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Fam151b is a mammalian homologue of the C. elegans menorin gene, which is involved in neuronal branching. The International Mouse Phenotyping Consortium (IMPC) aims to knock out every gene in the mouse and comprehensively phenotype the mutant animals. This project identified Fam151b homozygous knock-out mice as having retinal degeneration. We show they have no photoreceptor function from eye opening, as demonstrated by a lack of electroretinograph (ERG) response. Histological analysis shows that during development of the eye the correct number of cells are produced and that the layers of the retina differentiate normally. However, after eye opening at P14, Fam151b mutant eyes exhibit signs of retinal stress and rapidly lose photoreceptor cells. We have mutated the second mammalian menorin homologue, Fam151a, and homozygous mutant mice have no discernible phenotype. Sequence analysis indicates that the FAM151 proteins are members of the PLC-like phosphodiesterase superfamily. However, the substrates and function of the proteins remains unknown.

Brittle cornea syndrome (BCS) is a rare recessive condition characterised by extreme thinning of the cornea and sclera. BCS results from loss-of-function mutations in the poorly understood genes ZNF469 or PRDM5. In order to determine the function of ZNF469 and to elucidate pathogenic mechanisms, we used genome editing to recapitulate a human ZNF469 BCS mutation in the orthologous mouse gene Zfp469. Ophthalmic phenotyping showed that homozygous Zfp469 mutation causes significant central and peripheral corneal thinning arising from reduced stromal thickness. Expression of key components of the corneal stroma in primary keratocytes from Zfp469BCS/BCS mice is affected, including decreased Col1a1 and Col1a2 expression. This alters the collagen type I/collagen type V ratio and results in collagen fibrils with smaller diameter and increased fibril density in homozygous mutant corneas, correlating with decreased biomechanical strength in the cornea. Cell-derived matrices generated by primary keratocytes show reduced deposition of collagen type I, offering an in vitro model for stromal dysfunction. Work remains to determine whether modulating ZNF469 activity will have therapeutic benefit in BCS or in conditions such as keratoconus in which the cornea thins progressively. This article has an associated First Person interview with the first author of the paper., Summary: A mouse model of brittle cornea syndrome was created to elucidate molecular mechanisms underlying the pathology of this rare connective tissue disorder in which extremely thin corneas rupture, causing irreversible blindness.

PurposeAlbinism is a clinically and genetically heterogeneous condition. Despite analysis of the nineteen known genes, ∼30% patients remain unsolved. We aimed to identify new genes involved in albinism.MethodsWe sequenced a panel of genes with known or predicted involvement in melanogenesis in 230 unsolved albinism patients.ResultsWe identified variants in theDopachrome tautomerase(DCT) gene in two patients. One was compound heterozygous for a 14 bp deletion in exon 9 and c.118T>A p.(Cys40Ser). The second was homozygous for c.183C>G p.(Cys61Trp). Both patients had mild hair and skin hypopigmentation, and classical ocular features. CRISPR/Cas9 was used in C57BL/6J mice to create mutations identical to the missense mutations carried by the patients, along with one loss-of-function indel mutation. When bred to homozygosity the three mutations revealed hypopigmentation of the coat, milder for Cys40Ser compared to Cys61Trp or the frameshift mutation. Histological analysis identified significant hypopigmentation of the retinal pigmented epithelium (RPE) indicating that defective RPE melanogenesis could be associated with eye and vision defects.DCTloss of function in zebrafish embryos elicited hypopigmentation both in melanocytes and RPE cells.ConclusionsDCTis the gene for a new type of oculocutaneous albinism that we propose to name OCA8.

