Does evening sun increase the risk of skin cancer?

Abstract

Circadian clocks are time tracking systems for all organisms, including mammals, to adjust to the day/night cycle in regard to physiological behaviors. Usually, a 24-h periodicity is observed in the rhythms of mammals related to feeding, sleep and other behaviors. However, the cycle can also be reset by light or other external signals. Circadian regulation involves a central clock in the hypothalamus and peripheral clocks located in most mammalian tissues (1). Circadian regulation has been observed in a vast array of genes involved in cell cycle and growth, metabolism, survival, and DNA damage responses. In mammals, circadian oscillation of specific genes has been observed in most but not all organs. Sancar and coworkers showed that UV damage repair in mammals via the nucleotide excision repair (NER) pathway is regulated by circadian rhythm (2). The UV part of the solar spectrum is relatively small but has profound impact on human health, primarily because of the damage UV photons cause to the genome (3). UV causes multiple damages in DNA bases, the predominant among which are pyrimidine photoproducts (PPs) including cyclobutane pyrimidine dimers (CPDs) and (6-4) PPs (4). These lesions in the genome block replication (and transcription) and are highly toxic. Several specialized DNA translesion synthesis polymerases replicate these photoproducts (5) without using normal base pairing and often induce mutations at the photoproduct sites. Such mutations in tumor suppressor genes such as p53 and PTEN could lead to melanoma and nonmelanoma skin cancers (6, 7).