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Pour un article plus général, voir Albinisme.

L'albinisme ou amélanisme est une maladie génétique caractérisée chez l'humain par un déficit ou une absence de mélanine dans les yeux, la peau et les poils. Il existe plusieurs formes d'albinisme, certaines ne touchant que les yeux. L'albinisme est associé à plusieurs problèmes de vision, comme la photophobie, le nystagmus et l'amblyopie. Les personnes atteintes d'albinisme sont plus sensibles aux rayonnements du soleil, augmentant les risques de coups de soleil et de cancers de la peau.

Plusieurs gènes sont impliqués dans la pigmentation des yeux et de la peau. Il existe donc plusieurs formes d'albinisme.

Terminologie

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Le terme albinisme vient du latin albus, signifiant blanc.

Les personnes atteintes d'albinismes sont parfois appelées « albinos », mais ce terme est considéré comme péjoratif, raison pour laquelle il est parfois recommandé de plutôt parler de « personne atteinte d'albinisme ». En 2018, des associations internationales canadienne et africaines de défense des personnes atteintes d'albinisme proposent de remplacer les termes « albinisme » et « albinos » par « amélanisme » et « amélanique » du privatif a- voulant dire « sans mélanine ».

Symptômes

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Il existe deux catégories principales d'albinismes : les albinismes oculo-cutanés, touchant les yeux, la peau et les poils, et les albinismes oculaires ne touchant que les yeux.

Symptômes cutanés

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Il existe plusieurs types d'albinisme oculo-cutané en fonction de sa cause génétique[1].

La peau des personnes atteintes d'albinisme oculo-cutané est plus sensible aux rayonnements UV du soleil. Ainsi, elle a un risque plus élevé de brûlures liées à l'exposition au soleil[2].

Symptômes oculaires

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Les yeux des personnes atteintes d'albinisme sont clairs, mais peuvent devenir plus foncés avec l'âge[3]. Dans les photographies, les personnes atteintes d'albinisme semblent souvent avoir les yeux rouges, car la rétine est visible à travers l'iris[4].

La réduction ou l'absence de mélanine peut provoquer une anomalie du chiasma optique[2]. Elle peut aussi provoquer la photophobie et une diminution des capacités visuelles à cause de la dispersion de la lumière dans l'œil[2],[5], d'une hypoplasie fovéale et de dégâts à la rétine liés à l'exposition au soleil[2].

Il est courant pour des personnes atteinte d'albinisme de souffrir de nystagmus ou d'amblyopie souvent liée à un strabisme[2].

La photophobie provoque un inconfort lorsqu'exposés à des lumières puissantes, mais cet inconfort peut être réduit grâce à l'utilisation de lunettes de soleil[6].

Autres symptômes

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Dans des cas plus rares, comme pour le syndrome de Chediak-Higashi, l'albinisme peut être associé à des déficiences de la transportation des granules de mélanine, pouvant aussi affecter les granules présents dans les cellules du système immunitaire, augmentant la susceptibilité aux infections[7].

Genetics

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Family with albinism, showing inheritance

Oculocutaneous albinism is generally the result of the biological inheritance of genetically recessive alleles (genes) passed from both parents of an individual such as 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".[8] 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.[9][10] Some of these are associated with increased risk of skin cancer Modèle:Crossref.

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.[9] 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 X chromosomes.[11]

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.[12]

Enzyme

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The enzyme defect responsible for OCA1-type albinism is tyrosine 3-monooxygenase (tyrosinase), which synthesizes melanin from the amino acid tyrosine. [13]

Evolutionary theories

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It is suggested that the early genus Homo (humans in the broader sense) started to evolve in East Africa around 3 million years ago.[14] The dramatic phenotypic change from the ape-like Australopithecus to early Homo is hypothesized to have involved the extreme loss of body hair – except for areas most exposed to UV radiation, such as the head – 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 proto-humans 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 – those who first expressed the eumelanin-producing MC1R allele – 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 to excess vitamin D formation in the skin of people with less melanin, or simply natural selection due to mate preference and sexual selection.[14]

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.[14] It would follow, then, that there would be stronger selective forces acting on albino individuals in Africa than on albinos in Europe and the US.[15] In two separate studies in Nigeria, very few people with albinism appear to survive to old age. 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.[15]

However, it has also been theorized that albinism may have been able to spread in some Native American communities, because albino males were culturally revered and assumed as having divine origins. The very high incidence of albinism among the Hopi tribe has been frequently attributed to the privileged status of albino males in Hopi society, who were not required to perform physical work outdoors, shielding them from the harmful effects of UV radiation. This privileged status of albino males in Hopi society allowed them to reproduce with large numbers of non-albino women, spreading the genes that are associated with albinism.[16][17]

Diagnosis

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Modèle:More citations needed section Genetic testing can confirm albinism and what variety it is, but offers no medical benefits, except in the case of non-OCA disorders. Such disorders cause other medical problems in conjunction with albinism, and may be treatable. Genetic tests are currently available for parents who want to find out if they are carriers of ty-neg albinism. Diagnosis of albinism involves carefully examining a person's eyes, skin and hairs. Genealogical analysis can also help.

