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Albinism: Modern Molecular Diagnosis British Journal of Ophthalmology 1998;82:189–195 189 Br J Ophthalmol: first published as 10.1136/bjo.82.2.189 on 1 February 1998. Downloaded from PERSPECTIVE Albinism: modern molecular diagnosis Susan M Carden, Raymond E Boissy, Pamela J Schoettker, William V Good Albinism is no longer a clinical diagnosis. The past cytes and into which melanin is confined. In the skin, the classification of albinism was predicated on phenotypic melanosome is later transferred from the melanocyte to the expression, but now molecular biology has defined the surrounding keratinocytes. The melanosome precursor condition more accurately. With recent advances in arises from the smooth endoplasmic reticulum. Tyrosinase molecular research, it is possible to diagnose many of the and other enzymes regulating melanin synthesis are various albinism conditions on the basis of genetic produced in the rough endoplasmic reticulum, matured in causation. This article seeks to review the current state of the Golgi apparatus, and translocated to the melanosome knowledge of albinism and associated disorders of hypo- where melanin biosynthesis occurs. pigmentation. Tyrosinase is a copper containing, monophenol, mono- The term albinism (L albus, white) encompasses geneti- oxygenase enzyme that has long been known to have a cally determined diseases that involve a disorder of the critical role in melanogenesis.5 It catalyses three reactions melanin system. Each condition of albinism is due to a in the melanin pathway. The rate limiting step is the genetic mutation on a diVerent chromosome. The cutane- hydroxylation of tyrosine into dihydroxyphenylalanine ous hypopigmentation in albinism ranges from complete (DOPA) by tyrosinase, but tyrosinase does not act alone. absence of melanin to a minimal reduction in skin and hair There is a melanogenic complex in the melanocyte, colour.1 On the other hand, the ocular phenotypic charac- functioning as an integrated unit, which consists of tyrosi- teristics of most types of albinism are rather constant and nase, tyrosinase related protein 1 (TRP-1), TRP-2, include reduced visual acuity, nystagmus, pale irides that lysosome associated membrane protein 1, and melanocyte transilluminate, hypopigmented fundi, hypoplastic foveae, stimulating hormone receptors.6 TRP-1 and TRP-2 have and lack of stereopsis. Pleiotropy is exhibited by all types of multiple functions, one of which is to stabilise tyrosinase albinism; single gene mutations have eVects on diVerent activity.57 organ systems. The most obvious systems involved are the A putative tyrosinase transporter is situated in the limit- integument and the eyes, but the nervous, haematological, ing membrane of the pre-melanosome. The P polypeptide respiratory, and gastrointestinal systems may occasionally is the product of the pink eye dilution locus (named from be aVected. the mouse) and the counterpart on human chromosome http://bjo.bmj.com/ Three major mechanisms exist by which mutations exert 15 (15q11.2-13). It has been proposed,8 and refuted,9 that their eVect on the pigmentation process. Firstly, abnormal the P protein transports tyrosine into the melanosome. pigmentation may occur early in the pathway of diVeren- tiation of melanocytes. Melanoblasts from the neural crest Classification might be aVected along with cells from the same lineage, See Table 1. such as occurs in Waardenberg and Waardenberg– In the past, the classification of albinism has had a Hirschsprung syndrome where the melanocyte fails to dichotomous foundation, based on whether the phenotype on September 26, 2021 by guest. Protected copyright. populate certain areas during embryogenesis. Secondly, a was oculocutaneous or simply ocular. Recently, investiga- mutation can aVect the melanin biochemical pathway tions have become more sophisticated and some cases have causing a more specific abnormality. Oculocutaneous albi- been reclassified. For example, one third of cases initially nism type 1 (OCA 1) is an example of such a condition. labelled as OCA 2 can be reclassified as OCA 1B on the Thirdly, the biogenesis of intracellular organelles of diVer- basis of molecular genetic studies.10 OCA 1 and OCA 2 ent cells may need a common set of genes and one gene were originally separated on the basis of the hair bulb test, mutation could aVect seemingly unrelated cell types, which is now considered unreliable. We can now review the including the melanocyte. Chediak–Higashi and molecular classification of various albino conditions. Hermansky–Pudlak syndromes are disease examples. Oculocutaneous albinism Melanin physiology “Oculocutaneous albinism” refers to a heterogeneous Pigmented cells of the eye have one of two origins. The group of autosomal recessive disorders in which melanin is retinal pigment epithelium, the posterior and anterior iris reduced or absent. Pale skin and an increased risk of skin epithelium, and the outer pigmented and inner non- cancer, together with the previously mentioned ocular pigmented (so called) ciliary epithelium