Radiation-Induced Lens Opacity and Cataractogenesis: A Lifetime Study Using Mice of Varying Genetic Backgrounds

Recent epidemiological findings and reanalysis of historical data suggest lens opacities resulting from ionizing radiation exposures are likely induced at lower doses than previously thought. These observations have led to ICRP recommendations for a reduction in the occupational dose limits for the eye lens, as well as subsequent implementation in EU member states. The EU CONCERT LDLensRad project was initiated to further understand the effects of ionizing radiation on the lens and identify the mechanism(s) involved in radiation-induced cataract, as well as the impact of dose and dose-rate. Here, we present the results of a long-term study of changes to lens opacity in male and female adult mice from a variety of different genetic (radiosensitive or radioresistant) backgrounds, including mutant strains Ercc2 and Ptch1, which were assumed to be susceptible to radiation-induced lens opacities. Mice received 0.5, 1 and 2 Gy 60Co gamma-ray irradiation at dose rates of 0.063 and 0.3 Gy min-1. Scheimpflug imaging was used to quantify lens opacification as an early indicator of cataract, with monthly observations taken postirradiation for an 18-month period in all strains apart from 129S2, which were observed for 12 months. Opacification of the lens was found to increase with time postirradiation (with age) for most mouse models, with ionizing radiation exposure increasing opacities further. Sex, dose, dose rate and genetic background were all found to be significant contributors to opacification; however, significant interactions were identified, which meant that the impact of these factors was strain dependent. Mean lens density increased with higher dose and dose rate in the presence of Ercc2 and Ptch1 mutations. This project was the first to focus on low (<1 Gy) dose, multiple dose rate, sex and strain effects in lens opacification, and clearly demonstrates the importance of these experimental factors in radiobiological investigations on the lens. The results provide insight into the effects of ionizing radiation on the lens as well as the need for further work in this area to underpin appropriate radiation protection legislation and guidance.

2% Hämophilie-A-Patienten ohne Mutation im FVIII-Gen

In Deutschland leben rund 6000 Hämophilie-A-Patienten. Mit Screening-Methoden können ca. 97% der Mutationen erfasst werden. Für die restlichen Patienten werden alle kodierenden Regionen des FVIII-Gens sequenziert.

Von 1350 Patienten wurde bei 80 Patienten zunächst keine Mutation gefunden. In fünf Fällen war eine Inversion im Intron 1 verantwortlich, in 16 Patienten wurden bekannte und bei 19 Patienten neue Mutationen identifiziert. Von den neuen Mutationen lagen 14 in kodierenden Bereichen, die restlichen fünf befanden sich in flankierenden Bereichen von Exons. Neben den als kausal betrachteten Mutationen wurden fünf Polymorphismen identifiziert. Weitere Untersuchungen müssen klären, ob diese Polymorphismen Krankheitsbild und -verlauf beeinflussen.

Überraschenderweise wurde bei 23 der 80 Patienten keine kausale Mutation in den untersuchten Regionen identifiziert. Das bedeutet, dass in etwa 2% der Hämophilie-A-Patienten keine Mutation in der cDNA des FVIII-Gens gefunden werden kann. Daher müssen auch Mutationen in nicht kodierenden Regionen oder sogar in anderen Genen in Betracht gezogen werden. Eines dieser Gene kodiert für den von-Willebrand-Faktor (vWF). Wir identifizieren in zwei Fällen eine Mutation im vWF-Gen.

Charakterisierung von Faktor-VIII-Antikörperepitopen mit Faktor-VIII-Peptid-Bibliotheken

Etwa 30% der Patienten die an schwerer Hämophilie A leiden, entwickeln als Komplikation einer Substitutionstherapie gegen Faktor VIII gerichtete Antikörper, die die prokoagulatorische Aktivität von intravenös injizierten FVIII-Proteinen neutralisieren. In der Regel werden verschiedene Epitope auf dem FVIII-Molekül durch diese Antikörper gebunden. Die detaillierte Struktur dieser Epitope ist unbekannt. In dieser Studie werden Epitope auf dem Faktor-VIII-Molekül mit Hilfe festphasengebundener Peptid-Arrays identifiziert, auf denen die Aminosäuresequenz des FVIII in Oligopeptiden dargestellt wird. Die Bindung von FVIII-Antikörpern an bestimmte Peptidsequenzen zeigt potenzielle Epitopstrukturen. FVIII-Antikörper von Inhibitorpatienten und gesunden Blutspendern werden zurzeit mit dieser Methode charakterisiert. Identifizierte Epitope können zur Entwicklung neuer Therapiekonzepte führen, die dazu dienen, eine Hemmkörperentwicklung zu vermeiden bzw. bereits entstandene Hemmkörper besser zu eliminieren. Als Teilprojekt des Konsortiums »Hämophilie A« wird im Rahmen des Gesamtvorhabens »Genotyp-Phänotyp-Korrelation bei der Hämophilie A« untersucht, ob die Epitopcharakteristik mit dem Typ und der Position der FVIII-Genmutationen korreliert sowie Zusammenhänge zwischen Epitop und klinischem Verlauf der Krankheit bestehen. Zusätzlich wird der Einfluss der Epitope auf den therapeutischem Effekt und Erfolg der Immuntoleranzinduktion untersucht.

