Novel CACNA1F mutations in Japanese patients with incomplete congenital stationary night blindness

PURPOSE

Although it was reported that congenital stationary night blindness (CSNB) could be divided into complete and incomplete CSNB clinically in 1986, it was not until 1998 that the two types were found to be distinct clinical diseases by molecular genetic analysis. The purpose of this article is to report mutations in the retina-specific calcium channel alpha1-subunit gene (CACNA1F) in Japanese patients with incomplete CSNB and to describe the clinical features of these patients.

METHODS

Seven patients from five separate Japanese families with incomplete CSNB were examined. Genomic DNA was extracted from leukocytes of the peripheral blood, and all 48 exons of the CACNA1F were amplified by polymerase chain reaction and directly sequenced. A complete ophthalmic examination was performed, including best corrected visual acuity, slit lamp and fundus examinations, fundus photography, and electroretinography.

RESULTS

A mutation in the CACNA1F was identified in all the patients. The identified mutations were a missense mutation (Gly609Asp); a nonsense mutation (Arg913stop); a splice donor site mutation of G to C at nucleotide 2571+1; a G insertion at nucleotide 709, resulting in a frame shift with a predicted stop codon at codon 247; and a 4-bp deletion at nucleotides 271 to 274, with a replacement by an abnormal 34-bp sequence. Clinically, each patient had essentially normal fundi, mildly reduced corrected visual acuity, and slight myopia or hyperopia with astigmatism. Electrophysiologically, the mixed rod-cone ERG showed a negative configuration with recordable oscillatory potentials. The rod ERG was recordable but subnormal, and the cone and 30-Hz flicker ERGs were markedly depressed.

CONCLUSIONS

Five novel mutations were identified in the CACNA1F in five Japanese families with incomplete CSNB, leading to the conclusion that in most Japanese patients, incomplete CSNB is caused by a CACNA1F mutation.

Multifocal ERG findings in complete type congenital stationary night blindness

PURPOSE

To study the multifocal electroretinogram (mfERG) in patients with the complete type of congenital stationary night blindness (cCSNB), which is thought to be due to a defect in neurotransmission from the photoreceptors to the ON-bipolar cells.

METHODS

mfERGs were recorded with the VERIS recording system from four patients with cCSNB, none of whom had nystagmus. The stimulus array consisted of 61 hexagons, and the total recording time was approximately 4 minutes. The amplitudes and implicit times of the first- and second-order kernels of the local responses were compared with those from 20 myopic controls. Waveforms of the summed response from all locations were also compared between the two groups.

RESULTS

The first-order kernels of the mfERGs of cCSNB patients had normal amplitudes but delayed implicit times for nearly the whole field tested. The second-order kernel was severely attenuated in amplitude in cCSNB patients. The ratios of the second- to first-order kernel amplitudes were significantly reduced in cCSNB and clearly separated the cCSNB group from the control group without any overlap of the values.

CONCLUSIONS

The second-order kernel, which is involved in adaptative mechanism of the retina to repeated flashes, is selectively reduced in cCSNB. The delay of the implicit times of the first-order kernel in patients with cCSNB may be related to the severe amplitude reduction of the second-order kernel.

Complete form of X-linked congenital stationary night blindness: refined mapping and evidence of genetic homogeneity

A number of distinct, partly non-overlapping genetic loci have been reported for the complete type of X-linked congenital stationary night blindness (CSNB1), suggesting genetic heterogeneity. In order to refine the localization of the CSNB1 gene and to demonstrate genetic homogeneity, linkage analysis was performed in two large CSNB1 families. Clinical features consistent with the diagnosis of CSNB1 were documented in five patients from a German seven-generation kindred by full ophthalmological examination including psychophysical and electroretinographical testing. Haplotype analysis in 30 members of the large German family was performed with 38 polymorphic markers predominantly covering the critical region. Linkage analyses defined a locus for CSNB1 with flanking markers DXS8042 and DXS228, refining the interval to 2.5 cM in Xp11.4. In addition, two-point linkage analysis was carried out using the MLINK computer program. In agreement with meiotic breakpoints, lod scores of 3.0 and greater were obtained for markers located to the proximal site of the former 5 cM CSNB consensus interval. A large Dutch CSNB1 family was re-evaluated with markers from the Xp11.4 region, and supports the CSNB1 minimal interval found in the German family. Together with previous results from three unrelated families from Sweden, Sardinia and Great Britain, our results provide evidence of genetic homogeneity in the disorder. Subsequent mutation analyses in CSNB1 patients revealed no pathogenic sequence alterations in DFFRX and CASK genes, but retain candidates for other diseases mapping to that region.

