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Blue-cone monochromacy

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Blue cone monochromacy (BCM) is an inherited eye disease that causes severe color blindness, poor visual acuity, nystagmus, hemeralopia, and photophobia due to the absence of functional red (L) and green (M) cone photoreceptor cells in the retina. BCM is a recessive X-linked disease and almost exclusively affects XY karyotypes.

Blue cone monochromacy
Other namesX-linked achromatopsia
SpecialtyOphthalmology
Symptomspoor ability or inability to distinguish colours, poor visual acuity, nystagmus, hemeralopia
Usual onsetcongenital
Differential diagnosisincomplete achromatopsia
Treatmentdark lenses
Frequency1 in 100,000

Cause

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Cone cells are one kind of photoreceptor cell in the retina that are responsible for the photopic visual system and mediate color vision. The cones are categorized according to their spectral sensitivity:

  • LWS (long wave sensitive) cones are most sensitive to red light.
  • MWS (middle wave sensitive) cones are most sensitive to green light.
  • SWS (short wave sensitive) cones are most sensitive to blue light.

MWS and LWS cones are most responsible for visual acuity as they are concentrated in the fovea centralis region of the retina, which constitutes the very center of the visual field. Blue cone monochromacy is a severe condition in which the cones sensitive to red or green light are missing or defective, and only S-cones sensitive to blue light and rods which are responsible for night (scotopic) vision are functional.[1][2]

Symptoms

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A variety of symptoms characterize BCM:[2][3]

BCM symptoms are usually stationary, but some studies show evidence of disease progression.[4]

Poor Color Discrimination

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The color vision of Blue cone monochromats is severely impaired. However, interaction of the blue cones and rod photoreceptors in mesopic vision (twilight) may enable some level of dichromacy.[5]

Genetics

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Heredity

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Because Blue cone monochromacy shares many symptoms with achromatopsia, it was historically treated as a subset of achromatopsia, called x-linked achromatopsia or atypical incomplete achromatopsia. Both of these names differentiated BCM specifically by how its inheritance pattern deviated from other forms of achromatopsia. While other forms (ACHM) follow autosomal inheritance, BCM is X-Linked. Once the molecular biological basis of BCM was understood, the more descriptive term Blue cone monochromacy became dominant in the literature.

Genes

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The gene cluster responsible for BCM comprises 3 genes and is located at position Xq28, at the end of the q arm of the X chromosome.[6] The genes in the cluster are summarized in the following table:

Type OMIM Gene Locus Purpose
Locus Control Region 300824 LCR[1] Xq28 Acts as a promoter of the expression of the two opsin genes thereafter,[1] and ensures that only one of the two opsins (LWS or MWS) is expressed exclusively in each cone.[7]
LWS opsin 300822 OPN1LW Xq28 Encodes the LWS (red) photopsin protein.
MWS opsin 300821 OPN1MW Xq28 Encodes the MWS (green) photopsin protein.

Originating from a recent duplication event, the two opsins are highly homologous (very similar), having only 19 dimorphic sites (amino acids that differ),[8] and are therefore 96% similar.[9] Furthermore, only 7 of these dimorphic sites lead to a functional difference between the genes, i.e. that tune the opsin's spectral sensitivity. In comparison, these opsin genes are only 40% homologous (similar) to OPN1SW (encoding the SWS photopsin and located on chromosome 7) and "RHO" (encoding rhodopsin, and located on chromosome 3).[9] OPN1SW and rhodopsin are unaffected in BCM.

Mutations

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Since BCM is caused by non-functional M- and L-cones, it can result from the intersection of protanopia (no functional L-cones) and deuteranopia (no functional M-cones). Therefore the genetic causes of BCM include the genetic causes of protanopia and deuteranopia. These include (affecting either opsin gene):[9]

Data from the BCM International Patient Registry[12] shows that about 35% of Blue cone monochromacy stems from this 2-step process, where both genes are each affected by one of the above mutations.[9] The remaining 55% of Blue cone monochromats are caused by a deletion of the LCR.[9] In the absence of LCR, neither of the following two opsin genes are expressed.

Another disease of the retina that is associated with the position Xq28 is Bornholm Eye Disease (BED).[7] The point mutation W177R is a missense mutation that causes cone dystrophy when present on both opsin genes.[3]

Diagnosis

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Children 2 months and older can be identified as possible Blue cone monochromats from observing an aversion to light and/or nystagmus,[13] but are not sufficient for diagnosis, and especially not the differential diagnosis with achromatopsia. The differential diagnosis can be achieved in a few ways:

  • through reconstructing the family history to establish a x-linked mode of heredity[14][2][4]
  • with an electroretinogram (ERG), which measures the electrical response of photoreceptors to a visual stimulus of known wavelength. This can demonstrate the loss of function of the LWS and MWS cones.[15]
  • with a color vision test, either general in nature like the Farnsworth D-15[4] or Farnsworth Munsell 100 Hue test[15] or the Berson test, which is specifically designed to differentiate BCM from typical achromatopsia.[16]

Treatment

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Corrective visual aides and personalized vision therapy provided by Low Vision Specialists may help patients correct glare and optimize their remaining visual acuity. Tinted lenses for photophobia allow for greater visual comfort. A magenta (mixture of red and blue) tint allows for best visual acuity since it protects the rods from saturation while allowing the blue cones to be maximally stimulated.

