Albinism in humans (from the Latin albus, "white"; see extended etymology, also called achromia, achromasia, or achromatosis) is a congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes due to absence or defect of tyrosinase, a copper-containing enzyme involved in the production of melanin. It is the opposite of melanism. Unlike humans, other animals have multiple pigments and for these, albinism is considered to be a hereditary condition characterised by the absence of pigment in the eyes, skin, hair, scales, feathers or cuticle.
Albinism results from inheritance of recessive gene alleles and is known to affect all vertebrates, including humans. While an organism with complete absence of melanin is called an albino (UK /æpan>lˈbiËnoÊŠ/, or US /ælˈbaɪnoÊŠ/) an organism with only a diminished amount of melanin is described as leucistic or albinoid.
Albinism is associated with a number of vision defects, such as photophobia, nystagmus and amblyopia. Lack of skin pigmentation makes for more susceptibility to sunburn and skin cancers. In rare cases such as Chédiakâ€"Higashi syndrome, albinism may be associated with deficiencies in the transportation of melanin granules. This also affects essential granules present in immune cells leading to increased susceptibility to infection.
Signs and symptoms
In humans, there are two principal types of albinism: oculocutaneous, affecting the eyes, skin and hair, and ocular affecting the eyes only.
Most people with oculocutaneous albinism appear white or very pale, as the melanin pigments responsible for brown, black, and some yellow colorations are not present. Ocular albinism results in light blue eyes, and may require genetic testing to diagnose.
Because individuals with albinism have skin that entirely lacks the dark pigment melanin, which helps protect the skin from the sun's ultraviolet radiation, their skin can burn more easily from overexposure.
The human eye normally produces enough pigment to color the iris blue, green or brown and lend opacity to the eye. In photographs, those with albinism are more likely to demonstrate "red eye," due to the red of retina being visible through the iris. Lack of pigment in the eyes also results in problems with vision, both related and unrelated to photosensitivity.
Those afflicted with albinism are generally as healthy as the rest of the population (but see related disorders below), with growth and development occurring as normal, and albinism by itself does not cause mortality, although the lack of pigment blocking ultraviolet radiation increases the risk of melanomas (skin cancers) and other problems.
Visual problems
Development of the optical system is highly dependent on the presence of melanin, and the reduction or absence of this pigment in sufferers of albinism may lead to:
- Misrouting of the retinogeniculate projections, resulting in abnormal decussation (crossing) of optic nerve fibres
- Photophobia and decreased visual acuity due to light scattering within the eye (ocular straylight)
- Reduced visual acuity due to foveal hypoplasia and possibly light-induced retinal damage.
Eye conditions common in albinism include:
- Nystagmus, irregular rapid movement of the eyes back and forth, or in circular motion.
- Amblyopia, decrease in acuity of one or both eyes due to poor transmission to the brain, often due to other conditions such as strabismus.
- Optic nerve hypoplasia, underdevelopment of the optic nerve.
Some of the visual problems associated with albinism arise from a poorly developed retinal pigment epithelium (RPE) due to the lack of melanin. This degenerate RPE causes foveal hypoplasia (a failure in the development of normal foveae), which results in eccentric fixation and lower visual acuity, and often a minor level of strabismus.
The iris is a sphincter formed from pigmented tissue that contracts when the eye is exposed to bright light, to protect the retina by limiting the amount of light passing through the pupil. In low light conditions, the iris relaxes to allow more light to enter the eye. In albinistic subjects, the iris does not have enough pigment to block the light, thus the decrease in pupil diameter is only partially successful in reducing the amount of light entering the eye. Additionally, the improper development of the RPE, which in normal eyes absorbs most of the reflected sunlight, further increases glare due to light scattering within the eye. The resulting sensitivity (photophobia) generally leads to discomfort in bright light, but this can be reduced by the use of sunglasses and/or brimmed hats.
Genetics
Oculocutaneous albinism is generally the result of the biological inheritance of genetically recessive alleles (genes) passed from both parents of an individual for example OCA1 and OCA2. A mutation in the human TRP-1 gene may result in the deregulation of melanocyte tyrosinase enzymes, a change that is hypothesized to promote brown versus black melanin synthesis, resulting in a third oculocutaneous albinism (OCA) genotype, ″OCA3.″ Some rare forms are inherited from only one parent. There are other genetic mutations which are proven to be associated with albinism. All alterations, however, lead to changes in melanin production in the body. Some of these are associated with increased risk of skin cancer (see list of such genetic variations).
The chance of offspring with albinism resulting from the pairing of an organism with albinism and one without albinism is low. However, because organisms (including humans) can be carriers of genes for albinism without exhibiting any traits, albinistic offspring can be produced by two non-albinistic parents. Albinism usually occurs with equal frequency in both sexes. An exception to this is ocular albinism, which it is passed on to offspring through X-linked inheritance. Thus, ocular albinism occurs more frequently in males as they have a single X and Y chromosome, unlike females, whose genetics are characterized by two XÂ chromosomes.
There are two different forms of albinism: a partial lack of the melanin is known as hypomelanism, or hypomelanosis, and the total absence of melanin is known as amelanism or amelanosis.
Enzyme
The enzyme defect responsible for albinism is tyrosine 3-monooxegenase (tyrosinase), which synthesizes melanin from the amino acid tyrosine.
Diagnosis
Genetic testing can confirm albinism and what variety it is, but offers no medical benefits except in the cases of non-OCA disorders (see below) that cause albinism along with other medical problems which may be treatable. There is no 'cure' for Albinism. The symptoms of albinism can be assisted by various methods detailed below.
