Congenital stationary night blindness

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Congenital stationary night blindness
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Malfunction in transmission from the photoreceptors in the outer nuclear layer to bipolar cells in the inner nuclear layer underlies CSNB.
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Congenital stationary night blindness (CSNB) is a rare non-progressive retinal disorder. People with CSNB often have difficulty adapting to low light situations due to impaired photoreceptor transmission. These patients may also have reduced visual acuity, myopia, nystagmus, fundus abnormalities, and strabismus [1] [2] . CSNB has two forms -- complete, also known as type-1 (CSNB1), and incomplete, also known as type-2 (CSNB2), which are distinguished by the involvement of different retinal pathways. In CSNB1, downstream neurons called bipolar cells are unable to detect neurotransmission from photoreceptor cells. CSNB1 can be caused by mutations in various genes involved in neurotransmitter detection, including NYX. In CSNB2, the photoreceptors themselves have impaired neurotransmission function; this is caused primarily by mutations in the gene CACNA1F , which encodes a voltage-gated calcium channel important for neurotransmitter release. CSNB has been identified in horses and dogs as the result of mutations in TRPM1 (Horse, "LP") [3] , GRM6 (Horse, "CSNB2") [4] , and LRIT3 (Dog, CSNB) [5] .

Contents

Congenital stationary night blindness (CSNB) can be inherited in an X-linked, autosomal dominant, or autosomal recessive pattern, depending on the genes involved.

Two forms of CSNB can also affect horses, one linked to the leopard complex of equine coat colors and the other found in certain horse breeds. Both are autosomal recessives. [6] [7]

Symptoms and signs

The X-linked varieties of congenital stationary night blindness (CSNB) can be differentiated from the autosomal forms by the presence of myopia, which is typically absent in the autosomal forms. Patients with CSNB often have impaired night vision, myopia, reduced visual acuity, strabismus and nystagmus. Individuals with the complete form of CSNB (CSNB1) have highly impaired rod sensitivity (reduced ~300x) as well as cone dysfunction. Patients with the incomplete form can present with either myopia or hyperopia. [8]

Cause

CSNB is caused by malfunctions in neurotransmission from rod and cone photoreceptors to bipolar cells in the retina. [9] At this first synapse, information from photoreceptors is divided into two channels: ON and OFF. The ON pathway detects light onset, while the OFF pathway detects light offset. [10] The malfunctions in CSNB1 specifically affect the ON pathway, by hindering the ability of ON-type bipolar cells to detect neurotransmitter released from photoreceptors. [9] Rods, which are responsible for low-light vision, make contacts with ON-type bipolar cells only, while, cones, which are responsible for bright-light vision, make contacts with bipolar cells of both ON an OFF subtypes. [11] Because the low-light sensing rods feed only into the ON pathway, individuals with CSNB1 typically have problems with night vision, while vision in well-lit conditions is spared. [9] In CSNB2, release of neurotransmitter from photoreceptors is impaired, leading to involvement of both ON and OFF pathways.

The electroretinogram (ERG) is an important tool for diagnosing CSNB. The ERG a-wave, which reflects the function of the phototransduction cascade in response to a light flashes, is typically normal in CSNB patients, although in some cases phototransduction is also affected, leading to a reduced a-wave. The ERG b-wave, which primarily reflects the function of ON-bipolar cells, is greatly reduced in CSNB2 cases, and completely absent in CSNB1 cases. [9] [12]

Genetics

Only three rhodopsin mutations have been found associated with congenital stationary night blindness (CSNB). [13] Two of these mutations are found in the second transmembrane helix of rhodopsin at Gly-90 and Thr-94. Specifically, these mutations are the Gly90Asp [14] and the Thr94Ile, which has been the most recent one reported. [15] The third mutation is Ala292Glu, and it is located in the seventh transmembrane helix, in proximity to the site of retinal attachment at Lys-296. [16] Mutations associated with CSNB affect amino acid residues near the protonated Schiff base (PSB) linkage. They are associated with changes in conformational stability and the protonated status of the PSB nitrogen. [17]