Albinism can also be a feature of several syndromes:[18]

Management

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Since there is no cure for albinism, it is managed through lifestyle adjustments. People with albinism need to take care not to get sunburnt and should have regular healthy skin checks by a dermatologist.

For the most part, treatment of the eye conditions consists of visual rehabilitation. Surgery is possible on the extra-ocular muscles to decrease strabismus.[2] Nystagmus-damping surgery can also be performed, to reduce the "shaking" of the eyes back and forth.[19] The effectiveness of all these procedures varies greatly and depends on individual circumstances.

Glasses (often with tinted lenses), low vision aids, large-print materials, and bright angled reading lights can help individuals with albinism. Some people with albinism do well using bifocals (with a strong reading lens), prescription reading glasses, hand-held devices such as magnifiers or monoculars or wearable devices like eSight and Brainport.[6][20]

The condition may lead to abnormal development of the optic nerve and sunlight may damage the retina of the eye as the iris cannot filter out excess light due to a lack of pigmentation. Photophobia may be ameliorated by the use of sunglasses which filter out ultraviolet light.[21] 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".)

There are a number of national support groups across the globe which come under the umbrella of the World Albinism Alliance.[22]

Epidemiology

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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.[23] Today, the prevalence of albinism in sub-Saharan Africa is around 1 in 5,000, while in Europe and the US it is around 1 in 20,000 of the European derived population.[24][14] Rates as high as 1 in 1,000 have been reported for some populations in Zimbabwe and other parts of Southern Africa.[15]

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 (OCA 4);[25] and Ukerewe Island, the population of which shows a very high incidence of albinism.[26]

Society and culture

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Special status of albinos in Native American culture

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Modèle:CSS image crop In some Native American and South Pacific cultures, people with albinism have been traditionally revered, because they were considered heavenly beings associated with the sky.[27][28] Among various indigenous tribes in South America, albinos were able to live luxurious lives due to their divine status.[29] This special status was applied mainly to male albinos.[30] It has been theorized that the very high level of albinism among some Native American tribes can be attributed to sexual privileges given to male albinos, which allowed them to reproduce with large numbers of non-albino women in their tribes, leading to the spread of genes that are associated with albinism.[27]

Persecution of people with albinism

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Erreur : La version française équivalente de {{Main}} est {{Article détaillé}}. Humans with albinism often face social and cultural challenges (even threats), as the condition is often a source of ridicule, discrimination, or even fear and violence. It is especially socially stigmatised in many African societies. A study conducted in Nigeria on albino children stated that "they experienced alienation, avoided social interactions and were less emotionally stable. Furthermore, affected individuals were less likely to complete schooling, find employment, and find partners".[31] Many cultures around the world have developed beliefs regarding people with albinism.

In African countries such as Tanzania[32] and Burundi,[33][34] 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 witch doctors.[35] Numerous authenticated incidents have occurred in Africa during the 21st century.[36][37][38][39] 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.[40] Again in Tanzania and Burundi in 2010, the murder and dismemberment of a kidnapped albino child was reported from the courts,[33] as part of a continuing problem. The US-based National Geographic Society estimated that in Tanzania a complete set of albino body parts is worth US$75,000.[41][42]

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).[43]

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Erreur : La version française équivalente de {{Main}} est {{Article détaillé}}. 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, Hermeto Pascoal, Willie "Piano Red" Perryman, Kalash Criminel; actor-rapper Krondon, and fashion models Connie Chiu, Ryan "La Burnt" Byrne 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.[réf. nécessaire]

International Albinism Awareness Day

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Erreur : La version française équivalente de {{Main}} est {{Article détaillé}}. International Albinism Awareness Day was established after a motion was accepted on 18 December 2014 by the United Nations General Assembly, proclaiming that 13 June would be known as International Albinism Awareness Day as of 2015.[44] This was followed by a mandate created by the United Nations Human Rights Council that appointed Ms. Ikponwosa Ero, who is from Nigeria, as the first Independent Expert on the enjoyment of human rights by persons with albinism.[45]

