Albinism

Reading File

Finding Sources

Searching PubMed

albinism melanin genetics

Reading File

Loading Image

Reading File

Loading Image

I now have all the material needed for a comprehensive, well-illustrated answer.

Albinism

Albinism is a genetically heterogeneous group of disorders characterized by reduced or absent melanin production in the skin, hair, and/or eyes. It results from defects in melanin biosynthesis - primarily in the tyrosinase pathway.

Biochemical Basis

Melanin is synthesized from tyrosine in melanocytes within organelles called melanosomes. The key enzyme is tyrosinase (a copper-requiring enzyme), which catalyzes the first two steps in the conversion of tyrosine to melanin. Defects in this enzyme, or in proteins required for melanosome biogenesis, trafficking, and structure, result in albinism.

The pathway:

Tyrosine → (tyrosinase) → DOPA → (tyrosinase) → DOPAquinone → ... → Melanin

Lippincott's Biochemistry, 8th ed., p. 769

Classification

Albinism is broadly divided into two types:

Type	Structures affected
Oculocutaneous Albinism (OCA)	Eyes + skin + hair
Ocular Albinism (OA)	Eyes only

Most forms are autosomal recessive; ocular albinism is commonly X-linked.

Oculocutaneous Albinism (OCA) - The 7 Types

OCA1 - Tyrosinase Gene Mutations

Gene: TYR (chromosome 11q14) - encodes tyrosinase
Prevalence: ~40% of OCA worldwide; most common in Japanese and Europeans (~1:40,000)
Two subtypes:
- OCA1A: Complete absence of tyrosinase activity. White hair, white skin (does not tan), pink/translucent irises. VA decreased to 20/400. The most severe form.
- OCA1B ("yellow mutant"): Residual tyrosinase activity. Pigment can develop with age (from age 1-3 years); patients can tan. A temperature-sensitive variant (OCA1-TS) exists where the enzyme works only below 37°C - patients develop dark hair in cooler acral areas at puberty.

OCA2 - Most Common Worldwide

Gene: OCA2 (formerly P-gene, chromosome 15q12) - encodes an integral melanosomal membrane protein important for tyrosine transport and melanosomal biogenesis
Prevalence: ~50% of OCA worldwide; up to 1:4,000 in parts of Africa (most common form there)
Formerly called "tyrosinase-positive" or "brown OCA"
Newborns have pigmented hair; nevi and freckles are common; pink irises not usually seen
Pigmentation and visual acuity improve from infancy through adolescence
Important association: Deletions in the chromosomal region adjacent to the OCA2 gene cause Prader-Willi and Angelman syndromes

OCA3 - "Rufous" Albinism

Gene: TYRP1 (chromosome 9p23) - tyrosinase-related protein 1, a structural cofactor for tyrosinase
Primarily seen in Black African patients
Patients have red hair and reddish-brown skin (hence "rufous")
Visual abnormalities may not be detectable

OCA4

Gene: SLC45A2 (MATP gene) - membrane-associated transporter protein
Phenotypically identical to OCA2; variable hypopigmentation; nystagmus in many

OCA5

Mapped to chromosome 4q24
Described in a Pakistani family: golden hair, white skin, nystagmus, photophobia, impaired VA

OCA6

Gene: SLC24A5 - solute carrier protein important in melanosomal architecture
Found in diverse ethnicities; hair color ranges from white to dark brown

OCA7

Gene: C10orf11 - member of the leucine-rich repeat proteins
Andrews' Diseases of the Skin, pp. 1009-1010; Kanski's Clinical Ophthalmology 10th ed., p. 662; Emery's Medical Genetics, p. X

Ocular Features (Common to All Forms)

The ocular phenotype is the most consistent finding across all types of albinism, because melanin plays a critical developmental role in the eye.

