Color Blindness
By Jessie Wright
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.
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
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.
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.
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
Amino acid sequence
• Length: 809 aa
"MFKSLTKVNKVKPIGENNENEQSSRRNEEGSHPSNQSQQTTAQE
ENKGEEKSLKTKSTPVTSEEPHTNIQDKLSKKNSSGDLTTNPDPQNAAEPTGTVPEQK
EMDPGKEGPNSPQNKPPAAPVINEYADAQLHNLVKRMRQRTALYKKKLVEGDLSSPEA
SPQTAKPTAVPPVKESDDKPTEHYYRLLWFKVKKMPLTEYLKRIKLPNSIDSYTDRLY
LLWLLLVTLAYNWNCWFIPLRLVFPYQTADNIHYWLIADIICDIIYLYDMLFIQPRLQ
FVRGGDIIVDSNELRKHYRTSTKFQLDVASIIPFDICYLFFGFNPMFRANRMLKYTSF
FEFNHHLESIMDKAYIYRVIRTTGYLLFILHINACVYYWASNYEGIGTTRWVYDGEGN
EYLRCYYWAVRTLITIGGLPEPQTLFEIVFQLLNFFSGVFVFSSLIGQMRDVIGAATA
NQNYFRACMDDTIAYMNNYSIPKLVQKRVRTWYEYTWDSQRMLDESDLLKTLPTTVQL
ALAIDVNFSIISKVDLFKGCDTQMIYDMLLRLKSVLYLPGDFVCKKGEIGKEMYIIKH
GEVQVLGGPDGTKVLVTLKAGSVFGEISLLAAGGGNRRTANVVAHGFANLLTLDKKTL
QEILVHYPDSERILMKKARVLLKQKAKTAEATPPRKDLALLFPPKEETPKLFKTLLGG
TGKASLARLLKLKREQAAQKKENSEGGEEEGKENEDKQKENEDKQKENEDKGKENEDK
DKGREPEEKPLDRPECTASPIAVEEEPHSVRRTVLPRGTSRQSLIISMAPSAEGGEEV
LTIEVKEKAKQ"
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 membranespanning domain.
Bibliography
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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.
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OMIM:
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PubMed:
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“COLORBLINDNESS, PARTIAL, DEUTAN SERIES; CBD”
OMIM ID: 303800
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