Night Blindness, Congenital Stationary, Type 1a

A number sign (#) is used with this entry because X-linked complete congenital stationary night blindness (CSNB1A) can be caused by mutation in the NYX gene (300278), which encodes a small leucine-rich proteoglycan (SLRP) known as nyctalopin.

Description

Congenital stationary night blindness (CSNB) is a clinically and genetically heterogeneous group of nonprogressive retinal disorders that can be characterized by impaired night vision, decreased visual acuity, nystagmus, myopia, and strabismus. CSNB can be classified into 2 groups based on electroretinography (ERG) findings: the Schubert-Bornschein type is characterized by an ERG in which the b-wave is smaller than the a-wave, whereas the Riggs type is defined by proportionally reduced a- and b-waves. In addition, Schubert-Bornschein CSNB is associated with decreased visual acuity, myopia, and nystagmus, whereas in Riggs CSNB patients have visual acuity within the normal range and no symptoms of myopia and/or nystagmus (summary by Riazuddin et al., 2010). Additionally, Schubert-Bornschein CSNB can be subdivided into 'complete' and 'incomplete' forms (summary by Riazuddin et al., 2010).

Van Genderen et al. (2009) noted that standard flash ERG distinguishes a 'complete' form, also known as type 1 CSNB, from an 'incomplete' form, also known as type 2 CSNB (see CSNB2A, 300071). The complete form is characterized by the complete absence of rod pathway function, whereas the incomplete form is due to impaired rod and cone pathway function. Complete CSNB results from postsynaptic defects in depolarizing or ON bipolar cell signaling, whereas the hyperpolarizing or OFF bipolar cell pathway is intact.

Bijveld et al. (2013) noted that the term 'incomplete' CSNB refers to the less-impaired rod system function in CSNB2, whereas the more severely impaired cone system function results in a greater decrease in visual acuity, with a greater impact on a patient's daily life activities than the impairment in CSNB1. Thus, patients with so-called 'incomplete CSNB' actually experience more visual restrictions than those with 'complete CSNB,' which can be misleading to patients and their parents.

Genetic Heterogeneity of Congenital Stationary Night Blindness

Autosomal recessive forms of complete CSNB have been reported: CSNB1B (257270), caused by mutation in the GRM6 gene (604096); CSNB1C (613216), caused by mutation in the TRPM1 gene (603576); CSNB1D (613830), caused by mutation in the SLC24A1 gene (603617); and CSNB1E (614565), caused by mutation in the GPR179 gene (614515); CSNB1F (615058), caused by mutation in the LRIT3 gene (615004); CSNB1G (139330), caused by mutation in the GNAT1 gene (139330); and CSNB1H (617024), caused by mutation in the GNB3 gene (139130).

Autosomal dominant forms of complete CSNB that have been reported include CSNBAD1 (610445), caused by mutation in the RHO gene (180380); CSNBAD2 (163500), caused by mutation in the PDE6B gene (180072); and CSNBAD3 (610444), caused by mutation in the GNAT1 gene (139330).

In addition, an X-linked recessive form of incomplete CSNB (CSNB2A; 300071), caused by mutation in the CACNA1F gene (300110), has been reported.

A form of autosomal recessive CSNB in which all other visual functions are normal is designated Oguchi disease: Oguchi type 1 (258100) is caused by mutation in the SAG gene (181031), and Oguchi type 2 (613411) is caused by mutation in the RHOK gene (GRK1; 180381).

In 101 Dutch patients from 72 families diagnosed with CSNB, Bijveld et al. (2013) screened 6 known CSNB-associated genes and identified mutations in 94 patients. Of the 39 patients with CSNB1, 20 (51%) had mutations in the NYX gene, 10 (26%) in TRPM1, 4 in GRM6, and 2 in GPR179; no mutations were detected in 3 of these patients. Of the 62 patients diagnosed with CSNB2, 55 (89%) had mutations in the CACNA1F gene; no mutations were detected in 4 of these patients. Bijveld et al. (2013) stated that the electrophysiologic distinction between CSNB types 1 and 2 was thus confirmed by DNA analysis in 93% of the patients. In addition, 3 patients from the CSNB cohort, including 2 Dutch sibs originally reported by Littink et al. (2009), were found to be homozygous for a nonsense mutation in the CABP4 gene and to exhibit a distinct phenotype that Littink et al. (2009) designated 'congenital cone-rod synaptic disorder' (CRSD; 610427).