Neurotransmitter receptor density changes in Pitx3ak mice--a model relevant to Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder, characterized by alterations of nigrostriatal dopaminergic neurotransmission. Compared to the wealth of data on the impairment of the dopamine system, relatively limited evidence is available concerning the role of major non-dopaminergic neurotransmitter systems in PD. Therefore, we comprehensively investigated the density and distribution of neurotransmitter receptors for glutamate, GABA, acetylcholine, adrenaline, serotonin, dopamine and adenosine in brains of homozygous aphakia mice being characterized by mutations affecting the Pitx3 gene. This genetic model exhibits crucial hallmarks of PD on the neuropathological, symptomatic and pharmacological level. Quantitative receptor autoradiography was used to characterize 19 different receptor binding sites in eleven brain regions in order to understand receptor changes on a systemic level. We demonstrated striking differential changes of neurotransmitter receptor densities for numerous receptor types and brain regions, respectively. Most prominent, a strong up-regulation of GABA receptors and associated benzodiazepine binding sites in different brain regions and concomitant down-regulations of striatal nicotinic acetylcholine and serotonergic receptor densities were found. Furthermore, the densities of glutamatergic kainate, muscarinic acetylcholine, adrenergic α1 and dopaminergic D2/D3 receptors were differentially altered. These results present novel insights into the expression of neurotransmitter receptors in Pitx3(ak) mice supporting findings on PD pathology in patients and indicating on the possible underlying mechanisms. The data suggest Pitx3(ak) mice as an appropriate new model to investigate the role of neurotransmitter receptors in PD. Our study highlights the relevance of non-dopaminergic systems in PD and for the understanding of its molecular pathology.

Radiation cataractogenesis: a review of recent studies

The lens of the eye is recognized as one of the most radiosensitive tissues in the human body, and it is known that cataracts can be induced by acute doses of less than 2 Gy of low-LET ionizing radiation and less than 5 Gy of protracted radiation. Although much work has been carried out in this area, the exact mechanisms of radiation cataractogenesis are still not fully understood. In particular, the question of the threshold dose for cataract development is not resolved. Cataracts have been classified as a deterministic effect of radiation exposure with a threshold of approximately 2 Gy. Here we review the combined results of recent mechanistic and human studies regarding induction of cataracts by ionizing radiation. These studies indicate that the threshold for cataract development is certainly less than was previously estimated, of the order of 0.5 Gy, or that radiation cataractogenesis may in fact be more accurately described by a linear, no-threshold model.

[Perioperative discontinuation of antiplatelet therapy of patients with coronary stents. Reevaluating the risks]

According to the present guidelines, patients with coronary stents are to be treated with dual antiplatelet therapy. In case surgery is needed, the risk of a fatal stent thrombosis by withdrawing antithrombotics needs to be balanced in each individual case against the risk of haemorrhagic complications on continued antiplatelet medication. We present a case of fatal stent thrombosis and discuss the current evidence regarding perioperative continuation and interruption of antiplatelet therapy for this patient population. In summary the haemorrhagic risk with acetylsalicylic acid for secondary prevention seems very low, and it should be discontinued only in selected cases. Continued dual anticoagulation concepts are also discussed.

Sequencing of the factor 8(F8) coding regions in 10 Turkish hemophilia A patients reveals three novel pathological mutations, and one rediagnosis of von Willebrand's disease type 2N

The most common cause for severe cases of hemophilia A is the homologous recombination involving intron 22 and related sequences outside the F8 gene. F8 coding regions of the gene including the exon/intron junctions were sequenced in 10 Turkish hemophilia A patients all of whom have been typed negative for intron 22 inversion and who did not have a detectable change by DGGE analysis. Pathological changes including two novel deletions (c. 205del CT and c. 3699del ACAT), one novel missense mutation (9546A) and two recurrent missense mutations were observed in five patients. The c. 2110C > T is another novel pathological change affecting exonic splicing enhancer site in two patients. One of the remaining three patients had a recurrent vWD type 2N mutation in the F8 binding site of the vWF (C788R). The S1269S polymorphism (c. 3864A > C) detected phenotype. Conclusively, sequencing of the promoter and the coding regions of 10 hemophilia A patients contributes four novel pathological mutations to the F8 mutations list and reveals a rediagnosis of hemophilia A but is still not sufficient to confirm hemophilia A phenotype in two patients.

Analysis of mRNA in hemophilia A patients with undetectable mutations reveals normal splicing in the factor VIII gene

BACKGROUND

haemophilia A (HA) is characterized by partial or total deficiency of factor VIII (FVIII) protein activity. It is caused by a broad spectrum of mutations in the FVIII gene. Despite tremendous improvements in mutation screening methods, in about 2% of HA patients no DNA change could be found, even after sequencing the whole coding part of the FVIII gene including the flanking splice sites, as well as the promotor and the 3' UTR regions.

OBJECTIVES, PATIENTS AND METHODS

In the present study we performed a detailed RNA analysis of three groups of patients. The first included control patients with known splicing defects, the second included two patients with already identified nucleotide changes close to splicing sites, that could potentially alter the normal splicing process, and a third group of 11 unrelated patients whose genomic DNA have already been screened for mutations by DHPLC and direct sequencing with no mutation being identified.

RESULTS

Both candidate splice site mutations were shown to result in either skipping or alternative splicing of at least one exon, therefore these DNA changes must be considered as causal for the patients' HA phenotype. In contrast, no abnormalities on the RNA level were observed in any of 11 unrelated patients without mutations in the FVIII gene.

