The role of neural and optical factors in limiting visual resolution in myopia

The myopic growth process has the potential to modify both the optical and neural performance of the eye. We provide three simple models, based on different types of retinal stretching, to predict changes in neural resolution resulting from axial length increases in myopia. These predictions are compared to visual acuity (VA) measures in 34 subjects with refractive errors ranging from plano to -14 D. Our results show a reduction in VA with increasing myopia but not in a manner predicted by our models. We discuss the relative contribution of optical and neural factors to the reduction in visual resolution in myopia.

Tumour susceptibility and spontaneous mutation in mice deficient in Mlh1, Pms1 and Pms2 DNA mismatch repair

Germline mutations in the human MSH2, MLH1, PMS2 and PMS1 DNA mismatch repair (MMR) gene homologues appear to be responsible for most cases of hereditary non-polyposis colorectal cancer (HNPCC; refs 1-5). An important role for DNA replication errors in colorectal tumorigenesis has been suggested by the finding of frequent alterations in the length of specific mononucleotide tracts within genes controlling cell growth, including TGF-beta receptor type II (ref. 6), BAX (ref. 7) and APC (ref. 8). A broader role for MMR deficiency in human tumorigenesis is implicated by microsatellite instability in a fraction of sporadic tumours, including gastric, endometrial and colorectal malignancies. To better define the role of individual MMR genes in cancer susceptibility and MMR functions, we have generated mice deficient for the murine homologues of the human genes MLH1, PMS1 and PMS2. Surprisingly, we find that these mice show different tumour susceptibilities, most notably, to intestinal adenomas and adenocarcinomas, and different mutational spectra. Our results suggest that a general increase in replication errors may not be sufficient for intestinal tumour formation and that these genes share overlapping, but not identical functions.

Building effective partnerships: a national evaluation of the Cancer Information Service outreach program. Part 2

The Cancer Information Service's (CIS) nationally coordinated, regionally focused outreach program establishes partnerships with government, nonprofit, and private organizations to reach underserved and minority populations. A national random sample telephone survey, conducted with 867 partner organizations who had contact with the CIS in early 1996, assessed the types of CIS assistance used, satisfaction with and usefulness of these services, organizational characteristics, and the overall impact on partners' cancer-related programs. CIS partners provide programs for the medically underserved (79%) and specific racial and ethnic groups (72%) and address a range of cancer control issues. Partners use and highly value an array of CIS assistance. Over 90% are satisfied with the CIS services. Diffusion of accurate and science-based information, materials and resources is a key element of the outreach assistance. Two out of three partners are receiving late-breaking news and scientific information from the CIS and 86% are disseminating it to their constituents. The majority of those partners surveyed reported that they rely on the CIS to assist with their most important cancer programs. Sixty-eight percent of the partners rated the CIS assistance as important or very important to their cancer-related programs. These survey results indicate that the CIS outreach program provides key support to partners' cancer programs and has an impact on their efforts. The results support the effectiveness of a regionally based outreach program working in partnership with existing organizations at the state, county, and local level who are addressing the needs of underserved populations.

Novel monosaccharides as potent inhibitors of cell proliferation

The effects of several novel monosaccharides upon thymidine incorporation into both normal and tumour cells were investigated. The monosaccharide 2-deoxy-3-[1-(R)-(ethoxycarbonyl)ethyl]- alpha-D-allo-pyranose had the most inhibitory effect on proliferation, with the (S)-enantiomer having less inhibitory effects. The chiral centre at carbon-7 was found to be an important part of the molecule, as 2-deoxy-3-[methoxycarbonyl methyl]-alpha-D-allo-pyranose had greatly decreased anti-proliferative properties in comparison with the parent compound. In addition, the 2-deoxy structure at carbon-2 was also found to be important, as 3-[1-(S)-(ethoxycarbonyl)ethyl]-alpha-D-allo-hexopyranose had greatly decreased inhibitory properties in comparison with the parent compound. The results indicate that these novel monosaccharides possess potent anti-proliferative properties, related to their chiral carbon-7 and 2-deoxy carbon-2 structure and suggest that further substitutions of the functional group at carbon-7 may improve these properties and possibly produce inhibitor selectivity for tumour cells in preference to normal cells.