Pathophysiology

CSNB1

The complete form of X-linked congenital stationary night blindness, also known as nyctalopia, is caused by mutations in the NYX gene (Nyctalopin on X-chromosome), which encodes a small leucine-rich repeat (LRR) family protein of unknown function. [18] [19] This protein consists of an N-terminal signal peptide and 11 LRRs (LRR1-11) flanked by cysteine-rich LRRs (LRRNT and LRRCT). At the C-terminus of the protein there is a putative GPI anchor site. Although the function of NYX is yet to be fully understood, it is believed to be located extracellularly. A naturally occurring deletion of 85 bases in NYX in some mice leads to the "nob" (no b-wave) phenotype, which is highly similar to that seen in CSNB1 patients. [20] NYX is expressed primarily in the rod and cone cells of the retina. There are currently almost 40 known mutations in NYX associated with CSNB1, Table 1., located throughout the protein. As the function of the nyctalopin protein is unknown, these mutations have not been further characterized. However, many of them are predicted to lead to truncated proteins that, presumably, are non-functional.

Table 1. Mutations in NYX associated with CSNB1
MutationPositionReferences
NucleotideAmino acid
c.?-1_?-61del1_20delSignal sequence [19]
SplicingIntron 1 [21]
c.?-63_1443-?del21_481del [19]
c.48_64delL18RfsX108Signal sequence [21]
c.85_108delR29_A36delN-terminal LRR [18]
c.G91CC31SLRRNT [19]
c.C105AC35XLRRNT [19]
c.C169AP57TLRRNT [22]
c.C191AA64ELRR1 [22]
c.G281CR94PLRR2 [23]
c.301_303delI101delLRR2 [19]
c.T302CI101TLRR2 [23]
c.340_351delE114_A118delLRR3 [19] [21]
c.G427CA143PLRR4 [19]
c.C452TP151LLRR4 [18]
c.464_465insAGCGTGCCCGAGCGCCTCCTGS149_V150dup+P151_L155dupLRR4 [18]
c.C524GP175RLRR5 [19]
c.T551CL184PLRR6 [18]
c.556_618delinsH186?fsX260LRR6 [18]
c.559_560delinsAAA187KLRR6 [19]
c.613_621dup205_207dupLRR7 [18] [19]
c.628_629insR209_S210insCLRLRR7 [18]
c.T638AL213QLRR7 [18]
c.A647GN216SLRR7 [18] [21]
c.T695CL232PLRR8 [18]
c.727_738del243_246delLRR8 [19]
c.C792GN264KLRR9 [18]
c.T854CL285PLRR10 [18]
c.T893CF298SLRR10 [18]
c.C895TQ299XLRR10 [21]
c.T920CL307PLRR11 [19]
c.A935GN312SLRR11 [19]
c.T1040CL347PLRRCT [19]
c.G1049AW350XLRRCT [18]
c.G1109TG370VLRRCT [19]
c.1122_1457delS374RfsX383LRRCT [19] [21]
c.1306delL437WfsX559C-terminus [21]
LRR: leucine-rich repeat, LRRNT and LRRCT: N- and C-terminal cysteine-rich LRRs.

CSNB2

Figure 1. Schematic structure of CaV1.4 with the domains and subunits labeled. Alphasubunit calcium channel.png
Figure 1. Schematic structure of CaV1.4 with the domains and subunits labeled.

The incomplete form of X-linked congenital stationary night blindness (CSNB2) is caused by mutations in the CACNA1F gene, which encodes the voltage-gated calcium channel CaV1.4 expressed heavily in retina. [24] [25] One of the important properties of this channel is that it inactivates at an extremely low rate. This allows it to produce sustained Ca2+ entry upon depolarization. As photoreceptors depolarize in the absence of light, CaV1.4 channels operate to provide sustained neurotransmitter release upon depolarization. [26] This has been demonstrated in CACNA1F mutant mice that have markedly reduced photoreceptor calcium signals. [27] There are currently 55 mutations in CACNA1F located throughout the channel, Table 2 and Figure 1. While most of these mutations result in truncated and, likely, non-functional channels, it is expected that they prevent the ability of light to hyperpolarize photoreceptors. Of the mutations with known functional consequences, 4 produce channels that are either completely non-functional, and two that result in channels which open at far more hyperpolarized potentials than wild-type. This will result in photoreceptors that continue to release neurotransmitter even after light-induced hyperpolarization.