CONCLUSIONS

These findings exclude mutations that could be located deep in the introns and affecting either normal splicing or lead to mechanisms causing some unknown rearrangements of the FVIII gene. In fact, our results point to the presence of still unknown factor(s) causing HA, which might be either allelic or in the close proximity of the FVIII gene or non-allelic associated with other genetic loci that are involved in the processing of the FVIII protein.

[Characterization of factor VIII antibody epitopes from haemophilia A patients using cellulose bound FVIII peptide libraries]

Approximately 30% of patients suffering from severe haemophilia A develop antibodies against factor VIII (FVIII) neutralizing the effect of the pro-coagulant activity of intravenously injected FVIII as a complication of replacement therapy. Generally, various epitopes on the FVIII molecule are bound by these antibodies. The detailed structure of such epitopes is unknown. In this study epitopes on the FVIII molecule are identified using solid phase bound peptide arrays carrying the whole amino acid sequence of FVIII as small oligopeptides. The binding of FVIII antibodies by specific peptide sequences on the array indicates potential epitopes. FVIII antibodies of inhibitor patients and healthy blood donors are currently investigated by this method. Identified epitopes may lead to new concepts in therapy aiming at avoidance of inhibitor formation or improvement of inhibitor eradication. As participant of the 'haemophilia A' consortium dealing with genotype/phenotype correlation in haemophilia A we investigate, if the site or type of the mutation correlates with the epitopes, and if there is any relation between epitopes and clinical course. Furthermore, the influence of epitopes on therapeutical effects and the outcome of immune tolerance induction is under scrutiny.

[The national GTH haemophilia registry as database within the scope of the German human genome project]

The Committee of Haemophilia of the GTH has established a central registry for all German centers treating patients with haemophilia. The intention was to establish a suitable system for collecting and analyzing epidemiological data relevant to bleeding disorders. The registry provides the database within the scope of the German Human-Genome-Project. The set goal is the complete molecular characterization of the genetic mutations on chromosome X of haemophilia A patients in Germany and subsequent correlation with the phenotype. An electronic network is applied for communication. A Java-application was developed for online electronic data acquirement by the participating centers. Offline data entry and sending encrypted data carriers is possible, too. A high level of security is assured by personalized access. Data are anonymized and scrambled by secure encoding. The concept was confirmed by the official data security offices. A considerable improvement for the epidemiological sciences and a better basis on therapy for patients with bleeding disorders is expected. Furthermore the registry is available for other scientific projects.

[2% Haemophilia A patients without mutation in the FVIII gene]

In Germany, approximately 6,000 patients are suffering from haemophilia A. Screening methods cover 97% of the mutations. For the other patients the coding sequences of the FVIII gene have to be sequenced in total. Out of 1,350 patients, no mutation was observed in 80 patients. In 5 patients, we observed an inversion in intron 1. Known mutations were detected in 16 patients, and in 19 cases novel mutations were characterized (14 in coding regions and 5 in flanking introns). The mutations are mainly base pair substitutions, small deletions or insertions (max. 4 bp) and predicted to cause amino acid exchanges or frameshifts leading to premature stop codons. Moreover, 5 polymorphisms were identified in exons 14 and 26 as well as in introns 7 and 19. Further studies are necessary to identify their causative effects. Surprisingly, in 23 patients out of this subgroup of 80, no mutation was identified in the FVIII gene. Therefore, mutations in non-coding areas or even in other genes have to be considered responsible for the haemophilia A like phenotype. One of them codes for the von Willebrand factor (vWF). We confirmed in two of our cases mutations in the vWF gene.

[Significance of mutation analysis in patients with haemophilia A]

Haemophilia A represents the most frequent hereditary bleeding disorder in humans. The disease is caused by mutations within the factor VIII gene leading to decreased or absent factor VIII activities with a bleeding tendency depending on the degree of factor VIII deficiency. Nowadays, the causative mutations can be routinely detected and have substantially improved diagnostic and understanding of the pathophysiology of haemophilia A. Identification of the gene defects in haemophilic families have enabled fast and save carrier diagnosis. The correlation of the genetic defects with the clinical course revealed that the type of mutation represents the most important genetic predisposing factor for inhibitor formation, the most severe complication of treatment with factor VIII concentrates. Mitigated clinical courses of haemophilia A were shown to be due to special types of mutations or the presence of concomitant thrombophilic mutations. Molecular models of the factor VIII protein allowed to investigate the effects of specific mutations thus giving new insights in the structure/function relationship of the factor VIII molecule. These findings might promote the development of novel recombinant factor VIII concentrates with higher efficacy, longer half life and reduced immunogenicity.

[Gathering and evaluation of phenotype data of haemophilia A patients for correlation with genotype data]

Haemophilia A is caused by a genetic defect of the factor VIII gene resulting in complete or considerable functional loss of factor VIII molecule within blood. The high bleeding risk of patients can be prevented by intravenous injections of factor VIII protein. However, 25% of patients affected with severe haemophilia, develop factor VIII antibodies against the concentrate substituted. Within this study we try to comprise the phenotypic parameters (e. g. detailed documentation of disease course, basic laboratory values) and the therapy-associated data (e. g. applicated type and amount of factor VIII, number of substitutions, factor VIII recovery, inhibitor development and inhibitor elimination). We hope to identify differences of variable therapeutic treatments on course of disease as already identified for the factor VIII gene defects. At least we expect that certain mutations and mutation types, respectively, can be referred to typical phenotypes and similar course of treatment protocols.

