OMIM ID:
Retinitis Pigmentosa and Mental Retardation
Alternate Names
Defective Genes
Clinical Characteristics
Ocular Features
The lenses may have pleomorphic white axial opacities but in other patients can be totally opacified. Optic atrophy is present and vision may be reduced to light perception but nystagmus is absent. Evidence suggests that vision loss is progressive. Some patients have extensive posterior synechiae while others have been noted to have sluggish pupils. High myopia is a feature. The retinal pigmentation has a typical retinitis pigmentosa picture with attenuated retinal vessels and equatorial bone spicule pigmentation located in the midperiphery while the macula can have a bull’s eye appearance.
Systemic Features
Early development may seem normal but developmental milestones are usually delayed. Postnatal microcephaly and growth deficiency with mental retardation and early hypotonia are typical features. The mental retardation may be severe. Scoliosis and arachnodactyly have been noted and hypogonadism has been reported. Speech may not develop and mobility is sometimes limited.
Genetics
Inheritance
The family pattern suggests autosomal recessive inheritance. Homozygosity mapping has identified in a region of chromosome 8 (8q21.2-22.1) that overlaps the region for Cohen syndrome () but no specific mutated gene has been identified.
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.