See also

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References

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  1. ↑ « oculocutaneous albinism », sur Genetics Home Reference, Bethesda, Maryland, U.S. National Library of Medicine, National Institutes of Health, (consulté le )Modèle:Tertiary source
  2. ↑ a b c d e et f (en) Harold Chen, Atlas of genetic diagnosis and counseling, Totowa, New Jersey, Humana Press, , 37–40 p. (ISBN 9781588296818, lire en ligne)
  3. ↑ « Information Bulletin – Ocular Albinism » [archive du ], National Organization for Albinism and Hypopigmentation (consulté le )
  4. ↑ (en) Pragya Khanna, Essentials of Genetics, I. K. International, (ISBN 9789380026343, lire en ligne)
  5. ↑ Bastiaan Kruijt, Luuk Franssen, Liesbeth J. J. M. Prick, Johannes M. J. Van Vliet et Thomas J. T. P. Van Den Berg, « Ocular Straylight in Albinism », Optometry and Vision Science, vol. 88, no 5,‎ , E585–592 (PMID 21358444, DOI 10.1097/OPX.0b013e318212071e, S2CID 24992321)
  6. ↑ a et b Richard King, C. Gail Summers, James W. Haefemeyer et Bonnie LeRoy, « Facts About Albinism » [archive du ], Albinism.Med.UMN.edu, University of Minnesota, (consulté le )
  7. ↑ J. Kaplan, I. De Domenico et D. M. Ward, « Chediak-Higashi syndrome », Current Opinion in Hematology, vol. 15, no 1,‎ , p. 22–29 (PMID 18043242, DOI 10.1097/MOH.0b013e3282f2bcce, S2CID 43243529)
  8. ↑ R. E. Boissy, H. Zhao, W. S. Oetting, L. M. Austin, S. C. Wildenberg, Y. L. Boissy, Y. Zhao, R. A. Sturm, V. J. Hearing, R. A. King et J. J. Nordlund, « Mutation in and lack of expression of tyrosinase-related protein-1 (TRP-1) in melanocytes from an individual with brown oculocutaneous albinism: A new subtype of albinism classified as "OCA3" », American Journal of Human Genetics, vol. 58, no 6,‎ , p. 1145–1156 (PMID 8651291, PMCID 1915069)
  9. ↑ a et b Raymond E. Boissy, « Dermatologic Manifestations of Albinism », Medscape, eMedicine / WebMD, (consulté le )
  10. ↑ Online Mendelian Inheritance in Man, at Johns Hopkins University Modèle:Crossref.
  11. ↑ Chad Haldeman-Englert, « Sex-linked recessive », dans The ADAM Medical Encyclopedia, Bethesda, Maryland / Atlanta, Georgia, US, U.S. National Library of Medicine, National Institutes of Health / Ebix Inc., (lire en ligne) (consulté le )
  12. ↑ (en) James F. Frayne, Applications of Genetics, Montana Publications,
  13. ↑ M. B. Dolinska, E. Kovaleva, P. Backlund, P. T. Wingfield, B. P. Brooks et Y. V. Sergeev, « Albinism-Causing Mutations in Recombinant Human Tyrosinase Alter Intrinsic Enzymatic Activity », PLOS ONE, vol. 9, no 1,‎ , e84494 (PMID 24392141, PMCID 3879332, DOI 10.1371/journal.pone.0084494, Bibcode 2014PLoSO...984494D)
  14. ↑ a b c et d M. Greaves, « Was skin cancer a selective force for black pigmentation in early hominin evolution? », Proceedings of the Royal Society B: Biological Sciences, vol. 281, no 1781,‎ , p. 20132955 (PMID 24573849, PMCID 3953838, DOI 10.1098/rspb.2013.2955)
  15. ↑ a b et c E. S. Hong, H. Zeeb et M. H. Repacholi, « Albinism in Africa as a public health issue », BMC Public Health, vol. 6,‎ , p. 212 (PMID 16916463, PMCID 1584235, DOI 10.1186/1471-2458-6-212)
  16. ↑ (en) Philip W. Hedrick, Genetics of Populations, Jones & Bartlett Learning, , 2, 186 (ISBN 9780763747725, lire en ligne) :

    « "An often-cited example of pleiotropic effects of viability and male-mating success in humans is for albinism in Hopi Indians, a small tribe in northern Arizona." »
  17. ↑ (en) Russell P. Shuttleworth, The Pursuit of Sexual Intimacy for Men with Cerebral Palsy, University of California, San Francisco, (lire en ligne), p. 74 :

