OMIM ID:
Colorblindness-Achromatopsia 4
Alternate Names
Defective Genes
Clinical Characteristics
Ocular Features
The ocular phenotype in ACHM4 is similar to that of other forms of achromatopsia. Nystagmus, poor visual acuity, photophobia, and defects in color vision are usually present. Some subjects, however, retain some color discrimination, a condition referred to as incomplete achromatopsia. The ERG documents the absence of cone function but normal rod responses. The retina appears normal clinically.
Few families have been reported and the complete phenotype remains undocumented. For example, it has been reported that visual acuity weakens with age in some patients although it is uncertain if this is true of all cases.
Systemic Features
No systemic abnormalities are associated.
Genetics
Inheritance
This is an autosomal recessive disorder caused by mutations in GNAT2 located at 1p13. These mutations account for less than 2% of achromatopsia cases. The majority are caused by mutations in CNGA3 (25%), responsible for ACHM2 (216900) and CNGB3 (50%), causing ACHM3 (262300). Mutations in PDE6C (613093 ) causing ACHM5 are responsible for less than 2%. No doubt others will be found as many cases do not have mutations in these genes.
Pedigree
Autosomal recessive
In order for autosomal recessive disorders to be expressed, offspring generally must inherit two mutations, one from each carrier parent. Carriers with only one mutation, such as the parents, do not have clinical disease. Note that carrier parents can expect that 1 in 4 children (25%) will inherit both mutations and have the disorder, 2 in 4 children (50%) will be carriers like their parents, while 1 in 4 children (25%) inherit neither mutation.
In order for autosomal recessive disorders to be expressed, offspring generally must inherit two mutations, one from each carrier parent. Carriers with only one mutation, such as the parents, do not have clinical disease. Note that carrier parents can expect that 1 in 4 children (25%) will inherit both mutations and have the disorder, 2 in 4 children (50%) will be carriers like their parents, while 1 in 4 children (25%) inherit neither mutation.