Erfassung und Bewertung phänotypischer Parameter von Hämophilie-A-Patienten und Korrelation mit dem Genotyp

Die Hämophilie A wird durch einen genetischen Defekt des FVIII-Gens verursacht und führt entweder zum vollständigen Fehlen oder zu einem erheblichen Funktionsverlust des Gerinnungsfaktors VIII im Blut. Die erhöhte Blutungsneigung der betroffenen Patienten kann durch die intravenöse Gabe von Faktor-VIII-Protein verhindert werden. Allerdings entwickeln 25% der Patienten, die an einer schweren Hämophilie A leiden, Anti-Faktor-VIII-Antikörper gegen das substitutierte Konzentrat. In dieser Studie wird versucht, die phänotypischen Parameter (z.B. detaillierte Dokumentation des Krankheitsverlaufs, Basislaborwerte) und therapieassoziierten Daten (z.B. verwendetes Konzentrat, Konzentratverbrauch, Anzahl der Substitutionen, Faktor-VIII-Recovery sowie Inhibitorentwicklung und -eliminierung) von etwa 800 Patienten zu erfassen. Durch diese Erfassung kann der Effekt der unterschiedlichen Therapiemaßnahmen auf den Krankheitsverlauf bewertet werden. Einen besonderen Einfluss auf den Krankheitsverlauf und die Therapie hat der Faktor-VIII-Gendefekt, der ursächlich für die Hämophilie verantwortlich ist. Wir erwarten, dass bestimmte Mutationen bzw. Mutationstypen spezifischen Phänotypen und in etwa vergleichbaren therapeutischen Verläufen zugeordnet werden können.

Bedeutung der Mutationsdiagnostik bei Patienten mit Hämophilie A

Die Hämophilie ist die häufigste genetisch bedingte Form einer schweren Blutungsneigung. Ursächlich für die Erkrankung sind Defekte im Faktor-VIII-Gen, die zu verminderter Faktor-VIII-Aktivität mit einer vom Ausmaß der Störung abhängigen Blutungsneigung führen. Die ursächlichen Gendefekte können inzwischen routinemäßig identifiziert werden. Trotz der Vielfalt der Mutationen lassen sich mehrere Hotspots feststellen, z.B. Inversion im Intron 22, kleine Deletionen/Insertionen an Poly-A und Punktmutationen an CpG-Dinukleotiden. Bereiche geringer Mutationshäufigkeit sind z.B. im mittleren Abschnitt des Exons 14, der für die funktionell wenig bedeutende B-Domäne kodiert und praktisch keine Missense-Mutationen enthält. Die Mutationsdiagnostik verbesserte die Diagnostik und das Verständnis der Pathogenese der Hämophilie-A-Erkrankung. Nach Identifizierung des Gendefekts in einer Familie ist eine schnelle und sichere Konduktorinnendiagnostik möglich. Die Korrelation zwischen Gendefekt und klinischem Verlauf zeigte, dass die Art der Mutation einen wichtigen genetischen Prädispositionsfaktor für die zurzeit schwerste therapeutische Komplikation bei Hämophilie, die Hemmkörperbildung, darstellt. Ein relativ leichter Verlauf bei eigentlich schwerer Hämophilie A lässt sich mit speziellen Mutationen oder gleichzeitig vorliegenden thrombophiliefördernden Mutationen erklären. Molekulare Modelle des Faktor-VIII-Moleküls erlauben gezielt Auswirkungen von Mutationen zu untersuchen und so neue Struktur/ Funktionsbeziehungen zu erkennen. Dies könnte die Entwicklung zukünftiger rekombinanter Gerinnungspräparate mit veränderten Eigenschaften (z.B. höherer Wirkungsgrad, verlängerte Halbwertszeit, geringere Immunogenität) anstoßen.

Characterization of a mutation in the lens-specific MP70 encoding gene of the mouse leading to a dominant cataract

During an ethylnitrosourea mutagenesis screen, Aey5, a new mouse mutation exhibiting an autosomal dominant congenital cataract was isolated. The cataractous phenotype is visible at the eye opening and progresses to a nuclear and zonular cataract at 2 months of age with no difference in onset or severity between heterozygous and homozygous mutants. Histological analysis revealed that fiber cell differentiation continues at the lens bow region, but the cell nuclei do not degrade normally and remain in the deeper cortex. Further, the lens nucleus has clefts of various sizes while the remainder of the eye was morphologically normal. The mutation was mapped to chromosome 3 between the markers D3Mit101 and D3Mit77 near the connexin encoding genes Gja5 and Gja8. Sequence analysis revealed no differences in the Gja5 gene, but identified a T-->C mutation at position 191 in the Gja8 gene, which was confirmed by an additional Mva 12691 restriction site in the genomic DNA of homozygous mutants. This mutation results in Val-->Ala substitution at codon 64 of connexin50 (Cx50) also known as lens membrane protein 70 (MP70). Aey5 represents the second dominant mouse cataract mutant affecting Cx50, a membrane protein preferentially expressed in the lens. Since both mutations affect similar regions in the first extracellular domain this region appears to be critically important for its function in lens transparency.

Characterization of a new, dominant V124E mutation in the mouse alphaA-crystallin-encoding gene

PURPOSE

During an ethylnitrosourea (ENU) mutagenesis screening, mice were tested for the occurrence of dominant cataracts. The purpose of the study was morphologic description, mapping of the mutant gene, and characterization of the underlying molecular lesion in a particular mutant, Aey7.