Visual acuity: Severely reduced - typically < 6/60 (20/200 equivalent) in OCA1A, due to foveal hypoplasia (the fovea fails to develop properly without melanin signals during embryogenesis)

Nystagmus: Typically pendular and horizontal; worsens in bright light; lessens with age

Iris: Diaphanous and translucent ("pink-eyed" appearance) - in severe forms, the choroidal vasculature is visible through the iris, giving a red/pink color

Fundus: Severe hypopigmentation; large choroidal vessels are conspicuous; absent foveal pit and poorly formed perimacular arcades (see fundus image below)

Optic chiasm: Abnormal decussation - more fibers cross than normal (majority from each eye cross to the contralateral hemisphere), detectable on visual evoked potentials (crossed asymmetric VEPs)

Other: High refractive errors, strabismus, absent stereopsis, photophobia

Oculocutaneous albinism - red iris appearance due to choroidal vessel visibility through depigmented iris

Patient with OCA showing white eyebrows/lashes and red iris color (Lippincott Biochemistry, Fig. 20.21)

Fundus in OCA1 - severe hypopigmentation with visible choroidal vessels and foveal aplasia

Fundus in OCA1 showing severe hypopigmentation, conspicuous choroidal vessels, and foveal aplasia (Kanski's Ophthalmology, Fig. 15.44D)

Kanski's Clinical Ophthalmology 10th ed., pp. 662-664

Syndromic Associations

Several syndromes include albinism as a feature, with serious systemic consequences:

Hermansky-Pudlak Syndrome (HPS)

Inheritance: Autosomal recessive; lysosomal storage disease
Features: OCA + platelet dysfunction (impaired dense granules → easy bruising, excessive bleeding) + pulmonary fibrosis + granulomatous colitis
Can be life-threatening

Chediak-Higashi Syndrome

Inheritance: Autosomal recessive; mutation in LYST (lysosomal regulator protein)
Features: OCA + failure of phagolysosome formation → recurrent pyogenic infections (especially Staphylococcus) + giant cytoplasmic granules in leukocytes + neurological deterioration
Predisposes to accelerated lymphoma-like phase

Waardenburg Syndrome

Inheritance: Autosomal dominant
Features: White forelock, poliosis, synophrys (monobrow), sensorineural deafness, heterochromia iridis, sometimes limb abnormalities
Caused by mutations in PAX3, MITF, or SOX10 genes - not a true melanin synthesis defect but a melanocyte migration/development disorder
Kanski's Clinical Ophthalmology 10th ed., pp. 662-664; Andrews' Diseases of the Skin

Cutaneous Complications

Skin cancer risk: Significantly increased risk of basal cell carcinoma and squamous cell carcinoma (due to lack of melanin photoprotection)
Melanoma risk is also elevated
Dermoscopy is challenging due to absent pigment; reflectance confocal microscopy may help

Management

Concern	Intervention
Sun damage/skin cancer	Aggressive sun protection (SPF 50+ sunscreen, protective clothing, sun avoidance)
Vitamin D deficiency	Supplementation required due to sun avoidance
Visual impairment	Refractive correction; tinted lenses/sunglasses for photophobia
Nystagmus	May benefit from prism glasses; surgical options in select cases
Strabismus	Patching/surgery as appropriate
Skin surveillance	Regular dermatology review; biopsy any changing or atypical nevi

There is no cure; treatment is supportive and preventive. Gene therapy research is ongoing but not yet in clinical use.

Summary Table: OCA Types

Type	Gene	Chromosome	Key Features
OCA1A	TYR	11q14	Complete absence of melanin; most severe
OCA1B	TYR	11q14	Residual activity; pigment may develop
OCA2	OCA2 (P-gene)	15q12	Most common worldwide; tyrosinase-positive
OCA3	TYRP1	9p23	Red/rufous phenotype; mainly African
OCA4	SLC45A2	5p	Like OCA2; common in Japan
OCA5	unknown	4q24	Pakistani families; golden hair
OCA6	SLC24A5	15q21	Variable; diverse ethnicities
OCA7	C10orf11	10q22	Leucine-rich repeat protein

Sources:

Andrews' Diseases of the Skin (Clinical Dermatology), pp. 1009-1010
Kanski's Clinical Ophthalmology, 10th ed., pp. 662-664
Emery's Elements of Medical Genetics and Genomics
Lippincott Illustrated Reviews: Biochemistry, 8th ed., p. 769
The Developing Human (Clinically Oriented Embryology)

This is a shared conversation. Sign in to Orris to start your own chat.