1. Identification of a new animal model of human retinal dystrophy;characterization of the mutation causative of retinal dystrophy (Rdy) in Abyssinian cats. We have identified a new large animal model for human retinal disease by characterizing the mutation for a Rdy first described in an Abyssinian cat Rdy, an autosomal dominant, early-onset degeneration of the rod and cone systems. Disease expression in Rdy cats is comparable to young patients with congenital blindness (Leber congenital amaurosis (LCA) or Retinitis Pigmentosa,(RP)). Examination of 18 candidate genes of AD RP identified significant linkage (theta = 0, LOD= 5.8) to the candidate gene, cone-rod homeobox (CRX). Sequence analysis of genomic DNA and cDNA identified a single base pair deletion in exon 4 (n.546delC) of CRX in affected individuals, which generates a frame-shift in the transcript, introducing a premature stop codon truncating the putative CRX peptide and eliminating the critical transcriptional activation region. The CRX gene is crucial in early retinal development and maintenance, encoding a transcriptional activator controlling photoreceptor gene expression, development and maintenance. Mutations in the CRX gene have been characterized in several human retinal diseases exhibiting a wide range of clinical phenotypes. First described relative to its role in autosomal dominant cone rod dystrophy, mutations in CRX have since been described which are also causative of autosomal dominant RP and autosomal dominant and recessive LCA. The availability of a large animal model for a dominantly inherited eye disorder is particularly valuable. Gene related therapies prove a particular challenge with regard to dominant disorders as (1) the one functional gene may result in a haplo-insufficiency of product (2) a truncated or aberrant protein product may be causative of the pathology. 2. Identification of an domestic cat model of primary congenital glaucoma (PCG): mapping of PCG in a multi-generational cat pedigree. Glaucoma is a leading cause of vision impairment and vision loss worldwide. There is a need for an improved understanding of the pathogenesis and genetics of this condition that will lead to improved therapeutics for glaucoma. Animal models are critical in this search, especially large-eyed animal models. We collaborate with Matthew Ellinwood, Iowa State School of Vet. Medicine who maintains a pedigree of domestic cats (outbred) that segregates for human primary congenital glaucoma, a rare autosomal recessive ocular disorder that is generally observed at birth. The feline primary congenital glaucoma model is the only extant large-eyed primary congenital glaucoma model, and as such is an extraordinarily valuable as well as a unique resource. Linkage mapping in the large multi-generational pedigree identified significant linkage to the candidate gene LTBP2 (theta=0, LOD=18.08), identifying a new large animal model for human congenital glaucoma. 3. Wide-spread retinal degenerative disease mutation (rdAc) discovered among a large number of popular cat breeds. We have recently reported on the causative mutation for rdAc, in the CEP290 gene. Mutations in CEP290 have been reported in syndromic retinopathies in humans (Jouberts, Senior-Loken, Meckel-Gruber, Bardet-Biedl), as well as 30% of patients with Lebers congenital amaurosis. We conducted a survey among 41 cat breeds (846 individuals) to assess the possible incidence, frequency and clinical consequence of rdAc. The rdAc allele displays widespread distribution, observed in 16/43 (37%) breeds, exhibiting a high allele frequency (33%) in North American and European Siamese populations. Clinical evaluations demonstrated high concordance between rdAc pathology and the CEP290 (IVS50 + 9T greater than G) homozygous genotype (P = 1.1E-6), demonstrating clinical disease similar to affected Abyssinians/Somalis. Based on principals of population genetic theory,(Hardy-Weinberg equilibrium), 7-13% of the Siamese breed group could be affected with rdAc, and would be expected to be severely visually impaired by middle age, at approximately 5-6 years of age. This retinal degeneration is heretofore unreported in breeds other than the Abyssinian/Somali and poses a significant health risk particularly in the Siamese breed group. Alertness of the veterinary community and the present availability of commercial diagnostic testing could synergistically enable breeders to reduce the incidence of rdAc blindness in purebred cat populations. 4. On-going mapping and characterization of genes in the melanogenic pathway causative of phenotypic variation in the domestic cat. The genetics of coat color is a longstanding and rich model system which has led to a deeper understanding of an array of diverse biological processes, including differentiation of neural crest derivatives, cell migration, pigment synthesis, intracellular organelle transport and cell signaling. Much of the molecular machinery used by the pigmentary system is either shared by, or homologous to, molecules used in other physiological pathways. We have continued our research in basic biology of coat color pigmentation in the domestic cat with the mapping of the Silver locus, identifying a new genomic location distinct from that of any known reported gene associated with silver or hypo-pigmentation in mammals. A demonstrated lack of linkage to SILV, the strong candidate gene for silver, led to the initiation of a genome scan utilizing two pedigrees segregating for silver coat color. Linkage mapping defined a genomic region for Silver as a 3.3 Mb region, (95.87 Mb- 99.21 Mb) on chromosome D2, (peak LOD=10.5, theta =0) which displays conserved synteny to a genomic interval between 118.58 and 121.85 Mb on chromosome 10 in the human genome. In the domestic cat, mutations at the Silver locus suppress the development of pigment in the hair, but in contrast to other mammalian silver variants, there is an apparently greater influence on the production of pheomelanin than eumelanin pigment. The mapping of a novel locus for Silver offers much promise in identifying a gene that may help elucidate aspects of the pheomelanogenesis, a pathway that has been very elusive. 5. Defining and mapping coat pattern genes: multiple genomic regions implicated in domestic cat pattern. Increased understanding of cellular processes in the melanocyte has direct application to our understanding of melanoma. Genes involved in pattern formation likely act early in development and are involved in melanocyte differentiation, development and migration. Developmental timing likely plays an important role. These genes/pathways/interactions are totally unknown, largely because pattern formation variation in the laboratory mouse, the major model of coat pigmentation is not available. The domestic cat, unlike other patterned mammals, provides polymorphism for coat pattern (striped, spotted, swirled and absence of pattern which is dominant over all pattern). We have mapped one locus, responsible for the Abyssinian form (termed the Ticked locus), to a 3.8 Mb region on cat chromosome B1. A second locus which controls the Tabby alleles has been mapped to a 5 Mb genomic region on an independent chromosome. One or more additional loci act as modifiers and create a spotted coat by altering mackerel stripes. On the basis of our results we hypothesize that mammalian patterned coats are formed by two distinct processes: a spatially-oriented developmental mechanism that lays down a species-specific pattern of skin cell differentiation and a pigmentation-oriented mechanism that uses information from the pre-established pattern to regulate the synthesis of melanin profiles.