METHODS

Isolated lenses were photographed and histologic sections of the eye were analyzed according to standard procedures. Linkage analysis was performed with a set of microsatellite markers covering all autosomal chromosomes. cDNA was amplified after reverse transcription of lens mRNA. For PCR, cDNA or genomic DNA was used as a template.

RESULTS

Nuclear opacity and posterior suture anomaly were visible at eye opening and progressed to a nuclear and zonular cataract at 2 months of age. The opacity as well as the microphthalmia was more pronounced in the homozygotes than in the heterozygotes. The mutation was mapped to chromosome 17 between the markers D17Mit133 and D17Mit180. This position made the alphaA-crystallin-encoding gene (Cryaa) an excellent candidate gene. Sequence analysis revealed a mutation of a T to an A at position 371 in the Cryaa cDNA. The mutation was confirmed by an additional MnlI restriction site in the genomic DNA of homozygous mutants leading to replacement of Val with Glu at codon 124 affecting the C-terminal region of the alphaA-crystallin.

CONCLUSIONS

The Aey7 mutant represents the first dominant mouse cataract mutation affecting the Cryaa gene. The mutation leads to progressive opacification of the lens. Compared with the beta- and gamma-crystallin-encoding genes, mutations in the alpha-crystallin-encoding genes are rare.

Regulation of the human SIX3 gene promoter

A 2-kb promoter fragment of SIX3, a human transcription factor essential for vertebrate eye development, has been characterized in a gene reporter assay system. The peak of activity implies the 2-kb sequence of SIX3, whereas 5'-deletion constructs of the promoter decreases successively to 60% of the activity starting from the entire promoter. In contrast, cutting off 300 bp of the 3' promoter extinguishes its activity completely. Coexpression experiments of different other transcription factors illuminate the regulation of SIX3 during eye development: Pax6 activates the -703/-349 SIX3 promoter threefold, and PROX1 even eightfold. In contrast, Msx2 represses the entire SIX3 promoter. Furthermore, Six3 is regulated by its own negative feedback loop. In conclusion, SIX3 expression underlies a complex regulation, which is an important part to understand the network of transcription factors during eye development.

Aey2, a new mutation in the betaB2-crystallin-encoding gene of the mouse

PURPOSE

During an ethylnitrosourea (ENU) mutagenesis screen, mice were tested for the occurrence of dominant cataracts. One particular mutant was found that caused progressive opacity and was referred to as Aey2. The purpose of the study was to provide a morphologic description, to map the mutant gene, and to characterize the underlying molecular lesion.

METHODS

Isolated lenses were photographed, and histologic sections of the eye were analyzed according to standard procedures. Linkage analysis was performed using a set of microsatellite markers covering all autosomal chromosomes. cDNA from candidate genes was amplified after reverse transcription of lens mRNA.

RESULTS

The cortical opacification visible at eye opening progressed to an anterior suture cataract and reached its final phenotype as total opacity at 8 weeks of age. There was no obvious difference between heterozygous and homozygous mutants. The mutation was mapped to chromosome 5 proximal to the marker D5Mit138 (8.7 +/- 4.2 centimorgan [cM]) and distal to D5Mit15 (12.8 +/- 5.4 cM). No recombinations were observed to the markers D5Mit10 and D5Mit25. This position makes the genes within the betaA4/betaB-crystallin gene cluster excellent candidate genes. Sequence analysis revealed a mutation of T-->A at position 553 in the Crybb2 gene, leading to an exchange of Val for GLU: It affects the same region of the Crybb2 gene as in the Philly mouse. Correspondingly, the loss of the fourth Greek key motif is to be expected.

CONCLUSIONS

The Aey2 mutant represents the second allele of Crybb2 in mice. Because an increasing number of beta- and gamma-crystallin mutations have been reported, a detailed phenotype-genotype correlation will allow a clearer functional understanding of beta- and gamma-crystallins.

Ethylnitrosourea-induced mutation in mice leads to the expression of a novel protein in the eye and to dominant cataracts

A novel ENU-induced mutation in the mouse leading to a nuclear and zonular opacity of the eye lens (Aey1) was mapped to chromosome 1 between the markers D1Mit303 and D1Mit332. On the basis of the chromosomal position, the gamma-crystallin encoding gene cluster (Cryg) and the betaA2-crystallin encoding gene Cryba2 were tested as candidate genes. An A --> T mutation destroys the start codon of the Cryge gene in the mutants; this mutation was confirmed by the absence of a restriction site for NcoI in the corresponding genomic fragment of homozygous mutants. The next in-frame start codon is 129 bp downstream; this predicted truncated gammaE-crystallin consists of 131 amino acids, resulting in a molecular mass of 14 kD. However, another open reading frame was observed just 19 bp downstream of the regular Cryge start codon, resulting in a protein of 119 amino acids and a calculated molecular weight of 13 kD. Western blot analysis using polyclonal antibodies against gamma-crystallins or the novel Aey1-specific protein demonstrated the specific expression of the Aey1 protein in the cataractous lenses only; the truncated form of the gammaE-crystallin could not be detected. Therefore, it is concluded that the novel protein destroys the sensitive cellular structure of the eye lens.

