Clinical and genetic findings in Hungarian patients with X-linked juvenile retinoschisis

Abstract

Purpose: To determine clinical phenotypes, examine the age dependency of X-linked juvenile retinoschisis (XLRS), and identify mutations in the retinoschisis1 gene (RS1) in 13 Hungarian (Caucasian) families with this disease.

Methods: This study included 72 members in 13 families. Complete ophthalmological examinations, including optical coherence tomography (OCT) and full-field and multifocal electroretinography (ERG), were performed on 20 affected males, 13 female carriers, and 27 healthy controls. The patients were divided into two age groups (Group I <25 years and Group II >25 years), retrospectively, to assess the possible effects of age. Correlations among genotype, age, best corrected visual acuity (BCVA), OCT, and ERG results were analyzed. A modified classification scheme was done to identify the different phenotypes of the disease. In each of the 72 family members and 100 age-matched male controls, all exons and introns of RS1 were amplified by polymerase chain reaction (PCR) and directly sequenced.

Results: Foveal retinoschisis was detected in 25 eyes (62.5%) of patients by funduscopy, and in 29 eyes (72.5%) by OCT, while macular lamellar schisis was recognizable only by OCT in 30 eyes (75%) of patients. Foveal thickness (FT) and total macular volume were significantly increased in younger (Group I) patients only. For patients younger than 26 years, large inner nuclear central cysts were observable by OCT, while after 26 years, foveas were atrophic. White flecks and dots, which were like that seen in fundus albipunctatus, were detected in both eyes of one patient. In both patient groups, characteristically decreased b-waves of standard combined ERG were recorded without any significant difference between the patient groups. The BCVA and ERG parameters of all patients and the OCT of younger patients were significantly worse (p<0.05) than those of age-matched controls. A significant difference between the two age groups was found in case FT, total macular volume, and amplitudes of rod b-wave only. Moderate negative correlation (r=-0.54, p<0.001) was detected between age and FT, while only low negative correlation (r=-0.33, p<0.05) was detected between age and standard combined b-wave amplitudes of full-field ERG. BCVA LogMAR did not show any obvious correlation with age (r=-0.14, p=0.39) or with the type of mutation. Nine different mutations were identified in 25 male patients and 31 female carriers of 13 families: six known and one novel missense mutation (c.575C>T, p.Pro192Leu), one insertion mutation (c.579dupC, p.Ile194Hisfs29ext43), and one frameshift, causing splice site mutation (c.78+1G>C) were detected. These mutations were absent in the 100 age-matched male control samples.

Conclusions: Foveal cystic schisis was found more often by OCT than by funduscopy (+10%), while flat macular lamellar schisis was recognizable only by OCT. Advancing age inversely influenced the size of cavities (FT), and standard combined b-wave amplitudes of full-field ERG, while BCVA, response density, and implicit times of multifocal electroretinography did not show any obvious correlation with age. The atrophic stage of the disease was observable after 26 years of age. The lesions that appeared to be indicative of fundus albipunctatus were proven to be palisades between the splitted retinal layers. Our modified classification scheme was helpful in assessing the prevalence of disease types. In these Hungarian patients, one novel and eight known mutations were detected. The distribution of mutations in RS1 was different to that reported in the literature, because the greatest number of different mutations was in exon 6 instead of exon 4. Two mutation hot spots were found: between c.418-422 in exon 5 and between c.574-579 in exon 6. Genotype-phenotype correlation was not demonstrable.

Figure 1

Characteristic signs of XLRS. A: Fundus photograph of patient IV:2 of family 1 showing foveal schisis with spoke-wheel pattern. B: Fundus photograph of left eye of patient III:3 of family 3 showing bullous infero-temporal retinoschisis. C: OCT image (scan length: 6 mm, horizontal section) of patient IV:2 of family 1 showing foveal cystic retinoschisis in three different retinal layers (marked by red arrows) and the color-coded foveal thickness maps showing the eccentric fixation. GCL indicates ganglion cell layer, INL represents inner nuclear layer and PRL refers photoreceptor layer. D: OCT image of patient III:2 of family 5 showing foveal atrophy and the color-coded foveal thickness map showing the eccentric fixation. E: Standard combined response of full-field ERG showing decreased b-wave amplitude (“negative type” ERG) related to normal (see red arrow). F: Multifocal ERG with 61 first-order kernels showing decreased b-(P1) wave amplitudes in the central rings (marked by red).