The precise control of eye size is essential for normal vision. TMEM98 is a highly conserved and widely expressed gene which appears to be involved in eye size regulation. Mutations in human TMEM98 are found in patients with nanophthalmos (very small eyes) and variants near the gene are associated in population studies with myopia and increased eye size. As complete loss of function mutations in mouse Tmem98 result in perinatal lethality, we produced mice deficient for Tmem98 in the retinal pigment epithelium (RPE), where Tmem98 is highly expressed. These mice have greatly enlarged eyes that are very fragile with very thin retinas, compressed choroid and thin sclera. To gain insight into the mechanism of action we used a proximity labelling approach to discover interacting proteins and identified MYRF as an interacting partner. Mutations of MYRF are also associated with nanophthalmos. The protein is an endoplasmic reticulum-tethered transcription factor which undergoes autoproteolytic cleavage to liberate the N-terminal part which then translocates to the nucleus where it acts as a transcription factor. We find that TMEM98 inhibits the self-cleavage of MYRF, in a novel regulatory mechanism. In RPE lacking TMEM98, MYRF is ectopically activated and abnormally localised to the nuclei. Our findings highlight the importance of the interplay between TMEM98 and MYRF in determining the size of the eye., Author summary Having the correct eye size is important, too large and you will be short-sighted and too small and you will be far-sighted. Nanophthalmos, literally very small eye from the Greek, is a condition where the eye is very small but structurally normal. In addition to being farsighted such eyes are prone to glaucoma which can lead to loss of vision. Here we studied a protein called TMEM98 which is found in the membranes of the cells which form a layer at the back of eye called the retinal pigment epithelium (RPE). Mutations in TMEM98 have been found in nanophthalmos patients. Patients have one normal copy of the gene and one carrying a mutation. We removed Tmem98 from the RPE of mice in order to ascertain its function. We found, surprisingly, that rather than having small eyes this led to the development of very large eyes that were structurally fragile. We went on to identify protein partners of TMEM98 and found that it interacts with a protein called MYRF, mutations in which also cause nanophthalmos. This work demonstrates the importance of TMEM98 in eye size specification.

PURPOSEWe previously found a dominant mutation,Rwhs, causing white spots on the retina accompanied by retinal folds. Here we identify the mutant gene to beTmem98.In humans, mutations in the orthologous gene cause nanophthalmos. We modelled these mutations in mice and characterised the mutant eye phenotypes of these andRwhs.METHODSTheRwhsmutation was identified to be a missense mutation inTmem98by genetic mapping and sequencing. The humanTMEM98nanophthalmos missense mutations were made in the mouse gene by CRISPR-Cas9. Eyes were examined by indirect ophthalmoscopy and the retinas imaged using a retinal camera. Electroretinography was used to study retinal function. Histology, immunohistochemistry and electron microscopy techniques were used to study adult eyes.RESULTSAn I135T mutation ofTmem98causes the dominantRwhsphenotype and is perinatally lethal when homozygous. Two dominant missense mutations ofTMEM98, A193P and H196P are associated with human nanophthalmos. In the mouse these mutations cause recessive retinal defects similar to theRwhsphenotype, either alone or in combination with each other, but do not cause nanophthalmos. The retinal folds did not affect retinal function as assessed by electroretinography. Within the folds there was accumulation of disorganised outer segment material as demonstrated by immunohistochemistry and electron microscopy, and macrophages had infiltrated into these regions.CONCLUSIONSMutations in the mouse orthologue of the human nanophthalmos geneTMEM98do not result in small eyes. Rather, there is localised disruption of the laminar structure of the photoreceptors.

Purpose: We previously found a dominant mutation, Rwhs, causing white spots on the retina accompanied by retinal folds. Here we identify the mutant gene to be Tmem98. In humans, mutations in the orthologous gene cause nanophthalmos. We modeled these mutations in mice and characterized the mutant eye phenotypes of these and Rwhs. // Methods: The Rwhs mutation was identified to be a missense mutation in Tmem98 by genetic mapping and sequencing. The human TMEM98 nanophthalmos missense mutations were made in the mouse gene by CRISPR-Cas9. Eyes were examined by indirect ophthalmoscopy and the retinas imaged using a retinal camera. Electroretinography was used to study retinal function. Histology, immunohistochemistry, and electron microscopy techniques were used to study adult eyes. // Results: An I135T mutation of Tmem98 causes the dominant Rwhs phenotype and is perinatally lethal when homozygous. Two dominant missense mutations of TMEM98, A193P and H196P, are associated with human nanophthalmos. In the mouse these mutations cause recessive retinal defects similar to the Rwhs phenotype, either alone or in combination with each other, but do not cause nanophthalmos. The retinal folds did not affect retinal function as assessed by electroretinography. Within the folds there was accumulation of disorganized outer segment material as demonstrated by immunohistochemistry and electron microscopy, and macrophages had infiltrated into these regions. // Conclusions: Mutations in the mouse orthologue of the human nanophthalmos gene TMEM98 do not result in small eyes. Rather, there is localized disruption of the laminar structure of the photoreceptors.

