The mouse Cat4 locus maps to chromosome 8 and mutants express lens-corneal adhesion

Animal and cellular models of microphthalmia

Microphthalmia is a rare developmental eye disorder affecting 1 in 7000 births. It is defined as a small (axial length ⩾2 standard deviations below the age-adjusted mean) underdeveloped eye, caused by disruption of ocular development through genetic or environmental factors in the first trimester of pregnancy. Clinical phenotypic heterogeneity exists amongst patients with varying levels of severity, and associated ocular and systemic features. Up to 11% of blind children are reported to have microphthalmia, yet currently no treatments are available. By identifying the aetiology of microphthalmia and understanding how the mechanisms of eye development are disrupted, we can gain a better understanding of the pathogenesis. Animal models, mainly mouse, zebrafish and Xenopus, have provided extensive information on the genetic regulation of oculogenesis, and how perturbation of these pathways leads to microphthalmia. However, differences exist between species, hence cellular models, such as patient-derived induced pluripotent stem cell (iPSC) optic vesicles, are now being used to provide greater insights into the human disease process. Progress in 3D cellular modelling techniques has enhanced the ability of researchers to study interactions of different cell types during eye development. Through improved molecular knowledge of microphthalmia, preventative or postnatal therapies may be developed, together with establishing genotype-phenotype correlations in order to provide patients with the appropriate prognosis, multidisciplinary care and informed genetic counselling. This review summarises some key discoveries from animal and cellular models of microphthalmia and discusses how innovative new models can be used to further our understanding in the future.

Plain language summary

Animal and Cellular Models of the Eye Disorder, Microphthalmia (Small Eye) Microphthalmia, meaning a small, underdeveloped eye, is a rare disorder that children are born with. Genetic changes or variations in the environment during the first 3 months of pregnancy can disrupt early development of the eye, resulting in microphthalmia. Up to 11% of blind children have microphthalmia, yet currently no treatments are available. By understanding the genes necessary for eye development, we can determine how disruption by genetic changes or environmental factors can cause this condition. This helps us understand why microphthalmia occurs, and ensure patients are provided with the appropriate clinical care and genetic counselling advice. Additionally, by understanding the causes of microphthalmia, researchers can develop treatments to prevent or reduce the severity of this condition. Animal models, particularly mice, zebrafish and frogs, which can also develop small eyes due to the same genetic/environmental changes, have helped us understand the genes which are important for eye development and can cause birth eye defects when disrupted. Studying a patient's own cells grown in the laboratory can further help researchers understand how changes in genes affect their function. Both animal and cellular models can be used to develop and test new drugs, which could provide treatment options for patients living with microphthalmia. This review summarises the key discoveries from animal and cellular models of microphthalmia and discusses how innovative new models can be used to further our understanding in the future.

Analysis of Pax6 contiguous gene deletions in the mouse, Mus musculus, identifies regions distinct from Pax6 responsible for extreme small-eye and belly-spotting phenotypes

In the mouse Pax6 function is critical in a dose-dependent manner for proper eye development. Pax6 contiguous gene deletions were shown to be homozygous lethal at an early embryonic stage. Heterozygotes express belly spotting and extreme microphthalmia. The eye phenotype is more severe than in heterozygous Pax6 intragenic null mutants, raising the possibility that deletions are functionally different from intragenic null mutations or that a region distinct from Pax6 included in the deletions affects eye phenotype. We recovered and identified the exact regions deleted in three new Pax6 deletions. All are homozygous lethal at an early embryonic stage. None express belly spotting. One expresses extreme microphthalmia and two express the milder eye phenotype similar to Pax6 intragenic null mutants. Analysis of Pax6 expression levels and the major isoforms excluded the hypothesis that the deletions expressing extreme microphthalmia are directly due to the action of Pax6 and functionally different from intragenic null mutations. A region distinct from Pax6 containing eight genes was identified for belly spotting. A second region containing one gene (Rcn1) was identified for the extreme microphthalmia phenotype. Rcn1 is a Ca(+2)-binding protein, resident in the endoplasmic reticulum, participates in the secretory pathway and expressed in the eye. Our results suggest that deletion of Rcn1 directly or indirectly contributes to the eye phenotype in Pax6 contiguous gene deletions.