A summary of 20 CACNA1F mutations identified in 36 families with incomplete X-linked congenital stationary night blindness, and characterization of splice variants

Incomplete X-linked congenital stationary night blindness (CSNB) is a recessive, non-progressive eye disorder characterized by abnormal electroretinogram and psychophysical testing and can include impaired night vision, decreased visual acuity, myopia, nystagmus, and strabismus. Including the 20 families previously reported (Bech-Hansen et al. 1998b), we have now analyzed patients from a total of 36 families with incomplete CSNB and identified 20 different mutations in the calcium channel gene CACNA1F. Three of the mutations account for incomplete CSNB in two or more families, and a founder effect is clearly demonstrable for one of these mutations. Of the 20 mutations identified, 14 (70%) are predicted to cause premature protein truncation and six (30%) to cause amino acid substitutions or deletions at conserved positions in the alpha1F protein. In characterizing transcripts of CACNA1F we have identified several splice variants and defined a prototypical sequence based on the location of mutations in splice variants and comparison with the mouse orthologue, Cacnalf.

A case of a combination of Oguchi's disease and congenital retinoschisis

We report here a rare case of a combination of Oguchi's disease and congenital retinoschisis. A 36-year-old male patient presented with a decrease in vision in both eyes and night blindness. Indirect ophthalmoscopy revealed bilateral macular stellate striations and golden-gray discoloration of the retina outside the vascular arcades. This discoloration turned to a normal retina after complete adaptation to darkness (Mizuo-Nakamura phenomenon).

Development of a 1.4-Mb BAC/PAC contig and physical map within the critical region for complete X-linked congenital stationary night blindness in Xp11.4

A physical map internal to the markers DXS1368 and DXS228 was developed for the p11.4 region of the human X chromosome. Twenty-four BACs and 10 PACs with an average insert size of 149 kb were aligned to form a contig across an estimated 1.4 Mb of DNA. This contig, which has on average fourfold clone coverage, was assembled by STS and EST content analysis using 46 markers, including 8 ESTs, two retinally expressed genes, and 22 new STSs developed from BAC- and PAC-derived DNA sequence. The average intermarker distance was 30 kb. This physical map provides resources for high-resolution mapping as well as suitable clones for large-scale sequencing efforts in Xp11.4, a region known to contain the gene for complete X-linked congenital stationary night blindness.

A novel mutation within the rhodopsin gene (Thr-94-Ile) causing autosomal dominant congenital stationary night blindness

More than 100 mutations within the rhodopsin gene have been found to be responsible for some forms of retinitis pigmentosa, a progressive retinal degeneration characterized by night blindness and subsequent disturbance of day vision that may eventually result in total blindness. Congenital stationary night blindness (CSNB) is an uncommon inherited retinal dysfunction in which patients complain of night vision difficulties of a nonprogressive nature only and in which generally there is no involvement of day vision. We report the results of molecular genetic analysis of an Irish family segregating an autosomal dominant form of CSNB in which a previously unreported threonine-to-isoleucine substitution at codon 94 in the rhodopsin gene was found to segregate with the disease. Computer modeling suggests that constitutive activation of transducin by the altered rhodopsin protein may be a mechanism for disease causation in this family. Only two mutations within the rhodopsin gene have been previously reported in patients with congenital stationary night blindness, constitutive activation also having been proposed as a possible disease mechanism.