In an international effort to assign function to all genes, and to identify new models of human disease, cohorts of mice produced from mutant embryonic stem cells are being screened in large-scale phenotyping pipelines. Hundreds of mouse lines, each mutant for a different gene, have been analyzed for developmental, hematological, immunological, metabolic, neurological, reproductive, and sensory parameters, which, thus, provide a comprehensive assessment of the consequences of gene ablation.1 Included in the sensory analysis is slit-lamp examination of the anterior chamber of the eye, and indirect ophthalmoscopy to view the retina and retinal vessels. The phenotype data can be viewed online and the mice are freely available. We describe here the detailed pathological and functional analysis of a mouse line mutant in the sodium/hydrogen ion exchange protein gene, Slc9a8, which was identified as having a retinal defect and no other abnormality other than male sterility. Sodium/hydrogen ion exchange proteins (NHEs) or cotransporter solute channels are encoded by the Slc9 gene family, comprising nine members: Slc9a1-9, encoding NHE1-9. Proteins NHE1 to 5 are located in the plasma membrane while NHE6 to 9 are found in membranes of cell organelles (reviewed previously2). These proteins contain 12 membrane spanning domains and catalyze the electroneutral transport of cations down a concentration gradient. Typically sodium ions are transported in and hydrogen ions out of the cell or organelle. Simultaneous transport of anions creates osmotic pressure and results in influx of water, regulating the pH and volume of cells and organelles. Sodium/hydrogen ion exchange is involved in aqueous humor dynamics and maintenance of intraocular pressure (IOP). Inhibition of sodium/hydrogen ion exchange in mice3 reduces IOP and at least one cotransporter, NHE1, is found expressed in the ciliary body, where aqueous dynamics are regulated. The single yeast NHE is necessary for correct sorting of proteins to the vacuole4 and, indeed, depletion of NHE8 in mammalian cells disrupts endosome trafficking.5 Six of the Slc9 family members have been mutated in mice and give rise to a range of phenotypes, from hyperactivity or ataxia and seizures (Slc9a1 and 6)6–8 to relatively mild gastric secretion and absorption defects (Slc9a2, 3, and 4).9–11 The gene Slc9a8 is widely expressed in adult mouse tissues, but is enriched in kidney, intestine, muscle, liver, and testes.12,13 The gene is expressed in the kidney proximal tubules, where reabsorption of secreted bicarbonate requires NHE function, although this is mostly via Slc9a3 (NHE3). However, Slc9a8 is expressed at higher levels in the neonatal kidney, and may partially compensate for NHE3 in Slc9a3 knockout animals, which have only a mild absorption defect.14 The Slc9a8 gene previously has been knocked out and the authors report that the mice have normal levels of serum sodium ions. They also report an increase in the length of the intestine, but a reduction in the number of goblet cells in the mutant mice with a concomitant reduction in mucin secretion and pH.13,15 Furthermore, it has been reported recently that the gene is expressed in the epithelial cells of the conjunctiva, cornea, and lacrimal glands and may have a role in the protection of these ocular epithelia.16 The male mutants also are sterile. Here, we identified and characterized a previously undescribed retinal defect of Slc9a8 knockout mice and located the origin of the phenotype to be abnormalities in the retinal pigmented epithelium.

