OMIM ID:
Albinism, Oculocutaneous, Type II
Alternate Names
Defective Genes
Clinical Characteristics
Ocular Features
The iris and retina lack normal pigmentation and translucency of the iris can be demonstrated. Anomalous decussation of neuronal axons in the chiasm and foveal hypoplasia result in decreased visual acuity. Vision loss into the range of 20/100-20/200 does not progress after early childhood but is sometimes as good as 20/30. Nystagmus is often present from about 3-4 months of age although it is less severe than in type I oculocutaneous albinism (203100, 606952). The iris may darken to some extent with age. Strabismus has been reported. Significant refractive errors are often present and stereopsis is reduced. The VEP responses are altered and can be used to document abnormal chiasmal decussation.
Systemic Features
Melanin pigment is reduced in the skin and hair as well as the eyes. Individuals at birth may be misdiagnosed as OCA type I but it is common for pigmentation to increase in older individuals resulting in yellow or reddish-blond hair and the appearance of freckles and nevi. The skin may be creamy-white but this is often not as striking as in OCAI. It is possible for tanning to take place in some patients. This condition in Africans or African Americans is sometimes called brown oculocutaneous albinism (BOCA). There is an increased risk of skin cancer of all types.
Genetics
Inheritance
Type II is the most common type of oculocutaneous albinism and is especially prevalent among individuals of African heritage and in several Native American populations. It is an autosomal recessive condition caused by homozygous 2.7 kb deletions in the OCA2 gene (15q24.3-q12). Heterozygotes have normal pigmentation.
Oculocutaneous albinism type I (203100, 606952) is a separate disorder with many similar features caused by mutations in the TYR gene. Other types of autosomal recessive albinism are OCA3 (203290 ), and OCA4 (606574).
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.