Naturally occurring animal models with outer retina phenotypes

Abstract

Naturally occurring and laboratory generated animal models serve as powerful tools with which to investigate the etiology of human retinal degenerations, especially retinitis pigmentosa (RP), Leber congenital amaurosis (LCA), cone dystrophies (CD) and macular degeneration (MD). Much progress has been made in elucidating gene defects underlying disease, in understanding mechanisms leading to disease, and in designing molecules for translational research and gene-based therapy to interfere with the progression of disease. Key to this progress has been study of naturally occurring murine and canine retinal degeneration mutants. This article will review the history, phenotypes and gene defects of select animal models with outer retina (photoreceptor and retinal pigment epithelium) degeneration phenotypes.

Figure 1

N-terminal regions of the wt (top) and nob2 (middle, bottom) α1F channel subunit. The amino acuid sequences starts at the transloation initiator, M, in exon 1. The area shaded in blue is replaced by the transposon sequence shown in red. The middle sequence shows the truncation after residue 25 (adapted from (Doering et al., 2008). In the bottom sequence, the stop codon was removed by alternative splicing.

Figure 2

The murine Cep290 gene. In the rd16 mouse, exons 35-39 are deleted, thereby removing 298 amino acids in the C-terminal half of CEP290. The deletion joins exons 34 and 40 in-frame. In the rdAc cat gene, a single nucleotide polymorphism (SNP) generates a new splice site (blue), resulting in the truncation of cat CEP290 by 159 residues. The protein has multiple coiled-coil motifs (shaded in gray).

Figure 3

The Chx10/Vsx2 gene. In the orJ mouse, a nonsense codon in exon3 truncates the transcription factor in the center of the Hox domain. OAR (named after the initials of otp, aristaless, and rax) is a 14-aa motif identified within the C-terminal region of several paired-like homeodomain-containing proteins.

Figure 4

The CNGB3 gene and protein. The cd phenotype of the German Shorthaired pointer is caused by a missense mutation (D262N) in exon 6 (TM domain 2). The CNGB3 protein has 6 TM domains, a pore region (P), and a cNMP binding site. In the Alaskan Malamute, the entire CNGB3 structural gene is deleted (the canine CNGB3 is presently unavailable, therefore the defect is modeled on the mouse CNGB3 gene).

Figure 5

The Crb1 gene and protein. An exon 9 frameshift leads to a truncation removing the TM domain (gray). The extracellular domain of CRB1 has laminin G (globular) motifs, multiple EGF-like domains (not shown), and two cysteine-rich regions at the C- and N-termini (yellow). ‘Δ’ depicts the C-terminal deletion. Motifs were determined with Motif Scan at http://hits.isb-sib.ch/cgi-bin/PFSCAN.

Figure 6

The murine Gnat2 gene and protein. The location of the Cpfl3 mutation is indicated. Underneath, cartoon of the cTα protein. The numbers refer to amino acids predicted to be in contact with GTPγS in the nucleotide binding cleft (Noel et al., 1993). The mutant residue in cone Tα, D200N, is situated within the GTP binding cleft. Blue box, helical domain (HD). Yellow, β-sheets forming the GTPase domain.

Figure 7

The Rd4 gene defect. The Rd4 mouse chromosome 4 carries a large inversion encompassing most of the chromosome. The proximal breakpoint is in the centromere, the distal breakpoint is in intron 2 of the Gnb1 gene.

Figure 8

The chicken GNB3 gene and the retinopathy globe enlarged (rge) gene defect. The defect was determined to be a deletion of 3 bp in exon 6, deleting a single amino acid (D163).

Figure 9

The Grm6 gene and nob3/nob4 gene defects. The Nob3 mutant is generated by a SNP in intron 1 (C648T) that produces a new splice site. As a result, a new exon (X1a) is created, as shown in B (red sequence). Grm6(nob4) contains a missense mutation in exon 2 (S185P). The blue boxes 1-7 indicate the seven TM domains of mGluR6.

Figure 10

The gene defect of the retinal degeneration (rd) chicken, gene symbol GUCY1*B). Exons 4-7 were deleted and replaced by a 81 bp fragments with high sequence similarity to exon 9. The deletion removes the single transmembrane domain of guanylate cyclase 1 (Gucy2e in mouse). The defect is modeled on the mouse Gucy2e gene (the chicken GC1 gene has not been cloned and sequenced).

