Retinal Degeneration (Rhodopsin Mutation)
Encyclopedia
Retinal degeneration
is the deterioration of the retina
(1) caused by the progressive and eventual death of the cells of the retina.(2) There are several reasons for retinal degeneration, including artery or vein occlusion, diabetic retinopathy
, R.L.F./R.O.P. (retrolental fibroplasia/ retinopathy of prematurity
), or disease (usually hereditary).(3) 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(4). Of the retinal degenerative diseases retinitis pigmentosa
(RP) is a very important example.
Inherited retinal degenerative disorders in humans exhibit genetic
and phenotypic heterogeneity in their underlying causes and clinical outcomes* (5) (6) (7). These retinopathies
affect approximately one in 2000 individuals worldwide (7). A wide variety of causes have been attributed to retinal degeneration, such as disruption of genes that are involved in phototransduction, biosynthesis
and folding of the rhodopsin molecule, and the structural support of the retina.(6) Mutations in the rhodopsin gene
account for 25% to 30% (30% to 40% according to(9)) of all cases of autosomal dominant retinitis pigmentosa (adRP) (5) (20-22) in North America (10-12) There are many mechanisms of retinal degeneration attributed to rhodopsin mutations or mutations that involve or affect the function of rhodopsin. One mechanism of retinal degeneration is rhodopsin overexpression. Another mechanism, whereby a mutation caused a truncated rhodopsin, was found to affect rod function and increased the rate of photoreceptor degeneration.(9)
Photoreceptor cell death is the eventual outcome of retinal degeneration. Without proper function of the photoreceptor cells, vision
is not possible. Irreversible loss of these cells has been attributed as a cause of blindness
in many retinal degenerative disorders, including RP. The exact mechanism of photoreceptor cell death is not clearly understood. (5)(14) Among potential causes is the endocytosis
of stable complexes formed between rhodopsin and its regulatory protein arrestin
in certain mutants. (5) Various studies have also documented that over-expression of rhodopsin itself (mutations in genes involved in the termination of rhodopsin signaling activity have been shown to cause degeneration by persistent activation of the phototransduction cascade (14)) causes photoreceptor cell death and may induce photoreceptor cell loss in transgenic animals expressing truncated rhodopsin. Yet another mechanism may be prolonged photoreceptor responses and also abnormal rhodopsin deactivation may induce outer segment shortening and eventual photoreceptor death (9)
In RP photoreceptor cell death is believed to occur by programmed cell death or apoptosis
. (10) (11) (15)
.3 Autosomal dominant RP accounts for approximately 15% of these cases. (8) Autosomal dominant retinitis pigmentosa (ADRP) is a genetically heterogeneous group of inherited retinal degenerations that cause blindness in humans. (14)
RP begins with death of rod photoreceptor cells, which are the only cells in the retina to express rhodopsin and which express it as their most abundant protein. Eventually, loss of rod cells leads to loss of cone cells (cone photoreceptors), the mainstay of human vision (16). Symptoms of RP include loss of sensitivity to dim light, abnormal visual function, and characteristic bone spicule
deposits of pigment in the retina. Affected individuals progressively lose visual field
and visual acuity
, and photoreceptor cell death can ultimately lead to blindness. (9) A prominent early clinical feature of retinitis pigmentosa is the loss of night vision as a result of death of rod photoreceptor cells. Proper expression of the wild-type rhodopsin gene is essential for the development and sustained function of photoreceptor cells.(10)
Mutations in the human rhodopsin that affect its folding
, trafficking and activity are the most commonly encountered causes of retinal degeneration in afflicted patients. A single base-substitution at the codon position 23 in the human opsin gene (P23H) is the most common cause of ADRP in American patients. (6)(17) ADRP due to rhodopsin mutations has a wide range of clinical presentation and severity. Before 1991, phenotypic evidence pointed to different subsets of ADRP with varying prognoses
.(30-34) Molecular classification of ADRP and further sub-classification based on the region of the mutation in the rhodopsin gene allowed better prediction of a particular disease course. But even within these specific subsets, the prognosis is influenced by the specific mutation itself.(8)
(Rh1) that is the primary visual pigment (photopigment
