Melanin protects the skin against ultraviolet radiation-induced damage, thus reducing the risk of cutaneous cancers. Our primary objective is to elucidate the mechanisms that regulate melanin synthesis and characterize the pathogenesis of pigment-related disorders such as albinism. We will focus our efforts on the OCA2 gene, which is mutated in oculocutaneous albinism type 2, the most common form of albinism worldwide. The OCA2 gene is also linked to skin color variation and melanoma susceptibility. The precise function of the OCA2 gene product (P) is not known. We propose that P regulates maturation of tyrosinase, the key melanogenic enzyme, by redistributing intracellular glutathione (GSH) to the endoplasmic reticulum (ER). Mature tyrosinase has a specific tertiary structure, acquired by extensive folding in the ER, and GSH is required during this process. In the absence of functional P protein, tyrosinase fails to mature, significantly reducing melanin synthesis and skin pigmentation.
In Specific Aim 1, we will explore the role of GSH and the P protein in tyrosinase folding. Single nucleotide polymorphisms (SNPs) in the OCA2 gene have been associated with variation in skin and eye color as well as susceptibility to melanoma. We will therefore determine the impact of known coding region polymorphisms on P protein function. We predict that these SNPs reduce P activity such that melanin production is reduced to varying degrees, contributing to the broad range of normal skin pigmentation and accounting for association between SNPs and melanoma risk. OCA2 mutations cause accumulation of tyrosinase in the ER, triggering the unfolded protein response (UPR), a determinant of cell fate.
In Specific Aim 2, we will characterize the melanocyte UPR and determine how p-null melanocytes evade apoptosis that is typical of sustained UPR activation. UPR- mediated cell death has been implicated in diabetes and retinitis pigmentosa, and been targeted for the design of melanoma therapeutics. Our studies may identify novel UPR targets for treatment of these diseases.
In Specific Aim 3 we will screen for small molecules that compensate for lack of P protein function, determine the extent of phenotype correction by hit compounds and characterize their mechanism/s of action. We have successfully identified agents that modulate pigmentation in wild-type cells using this approach and recently identified 16 compounds that induce pigmentation in p-null cells. Selection criteria were: ability to induce melanin synthesis, lack of toxicity and molecular weight <500 to facilitate percutaneous penetration. Validated compounds will be tested on p-null mice to confirm increased pigmentation after topical application. These agents will be vital for treatment of OCA2 and may augment normal skin pigmentation. These experiments will greatly advance our understanding of the pathogenesis of OCA2 and regulation of normal skin pigmentation (a major risk factor for melanoma). We may also identify novel therapeutics for albinism and UPR-related targets for the treatment of diabetes, retinitis pigmentosa and melanoma.
Mutations in the oculocutaneous albinism type 2 (OCA2) gene, which account for the most common form of albinism worldwide, are particularly prevalent among individuals of African-American (1 in 10, 000) and Native American (1 in 28 to 1 in 6,500) descent, while polymorphisms associate with variation in skin and eye pigmentation and susceptibility to melanoma, the deadliest form of skin cancer. Our studies aim to characterize a role for OCA2 to improve our understanding of albinism as well as normal skin pigmentation and to identify pharmacologic therapies for OCA.
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