Oculocutaneous Albinism

2015-04-01   Kunal Ray , Mainak Sengupta , Kausik Ganguly 

Academy of Scientific and Innovative Research (AcSIR), New Delhi # (KR); Department of Genetics, University of Calcutta, Kolkata (MS, KG), India. kunalray@gmail.com; sengupta.mainak@gmail.com; kausikganguly6@gmail.com

Identity

Name

Oculocutaneous Albinism

Alias

Albinism, Oculocutaneous Albinism type 1 (OCA1) , Oculocutaneous Albinism type 2 (OCA2) , Oculocutaneous Albinism type 3 (OCA3) , Oculocutaneous Albinism type 4 (OCA4) , Oculocutaneous Albinism type 5 (OCA5) , Oculocutaneous Albinism type 6 (OCA6) , Oculocutaneous Albinism type 7 (OCA7) , Tyrosinase-Negative Albinism , Tyrosinase-Positive Albinism

Note

Oculocutaneous Albinism (OCA) is a group of congenital developmental disorder characterized by complete or partial loss of melanin in skin, hair and eye. OCA is caused due to defects in genes associated with melanin biosynthetic pathway. Depending on the gene mutated, OCA can be classified into Oculocutaneous Albinism type 1 (OCA1), Oculocutaneous Albinism type 2 (OCA2), Oculocutaneous Albinism type 3 (OCA3) and Oculocutaneous Albinism type 4 (OCA4), Oculocutaneous Albinism type 5 (OCA5), Oculocutaneous Albinism type 6 (OCA6) and Oculocutaneous Albinism type 7 (OCA7). OCA1 affects 1 per 40000 individuals in most populations (King et al., 2001) but is very uncommon among African-Americans. The overall prevalence of OCA2 is estimated to be 1:36000 in USA, but it is a lot more common among the African Americans with a prevalence of 1:10000 (Okoro, 1975). In fact, OCA2 affects 1 in 3900 of the population in the southern parts of Africa. OCA3 or Rufous oculocutaneous albinism has been estimated to affect 1:8500 individuals in Africa; however, it is very rare in any other populations as per published literature. OCA4 has been found to be the second largest rare form of albinism after OCA1 in Japan. The 3 other forms of OCA are very rare. There is a single report till date for OCA5 in a consanguineous Pakistani family [Kausar et al., 2013]. A few distinct cases of SLC24A5 mediated OCA6 - one from India, one from China, two patients from France, three from Portugal, one from Belgium and one from Syria, have been reported till date [Mondal et al., 2012; Wei et al., 2013; Fanny et al., 2014]. Nine Faroese patients and one Danish patient of Lithuanian origin harbored mutations in C10ORF11 gene representing OCA7 [ Gronskov et al., 2014]. It is worth mentioning that in countries like India where endogamy prevails, the incidence of OCA would be higher than the world average. In fact, in India, a preponderance of homozygous mutations is found and OCA1 is caused majorly because of founder mutations (Chaki et al., 2005; Chaki et al. 2006).

Inheritance

OCA is inherited in an autosomal recessive mode. However, recently, it has been hypothesized that the clinical spectrum of OCA depends on the pigmentation threshold of the patient. In genotypically darker complexion individuals such as in Africans, two mutations are needed to completely shut off the high pigmentation background; whereas in individuals with lighter complexion such as Caucasians, OCA can be manifested by the presence of one mutation and one hypomorphic allele (Chiang et al., 2008).

