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Albinism - A Comprehensive Teaching Guide

1. Definition

Albinism is a genetically heterogeneous group of disorders of melanin synthesis characterized by a reduction or complete absence of melanin pigment in the skin, hair, and/or eyes. The core defect lies in melanocyte function - the cells are present in normal numbers but fail to produce adequate melanin.

Kanski's Clinical Ophthalmology, 10th ed.
Andrews' Diseases of the Skin

2. Biochemical Basis

The fundamental pathway involves the amino acid tyrosine:

Tyrosine  →(Tyrosinase)→  DOPA  →(Tyrosinase)→  Dopaquinone  →  Melanin

Tyrosinase is a copper-containing enzyme that catalyzes the first two steps. Any mutation disrupting this enzyme, its processing, or the melanosomal machinery where it operates leads to albinism. Melanin normally pigments skin, hair, the iris, retinal pigment epithelium (RPE), and the choroid.

Lippincott Biochemistry, 8th ed.

3. Classification

Albinism is classified into two broad categories:

A. Oculocutaneous Albinism (OCA)

Eyes, skin, AND hair all affected. Autosomal recessive. Seven recognized subtypes:

Type	Gene	Key Features
OCA1A	TYR (chromosome 11q14)	Complete absence of tyrosinase activity; white hair/skin throughout life; VA ~20/400; most severe
OCA1B	TYR	Reduced (not absent) tyrosinase; pigment may develop after age 1-3 years; can tan; "yellow mutant" albinism
OCA2	OCA2 / P-gene (chromosome 15q12)	Most common globally (~50% of OCA); formerly "tyrosinase-positive"; broad phenotype; prevalent in Africa (1:4,000); associated with Prader-Willi/Angelman deletions
OCA3	TYRP1 (chromosome 9p23)	"Rufous" or red OCA; seen especially in Black Africans; reddish-brown skin and red hair; mild visual defects
OCA4	SLC45A2 / MATP	Phenotypically similar to OCA2; variable hypopigmentation; common in East Asia
OCA5	4q24 locus	Rare; described in a Pakistani family; golden hair, white skin
OCA6	SLC24A5	Diverse ethnicities; hair ranges from white to dark brown
OCA7	C10orf11	Rare; leucine-rich repeat protein

B. Ocular Albinism (OA)

Eyes only affected. Skin and hair pigmentation is near-normal or only mildly reduced.

OA1 (X-linked): Most common form of ocular albinism. Gene: GPR143 (chromosome Xp22.2). GPR143 encodes a G-protein-coupled receptor involved in melanosome biogenesis. Melanosomes are decreased in number but enlarged in size (macromelanosomes visible on electron microscopy). Prevalence: ~1:50,000-60,000.

Note: Because OA is X-linked, it primarily affects males. Female carriers show a mosaic "mud-splattered" appearance of the retinal pigment epithelium.

4. Ocular Findings (Core Pathology)

This is the clinically significant part of albinism. ALL forms - OCA and OA - share the same ocular phenotype because melanin is required for normal eye development:

Visual Acuity

Reduced VA is universal, typically 20/100 to 20/400 in OCA1A
Caused by foveal hypoplasia - the foveal pit fails to develop (requires melanin during embryogenesis)
The macula shows absent perimacular vascular arcades

Nystagmus

Pendular and horizontal
Worsens in bright illumination
Tends to lessen with age

Iris Translucency / "Pink Eye"

The iris lacks melanin and is diaphanous
Light passes straight through, giving the characteristic pink-red iris color (from the underlying blood vessels and choroidal circulation)

Oculocutaneous albinism - characteristic red/pink iris appearance due to complete lack of melanin pigment

Figure: Patient with OCA showing white eyebrows/lashes and characteristic red irises - Lippincott Biochemistry

Fundus Appearance

Severely hypopigmented fundus with prominent choroidal vessels visible through the RPE
Foveal aplasia or hypoplasia

Marked iris translucency in OCA - slit-lamp view showing choroidal vessels visible through the depigmented iris

Figure: Marked iris translucency in OCA1 - Kanski's Clinical Ophthalmology

Optic Chiasm Misrouting

Normally, about 60% of optic nerve fibers cross at the chiasm (nasal fibers)
In albinism, an abnormally high proportion of fibers cross (sometimes >90%)
This results in reduced uncrossed fibers and abnormal visual evoked potentials (VEP) with crossed asymmetry
Clinically produces: strabismus, reduced stereoscopic vision, absence of normal binocular responses

