Linkage between the X-linked retinitis pigmentosa locus and the L1.28 locus

Visual Function in Carriers of X-Linked Retinitis Pigmentosa

PURPOSE

To determine the frequency and severity of visual function loss in female carriers of X-linked retinitis pigmentosa (XLRP).

DESIGN

Case series.

PARTICIPANTS

Two hundred seventy-six XLRP carriers with cross-sectional data (n = 242) and longitudinal data (n = 34; median follow-up, 16 years; follow-up range, 3-37 years). Half of the carriers were from RPGR- or RP2-genotyped families.

METHODS

Retrospective medical records review.

MAIN OUTCOME MEASURES

Visual acuities, visual field areas, final dark adaptation thresholds, and full-field electroretinography (ERG) responses to 0.5-Hz and 30-Hz flashes.

RESULTS

In genotyped families, 40% of carriers showed a baseline abnormality on at least 1 of 3 psychophysical tests. There was a wide range of function among carriers. For example, 3 of 121 (2%) genotyped carriers were legally blind because of poor visual acuity, some as young as 35 years. Visual fields were less affected than visual acuity. In all carriers, the average ERG amplitude to 30-Hz flashes was approximately 50% of normal, and the average exponential rate of amplitude loss over time was half that of XLRP males (3.7%/year vs. 7.4%/year, respectively). Among obligate carriers with affected fathers, sons, or both, 53 of 55 (96%) had abnormal baseline ERG results. Some carriers who initially had completely normal fundi in both eyes went on to experience moderately decreased vision, although not legal blindness. Among carriers with RPGR mutations, those with mutations in ORF15, compared with those in exons 1-14, had worse final dark adaptation thresholds and lower 0.5-Hz and 30-Hz ERG amplitudes.

CONCLUSIONS

Most carriers of XLRP had mildly or moderately reduced visual function but rarely became legally blind. In most cases, obligate carriers could be identified by ERG testing. Carriers of RPGR ORF15 mutations tended to have worse visual function than carriers of RPGR exon 1 through 14 mutations. Because XLRP carrier ERG amplitudes and decay rates over time were on average half of those of affected men, these observations were consistent with the Lyon hypothesis of random X-inactivation.

Retinal photoreceptor dystrophies LI. Edward Jackson Memorial Lecture

PURPOSE

To assess the state of knowledge of photoreceptor dystrophies.

METHODS

The current literature concerning photoreceptor dystrophies is reviewed, and their potential impact on concepts of pathogenesis of disease and clinical practice is assessed.

RESULTS

As a result of cooperative investigative work between researchers in various disciplines, major advances in the classification of retinal photoreceptor dystrophies have been made. Until recently, classification of retinal dystrophies was based on clinical observation alone, and it was evident that this method was imprecise and of limited value. Largely through the work of molecular biologists, it has been shown that diseases clinically indistinguishable from one another may be a result of mutations on a variety of genes; conversely, different mutations on a single gene may give rise to a variety of phenotypes. It is reassuring that it is possible to generate concepts as to potential pathogenetic mechanisms that exist in retinal dystrophies in light of this new knowledge. More important for the clinician is the potential impact on clinical practice. There is as yet no therapy by which the course of most of these disorders can be modified. However, there is a considerable body of work in which therapeutic intervention is being explored, and many researchers now see treatment as a justifiable objective of their work.

CONCLUSIONS

Knowledge of the causative mutation is of value to the clinician in that it provides a precise diagnosis and allows the distribution of the abnormal gene to be documented fully within a family. To take full advantage of the opportunities provided by current research, clinical practice will have to be modified, particularly if therapy can be justified.

XLPRA: a canine retinal degeneration inherited as an X-linked trait

Breeding studies are reported of a previously undescribed hereditary retinal degeneration identified in the Siberian Husky breed of dog. This disorder clinically resembles the previously reported autosomal recessive canine hereditary retinal degenerations collectively termed progressive retinal atrophy (PRA). However, the pedigree of the propositus, a male Siberian Husky, exhibited an X-linked pattern of transmission. This dog was outcrossed to three phenotypically normal female laboratory Beagles and two of their F1 daughters were bred to a phenotypically normal male Beagle, producing affected males in the F2 generation. Subsequent inbreedings produced further affected males and affected females as well. X-linked transmission was established by exclusion of alternative modes of inheritance and, consequently, the disease has been termed X-linked progressive retinal atrophy (XLPRA). This is the first reported X-linked retinal degeneration in an animal. Because of the many similarities of PRA in dogs to retinitis pigmentosa (RP) in humans, this new disease may not only represent the first animal model of X-linked RP (XLRP) but may well be a true homolog of one of the XLRP loci (RP2, RP3, RP6). It is the first retinal degeneration in dogs that can be assigned to an identified canine chromosome, and the first for which linkage mapping offers a realistic approach to proceed by positional cloning towards identifying the responsible gene locus.