Omim

203100 , 203200 , 203290 , 605962 , 606574

Mesh

D016115

Orphanet

55 Oculocutaneous albinism

Umls

C0078918

Clinics

Note

OCA is characterized by partial or total absence of melanin in the skin, hair and eyes at birth. The absence of optimum content of melanin during embryogenesis acts as a cue to trigger defects in eye development that cannot be corrected. A portion of the retinal ganglion cell (RGC) axons, originally destined to the ipsilateral hemisphere of the dorsal lateral geniculate nuclei (dLGN) of the midbrain, misproject to the contralateral side, thereby resulting in the disruption of binocular vision (Lund, 1965; Guillery, 1971; Cooper and Pettigrew, 1979; Drager and Olsen, 1980; Lavado and Montoliu, 2006). The developmental defect concerned with abnormal nerve fibre projection has been discussed in details in a review by Ray et al., 2007 (Ray et al., 2007).

Phenotype and clinics

The reduction of melanin in peripheral retina results in a stereoscopic set of developmental defects in neuronal migration in the visual pathways leading to foveal hypoplasia, abnormal routing of the nerve fibers from the eye to brain with consequent low vision (reduced visual acuity usually in the range 20\/60 to 20\/400 and refractive errors), photophobia, iris transillumination, nystagmus and strabismus. An OCA affected person is considered legally blind if he\/she has a visual acuity of 20\/200 (6\/60) or less. The degree of severity of the eye features as well as skin pigmentation varies with the different subtypes of albinism. Due to loss of pigmentation, the iris looks hazel or light blue or in extreme cases as in OCA1, it is translucent to such an extent that it appears pink or red in ambient light. The skin remains white or only become slightly pigmented with time in case of OCA1 whereas the patients suffering from the other 3 types of albinism have residual pigmentation and look pinkish or yellowish. If unprotected from sun rays, hypopigmented skin in the albinistic individuals may develop erythema. Moreover, the reduction in melanin pigment in the skin results in an increased sensitivity to UV induced skin damage and subsequently non-melanotic skin cancers.
It must be stated here that based on the severity of pigment loss, the most severe form of OCA viz. OCA1 can be sub-classified into two categories - (a) OCA1A: when the tyrosinase enzyme activity is completely lacking, and (b) OCA1B: when some residual activity is retained. The visual acuity of the OCA1A patients is greatly reduced; the degree of nystagmus, strabismus, photophobia are usually severe and the translucent iris that appear pink early in life, often become gray-blue with age. In case of OCA1A there is an absence of pigmentation throughout the patients life. In contrast, in OCA1B, although there is little or no apparent melanin at birth, progressive melanization might occur with time. The range of pigmentation in OCA1B varies from little cutaneous pigment to nearly normal skin color and the phenotype is often influenced by ethnicity. OCA1B is called yellow OCA due to the color of the hair, produced by pheomelanin synthesis.

Neoplastic risk

The loss of pigment often leads to non-melanotic skin cancers in form of Squamous Cell Carcinoma (SCC) and Basal Cell Carcinoma (BCC) (Mabula et al., 2012). Melanoma is rare in albino patients (Pehamberger et al., 1984).

Treatment

No specific treatment is available for OCA. Attention must be paid to avoidance of direct sun exposure.

Evolution

OCA1B patients although are born with almost no pigmentation, show a progressive melanization with age.

Prognosis

With proper protection from direct exposure from sun and visual aids like dome magnifiers, reading glasses, hand-held and stand magnifiers, patients should be able to lead a normal life.

Genes involved and Proteins

Note

OCA can be classified into seven major types viz. Oculocutaneous Albinism type 1 (OCA1), Oculocutaneous Albinism type 2 (OCA2), Oculocutaneous Albinism type 3 (OCA3) and Oculocutaneous Albinism type 4 (OCA4), Oculocutaneous Albinism type 5 (OCA5), Oculocutaneous Albinism type 6 (OCA6) and Oculocutaneous Albinism type 7 (OCA7). Each of the classical subtypes is caused due to defects in 7 different genes independently. Table 1 shows the genes involved in OCA:
Table 1: Causal genes and specific symptoms for 7 classical OCA syndromes.
GeneSyndromeSpecific symptoms
TYROCA1 (A and B)

OCA1A: The skin, hair, eyelashes and eyebrows are white, Irises completely translucent. Visual acuity is 1\/10 or less with intense photophobia. OCA1 symptoms do not vary with age or race. Amelanotic nevi may be present.OCA1B: In OCA1B, the hair and skin may develop some pigment with time and the irises may appear to green\/brown. Visual acuity is 2\/10.