Antagonistic action of Six3 and Prox1 at the gamma-crystallin promoter

Gamma-crystallin genes are specifically expressed in the eye lens. Their promoters constitute excellent models to analyse tissue-specific gene expression. We investigated murine CRYGE/f promoters of different length in lens epithelial cell lines. The most active fragment extends from position -219 to +37. Computer analysis predicts homeodomain and paired-domain binding sites for all rodent CRYGD/e/f core promoters. As examples, we analysed the effects of Prox1 and Six3, which are considered important transcription factors involved in lens development. Because of endogenous Prox1 expression in N/N1003A cells, a weak stimulation of CRYGE/f promoter activity was found for PROX1. In contrast, PROX1 stimulated the CRYGF promoter 10-fold in CD5A cells without endogenous PROX1. In both cell lines Six3 repressed the CRYGF promoter to 10% of its basal activity. Our cell transfection experiments indicated that CRYG expression increases as Six3 expression decreases. Prox1 and Six3 act antagonistically on regulation of the CRYGD/e/f promoters. Functional assays using randomly mutated gammaF-crystallin promoter fragments define a Six3-responsive element between -101 and -123 and a Prox1-responsive element between -151 and -174. Since Prox1 and Six3 are present at the beginning of lens development, expression of CRYGD/e/f is predicted to remain low at this time. It increases as Six3 expression decreases during ongoing lens development.

Characterization of a 1-bp deletion in the gammaE-crystallin gene leading to a nuclear and zonular cataract in the mouse

PURPOSE

A previous study had found a mouse mutant to have bilateral nuclear cataract with zonular opacity after paternal irradiation with gamma-rays. The mutation was then demonstrated to be allelic with the Cat2 group of dominant cataract mutations and was referred to as Cat2(nz) in a later study. Because several members of this group have been confirmed as mutations in the gene cluster coding for gamma-crystallins (CRYG:), these genes were now tested as candidates for Cat2(nz).

METHODS

All six gamma-crystallin-encoding genes were amplified by polymerase chain reaction (PCR) from cDNA or genomic DNA and sequenced. An antibody against the changed protein was developed and used for Western blot analysis. The mutant was also characterized morphologically.

RESULTS

A 1-bp deletion in exon 2 of the gammaE-crystallin-encoding gene CRYGE: was causative of the cataract phenotype. This particular mutation is therefore referred to as CRYGE:(nz). The predicted frameshift after codon 29 led to a changed amino acid sequence of 96 amino acids. The altered 13-kDa protein was expressed in the eye lens as demonstrated by Western blot analysis. Cataracts became visible at day 18.5 of embryonic development and reached the final phenotype at 2 weeks after birth.

CONCLUSIONS

The CRYGE:(nz) is the sixth mutation in the mouse that has been reported so far to affect the CRYG: gene cluster, which demonstrates its importance for lens transparency.

Deletion in the promoter region and altered expression of Pitx3 homeobox gene in aphakia mice

Mouse aphakia (ak) is a recessive phenotype that spontaneously occurs in the 129/Sv-SlJ strain and is characterized by small eyes that lack a lens. We have recently identified a homeobox-containing gene, Pitx3, and have shown that it is expressed in the developing lens and maps to chromosome 19 close to ak in mouse. Human PITX3 gene was found to underlie anterior segment dysgenesis and cataracts. We have now obtained the entire sequence of the mouse Pitx3 gene including 10 kb of the 5' region and 5 kb of the 3' region. Of several microsatellite repeat regions identified within the Pitx3 sequence, one was informative for linkage analysis. No recombination was observed between ak and the Pitx3 marker, indicating that these two loci are closely linked (0.2 +/- 0.2 cM). Additionally, Pitx3 transcripts were not detected in the ak/ak mice either in the lens placode or at later developmental stages of the lens by in situ hybridization. Since no differences were previously found between ak/ak and wild-type sequences in the Pitx3 coding region, we hypothesized that an etiologic mutation is located in the promoter or other regulatory regions. To test this hypothesis we studied the 5' flanking region of the Pitx3 gene. This analysis revealed a deletion of 652 bp located 2.5 kb upstream from the start point of the Pitx3 5' UTR sequence in ak/ak mice. The deletion co-segregated with the ak mutation and was not detected in 16 samples from 10 different mouse strains including the founder strains. Analysis of the 652 bp region identified sequences similar to consensus binding sites for transcription factors AP-2 and Maf that were shown to play a critical role in lens determination. These lines of evidence suggest that the abnormal ocular development in the aphakia mouse is due to the deletion upstream of the Pitx3 gene.

Up-regulation of novel intermediate filament proteins in primary fiber cells: an indicator of all vertebrate lens fiber differentiation?

The early embryonic development and expression patterns of the eye lens specific cytoskeletal proteins, CP49 and CP95, were determined for the chick and were found to be similar in both human and mouse. These proteins, as well as their homologs in other species, are obligate polymerization partners which form unique filamentous structures termed "beaded filaments." CP49 and CP95 appeared as protein products after 3 days of embryonic development in the chick during the elongation of primary fiber cells. Although limited data were obtained for human embryos at these early developmental timepoints, they were consistent with the interpretation that the up-regulation of these lens specific proteins began only after the initiation of lens vesicle closure. In situ hybridization with the mouse lens confirmed that message levels for beaded filament proteins were greatly elevated in differentiating primary fiber cells. Nuclease protection assays established that mRNA levels for CP49 remained relatively constant while CP95 mRNA levels increased once the process of secondary fiber formation was under way. Although present in relatively low abundance, the mRNA for a unique splice variant of CP49, CP49(INS), was also detected early in embryonic development and into adulthood. Peptide-specific antibodies directed against unique predicted sequences were able to confirm the protein expression of CP49(INS) in both embryonic and adult chick lens cells. These data present the first detailed study of the expression of CP49 and CP95 during early lens development. They suggest that the up-regulated expression of CP49 and CP95 could serve as pan-specific markers for all vertebrate lens fiber development.