Co-operative roles for E-cadherin and N-cadherin during lens vesicle separation and lens epithelial cell survival

The classical cadherins are known to have both adhesive and signaling functions. It has also been proposed that localized regulation of cadherin activity may be important in cell assortment during development. In the context of eye development, it has been suggested that cadherins are important for separation of the invaginated lens vesicle from the surface ectoderm. To test this hypothesis, we conditionally deleted N-cadherin or E-cadherin from the presumptive lens ectoderm of the mouse. Conditional deletion of either cadherin alone did not produce a lens vesicle separation defect. However, these conditional mutants did exhibit common structural deficits, including microphthalmia, severe iris hyperplasia, persistent vacuolization within the fibre cell region, and eventual lens epithelial cell deterioration. To assess the co-operative roles of E-cadherin and N-cadherin within the developing lens, double conditional knockout embryos were generated. These mice displayed distinct defects in lens vesicle separation and persistent expression of another classical cadherin, P-cadherin, within the cells of the persistent lens stalk. Double mutant lenses also exhibited severe defects in lens epithelial cell adhesion and survival. Finally, the severity of the lens phenotype was shown to be sensitive to the number of wild-type E- and N-cadherin alleles. These data suggest that the co-operative expression of both E- and N-cadherin during lens development is essential for normal cell sorting and subsequent lens vesicle separation.

Relationship of Pax6 activity levels to the extent of eye development in the mouse, Mus musculus

In this study we extend the mouse Pax6 mutant allelic series to include a homozygous and hemizygous viable hypomorph allele. The Pax6(132-14Neu) allele is a Phe272Ile missense mutation within the third helix of the homeodomain. The mutant Pax6 homeodomain shows greatly reduced binding activity to the P3 DNA binding target. Glucagon-promoter activation by the entire mutant Pax6 product of a reporter gene driven by the G1 paired and homeodomain DNA binding target was slightly increased. We constructed mutant Pax6 genotypes such that Pax6 activity ranged between 100 and 0% and show that the extent of eye development is progressively reduced as Pax6 activity decreased. Two apparent thresholds identify three groups in which the extent of eye development abruptly shifted from complete eye at the highest levels of Pax6 to a rudimentary eye at intermediate levels of Pax6 to very early termination of eye development at the lowest levels of Pax6. Of the two Pax6-positive regions that participate in eye development, the surface ectoderm, which develops into the lens vesicle and the cornea, is more sensitive to reduced levels of Pax6 activity than the optic vesicle, which develops into the inner and outer retinal layers.

Cell autonomous roles for AP-2alpha in lens vesicle separation and maintenance of the lens epithelial cell phenotype

In this study, we have created a conditional deletion of AP-2alpha in the developing mouse lens (Le-AP-2alpha mutants) to determine the cell-autonomous requirement(s) for AP-2alpha in lens development. Embryonic and adult Le-AP-2alpha mutants exhibited defects confined to lens placode derivatives, including a persistent adhesion of the lens to the overlying corneal epithelium (or lens stalk). Expression of known regulators of lens vesicle separation, including Pax6, Pitx3, and Foxe3 was observed in the Le-AP-2alpha mutant lens demonstrating that these genes do not lie directly downstream of AP-2alpha. Unlike germ-line mutants, Le-AP-2alpha mutants did not exhibit defects in the optic cup, further defining the tissue specific role(s) for AP-2alpha in eye development. Finally, comparative microarray analysis of lenses from the Le-AP-2alpha mutants vs. wild-type littermates revealed differential expression of 415 mRNAs, including reduced expression of genes important for maintaining the lens epithelial cell phenotype, such as E-cadherin.

Ocular defects associated with a null mutation in the mouse arylamine N-acetyltransferase 2 gene

