- Variable inheritance (locus heterogeneity); vs ocular albinism (x-linked recessive).
- Congenital deficiency of either
- tyrosinase (can’t make melanin from tyrosine; aut-rec);
- defective tyrosine transporters (dec. tyrosine = dec. melanin); can also be from lack of migration of neural crest cells.
- Increased. risk skin cancer
A newborn has milky white skin, white hair, and redappearing eye color (see the fi gure below). This disorder most often results from a defect in which of the following enzymes?
(A) Phenylalanine hydroxylase
(B) NADPH oxidase
(C) Dihydrofolate reductase
(E) Homogentisic acid oxidase
2 The answer is D:
Tyrosinase. The child has albinism, a lack of pigment in the skin cells, which is produced by melanocytes. Melanocyte tyrosinase (a different isozyme than the neuronal tyrosinase that produces DOPA for catecholamine biosynthesis) is defective in albinism. The DOPA produced is then used for pigment production. A lack of phenylalanine hydroxylase leads to PKU. A lack of dihydrofolate reductase is most likely a lethal event as there are no reported cases of a lack of this enzyme. Tetrahydrofolate is not required for the conversion of tyrosine to DOPA in melanocytes. NADPH oxidase generates superoxide, which is not part of this pathway. Homogentisic acid is part of the phenylalanine and tyrosine degradation pathways, and is not involved in albinism.
Pick out the correct statement regarding albinism:
- Increased tyrosinase activity will be observed in melanocytes
- It results in photophobia
- Mental retardation is the main characteristic feature in albinism
- Skin has high pigmentation in albinism
ANS: B. Albinism is due to defect in tyrosinase enzyme, which is required for the synthesis of melanin in melanocytes from tyrosine. Lack of melanin pigment in the eyes, albinos sensitive to sunlight (photophobia), skin has low pigmentation. Mental retardation is not observed in albinism.