11-cis retinol dehydrogenase mutations as a major cause of the congenital night-blindness disorder known as fundus albipunctatus

PURPOSE

Patients with fundus albipunctatus uniformly experience difficulty with vision at night. Their retinas are spotted with characteristic light yellow flecks of unknown composition that typically spare the macula. A defect in the transport or utilization of visual cycle retinoids is thought to underlie this recessive disorder with variable clinical expression. To elucidate the molecular defect we considered the genes for interphotoreceptor retinoid-binding protein (RBP3) and 11-cis retinol dehydrogenase (RDH5) as candidates for this disease.

METHODS

We examined two unrelated families with fundus albipunctatus. The diagnosis was determined clinically and RBP3 and RDH5 were analyzed by molecular screening methods and direct genomic sequencing.

RESULTS

Each family had two affected members with typical fundus albipunctatus. The affected members were siblings born to unaffected parents who were seventh cousins in the first family and unrelated in the second family. The probands from both families were clinically similar except for the fundus dots that were more extensive in the second family to the point of involving the parafoveal region. In the initial phase of genetic screening RBP3 defects were ruled-out as the cause of the disease in both families. In contrast, RDH5 mutations were found in the affected siblings in both families. The proband in one had a homozygotic Gly238Trp missense mutation (GGG -> TGG) involving exon 4 and in the other carried compound heterozygotic changes Arg280His (CGC -> CAC) and Ala294Pro (GCC -> CCC) in exon 5. The disease phenotype was only manifested in family members with two abnormal RDH5 alleles consistent with autosomal recessive inheritance in both pedigrees.

CONCLUSIONS

These findings strongly implicate defects of RDH5 as the cause of fundus albipunctatus and point to a heterogeneity of RDH5 mutations in this form of congenital stationary night blindness with variable expressivity.

Blue-on-yellow perimetry in the complete type of congenital stationary night blindness

PURPOSE

To resolve the discrepancy between nonrecordable full-field short wavelength cone electroretinograms (S-cone ERGs) and the presence of normal color vision in patients with the complete type of congenital stationary night blindness (CSNB1).

METHODS

Conventional white-on-white (W-W) perimetry, blue-on-yellow (B-Y) perimetry, and the Farnsworth-Munsell 100-hue test were performed in five patients with CSNB1. Diagnosis of CSNB1 was made by clinical and electrophysiological examinations. Twelve normal, age-matched control subjects and an additional 7 normal, highly myopic subjects were tested.

RESULTS

Color vision was normal in all the CSNB1 patients by the Farnsworth-Munsell 100-hue test. B-Y perimetry demonstrated that blue cone sensitivity in CSNB1 was normal in the fixation area, but the mean sensitivities of the entire 60 degrees field, the central 0 degrees-to-15 degrees, and 15 degrees-to-30 degrees ring were significantly decreased compared with the normal and myopic subjects. The sensitivity difference between 15 degrees-to-30 degrees and 0 degrees-to-15 degrees in B-Y perimetry increased significantly in CSNB1 compared with both normal and myopic control subjects.

CONCLUSIONS

Our perimetric results demonstrated that the S-cone function in CSNB1 is preserved only in the fovea and becomes abnormal toward the peripheral retina. This accounts for the normal color vision that tests mainly foveal function and the nonrecordable S-cone ERGs that arise mainly from peripheral retina.

Localization of the mouse nob (no b-wave) gene to the centromeric region of the X chromosome

PURPOSE

To determine the position on the X chromosome of the gene responsible for a spontaneous mouse mutation, nob (no b-wave), which matches the phenotype of complete X-linked congenital stationary night blindness (CSNB) type 1 in human.

METHODS

Inter- and intraspecific pedigrees were generated, and the phenotype of each mouse was scored on the basis of either the presence or the absence of an electroretinographic b-wave. DNA was isolated from a tail biopsy from each mouse and was used to determine the genotype at various polymorphic markers on the X chromosome. LOD scores (Z) between the nob phenotype and each marker were calculated to determine the most probable location of the nob gene.

RESULTS

A total of 174 informative offspring were analyzed. The nob gene is tightly linked to DXMit103 with a maximum LOD score of 25.9 at a recombination fraction of zero. This marker is located at 4.2 cM on the X chromosome of the mouse map. Haplotype analyses of several recombinant chromosomes in the region indicates that the nob gene maps between DXMit54 (3.8 cM) and Ube1x (5.7 cM).