The xenobiotic metabolizing enzyme, mouse arylamine N-acetyltransferase type 2 (Nat2), is expressed during embryogenesis from the blastocyst stage and in the developing neural tube and eye. Mouse Nat2 is widely believed to have an endogenous role distinct from xenobiotic metabolism, and polymorphisms in the human ortholog have been implicated in susceptibility to spina bifida and orofacial clefting. The developmental role of Nat2 was investigated using transgenic Nat2 knockout/lacZ knockin (Nat2 (tm1Esim)) mice. The transgene was bred onto an A/J background and offspring were scored for developmental defects at weaning. After backcross generation eight, an ocular defect, ranging from cataract to microphthalmia and anophthalmia, was recorded among offspring of backcross and intercross pairs. Histologic analysis of cataract cases revealed a failure of the lens to separate from the cornea and plaques within the lens tissue. While Nat2 ( -/- ) mice have been described as overtly aphenotypic, the presence of a Nat2 null allele in one or both parents can result in ocular defects. These ocular phenotypes and their association with Nat2 genotype indicate that the Nat2 locus may be responsible for the previously described microphthalmic Cat4 phenotype and implicate the orthologous human NAT as a phenotypic modifier of microphthalmia and anophthalmia.

A heterozygous c-Maf transactivation domain mutation causes congenital cataract and enhances target gene activation

MAF, one of a family of large Maf bZIP transcription factors, is mutated in human developmental ocular disorders that include congenital cataract, microcornea, coloboma and anterior segment dysgenesis. Expressed early in the developing lens vesicle, it is central to regulation of lens crystallin gene expression. We report a semi-dominant mouse c-Maf mutation recovered after ENU mutatgenesis which results in the substitution, D90V, at a highly conserved residue within the N-terminal 35 amino-acid minimal transactivation domain (MTD). Unlike null and loss-of-function c-Maf mutations, which cause severe runting and renal abnormalities, the phenotype caused by the D90V mutation is isolated cataract. In reporter assays, D90V results in increased promoter activation, a situation similar to MTD mutations of NRL that also cause human disease. In contrast to wild-type protein, the c-Maf D90V mutant protein is not inhibited by protein kinase A-dependent pathways. The MTD of large Maf proteins has been shown to interact with the transcriptional co-activator p300 and we demonstrate that c-Maf D90V enhances p300 recruitment in a cell-type dependent manner. We observed the same for the pathogenic human NRL MTD mutation S50T, which suggests a common mechanism of action.

Type IV procollagen missense mutations associated with defects of the eye, vascular stability, the brain, kidney function and embryonic or postnatal viability in the mouse, Mus musculus: an extension of the Col4a1 allelic series and the identification of the first two Col4a2 mutant alleles

The basement membrane is important for proper tissue development, stability, and physiology. Major components of the basement membrane include laminins and type IV collagens. The type IV procollagens Col4a1 and Col4a2 form the heterotrimer [alpha1(IV)]2[alpha2(IV)], which is ubiquitously expressed in basement membranes during early developmental stages. We present the genetic, molecular, and phenotypic characterization of nine Col4a1 and three Col4a2 missense mutations recovered in random mutagenesis experiments in the mouse. Heterozygous carriers express defects in the eye, the brain, kidney function, vascular stability, and viability. Homozygotes do not survive beyond the second trimester. Ten mutations result in amino acid substitutions at nine conserved Gly sites within the collagenous domain, one mutation is in the carboxy-terminal noncollagenous domain, and one mutation is in the signal peptide sequence and is predicted to disrupt the signal peptide cleavage site. Patients with COL4A2 mutations have still not been identified. We suggest that the spontaneous intraorbital hemorrhages observed in the mouse are a clinically relevant phenotype with a relatively high predictive value to identify carriers of COL4A1 or COL4A2 mutations.

Replicated anterior zeugopod (raz): a polydactylous mouse mutant with lowered Shh signaling in the limb bud

A unique limb phenotype is described in a radiation-induced mutant mouse resulting from an inversion of a proximal segment of chromosome 5. The limb phenotype in the homozygous mutant presents with two anterior skeletal elements in the zeugopod but no posterior bone, hence the name replicated anterior zeugopod, raz. The zeugopod phenotype is accompanied by symmetrical central polydactyly of hand and foot. The chromosomal inversion includes the Shh gene and the regulatory locus, located ∼1 Mb away, within the Lmbr1 gene. In homozygous mutants, the expression of Shh mRNA and Shh protein is severely downregulated to about 20% of wild-type limb buds, but Shh expression appears normal throughout the remainder of the embryo. Correspondingly, Gli3 expression is upregulated and posteriorly expanded in the raz/raz limb bud. We propose that the double anterior zeugopod and symmetrical central polydactyly are due to an increased and uniform concentration of the Gli3 repressor form because of lowered Shh signaling.

