Albinism classification
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief:
Overview
Classification
Type
About 1 in 17,000 human beings has some type of albinism, although up to 1 in 70 is a carrier of albinism genes.[1]
There are two main categories of albinism in humans:
- In oculocutaneous albinism (despite its Latin-derived name meaning "eye-and-skin" albinism), pigment is lacking in the eyes, skin and hair. (The equivalent mutation in non-humans also results in lack of melanin in the fur, scales or feathers.)
- In ocular albinism, only the eyes lack pigment. People with oculocutaneous albinism can have anywhere from no pigment at all to almost-normal levels. People who have ocular albinism have generally normal skin and hair color, and many even have a normal eye appearance.
While there is only one major type of ocular albinism, there are several varieties of oculocutaneous albinism (and disorders which produce the same or similar results), some of which have subtypes. Some are easily distinguished by appearance, but in most cases, genetic testing is the only way to be certain.
Oculocutaneous albinism family
- Oculocutaneous albinism type 1 (OCA1) (Online Mendelian Inheritance in Man (OMIM) 203100) is the type with (usually) the least amount of pigment. People with this type generally have very pale skin, white to yellow hair (depending on subtype, see below) and light blue eyes; however there are cases in which the irises can appear pink or violet, depending on the amount of blue (non-melanin)[citation needed] pigment present in the irises and the level and direction of light available to the observer. OCA1 is caused by an alteration of the tyrosinase gene, and can occur in two variations. The first is OCA1a, and means that the organism cannot develop pigment at all. The hair is usually white (often translucent) and the skin very pale. Vision usually ranges from 20/200 to 20/400. The second is OCA1b, which has several subtypes itself. Some individuals with OCA1b can tan and also develop pigment in the hair.[2]One subtype of OCA1b is called OCA1b TS (temperature sensitive), where the tyrosinase can only function below a certain temperature, which causes the body hair in cooler body regions to develop pigment (i.e. get darker). (An equivalent mutation produces the coat pattern in Siamese cats[citation needed].) Another variant of OCA1b, called Albinism, yellow mutant type (OMIM: 606952) is more common among the Amish than in other populations, and results in blonde hair and the eventual development of skin pigmentation during infancy, though at birth is difficult to distinguish from other types.[3] About 1 in 40,000 people have some form of OCA1.
- Oculocutaneous albinism type 2 (OCA2) (OMIM: 203200), the most common type of albinism, is caused by mutation of the P gene. People with OCA2 generally have more pigment and better vision than those with OCA1, but cannot tan like some with OCA1b. A little pigment can develop in freckles or moles.[2] People with OCA2 usually have fair skin but not as pale as OCA1, and pale blonde to golden or reddish-blonde hair, and most commonly blue eyes. Affected people of African descent usually have a different phenotype (appearance): yellow hair, pale skin, and blue, gray or hazel eyes. About 1 in 15,000 people have OCA2.
- Oculocutaneous albinism type 3 (OCA3, or rufous albinism) (OMIM: 203290) has only been partially researched and documented. It is caused by mutation of the tyrosinase-related protein-1 (Tyrp1) gene. Cases have been reported in Africa and New Guinea. Affected individuals typically have red hair, reddish-brown skin and blue or gray eyes. Variants may be the recently-identified minimal pigment type albinism (OMIM: 203280) and rufous oculocutaneous albinism (ROCA or xanthism) (OMIM: 278400). The incidence rate of OCA3 is unknown.
- Oculocutaneous albinism type 4 (OCA4) (OMIM: 606574) is very rare outside of Japan, where OCA4 accounts for 24% of albinism cases. OCA4 can only be distinguished from OCA2 through genetic testing, and is caused by mutation of the membrane-associated transporter protein (MATP) gene.