Table 2. Mutations in CACNA1F associated with CSNB2
MutationPositionEffectReferences
NucleotideAmino Acid
c.C148T R50X N-terminus [28]
c.151_155delAGAAA R51PfsX115 N-terminus [29]
c.T220C C74R N-terminus [29]
c.C244T R82X N-terminus [28] [29]
c.466_469delinsGTAGGGGTGCT
CCACCCCGTAGGGGTGCTCCACC
S156VdelPinsGVKHOVGVLH D1S2-3 [28] [30] [31]
Splicing Intron 4 [28]
c.T685C S229P D1S4-5 [29]
c.G781A G261R D1-pore [29]
c.G832T E278X D1-pore [21] [32]
c.904insG R302AfsX314 D1-pore [30]
c.951_953delCTT F318del D1-pore [28]
c.G1106A G369D D1S6 Activates ~20mV more negative than wild-type, increases time to peak current and decreases inactivation, increased Ca2+ permeability. [24] [26] [28] [29] [33]
c.1218delC W407GfsX443 D1-2 [25] [28] [32]
c.C1315T Q439X D1-2 [29]
c.G1556A R519Q D1-2 Decreased expression [24] [34]
c.C1873T R625X D2S4 [28] [29]
c.G2021A G674D D2S5 [26] [28] [30]
c.C2071T R691X D2-pore [22]
c.T2258G F753C D2S6 [29]
c.T2267C I756T D2S6 Activates ~35mV more negative than wild-type, inactivates more slowly [35]
Splicing Intron 19 [29]
c.T2579C L860P D2-3 [29]
c.C2683T R895X D3S1-2 [21] [22] [25] [28]
Splicing Intron 22 [29] [30]
Splicing Intron 22 [29]
c.C2783A A928D D3S2-3 [26] [28]
c.C2905T R969X D3S4 [24] [29]
c.C2914T R972X D3S4 [32]
Splicing Intron24 [28]
c.C2932T R978X D3S4 [30]
c.3006_3008delCAT I1003del D3S4-5 [28]
c.G3052A G1018R D3S5 [29]
c.3125delG G1042AfsX1076 D3-pore [28]
c.3166insC L1056PfsX1066 D3-pore [24] [25] [28] [29]
c.C3178T R1060W D3-pore [24] [29]
c.T3236C L1079P D3-pore Does not open without BayK, activates ~5mV more negative than wild-type [29] [33]
c.3672delC L1225SfsX1266 D4S2 [25] [28]
c.3691_3702del G1231_T1234del D4S2 [24] [29]
c.G3794T S1265I D4S3 [22]
c.C3886A R1296S D4S4 [22]
c.C3895T R1299X D4S4 [25] [28] [29]
Splicing Intron 32 [29]
c.C4075T Q1359X D4-pore [24] [29]
c.T4124A L1375H D4-pore Decreased expression [24] [29] [34]
Splicing Intron 35 [29]
c.G4353A W1451X C-terminus Non-functional [25] [26] [28] [33]
c.T4495C C1499R C-terminus [29]
c.C4499G P1500R C-terminus [29]
c.T4523C L1508P C-terminus [29]
Splicing intron 40 [28]
c.4581delC F1528LfsX1535 C-terminus [36]
c.A4804T K1602X C-terminus [24] [29]
c.C5479T R1827X C-terminus [29]
c.5663delG S1888TfsX1931 C-terminus [28]
c.G5789A R1930H C-terminus [22]

Diagnosis

Night blindness is a symptom in many patients and diagnosis often occurs through the use of various tests including a electroretinogram to reveal any impairment in the retina "as a whole" [37] [38] [39] . Tests performed can also include a visual field examination, Fundoscopic examination, and slit-lamp microscopy in addition to measurements provided by the electroretinogram (ERG) [40] [41] [42] .

