murine autosomal dominant mutations in impdh1 (110) and rho (111–113, 119) have been successfully treated through suppression of mutant alleles and replacement with wild-type transcript, using

Figure 3

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Integration of green fluorescent green (GFP) positive donor cells into the neuroretina and the retinal pigment epithelium (RPE) layer. Immunofluorescence microscopy at two weeks posttransplantation of GFP-labeled feline neural precursor cells. (a) Radially oriented GFPþ cell of bipolar morphology in the inner nuclear layer (INL), extending one process to the outer plexiform layer and the other into the inner plexiform layer (IPL). (b) Confocal reconstruction of the cell in a. (c–e) Images of GFPþ cellular profiles from the same area of two consecutive sections. One profile (small arrow) is radially oriented and appears to contribute cytoplasmic extensions to the outer limiting membrane. Another profile (large arrow) lies in the vicinity of the outer plexiform layer and appears to be horizontally oriented. (e) Confocal reconstruction shows that the cytoplasmic extensions at the outer limiting membrane seen in c derive from the radially oriented profile seen in d. (f) GFPþ profile (arrow) that appears to be in the RPE layer. Reproduced with permission from Reference 126. Abbreviation: ONL, outer nuclear layer.

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AAV-based shRNA or RNAi vector therapy, respectively. A third murine model for ADRP used a similar siRNA AAV-mediated vector therapy to suppress the expression of mutant peripherin 2 and to express the introduced wild-type peripherin 2 (114). These examples of AAV-based gene therapies have demonstrated successful rescue from photoreceptor cell degeneration in both autosomal recessive and dominant vision disorders. Nonviral gene delivery methods, which employ physical and chemical methods for transgene introduction (electroporation, iontophoresis, liposomes, polymers, compacted nanoparticles), are also being investigated in animal models for the treatment of autosomal dominant vision disorders (for review, see Reference 120). Finally, an example of non-DNA therapy is the oral administration of a trafficking protein, curcumin, in rat for a rho autosomal dominant disorder (115). This nongene-therapy treatment method has demonstrated the restoration of a functional vision cascade in the rat. Recent developments in stem cell therapy show promise for the treatment of retinal degenerative disease. Cat models of hereditary retinal dystrophies will be especially valuable to examine the efficacy of these developing methodologies over extended time periods. Stem cells derived from three sources—embryonic, adult, and induced pluripotent stem cells—have shown promise to repair or replace neuroretinal cells (for review, see Reference 121). The secretion of neurotrophic factors from transplanted adult stem cells has demonstrated a protective role for degenerative conditions of the RPE, photoreceptors, and ganglion cells. Induced pluripotent stem cells, which are generated from the reprogramming of adult somatic cells (for review, see Reference 122), offer the increased advantage of development of patient-specific, isogenic lines, thereby decreasing the chance of host rejection of introduced cells, and eliminate ethical issues related to the use of cells derived from human embryos. The generation of both photoreceptor and retinal pigment epithelial phenotypes from induced pluripotent stem cells has recently been demonstrated (123, 124). The first human clinical trials examining the efficacy of human embryonic stem cells for two retinal diseases associated with degeneration of the RPE, Stargardt’s macular dystrophy and age-related macular degeneration, recently reported encouraging results (125). So far, the rdAc model has been used for transplantation of neural precursor cells (Figure 3) and for fetal retinal sheet transplantation studies (Figure 4) (126, 127). Additional work is needed to

a

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Figure 4 Scanning-laser ophthalmoscopy images were obtained from the nasal part of the central tapetal fundus illustrating the fundus appearance of retinal sheet allograft–transplanted cats during (a) native infrared imaging, (b) fluorescein angiography (FA), and (c) indocyanine green angiography (ICGA). (a) Autofluorescence image obtained before injection of dye, (b) arterio-venous phase of FA depicting mainly the retinal circulation. (c) ICGA image allowing visualization of both the choroidal and the retinal circulation. Reproduced with permission from Seiler et al. (2009) Vet Ophthalmol. 12: 158-169.

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optimize and promote long-term graft survival and cellular connectivity between host and transplant tissues. An important finding in this work is, however, that the size of the cat eye makes the surgical approach, and the clinical follow-up using more advanced ophthalmic studies (Figure 4) and ERG examinations, more analogous to those used with human patients. This is certainly an advantage over using rodents in this type of research.


CONCLUSIONS The development of genomics resources in the cat has brought to reality the promise of the cat as a strong comparative model for prevention, diagnostics, and treatment studies. Long-lived, large animal models of homologous human disease are critical for testing the safety and efficacy of new therapeutic modalities. The domestic cat is emerging as a promising resource of phenotypically defined genetic variation of important biomedical significance. Feline models for specific human ophthalmic diseases may thus become the necessary last step in the process of translational research, i.e., the ultimate link in the complicated research process of evolving from bench to bedside.

DISCLOSURE STATEMENT The authors are not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review.