Clinical Features

Night blindness is a symptom of several chorioretinal degenerations. (According to the interpretation of some, particularly French-speaking writers, nyctalopia means literally 'seeing at night' and hemeralopia means 'seeing in the day;' hence, nyctalopia is 'dayblindness,' e.g., total colorblindness (216900), and hemeralopia is 'night blindness.' See later for a discussion of the derivation of these 2 terms, including the idea that the syllable 'al,' coming from a Greek root for 'blind' or 'obscure,' actually makes nyctalopia mean night blindness.) The distinctive feature of the mutation listed here is the stationary nature of the night blindness. There is an autosomal dominant variety reported in many families, of which the most famous is that descendant from Jean Nougaret, born in Provence in 1637, and studied by Cunier (1838), Nettleship (1909, 1912) and others (see 610444). The X-linked form is distinguished from the autosomal form by the association of myopia.

Morton (1893) described a family with X-linked myopia and night blindness. Fraser and Friedmann (1967) described a family from the same area near Cardiff, Wales. Myopia also occurs with external ophthalmoplegia (311000) and possibly as an uncomplicated X-linked recessive (310460). Worth (1906) reported 4 families with myopia which apparently was X-linked. At Nettleship's suggestion, he looked for associated night blindness and found it in the affected members of only 1 of the families. In Oswald's family with myopia transmitted in a pattern otherwise consistent with X-linked inheritance, apparent male-to-male transmission occurred in the first generation. Francois and De Rouck (1965) described 2 families with 'degenerative' myopia transmitted as an X-linked recessive. In 1 of the families congenital hemeralopia was associated. Scotopic vision is abnormal in this disorder from a putative defect in neurotransmission from photoreceptors to middle retinal neurons.

Moro et al. (1982) described a large 4-generation Sicilian family segregating severe myopia associated with nyctalopia as an X-linked trait. The authors noted that night blindness was present in all 4 affected family members in whom it was sought and stated that it 'probably also existed' in the other 11 patients. Exotropia, esotropia, and hypertropia were also observed in affected individuals who underwent examination.

Miyake et al. (1986) suggested that congenital stationary night blindness can be divided into 2 types, which they called 'complete' and 'incomplete.' In the complete type, rod function is absent, whereas in the incomplete type, there is residual, recordable rod function. Complete and incomplete types did not occur within the same pedigree (Miyake et al., 1986). Khouri et al. (1988), however, reported a family in which 3 of 5 affected members had hyperopia and could be classified as incomplete type, while a fourth member with myopia was more consistent with the complete type. They repeated the suggestion that the association between myopia and night blindness is 'consistent with close linkage of the genes coding for these 2 traits on the X chromosome.' In fact, the great rarity of X-linked night blindness without myopia makes the explanation of linkage unlikely. Furthermore, their explanation for the occurrence of both myopia and hyperopia in the same family, i.e., a crossing-over between 2 genes, seems unlikely. Another suggestion they made seems more likely, namely, that 'an autosomal dominant hyperopic gene masked the myopic gene.'

Musarella et al. (1989) studied 7 multigeneration families with complete congenital stationary night blindness (symbolized CSNB1) and 1 family with the incomplete disorder (CSNB2; see 300071). In general, X-linked congenital stationary night blindness is a nonprogressive retinal disorder resulting from a presumptive defect of neurotransmission between the photoreceptors and the bipolar cells. The complete form lacks rod function by ERG and dark adaptometry and is accompanied by refractive error ranging from mild to severe myopia. The incomplete type shows some rod function on scotopic testing and is accompanied by refraction ranging from moderate hyperopia to moderate myopia.

Other Features

White (1940) found a value of recombination apparently exceeding 50% between the loci for colorblindness and myopia with night blindness. Haldane (1948) took this to mean that the 'genetical map of the human X chromosome is likely to be as long as that of Drosophila melanogaster.' From the information now available that the color blindness loci are located on the tip of the long arm of the X chromosome and the myopia with night blindness on the short arm, the findings of White (1940) and the interpretation of Haldane (1948) are validated.