- Hermansky-Pudlak syndrome (HPS) (OMIM: 203300) is not a type of OCA, technically, but has similar features. HPS has a great range of degrees of pigmentation, from OCA1a-like to almost-normal coloring. Vision usually ranges from 20/60 to 20/200. Apart from the hypopigmentation and impaired vision, people with HPS lack dense bodies in their blood platelets which are responsible for releasing clotting factors. For this reason, HPS patients bruise easily and have a hard time stopping bleeding once it begins (bleeding diathesis, similarly to hemophilia). HPS has seven known forms (HPS-1 through HPS-7), each caused by a different autosomal recessive gene mutation. HPS-1 and HPS-4 may also include pulmonary fibrosis, or scarring of lung tissue that prevents the necessary expansion and contraction during breathing. It is believed that this is due to a buildup of fatty ceroid in the lungs. Colitis, or inflammation in the large intestine, is another symptom of most types of HPS, which may cause diarrhea, nausea, and blood in the stool. HPS is rare generally, but affects 1 in 1800 Puerto Ricans, and is typically fatal by middle age.
- Chediak-Higashi syndrome (CHS) (OMIM: 214500), like HPS, is not technically a form of OCA, but produces similar results. CHS, caused by mutation of the LYST gene, is very rare, and is associated with other medical problems, such as immune system dysfunction that leads to a high infant mortality rate, HPS-like hemophilia, and neurological problems, among many others, in 85% of sufferers.
- Griscelli syndrome (GS) is similar to CHS in symptoms (and also very rare). It is divided into three types, GS1 (OMIM: 214450), GS2 (OMIM: 607624) and GS3 (OMIM: 609227). Each type is due to a different autosomal recessive gene mutation. Type 1 produces mainly neurological problems in addition to albinism, while type 2 produces mainly immunological issues as well as the hypopigmentation, and type 3 only evidences hypomelanosis without either of the other sorts of problems. People affected by GS differ in appearance from those with OCA, having silvery-grey hair. A fourth and even rarer variant, partial albinism and immunodeficiency syndrome (PAID) (OMIM: 604228), has been identified and requires further study. An additional type called Elejalde syndrome (OMIM: 256710) may exist, but some researchers believe it is actually simply GS1. GS2, because of its immune system effects, results in a very high mortality rate among children and young adults that have it.
Ocular albinism family
- Ocular albinism, type 1 (OA1) (OMIM: 300500), also known as Nettleship-Falls syndrome, is the most common variety of ocular albinism, which affects the eyes but generally not the skin or hair. OA1 is usually associated with nystagmus, and difficult to otherwise detect in females; males show more readily observable symptoms. There are several other identified types of OA, though researchers are not all agreed on the distinctions and classification. Most are caused by a mutation in a gene on the X chromosome, and are X-linked recessive traits.
- Ocular albinism, type 2 (OA2) (OMIM: 300600), also known as Forsius-Eriksson syndrome or "Åland Island eye disease", mostly only affects males though females are often carriers and can sometimes be symptomatic; it is frequently linked with protanopic dichromacy (a form of color blindness) and with night blindness (nyctalopia).
- Ocular albinism, type 3 (OA3) (OMIM: 203310), also known as ocular albinism, autosomal recessive (OAR) is a non-X-linked variant, which may be more common among the Amish than in other populations.
- Ocular albinism with sensorineural deafness (OASD) (OMIM: 300650), and as its name implies is associated with loss of hearing.
- Waardenburg syndrome, type 2, with ocular albinism (WS2-OA) (OMIM: 103470) is a rare non-X-linked recessive gene variant.
The skin color of people affected by OA can be slightly lighter than those of the rest of their families. The eye color can vary greatly, and in some cases only examination of the retina or genetic testing can reveal OA for certain. Some form of OA afflicts 1 in 50,000 people, though certain isolated populations are at greater risk.
Other types
Other rare variants of albinism are theorized (by ongoing research as of 2007) to exist, such as
- Albinism-deafness syndrome (ADFN) (OMIM: 300700, which may actually be closer related to vitiligo); it is predominantly observed among Hopi Native Americans (with an incidence estimated at[citation needed] 1 in 200 individuals)
- Recessive total albinism with congenital deafness (OMIM: 220900)
- Albinism black-lock cell-migration disorder syndrome (ABCD)
References
- ↑ "Albinism" by Carol A. Turkington at answers.com
- ↑ 2.0 2.1 "Facts about Albinism", by Dr. Richard King et al.
- ↑ "Ocular Manifestations of Albinism", by Dr. Mohammed O. Peracha, at eMedicine, 13 September 2005; retrieved 31 March 2007