Mutation in the betaA3/A1-crystallin encoding gene Cryba1 causes a dominant cataract in the mouse

During the mouse ENU mutagenesis screen, mice were tested for the occurrence of dominant cataracts. One particular mutant was discovered as a progressive opacity (Po). Heterozygotes show opacification of a superficial layer of the fetal nucleus, which progresses and finally forms a nuclear opacity. Since the homozygotes have already developed the total cataract at eye opening, the mode of inheritance is semidominant. Linkage analysis was performed using a set of genome-wide microsatellite markers. The mutation was mapped to chromosome 11 distal of the marker D11Mit242 (9.3 +/- 4.4 cM) and proximal to D11Mit36 (2.3 +/- 2.3 cM). This position makes the betaA3/A1-crystallin encoding gene Cryba1 an excellent candidate gene. Mouse Cryba1 was amplified from lens mRNA. Sequence analysis revealed a mutation of a T to an A at the second base of exon 6, leading to an exchange of Trp by Arg. Computer analysis predicts that the fourth Greek key motif of the affected betaA3/A1-crystallin will not be formed. Moreover, the mutation leads also to an additional splicing signal, to the skipping of the first 3 bp of exon 6, and finally to the deletion of the Trp residue. Both types of mRNA are present in the homozygous mutant lenses. The mutation will be referred to as Cryba1(po1). This particular mouse mutation provides an excellent animal model for a human congenital zonular cataract with suture opacities, which is caused by a mutation in the homologous gene.

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.

Isolation and embryonic expression of the novel mouse gene Hic1, the homologue of HIC1, a candidate gene for the Miller-Dieker syndrome

The human gene HIC1 (hypermethylated in cancer) maps to chromosome 17p13.3 and is deleted in the contiguous gene disorder Miller-Dieker syndrome (MDS) [Makos-Wales et al. (1995) Nature Med., 1, 570-577; Chong et al. (1996) Genome Res., 6, 735-741]. We isolated the murine homologue Hic1, encoding a zinc-finger protein with a poxvirus and zinc-finger (POZ) domain and mapped it to mouse chromosome 11 in a region exhibiting conserved synteny to human chromosome 17. Comparison of genomic and cDNA sequences predicts two exons for the murine Hic1. The second exon exhibits 88% identity to the human HIC1 on DNA level. During embryonic development, Hic1 is expressed in mesenchymes of the sclerotomes, lateral body wall, limb and cranio-facial regions embedding the outgrowing peripheral nerves during their differentiation. During fetal development, Hic1 additionally is expressed in mesenchymes apposed to precartilaginous condensations, at many interfaces to budding epithelia of inner organs, and weakly in muscles. We observed activation of Hic1 expression in the embryonic anlagen of many tissues displaying anomalies in MDS patients. Besides lissencephaly, MDS patients exhibit facial dysmorphism and frequently additional birth defects, e.g. anomalies of the heart, kidney, gastrointestinal tract and the limbs (OMIM 247200). Thus, HIC1 activity may correlate with the defective development of the nose, jaws, extremities, gastrointestinal tract and kidney in MDS patients.

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.

Three murine cataract mutants (Cat2) are defective in different gamma-crystallin genes

A number of murine cataract mutations have been localized to chromosome 1 close to the gamma-crystallin gene cluster (Cryg) (Everett et al., 1994, Genomics 20: 429-434; Löster et al., 1994, Genomics 23: 240-242). Based on the size of the mapping or allelism tests they have not been shown to be genetically distinct and have been assigned to locus symbol Cat2. Here we assign three mutations to the respective gamma-crystallin gene. Using a systematic candidate gene approach to analyze the entire Cryg cluster, an A-->G transition was found in exon 2 of Cryga for the ENU-436 mutation and is designated Cryga1Neu. The mutant allele Crygbnop (formerly Cat2(nop)) is caused by a replacement of 11 bp by 4 bp in the third exon of Crygb, while a C-->G transversion in exon 3 of Cryge has been found for the Cryget (formerly Cat2(t)) mutation. For the mutation Cryga1Neu, an Asp-->Gly exchange is deduced, whereas the mutations Crygbnop and Cryget lead to the formation of in-frame stop codons and give rise to truncated proteins of 144 and 143 amino acids, respectively. The effects of the mutations upon gamma-crystallin structure are likely to be quite different. The Cryga1Neu mutation is expected to affect the link between Greek-key motifs 2 and 3, whereas both Crygbnop and Cryget mutations are supposed to truncate the fourth Greek-key motif. All three mutations are predicted to alter protein folding of the gamma-crystallins and result in lens cataract, but the phenotype for each is quite distinctive.