Molecular characterization of Pax6(2Neu) through Pax6(10Neu): an extension of the Pax6 allelic series and the identification of two possible hypomorph alleles in the mouse Mus musculus

Phenotype-based mutagenesis experiments will increase the mouse mutant resource, generating mutations at previously unmarked loci as well as extending the allelic series at known loci. Mapping, molecular characterization, and phenotypic analysis of nine independent Pax6 mutations of the mouse recovered in mutagenesis experiments is presented. Seven mutations result in premature termination of translation and all express phenotypes characteristic of null alleles, suggesting that Pax6 function requires all domains to be intact. Of major interest is the identification of two possible hypomorph mutations: Heterozygotes express less severe phenotypes and homozygotes develop rudimentary eyes and nasal processes and survive up to 36 hr after birth. Pax6(4Neu) results in an amino acid substitution within the third helix of the homeodomain. Three-dimensional modeling indicates that the amino acid substitution interrupts the homeodomain recognition alpha-helix, which is critical for DNA binding. Whereas cooperative dimer binding of the mutant homeodomain to a paired-class DNA target sequence was eliminated, weak monomer binding was observed. Thus, a residual function of the mutated homeodomain may explain the hypomorphic nature of the Pax6(4Neu) allele. Pax6(7Neu) is a base pair substitution in the Kozak sequence and results in a reduced level of Pax6 translation product. The Pax6(4Neu) and Pax6(7Neu) alleles may be very useful for gene-dosage studies.

NIMA-related kinases: isolation and characterization of murine nek3 and nek4 cDNAs, and chromosomal localization of nek1, nek2 and nek3

The Aspergillus NIMA kinase plays a key role in controlling entrance into mitosis, and recent evidence suggests that mammalian NIMA-related kinases perform similar functions. We report here the cloning of the mouse nek3 and nek4 genes. Mouse nek3 is probably the ortholog of the partially sequenced, human nek3, whereas murine nek4 cDNA is probably the ortholog of human STK2. Nek4 is highly conserved between mouse and human, whereas Nek3 is somewhat less conserved (96.5 and 88% identity in the kinase domains, respectively). Northern analysis shows preferential expression of nek3 in mitotically active tissue, whereas nek4 is highly abundant in the testis. Within the developing testicular germ cells, in-situ analysis demonstrated that nek1, 2 and 4 exhibit differential patterns of expression, suggesting overlapping, but non-identical functions. Linkage analysis, using the mouse recombinant inbred strain panel (BXD), was used to localize nek1, 2 and 3. nek1 was mapped between Cpe and D8Mit8 on chromosome 8 at around 32cM, nek2 was mapped to the distal region of chromosome 1, and nek3 was mapped to the most centromeric region of chromosome 8.

Mouse models of congenital cataract

Mouse mutants affecting lens development are excellent models for corresponding human disorders. The mutant aphakia has been characterised by bilaterally aphakic eyes (Varnum and Stevens, J Hered 1968;59:147-50); the corresponding gene was mapped to chromosome 19 (Varnum and Stevens, Mouse News Lett 1975;53:35). Recent investigations in our laboratory refined the linkage of 0.6 cM proximal to the marker D19Mit10. Several candidate genes have been excluded (Chuk1, Fgf8, Lbp1, Npm3, Pax2, Pitx3). The Cat3 mutations are characterised by vacuolated lenses caused by alterations in the initial secondary lens fibre cell differentiation. Secondary malformations develop at the cornea and iris, but the retina remains unaffected. The mutation has been mapped to chromosome 10 close to the markers D10Mit41 and D10Mit95. Several candidate genes have been excluded (Dcn, Elk3, Ldc, Mell8, Tr2-11). The series of Cat2 mutations have been mapped close to the gamma-crystallin genes (Cryg; Löster et al., Genomics 1994;23:240-2). The Cat2nop mutation is characterised by a mutation in the third exon of Crygb leading to a truncated gamma B-crystallin and the termination of lens fibre cell differentiation. The Cat2 mutants are interesting models for human cataracts caused by mutations in the human CRYG genes at chromosome 2q32-35.

