Color Blindness

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Color Blindness

• By Jessie Wright

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Let there be light!!!
The normal human retina contains two kinds of
light sensitive cells the rod cells (active only
in low light) and the cone cells (active in
normal daylight and responsible for color
perception).

• Normally, there are three kinds of
• cones (each one sensitive to a
• specific range of wavelengths)
• "red" cones (64)
• "green" cones (32)
• "blue" cones (2)

The different kinds of inherited color
blindness result from partial or complete loss of
function of one or more of the different cone
systems.
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Different Types of Color Blindness

• Monochromacy occurs when two or all three of the
  cone pigments are missing and color and lightness
  vision is reduced to one dimension.
• Total color blindness
• Dichromacy occurs when only one of the cone
  pigments is missing and color is reduced to two
  dimensions.
• Partial color blindness
• red-green
• blue-yellow

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Total Color Blindness

• Also known as rod monochromacy, complete
  achromatopsia, and
• typical monochromacy.
• A rare, non-progressive inability to distinguish
  any colors as a
• result of absent or nonfunctioning retinal cones.
  
• See everything as white, black, or some shade of
  gray
• Typically caused by a missense mutation (a
  switched amino acid)
• in the CNGB3 gene.

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CYCLIC NUCLEOTIDE-GATED CHANNEL, BETA-3 (CNGB3)

• Classic achromatopsia results from a complete
  loss of CNGB3 function.
• CNGB3 encodes for the beta subunit of the cone
  cyclic nucleotide-gated cation channel, found
  photoreceptor plasma membranes.
• Upon activation by cGMP, it leads to an opening
  of cation channels which thereby cause a
  depolarization of rod photoreceptors. This gene
  is essential for the generation of light-evoked
  electrical responses in cones.
• CNGB3 is not required for vital processes
  outside the visual system.

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CNGB3 Cont.

• The human CNGB3 gene consists of 18 exons
• distributed over 200 kb of genomic sequence.
• Gene type protein coding
• Domain similar to ion transport protein

Cytogenetic locus chromosome 8 Location
8q21-q22
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Amino acid sequence

• Length 809 aa
• "MFKSLTKVNKVKPIGENNENEQSSRRNEEGSHPSNQSQQTTAQE
  ENKGEEKSLKTKSTPVTSEEPHTNIQDKLSKKNSSGD
  LTTNPDPQNAAEPTGTVPEQK
  EMDPGKEGPNSPQNKPPAAPVINEYADAQLHNLVKRMRQRTALYKKKLVE
  GDLSSPEA SPQTAKPTAVPPVKESDDKPTE
  HYYRLLWFKVKKMPLTEYLKRIKLPNSIDSYTDRLY
  LLWLLLVTLAYNWNCWFIPLRLVFPYQTADNIHYWLIADIICDI
  IYLYDMLFIQPRLQ
  FVRGGDIIVDSNELRKHYRTSTKFQLDVASIIPFDICYLFFGFNPMFRAN
  RMLKYTSF FEFNHHLESIMDKAYIYRVIRT
  TGYLLFILHINACVYYWASNYEGIGTTRWVYDGEGN
  EYLRCYYWAVRTLITIGGLPEPQTLFEIVFQLLNFFSGVFVFS
  SLIGQMRDVIGAATA
  NQNYFRACMDDTIAYMNNYSIPKLVQKRVRTWYEYTWDSQRMLDESDLLK
  TLPTTVQL ALAIDVNFSIISKVDLFKGCDT
  QMIYDMLLRLKSVLYLPGDFVCKKGEIGKEMYIIKH
  GEVQVLGGPDGTKVLVTLKAGSVFGEISLLAAGGGNRRTANVVA
  HGFANLLTLDKKTL
  QEILVHYPDSERILMKKARVLLKQKAKTAEATPPRKDLALLFPPKEETPK
  LFKTLLGG TGKASLARLLKLKREQAAQKKE
  NSEGGEEEGKENEDKQKENEDKQKENEDKGKENEDK
  DKGREPEEKPLDRPECTASPIAVEEEPHSVRRTVLPRGTSRQSL
  IISMAPSAEGGEEV
• LTIEVKEKAKQ"

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Mutation

• The genetic basis for achromatopsia is found on
  chromosome 8 (location 8q21-q22) where there is a
  recessive point mutation in CNGB3 that changes
  serine at residue 435 to phenylalanine in a
  highly conserved site in the S6 membrane-spanning
  domain.

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Bibliography

• Bookshelf
• Samir S Deeb, PhD, Arno G Motulsky, MD,
  Red-Green Color Vision Defects, GeneReviews,
  September 19, 2005.
• Berg, Jeremy M. 32.3.5. Rearrangements in the
  Genes for the Green and Red Pigments Lead to
  Color Blindness, Biochemistry 5th edition,
  W.H. Freeman and Company, 2002.
• OMIM
• COLORBLINDNESS, PARTIAL, DEUTAN SERIES CBD
• OMIM ID 303800
• PubMed
• Harrison, R. Hoefnagel, D. Hayward, J. N.
  Congenital total color blindness. Arch.
  Ophthal. 64 685-692, 1960. PubMed ID 13711836
• Botstein, D. The molecular biology of color
  vision. (Editorial) Science 232 142-143, 1986.
  PubMed ID 2937146
• Reyniers, E. Van Thienen, M.-N. Meire, F. De
  Boulle, K. Devries, K. Kestelijn, P. Willems,
  P. J. Gene conversion between red and defective
  green opsin gene in blue cone monochromacy.
  Genomics 29 323-328, 1995. PubMed ID 8666378
• Winderickx, J. Sanocki, E. Lindsey, D. T.
  Teller, D. Y. Motulsky, A. G. Deeb, S. S.
• Defective colour vision associated with a
  missense mutation in the human green visual
  pigment gene. Nature Genet. 1 251-256, 1992.
  PubMed ID 1302020