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177

Annual Review of Animal Biosciences


Volume 1, 2013

Contents After 65 Years, Research Is Still Fun William Hansel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Cross Talk Between Animal and Human Influenza Viruses Makoto Ozawa and Yoshihiro Kawaoka . . . . . . . . . . . . . . . . . . . . . . . . . 21 Porcine Circovirus Type 2 (PCV2): Pathogenesis and Interaction with the Immune System Xiang-Jin Meng . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Evolution of B Cell Immunity David Parra, Fumio Takizawa, and J. Oriol Sunyer . . . . . . . . . . . . . . . . . . 65 Comparative Biology of gd T Cell Function in Humans, Mice, and Domestic Animals Jeff Holderness, Jodi F. Hedges, Andrew Ramstead, and Mark A. Jutila . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Genetics of Pigmentation in Dogs and Cats Christopher B. Kaelin and Gregory S. Barsh . . . . . . . . . . . . . . . . . . . . . . 125 Cats: A Gold Mine for Ophthalmology Kristina Narfström, Koren Holland Deckman, and Marilyn Menotti-Raymond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 Comparative Aspects of Mammary Gland Development and Homeostasis Anthony V. Capuco and Steven E. Ellis . . . . . . . . . . . . . . . . . . . . . . . . . . 179 Genetically Engineered Pig Models for Human Diseases Randall S. Prather, Monique Lorson, Jason W. Ross, Jeffrey J. Whyte, and Eric Walters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 Accelerating Improvement of Livestock with Genomic Selection Theo Meuwissen, Ben Hayes, and Mike Goddard . . . . . . . . . . . . . . . . . . 221

vi

Integrated Genomic Approaches to Enhance Genetic Resistance in Chickens Hans H. Cheng, Pete Kaiser, and Susan J. Lamont . . . . . . . . . . . . . . . . . . 239 Conservation Genomics of Threatened Animal Species Cynthia C. Steiner, Andrea S. Putnam, Paquita E.A. Hoeck, and Oliver A. Ryder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261


Phytase, A New Life for an “Old” Enzyme Xin Gen Lei, Jeremy D. Weaver, Edward Mullaney, Abul H. Ullah, and Michael J. Azain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 Effects of Heat Stress on Post-Absorptive Metabolism and Energetics Lance H. Baumgard and Robert P. Rhoads Jr. . . . . . . . . . . . . . . . . . . . . 311 Epigenetics: Setting Up Lifetime Production of Cows by Managing Nutrition R.N. Funston and A.F. Summers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339 Systems Physiology in Dairy Cattle: Nutritional Genomics and Beyond Juan J. Loor, Massimo Bionaz, and James K. Drackley . . . . . . . . . . . . . . 365 In Vivo and In Vitro Environmental Effects on Mammalian Oocyte Quality Rebecca L. Krisher . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 The Equine Endometrial Cup Reaction: A Fetomaternal Signal of Significance D.F. Antczak, Amanda M. de Mestre, Sandra Wilsher, and W.R. Allen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 The Evolution of Epitheliochorial Placentation Anthony M. Carter and Allen C. Enders . . . . . . . . . . . . . . . . . . . . . . . . . 443 The Role of Productivity in Improving the Environmental Sustainability of Ruminant Production Systems Judith L. Capper and Dale E. Bauman . . . . . . . . . . . . . . . . . . . . . . . . . . 469 Making Slaughterhouses More Humane for Cattle, Pigs, and Sheep Temple Grandin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491

Contents

vii

Circulating tumor DNA: a resuscitative gold mine?

Cancer epidemiology. Gold mine in east Germany.

Alpinia: the gold mine of future therapeutics.

Niacin Alternatives for Dyslipidemia: Fool's Gold or Gold Mine? Part II: Novel Niacin Mimetics.

Green Synthesis of Gold Nanoparticles by a Metal Resistant Arthrobacter nitroguajacolicus Isolated From Gold Mine.

Psychiatric nursing researchers: canaries in the gold mine?

Health information on the Internet: gold mine or minefield?

Determining the radon exhalation rate from a gold mine tailings dump by measuring the gamma radiation.

Lipopeptides from Bacillus and Paenibacillus spp.: A Gold Mine of Antibiotic Candidates.

Half-century archives of occupational medical data on French nuclear workers: a dusty warehouse or gold mine for epidemiological research?

Rationale for a program in community ophthalmology.

Tests for malingering in ophthalmology.

Editorial: Neuro-ophthalmology and medical ophthalmology. A dialogue.

Toxicity in semiarid sediments influenced by tailings of an abandoned gold mine.

Ophthalmology.

Ophthalmology.

Ophthalmology.

Ophthalmology.

Ophthalmology.

Ophthalmology.

Ophthalmology.

Modelling of Radiological Health Risks from Gold Mine Tailings in Wonderfonteinspruit Catchment Area, South Africa.

Characterizing Particle Size Distributions of Crystalline Silica in Gold Mine Dust.

Estimation of mercury content in tailings of the gold mine area of Poconé, Mato Grosso, Brazil.