Figure 2

Identified mutations, their frequency in the RS1 gene, and the retinoschisis protein. In the gene diagram, exons are represented by bars (gray numbers indicate their numbers; black numbers indicate their size in nucleotides), and introns by lines. Mutations and their localizations in the gene and the protein are marked by gray lines. In the protein diagram, leader sequence is abbreviated LS. Frequency of different types of mutations within each exon and intron are indicated by red bars.

Figure 3

Pedigrees of 13 Hungarian families with XLRS and identified mutations in the RS1 gene. Black boxes represent affected males, while circles with a black dot in the center represent carrier females by sequence analysis. Circles with an open dot represent females those who are expected to be obligate carrier females. Red arrows point to probands. Slashed boxes are deceased family members. Blue stars mark family members who underwent complete clinical examinations. The mutation screening of RS1 was performed in all family members except probable XLRS carriers, who did not want or could not take part in the study. Patients II:2 and II:3 in family 2 are fraternal twins, while patients II:3 and II:4 in family 12 are identical twins.

Figure 4

Correlation between age and foveal thickness in patients with XLRS. Black dots indicate eyes in the cystic stage, white dots represent eyes in the atrophic stage. Green-highlighted horizontal line represents the normal foveal thickness±SD. Red vertical line represents the time (26 years of age) after which no cysts were observed. Moderate negative correlation was detected between age and foveal thickness.

Figure 5

Fundus photography of patient III:3 from family 3 with c.214G>A missense mutation in the RS1 gene. Huge foveal schisis in both eyes and pronounced white flecks, characteristic of fundus albipunctatus, were seen in the right eye and mild white dots in the left eye between the superior and inferior temporal vessels. The huge foveal schisis extended up to the vascular arcades. Pronounced bullous peripheral retinoschisis were detected in the inferotemporal part in both eyes.

Figure 6

Correlation between age and standard combined b-wave amplitudes and between age and standard combined b/a-wave ratio in patients with XLRS. Black points represent eyes. A: Low negative correlation was detected between age and standard combined b-wave amplitudes of full-field ERG. B: There was no correlation detectable between age and standard combined b/a-wave ratio of full-field ERG. Negative type ERG (b/a ratio ≤1) was detected in half of the group of eyes.

Figure 7

Mean values of response densities and implicit times of mfERGs for five eccentric rings in controls, carriers, and patients. Gray area represents standard deviation. Red lines highlight significant changes compared with the controls. A: Response densities of patients were significantly decreased (p<0.05) in all rings (especially in central rings) compared with the controls, with no significant difference found between the two patient groups. B: Implicit times of patients were significantly increased (p<0.05) only in the peripheral part of the examined retinal area (rings 3–5) compared with the controls, with no significant difference observed between the two patient groups. In carriers, response densities and implicit times were within the normal range.

Figure 8

Distribution of different mutations with relevant BCVA (A), FT (B), standard combined b-wave amplitudes of full field ERG (C), and age. Age (yrs) is showed by numbers beside symbols characterizing the type of mutations in each eye. Mean values of controls are shown by the horizontal black broken line and the ±SD by a horizontal dark gray stripe. A: BCVA values belonging to a certain mutation were variable irrespective of age. The best and the worst BCVA belonged to two adjoining mutations (c.422G>A, c.421C>T). B: FT values change irrespectively of the type of mutation, but after 26 years of age, almost exclusively the atrophic form is detectable by OCT. C: The decrease of standard combined b-wave amplitudes belonging to a certain type of mutations were also different, irrespective of age.