) of rod photoreceptors (which are the only cells in the retina to express rhodopsin and which express it as their most abundant protein(16)) and forms an integral part of the visual cascade.(8)(13)(10) It is a G-protein-coupled receptor activated by light that initiates the phototransduction cascade (visual transduction cascade taking place in photoreceptor rod outer segments (13).) converting light signals to electrophysiological signals in retinal neurons. This photo-activated signal transduction process is essential for vision. (10)
The structure and function of rhodopsin and the gene encoding it have been the subjects of intense scrutiny for many years because rhodopsin serves as a useful model for understanding the largest receptor family in the human genome, the G protein-coupled receptors, and because defects in the rhodopsin gene are the most common cause of the most common inherited blinding disease, retinitis pigmentosa. (16) (18)(19)
for numerous alleles linked to the neurodegenerative disease retinitis pigmentosa. (16) Mutations in the rhodopsin gene account for 25% to 30% (30% to 40% according to (9)) of all cases of autosomal dominant retinitis pigmentosa (ADRP).(8)(10)(20-22) Over 100 distinct mutations in the light-sensing molecule rhodopsin are known to cause (adRP). (6)(9)(10)(13)(14) Most of these mutations are missense mutations affecting single amino acid
residues in the rhodopsin protein. (10)(24) These mutations affect rhodopsin transport to the outer segments of rod photoreceptor cells, rhodopsin folding, and rhodopsin endocytosis. Mutations in the human rhodopsin that affect its folding, trafficking and activity are the most commonly encountered causes of retinal degeneration in patients afflicted with RP. (6) A single base-substitution mutation of codon 23 of the rhodopsin gene in which proline
is changed to histidine
(Pro23His) in the human opsin
gene accounts for the largest fraction of rhodopsin mutations observed in the United States and is the most common cause of ADRP in American patients.(6)(8)(17)
In 1990, the Pro23His mutation of the rhodopsin gene was reported as the first mutation associated with RP.(8)(20)(21)(24-26) This mutation has been described only in the United States, where it continues to be the most commonly described gene defect in RP.
The phenotype of the RP associated with the P23H mutation is characteristically relatively mild but variable.(8)(20-22)(24-28)
Multiple studies have demonstrated that the degree of severity of a given mutation in the rhodopsin gene is based in large part on its position in the rhodopsin molecule—intradiscal, transmembrane, or cytoplasmic.(29-36) Intradiscal mutations tend to be less severe, whereas mutations affecting cytoplasmic domains and retinol binding sites tend to be very severe.(30)(36) The severity of cytoplasmic mutations affecting maintenance of photoreceptor cell polarity, such as those at the C-terminus (sorting signal of rhodopsin) may result from inappropriate intracellular transport of the molecule. (8)(13)(37)
Some research has described transgenic mouse mutants that cause degeneration by prolonged activation of the phototransduction cascade.(38) In this research null mutations in the rhodopsin kinase
(39) and arrestin(40) genes, each of which plays a role in terminating rhodopsin activity, caused light-dependent retinal degeneration.(14)
s (fs 341del and fs 341-343del) are predicted to add additional residues to the C terminus,(43) whereas Q344ter results in a C-terminal truncation
, and an intron
splice mutation (N88) is thought to remove the entire C-terminal tail of rhodopsin.(9)(41)(42)
Data indicate that expression of truncated rhodopsin negatively affects both photoreceptor function and health, compromising rod cell survival.The presence of truncated opsin may impair synaptic transmission or other cellular processes and eventually cause cell death. Large quantities of mislocalized opsin may decrease the availability of functional proteins in regions where truncated opsin is concentrated. The presence of truncated rhodopsin in the outer segments causes functional abnormalities and localization of rhodopsin in the outer and/or inner segments induces increased photoreceptor cell death (9)
Missense mutations in the opsin gene affecting the R135 and K296 residues of the protein product cause ADRP and result in accumulation of Rhodopsin-Arrestin complexes in the photoreceptor cell (6)(46)(47). The R135 mutant rhodopsin is noted to form stable complex with arrestin and undergo endocytosis resulting in aberrant endocytic vesicles in HEK cell culture system (47). Similarly, the K296E rhodopsin is observed to bind the visual arrestin with high affinity. This abnormal interaction is demonstrated to have pathological consequences in the retina. Besides this, the stable rhodopsin and arrestin complexes are shown to mislocalize and accumulate in the inner segments of rod photoreceptors of the mouse model of ADRP.(6)
