AIMS/BACKGROUND To characterise clinically a large kindred segregating retinitis pigmentosa and sensorineural hearing impairment in an autosomal dominant pattern and perform genetic linkage studies in this family. Extensive linkage analysis in this family had previously excluded the majority of loci shown to be involved in the aetiologies of RP, some other forms of inherited retinal degeneration, and inherited deafness.
METHODS Members of the family were subjected to detailed ophthalmic and audiological assessment. In addition, some family members underwent skeletal muscle biopsy, electromyography, and electrocardiography. Linkage analysis using anonymous microsatellite markers was performed on DNA samples from all living members of the pedigree.
RESULTS Patients in this kindred have a retinopathy typical of retinitis pigmentosa in addition to a hearing impairment. Those members of the pedigree examined demonstrated a subclinical myopathy, as evidenced by abnormal skeletal muscle histology, electromyography, and electrocardiography. LOD scores of Zmax = 3. 75 (Θ = 0. 10), Zmax = 3. 41 (Θ = 0. 10), and Zmax = 3. 25 (Θ = 0. 15) respectively were obtained with the markers D9S118, D9S121, and ASS, located on chromosome 9q34-qter, suggesting that the causative gene in this family may lie on the long arm (q) of chromosome 9.
CONCLUSIONS These data indicate that the gene responsible for the phenotype in this kindred is located on chromosome 9q. These data, together with evidence that a murine deafness gene is located in a syntenic area of the mouse genome, should direct the research community to consider this area as a candidate region for retinopathy and/or deafness genes.
- retinitis pigmentosa
- sensorineural deafness
- genetic characterisation.
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The term retinitis pigmentosa (RP), although semantically incorrect, describes a variety of retinal degenerations characterised by a number of common clinical features such as night blindness, peripheral visual field loss, optic disc pallor, retinal vascular attenuation, intraretinal pigment deposition, and electroretinographic amplitudes which are reduced or non-detectable.1 The aetiological basis of most inherited retinal degenerations has remained obscure until recent years.
Molecular genetic research resulting in the localisation and mutation analysis of genes responsible for some forms of RP has helped to elucidate the basic pathophysiology of these inherited retinopathies. To date, mutations in four genes have been shown to be responsible for autosomal dominant or recessive forms of non-syndromic RP—that is, rhodopsin,2 3 peripherin/rds,4rom1,5 β pde,6 and rod α cGMP gated channel.7
Five genetic loci for Usher syndrome, the concomitant inheritance of RP and deafness in an autosomal recessive inheritance pattern, have been established—that is, 1q32 (type 2A),8 3q21-q25 (type 3),9 11p15.1 (type 1C),10 11q13.5 (type 1B),11 and 14q32 (type 1).12 Mutations in the myosin VIIa gene have recently been shown to be responsible for symptoms in five families with type IB Usher syndrome.13
Here we describe an Irish kindred cosegregating RP and deafness in an autosomal dominant fashion and report molecular genetic findings which suggest that the responsible gene lies on the long arm (q) of chromosome 9.
Patients and methods
Family TCD ZMK 92 (Fig 1) is a five generation Irish kindred in which classic retinitis pigmentosa and partial deafness cosegregate in a manner compatible with autosomal dominant inheritance. Most members of the family were clinically characterised in the research department of the Eye and Ear Hospital, Dublin, using procedures to be described. A number of elderly patients with advanced retinopathy and hearing loss were examined in their homes with direct ophthalmoscopy.
Twenty nine individuals were clinically assessed. DNA was extracted from blood samples drawn from all individuals in the pedigree, including spouses of affected people with offspring, using a standard protocol.14
A nuclear family unit, consisting of an affected mother and her three affected offspring, two daughters and a son (IV7, V4, V5, and V6 in Fig1), underwent electromyography (EMG) and electrocardiography (ECG). In addition, the mother had a quadriceps femoris muscle biopsy performed with a local anaesthetic. This biopsy specimen was subjected to light and electron microscopy.
Patients who were assessed in the research department had best corrected Snellen visual acuity measured. Goldmann perimetry, using IV4e, I4e, and O4e targets, was then performed followed by assessment of colour vision using the Farnsworth Munsell 100 Hue test under standardised conditions.
Electroretinograms (ERGs) were recorded using a protocol based on that proposed by the International Standardisation Committee.15Briefly, the patient’s pupils were dilated with tropicamide BP 1% and phenylephrine BP 10%. The subject was then dark adapted for at least 30 minutes. Following corneal anaesthesia induced with benoxinate BP 0. 4%, corneal contact lens electrodes of the Henkes type were inserted under dim red illumination. A Ganzfeld bowl was used to present a series of flashes produced by an OS 5 photostimulator located above and slightly behind the subject’s head. The intensities of the flashes were altered by a combination of varying the output of the photostimulator and the interposition of neutral density filters. Wave forms were analysed using the Medelec ‘Mistral’ data averaging and display system. The intensity of the flash at the patient's eyes was measured using a Gossen ‘Mastersix’ photometer. The standard flash produced by this protocol (setting 16) was 1.5 candelas (cd)/m2 per second. Wave forms were analysed using the standard conventions.16
The responses of the dark adapted eye to a series of single flashes of increasing intensity, starting with a flash of −3. 5 log units intensity, were used to assess rod function. A light of intensity 22 cd/m2 was then used to illuminate the interior of the bowl and the patient was light adapted for 10 minutes. Cone responses to a series of maximal intensity flashes, presented singly and at 30 Hz flicker in the presence of the background illumination, were then measured.