Phenotype-genotype correlations in X linked retinitis pigmentosa

Retinitis pigmentosa (RP) represents a group of clinically heterogeneous retinal degenerations in which all modes of inheritance have been described. We have previously found two different clinical profiles in X linked RP as a function of age and mode of onset. The first clinical form has very early onset with severe myopia. The second form starts later with night blindness with mild myopia or none. At least two genes have been identified in X linked forms, namely RP2 (linked to DXS7, DXS255, and DXS14) and RP3 (linked to DXS84 and OTC) on the short arm of the X chromosome. In order to contribute to phenotype-genotype correlations in X linked RP, we tested the hypothesis that the two clinical profiles could be accounted for by the two different gene loci. The present study provides evidence for linkage of the clinical form with early myopia as the onset symptom with the RP2 gene (pairwise linkage to DXS255: Z = 3.13 at theta = 0), while the clinical form with later night blindness as the onset symptom is linked to the RP3 gene (pairwise linkage to OTC: Z = 4.16 at theta = 0).

Multipoint linkage analysis and heterogeneity testing in 20 X-linked retinitis pigmentosa families

Using multipoint linkage analysis in 20 families segregating for X-linked retinitis pigmentosa (XLRP), the lod scores on a map of eight RFLP loci were obtained. Our results indicate that under the hypothesis of homogeneity the maximal multipoint lod score supports one disease locus located slightly distal to OTC at Xp21.1. Heterogeneity testing for two XLRP loci suggested that a second XLRP locus may be located 8.5 cM proximal to DXS28 at Xp21.3. Further heterogeneity testing for three disease loci failed to detect a third XLRP locus proximal to DXS7 in any of our 20 XLRP families.

Retinitis pigmentosa: genetic mapping in X-linked and autosomal forms of the disease

Retinitis pigmentosa (RP) is an hereditary degenerative disease of the retina and a major cause of visual impairment, prevalence estimates ranging from 1 in 3000 to 1 in 7000. The condition may segregate as an autosomal dominant, autosomal recessive or an X-linked recessive trait and it may also occur on a sporadic basis in up to 50% of cases. In the autosomal dominant form, close linkage to the DNA marker C17 (D3S47) was recently established in a large family of Irish origin displaying early-onset disease (McWilliam et al. 1989), multipoint analysis indicating the gene for rhodopsin as a likely candidate (Farrar et al. 1990). In that gene, a C----A transversion in codon 23, resulting in a proline----histidine substitution has now been identified in 17 of 148 unrelated ADRP patients in the United States (Dryja et al. 1990). This mutation is absent however in the original Irish pedigree (it is also absent in 21 other dominant Irish pedigrees, representing approximately 70% of the estimated ADRP population) indicating that another mutation, either in rhodopsin itself, or in a gene very closely linked to rhodopsin is responsible for the disease in that family. Analysis of other dominant pedigrees using the C17 and/or rhodopsin probes has indicated either tight linkage (Bhattacharya, Personal Communication), looser linkage, possibly indicative of a second locus on 3q (Olsson et al. 1990) or no linkage (Farrar et al. 1990, Blanton et al. 1990, Inglehearn et al. 1990). Extensive genetic heterogeneity thus exists in the autosomal dominant form of this disease, and in the light of these new observations, earlier tentative evidence for linkage of ADRP to the Rhesus locus on chromosome 1 will be re-evaluated. A locus for type II Usher syndrome (classical RP combined with congenital pedial deafness, and normal vestibular function) has now been established on the long arm of chromosome 1 (Kimberling et al. 1990). Type I Usher families, in which hearing loss is more profound and vestibular function absent, do not segregate with the same chromosome 1q markers, indicating the existence of another, as yet unlocated gene. In the X-linked form of the disease, two genes, XLRP2 and XLRP3, have been located on the proximal short arm of the X chromosome using a combination of physical and linkage mapping techniques, and there is some evidence to suggest a possible third locus more distally located.(ABSTRACT TRUNCATED AT 400 WORDS)

Mapping of the X-linked recessive retinitis pigmentosa gene. A review

X-linked retinitis pigmentosa is caused by (a) mutation(s) on the X chromosome. One of the problems encountered in the genetic counselling of this disease is the identification of carrier females who appear ophthalmologically and electrodiagnostically normal. Despite normal testing these women are at risk of transmitting the XLRP gene to their children. Since the biochemical basis of XLRP is unknown, prenatal diagnosis and definitive carrier detection remain elusive. Existing methods of diagnosis and carrier detection are subject to limitations and are dependent on X-inactivation. The application of recombinant DNA probes to families with XLRP has provided a large number of genetic marker loci at the level of DNA. These markers are called restriction fragment length polymorphisms (RFLPs). By analysis of linkage relationships in affected kindreds, the XLRP gene(s) has (have) been localized to two subregions of the short arm of the X chromosome, Xp11 and Xp21. These findings suggest that there may be more than one retinitis pigmentosa gene on the X chromosome. Until further families are studied to clarify the localization(s) of XLRP, neither locus can be excluded if prenatal diagnosis and accurate carrier detection is to be established.