Using mutants that are defective in late endosome to lysosome trafficking, it was shown that rhodopsin accumulates in endosomal compartments in these mutants and leads to light-dependent retinal degeneration. It was also shown that the internalized rhodopsin in dying photoreceptor cells were not degraded but instead showed characteristics of insoluble proteins. This data led to the implication of rhodopsin buildup in the late endosomal system as a novel trigger of death of photoreceptor neurons. Thus failure to degrade internalized rhodopsin in a timely manner triggers cell death of photoreceptor neurons, suggesting that lysosomal turnover of rhodopsin is vital in maintaining photoreceptor viability. (6)
The precise mechanisms regulating the pro-cell death signaling pathways and their interconnection with endocytosis is not well understood. It is speculated that a component innate to the endolysosomal system plays a crucial role in regulating the cell death signals emanating from the endosomes. This component senses the accumulation of rhodopsin and then engages the proper machinery to execute cell death in the retina. Failure of proper protein degradation and resultant subsequent accumulation of proteins, as in the case of rhodopsin-arrestin complexes, is a well-recognized cause of cell death in many neurodegenerative disorders, such as retinitis pigmentosa (6)(48).
, proper function of these proteins is necessary for spliceosome
performance. (10)
The rhodopsin transcript is a pre-mRNA splicing substrate affected by PRPF31 protein, meaning that rhodopsin (RHO) is among the target splicing substrate genes for PRPF31. Thus it can be understood that mutations in PRPF31 can cause alternative, potentially non-functional, forms of the rhodopsin protein. It was shown that expression of these mutant PRPF31 proteins significantly reduced rhodopsin expression in cultured retinal cells and induced apoptosis of retinal cells, establishing a link between mutations in proteins involved in pre-mRNA splicing and the expression of a critical retina-specific gene, rhodopsin. (10)
This shows that non-rhodopsin mutations may also be critical in presentation of retinal degenerative disorders. It suggests a mechanism for retinal degeneration caused by non-retinal-specific genes, such as PRPF31 mutations. (10)
Retinal degeneration
The term retinal degeneration may refer to:* One of several eye diseases or eye disorders in humans* Progressive retinal atrophy, an eye disease in dogs-See also:*List of human eye diseases and disorders...
is the deterioration of the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
(1) caused by the progressive and eventual death of the cells of the retina.(2) There are several reasons for retinal degeneration, including artery or vein occlusion, diabetic retinopathy
Diabetic retinopathy
Diabetic retinopathy is retinopathy caused by complications of diabetes mellitus, which can eventually lead to blindness....
, R.L.F./R.O.P. (retrolental fibroplasia/ retinopathy of prematurity
Retinopathy of prematurity
Retinopathy of prematurity , previously known as retrolental fibroplasia , is an eye disease that affects prematurely-born babies. It is thought to be caused by disorganized growth of retinal blood vessels which may result in scarring and retinal detachment. ROP can be mild and may resolve...
), or disease (usually hereditary).(3) These may present in many different ways such as impaired vision, night blindness, retinal detachment
Retinal detachment
Retinal detachment is a disorder of the eye in which the retina peels away from its underlying layer of support tissue. Initial detachment may be localized, but without rapid treatment the entire retina may detach, leading to vision loss and blindness. It is a medical emergency.The retina is a...
, light sensitivity, tunnel vision
Tunnel vision
Tunnel vision is the loss of peripheral vision with retention of central vision, resulting in a constricted circular tunnel-like field of vision.- Medical / biological causes :Tunnel vision can be caused by:...
, and loss of peripheral vision
Peripheral vision
Peripheral vision is a part of vision that occurs outside the very center of gaze. There is a broad set of non-central points in the field of view that is included in the notion of peripheral vision...
to total loss of vision(4). Of the retinal degenerative diseases retinitis pigmentosa
Retinitis pigmentosa
Retinitis pigmentosa is a group of genetic eye conditions that leads to incurable blindness. In the progression of symptoms for RP, night blindness generally precedes tunnel vision by years or even decades. Many people with RP do not become legally blind until their 40s or 50s and retain some...
