The mouse eye lens obsolescence (Elo) mutant: studies on crystallin gene expression and linkage analysis between the mutant locus and the gamma-crystallin genes

Lop12, a mutation in mouse Crygd causing lens opacity similar to human Coppock cataract

A new cataract mutation was discovered in an ongoing program to identify new mouse models of hereditary eye disease. Lens opacity 12 (Lop12) is a semidominant mutation that results in an irregular nuclear lens opacity similar to the human Coppock cataract. Lop12 is associated with a small nonrecombining segment that maps to mouse Chromosome 1 close to the eye lens obsolescence mutation (Cryge(Cat2-Elo)), a member of the gamma-crystallin gene cluster (Cryg). Using a systemic candidate gene approach to analyze the entire Cryg cluster, a G to A transition was found in exon 3 of Crygd associated with the Lop12 mutation and has been designated Crygd(Lop12). The mutation Crygd(Lop12) leads to the formation of an in-frame stop codon that produces a truncated protein of 156 amino acids. It is predicted that the defective gene product alters protein folding of the gamma-crystallin(s) and results in lens opacity.

Developmental regulation and cell type-specific expression of the murine gamma F-crystallin gene is mediated through a lens-specific element containing the gamma F-1 binding site

The mouse gamma F-crystallin gene, one of six differentially regulated members of the gamma-crystallin gene family, is expressed exclusively in central nuclear fiber cells of the adult lens. The expression of this gene is controlled through regulatory elements contained in two upstream enhancers and the proximal promoter. Here we show that while the upstream enhancers and the proximal promoter could each direct gene expression in fiber cells formed at early stages of lens growth and development, cooperation between these elements is required to achieve expression in fiber cells formed at later stages. Evidence is provided that cooperative interaction between these elements modulates gene expression by increasing promoter strength. We also show that sequences within the proximal promoter region that bind lens cell nuclear factor gamma F-1 are sufficient to elicit gene expression in central nuclear fiber cells of the adult lens.

A frameshift mutation in the gamma E-crystallin gene of the Elo mouse

The murine Elo (eye lens obsolescence) mutation confers a dominant phenotype characterized by malformation of the eye lens. The mutation maps to chromosome 1, in close proximity to the gamma E-crystallin gene which is the 3'-most member of the gamma-crystallin gene cluster. We have analysed the sequence of this gene from the Elo mouse and identified a single nucleotide deletion which destroys the fourth and last "Greek key" motif of the protein. This mutation is tightly associated with the phenotype, as no recombination was detected in 274 meioses. In addition, the mutant mRNA is present in the affected lens, providing further support for our hypothesis that the deletion is responsible for the dominant Elo phenotype.

Temporal regulation of six crystallin transcripts during mouse lens development

Using the polymerase chain reaction (PCR) and RNA blot analysis, we have examined the differential expression patterns of the gamma-crystallins during lens development. Since only four of these genes had been previously characterized, the cDNAs for the remaining two genes, gamma C and gamma F, were isolated and sequenced. The steady-state mRNA profiles were then determined by RNA blot analysis of samples from embryonic stages to 180 days after birth, with gene-specific probes for gamma A, gamma B, gamma C, and gamma D, and a common probe for gamma E and gamma F. Due to the paucity of mismatches between the gamma E and gamma F-crystallin genes, the PCR technique was exploited to determine their relative abundance. The data showed that while all six gamma-crystallin genes were expressed in the embryonic lens, they were differentially regulated during development. At early stages, the levels of gamma B and gamma C mRNAs were found to be relatively low in comparison to those for gamma A, gamma D, gamma E and gamma F. After 30-40 days, however, the levels of gamma A, gamma E, and gamma F mRNAs declined rapidly, and the gamma B, gamma C and gamma D transcripts became the major gamma-crystallin mRNA species. The utility of the PCR technique in studying the relative abundance of steady-state gamma-crystallin mRNAs was also investigated.

Defect of a fiber cell-specific 94-kDa protein in the lens of inherited microphthalmic mutant mouse Elo

Deficiency in a 94,000-dalton protein in the non-crystallin fraction from the Elo mouse lens was shown. To perform further investigations, we raised an antibody against the 94,000-dalton protein isolated from normal mouse lens. Western blot analysis with the antibody indicated that the protein was only present in the lens and not in the brain, lung, heart, liver, and kidney. In the lens, it was unique to the cortex and nucleus fractions, not being present in the epithelial cells. Furthermore, it was observed in the water-soluble fraction as well as in the urea-soluble fraction. The antibody weakly but clearly reacted with the chick CP97 lens peptide, a fiber cell-specific protein, and anti-CP97 antibody also reacted with the 94,000-dalton protein. From these results, we concluded that the protein corresponds to CP97 cytoskeletal protein in the mouse lens. The protein was deficient in the lenses from Elo mice, but microphthalmic lenses from CTA mice contained a normal level.

