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Complete achromatopsia or rod monochromatism is a stationary cone dystrophy, with an incidence of ~1 in 30 000, in which functional cones are absent from the retina.1,2 Affected individuals usually present in infancy with nystagmus, poor visual acuity (6/60–6/36), photophobia, and complete colour blindness. Fundal examination is normal, but electroretinography reveals absent photopic (cone) responses and normal scotopic (rod) responses. Individuals with incomplete achromatopsia retain some colour vision.3
Achromatopsia is recessively inherited and genetically heterogeneous. To date, three achromatopsia genes have been characterised, the first two described being CNGA34–6 and CNGB3,7–9 located at chromosome positions 2q11 and 8q21 respectively. CNGA3 and CNGB3 respectively code for the α and β subunits of the cGMP gated cation channel in cone cells. Recently, mutations within a third gene, GNAT2, which encodes the α subunit of cone transducin, have also been shown to cause achromatopsia.10,11 All these genes encode crucial components of cone specific phototransduction.
Studies of patients with achromatopsia have revealed more than 40 disease causing mutations in CNGA3, the majority being missense mutations.5,6,12 In the patient groups studied, four CNGA3 mutations (Arg277Cys, Arg283Trp, Arg436Trp, and Phe547Leu) accounted for 40% of all mutant CNGA3 alleles. In contrast, only eight different mutations of CNGB3 have been identified to date, with six of these being nonsense mutations.8,9 The most common mutation is a 1 bp frameshift deletion, 1148delC, which accounts for 84% (199/237) of CNGB3 mutations.14
Current estimates suggest that mutations in CNGB3 account for 40–50% of achromatopsia,14 with mutations in CNGA3 contributing a further 20%.5 There is therefore a significant proportion of patients for whom neither CNGA3 nor CNGB3 mutations can be found (~30%). GNAT2 is the third gene to be implicated in achromatopsia, …
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