CONCLUSIONS

The genetic position of the mouse nob gene overlaps the homologous region in human that contains the locus for CSNB1 and excludes the region of CSNB2. Further studies are planned to identify the mouse nob gene and to evaluate it as a candidate for CSNB1.

Ectopic transcription and the possibility of RNA editing of the human arrestin gene

PURPOSE

To investigate the ectopic transcription of arrestin in various human organs and tissues.

METHODS

Reverse transcriptase-polymerase chain reaction was used to investigate the ectopic transcription of the arrestin gene in the retina, anterior capsule of the lens, iris, bulbar conjunctiva, muscle, skin, placenta, intestine, and human blood. In addition, we examined the expression of arrestin mRNA isolated from whole blood cells of patients with Oguchi disease who had the 1147delA mutation in the arrestin gene.

RESULTS

Arrestin was expressed at the mRNA level in all our samples. The mRNA isolated from the blood cells of Oguchi patients was not associated with the same 1147delA mutation as was reported earlier in the arrestin gene but had the normal sequence.

CONCLUSIONS

These results demonstrate the ectopic transcription of the arrestin gene in various tissues and also suggest the possibility of RNA editing in the blood cells of Oguchi patients.

Complete congenital stationary night blindness maps on Xp11.4 in a Sardinian family

X-linked congenital stationary night blindness (CSNBX) is a hereditary non-progressive retinal disorder, which can appear in two different clinical forms, complete and incomplete, associated with CSNB1 and CSNB2 loci on Xp. We describe a Sardinian family with complete CSNBX and define better the limits of the CSNB1 genetic locus on Xp11.4 through linkage analysis. Haplotype analysis showed two key recombinants, which restrict the CSNB1 locus to a region of about 3 cM limited by markers DSX1068 and DSX6810 respectively. The locus that we describe is included in the CSNB1 locus defined by previous reports referring to the same clinical form of the disease. These results, in addition to other recent mapping reports about families from different geographical areas, confirm the genetic homogeneity of X-linked complete CSNB.

Evidence supportive of a functional discrimination between photopic oscillatory potentials as revealed with cone and rod mediated retinopathies

We report on a family where four of the eleven children presented with reduced visual acuities, a red-green deficit at the Farnsworth-Munsel FM 100-hue test, normal appearing fundi and unexpected electroretinographic findings. Light- (photopic) and dark- (scotopic) adapted electroretinograms (ERG) and oscillatory potentials (OPs) were obtained following an accepted standard protocol. The b-wave of their photopic ERG was significantly more attenuated than the a-wave due to the specific abolition of OP4, while the amplitudes of OP2 and OP3 were within the normal range, giving to the b-wave a truncated appearance reminiscent of that seen in congenital stationary night blindness (CSNB) with myopia. Interestingly in the latter condition, which is believed to result from an ON-retinal pathway anomaly, it is OP2 and OP3 which are specifically abolished while OP4 is of normal amplitude thus resulting in an OP response pattern which complements that seen with our patients. Also of interest is the fact that, in our patients, the amplitude of the dark-adapted OP2 was, on average, 240% larger than that measured in light-adaptation while, in normal, a non-significant 14% increase is noted; a finding which is in keeping with other studies reporting supernormal scotopic ERGs in some forms of cone dystrophies. Based on the photopic OP response pattern, our patients represent the electrophysiological complement of patients affected with CSNB. Interestingly their symptoms are also complementary, a finding which could support a functional discrimination between the photopic OPs.

Localization of a gene for incomplete X-linked congenital stationary night blindness to the interval between DXS6849 and DXS8023 in Xp11.23

Congenital stationary night blindness (CSNB) is a nonprogressive retinal disorder characterized by night blindness, nystagmus, myopia, a variable decrease in visual acuity, an abnormal electroretinographic response, and a disturbance in dark adaptation. Two forms of X-linked CSNB have been defined, complete CSNB in which rod function is extinguished, and incomplete CSNB in which rod function is reduced but not extinguished, as seen by electroretinography and dark adaptometry. In studying a large family of Mennonite ancestry, we have confirmed linkage between the locus (CSNB2) for incomplete CSNB and genetic markers in the Xp11 region. In particular, lod scores of 12.25 and 15.26 at zero recombination were observed between CSNB2 and the markers DXS573 and DXS255. Detailed analysis of critical recombinant chromosomes in this extended family have refined the minimal region for the CSNB2 locus to the interval between DXS6849 and DXS8023 in Xp11.23.