Non-allelic mutations in X-linked retinitis pigmentosa

Using RFLP studies, the disease locus in two X-linked retinitis pigmentosa families was found to be centromeric to DXS7 in one family and telomeric to DXS7 in another, suggesting non-allelic heterogeneity.

Linkage analysis of a large Latin-American family with X-linked retinitis pigmentosa and metallic sheen in the heterozygote carrier

An extended linkage analysis was performed on the large Latin-American kindred with X-linked retinitis pigmentosa (XLRP) and metallic sheen in the heterozygous carrier studied and reported previously by R.L. Nussbaum et al. (1985, Hum. Genet. 70:45-50) and on a smaller family with the same XLRP variant. In these kindreds the XLRP locus shows close linkage with Xp21 marker loci OTC and DXS206. The results of this linkage analysis agree with the observations made by Nussbaum et al. (1985) that an XLRP locus is distal to DXS7.

Diagnosis of genetic disorders at the DNA level

In the past 10 years considerable progress has been made in the diagnosis of hereditary disorders at the DNA level. Many monogenic disorders can now be examined at the gene level; such examination has led to a better understanding of the molecular basis of these disorders and made carrier detection and prenatal diagnosis possible. Each year, more and more monogenic disorders can be added to the list of diseases that can be diagnosed by DNA analysis. Future research will be devoted to the identification of genes responsible for other known monogenic hereditary disorders, the elucidation of the molecular lesion associated with chromosomal abnormalities, and the characterization of the genes and gene defects involved in the common multifactorial diseases. The goal of diagnosis is the identification of the genetic defect in affected patients, persons destined to be affected, and carriers.

Cellular immune status in retinitis pigmentosa

Previous reports have suggested a role of cellular and/or humoral immunity in retinitis pigmentosa. Because of the controversial nature of many of these reports, the authors undertook a detailed investigation of cellular and humoral immunity in a well-characterized group of 47 persons with retinitis pigmentosa of various heritability patterns and a similar number of age- and sex-matched controls. The authors found two changes in lymphocyte subsets. Retinitis pigmentosa patients had significantly elevated Leu 3A-positive lymphocytes (CD4 or T-helper cells) and significantly fewer Leu 2A-positive lymphocytes (CD8 or T-suppressor cells) than controls, although the total numbers of T cells did not differ between the two groups. A small but significant number of retinitis pigmentosa patients expressed interleukin 2 (IL-2) antigens on their lymphocytes as compared with none of the controls. The authors saw no differences between the retinitis pigmentosa and control groups in the inducibility and secretion of gamma-interferon or IL-2. Concentrations of immunoglobulins G, A, and M did not differ between the two groups. The link between immune system alterations and the retinitis pigmentosa process remains tenuous.

Clinical investigation of retinitis pigmentosa

The diagnostic features and future research directions of retinitis pigmentosa were documented in this update and review of the subject. An extensive and current bibliography is provided.

Two different genes for X-linked retinitis pigmentosa

Linkage analysis was carried out in three large multigenerational kindreds with X-linked retinitis pigmentosa using DNA markers on Xp. About 10% recombination has been found between the retinitis pigmentosa locus (RP2) and the marker locus DXS7, assigned to band Xp11.3, which was reported earlier to be closely linked to RP2 in several independent families. In the kindreds described in this paper, however, RP2 shows close linkage and no recombination with the marker loci OTC and DXS148, both assigned to Xp21, indicating that, contrary to previous linkage studies, there is evidence of an RP locus distal to DXS7. This suggests that X-linked retinitis pigmentosa is genetically heterogeneous, i.e., caused by mutations at different loci.

Linkage analysis of X linked retinitis pigmentosa in the Irish population

There is significant evidence for genetic and phenotypic heterogeneity in X linked retinitis pigmentosa (XLRP). We have studied the linkage of XLRP in four Irish families to a number of polymorphic DNA markers. We report linkage between the DXS7 (L1.28) locus and the XLRP locus (Z = 3.445, theta = 0.00). Combined with the previously published data on British and Danish families, the genetic distance between the DXS7 and XLRP loci is now estimated at 5 cM with a maximum lod score of 13.026 and a 1-lod confidence interval of 0.75 to 9.5 cM. Linkage was also observed between 754 and XLRP (Z = 3.41, theta = 0.00) and between pERT87 and XLRP (Z = 1.37, theta = 0.1). The heterogeneity of XLRP is discussed in relation to these observations.