Murine gamma E-crystallin is distinct from murine gamma 2-crystallin

The murine gamma E-crystallin-encoding gene (gamma E-cry) was isolated from a genomic DNA library. The nucleotide (nt) sequence was determined of 1100 bp upstream from the first exon to the polyadenylation site, comprising more than 3600 bp. The gene was characterized by phylogenetic nt sequence analysis in context with the already described gamma-cry genes from rat, mouse and human. The gamma E-cry genes (mouse and rat) are clearly separate from the corresponding gamma F-cry genes. Based on the phylogeny, the discussion about the murine gamma 2-cry classification as gamma F-cry [Bloemendal et al., Exp. Eye Res. 48 (1989) 465-466] is resolved. The murine gamma E-cry gene has characteristics similar to other genes from the gamma-cry gene family, except for an 18-fold repeat of the sequence, 5'-CTCAG, located at the 3'-end of intron B. There is no similar repeat structure in any other gamma-cry gene. No binding site for a common transcription factor could be detected among the 1100 bp of the 5'-region.

Restriction fragment length polymorphisms associated with the gene for the major intrinsic protein of eye-lens fibre cell membranes in mice with hereditary cataracts

Cloned cDNAs coding for eye-lens fibre cell-membrane proteins, MIP and MP70, were used to detect restriction fragment length polymorphisms (RFLPs) in genomic DNA from inbred mice with autosomally inherited cataracts. Whereas distinct RFLPs associated with the MIP gene were identified in the Cba Cat and Nct mutants, no such genetic variation was associated with the MP70 gene. RFLPs associated with the mouse MIP gene may provide informative DNA markers in gene linkage studies of murine hereditary cataracts.

Interaction of a lens cell transcription factor with the proximal domain of the mouse gamma F-crystallin promoter

The elements regulating lens-specific expression of the mouse gamma F-crystallin gene were examined. Here we show that mouse gamma F-crystallin sequences -67 to +45 contain a low basal level of lens-specific promoter activity and that sequences -67 to -25, which are highly conserved among different gamma-crystallin genes, are able to function as a strong transcriptional activator when duplicated and placed upstream of the TATA box. We also show that nuclear factors from lens and nonlens cells are able to form different complexes with sequences centered at -46 to -36 and demonstrate that binding of the factor from lens cells correlates with lens-specific promoter activity of the mouse gamma F-crystallin gene.

Interaction of a lens cell transcription factor with the proximal domain of the mouse gamma F-crystallin promoter

The elements regulating lens-specific expression of the mouse gamma F-crystallin gene were examined. Here we show that mouse gamma F-crystallin sequences -67 to +45 contain a low basal level of lens-specific promoter activity and that sequences -67 to -25, which are highly conserved among different gamma-crystallin genes, are able to function as a strong transcriptional activator when duplicated and placed upstream of the TATA box. We also show that nuclear factors from lens and nonlens cells are able to form different complexes with sequences centered at -46 to -36 and demonstrate that binding of the factor from lens cells correlates with lens-specific promoter activity of the mouse gamma F-crystallin gene.

Histological and biochemical characterization of the murine cataract mutant Nop

Nop, a spontaneous murine dominant cataract mutation, was detected by slit lamp investigations and preliminary characterized as a nuclear opacity. Histological investigations confirmed these findings and revealed additionally polar cataracts with vacuolization. In contrast to wild-type lenses, the nuclei of the cortical cells could also be detected in the area of the lens nucleus in Nop lenses. No other pathological alterations were found in the eyes. Lens wet and dry weights, as well as the content of water-soluble lens proteins, were reduced in heterozygous and homozygous mutants. The body weight was only slightly altered, indicating a rather lens-specific growth retardation. Some parameters concerning the osmotic state of the lens were changed, however, only in the homozygous mutants. Electrophoresis of the water-soluble lens proteins of the mutants revealed either additional bands, not present in the wild types, or bands of overrepresented proteins only slightly present in wild-type lenses. The changes might be related to the reduced amount of gamma-crystallins, which alters the composition of lenticular proteins in the mutants. Northern blots probed with cDNA specific for alpha-, beta- or gamma-crystallin genes suggested a reduced transcription of the gamma-crystallin genes. In contrast, the transcription of alpha- and beta-crystallins appeared to be similar in wild type and the mutants. The selective reduced amount of gamma-crystallin specific RNA can be discussed as a biochemical indicator for the histologically observed changes of differentiation in the cataractous Nop lenses.

Multiple regulatory elements of the murine gamma 2-crystallin promoter

Crystallins are the major water-soluble proteins of the vertebrate eye lens. These lens-specific proteins are encoded by several multi-gene families whose expression is differentially regulated during development. Our previous studies showed that the mouse gamma 2-crystallin promoter is active on transfection into lens-explant cultures derived from 14-day-old chick embryos but not on transfection into a variety of non-lens cells. In this study, transient expression data show that a sequence of 226 nucleotides upstream from the transcription start site is sufficient for activity of this promoter in the chicken lens cells. This sequence can be further divided into two domains, A and B, both of which are required for promoter function. Domain A (nucleotide -68 to -18) contains the TATA box and sequence motifs that are conserved in all gamma-crystallin promoters. Domain B (-226 to -120) consists of three regions. One of these regions contains an element with dyad symmetry and a sequence similar to the octamer motif. The second region contains an enhancer core consensus sequence. Two "enhancer-like" activities have been detected, one in Domain B and a second in a more distal region (-392 to -278) that does not appear to be required for promoter activity in transfection assays.