OCA2OCA2While the degree of cutaneous pigment and iris color may vary, the newborn with OCA2 nearly always have pigmented hair. Nevi and freckles are commonly found in the skin. Visual acuity is better than in OCA1, and can reach 3\/10. Africans with OCA2 appear with light brown hair and skin, and gray irises.
TYRP1OCA3Among Africans OCA3 affected individuals have red hair and reddish brown skin (xanthism). Visual anomalies are less severe and often not detectable.
SLC45A2OCA4The clinical findings of OCA are very similar to that of OCA2
Locus 4q24(gene not identified) OCA5The patients have been reported with golden hair, white skin, nystagmus, photophobia, foveal hypoplasia, and impaired visual acuity
SLC24A5 OCA6Patients are generally characterized by light hair at birth that darkens with age, white skin, transparent irides, photophobia, nystagmus, foveal hypoplasia and reduced visual acuity. In a man from eastern India who had extreme hypopigmentation resulting in pinkish-white skin, but with dark brown hair and brown irides, homozygosity for a 4-bp insertion in the SLC24A5 gene was observed (Mondal et al,, 2012)
C10ORF11 OCA7Patients have a light skin pigmentation that is reported as lighter than their relatives. Hair color ranges from light blond to dark brown. Eye findings include nystagmus, iris transillumination, visual acuity ranging from 6\/9 to 3\/60 and very sparse peripheral ocular fundus pigmentation.

Alias

Monophenol monooxygenase, SHEP3, Tumor rejection antigen AB, LB24-AB, SK29-AB.

Note

TYR codes for Tyrosinase protein, the rate limiting enzyme of melanin biosynthetic pathway.

Description

The human tyrosinase gene consists of 5 exons and spans about 65 kb of the genome.

Transcription

It encodes a 2082 bp transcript (Accession No: NM_000372.4).

Pseudogene

TYR-like segment (TYRL, 11p11.2, MIM 191270) is a pseudogene of TYR, which contains sequences very similar to exons IV and V of TYR gene. It is hypothesized that duplication of TYR exons IV and V regions followed by 11q:11p translocation has given rise to the TYRL segment.

Description

TYR (monophenol monoxygenase EC 1.14.18.1) encodes a ~80 kDa glycoprotein (Accession No: NP_000363.1) composed of 529 amino acids. TYR is a melanosomal membrane bound glycoenzyme with a type-3 copper active site. The mature TYR polypeptide includes an 18-amino acid long N-terminal signal peptide, six N-glycosylation sites, two copper binding sites (CuA and CuB) and one transmembrane (TM) domain followed by a relatively short carboxyl tail.

Expression

TYR is mainly expressed in two cell types: (a) Melanocytes that are derived from neural crest cells colonizing within iris, cochlea, skin and choroids, and (b) Retinal pigment epithelial (RPE) cells that are derived from the optic cup. During mouse embryogenesis, the expression of TYR could be first detected from +16.5 days post coitum onwards in the skin melanocytes and from +10.5 days postcoitum onwards in the RPE cells (Beermann et al., 1992).

Localisation

TYR is a melanosomal membrane protein and the TM region anchors the bulk of the protein inside the melanosomal lumen.

Function

TYR catalyzes the rate limiting steps of melanin biosynthesis viz. hydroxylation of L-tyrosine to L-DOPA and oxidation of L-DOPA to DOPAquinone. It also catalyzes the conversion of 5,6 dihydroxyindole to Indole 5,6 Quinone and 5,6,dihydroxyindole carboxylic acid to Indole 5,6 quinone carboxylic acid.