Two colour dark adaptometry using a Goldmann–Weekers dark adaptometer and red and blue filters was performed on selected individuals.17 Following pupillary dilatation as described above, the subject’s left eye was bleached by exposure to a light of 2650 apostilbs for 5 minutes. Thresholds to two 2° targets, one blue and the other red, presented at 15° above fixation were measured at 2.5 minute intervals over a 30 minute period. Flashing targets were presented at the rate of 1 second on, 1 second off. The true threshold was taken as the average of the on threshold and the off threshold for each testing.
Anterior segment biomicroscopy and applanation tonometry were then carried out followed by direct and indirect funduscopy. Most individuals had colour retinal photography.
Pure tone audiometry was carried out in the audiology department at the Royal Victoria Eye and Ear Hospital.
Microsatellite markers covering extensive regions of the genome were analysed in the pedigree. Two point LOD scores were calculated using the program liped for IBM PC/At (October 1987).18 Data were entered using the data management program linksys (version 4.11).19
The retinopathy in TCD ZMK 92 is marked by extreme variability in the age of onset and the severity of affectation. This ranges from legal blindness (as assessed by loss of central visual acuity and of peripheral visual field) by early 20s in one individual to only mild night vision difficulties in the early 60s in another. Most individuals become aware of mild hearing loss and night vision problems in the third decade with subsequent symptoms as a result of loss of peripheral vision in the fourth decade. The hearing loss and visual symptoms are slowly progressive. Posterior subcapsular lens opacities requiring surgery were noted in some individuals in their late 40s and early 50s.
Electroretinographic findings demonstrate the wide variability in affected status. The rod isolated responses, in individuals in whom these responses can be recorded, tend to be significantly delayed in latency and reduced in amplitude (Fig 2). Single flash cone responses are similarly delayed in timing and reduced in amplitude (Fig 3).
Two colour dark adaptometry profiles in individuals with impaired but electroretinographically recordable retinal function demonstrate a rod mediated profile but at thresholds significantly above normal values (Fig 4).
Audiometric analysis of some members of the family shows a moderate to severe sensorineural deafness particularly to middle range frequencies (Fig 5), with evidence of ‘recruitment’—that is, marked distortion and physical discomfort precipitated by loud noise, suggesting that the primary site of hearing impairment may lie in the organ of Corti.
Funduscopy in young members of the pedigree with significantly abnormal ERG findings showed subtle diffuse retinal pigment epithelial thinning and pigmentary mottling (Fig 6). Older affected individuals showed all the classic features of retinitis pigmentosa with waxy disc pallor, arteriolar narrowing, and bone spicule pigmentary deposition in all four retinal quadrants.
Two individuals in this family suffer moderate to severe mental handicap in addition to visual and auditory symptoms. Figure 7 shows the fundus picture of the youngest of these patients. The retinopathy in both cases is distinguished by marked optic disc pallor. The disc pallor noted in other affected individuals is not as striking.
None of the affected individuals in this family was found to have external ocular muscle movement disorders suggestive of the Kearns–Sayre syndrome.
In the four individuals on whom electrocardiography was performed, ECG changes were noted. All showed right axis deviation. Significantly, the youngest affected child, aged 13 years, in addition to having the most compromised retinal function of the three children as measured by ERG, also had the most serious ECG changes. These consisted of inferoseptal and inferolateral ST depression in addition to right axis deviation, suggestive of global concentric hypertrophy (Fig 8). Electromyography demonstrated short, small motor unit action potentials, consistent with myopathy in the mother and two of her three affected children. Electron microscopy of the muscle biopsy obtained from the mother showed an abnormally high number of mitochondria beneath the sarcolemmal membrane (Fig 9A) when compared with a biopsy obtained from an unrelated normal individual (Fig 9B). While no abnormal forms were seen, the significantly increased number of mitochondria seen was consistent with a mitochondrial myopathy.
A high resolution karyotype was performed on four affected individuals from the family. No abnormalities were observed (data not shown).
Table 1 shows two point LOD (Log of theOdds) scores obtained in the family for a number of markers linked to various retinitis pigmentosa, Usher syndrome, maculopathy, and deafness loci. A LOD score of −2. 0 or less indicates exclusion of the locus concerned as a site likely to harbour the disease gene. Conversely, a score of 3.0 or more provides significant evidence of linkage. Various regions of interest have been excluded, including areas of linkage to loci of ophthalmic and audiological relevance.