Suppression of lens gamma-crystallin encoded by Len-1 in the mutant mouse with eye lens obsolescence (Elo)

Eye lens obsolescence (Elo) is a heritable, autosomal dominant eye disorder in mice. The Elo mutation lies close to the Len-1 locus on mouse chromosome 1, and it was speculated that the Elo locus affects the expression level of gamma-crystallin. To clarify this, we used the BASO-DALT system to compare the amount of lens gamma-crystallin between Elo and normal mice. The lens extracts from 10-day-old Elo mice revealed the same profiles and expression levels of alpha- and beta-crystallins as those of +/+ mice. Among gamma-crystallins, the amounts of the components encoded by Len-1 were lower than wild-type levels, but that encoded by Len-2 was not in the Elo mouse. NaDodSO4-gel analysis revealed that the lower gamma-crystallin level could be detected as early as the 14-day stage in the lens of the Elo mouse embryo. The suppression of gamma-crystallin gene expression in Elo mice was examined at the mRNA level. The concentration of gamma 2-crystallin mRNA in Elo mice lens was estimated at 1/13 of that of normal lens, indicating that a small amount of transcript production from gamma-crystallin genes may cause the low level of gamma-crystallin in Elo lens.

Mapping of mouse gamma crystallin genes on chromosome 1

Restriction fragments analysis of DNA from mouse-hamster somatic-cell hybrid clones revealed that a mouse gamma crystallin cDNA hybridized to genomic sequences located on mouse chromosome 1. Identification of restriction fragment length polymorphisms (RFLPs) in the gamma crystallin sequences of inbred strains of mice permitted the further localization of the gamma crystallin genes (Cryg) to the proximal region of chromosome 1 closely linked to the loci encoding isocitrate dehydrogenase (Idh-1), a low molecular weight (LM) crystallin protein polymorphism (Len-1), and fibronectin (Fn-1). A single recombinant was observed between Len-1 and an RFLP in the gamma crystallin gene family, consistent with the hypothesis that Len-1 is one of the several structural loci encoding gamma crystallin genes. Len-1 is probably located on the centromeric end of the Cryg gene family. Linkage of Idh-1, Cryg, and Fn-1 in mice extends the syntenic relationship of those loci to the human, bovine, and rodent genomes and may define a chromosomal region that is generally conserved among mammals. The map position of Cryg, near the eye lens obsolescence (Elo) locus, was confirmed by the discovery that the restriction fragment patterns of gamma crystallin sequences differed between strain C3H/HeJ and the congenic anophthalmic mutant strain, C3H.Elo. Therefore, the gamma crystallin genes were cotransferred with the mutant Elo gene in the derivation of C3H.Elo. The results establish that LEN-1 is a marker for the gamma crystallin gene family, position the gamma crystallin gene family relative to other markers on mouse chromosome 1, and provide additional evidence that the Elo mutation is encoded at a locus closely linked to the gamma crystallin gene cluster. This study found no evidence of recombination hot spots within the gamma crystallin gene cluster.

The ontogeny of gamma-crystallin mRNAs in CatFraser mice

Mice which are either homozygous or heterozygous for the CatFraser mutation have ocular cataracts accompanied by selective reduction of the γ-crystallins, a homologous family of proteins present in the lens and encoded by a family of tightly linked genes. We measured the concentrations of four different mRNAs, each encoding a different γ-crystallin, in the lenses of homozygous CatFraser mice and in normal controls at various stages of development by preparing Northern blots from lens RNA, probing with RNAs complementary to each of the four messages and densitometry of the bands thus generated. The results show that, for each of these messages, the ontogenetic patterns observed in normal mice are retained in the mutant, but at much lower concentrations.

The murine cataractogenic mutation, Cat Fraser, segregates independently of the gamma crystallin genes

The murine mutation, Cat Fraser (CatFr), causes dominantly inherited ocular cataracts. Lenses of adult mice bearing this mutation contain reduced amounts of all seven γ-crystallin proteins and their corresponding transcripts. Levels of other lens proteins and transcripts appear normal and no extra-ocular effects of the mutation have been observed. The selective effect of this mutation on the γ-crystallins is consistent with the possibility that the site at which it occurs is involved in the coordinated regulation of the family of genes which encodes them. We have shown that several restriction fragment length polymorphisms in the γ-crystallin genes segregate independently of the CatFr mutation. Therefore, despite its selective effect on the expression of the γ-crystallin genes, the mutation is not linked to them. This observation rules out the possibility that the mutation is in a cis-acting regulatory site.