Dilp1 is a semi-dominant mouse mutation that causes dilated pupils when heterozygous and is lethal when homozygous. We report here that it is caused by a point mutation that introduces a stop codon close to the start of the coding sequence of the paired-like homeobox transcription factor Phox2b. Mice carrying a targeted allele of Phox2b also have dilated pupils and the two alleles do not complement. Phox2b is necessary for the development of the autonomic nervous system and when absent one of the consequences is that all parasympathetic ganglia fail to form. Constriction of the pupil is a parasympathetic response mediated by the ciliary ganglion and we find that in Phox2b heterozygous mutants it is highly atrophic. The development of other parasympathetic and sympathetic ganglia appears to be largely unaffected indicating that the ciliary ganglion is exquisitely sensitive to a reduction in dose of this transcription factor. PHOX2B has been implicated in human disease. Mutations, principally leading to polyalanine expansions within the protein, have been found in patients with congenital central hypoventilation syndrome (CCHS), the cardinal feature of which is an inability to breathe unassisted when asleep. Additionally, some CCHS patients have ocular abnormalities, including pupillary defects, although they principally have constricted rather than dilated pupils. The apparent phenotypic differences observed between mice carrying a loss-of-function mutation of Phox2b and CCHS patients indicate that PHOX2B mutations found in CCHS patients, all of which can produce proteins with intact DNA-binding domains, are gain-of-function mutations that alter rather than abolish protein function.

Mutations in the LIM-homeodomain transcription factor LMX1B cause nail-patella syndrome, an autosomal dominant pleiotrophic human disorder in which nail, patella and elbow dysplasia is associated with other skeletal abnormalities and variably nephropathy and glaucoma. It is thought to be a haploinsufficient disorder. Studies in the mouse have shown that during development Lmx1b controls limb dorsal-ventral patterning and is also required for kidney and eye development, midbrain-hindbrain boundary establishment and the specification of specific neuronal subtypes. Mice completely deficient for Lmx1b die at birth. In contrast to the situation in humans, heterozygous null mice do not have a mutant phenotype. Here we report a novel mouse mutant Icst, an N-ethyl-N-nitrosourea-induced missense substitution, V265D, in the homeodomain of LMX1B that abolishes DNA binding and thereby the ability to transactivate other genes. Although the homozygous phenotypic consequences of Icst and the null allele of Lmx1b are the same, heterozygous Icst elicits a phenotype whilst the null allele does not. Heterozygous Icst causes glaucomatous eye defects and is semi-lethal, probably due to kidney failure. We show that the null phenotype is rescued more effectively by an Lmx1b transgene than is Icst. Co-immunoprecipitation experiments show that both wild-type and Icst LMX1B are found in complexes with LIM domain binding protein 1 (LDB1), resulting in lower levels of functional LMX1B in Icst heterozygotes than null heterozygotes. We conclude that Icst is a dominant-negative allele of Lmx1b. These findings indicate a reassessment of whether nail-patella syndrome is always haploinsufficient. Furthermore, Icst is a rare example of a model of human glaucoma caused by mutation of the same gene in humans and mice.

Mutations in the LIM-homeodomain transcription factor LMX1B cause nail-patella syndrome, an autosomal dominant pleiotrophic human disorder in which nail, patella and elbow dysplasia is associated with other skeletal abnormalities and variably nephropathy and glaucoma. It is thought to be a haploinsufficient disorder. Studies in the mouse have shown that during development Lmx1b controls limb dorsal-ventral patterning and is also required for kidney and eye development, midbrain-hindbrain boundary establishment and the specification of specific neuronal subtypes. Mice completely deficient for Lmx1b die at birth. In contrast to the situation in humans, heterozygous null mice do not have a mutant phenotype. Here we report a novel mouse mutant Icst, an N-ethyl-N-nitrosourea-induced missense substitution, V265D, in the homeodomain of LMX1B that abolishes DNA binding and thereby the ability to transactivate other genes. Although the homozygous phenotypic consequences of Icst and the null allele of Lmx1b are the same, heterozygous Icst elicits a phenotype whilst the null allele does not. Heterozygous Icst causes glaucomatous eye defects and is semi-lethal, probably due to kidney failure. We show that the null phenotype is rescued more effectively by an Lmx1b transgene than is Icst. Co-immunoprecipitation experiments show that both wild-type and Icst LMX1B are found in complexes with LIM domain binding protein 1 (LDB1), resulting in lower levels of functional LMX1B in Icst heterozygotes than null heterozygotes. We conclude that Icst is a dominant-negative allele of Lmx1b. These findings indicate a reassessment of whether nail-patella syndrome is always haploinsufficient. Furthermore, Icst is a rare example of a model of human glaucoma caused by mutation of the same gene in humans and mice., Author Summary Nail-patella syndrome is a human genetic disease caused by an inactivating mutation in one copy of a gene called LMX1B, with the amount of protein produced from the remaining copy of the gene not being enough for normal function. Patients with this disease have malformations of their nails, elbows and kneecaps. Some patients also develop kidney disease and glaucoma. LMX1B controls where and when other genes are expressed and it is important during development. Studies in mice have shown that complete absence of Lmx1b is lethal at birth. In contrast to humans, mice with only one copy of the gene are normal. Here we describe a new mutant mouse, Icst, which has a mutation in Lmx1b that abolishes the ability of the protein to bind near genes that it controls. Mice with one normal and one copy of Lmx1b with the Icst mutation have eye defects and some die shortly after birth probably due to kidney failure. Therefore having one functional and one mutant copy of Lmx1b is more detrimental than having a half dose of functional protein. The Icst mouse is a model of human glaucoma where mutation of the same gene causes glaucoma in humans and mice.