(RP) is a very important example.
Inherited retinal degenerative disorders in humans exhibit genetic
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
and phenotypic heterogeneity in their underlying causes and clinical outcomes* (5) (6) (7). These retinopathies
Retinopathy
Retinopathy is a general term that refers to some form of non-inflammatory damage to the retina of the eye. Frequently, retinopathy is an ocular manifestation of systemic disease.-Pathophysiology:Causes of retinopathy are varied:...
affect approximately one in 2000 individuals worldwide (7). A wide variety of causes have been attributed to retinal degeneration, such as disruption of genes that are involved in phototransduction, biosynthesis
Biosynthesis
Biosynthesis is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step...
and folding of the rhodopsin molecule, and the structural support of the retina.(6) Mutations in the rhodopsin gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
account for 25% to 30% (30% to 40% according to(9)) of all cases of autosomal dominant retinitis pigmentosa (adRP) (5) (20-22) in North America (10-12) There are many mechanisms of retinal degeneration attributed to rhodopsin mutations or mutations that involve or affect the function of rhodopsin. One mechanism of retinal degeneration is rhodopsin overexpression. Another mechanism, whereby a mutation caused a truncated rhodopsin, was found to affect rod function and increased the rate of photoreceptor degeneration.(9)
- *For example, a single peripherinPeripherinPeripherin is a protein that in humans is encoded by the PRPH gene.Peripherin is a type III Intermediate filament protein expressed mainly in neurons of the peripheral nervous system. It is also found in neurons of the central nervous system that have projections toward peripheral structures, such...
/RDS splice site mutation was identified as the cause of retinopathyRetinopathyRetinopathy is a general term that refers to some form of non-inflammatory damage to the retina of the eye. Frequently, retinopathy is an ocular manifestation of systemic disease.-Pathophysiology:Causes of retinopathy are varied:...
in eight families; the phenotype in these families ranged from retinitis pigmentosa to macular degenerationMacular degenerationAge-related macular degeneration is a medical condition which usually affects older adults and results in a loss of vision in the center of the visual field because of damage to the retina. It occurs in “dry” and “wet” forms. It is a major cause of blindness and visual impairment in older adults...
.(7)
Photoreceptor cell death
Visual perception
Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...
is not possible. Irreversible loss of these cells has been attributed as a cause of blindness
Blindness
Blindness is the condition of lacking visual perception due to physiological or neurological factors.Various scales have been developed to describe the extent of vision loss and define blindness...
in many retinal degenerative disorders, including RP. The exact mechanism of photoreceptor cell death is not clearly understood. (5)(14) Among potential causes is the endocytosis
Endocytosis
Endocytosis is a process by which cells absorb molecules by engulfing them. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma or cell membrane...
of stable complexes formed between rhodopsin and its regulatory protein arrestin
Arrestin
Arrestins are a small family of proteins important for regulating signal transduction.-Function:Arrestins were first discovered as a part of a conserved two-step mechanism for regulating the activity of G protein-coupled receptors in the visual rhodopsin system by Hermann Kühn and co-workers and...
in certain mutants. (5) Various studies have also documented that over-expression of rhodopsin itself (mutations in genes involved in the termination of rhodopsin signaling activity have been shown to cause degeneration by persistent activation of the phototransduction cascade (14)) causes photoreceptor cell death and may induce photoreceptor cell loss in transgenic animals expressing truncated rhodopsin. Yet another mechanism may be prolonged photoreceptor responses and also abnormal rhodopsin deactivation may induce outer segment shortening and eventual photoreceptor death (9)
In RP photoreceptor cell death is believed to occur by programmed cell death or apoptosis
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
. (10) (11) (15)
Retinitis Pigmentosa
Retinitis pigmentosa is a progressive neurodegenerative disorder(16), which affects 1 in 3,000 individuals (6) and affects between 50000 and 100000 people in the United StatesUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
.3 Autosomal dominant RP accounts for approximately 15% of these cases. (8) Autosomal dominant retinitis pigmentosa (ADRP) is a genetically heterogeneous group of inherited retinal degenerations that cause blindness in humans. (14)
RP begins with death of rod photoreceptor cells, which are the only cells in the retina to express rhodopsin and which express it as their most abundant protein. Eventually, loss of rod cells leads to loss of cone cells (cone photoreceptors), the mainstay of human vision (16). Symptoms of RP include loss of sensitivity to dim light, abnormal visual function, and characteristic bone spicule
Spicule
Spicules are tiny spike-like structures of diverse origin and function found in many organisms, such as the copulatory spicules of certain nematodes or the grains on the skin of some frogs.In sponges, spicules perform a structural function....