    « "The apparent reason for this phenomenon is that albino Hopi men developed a sexual advantage over non-albino men." »
  18. ↑ (en) « Albinism (Concept Id: C0001916) », sur www.ncbi.nlm.nih.gov (consulté le )
  19. ↑ J. Lee, « Surgical management of nystagmus », Journal of the Royal Society of Medicine, vol. 95, no 5,‎ , p. 238–241 (PMID 11983764, PMCID 1279676, DOI 10.1177/014107680209500506)
  20. ↑ Arielle Pardes, « The Wearables Giving Computer Vision to the Blind », {{Article}} : paramètre « périodique » manquant, Condé Nast,‎ (lire en ligne, consulté le )
  21. ↑ « Albinism causes », sur News-Medical.net, (consulté le )
  22. ↑ Anon, « World Albinism Alliance » [archive du ], WAA, (consulté le )
  23. ↑ K. Gronskov, J. Ek et K. Brondum-Nielsen, « Oculocutaneous albinism », Orphanet Journal of Rare Diseases, vol. 2,‎ , p. 43 (PMID 17980020, PMCID 2211462, DOI 10.1186/1750-1172-2-43)
  24. ↑ « About Albinism and Hypopigmentation », Albinism.org, National Organization for Albinism and Hypopigmentation,
  25. ↑ « Entry - #606574 - Albinism, Oculocutaneous, Type IV; OCA4 - OMIM », sur omim.org (consulté le )
  26. ↑ « Ukerewe Albino Society » [archive du ], Southern Africa's Children, (consulté le )
  27. ↑ a et b (en) Philip W. Hedrick, « Hopi Indians, cultural selection, and albinism », American Journal of Physical Anthropology, vol. 121, no 2,‎ , p. 151–156 (ISSN 0002-9483, PMID 12740958, DOI 10.1002/ajpa.10180, lire en ligne)
  28. ↑ (en) Paul Martin Lester, Visual Communication: Images with Messages, Thomson/Wadsworth, (ISBN 9780534562441, lire en ligne), p. 17
  29. ↑ (en) Jennifer Kromberg et Prashiela Manga, Albinism in Africa: Historical, Geographic, Medical, Genetic, and Psychosocial Aspects, Academic Press, (ISBN 9780128133170, lire en ligne), p. 58
  30. ↑ (en) James J. Nagle, Heredity and Human Affairs, Times Mirror/Mosby, (ISBN 9780801636264, lire en ligne), p. 291 :

    « "Albino men in particular have been socially accepted in the dark-skinned Hopi society." »
  31. ↑ P. Magna, « Biology and genetics of Oculocutaneous albinism and vitiligo-common pigmentation disorders in Southern Africa », South African Medical Journal, vol. 103, no 1,‎ , p. 984–988 (PMID 24300644, DOI 10.7196/samj.7046, lire en ligne)
  32. ↑ (en) « Living in fear: Tanzania's albinos », BBC News, British Broadcasting Corporation,‎ (lire en ligne)
  33. ↑ a et b (en) « Burundi albino boy 'dismembered' », BBC News,‎ (lire en ligne)
  34. ↑ (en) « Burundian albino murders denied », BBC News,‎ (lire en ligne)
  35. ↑ (en) Franck Vogel, « Zeru, Zeru: Being Albino in Tanzania », Visura Magazine, Westford, Vermont, Foto Visura,‎ (lire en ligne)
  36. ↑ (en) « Man 'tried to sell' albino wife », BBC News,‎ (lire en ligne)
  37. ↑ (en) « Tanzania albinos targeted again », BBC News,‎ (lire en ligne)
  38. ↑ (en) Vicky Ntetema, « In hiding for exposing Tanzania witchdoctors », BBC News,‎ (lire en ligne)
  39. ↑ (en) « Mothers hacked in albino attacks », BBC News,‎ (lire en ligne)
  40. ↑ (en) « Death for Tanzania albino killers », BBC News,‎ (lire en ligne)
  41. ↑ (en) Sasha Ingber, « Pictures: Inside the Lives of Albinos in Tanzania », National Geographic News, Washington DC, National Geographic Society,‎ (lire en ligne)
  42. ↑ Tanzanians with albinism targeted for witchcraft (), UNICEF
  43. ↑ Lewis Machipisa, « The Last Minority Group to Find a Voice » [archive du ], IPSNews.net, Inter Press Service News Agency, (consulté le )
  44. ↑ « International Albinism Awareness Day »
  45. ↑ « Independent Expert on the enjoyment of human rights by persons with albinism », OHCHR.org, Geneva, Office of the High Commissioner for Human Rights, United Nations
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