Using targeted large deletions and high-efficiency N-ethyl-N-nitrosourea mutagenesis for functional analyses of the mammalian genome

The Human Genome Project has generated nucleotide sequences from an estimated 80,000 to 100,000 genes, only a small fraction of which have a known role. Nucleotide sequence information alone is insufficient to predict gene function. One of the most powerful ways of revealing gene function, as demonstrated in bacteria, worms, yeast, and flies, is to generate mutations and characterize them at both the phenotypic and the molecular levels. Given the physiological and anatomical parallels between mouse and human, genotype-phenotype relationships established in mice can be extrapolated to human syndromes. A new method is described for functional genetic analyses in the mouse that uses loxP/Cre engineering to generate coat color-tagged large deletions. The haploid regions can then be dissected by mutagenesis with N-ethyl-N-nitrosourea in phenotype-driven screens to obtain functional information on genes in any desired region of the mouse genome.

Role of DNA mismatch repair in the cytotoxicity of ionizing radiation

The DNA mismatch repair (MMR) system in mammalian cells not only serves to correct base mispairs and other replication errors, but it also influences the cellular response to certain forms of DNA damage. Cells that are deficient in MMR are relatively resistant to alkylation damage because, in wild-type cells, the MMR system is thought to promote toxicity via futile repair of alkylated mispairs. Conversely, MMR-deficient cells are sensitive to UV light, possibly due to the requirement for MMR factors in transcription-coupled repair of active genes. MMR deficiency has been associated with familial and sporadic carcinomas of the colon and other sites, and so, we sought to determine the influence of MMR status on cellular response to ionizing radiation, an agent commonly used for cancer therapy. Fibroblast cell lines were established from transgenic mice carrying targeted disruptions of one of three MMR genes in mammalian cells: Pms2, Mlh1, or Msh2. In comparison to wild-type cell lines from related mice, the Pms2-, Mlh1-, or Msh2-nullizygous cell lines were found to exhibit higher levels of clonogenic survival following exposure to ionizing radiation. Because ionizing radiation generates a variety of lesions in DNA, the differences in survival may reflect a role for MMR in processing a subset of these lesions, such as damaged bases. These results both identify a new class of DNA-damaging agents whose effects are modulated by the MMR system and may help to elucidate pathways of radiation response in cancer cells.

Effects of refractive error on detection acuity and resolution acuity in peripheral vision

PURPOSE

To evaluate the effect of refractive error on detection acuity and resolution acuity in peripheral vision.

METHODS

Detection acuity, defined as the highest spatial frequency for which luminance gratings can be discriminated from a uniform field, and resolution acuity, defined as the highest spatial frequency for which spatial patterns are perceived veridically, was determined for vertical and horizontal gratings located at 20 degrees, 30 degrees, and 40 degrees of eccentricity. Resolution was also measured for tumbling-E discrimination at these locations. Refractive state of the eye for test targets was manipulated by introducing an ophthalmic trial lens into the line of sight for the stimulus while holding accommodative state fixed.

RESULTS

Detection acuity in the periphery varied significantly with the amount of optical defocus, whereas acuity for grating resolution or letter discrimination was unaffected by defocus over a large range (up to 6 D). These results are consistent with the working hypothesis that detection acuity in the periphery is limited by contrast insufficiency under normal viewing conditions, but resolution is limited by ambiguity because of neural undersampling.

CONCLUSIONS

The large depth of focus for resolution acuity measured for peripheral vision indicates that spatial resolution is likely to remain sampling-limited even when peripheral refractive errors are not fully corrected, thus relaxing the methodologic requirements for obtaining noninvasive estimates of neural sampling density of the living eye in a clinical setting.