Other Ocular Features

High refractive errors (various types)
Positive angle kappa
Absent stereopsis
Photophobia

5. Syndromic Forms (Albinism + Systemic Disease)

Some rare syndromes combine albinism with serious systemic features:

Hermansky-Pudlak Syndrome (HPS)

Autosomal recessive
Lysosomal storage disease affecting the biogenesis of lysosome-related organelles
Triad: OCA + platelet dysfunction (bleeding) + ceroid lipofuscin storage
Complications: pulmonary fibrosis (major cause of death), granulomatous colitis
Multiple subtypes (HPS1-11); HPS1 common in Puerto Ricans and Japanese

Chediak-Higashi Syndrome

Autosomal recessive; mutation in LYST (lysosomal trafficking regulator)
Failure of phagolysosome formation
Triad: OCA + recurrent pyogenic infections (especially Staphylococcus) + peripheral neuropathy
Neutrophils have giant granules visible on blood smear
Risk of accelerated hemophagocytic lymphohistiocytosis (HLH) phase

Waardenburg Syndrome

Autosomal dominant
PAX3, MITF, EDN3 gene mutations
Features: white forelock, poliosis, synophrys ("monobrow"), sensorineural deafness, sometimes heterochromia irides

6. Genetics Summary

Feature	OCA	OA1
Inheritance	Autosomal recessive	X-linked recessive
Primary defect	Tyrosinase pathway / melanosome proteins	GPR143 melanosome receptor
Skin + hair	Hypopigmented to normal	Normal to mildly diluted
Eyes	Severely affected	Severely affected
Melanocyte number	Normal	Normal
Melanosome size	Normal	Enlarged (macromelanosomes)

7. Clinical Consequences and Complications

Skin Cancer Risk

All OCA patients lack UV-protective melanin
Greatly increased risk of squamous cell carcinoma (SCC) and basal cell carcinoma (BCC)
Melanoma risk also elevated but harder to detect clinically
Dermoscopy is challenging due to lack of pigment; reflectance confocal microscopy may help

Visual Disability

Most significant functional impairment
Low vision aids, tinted lenses, and contact lenses can reduce photophobia

8. Management

Ophthalmological

Regular ophthalmology review - early referral is critical
Refractive correction (spectacles/contact lenses)
Tinted lenses / sunglasses to reduce photophobia
VA is often stable through life and may improve slightly into adulthood (especially OCA1B, OCA2)
Strabismus surgery considered on case-by-case basis

Dermatological

Sun protection is the cornerstone: broad-spectrum sunscreen, protective clothing, sun avoidance
Annual skin examination for premalignant/malignant lesions
Vitamin D supplementation - required because sun avoidance limits cutaneous synthesis

Genetic Counseling

Genetic testing available for TYR, OCA2, TYRP1, SLC45A2, GPR143 and other loci
Carrier testing and prenatal diagnosis possible
Molecular genetics now allows precise subtyping (OCA1-7)

9. Differential Diagnosis

Condition	Distinguishing Feature
Piebaldism	Patchy depigmentation (not global); white forelock; normal eyes
Vitiligo	Acquired; patchy; normal eyes; autoimmune
Phenylketonuria (PKU)	Hypopigmentation + intellectual disability + elevated phenylalanine; no eye misrouting
Prader-Willi / Angelman	May have OCA2 gene deletion causing hypopigmentation; neurological features dominate
Ocular albinism carriers	Mosaic RPE "mud-splattered" fundus; normal systemic pigmentation

10. Key Mnemonics

"FONS" = Fundus hypopigmentation, Optic chiasm misrouting, Nystagmus, Strabismus - the four cardinal ocular signs of albinism.

OCA types by gene: TYR (1), P-gene/OCA2 (2), TYRP1 (3), MATP/SLC45A2 (4), SLC24A5 (6) - "TPTMS"

Sources

Kanski's Clinical Ophthalmology, 10th edition - pp. 662-664
Andrews' Diseases of the Skin (Clinical Dermatology) - Chapter 36
Lippincott Illustrated Reviews: Biochemistry, 8th ed. - p. 769
Emery's Elements of Medical Genetics and Genomics
Fitzpatrick's Dermatology, Volumes 1-2
The Developing Human (Clinically Oriented Embryology)

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