Homology

TYR, Tyrosinase Related Protein 1 (TYRP1) and Tyrosinase Related Protein 2 (TYRP2\/DCT) represent a family of closely related gene products (with almost 40% amino acid identity) that share a common tertiary structure (Jimenez-Cervantes et al., 1998; Kobayashi et al., 1998). These have been grouped together to form the TYRP family of genes.

Germinal

TYR mutations are responsible for OCA1. A few OCA2 mutations have been associated with autosomal recessive ocular albinism (AROA). OCA1 is an endoplasmic reticulum retention (ER) disorder and all the missense mutations that have been functionally characterized have yielded ER -retained proteins.

Alias

P, Pink-eyed dilution protein homolog, D15S12, Melanocyte-specific transporter protein, EYCL, EYCL2, EYCL3, BOCA, BEY, BEY1, BEY2, HCL3, PED, SHEP1, oculocutaneous albinism II (pink-eye dilution (murine) homolog)1

Note

OCA2 codes for OCA2 protein, hypothesized to be involved in the transport of tyrosine, the precursor to melanin synthesis, within the melanocyte.

Description

The human OCA2 gene consists of 24 exons and spans ~344.5 kb of the genome.

Transcription

It encodes a 3154 bp transcript (Accession No: NM_000275.2).

Description

OCA2 encodes a ~110 kDa protein (Accession No: NP_000266.2) composed of 838 amino acids. The OCA2 protein is thought to be a melanosomal multipass integral membrane protein (with 12 predicted transmembrane domains) involved in small molecule transport, specifically tyrosine - a precursor of melanin.

Expression

Due to its localization in the melanosomal membrane, OCA2 is thought to be expressed in the melanocytes.

Localisation

OCA2 is hypothesized to be present in the melanosomal membrane of the melanocytes.

Function

The precise function of OCA2 has not been elucidated till date. However, the potential functions include: a) normal biogenesis of melanosomes (Rosemblat et al., 1998; Orlow and Brilliant et al., 1999); b) for normal processing and transport of tyrosinase and other melanosomal proteins (Puri et al., 2000; Manga et al., 2001; Toyofuku et al., 2002; Chen et al., 2002); and c) maintenance of an acidic pH in melanosomes (Ni-Komatsu and Orlow, 2006).

Homology

Its sequence predicts that OCA2 has a homology to a superfamily of permeases (Rinchik et al., 1993; Lee et al., 1995).

Germinal

OCA2 mutations are responsible for OCA2. A few OCA2 mutations have been associated with autosomal recessive ocular albinism (AROA) too.

Alias

RP11-3L8.1, CAS2, CATB, GP75, SHEP11, TRP, TYRP, b-PROTEIN

Note

TYRP1 codes for TYRP1 protein, hypothesized to be involved in melanin synthesis, stabilization of tyrosinase and modulating its catalytic activity, maintenance of melanosome structure and affects melanocyte proliferation and melanocyte cell death. Defects in this gene are the cause of rufous oculocutaneous albinism and oculocutaneous albinism type III.

Description

The human TYRP1 gene consists of 8 exons and spans ~24.8 kb of the genome.

Transcription

It encodes a 2876 bp transcript (Accession No: NM_000550.2).

Description

TYRP1 encodes a 60.7 kDa protein (Accession No: NP_000541.1) composed of 537 amino acids. The TYRP1 protein is thought to be a melanosomal membrane single-pass type I membrane protein.

Expression

TYRP1, similar to TYR, is mainly expressed in two cell types: (a) Melanocytes that are derived from neural crest cells colonizing within iris, cochlea, skin and choroids, and (b) Retinal pigment epithelial (RPE) cells that are derived from the optic cup.

Localisation

TYRP1 is hypothesized to be localized in melanosome membrane.

Function

Oxidation of 5,6-dihydroxyindole-2-carboxylic acid (DHICA) into indole-5,6-quinone-2-carboxylic acid. May regulate or influence the type of melanin synthesized.