Polymorphism in the murine Tr2-11 gene encoding an orphan receptor, and its exclusion as a candidate gene for the cataract mutation Cat3

Since the dominant cataract mutation Cat3 was mapped very closely to the murine nuclear receptor TR2-11 gene locus, the corresponding coding region was amplified by PCR using either genomic DNA or eye-derived cDNA of wild-type (C3Hx102)F1 and of homozygous Cat3 cataract animals. The analysis of the complete coding sequences showed no differences. Additionally, the expression level was very similar. Therefore, Tr2-11 was excluded as a candidate for the Cat3 mutation. Surprisingly, the obtained sequences exhibited significant alterations to the murine Tr2-11 sequence reported previously (Lee et al., Genomics 30, 1995, 46-52). The differences in the DNA sequence predict remarkable secondary and tertiary structure alterations of the corresponding protein. The structure model of the new Tr2-11 protein is very similar to related receptors.

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.

Cat3vl and Cat3vao cataract mutations on mouse chromosome 10: phenotypic characterization, linkage studies and analysis of candidate genes

Cat3vl and Cat3vao are two allelic, dominant cataract mutations that arose independently in the F1 generation after gamma-irradiation of male mice. The cataracts are already present at birth. Examination of the eyes with a slit lamp revealed completely vacuolated lenses in Cat3vl mutants and anteriorly located opacity in Cat3vao mutants. The appearance of the opacities does not differ between the individuals or between heterozygotes and homozygotes. Penetrance of the mutations is complete. Viability and fertility of the mutants are normal except in the case of the Cat3vl homozygotes. Cat3vao was assigned to the distal part of mouse chromosome 10, 3.2 +/- 0.9 cM away from the visible marker Steel (SlgbH). Using polymorphic markers the following locus order was found: D10Mit230-(0.2 +/- 0.1 cM)-Cat3vao-(2.5 +/- 0.6 cM)-D10Mit70. No recombinants were found between Cat3vao and the markers D10Mit4l and D10Mit95 among 921 offspring. The results exclude allelism of Cat3vao with CatLop or To2, which also map to chromosome 10. Candidate genes were tested by examination of their expression in the eye of newborn mice and by analysis of cDNA sequences. So far, negative results have been obtained for the genes encoding the proteoglycans lumican and decorin, the nuclear orphan receptor Tr2-11 and the transcription factor Elk3. Based on syntenic homology of the Cat3 region to the human chromosome 12q, the Cat3 mutants are discussed as mouse models for cornea plana congenita in man. The recovery of the Cat3 mutations demonstrates the importance of the corresponding locus for proper eye development.

The crystallins: genes, proteins and diseases

The crystallins were discovered as the structural proteins of the vertebrate eye lens in the last century by C.T. Mörner (Z. Physiol. Chem. 18, 1893, 61-106). Since that time the mammalian crystallins referred to as alpha-, beta-, and gamma-crystallins have been characterized with respect to their genetic organization, the regulation of their expression pattern and their participation in several diseases. Moreover, some crystallins have also been discovered outside the eye. Evolutionary analysis has demonstrated the relationship of crystallins to proteins involved in protection against stress. The alpha-crystallins are considered to be molecular chaperones and members of the small heat shock protein family; they have autokinase activity and are involved in the gamma-crystallin gene activation. The alpha-crystallins are associated with a broad variety of neurological disorders. The beta/gamma-crystallin superfamily is characterized by four greek key motifs. The various N- and C-terminal extensions of the beta/gamma-crystallins are mainly responsible for their distinct biophysical and biochemical properties. Modifications in the beta/gamma-crystallins or mutations in their genes lead to opacification of the eye lens (cataract). Other proteins found to be expressed at relatively high levels in the lens are characterized bytheir strong relationship to well-known enzymes. They are referred to as enzyme-crystallins, and as one example, the xi-crystallin will be discussed. It has evolved from a quinone oxidoreductase using a lens-specific promoter, and a mutation in xi-crystallin is involved in cataract formation.

Autokinase activity of alpha-crystallin inhibits its specific interaction with the DOTIS element in the murine gamma D/E/F-crystallin promoter in vitro

In a previous report we demonstrated the in vitro interaction of alpha-crystallin with an element downstream of the transcriptional initiation site (DOTIS) of the murine gamma E-crystallin promoter (Pietrowski et al., 1994, Gene 144, 171-178). The aim of the present study was to investigate the influence of phosphorylation on this particular interaction. We could demonstrate that the autophosphorylation of alpha-crystallin leads to a complete loss of interaction with the DOTIS element, however, PKA-dependent phosphorylation of alpha-crystallin is without effect on the interaction. It is hypothesized that the autophosphorylation of alpha-crystallin might be involved in regulatory mechanisms of the murine gamma D/E/F-crystallin gene expression.

Detection of a point mutation (A to G) in exon 5 of the murine Mgf gene defines a novel allele at the Steel locus with a weak phenotype

A new mutation at the locus encoding the mast cell growth factor (Mgf) is described and designated as MgfSl-3Neu. Homozygous mutants have a light grey fur, sometimes with white patches. Homozygotes are fertile, but with reduced litter size, when mated inter se. Analysis of haematological parameters indicated no difference between mutant and wild-type mice. Sequence analysis of the cDNA obtained from the brain of homozygous mutants revealed an A-->G exchange at position 400 leading to a predicted amino acid exchange from Asn-->Leu at position 122. As a consequence of the predicted amino acid exchange an extension of the alpha-helical context and a decreased hydropathicity of the region at positions 101-125 can be deduced. This single amino acid exchange is outside of the known important domains of MGF and explains the weak phenotype of MgfSl-3Neu.