deposits of pigment in the retina. Affected individuals progressively lose visual field
Visual field
The term visual field is sometimes used as a synonym to field of view, though they do not designate the same thing. The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments", while 'field of view' "refers to the physical...
and visual acuity
Visual acuity
Visual acuity is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....
, and photoreceptor cell death can ultimately lead to blindness. (9) A prominent early clinical feature of retinitis pigmentosa is the loss of night vision as a result of death of rod photoreceptor cells. Proper expression of the wild-type rhodopsin gene is essential for the development and sustained function of photoreceptor cells.(10)
Mutations in the human rhodopsin that affect its folding
Folding
Fold or folding may refer to:* Paper folding, the art of folding paper* Book folding, in book production* Skin fold, an area of skin that folds* Fold , in the game of poker, to discard one's hand and forfeit interest in the current pot...
, trafficking and activity are the most commonly encountered causes of retinal degeneration in afflicted patients. A single base-substitution at the codon position 23 in the human opsin gene (P23H) is the most common cause of ADRP in American patients. (6)(17) ADRP due to rhodopsin mutations has a wide range of clinical presentation and severity. Before 1991, phenotypic evidence pointed to different subsets of ADRP with varying prognoses
Prognosis
Prognosis is a medical term to describe the likely outcome of an illness.When applied to large statistical populations, prognostic estimates can be very accurate: for example the statement "45% of patients with severe septic shock will die within 28 days" can be made with some confidence, because...
.(30-34) Molecular classification of ADRP and further sub-classification based on the region of the mutation in the rhodopsin gene allowed better prediction of a particular disease course. But even within these specific subsets, the prognosis is influenced by the specific mutation itself.(8)
Rhodopsin and its function in vision
Rhodopsin is a transmembrane proteinTransmembrane protein
A transmembrane protein is a protein that goes from one side of a membrane through to the other side of the membrane. Many TPs function as gateways or "loading docks" to deny or permit the transport of specific substances across the biological membrane, to get into the cell, or out of the cell as...
(Rh1) that is the primary visual pigment (photopigment
Photopigment
Photopigments are unstable pigments that undergo a chemical change when they absorb light. The term is generally applied to the non-protein chromophore moiety of photosensitive chromoproteins, such as the pigments involved in photosynthesis and photoreception...
) of rod photoreceptors (which are the only cells in the retina to express rhodopsin and which express it as their most abundant protein(16)) and forms an integral part of the visual cascade.(8)(13)(10) It is a G-protein-coupled receptor activated by light that initiates the phototransduction cascade (visual transduction cascade taking place in photoreceptor rod outer segments (13).) converting light signals to electrophysiological signals in retinal neurons. This photo-activated signal transduction process is essential for vision. (10)
The structure and function of rhodopsin and the gene encoding it have been the subjects of intense scrutiny for many years because rhodopsin serves as a useful model for understanding the largest receptor family in the human genome, the G protein-coupled receptors, and because defects in the rhodopsin gene are the most common cause of the most common inherited blinding disease, retinitis pigmentosa. (16) (18)(19)
Rhodopsin Mutation
The human rhodopsin gene is the locusLocus (genetics)
In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...
