Further linkage data on Norrie disease

Familial exudative vitreoretinopathy and related retinopathies

Familial exudative vitreoretinopathy (FEVR) is a rare inherited disorder of retinal angiogenesis. Cases can be autosomal dominant, autosomal recessive, or X-linked. FEVR patients have an avascular peripheral retina which, depending on the degree of ischaemia, causes the secondary complications of the disease. Expressivity may be asymmetric and is highly variable. Five genes have been identified that when mutated, cause FEVR; NDP (X-linked), FZD4 (autosomal dominant and recessive), LRP5 (autosomal dominant and recessive), TSPAN12 (autosomal dominant and recessive), and ZNF408 (autosomal dominant). Four of these genes have been shown to have a central role in Norrin/Frizzled4 signalling, suggesting a critical role for this pathway in retinal angiogenesis. In addition to the ocular features, LRP5 mutations can cause osteopenia and osteoporosis. All FEVR patients in whom molecular testing is not easily accessible should have dual energy X-ray absorptiometry (DEXA) scans to assess bone mineral density, as treatment can be initiated to reduce the risk of bone fractures.

Inversion (X)(p11.4q22) associated with Norrie disease in a four generation family

We report on a 4-generation family in which Norrie disease occurs together with a pericentric inversion of the X chromosome in all affected males and carrier females. The breakpoint in the short arm of the X chromosome appears to be at the purported location of the Norrie disease gene. This is the second report of an association between Norrie disease and a chromosome aberration involving Xp11, and the first report of a specific gene disruption, thus physical gene location, due to a pericentric chromosome inversion.

Clinical reinvestigation and linkage analysis in the family with Episkopi blindness (Norrie disease)

We present the results of a clinical and genetic reinvestigation of the Cypriot family affected by an X chromosomally inherited eye disease originally published by Taylor et al, who coined the term Episkopi blindness. The pedigree was extended to 160 members, including 16 affected males out of 48 males at risk for the disease, most of whom were seen by one of us (PA). Affected males are blind with no associated symptoms and apparently are not mentally retarded. Thirty-nine family members agreed to blood sampling for genetic investigations. RFLP analysis was performed using probes from the region known to be deleted in some Norrie patients and polymorphic markers (DXS77, DXS7, MAOA, DXS255) from the proximal short arm of the X chromosome. There was no deletion for any of the probes in the affected males. Linkage analysis yielded positive lod scores for all informative markers (Z (DXS255, theta = 0) = 6.54, Z (MAOA, theta = 0) = 2.23, Z (DXS7, theta = 0) = 2.13). Thus, the conclusion that Episkopi blindness and Norrie disease (NDP, MIM *310600) are the same entity based on clinical evidence is now reinforced by gene mapping.

Isolation of a candidate gene for Norrie disease by positional cloning

The gene for Norrie disease, an X-linked disorder characterized by progressive atrophy of the eyes, mental disturbances and deafness, has been mapped to chromosome Xp11.4 close to DXS7 and the monoamine oxidase (MAO) genes. By subcloning a YAC with a 640 kilobases (kb) insert which spans the DXS7-MAOB interval we have generated a cosmid contig which extends 250 kb beyond the MAOB gene. With one of these cosmids, microdeletions were detected in several patients with Norrie disease. Screening of cDNA libraries has enabled us to isolate and sequence a likely candidate gene for Norrie disease which is expressed in retina, choroid and fetal brain. No homologous sequences were found in DNA and protein databases indicating that this cDNA is part of a gene encoding a 'pioneer' protein.

Localisation of the gene for Norrie disease to between DXS7 and DXS426 on Xp

A highly informative microsatellite marker, DXS426, which maps proximal to DXS7 in the interval Xp11.4-Xp11.23, has been used to refine further the localisation of the gene for Norrie disease (NDP). The results from a multiply informative crossover localize the NDP gene proximal to DXS7. In conjunction with information from 2 NDP patients who have a deletion for DXS7 but not for DSX426, our data indicate that the NDP gene lies between DXS7 and DXS426 on proximal Xp.

Audiometric tests in gene carriers of Norrie's disease

The purpose of the present study was to validate the reliability of audiometric testing procedures in the identification of carriers of Norrie's disease by using DNA analysis. Potential female carriers of Norrie's disease were investigated by means of Békésy tracing audiometry and stapedial reflex thresholds previously found to reveal carriers of recessively inherited hearing impairment by means of peculiar 'dips' and elevation in the thresholds. Relating the audiometric test results to the outcome of the DNA analysis the previous findings could not be confirmed. In addition no concordance between the two audiometric test procedures was demonstrated, and Békésy threshold tracing exhibited poor repeatability and sensitivity. It is concluded that none of these audiometric test procedures are feasible for detection of carriers of an X-linked hearing disorder, such as Norrie's disease.