Interaction between sub- and supra-Nyquist spatial frequencies in peripheral vision

In peripheral vision, high-frequency gratings beyond the Nyquist limit are visible as aliased patterns but, as shown previously, their visibility can be masked by superimposed sub-Nyquist gratings. Is the converse also true? Can supra-Nyquist gratings affect the detectability of sub-Nyquist gratings? In this study, we investigated the masking effect of high contrast, supra-Nyquist components of a compound grating on the contrast detection of sub-Nyquist components by employing a temporal three-alternative, forced-choice (3AFC) masking paradigm. We found that high-frequency, aliased gratings with contrast just 2 or 3 times above threshold can have a powerful masking effect on low-frequency, resolved gratings in peripheral vision. This result was surprising because prior results from sub-Nyquist masking studies in the fovea and the periphery have indicated that masking occurs only when the mask contrast is at least 5 times greater than threshold. Strong masking by supra-Nyquist gratings that are only just visible may be accounted for by an irregular sampling model in which the alias of the mask is distributed over a band of frequencies in the sub-Nyquist range. Furthermore, if undersampling is the explanation for the results of this study, then masking must occur after spatial sampling.

Use of liquid-crystal adaptive-optics to alter the refractive state of the eye

We evaluated the potential of wavefront shaping with liquid-crystals for modulating the eye's refractive state. A spatial light modulator with 127 liquid crystals cells was imaged in the entrance pupil of the eye and programmed to induce prismatic, spherical, and astigmatic refractive changes. Psychophysical evaluation of these optical effects was in agreement with expectations for prisms up to approximately 0.08 D and for lenses up to approximately 1.5 D. These maximum dioptric values represent wavefront retardation of about 3 to 4 wavelengths of 584 nm light across a 3-mm diameter pupil. Optical aliasing of high-power prisms was traced to spatial undersampling of the wavefront retardation function by the discrete array of liquid crystal cells. Undersampling may also be the factor which limits the useful dioptric range of the technique.

Spherical aberration of the reduced schematic eye with elliptical refracting surface

We extend the single-surface schematic-eye model of ocular chromatic aberration to account for spherical aberration of the eye. This extension is accomplished by allowing the model's single refracting surface to be a member of the family of ellipses with variable shape parameter (eccentricity). The resulting model, dubbed the "Indiana Eye," may have either positive or negative spherical aberration of varying degree, depending upon the numerical value of the shape parameter. Spherical aberration of the model eye is well described by third-order optical theory for shape parameters in the range 0 < or = p < or = 0.7, but requires fifth-order theory for an accurate description over the parametric range 0.7 < p < or = 1.0. An improved technique was devised for fitting the model to published measurements of ray aberrations while avoiding errors of estimation of the degree of spherical aberration present in eyes which also manifest odd-symmetric aberrations, such as coma. A shape parameter value of approximately p = 0.6 provided the best fit of the model to selected data from the literature.

Wavelength-dependent magnification and polychromatic image quality in eyes corrected for longitudinal chromatic aberration

Theoretical calculations using a simple model eye in combination with achromatizing lenses or artificial pupils show that correcting wavelength-dependent refractive error or its effects can exaggerate wavelength-dependent magnification by up to a factor of 7. These calculations are confirmed experimentally, and their effects on retinal image quality are modeled. Because of the increased wavelength-dependent magnification, gains in polychromatic image quality produced by correcting wavelength-dependent refractive error (or minimizing its effects with small pupils) are generally restricted to a small region of the retina.

Visual performance and patient preference: a comparison of anti-reflection coated and uncoated spectacle lenses

BACKGROUND

Anti-reflection (AR) coatings of spectacle lenses are designed to reduce the reflected images often found in uncoated lenses. This study examined the visual impact of a specific AR coating.

METHODS

First, using a survey, it was determined if patients were aware of the AR coating and, if given an identical pair of lenses without the AR coating, which lenses were preferred and why. Second, in the laboratory, changes in visual performance were evaluated when an anti-reflection coating was applied to a lens.

RESULTS

Verbal reports were obtained from 18 patients on both types of lenses. All patients were aware the AR coating reduced unwanted reflections, and many reported reductions in visual problems associated with glare. Within the laboratory, 28 patients were tested on a battery of clinical vision tests. Under normal room illumination, visual acuity, grating-contrast sensitivity, and letter-contrast sensitivity were identical for both coated or uncoated lenses. In a second experiment, the effects of AR coating on contrast sensitivity were examined when subjects viewed a dimly illuminated target while their eye and face were brightly illuminated. Under these conditions, significantly better (a factor of two) contrast sensitivity was observed with AR-coated lenses.

CONCLUSIONS

On the basis of these findings, it is proposed that drivers should wear AR-coated spectacle lenses at night.