Homology

Belongs to the tyrosinase family. Homologous to murine brown locus.

Germinal

TYRP1 mutations are responsible for OCA3.

Alias

Membrane associated transporter protein, MATP, MELANOMA ANTIGEN AIM1, AIM1

Note

SLC45A2 codes for SLC45A2 protein, hypothesized to be involved in the transport of substances required for melanin biosynthesis within the melanocyte.

Description

The human SLC45A2 gene consists of 7 exons and spans 40.1 kb of the genome.

Transcription

It encodes a 1734 bp transcript (Accession No: NM_016180.3).

Description

SLC45A2 encodes a ~58 kDa protein (Accession No: NP_057264.3) and composed of 530 amino acids. The protein is thought to be a melanosomal multipass membrane protein (contains 12 putative transmembrane domains) involved in small molecule transport.

Expression

Expressed in most melanoma cell lines and melanocytes.

Localisation

SLC45A2 is hypothesized to be present in the melanosomal membrane of the melanocytes.

Function

The precise function of SLC45A2 has not been elucidated till date. Studies on Medaka fish show that the SLC45A2\/MATP plays an important role in pigmentation and probably functions as a membrane transporter in melanosomes (Fukamachi et al., 2001).

Homology

Belongs to the glycoside-pentoside-hexuronide (GPH) cation symporter transporter (TC 2.A.2) family.

Germinal

SLC45A2 mutations are responsible for OCA4.

Alias

Oculocutaneous Albinism 5 (Autosomal Recessive)

Note

This chromosomal locus harbors 14 genes, none of which are directly responsible for melanin biosynthesis

Description

4q24 harbors 14 genes (CENPE, TACR3, CXXC4, TET2, PPA2, INTS12, GSTCD, NPNT, AIMP1, DKK2, PAPSS1, SGMS2, CYP2U1, HADH) with linkage interval of 3.84Mb.

Transcription

The underlying gene causing OCA5 has not yet been identified.

Function

4q24 is a chromosomal locus harbouring about 14 genes in it with a linkage interval of 3.84 Mb. Though none of those 14 genes are known to have any role in melanin synthesis, but studies are being done to ascertain the role of this locus and to identify the underlying gene.

Germinal

Though inheritance indicates germinal origin of the mutations, the causal gene is yet to be recognized.

Alias

Solute Carrier Family 24 (Sodium\/Potassium\/Calcium Exchanger), Member 5, JSX, OCA6

Note

SLC24A5 codes for a cation exchanger which is probably involved in ion transport in melanosomes.

Description

The human SLC24A5 gene consists of 5 exons and spans 21.701 kb of the genome

Transcription

This gene has 4 transcripts (splice variants), three of which either skips an exon or are truncated versions. The actual protein coding transcript is 1617 bp long (NM_205850.2).

Description

SLC24A5 codes for a cation exchanger which is 500 amino acid long (NP_995322). This protein is an intracellular potassium-dependent sodium\/calcium exchanger with 2 large hydrophilic loops and 2 sets of multiple trans-membrane-spanning segments.

Expression

Due to its localization in the melanosomal membrane, OCA2 is thought to be expressed in the melanocytes.

Localisation

SLC24A5 is hypothesized to be present in the trans-Golgi network of melanocytes.

Function

The precise function of SLC24A5 has not been elucidated till date. However, the potential functions include: (a) transporting 1 Ca2+ and 1 K+ to the melanosome in exchange for 4 cytoplasmic Na+ [Lamason RL et al., 2005]; (b) Influencing natural variation in skin pigmentation via a novel, unknown mechanism affecting cellular sterol levels [Wilson S et al., 2013].

Homology

It belongs to Solute Carrier Family 24 (http:\/\/www.guidetopharmacology.org\/GRAC\/FamilyDisplayForward?familyId=202).

Germinal

SLC24A5 mutations are responsible for OCA6.