Ornithine aminotransferase (OAT): recombination between an X-linked OAT sequence (7.5 kb) and the Norrie disease locus

A human ornithine aminotransferase (OAT) locus has been mapped to the Xp11.2, as has the Norrie disease locus. We used a cDNA probe to investigate a 3-generation UCLA family with Norrie disease; a 4.2-kb RFLP was detected and a maximum lod score of 0.602 at zero recombination fraction was calculated. We used the same probe to study a second multigeneration family with Norrie disease from Utah. A different RFLP of 7.5 kb in size was identified and a recombinational event between the OAT locus represented by this RFLP and the disease loci was observed. Linkage analysis of these two loci in this family revealed a maximum load score of 1.88 at a recombination fraction of 0.10. Although both families have affected members with the same disease, the lod scores are reported separately because the 4.2- and 7.5-kb RFLPs may represent two different loci for the X-linked OAT.

Physical fine-mapping of a deletion spanning the Norrie gene

Norrie disease (ND), atrophia bulborum hereditaria, is caused by a gene defect on the proximal short arm of the X-chromosome. As shown by us and others, microdeletions spanning the DXS7 locus are not uncommon in this disorder, and there is recent evidence that, at least in some of the Norrie deletion patients, the monoamine oxidase (MAO) A and B genes are deleted as well. Molecular hybridization experiments with 19 cloned DNA fragments have enabled us to construct a preliminary long-range restriction map around DXS77, DXS7, MAO-A and MAO-B, and to localize the distal end point of an ND deletion between DXS77 and DXS7.

Carrier detection and prenatal diagnosis in Norrie disease

We report the use of DNA probes to determine carrier status in three young women from a large kindred with Norrie disease. One of the women requested prenatal diagnosis during pregnancy. In this pedigree, Norrie disease was not characterized by a deletion at DXS7.

Microdeletion in the X-chromosome and prenatal diagnosis in a family with Norrie disease

We have studied a three-generation family in which Norrie disease is segregating and have performed prenatal diagnosis on the fetus of an obligatory carrier. Deletions at loci DXS7 and DXS77 defined by probes L1.28, L1.28-p59, and pX59 were detected in the affected male. DNA studies of chorionic villus biopsy material indicated that the male fetus had inherited the normal allele from the carrier mother. This prediction was confirmed on eye examination at age 5 months.

Norrie disease as part of a complex syndrome explained by a submicroscopic deletion of the X chromosome

A 15-year-old male patient with the typical ocular symptoms of Norrie disease is described. Additionally, he presents severe mental retardation, growth disturbances, hypogonadism, and increased susceptibility to infections. This complex syndrome is apparently segregating through three generations: four other male relatives of the patient were blind from birth and died from recurrent infections between the ages of three to 15 months. The DNA sequence of the DXS7 locus (L1.28 probe), known to be closely linked to the Norrie gene, was not found in the patient's DNA. This result suggests that the more complex clinical picture seen is the result of a deletion of the X chromosome spanning DXS7, the Norrie gene, and several neighbouring loci. A detailed clinical description of the patient is given and compared to that of similar cases.

Prenatal exclusion of Norrie disease with flanking DNA markers

Three polymorphic DNA markers linked to the locus of Norrie disease were used for indirect genotype analysis in a ten-wk-old fetus at risk for the disease. When haplotypes of the family members and the estimated recombination frequency between Norrie gene and each of the DNA marker loci DXS7, DXS84, and DXS146 were taken into account, the risk that the fetus had inherited the mutation was about 1%.

First demonstration of recombination between the gene for Norrie disease and probe L1.28

Norrie disease is an X-linked trait thought to be tightly linked to the anonymous probe L1.28. Here we report the first recombinational event among 24 informative meioses. This indicates there is a 4% (95% C.L. 0.1%-21%) error rate introduced by meiotic crossovers in carrier or prenatal diagnosis based on linkage between Norrie disease and L1.28.

Recombinational event between Norrie disease and DXS7 loci

We have identified a family affected with X-linked recessive Norrie disease, in which a recombinational event occurred between the disease locus and the DXS7 locus identified by the probe L1.28. The addition of our family brings the total of published informative families to seven, with a maximum lod score of 7.58 at a recombination frequency of 0.038 +/- 0.036. This finding indicates that the L1.28 probe is useful but may not be completely reliable for prenatal diagnosis and that the gene for Norrie disease is not within the DNA sequence identified by the L1.28 probe.

Norrie disease resulting from a gene deletion: clinical features and DNA studies

We describe a family in which two boys with Norrie disease have a deletion of the DXS7 locus. The deletion does not extend as far distally as the OTC or DXS84 loci. A full clinical description of the patients is given to help establish the range of manifestations of Norrie disease. There is no evidence of any other X linked disease in our patients.