Background Some abnormalities of mouse corneal epithelial maintenance can be identified by the atypical mosaic patterns they produce in X-chromosome inactivation mosaics and chimeras. Human FLNA/+ females, heterozygous for X-linked, filamin A gene (FLNA) mutations, display a range of disorders and X-inactivation mosaicism is sometimes quantitatively unbalanced. FlnaDilp2/+ mice, heterozygous for an X-linked filamin A (Flna) nonsense mutation have variable eye, skeletal and other abnormalities, but X-inactivation mosaicism has not been investigated. The aim of this study was to determine whether X-inactivation mosaicism in the corneal epithelia of FlnaDilp2/+ mice was affected in any way that might predict abnormal corneal epithelial maintenance. Results X-chromosome inactivation mosaicism was studied in the corneal epithelium and a control tissue (liver) of FlnaDilp2/+ and wild-type (WT) female X-inactivation mosaics, hemizygous for the X-linked, LacZ reporter H253 transgene, using β-galactosidase histochemical staining. The corneal epithelia of FlnaDilp2/+ and WT X-inactivation mosaics showed similar radial, striped patterns, implying epithelial cell movement was not disrupted in FlnaDilp2/+ corneas. Corrected stripe numbers declined with age overall (but not significantly for either genotype individually), consistent with previous reports suggesting an age-related reduction in stem cell function. Corrected stripe numbers were not reduced in FlnaDilp2/+ compared with WT X-inactivation mosaics and mosaicism was not significantly more unbalanced in the corneal epithelia or livers of FlnaDilp2/+ than wild-type Flna+/+ X-inactivation mosaics. Conclusions Mosaic analysis identified no major effect of the mouse FlnaDilp2 mutation on corneal epithelial maintenance or the balance of X-inactivation mosaicism in the corneal epithelium or liver.

Renal-coloboma syndrome, also known as papillorenal syndrome, is an autosomal dominant human disorder in which optic disc coloboma is associated with kidney abnormalities. Mutations in the paired domain transcription factor PAX2 have been found to be the underlying cause of this disease. Disease severity varies between patients, and in some cases, renal hypoplasia has been found in the absence of any retinal defects. Here we report an N-ethyl-N-nitrosourea-induced mouse mutation, Opdc, which is an isoleucinetothreonine missense mutation, I40T, in the first α-helix of the Pax2 paired domain. The mutant protein binds target DNA sequences less strongly than the wild-type protein and acts poorly to transactivate target promoters in culture. The phenotypic consequence of this mutation on the development of the eye and ear is similar to that reported for null alleles of Pax2. However, in homozygotes, cerebellar development is normal on a genetic background in which loss of Pax2 results in failure of cerebellar formation. Moreover, there is a genetic background effect on the heterozygous phenotype such that on some strain backgrounds, kidney development is unaffected. Opdc is the first hypomorphic mutation reported for Pax2 that differs in phenotype from loss-of-function mutations. These results suggest that PAX2 is a strong candidate gene for cases in which human patients have optic disc coloboma not associated with renal dysplasia.