Embryonic lethality and radiation hypersensitivity mediated by Rad51 in mice lacking Brca2

Inherited mutations in the human BRCA2 gene cause about half of the cases of early-onset breast cancer. The embryonic expression pattern of the mouse Brca2 gene is now defined and an interaction identified of the Brca2 protein with the DNA-repair protein Rad51. Developmental arrest in Brca2-deficient embryos, their radiation sensitivity, and the association of Brca2 with Rad51 indicate that Brca2 may be an essential cofactor in the Rad51-dependent DNA repair of double-strand breaks, thereby explaining the tumour-suppressor function of Brca2.

Entoptic evaluation of diabetic retinopathy

PURPOSE

Studies using optimized entoptic viewing of the parafoveal retinal vasculature have shown that normal subjects see their own capillaries with greater detail in the fovea than seen typically in fluorescein angiography. The authors have extended these investigations to persons with diabetes to evaluate the sensitivity, specificity, and accuracy with which they can detect and locate their own parafoveal retinal defects untrained.

METHODS

A vascular entoptoscope using Maxwellian view optics creates a high-contrast entoptic view of retinal vasculature abnormalities in the parafoveal area. Using a double-masked protocol, 70 patients with diabetes and 29 control subjects described, drew, and quantified their entoptic image. These entoptic records were compared to angiograms and color photographs obtained immediately after the entoptic evaluation.

RESULTS

Angiograms or color photographs or both showed that 61 of 70 patients with diabetes had retinal defects (e.g., microaneurysms or exudates or both) within the field of view of the Vascular Entoptoscope (8.1 degrees or 11.6 degrees circular field depending on the Vascular Entoptoscope used: parafoveal area subtends approximately 9.7 degrees). Of these 61 patients with diabetes, 51% (31) observed dark "spots" or "blobs" in the entoptic field corresponding to retinal defects in the angiograms or photographs or both. Seven (18%) of the 38 patients (9 patients with diabetes and 29 control subjects without defects in the entoptic field) said they saw something when angiograms or photographs or both showed nothing (false-positive). Thus, the sensitivity and specificity (using angiograms or photographs or both as the gold standard) with which untrained patients with diabetes detect their own parafoveal area defects are 51% and 82%, respectively. Superimposition of the entoptic image (as drawn by the patient) and the angiograms or color photography or both often showed excellent correspondence. Most (22 of 29) of the control subjects and more than half (40 of 70) the patients with diabetes were able to quantify the size of their foveal avascular zone (FAZ) from the entoptic view, whereas only 22 of 70 of the capillary loops defining the FAZ were visible in the optimal frame of the capillary phase of the fluorescein angiogram. As reported previously in a smaller sample, large FAZs often were associated with poor visual acuity.

CONCLUSIONS

More than half the untrained patients with diabetes were able to visualize their own parafoveal retinopathy entoptically, and most untrained patients with diabetes and control subjects where able to quantify the size of their FAZ. Patients and control subjects without parafoveal defects rarely report defects not visible photographically. Patients can be trained to detect their defects. Clinical entoptic monitoring will require verification that patients can detect changes in their retinopathy. Entoptic testing is low cost, noninvasive, and can be performed as often as needed at no risk to the patient. It is, therefore, a promising research technique for subjective monitoring of the early natural history of parafoveal area disease processes.

Murine Brca2: sequence, map position, and expression pattern

Mutations in the human BRCA2 gene are responsible for about 45% of hereditary early onset breast cancer. Recently, the human BRCA2 gene was cloned, and several germline mutations were identified. Here we describe the cloning of the mouse homologue of BRCA2. The mouse cDNA sequence predicts a 3328-amino-acid Brca2 protein, 90 amino acids shorter than the human protein. The overall identity between the mouse and the human proteins is 59%, while the similarity is 72%. At the nucleotide level the homology is 74%. By comparing the amino acid sequences of the two homologues we have identified five highly conserved novel domains that may be functionally significant. Brca2 has been mapped to the distal end of mouse chromosome 5, a region of the mouse genome that contains other genes that also map to human chromosome 13q12-q13, confirming the conservation of this linkage group between the two species. Expression of Brca2 was detected in midgestation embryos and adult testis, thymus, and ovary.