Loss-of-function mutations in a calcium-channel alpha1-subunit gene in Xp11.23 cause incomplete X-linked congenital stationary night blindness

X-linked congenital stationary night blindness (CSNB) is a recessive non-progressive retinal disorder characterized by night blindness, decreased visual acuity, myopia, nystagmus and strabismus. Two distinct clinical entities of X-linked CSNB have been proposed. Patients with complete CSNB show moderate to severe myopia, undetectable rod function and a normal cone response, whereas patients with incomplete CSNB show moderate myopia to hyperopia and subnormal but measurable rod and cone function. The electrophysiological and psychophysical features of these clinical entities suggest a defect in retinal neurotransmission. The apparent clinical heterogeneity in X-linked CSNB reflects the recently described genetic heterogeneity in which the locus for complete CSNB (CSNB1) was mapped to Xp11.4, and the locus for incomplete CSNB (CSNB2) was refined within Xp11.23 (ref. 5). A novel retina-specific gene mapping to the CSNB2 minimal region was characterized and found to have similarity to voltage-gated L-type calcium channel alpha1-subunit genes. Mutation analysis of this new alpha1-subunit gene, CACNA1F, in 20 families with incomplete CSNB revealed six different mutations that are all predicted to cause premature protein truncation. These findings establish that loss-of-function mutations in CACNA1F cause incomplete CSNB, making this disorder an example of a human channelopathy of the retina.

An L-type calcium-channel gene mutated in incomplete X-linked congenital stationary night blindness

The locus for the incomplete form of X-linked congenital stationary night blindness (CSNB2) maps to a 1.1-Mb region in Xp11.23 between markers DXS722 and DXS255. We identified a retina-specific calcium channel alpha1-subunit gene (CACNA1F) in this region, consisting of 48 exons encoding 1966 amino acids and showing high homology to L-type calcium channel alpha1-subunits. Mutation analysis in 13 families with CSNB2 revealed nine different mutations in 10 families, including three nonsense and one frameshift mutation. These data indicate that aberrations in a voltage-gated calcium channel, presumably causing a decrease in neurotransmitter release from photoreceptor presynaptic terminals, are a frequent cause of CSNB2.

Evidence for genetic heterogeneity in X-linked congenital stationary night blindness

X-linked congenital stationary night blindness (CSNB) is a nonprogressive retinal disorder characterized by disturbed or absent night vision; its clinical features may also include myopia, nystagmus, and impaired visual acuity. X-linked CSNB is clinically heterogeneous, and it may also be genetically heterogeneous. We have studied 32 families with X-linked CSNB, including 11 families with the complete form of CSNB and 21 families with the incomplete form of CSNB, to identify genetic-recombination events that would refine the location of the disease genes. Critical recombination events in the set of families with complete CSNB have localized a disease gene to the region between DXS556 and DXS8083, in Xp11.4-p11.3. Critical recombination events in the set of families with incomplete CSNB have localized a disease gene to the region between DXS722 and DXS8023, in Xp11.23. Further analysis of the incomplete-CSNB families, by means of disease-associated-haplotype construction, identified 17 families, of apparent Mennonite ancestry, that share portions of an ancestral chromosome. Results of this analysis refined the location of the gene for incomplete CSNB to the region between DXS722 and DXS255, a distance of 1.2 Mb. Genetic and clinical analyses of this set of 32 families with X-linked CSNB, together with the family studies reported in the literature, strongly suggest that two loci, one for complete (CSNB1) and one for incomplete (CSNB2) X-linked CSNB, can account for all reported mapping information.

[Clinical features and genetic analysis in a family with X-linked incomplete congenital stationary night blindness (CSNBi)]

PURPOSE

We describe particular clinical features in a three-generation family with X-linked CSNBi and present the genetic analysis.