Inheritance

Ruttum et al. (1992) presented studies of 4 affected females from a 5-generation family with X-linked CSNB1. Each of the manifesting females was the daughter of a different, asymptomatic, carrier mother. None of the 14 daughters of the 9 affected males showed signs or symptoms of CSNB. Uneven X-chromosome lyonization was considered the most likely reason for the manifestation of the disorder in these females. They mentioned but dismissed the possibility of genomic imprinting as the basis. If only 1 manifesting female were involved in such a family, explanations such as loss of the paternal X in the 45,X Turner syndrome or uniparental disomy (UPD) would be possible. They stated that uniparental disomy had not been documented for an X-linked disease and would be an unlikely recurring event. (Quan et al. (1997) demonstrated UPD of the entire X chromosome in a female with Duchenne muscular dystrophy.)

Dry et al. (1993) reported a family in which 2 male cousins showed clinical variation that resulted in diagnostic difficulties: one had congenital nystagmus and myopia, while the other was initially thought to have retinitis pigmentosa with optic atrophy and was hyperopic. The diagnosis of X-linked congenital stationary night blindness was established by clinical, psychophysical, and electrophysiologic criteria. With DNA markers, it was shown that the 2 affected males inherited the same haplotype from their carrier mothers, thus excluding the possibility that a myopia gene in linkage disequilibrium with CSNB1 had recombined with this locus.

Mapping

In 7 families with CSNB1, Musarella et al. (1989) found evidence of linkage to markers in the vicinity of Xp11.3; no recombination was found with DXS7 (L1.28) in 22 opportunities, giving a maximum lod score of 7.35 at theta = 0.00. Linkage information was inconclusive in the 1 family with incomplete night blindness, CSNB2 (the family studied by Khouri et al., 1988). Bech-Hansen et al. (1989, 1990) found linkage suggesting location of the CSNB locus within Xp11. Gal et al. (1989, 1989) likewise assigned the locus to Xp11.3 by family linkage studies. Musarella et al. (1989) mapped the gene to Xp11.3 by family linkage studies and suggested that the locus is distal to TIMP (305370) and proximal to OTC (300461). Pearce and Bech-Hansen (1990) presented further arguments favoring location of the CSNB1 gene in the Xp11 region.

Pearce et al. (1990) reported patients with complete and incomplete forms of CSNB in the same family. One of these families was included in the study of Bech-Hansen et al. (1990). Bech-Hansen et al. (1992) reported a family in which a recombinant demonstrated that the CSNB1 locus is located proximal to DXS7. Bergen et al. (1994) described a family classified as incomplete CSNB, or CSNB2, in which a key recombinant allowed assignment of the gene to a site proximal to monoamine oxidase B (MAOB; 309860). The issue of whether CSNB1 and CSNB2 and even Aland Island eye disease (AIED; 300600) are allelic disorders was raised but not solved by the study.

Boycott et al. (1998) studied 32 families with X-linked CSNB, including 11 families with the complete form of CSNB and 21 families with the incomplete form. Critical recombination events in the families with complete CSNB localized a disease gene to the region between DXS556 and DXS8083, in Xp11.4-p11.3. The critical recombination events in the set of families with incomplete CSNB 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 shared portions of an ancestral chromosome. The 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 2 loci, 1 for complete and 1 for incomplete (CSNB2) X-linked CSNB, can account for all reported mapping information.

Rozzo et al. (1999) studied a Sardinian family with complete X-linked congenital stationary night blindness and defined better the limits of the CSNB1 locus on Xp11.4 through linkage analysis. Two key recombinants served to restrict the CSNB1 locus to an approximately 3-cM region between markers DXS1068 and DXS6810. Comparison of their results with those of other mapping studies in families from different geographic areas confirmed the genetic homogeneity of X-linked complete CSNB.