Aliased frequencies enable the discrimination of compound gratings in peripheral vision

Although gratings beyond the Nyquist limit of the peripheral retina are visible as aliased percepts when presented in isolation, the reported lack of aliasing for targets with complex spectra (edges, lines, letters) suggests that aliased frequency components are invisible in the presence of sub-Nyquist image components. We tested this hypothesis by systematically exploring a range of stimulus parameters in search of conditions which would enable subjects to detect the supra-Nyquist components of a compound grating. A three-alternative forced-choice masking paradigm was used, which required subjects to discriminate a 2.5 deg patch of compound grating (mask + test) from a simple grating (mask only). Using a 2 c/deg grating as the masking component, which is well below the 4 c/deg Nyquist limit to veridical perception at 20 deg in the horizontal nasal field, we varied the spatial frequency of the test grating over a range extending above and below the Nyquist frequency. We found that aliased, supra-Nyquist components are reliably detected in the presence of high contrast, sub-Nyquist gratings, provided they have sufficient contrast. Contrast threshold for detecting the aliased gratings was higher when presented as a component of the compound grating than when presented in isolation. This masking effect of the sub-Nyquist component was not specific to supra-Nyquist test targets, however, since a similar masking effect also occurred for sub-Nyquist targets. These results suggest that the invisibility of aliasing described previously for edges or square wave gratings is due primarily to the combination of the low amplitudes of supra-Nyquist harmonics in such stimuli and a high contrast threshold of the peripheral retina. The additional masking effects of the sub-Nyquist, fundamental component of a square wave on the supra-Nyquist harmonics make the detection of aliasing impossible even for very high contrast square wave gratings presented in the periphery.

A prospective evaluation of central venous blood flow using Doppler ultrasound in patients with a long-term central venous catheter

Using Doppler ultrasound, we prospectively evaluated the relationship between central venous blood flow and the development of central venous thrombosis in ten patients with a long term central line. The presence of turbulent blood flow around the catheter was followed by the development of central venous thrombosis in two patients but neither had subsequent clinical sequelae. More surprisingly, however, normal blood flow was demonstrated in eight patients, two of whom subsequently developed intravascular thrombi.

Altered segmental identity and abnormal migration of motor neurons in mice lacking Hoxb-1

Segmentation of the vertebrate hindbrain into rhombomeres is important for the anterior-posterior arrangement of cranial motor nuclei and efferent nerves. Underlying this reiterated organization, Hox genes display segmentally restricted domains of expression, such as expression of Hoxb-1 (refs 5, 6) in rhombomere 4 (r4). Here we report that absence of Hoxb-1 leads to changes in r4 identity. In mutant mouse embryos, molecular markers indicate that patterning of r4 is initiated properly but not maintained. Cellular analysis by DiI tracing reveals that the r4-specific facial branchiomotor (FBM) and contralateral vestibuloacoustic efferent (CVA) neurons are incorrectly specified. In wild-type mice CVA neurons migrate from r4 into the contralateral side, and we found in lineage analysis that FBM neurons migrate from r4 into r5. In mutants, motor neurons differentiate but the CVA and FBM neurons fail to migrate into their proper positions. Instead, they form a motor nucleus which migrates atypically, and there is a subsequent loss of the facial motor nerve. These results demonstrate that, as a part of its role in maintaining rhombomere identity, Hoxb-1 is involved in controlling migratory properties of motor neurons in the hindbrain.

Requirement of the paraxis gene for somite formation and musculoskeletal patterning

The segmental organization of the vertebrate embryo is first apparent when somites form in a rostrocaudal progression from the paraxial mesoderm adjacent to the neural tube. Newly formed somites appear as paired epithelial spheres that become patterned to form vertebrae, ribs, skeletal muscle and dermis. Paraxis is a basic helix-loop-helix transcription factor expressed in paraxial mesoderm and somites. Here we show that in mice homozygous for a paraxis null mutation, cells from the paraxial mesoderm are unable to form epithelia and so somite formation is disrupted. In the absence of normal somites, the axial skeleton and skeletal muscle form but are improperly patterned. Unexpectedly, however, we found that formation of epithelial somites was not required for segmentation of the embryo or for the establishment of somitic cell lineages. These results demonstrate that paraxis regulates somite morphogenesis, and that the function of somites is to pattern the axial skeleton and skeletal muscles.