METHOD

The diagnosis of CSNBi was established on clinical and electrophysiological criteria. Polymorphic DNA markers of the Xp region were analyzed by fluorescent polymerase chain reaction.

RESULTS

Clinical findings evidenced an atypical association of both myopia and hyperopia in the same brotherhood. The most interesting feature in this family was the observation of major worsening of the clinical shape between the first and the third generation of affected individuals. DNA analysis did not show significant linkage between the disease and markers of the Xp11-p21 region. Southern analysis did not show expansion of trinucleotide repeat CAG/CTG and CCG/CGG over the three generation.

CONCLUSION

Haplotypic analysis together with clinical observations allow to exclude the existence of a myopia gene closely linked to the CSNB2 locus. The clinical anticipation observed in this family does not seem to be linked with trinucleotide repeat expansion CAG/CTG or CCG/CGG.

Biochemical evidence for pathogenicity of rhodopsin kinase mutations correlated with the oguchi form of congenital stationary night blindness

Rhodopsin kinase (RK), a rod photoreceptor cytosolic enzyme, plays a key role in the normal deactivation and recovery of the photoreceptor after exposure to light. To date, three different mutations in the RK locus have been associated with Oguchi disease, an autosomal recessive form of stationary night blindness in man characterized in part by delayed photoreceptor recovery [Yamamoto, S. , Sippel, K. C., Berson, E. L. & Dryja, T. P. (1997) Nat. Genet. 15, 175-178]. Two of the mutations involve exon 5, and the remaining mutation occurs in exon 7. Known exon 5 mutations include the deletion of the entire exon sequence [HRK(X5 del)] and a missense change leading to a Val380Asp substitution in the encoded product (HRKV380D). The mutation in exon 7 is a 4-bp deletion in codon 536 leading to premature termination of the encoded polypeptide [HRKS536(4-bp del)]. To provide biochemical evidence for pathogenicity of these mutations, wild-type human rhodopsin kinase (HRK) and mutant forms HRKV380D and HRKS536(4-bp del) were expressed in COS7 cells and their activities were compared. Wild-type HRK catalyzed light-dependent phosphorylation of rhodopsin efficiently. In contrast, both mutant proteins were markedly deficient in catalytic activity with HRKV380D showing virtually no detectible activity and HRKS536(4-bp del) only minimal light-dependent activity. These results provide biochemical evidence to support the pathogenicity of the RK mutations in man.

Localization of CSNBX (CSNB4) between the retinitis pigmentosa loci RP2 and RP3 on proximal Xp

PURPOSE

Proximal Xp harbors many inherited retinal disorders, including retinitis pigmentosa (RP) and congenital stationary night blindness, both of which display genetic heterogeneity. X-linked congenital stationary night blindness (CSNBX) is a nonprogressive disease causing night blindness and reduced visual acuity. Distinct genetic loci have been reported for CSNBX at Xp21.1, which is potentially allelic with the RP3 gene, and at Xp11.23, which is potentially allelic with the RP2 gene. The study to identify the RP2 gene led to an extended study of families with potentially allelic diseases that include CSNBX.

METHODS

Haplotype analysis of a family diagnosed with CSNBX was performed with 17 polymorphic markers on proximal Xp covering previously identified loci for CSNBX and XLRP. Two-point and multipoint lod scores were calculated.

RESULTS

Informative recombinations in this family define a locus for CSNBX (CSNB4) with flanking markers DXS556 and DXS8080 on Xp11.4 to Xp11.3, an interval spanning approximately 5 to 6 cM. A maximum lod score of 3.2 was calculated for the locus order DXS556-1 cM-(CSNB4-DXS993)-2 cM-DXS1201.

CONCLUSIONS

The results describe a new localization for CSNBX (CSNB4) between the RP2 and RP3 loci on proximal Xp. CSNB4 is not allelic with any previously reported XLRP loci; however, the interval overlaps the locus reported to contain the cone dystrophy (COD1) gene, and both diseases are nonrecombinant with DXS993. Because mutations in the RPGR gene to date account for disease in only a small proportion of RP3 families, the possibility that this new locus (CSNB4) also segregates with an as yet unidentified XLRP locus cannot be excluded.