Molecular Genetics

In a study of a large Mennonite family with CSNB1, Bech-Hansen and Pearce (1993) found that 3 of 5 sisters in 1 sibship had typical manifestations of the disorder. All of the sons of these 3 sisters were affected. Each of the 2 nonmanifesting sisters had at least 1 unaffected son. Analysis of Xp markers in the Xp21.1-p11.22 region showed that the 2 sisters who were unaffected had inherited the same maternal X chromosome, designated M2. Two of the daughters who manifested CSNB had inherited the other maternal X chromosome, designated M1. The third manifesting sister inherited a recombinant X chromosome with a crossover between TIMP and DXS255, which suggested that the CSNB1 locus lies proximal to TIMP, a conclusion contrary to one mentioned earlier. One of the affected daughters' sons had inherited the maternal M1 X chromosome, a finding consistent with that chromosome carrying a mutant CSNB gene; the other affected sons inherited the grandfather's X chromosome, thus supporting homozygosity of the mothers. Molecular analysis of DNA from the 3 sisters with manifestations of CSNB yielded results consistent with their being homozygous and with their mother being a carrier of CSNB1. Bech-Hansen et al. (1998) subsequently identified a frameshift mutation in exon 27 of the CACNA1F gene in 15 different families with incomplete CSNB (CSNB2; 300071); these 15 families were established by haplotype analysis to be related by a founder effect (Boycott et al., 1998; see 300110.0003).

Bech-Hansen et al. (2000) studied 22 families with X-linked complete CSNB in which affected males have night blindness, some photopic vision loss, and a defect of the ON-pathway. They found 14 different mutations, including 1 founder mutation, in 7 families from the United States, in a novel candidate gene, NYX (300278). NYX, which encodes a glycosylphosphatidyl (GPI)-anchored protein called nyctalopin, is a unique member of the small leucine-rich proteoglycan (SLRP) family. The role of other SLRP proteins suggests that mutant nyctalopin disrupts developing retinal interconnections involving the ON-bipolor cells, leading to the visual losses seen in patients with complete CSNB. NYX was the second member of the SLRP family to be associated with a human genetic disease. Keratocan (KERA; 603228) had been identified as the cause of autosomal recessive cornea plana.

Pusch et al. (2000) identified mutations in the NYX gene in patients with CSNB1. The gene was partially deleted in 3 families, and mutation analysis in 21 further families detected another 13 different mutations. They found the NYX gene to be expressed at low levels in tissues including retina, brain, testis, and muscle.

In affected members from 2 unrelated Chinese families with X-linked congenital stationary night blindness and high myopia, Xiao et al. (2006) identified 2 different mutations in the NYX gene (300278.0005; 300278.0006).

Nomenclature

Note on the use of the terms hemeralopia and nyctalopia: in several places in Mendelian Inheritance in Man, hemeralopia has been defined as night blindness and nyctalopia as dayblindness. Acland (1994) pointed out that this terminology is controversial. In general, in the English-speaking world, ophthalmologists understand hemeralopia to mean dayblindness, i.e., loss of photopic vision, as in total colorblindness, and nyctalopia to mean night blindness. This is particularly true of British-trained ophthalmologists, for whom the standard authority, the 'System of Ophthalmology' by Duke-Elder (1963), discussed at some length the 'correct' derivation, meaning, and usage of the terms. Furthermore, all standard English dictionaries (Oxford, Merriam Webster, Random House, McGraw-Hill Dictionary of Scientific and Technical Terms, Dorland's Medical Dictionary, etc.) agree with this usage. Skinner (1970) gave the derivation of the terms, as accepted officially by the Royal College of Physicians in London in the 18th century, as being from the Greek roots 'hemer(a)' [day] or 'nyktos' [night], plus 'al(aos)' [blind or obscure] and 'ops' [eye]. In the French literature, the terminology and 'accepted' derivations are exactly as in MIM and opposite to standard English usage. Although MIM does not wish to become involved in the sometimes heated discussion between anglophones and francophones on this subject, it does wish to lay out the problem and recommend that it is time to get rid of both terms and use the correctly understandable terms dayblindness and night blindness.

Animal Model

Gregg et al. (2003) studied the naturally occurring mouse mutant nob (no b-wave) to determine whether it provided an animal model for the complete form of human X-linked congenital stationary night blindness. They found that the nob phenotype was caused by an 85-bp deletion in the mouse Nyx gene. Behavioral testing showed that the nob mice had a significant decrease in visual sensitivity. Gregg et al. (2003) concluded that the nob mouse is a model for human CSNB1.