for numerous alleles linked to the neurodegenerative disease retinitis pigmentosa. (16) Mutations in the rhodopsin gene account for 25% to 30% (30% to 40% according to (9)) of all cases of autosomal dominant retinitis pigmentosa (ADRP).(8)(10)(20-22) Over 100 distinct mutations in the light-sensing molecule rhodopsin are known to cause (adRP). (6)(9)(10)(13)(14) Most of these mutations are missense mutations affecting single amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
residues in the rhodopsin protein. (10)(24) These mutations affect rhodopsin transport to the outer segments of rod photoreceptor cells, rhodopsin folding, and rhodopsin endocytosis. Mutations in the human rhodopsin that affect its folding, trafficking and activity are the most commonly encountered causes of retinal degeneration in patients afflicted with RP. (6) A single base-substitution mutation of codon 23 of the rhodopsin gene in which proline
Proline
Proline is an α-amino acid, one of the twenty DNA-encoded amino acids. Its codons are CCU, CCC, CCA, and CCG. It is not an essential amino acid, which means that the human body can synthesize it. It is unique among the 20 protein-forming amino acids in that the α-amino group is secondary...
is changed to histidine
Histidine
Histidine Histidine, an essential amino acid, has a positively charged imidazole functional group. It is one of the 22 proteinogenic amino acids. Its codons are CAU and CAC. Histidine was first isolated by German physician Albrecht Kossel in 1896. Histidine is an essential amino acid in humans...
(Pro23His) in the human opsin
Opsin
Opsins are a group of light-sensitive 35–55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical...
gene accounts for the largest fraction of rhodopsin mutations observed in the United States and is the most common cause of ADRP in American patients.(6)(8)(17)
In 1990, the Pro23His mutation of the rhodopsin gene was reported as the first mutation associated with RP.(8)(20)(21)(24-26) This mutation has been described only in the United States, where it continues to be the most commonly described gene defect in RP.
The phenotype of the RP associated with the P23H mutation is characteristically relatively mild but variable.(8)(20-22)(24-28)
Multiple studies have demonstrated that the degree of severity of a given mutation in the rhodopsin gene is based in large part on its position in the rhodopsin molecule—intradiscal, transmembrane, or cytoplasmic.(29-36) Intradiscal mutations tend to be less severe, whereas mutations affecting cytoplasmic domains and retinol binding sites tend to be very severe.(30)(36) The severity of cytoplasmic mutations affecting maintenance of photoreceptor cell polarity, such as those at the C-terminus (sorting signal of rhodopsin) may result from inappropriate intracellular transport of the molecule. (8)(13)(37)
Some research has described transgenic mouse mutants that cause degeneration by prolonged activation of the phototransduction cascade.(38) In this research null mutations in the rhodopsin kinase
Kinase
In chemistry and biochemistry, a kinase is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates, a process referred to as phosphorylation. Kinases are part of the larger family of phosphotransferases...
(39) and arrestin(40) genes, each of which plays a role in terminating rhodopsin activity, caused light-dependent retinal degeneration.(14)
Rhodopsin C-terminal mutations
A fraction of rhodopsin mutations alter the C-terminal tail of the protein, such as the point mutations P347L, P347S, P347R, and V345M.3–5 In addition, two frameshift mutationFrameshift mutation
A frameshift mutation is a genetic mutation caused by indels of a number of nucleotides that is not evenly divisible by three from a DNA sequence...
s (fs 341del and fs 341-343del) are predicted to add additional residues to the C terminus,(43) whereas Q344ter results in a C-terminal truncation
Truncation
In mathematics and computer science, truncation is the term for limiting the number of digits right of the decimal point, by discarding the least significant ones.For example, consider the real numbersThe result would be:- Truncation and floor function :...
, and an intron
Intron
An intron is any nucleotide sequence within a gene that is removed by RNA splicing to generate the final mature RNA product of a gene. The term intron refers to both the DNA sequence within a gene, and the corresponding sequence in RNA transcripts. Sequences that are joined together in the final...