Treatment
For the most part, treatment of the eye conditions consists of visual rehabilitation. Surgery is possible on the extra-ocular muscles to decrease strabismus. Nystagmus-damping surgery can also be performed, to reduce the "shaking" of the eyes back and forth. The effectiveness of all these procedures varies greatly and depends on individual circumstances.
Glasses and other vision aids, large-print materials as well as bright but angled reading lights, can help individuals with albinism, even though their vision cannot be corrected completely. Some people with albinism do well using bifocals (with a strong reading lens), prescription reading glasses, and/or hand-held devices such as magnifiers or monoculars. Contact lenses may be colored to block light transmission through the iris. But in the case of nystagmus this is not possible, due to the irritation that is caused by the movement of the eyes. Some use bioptics, glasses which have small telescopes mounted on, in, or behind their regular lenses, so that they can look through either the regular lens or the telescope. Newer designs of bioptics use smaller light-weight lenses. Some US states allow the use of bioptic telescopes for driving motor vehicles. (See also NOAH bulletin "Low Vision Aids".)
It is important to remember there is no cure for Albinism which is not a sickness or disease. People with Albinism need to take care not to sun-burn and should have regular healthy skin checks by a dermatologist.
To support those with albinism, and their families, National Organization for Albinism and Hypopigmentation can provide a network of resources and information.
Epidemiology
Albinism affects people of all ethnic backgrounds; its frequency worldwide is estimated to be approximately one in 17,000. Prevalence of the different forms of albinism varies considerably by population, and is highest overall in people of sub-Saharan African descent.
Society and culture
In physical terms, humans with albinism commonly have visual problems and need sun protection. They often face social and cultural challenges (even threats), as the condition is often a source of ridicule, discrimination, or even fear and violence. Many cultures around the world have developed beliefs regarding people with albinism.
In African countries such as Tanzania and Burundi, there has been an unprecedented rise in witchcraft-related killings of people with albinism in recent years, because their body parts are used in potions sold by witchdoctors. Numerous authenticated incidents have occurred in Africa during the 21st Century. For example, in Tanzania, in September 2009, three men were convicted of killing a 14-year-old albino boy and severing his legs in order to sell them for witchcraft purposes. Again in Tanzania and Burundi in 2010, the murder and dismemberment of a kidnapped albino child was reported from the courts, as part of a continuing problem. National Geographic estimates that in Tanzania a complete set of albino body parts is worth $75,000.
Another harmful and false belief is that sex with an albinistic woman will cure a man of HIV. This has led, for example in Zimbabwe, to rapes (and subsequent HIV infection).
Certain ethnic groups and populations in isolated areas exhibit heightened susceptibility to albinism, presumably due to genetic factors. These include notably the Native American Kuna, Zuni and Hopi nations (respectively of Panama, New Mexico and Arizona); Japan, in which one particular form of albinism is unusually common; and Ukerewe Island, the population of which shows a very high incidence of albinism.
Famous people with albinism include historical figures such as Oxford don William Archibald Spooner; actor-comedian Victor Varnado; musicians such as Johnny and Edgar Winter, Salif Keita, Winston "Yellowman" Foster, Brother Ali, Sivuca, Willie "Piano Red" Perryman; and fashion models Connie Chiu and Shaun Ross. Emperor Seinei of Japan is thought to have been an albino because he was said to have been born with white hair.
Evolutionary theories
It is suggested that the early hominin evolved in East Africa around 3 million years ago. The dramatic phenotypic change from primate to early hominin is hypothesized to have involved the extreme loss of body hair â€" except for areas most exposed to UV radiation, such as the head â€" to allow for more efficient thermoregulation in the early hunter-gatherers. The skin that would have been exposed upon general body hair loss in these early hominins would have most likely been non-pigmented, reflecting the pale skin underlying the hair of our chimpanzee relatives. A positive advantage would have been conferred to early hominids inhabiting the African continent that were capable of producing darker skin â€" those who first expressed the eumelanin-producing MC1R allele â€" which protected them from harmful epithelium-damaging ultraviolet rays. Over time, the advantage conferred to those with darker skin may have led to the prevalence of darker skin on the continent. The positive advantage, however, would have had to be strong enough so as to produce a significantly higher reproductive fitness in those who produced more melanin. The cause of a selective pressure strong enough to cause this shift is an area of much debate. Some hypotheses include the existence of significantly lower reproductive fitness in people with less melanin due to lethal skin cancer, lethal kidney disease due excess vitamin D formation in the skin of people with less melanin, or simply natural selection due to mate preference and sexual selection.
When comparing the prevalence of albinism in Africa to its prevalence in other parts of the world, such as Europe and the United States, the potential evolutionary effects of skin cancer as a selective force due to its effect on these populations may not be insignificant. The prevalence of albinism in some ethnic groups in sub-Saharan Africa is around 1 in 5,000, while in Europe and the US it is 1 in 20,000. It would follow, then, that there would be stronger selective forces acting on albino populations in Africa than on albino populations in Europe and the US. Rates as high as 1 in 1,000 have been reported for some populations in Zimbabwe and other parts of Southern Africa. In two separate studies in Nigeria, people suffering from albinism were found to be of reproductively significant age more often than not. One study found that 89% of people diagnosed with albinism are between 0 and 30 years of age, while the other found that 77% of albinos were under the age of 20.
In other organisms
Albinism and other types of pigment mutations also occur in the animal and plant kingdoms.
See also
References
External links
- Albinism explained
- GeneReview/NCBI/NIH/UW entry on Oculocutaneous Albinism Type 2
- GeneReview/NCBI/NIH/UW entry on Oculocutaneous Albinism Type 4
- Albinism
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