are all derived phenotype, are present. from the neuroectoderm of the primitive forebrain. On the other hand, melanocytes in the iris stroma, ciliary stroma, OCA 1 and choroid are derived from the neural crest. Melano- OCA 1 results from mutations in the tyrosinase gene blasts from the neural crest migrate to the skin, brain, inner aVecting its synthesis and/or catalytic activity.11 Three steps ear, and uveal tract to develop into melanocytes.2–4 in melanin biosynthesis are catalysed by tyrosinase.12 OCA The biochemical pathway converting tyrosine to mela- 1 is further divided into OCA 1A and OCA 1B. DiVerent nin is shown in Figure 1. The pigment granule (melano- types of mutations of the gene for tyrosinase explain the some) is an intracellular organelle produced by melano- two types of OCA 1. The gene position is on chromosome 190 Carden, Boissy, Schoettker, Good Br J Ophthalmol: first published as 10.1136/bjo.82.2.189 on 1 February 1998. Downloaded from Tyrosine tyrosinase (tyrosine hydroxylase activity) dihydroxyphenylalanine (DOPA) tyrosinase (dopa oxidase activity) DOPAquinone glutathione or cysteine spontaneous cysteinyl DOPA LeucoDOPAchrome spontaneous intermediates DOPAchrome DOPAchrome tautomerase (TRP-2) 5,6-dihydroxyindole 5,6-dihydroxyindole carboxylic acid tyrosinase or (peroxidase) DHICA oxidase Indole-5,6-quinone indole-5,6-quinone carboxylic acid pheomelanins (yellow and red polymers) eumelanins (black and brown polymers) Figure 1 Biosynthesis of melanin. http://bjo.bmj.com/ 11 (11q14-21).13 OCA 1A is the most severe form of OCA. Fifty five mutations of the tyrosinase (TYR) gene have Tyrosinase activity is completely absent and there is no been found to date.14 melanin in the skin or eyes (fig 2). Visual acuity is The phenotype of OCA 1B is indistinguishable from decreased to 20/400.11 Nonsense, frameshift, and missense OCA 1A at birth. Tyrosinase activity modulates pigmenta- mutations account for OCA 1A. OCA 1B has a greatly tion and causes a darker skinned phenotype during diminished, but not absent, level of tyrosinase. OCA 1B childhood.11 OCA 1B was originally described in the individuals may exhibit an increase in skin, hair, and eye Amish people,15 16 who are highly inbred. The condition pigment with age and do tan with sun exposure (Fig 2). was formerly referred to as “yellow mutant.” on September 26, 2021 by guest. Protected copyright. Table 1 Conditions of albinism COMPOUND HETEROZYGOTES OCA 1A/1B compound heterozygotes share phenotypic Type of albinism Gene position attributes of OCA 1A and 1B. OCA 1 phenotypes can OCA 1 11q14-21 result from homozygosity or compound heterozygosity of OCA 1A the alleles. In white people there is no one mutant TYR OCA 1B allele that is responsible for most of the mutations. Thus, if OCA 2 15q11-13 OCA 3 9p23 the parents are not related then the patient will probably be 14 OA 1 Xp22.3-22.2 a compound heterozygote. ?OA 2 An OCA 1A patient must have two type OCA 1A alleles; Chediak–Higashi 1q42-44 Hermansky–Pudlak 10q23.1-23.3 an OCA 1B can have two type OCA 1B alleles or one type 1A and one type 1B allele. This explains why the Contiguous gene syndromes phenotype of OCA 1B has such a broad spectrum. Even a Prader–Willi (OCA 2) 15q11-13 Angelman (OCA 2) 15q11-13 small amount of tyrosinase activity will alter the pheno- Kallmann syndrome (OA 1) Xp22.3-22.4 typic expression.14 Late onset sensorineural deafness Xp22 (OA 1) Xp22 X linked ichthyosis and Kallmann syndrome (OA 1) Xp22 OCA 2 Microphthalmia and linear skin defects (OA 1) Xp22 OCA 2 has a prevalence of 1:15 000. Patients with OCA 2 Aicardi Xp22 were formerly referred to as “tyrosinase positive” oculocu- Goltz Xp22 Waardenburg 1 2q35-q37.3 taneous albinos because the mutation causing OCA 2 does Waardenburg 2 3p12 not aVect the tyrosinase gene, instead aVecting the P Waardenburg 3 2q35-q37.3 81718 Warrdenburg–Hirschsprung 13q22 polypeptide. The disease is also autosomal recessive, but coded on a diVerent chromosome from OCA 1—that Albinism: modern molecular diagnosis 191 mutation results in the disregulation of tyrosinase activity Br J Ophthalmol: first published as 10.1136/bjo.82.2.189 on 1 February 1998. Downloaded from and the synthesis of brown rather than black melanin.21 In mice, TRP-1 catalytically functions as a dihydroxyindole carboxylic acid (DHICA) oxidase24 25 and may aid in stabi- lisation of tyrosinase and other melanosomal enzymatic processes.7 There may be an interaction with TRP-1 and tyrosinase to aVect tyrosinase hydroxylase activity. TRP-1 may also upregulate the availability of L-DOPA during the onset of melanogenesis.26 27 Ocular albinism OCULAR ALBINISM 1 X linked recessive ocular albinism (Nettleship–Falls type) is categorised as ocular albinism type 1 (OA 1). It has a prevalence of 1:50 000 in the United States.11 The OA 1 locus is 5p22-3.28–32 Charles et al 32 looked at 72 cases of OA 1 and found nystagmus (in all cases except one), reduced visual acuity (to 6/36 in most cases), refractive errors Figure 2 Photograph of child with oculocutaneous albinism 1A.
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