Effective immunization against influenza in pediatric renal transplant recipients

Viral infections such as influenza are an important cause of morbidity following organ transplantation. We evaluated the immunogenicity of a commercially available influenza vaccine in pediatric renal transplant recipients in a two-phase, prospective study. In phase one, 47 transplant patients and seven control subjects with bronchopulmonary dysplasia received influenza vaccine. Sera were collected at the time of vaccination and 6 wk later. In phase two, sera from 18 transplant recipients and 47 healthy adults who had received the same vaccine were collected 6-12 months after vaccination. Antibody titers to the A/Taiwan/1/86 antigen were measured with hemagglutination inhibition assay in both phases of the study. Vaccine was well tolerated in all subjects. No vaccinated patient required hospitalization for complications of influenza infection. Vaccination did not increase the frequency of acute allograft rejection. In phase one, 43 patients (91%) and 5 controls (71%) either seroconverted (developed a fourfold or greater rise in titer), or developed post-vaccination titers > or = 1:160 (p = NS). Among the transplant recipients, non-seroconverters had a higher pre-vaccination geometric mean antibody titer (GMT) than those who seroconverted. Seroconversion developed independently of whether patients received double or triple immunosuppression. In phase two, post-vaccination GMT were similar for patients and control subjects at 11.5 and 8 months post-vaccination, respectively. In our study, influenza vaccination produced equivalent humoral immunity in transplant recipients and normal subjects. Routine influenza vaccination should be performed annually in this high-risk population.

The tumorigenic potential and cell growth characteristics of p53-deficient cells are equivalent in the presence or absence of Mdm2

The Mdm2 oncoprotein forms a complex with the p53 tumor suppressor protein and inhibits p53-mediated regulation of heterologous gene expression. Recently, Mdm2 has been found to bind several other proteins that function to regulate cell cycle progression, including the E2F-1/DP1 transcription factor complex and the retinoblastoma tumor-suppressor protein. To determine whether Mdm2 plays a role in cell cycle control or tumorigenesis that is distinct from its ability to modulate p53 function, we have examined and compared both the in vitro growth characteristics of p53-deficient and Mdm2/p53-deficient fibroblasts, and the rate and spectrum of tumor formation in p53-deficient and Mdm2/p53-deficient mice. We find no difference between p53-deficient fibroblasts and Mdm2/p53-deficient fibroblasts either in their rate of proliferation in culture or in their survival frequency when treated with various genotoxic agents. Cell cycle studies indicate no difference in the ability of the two cell populations to enter S phase when treated with DNA-damaging agents or nucleotide antimetabolites, and p53-deficient fibroblasts and Mdm2/p53-deficient fibroblasts exhibit the same rate of spontaneous immortalization following long-term passage in culture. Finally, p53-deficient mice and Mdm2/p53-deficient mice display the same incidence and spectrum of spontaneous tumor formation in vivo. These results demonstrate that deletion of Mdm2 has no additional effect on cell proliferation, cell cycle control, or tumorigenesis when p53 is absent.

Consequences of monocular diplopia for the contrast sensitivity function

Though the human eye generally creates a single image on the retina, the literature contains many examples showing perceptual monocular diplopia. Previously, monocular diplopia resulting from astigmatic defocus has been demonstrated to cause a notch (local minimum) in the contrast sensitivity function (CSF). We examine Verhoeff's (1900) model which explains how monocular diplopia can occur through an interaction between defocus and common ocular aberrations. From the measured ocular transverse aberration function and from the measured monocular diplopia of three cyclopleged subjects we predicted multiple notches in the CSF with hyperopic spherical defocus. Monochromatic and polychromatic CSF were measured for vertical gratings with best refraction and with simulated myopia and hyperopia. Multiple notches in CSF were observed experimentally. Notches in the polychromatic CSF were smaller and broader than those found in the monochromatic CSF. Our aberration model was successful in predicting notches in the CSF with hyperopic spherical defocus. The implications for clinical measurement of CSF are discussed.