Footnotes

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  2. Zeitz, Christina; Robson, Anthony G.; Audo, Isabelle (2015-03-01). "Congenital stationary night blindness: An analysis and update of genotype–phenotype correlations and pathogenic mechanisms". Progress in Retinal and Eye Research. 45: 58–110. doi:10.1016/j.preteyeres.2014.09.001. ISSN   1350-9462. PMID   25307992.
  3. Bellone RR, Holl H, Setaluri V, Devi S, Maddodi N, Archer S, et al. (2013-10-22). "Evidence for a retroviral insertion in TRPM1 as the cause of congenital stationary night blindness and leopard complex spotting in the horse". PLOS ONE. 8 (10): e78280. Bibcode:2013PLoSO...878280B. doi: 10.1371/journal.pone.0078280 . PMC   3805535 . PMID   24167615.
  4. Hack YL, Crabtree EE, Avila F, Sutton RB, Grahn R, Oh A, et al. (March 2021). "Whole-genome sequencing identifies missense mutation in GRM6 as the likely cause of congenital stationary night blindness in a Tennessee Walking Horse". Equine Veterinary Journal. 53 (2): 316–323. doi: 10.1111/evj.13318 . PMID   32654228. S2CID   220500585.
  5. Das RG, Becker D, Jagannathan V, Goldstein O, Santana E, Carlin K, et al. (October 2019). "Genome-wide association study and whole-genome sequencing identify a deletion in LRIT3 associated with canine congenital stationary night blindness". Scientific Reports. 9 (1): 14166. Bibcode:2019NatSR...914166D. doi:10.1038/s41598-019-50573-7. PMC   6775105 . PMID   31578364.
  6. "Appaloosa Panel 2 | Veterinary Genetics Laboratory". vgl.ucdavis.edu. Retrieved 11 October 2022.
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Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene and a G-protein-coupled receptor (GPCR). It is the opsin of the rod cells in the retina and a light-sensitive receptor protein that triggers visual phototransduction in rods. Rhodopsin mediates dim light vision and thus is extremely sensitive to light. When rhodopsin is exposed to light, it immediately photobleaches. In humans, it is regenerated fully in about 30 minutes, after which the rods are more sensitive. Defects in the rhodopsin gene cause eye diseases such as retinitis pigmentosa and congenital stationary night blindness.

Achromatopsia, also known as Rod monochromacy, is a medical syndrome that exhibits symptoms relating to five conditions, most notably monochromacy. Historically, the name referred to monochromacy in general, but now typically refers only to an autosomal recessive congenital color vision condition. The term is also used to describe cerebral achromatopsia, though monochromacy is usually the only common symptom. The conditions include: monochromatic color blindness, poor visual acuity, and day-blindness. The syndrome is also present in an incomplete form that exhibits milder symptoms, including residual color vision. Achromatopsia is estimated to affect 1 in 30,000 live births worldwide.

<span class="mw-page-title-main">Retinitis pigmentosa</span> Gradual retinal degeneration leading to progressive sight loss

Retinitis pigmentosa (RP) is a genetic disorder of the eyes that causes loss of vision. Symptoms include trouble seeing at night and decreasing peripheral vision. As peripheral vision worsens, people may experience "tunnel vision". Complete blindness is uncommon. Onset of symptoms is generally gradual and often begins in childhood.

<span class="mw-page-title-main">Rod cell</span> Photoreceptor cells that can function in lower light better than cone cells

Rod cells are photoreceptor cells in the retina of the eye that can function in lower light better than the other type of visual photoreceptor, cone cells. Rods are usually found concentrated at the outer edges of the retina and are used in peripheral vision. On average, there are approximately 92 million rod cells in the human retina. Rod cells are more sensitive than cone cells and are almost entirely responsible for night vision. However, rods have little role in color vision, which is the main reason why colors are much less apparent in dim light.

<span class="mw-page-title-main">Melanopsin</span> Mammalian protein found in Homo sapiens

Melanopsin is a type of photopigment belonging to a larger family of light-sensitive retinal proteins called opsins and encoded by the gene Opn4. In the mammalian retina, there are two additional categories of opsins, both involved in the formation of visual images: rhodopsin and photopsin in the rod and cone photoreceptor cells, respectively.

<span class="mw-page-title-main">Nyctalopia</span> Condition making it difficult or impossible to see in relatively low light

Nyctalopia, also called night-blindness, is a condition making it difficult or impossible to see in relatively low light. It is a symptom of several eye diseases. Night blindness may exist from birth, or be caused by injury or malnutrition. It can be described as insufficient adaptation to darkness.

<span class="mw-page-title-main">Electroretinography</span>

Electroretinography measures the electrical responses of various cell types in the retina, including the photoreceptors, inner retinal cells, and the ganglion cells. Electrodes are placed on the surface of the cornea or on the skin beneath the eye to measure retinal responses. Retinal pigment epithelium (RPE) responses are measured with an EOG test with skin-contact electrodes placed near the canthi. During a recording, the patient's eyes are exposed to standardized stimuli and the resulting signal is displayed showing the time course of the signal's amplitude (voltage). Signals are very small, and typically are measured in microvolts or nanovolts. The ERG is composed of electrical potentials contributed by different cell types within the retina, and the stimulus conditions can elicit stronger response from certain components.