Gene therapy

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Main article: Gene therapy for color blindness

There is no cure for Blue cone monochromacy. However, there are prospective gene therapy treatments which are currently being evaluated for safety and efficacy. Gene therapy is a general treatment for genetic disorders; it uses viral vectors to carry typical genes into cells (e.g. cone cells) that are not able to express functional genes (e.g. photopsins). It may be possible to restore color vision by adding missing opsin genes – or a functional copy of the entire gene complex – into the cone cells. In 2015, a team at the University of Pennsylvania evaluated possible outcome measures of BCM gene therapy.[17] Since 2011, several studies have performed gene therapy for BCM on mouse and rat models.[18]

Epidemiology

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BCM affects approximately 1/100,000 individuals.[14] The disease affects males much more than females due to its recessive X-linked nature, while females usually remain unaffected carriers of the BCM trait.[6]

History

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Prior to the 1960s, Blue cone monochromacy was treated as a subset of achromatopsia. The first detailed description of achromatopsia was given in 1777, where the subject of the description:

...could never do more than guess the name of any color; yet he could distinguish white from black, or black from any light or bright color...He had 2 brothers in the same circumstances as to sight; and 2 brothers and sisters who, as well as his parents, had nothing of this defect.

— J. Huddart, "Article Title", An account of persons who could not distinguish colours (1777)[19]

In 1942, Sloan first distinguished typical and atypical achromatopsia, differentiated mainly on the inheritance patterns.[20] In 1953, Weale theorized that the atypical achromatopsia must stem from cone-monochromatism, but estimated a prevalence of only 1 in 100 million.[21] In the early 1960s, the inheritance of atypical achromatopsia led to a name change to x-linked achromatopsia, and at the same time, several studies demonstrated that Blue cone monochromats retain some Blue yellow color vision.[22][23] A significant discovery was announced in 1989 (and 1993) by Nathans et al.[1][2] who identified the genes which cause Blue cone monochromacy.