Cataract mutations and lens development

The lens plays an essential role for proper eye development. Mouse mutants affecting lens development are excellent models for corresponding human disorders. Moreover, using mutations in particular genes the process of eye and lens development can be dissected into distinct steps. Therefore, three mouse mutants will be described in detail and discussed affecting three essential stages: formation of the lens vesicle, initiation of secondary lens fiber cell formation, and terminal differentiation of the secondary fiber cells. The mutant aphakia (ak) has been characterized by bilaterally apakic eyes [Varnum and Stevens (1968) J. Hered. 59, 147-150], and the corresponding gene was mapped to chromosome 19 [Varnum and Stevens (1975) Mouse News Letters 53, 35]. Recent investigations in our laboratory refined the linkage 0.6 +/- 0.3 N cm proximal to the microsatellite marker D19Mit10. The linked gene Pax2, responsible for proper development of the posterior part of the eye and the optic nerve, was excluded as candidate gene by sequence analysis. Histological analysis of the homozygous ak mutants revealed a persisting lens stalk and subsequently the formation of lens rudiments. The lens defects led to irregular iris development and retinal folding. Congenital aphakia is known as a rare human anomaly. Besides a corneal dystrophy (CDTB), no corresponding disease is localized at the homologous region of human chromosome 10q23. The Cat3 mutations are characterized by vacuolated lenses caused by alterations in the beginning of secondary lens fiber cell differentiation at embryonic day 12.5. Secondary malformations develop at the cornea and the iris, but the retina remains unaffected. Two mutant alleles of the Cat3 locus have been mapped to mouse chromosome 10 very close to the microsatellite markers D10Mit41 and D10Mit95 (less than 0.3 cM). Since Cat3 is mapped to a position, which is homologous to human chromosome 12q21-24, the disorder cornea plana congenita can be considered as a candidate disease. The series of Cat2 mutations have been mapped close to the locus encoding the gamma-crystallin gene cluster Cryg [Löster et al. (1994) Genomics 23, 240-242]. The Cat2nop mutation is characterized by a deletion of 11 bp and an insertion of 4 bp in the 3rd exon of Crygh leading to a truncated gamma B-crystallin. The defect in the Crygh gene is causative for the stop of lens fiber cell differentiation from embryonic day 15.5 onward. Besides the lens, no further ocular tissue is affected. The Cat2 mouse mutants are interesting models for human cataracts caused by mutations in the gamma-crystallin genes at human chromosome 2q32-35. The ak, Cat3 and Cat2 mutants are discussed in the context of other mutants affecting early eye and lens development. Additionally, human congenital cataracts are discussed, which have been characterized similar to the mouse models. The overview of the three types of mutants demonstrates that genes, which affect the early eye development, e.g. at the lens vesicle stage, have consequences for the development of the whole eye. In contrast, if the mutation influences later steps of lens differentiation, the consequences are restricted to the lens only. These data indicate a decreasing effect of the lens for the regulation of eye development during embryogenesis.

Aphakia (ak), a mouse mutation affecting early eye development: fine mapping, consideration of candidate genes and altered Pax6 and Six3 gene expression pattern

The homozygous mouse mutant aphakia (ak) has been characterized by bilaterally aphakic eyes without a pupil [Varnum DS, Stevens, LC (1968): J Hered 59:147-150]. The mutation was mapped to chromosome 19 [Varnum DS, Stevens, LC (1975): Mouse News Lett 53:35]. Our linkage studies yielded a precise localization of the ak gene 0.6 +/- 0.3 cM proximal to the microsatellite marker D19Mit10 and 0.7 +/- 0.4 cM distal to D19Mit4 and D19Mit91. No recombination was found with the marker D19Mit9 among 418 backcross offspring tested. The developmental control gene Pax2 mapped 11.0 +/- 3.5 cM proximal to ak and is excluded as a candidate gene. Sequence analysis of Fgf8 and Chuk1, which are localized close to the marker D19Mit10, detected no mutations in the ak/ak mutants. Histological analysis of homozygous mutants suggested the arrest of lens development at the lens stalk stage, a transient morphological structure during the formation of the lens vesicle. In the lens remnants, Pax6 and Six3 are expressed, whereas in the persisting lens stalk only Pax6 was detected. The expression pattern of Pax2 appeared normal; Cryaa expression could not be detected. As a consequence of the arrested lens development, other ocular tissues that require for their development information from the intact lens, such as iris, ciliary muscle, retina, and vitreous body, are absent or formed abnormally.