The intracellular target of diphtheria toxin is a modified histidine residue, diphthamide, in the translation elongation factor, eEF2 (also known as EFT1). This enigmatic modification occurs in all eukaryotes and is produced in yeast by the action of five gene products, DPH1 to DPH5. Sequence homologues of these genes are present in all sequenced eukaryotic genomes and, in higher eukaryotes, there is functional evidence for DPH1, DPH2, DPH3 and DPH5 acting in diphthamide biosynthesis. We identified a mouse that was mutant for the remaining gene, Dph4. Cells derived from homozygous mutant embryos lacked the diphthamide modification of eEF2 and were resistant to killing by diphtheria toxin. Reporter-tagged DPH4 protein localized to the cytoskeleton, in contrast to the localization of DPH1 and consistent with evidence that DPH4 is not part of a proposed complex containing DPH1, DPH2 and DPH3. Mice that were homozygous for the mutation were retarded in growth and development, and almost always die before birth. Those that survive long enough had preaxial polydactyly, a duplication of digit 1 of the hind foot. This same defect has been seen in embryos that were homozygous for mutation of DPH1, suggesting that lack of diphthamide on eEF2 could result in translational failure of specific proteins, rather than a generalized translation downregulation.

The X-linked gene filamin A (FIna) encodes a widely expressed actin-binding protein that crosslinks actin into orthogonal networks and interacts with a variety of other proteins including membrane proteins, integrins, transmembrane receptor complexes and second messengers, thus forming an important intracellular signalling scaffold. Heterozygous loss of function of human FLNA causes periventricular nodular heterotopia in females and is generally lethal (cause unknown) in hemizygous males. Missense FLNA mutations underlie a spectrum of disorders affecting both sexes that feature skeletal dysplasia accompanied by a variety of other abnormalities. Dilp2 is an X-linked male-lethal mouse mutation that was induced by N-ethyl-N-nitro-sourea. We report here that Dilp2 is caused by a T-to-A transversion that converts a tyrosine codon to a stop codon in the FIna gene (Y2388X), leading to absence of the FIna protein and male lethality because of incomplete septation of the outflow tract of the heart, which produces common arterial trunk. A proportion of both male and female mutant mice have other cardiac defects including ventricular septal defect. In addition, mutant males have midline fusion defects manifesting as sternum and palate abnormalities. Carrier females exhibit milder sternum and palate defects and misshapen pupils. These results define crucial roles for FIna in development, demonstrate that X-linked male lethal mutations can be recovered from ENU mutagenesis screens and suggest possible explanations for lethality of human males hemizygous for null alleles of FLNA.

Members of the type IV collagen family are essential components of all basement membranes (BMs) and define structural stability as well as tissue-specific functions. The major isoform, alpha1.alpha1.alpha2(IV), contributes to the formation of many BMs and its deficiency causes embryonic lethality in mouse. We have identified an allelic series of three ENU induced dominant mouse mutants with missense mutations in the gene Col4a1 encoding the alpha1(IV) subunit chain. Two severe alleles (Bru and Svc) have mutations affecting the conserved glycine residues in the Gly-Xaa-Yaa collagen repeat. Bru heterozygous mice display defects similar to Axenfeld-Rieger anomaly, including iris defects, corneal opacity, vacuolar cataracts, significant iris/corneal adhesions, buphthalmos and optic nerve cupping, a sign indicative of glaucoma. Kidneys of Bru mice have peripheral glomerulopathy characterized by hypertrophy and hyperplasia of the parietal epithelium of Bowman's capsule. A milder allele (Raw) contains a mutation in the Yaa residue of the collagen repeat and was identified by a silvery appearance of the retinal arterioles. All phenotypes are associated with BM defects that affect the eye, kidney and other tissues. This allelic series shows that mutations affecting the collagen domain cause dominant negative effects on the expression and function of the major collagen IV isoform alpha1(IV), and pathological effects vary with the individual mutations.