Alias

Chromosome 10 Open Reading Frame, OCA7

Note

C10Orf11 codes for a leucine rich repeat containing protein.

Description

The human C10ORF11 gene consists of 6 exons and spans 959 kb of the genome.

Transcription

C10ORF11 encodes 16 splice variants of which 4 are protein coding and rest are processed transcripts. Of those 16, transcript variant 2 (Accession No: NM_032024.4) is 909 bp long and codes for a 198 amino acids long peptide.

Description

C10ORF11 encodes a 22.5 kDa protein (Accession No: NP_114413.1) composed of 198 amino acids. The sub-cellular localization of leucine rich repeat containing protein C10ORF11 is not clear.

Expression

This protein was found in melanoblast of embryo and melanocytes of fetus, but not in retinal pigment epithelial cells.

Localisation

C10ORF11 is hypothesized to be present in the melanosome, but not in retinal pigment epithelial cells.

Function

The precise function of C10ORF11 has not been elucidated till date. However, there is some evidence that it may play some role in melanocyte differentiation (Grønskov K et al., 2013).

Germinal

C10ORF11 mutations are responsible for OCA7.

Bibliography

Pubmed IDLast YearTitleAuthors
15573871992Expression of the mouse tyrosinase gene during embryonic development: recapitulation of the temporal regulation in transgenic mice.Beermann F et al
160562192005Higher prevalence of OCA1 in an ethnic group of eastern India is due to a founder mutation in the tyrosinase gene.Chaki M et al
169077082006OCA1 in different ethnic groups of india is primarily due to founder mutations in the tyrosinase gene.Chaki M et al
120580622002Pink-eyed dilution protein controls the processing of tyrosinase.Chen K et al
195337892009Oculocutaneous albinism spectrum.Chiang PW et al
4897821979The retinothalamic pathways in Siamese cats.Cooper ML et al
74106001980Origins of crossed and uncrossed retinal projections in pigmented and albino mice.Dräger UC et al
114795962001Mutations in the gene encoding B, a novel transporter protein, reduce melanin content in medaka.Fukamachi S et al
233954772013Mutations in c10orf11, a melanocyte-differentiation gene, cause autosomal-recessive albinism.Grønskov K et al
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230505612013OCA5, a novel locus for non-syndromic oculocutaneous albinism, maps to chromosome 4q24.Kausar T et al
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240669602014Increasing the complexity: new genes and new types of albinism.Montoliu L et al
239859942014SLC24A5 mutations are associated with non-syndromic oculocutaneous albinism.Morice-Picard F et al
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100684801999The pink-eyed dilution locus controls the biogenesis of melanosomes and levels of melanosomal proteins in the eye.Orlow SJ et al
64909971984Dysplastic nevus syndrome with multiple primary amelanotic melanomas in oculocutaneous albinism.Pehamberger H et al
109981312000Aberrant pH of melanosomes in pink-eyed dilution (p) mutant melanocytes.Puri N et al
173559132007Tyrosinase and ocular diseases: some novel thoughts on the molecular basis of oculocutaneous albinism type 1.Ray K et al
84214971993A gene for the mouse pink-eyed dilution locus and for human type II oculocutaneous albinism.Rinchik EM et al
95218521998Melanosomal defects in melanocytes from mice lacking expression of the pink-eyed dilution gene: correction by culture in the presence of excess tyrosine.Rosemblat S et al
120285862002The etiology of oculocutaneous albinism (OCA) type II: the pink protein modulates the processing and transport of tyrosinase.Toyofuku K et al
233644762013Exome sequencing identifies SLC24A5 as a candidate gene for nonsyndromic oculocutaneous albinism.Wei AH et al
232248732013NCKX5, a natural regulator of human skin colour variation, regulates the expression of key pigment genes MC1R and alpha-MSH and alters cholesterol homeostasis in normal human melanocytes.Wilson S et al

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