Table 2 shows two point LOD scores obtained with a number of markers on chromosome 9q34-qter. The markers D9S118, D9S121, and ASS showed positive LOD scores of 3.75 at a recombination ratio (θ) of 0.10, 3.41 (θ = 0.10), and 3. 25 (θ = 0.15), respectively, suggesting that the causative gene in this family lies on chromosome 9q. Other, less statistically significant LOD scores were obtained in the same region.
The pedigree TCD ZMK 92 is a large kindred segregating retinitis pigmentosa and sensorineural hearing loss in a pattern consistent with autosomal dominant inheritance. To our knowledge this is the first extensive family of this type to be clinically characterised and subjected to molecular genetic analysis.
The association of a pigmentary retinopathy with hearing loss has been known for many decades. The most common syndrome in which this association is found is that described by Usher in 1914, which is characterised by an autosomal recessive mode of inheritance. Deafness in Usher syndrome is typically congenital in onset and may be partial or complete. Karjalainen et al 20 described a number of cases of Usher syndrome in which the hearing loss had its onset at puberty and was progressive (type III Usher syndrome), a pattern which is also found in our family. Karjalainen’s cases however had an autosomal recessive inheritance pattern.
As may be seen in Figure 1, there is no evidence of male to male transmission in TCD ZMK 92. This is explained by the fact that most affected males in this family have not married and produced offspring. The possibility of an X linked gene operating in this pedigree was discounted by significant exclusion obtained with X chromosome markers.21 The clinical status of patient I1 (Fig 1) is anecdotal, the individual in question having died at the end of the last century. If this individual were unaffected it would raise the possibility of mitochondrial inheritance in this pedigree.
Significantly positive LOD scores—that is, >3. 0, were observed on chromosome 9q with the markers D9S118, D9S121, and ASS at a recombination ratio (θ) of 0. 10 in each case. Analysis of this family is ongoing in order to find markers at 0 recombination. The number of microsatellite markers in the area is limited, as this region of 9q is sparsely mapped. Presently, the human genetic linkage map22 leaves a 14 centiMorgan (cM) gap between D9S158, the most distal marker, and D9S164, the next most distal one. In addition, the informativeness of markers within the family is reduced by the fact that the spouses of two ancestral affected people were siblings (Fig1).
Mice homozygous for the ‘whirler’ (wi) mutation exhibit the ‘shaker-waltzer’ syndrome consisting of deafness, circling movements, and tossing of the head. Defects in the membranous labyrinth are thought to be responsible for the deafness. It may be of significance that the whirler locus is located on mouse chromosome 4, which is syntenic with human chromosome 9q32-q34.23 The human homologue of this gene would be a candidate for the pathology noted in family TCD ZMK 92. Alternatively, a family of genes with ophthalmic and/or auditory functions may be located in this region.
The findings of abnormalities on ECG, EMG, and skeletal muscle biopsy were unexpected. A number of possible explanations present themselves for consideration. The causative gene, at present unknown, may have a function in muscle physiology. Individuals with Duchenne muscular dystrophy (DMD) have been documented as having abnormal retinal electrophysiology.24 It has been postulated that the DMD protein is also functional in retinal cells. Mutated myosins have been shown to be responsible for type 1B Usher syndrome.13Myosins are one of the structural proteins involved in muscle contraction. It is possible that mutations in similar contractile proteins might cause hearing and vision problems in addition to muscle abnormalities.
An intriguing possibility in our family is co-segregation of genomic and mitochondrial DNA mutations. A similar possibility has been invoked for the variation in phenotype noted in Leber’s hereditary optic atrophy.25 We are presently examining the mitochondrial DNA in order to investigate this possibility.
The data presented in this paper will be of benefit to the research community in directing attention to chromosome 9q as a candidate region for genes responsible for human retinopathies and/or hearing deficits.
This work was supported by the Wellcome Trust, the US Foundation Fighting Blindness, RP Ireland Fighting Blindness, the British RP Association, the Science Programme of the European Union, and the Ulverscroft Foundation (UK).
Clinical evaluation of family members of TCD ZMK 92 was carried out at the Research Foundation, Eye and Ear Hospital, Dublin 2, Ireland with the assistance of Miss Hillary Dempsey. Audiometry was performed by Miss Mary Glennan and Miss Judy Nugent. Retinal photography was performed by Mr Stephen Travers.
We would also like to thank Professor Jean Weissenbach and Dr Sabine Fauré for provision of marker data before their official release.
Electrocardiography and muscle biopsy were carried out at the Adelaide Hospital, Peter Street, Dublin 2. The muscle biopsy was performed by Mr F Keane, FRCS. Electron microscopy of the muscle biopsy specimen was carried out by Miss Anne Mynes and Dr Michael Farrell at the Electron Microscopy Unit, St James’ Hospital, Dublin. The electromyography was performed at St James’ Hospital, Dublin.
High resolution karyotyping was performed by Dr Ray Stallings at the National Centre for Medical Genetics, Our Lady's Hospital for Sick Children, Crumlin, Dublin 12.
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