splice mutation (N88) is thought to remove the entire C-terminal tail of rhodopsin.(9)(41)(42)
Data indicate that expression of truncated rhodopsin negatively affects both photoreceptor function and health, compromising rod cell survival.The presence of truncated opsin may impair synaptic transmission or other cellular processes and eventually cause cell death. Large quantities of mislocalized opsin may decrease the availability of functional proteins in regions where truncated opsin is concentrated. The presence of truncated rhodopsin in the outer segments causes functional abnormalities and localization of rhodopsin in the outer and/or inner segments induces increased photoreceptor cell death (9)
Rhodopsin Endocytosis
Like many other G protein-coupled receptors, the rhodopsin protein undergoes endocytosis following activation (6)(44)(45). Perturbation of endocytic regulation of rhodopsin has deleterious effects on photoreceptor cell physiology. In certain mutants, rhodopsin and its regulatory protein arrestin form stable complexes. As mentioned these complexes have a fatal effect; when taken up during endocytosis these complexes causes photoreceptor cell death, because the internalized rhodopsin is not degraded in the lysosome but instead accumulates in the late endosomes. The formation of toxic Rhodopsin- Arrestin complexes is also reported for mutants of human rhodopsin associated with severe forms of ADRP (6)(46)(47). For example, mutations at Arg135 are associated with severe forms of retinitis pigmentosa and exhibit a high affinity for arrestin, undergo endocytosis, and display endosomal abnormalities.(6)Missense mutations in the opsin gene affecting the R135 and K296 residues of the protein product cause ADRP and result in accumulation of Rhodopsin-Arrestin complexes in the photoreceptor cell (6)(46)(47). The R135 mutant rhodopsin is noted to form stable complex with arrestin and undergo endocytosis resulting in aberrant endocytic vesicles in HEK cell culture system (47). Similarly, the K296E rhodopsin is observed to bind the visual arrestin with high affinity. This abnormal interaction is demonstrated to have pathological consequences in the retina. Besides this, the stable rhodopsin and arrestin complexes are shown to mislocalize and accumulate in the inner segments of rod photoreceptors of the mouse model of ADRP.(6)
Using mutants that are defective in late endosome to lysosome trafficking, it was shown that rhodopsin accumulates in endosomal compartments in these mutants and leads to light-dependent retinal degeneration. It was also shown that the internalized rhodopsin in dying photoreceptor cells were not degraded but instead showed characteristics of insoluble proteins. This data led to the implication of rhodopsin buildup in the late endosomal system as a novel trigger of death of photoreceptor neurons. Thus failure to degrade internalized rhodopsin in a timely manner triggers cell death of photoreceptor neurons, suggesting that lysosomal turnover of rhodopsin is vital in maintaining photoreceptor viability. (6)
The precise mechanisms regulating the pro-cell death signaling pathways and their interconnection with endocytosis is not well understood. It is speculated that a component innate to the endolysosomal system plays a crucial role in regulating the cell death signals emanating from the endosomes. This component senses the accumulation of rhodopsin and then engages the proper machinery to execute cell death in the retina. Failure of proper protein degradation and resultant subsequent accumulation of proteins, as in the case of rhodopsin-arrestin complexes, is a well-recognized cause of cell death in many neurodegenerative disorders, such as retinitis pigmentosa (6)(48).
Non-Rhodopsin Mutations
Mutations non-retina-specific ADRP genes that encode for proteins essential for pre-mRNA splicing may be a major cause of ADRP. Proteins required for the formation of stable U4/U6 snRNPs and for assembly of the U4/U6.U5 tri-snRNP have been identified, including HPRP3, PRPC8, and PRPF31PRPF31
PRP31 pre-mRNA processing factor 31 homolog , also known as PRPF31, is a protein which in humans is encoded by the PRPF31 gene.- Function :PRPF31 is the gene coding for the splicing factor hPRP31...
, proper function of these proteins is necessary for spliceosome
Spliceosome
A spliceosome is a complex of snRNA and protein subunits that removes introns from a transcribed pre-mRNA segment. This process is generally referred to as splicing.-Composition:...
performance. (10)
The rhodopsin transcript is a pre-mRNA splicing substrate affected by PRPF31 protein, meaning that rhodopsin (RHO) is among the target splicing substrate genes for PRPF31. Thus it can be understood that mutations in PRPF31 can cause alternative, potentially non-functional, forms of the rhodopsin protein. It was shown that expression of these mutant PRPF31 proteins significantly reduced rhodopsin expression in cultured retinal cells and induced apoptosis of retinal cells, establishing a link between mutations in proteins involved in pre-mRNA splicing and the expression of a critical retina-specific gene, rhodopsin. (10)
This shows that non-rhodopsin mutations may also be critical in presentation of retinal degenerative disorders. It suggests a mechanism for retinal degeneration caused by non-retinal-specific genes, such as PRPF31 mutations. (10)