We have carried out a genome-wide screen for novel N-ethyl-N-nitrosourea-induced mutations that give rise to eye and vision abnormalities in the mouse and have identified 25 inherited phenotypes that affect all parts of the eye. A combination of genetic mapping, complementation and molecular analysis revealed that 14 of these are mutations in genes previously identified to play a role in eye pathophysiology, namely Pax6, Mitf, Egfr and Pde6b. Many of the others are located in genomic regions lacking candidate genes and these define new loci. Four of the mutants display a similar phenotype of dilated pupils but do not appear to be allelic, and at least two of these are embryonic lethal when homozygous. This collection of eye mutations will be valuable for understanding gene function, for dissecting protein function and as models of human eye disease.

WOS: 000293338600004, PubMed ID: 21750680, Ophthalmo-acromelic syndrome (OAS), also known as Waardenburg Anophthalmia syndrome, is defined by the combination of eye malformations, most commonly bilateral anophthalmia, with post-axial oligosyndactyly. Homozygosity mapping and subsequent targeted mutation analysis of a locus on 14q24.2 identified homozygous mutations in SMOC1 (SPARC-related modular calcium binding 1) in eight unrelated families. Four of these mutations are nonsense, two frame-shift, and two missense. The missense mutations are both in the second Thyroglobulin Type-1 (Tg1) domain of the protein. The orthologous gene in the mouse, Smoc1, shows site-and stage-specific expression during eye, limb, craniofacial, and somite development. We also report a targeted pre-conditional gene-trap mutation of Smoc1 (Smoc(1tm1a)) that reduces mRNA to similar to 10% of wild-type levels. This gene-trap results in highly penetrant hindlimb post-axial oligosyndactyly in homozygous mutant animals (Smoc(1tm1a/tm1a)). Eye malformations, most commonly coloboma, and cleft palate occur in a significant proportion of Smoc(1tm1a/tm1a) embryos and pups. Thus partial loss of Smoc-1 results in a convincing phenocopy of the human disease. SMOC-1 is one of the two mammalian paralogs of Drosophila Pentagone, an inhibitor of decapentaplegic. The orthologous gene in Xenopus laevis, Smoc-1, also functions as a Bone Morphogenic Protein (BMP) antagonist in early embryogenesis. Loss of BMP antagonism during mammalian development provides a plausible explanation for both the limb and eye phenotype in humans and mice., Medical Research Council (UK)Medical Research Council UK (MRC); Medical Research CouncilMedical Research Council UK (MRC) [MC_U127561093, MC_PC_U127561112, MC_U127561112], Funding for this project was provided as an intramural program grant from the Medical Research Council (UK). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

PURPOSE. To identify the underlying molecular defects causing retinal degeneration in seven N-ethyl-N-nitrosourea (ENU) induced mutant alleles of the Pde6b gene and to analyze the timescale of retinal degeneration in these new models of retinitis pigmentosa. METHODS. Conformation sensitive capillary electrophoresis and DNA sequencing were used to identify the mutations in the Pde6b gene. Visual acuity testing was performed with a visualtracking drum at ages ranging from postnatal day 25 to week 10. Retinal examination was performed with an indirect ophthalmoscope. Animals were killed and eyes were prepared for histologic analysis. RESULTS. Point mutations in the seven new alleles of Pde6b were identified: Three generated premature stop codons, two were missense mutations, and two were splice mutations. The three stop codon mutants and one of the splice mutants had phenotypes indistinguishable from the Pde6b rd1 mouse in rapidity of onset of retinal degeneration, suggesting that they are null alleles. However, the remaining alleles showed slower onset of retinal degeneration, as determined by visual acuity testing, fundus examination, and histology, indicating that they are hypomorphic alleles. CONCLUSIONS. These data demonstrate a correlation between genotype and phenotype. Four of the mutants with severe genetic lesions have rapid onset of retinal degeneration, as determined by fundus examination. These mice were indistinguishable from Pde6b rd1 mice, which are effectively blind by 3 weeks of age. In contrast, the milder genetic lesions show a slower progression of the disease and provide the community with models that more closely mimic human retinitis pigmentosa. (Invest Ophthalmol Vis Sci. 2005;46:3443‐3450)