Rhodopsin kinase is a serine/threonine-specific protein kinase involved in phototransduction. This enzyme catalyses the following chemical reaction:

<span class="mw-page-title-main">Photoreceptor cell-specific nuclear receptor</span> Protein-coding gene in the species Homo sapiens

The photoreceptor cell-specific nuclear receptor (PNR), also known as NR2E3, is a protein that in humans is encoded by the NR2E3 gene. PNR is a member of the nuclear receptor super family of intracellular transcription factors.

<span class="mw-page-title-main">TRPM1</span> Protein-coding gene in the species Homo sapiens

Transient receptor potential cation channel subfamily M member 1 is a protein that in humans is encoded by the TRPM1 gene.

<span class="mw-page-title-main">Metabotropic glutamate receptor 6</span> Mammalian protein found in humans

Glutamate receptor, metabotropic 6, also known as GRM6 or mGluR6, is a protein which in humans is encoded by the GRM6 gene.

Ca<sub>v</sub>1.4 Protein-coding gene in humans

Cav1.4 also known as the calcium channel, voltage-dependent, L type, alpha 1F subunit (CACNA1F), is a human gene.

<i>CRX</i> (gene) Protein-coding gene in the species Homo sapiens

Cone-rod homeobox protein is a protein that in humans is encoded by the CRX gene.

<span class="mw-page-title-main">PDE6B</span> Protein-coding gene in the species Homo sapiens

Rod cGMP-specific 3',5'-cyclic phosphodiesterase subunit beta is the beta subunit of the protein complex PDE6 that is encoded by the PDE6B gene. PDE6 is crucial in transmission and amplification of visual signal. The existence of this beta subunit is essential for normal PDE6 functioning. Mutations in this subunit are responsible for retinal degeneration such as retinitis pigmentosa or congenital stationary night blindness.

<span class="mw-page-title-main">SAG (gene)</span>

S-arrestin is a protein that in humans is encoded by the SAG gene.

<i>NRL</i> (gene) Protein-coding gene in the species Homo sapiens

Neural retina-specific leucine zipper protein is a protein that in humans is encoded by the NRL gene.

<span class="mw-page-title-main">Nyctalopin</span> Protein-coding gene in the species Homo sapiens

Nyctalopin is a protein located on the surface of photoreceptor-to-ON bipolar cell synapse in the retina. It is composed of 481 amino acids. and is encoded in human by the NYX gene. This gene is found on the chromosome X and has two exons. This protein is a leucine-rich proteoglycan which is expressed in the eye, spleen and brain in mice. Mutations in this gene cause congenital stationary night blindness in humans (CSNB). which is a stable retinal disorder. The consequence of this mutation results in an abnormal night vision. Nyctalopin is critical due to the fact that it generates a depolarizing bipolar cell response due to the mutation on the NYX gene. Most of the time, CSNB are associated to hygh myopia which is the result of a mutation on the same gene. Several mutations can occur on the NYX gene resulting on many form of night blindness in humans. Some studies show that these mutations are more present in Asian population than in Caucasian population. A mouse strain called nob carries a spontaneous mutation leading to a frameshift in this gene. These mice are used as an animal model for congenital stationary night blindness.

<span class="mw-page-title-main">Leopard complex</span> Coat pattern in horses

The leopard complex is a group of genetically related coat patterns in horses. These patterns range from progressive increases in interspersed white hair similar to graying or roan to distinctive, Dalmatian-like leopard spots on a white coat. Secondary characteristics associated with the leopard complex include a white sclera around the eye, striped hooves and mottled skin. The leopard complex gene is also linked to abnormalities in the eyes and vision. These patterns are most closely identified with the Appaloosa and Knabstrupper breeds, though its presence in breeds from Asia to western Europe has indicated that it is due to a very ancient mutation.

<span class="mw-page-title-main">Retinal degeneration (rhodopsin mutation)</span> Retinopathy

Retinal degeneration is a retinopathy which consists in the deterioration of the retina caused by the progressive death of its cells. There are several reasons for retinal degeneration, including artery or vein occlusion, diabetic retinopathy, R.L.F./R.O.P., or disease. These may present in many different ways such as impaired vision, night blindness, retinal detachment, light sensitivity, tunnel vision, and loss of peripheral vision to total loss of vision. Of the retinal degenerative diseases retinitis pigmentosa (RP) is a very important example.

Retinal gene therapy holds a promise in treating different forms of non-inherited and inherited blindness.