Figure 11

The Mertk gene and protein. In the RCS rat, exon 2 is deleted causing a frameshift, deleting all of the molecule after codon 19. MERTK has a protein kinase domain (yellow), immunoglubulin-like domains (IG), and fibronectin (FN) domains.

Figure 12

The Mfrp gene (rd6). Exon 4 (red) is skipped in the rd6 mouse. The Mfrp is flanked by C1qtnf5, such that the 3′-UTR of Mfrp also is the 5′-UTR of C1qtnf5 consisting of 2 exons. A bicistronic mRNA is transcribed where C1qtnf5 mRNA is part of the 3′UTR of Mfrp. The MFRP protein has a transmembrane domain (TM), an extracellular cysteine-rich frizzled domain (fz) and two CUB domain profiles (named after initials of Cir, Cis, uEGF, and bone morphogenetic protein) (Stohr et al., 2002).

Figure 13

The mouse Myo7a gene and protein. Missense mutations sh16J (exon 7) and sh1 (exon 13) are located in the myosin head region. A third mutation, Q720ter, was generated by ENU mutagenesis. R and E in the protein schematic are arginine- and glutamic acid-rich regions. Myth4 are predicted microtubule-binding domains, Ferm domains participate in the linkage of cytoplasmic proteins to the membrane.

Figure 14

The Nr2e3 gene and protein. Multiple mutations, including a nonsense mutation, silence exons 4 and 5 that encode part of a ligand-binding domain.

Figue 15

The mouse Nyx gene and protein. The 85 bp deletion in exon 3 truncates the protein, eliminating a predicted GPI anchor at the C-terminal region.

Figure 16

The canine PDE6a gene. The rcd3 PDE6a gene defect is a deletion of 1 nt in exon 15 (CAT domain, red). Also shown are two missense mutations (V685M, D670G) generated by ENU mutagenesis in exons 18 and 19 of the mouse (canine and mouse PDE6a gene structures are identical). GAF1 and GAF2 contain noncatalytic cGMP binding sites. Fa denotes farnesylation of the C-terminal cysteine.

Figure 17

Transverse section of rodless mouse retina. Left, a histological section generated by Clyde E. Keeler, re-photographed in 1993. The section, gifted to Richard Sidman in the 1960s, was used for DNA extraction and PCR amplification (Pittler et al., 1993). Right, camera lucida drawing of rodless retina published by Clyde E. Keeler, 1924.

Figure 18

The mouse Pde6b gene and protein. The rd1 Pde6b gene carries a proviral insertion in intron 1and a disease-causing stop codon in exon 7. Exon 21 mutations found in rcd1 Irish setter and Sloughi dog are shown in blue. Relevant PDE6B domains: GAF domains (black), catalytic domain (red) and prenylated C-terminus (yellow). GAF domains are cyclic GMP binding sites, named after proteins that contain them: cGMP-specific and -regulated cNMP PDEs, Adenylyl cyclase, and E.coli transcription factor FhlA.

Figure 19

The Prph2 gene and rds mutation. The gene product is a glycoprotein with four TM domains. A large insert of foreign DNA into exon 2 produces the Prph2 null allele. The red bar designates a sorting signal required for transport to the OS (Tam et al., 2004).

Figure 20

The rd3 gene and proteins. The RD3 protein has no recognizable motifs. The mutation truncates the protein by 89 residues.

Figure 21

The Rpe65 gene. The Briard gene defect is a 4 nt deletion in exon 5. The rd12 mouse carries a stop codon in exon 3 (canine and murine Rpe65 genes structures are identical).

Figure 22

The canine Rpgr gene has a complex splicing pattern. The transcript containing the RCC1 (sequence similarity to regulator of chromatin condensation) domain encoded by exons 1-11, and the exon ORF14/15 is shown underneath the gene structure. Mutations causative for XLPRA1 and XLPRA2 are both located in ORF14/15 within a 100-bp interval. The “constitutive” transcript consisting of exons 1-13 and 16-19 is not depicted.

Figure 23

The canine Rpgrip gene. A 44 bp insertion (shaded blue), a stretch of 29 A flanked by two 15 bp perfect repeats, alters the reading frame leading to a premature stop in exon 3 (shaded green).

Figure 24

The tubby gene defect is a G>T conversion of the first nucleotide of intron 11 (pos. 18,626 of the murine Tub gene, numbering starting with ATG), abolishing the donor splice site and deleting exon 12. NLS, putative nuclear localization signals (K₃₉KKR, R₃₀₁KRKKsK). PIP2, phosphatidylinositol(4,5)bisphosphate binding site.