Cosegregation of X-linked retinitis pigmentosa and hemophilia A

We examined a family pedigree in which retinitis pigmentosa and hemophilia A were inherited in an X-linked manner. Six female carriers were identified by electroretinography. Results of ophthalmoscopic examination were normal in two women, while four displayed marked variability in phenotypic expression. Six of seven males identified with retinitis pigmentosa had hemophilia A. One asymptomatic boy had a markedly abnormal electroretinogram despite normal ophthalmoscopic examination. Pedigree analysis showed a high recombination rate, which would be expected as these two genes are known to be at opposite arms of the X chromosome.

Analysis of linkage relationships of X-linked retinitis pigmentosa with the following Xp loci: L1.28, OTC, 754, XJ-1.1, pERT87, and C7

A number of variants of X-linked retinitis pigmentosa (XLRP) have been described. In one variant, listed in the McKusick (McK) catalogue (McKusick 1983) as entry no. 30320, the heterozygotes exhibit a golden metallic or tapetal reflex. Three large pedigrees segregating for XLRP with the characteristic tapetal reflex in the heterozygotes were examined, and the linkage between the XLRP locus and Xp loci, L1.28, OTC, 754, XJ-1.1, pERT87 and C7 was measured. The strongest linkage was found to be between the XLRP locus and OTC. In addition, recombinational evidence drawn from the three pedigrees suggests that the XLRP locus is distal to L1.28 and proximal to 754. This putative location of the XLRP gene between L1.28 and 754 taken together with the tight linkage to OTC, a locus already located between L1.28 and 754, leads us to propose a gene order of centromere-L1.28-OTC/XLRP-754-telomere.

Molecular genetic approaches to the analysis of human ophthalmic disease

In this review of the recent literature, the contribution that the new techniques of molecular genetics has made in the analysis and diagnosis of human ophthalmic conditions is presented and discussed. Among the disorders reviewed are X-linked retinitis pigmentosa, Norrie's disease, gyrate atrophy and retinoblastoma, and there are also sections on crystallins and visual pigments.

Localization of the ornithine aminotransferase gene and related sequences on two human chromosomes

We have used a full length cDNA clone to determine the chromosomal location of the gene encoding human ornithine aminotransferase (OAT), a mitochondrial matrix enzyme. Southern blot analysis of Sca I-digested DNA from 34 human-mouse somatic cell hybrids revealed 11 human fragments. Three fragments mapped to chromosome 10q23-10qter, confirming the previous provisional assignment of the functional gene to this autosome by analysis of OAT expression in somatic cell hybrids (O'Donnell et al. 1985). The remaining eight fragments were assigned to the X chromosome, and regionally assigned to Xp21-Xp11 by use of an X-chromosome mapping panel. These X chromosome sequences could represent pseudogenes, or related members of a multigene family. Two of the X chromosome fragments are alternate alleles of a restriction fragment length polymorphism (RFLP) making this OAT-related locus an excellent genetic marker. The RFLP may now be used to determine any possible relationship between this locus and several X-linked eye defects.

Choroideremia: further evidence for assignment of the locus to Xq13-Xq21

Choroideremia is an X-linked hereditary retinal dystrophy leading to blindness in early adulthood. RFLP analyses in three Danish families were consistent with close linkage between choroideremia and the locus DXYS1, located at Xq13-Xq21. Measurable linkage was found between choroideremia and DXS17, at Xq22. Furthermore, choroideremia was diagnosed in a boy with an interstitial deletion at Xq13-Xq21, strongly suggesting the assignment of the locus for choroideremia to this region of the X chromosome. The deletion also covered DXYS1, but did not include DXS17.

Genetic linkage between X-linked retinitis pigmentosa and DNA probe DXS7 (L1.28): further linkage data, heterogeneity testing, and risk estimation

Further linkage data relating X-linked retinitis pigmentosa and DNA probe DXS7 (L1.28) is presented in this paper. The current mean estimate of the recombination fraction (theta) including this and all published data, is 0.09, with confidence limits 0.04 to 0.17 (maximum Lod score of 14.01 at a theta of 0.08). There is no evidence for heterogeneity of recombination fraction between the 13 families for which data are available. However, it is argued that heterogeneity should be assumed to exist for the purposes of risk estimation. Mean estimates and variances of risk are calculated for hypothetical families each with different linkage data. In families in which no recombination has been observed, the mean and variance of risk are sufficiently small for the clinical use of this probe to be acceptable to many.