References

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  1. ^ a b c d Nathans, J; Davenport, C M; Maumenee, I H; Lewis, R A; Hejtmancik, J F; Litt, M; Lovrien, E; Weleber, R; Bachynski, B; Zwas, F; Klingaman, R; Fishman, G (1989). "Molecular genetics of human blue cone monochromacy". Science. 245 (4920): 831–838. Bibcode:1989Sci...245..831N. doi:10.1126/science.2788922. PMID 2788922. S2CID 13093786.
  2. ^ a b c d Nathans, J; Maumenee, I H; Zrenner, E; Sadowski, B; Sharpe, L T; Lewis, R A; Hansen, E; Rosenberg, T; Schwartz, M; Heckenlively, J R; Trabulsi, E; Klingaman, R; Bech-Hansen, N T; LaRoche, G R; Pagon, R A; Murphey, W H; Weleber, R G (1993). "Genetic heterogeneity among blue cone monochromats". Am. J. Hum. Genet. 53 (5): 987–1000. PMC 1682301. PMID 8213841.
  3. ^ a b Gardner, J C; Webb, T R; Kanuga, N; Robson, A G; Holder, G E; Stockman, A; Ripamonti, C; Ebenezer, N D; Ogun, O; Devery, S; Wright, G A; Maher, E R; Cheetham, M E; Moore, A T; Michaelides, M; Hardcastle, A J (2010). "X-Linked Cone Dystrophy Caused by Mutation of the Red and Green Cone Opsins". Am. J. Hum. Genet. 87 (1): 26–39. doi:10.1016/j.ajhg.2010.05.019. PMC 2896775. PMID 20579627.
  4. ^ a b c Michaelides, M; Johnson, S; Simunovic, M P; Bradshaw, K; Holder, G; Mollon, J D; Moore, A T; Hunt, D M (2005). "Blue cone monochromatism: a phenotype and genotype assessment with evidence of progressive loss of cone function in older individuals". Eye (Lond). 19 (1): 2–10. doi:10.1038/sj.eye.6701391. PMID 15094734.
  5. ^ Reitner, A; Sharpe, L T; Zrenner, E (1991). "Is colour vision possible with only rods and Blue sensitive cones?". Nature. 352 (6338): 798–800. Bibcode:1991Natur.352..798R. doi:10.1038/352798a0. PMID 1881435. S2CID 4328439.
  6. ^ a b Alpern M, Lee GB, Maaseidvaag F, Miller SS (January 1971). "Colour vision in blue cone 'monochromacy'". J. Physiol. 212 (1): 211–33. doi:10.1113/jphysiol.1971.sp009318. PMC 1395698. PMID 5313219.
  7. ^ a b c Neitz, J; Neitz, M (2011). "The genetics of normal and defective color vision". Vision Res. 51 (7): 633–651. doi:10.1016/j.visres.2010.12.002. PMC 3075382. PMID 21167193.
  8. ^ Neitz, Maureen (1 May 2000). "Molecular Genetics of Color Vision and Color Vision Defects". Archives of Ophthalmology. 118 (5): 691–700. doi:10.1001/archopht.118.5.691. PMID 10815162.
  9. ^ a b c d e f g h i Gardner, Jessica C.; Michaelides, Michel; Holder, Graham E.; Kanuga, Naheed; Webb, Tom R.; Mollon, John D.; Moore, Anthony T.; Hardcastle, Alison J. (1 May 2009). "Blue cone monochromacy: Causative mutations and associated phenotypes". Molecular Vision. 15: 876–884. ISSN 1090-0535. PMC 2676201. PMID 19421413.
  10. ^ Winderickx J, Sanocki E, Lindsey DT, Teller DY, Motulsky AG, Deeb SS (July 1992). "Defective colour vision associated with a missense mutation in the human green visual pigment gene". Nat. Genet. 1 (4): 251–6. doi:10.1038/ng0792-251. PMID 1302020. S2CID 23127406.
  11. ^ Ladekjaer-Mikkelsen, A S; Rosenberg, T; Jørgensen, A L (1996). "A new mechanism in blue cone monochromatism". Hum. Genet. 98 (4): 403–408. doi:10.1007/s004390050229. PMID 8792812. S2CID 11799731.
  12. ^ "Patient Registry – Blue Cone Monochromacy".
  13. ^ Alpern, M; Falls, H F; Lee, G B (1960). "The enigma of typical total monochromacy". Am. J. Ophthalmol. 50 (5): 996–1012. doi:10.1016/0002-9394(60)90353-6. PMID 13682677.
  14. ^ a b Kohl, S; Hamel, C P (2011). "Clinical utility gene card for: blue cone monochromatism". Eur. J. Hum. Genet. 19 (6): 732. doi:10.1038/ejhg.2010.232. PMC 3110038. PMID 21267011.
  15. ^ a b Ayyagari, R; Kakuk, L E; Bingham, E L; Szczesny, J J; Kemp, J; Toda, Y; Felius, J; Sieving, P A (2000). "Spectrum of color gene deletions and phenotype in patients with blue cone monochromacy" (PDF). Hum. Genet. 107 (1): 75–82. doi:10.1007/s004390000338. hdl:2027.42/42266. PMID 10982039. S2CID 8527902.
  16. ^ Berson EL, Sandberg MA, Rosner B, Sullivan PL (June 1983). "Color plates to help identify patients with blue cone monochromatism". Am. J. Ophthalmol. 95 (6): 741–7. doi:10.1016/0002-9394(83)90058-2. PMID 6602551.
  17. ^ Luo, X; Cideciyan, AV; Iannaccone, A; Roman, A J; Ditta, L C; Jennings, B J; Yatsenko, S; Sheplock, R; Sumaroka, A; Swider, M; Schwartz, S B; Wissinger, B; Kohl, S; Jacobson, S G (2015). "Blue cone monochromacy: visual function and efficacy outcome measures for clinical trials". PLOS ONE. 10 (4): e0125700. Bibcode:2015PLoSO..1025700L. doi:10.1371/journal.pone.0125700. PMC 4409040. PMID 25909963.
  18. ^ Zhang, Y; Deng, WT; Du, W; Zhu, P; Li, J; Xu, F; Sun, J; Gerstner, C D; Baehr, W; Boye Sanford, L; Zhao, C; Hauswirth, W W; Pang, J (2017). "Gene-based Therapy in a Mouse Model of Blue Cone Monochromacy". Scientific Reports. 7 (6690): 6690. Bibcode:2017NatSR...7.6690Z. doi:10.1038/s41598-017-06982-7. PMC 5532293. PMID 28751656.
  19. ^ Huddart, J (1777). "An account of persons who could not distinguish colours". Philos. Trans. R. Soc. 67: 260. doi:10.1098/rstl.1777.0015. S2CID 186212155.
  20. ^ Sloan, LL; Newhall, SM (1942). "Comparison of cases of atypical and typical achromatopsia". American Journal of Ophthalmology. 25 (8): 945–961. doi:10.1016/S0002-9394(42)90594-4.
  21. ^ Weale, RA (1953). "Cone Monochromatism". The Journal of Physiology. 121 (3): 548–569. doi:10.1113/jphysiol.1953.sp004964. PMC 1366097. PMID 13097391.
  22. ^ Blackwell, H R; Blackwell, O M (1961). "Rod and cone receptor mechanisms in typical and atypical congenital achromatopsia". Vision Res. 1 (1–2): 62–107. doi:10.1016/0042-6989(61)90022-0.
  23. ^ Spivey, B E (1965). "The X-linked recessive inheritance of atypical monochromatism". Arch. Ophthalmol. 74 (3): 327–333. doi:10.1001/archopht.1965.00970040329007. PMID 14338644.
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