OMIM ID:
Cataracts 13, Congenital, in Adult i RBC Phenotype
Alternate Names
Defective Genes
Clinical Characteristics
Ocular Features
The only ocular signs are dense nuclear cataracts with vision in the range of count fingers or light perception. The opacities are usuallyrecognized at birth. Some patients develop nystagmus.
Systemic Features
The Ii blood group is associated with cataracts, most strongly in Asians. The RBC antigens are developmentally regulated. Fetal and neonatal RBCs express the i form of the antigen which is gradually converted enzymatically to the I form that normally persists in adults. The normal Ii adult phenotype is reached before the age of two years.
Genetics
Inheritance
Homozygous or compound heterozygous deletions of the GCNT2 gene (6p24.3) produce the adult i phenotype (110800) which may or may not be associated with lens opacities.
The GCNT2 gene codes for 3 variants of its product. Mutations that eliminate all 3 GCNT2 variants result in the adult i phenotype in association with congenital cataracts.
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.