Shedding light on the role of circadian disruption in breast cancer etiology

Abstract

In this issue of the journal, Papantoniou et al. [1] examine the association between night shift work and the risk for clinical subtypes of breast cancer in a large populationbased case–control study in Spain. The authors assessed different types of night work in a variety of occupations. In addition to lifetime cumulative exposure, they also focused on exposure in critical time periods (before versus after first full-term pregnancy) and evaluated the effect of chronotype (an individual characteristic that correlates with personal preference for morning or evening activity). They report that having ever performed night shift could be associated with a small increase in breast cancer risk (OR 1.18; 95 % CI 0.97–1.44). Neither chronotype, nor exposure to night shift before a first full-term pregnancy appeared to materially affect the associations with breast cancer risk. The risk for the disease appeared to be higher for estrogen and progestogen receptor-positive tumors. The interest on the effects of circadian disruption on breast cancer risk was stimulated in the late 2000s. At that time, on the basis of sufficient evidence from experimental studies and limited evidence from epidemiological studies, the International Agency for Research on Cancer (IARC) classified ‘‘shift work that involves circadian disruption’’ as a probable human carcinogen (Group 2A) [2, 3]. Between 2007 and 2011, close to 100 breast cancer cases were recognized by the Danish National Board of Occupational Injuries as disease caused by night work and became eligible for compensation, a decision that generated a lot of attention worldwide [4, 5]. In 2012, the American Medical Association broadened the topic and released a policy statement on the health hazards of light at night in general [6]. The biological mechanisms by which night shifts can affect the circadian rhythm are far from fully understood, but light exposure at night and decreased length of sleep have both been implicated [7]. Exposure to light at night time can reset the circadian phase and suppress the nocturnal rise in melatonin secretion by the pineal gland [8]. In turn, the melatonin-induced suppression of tumor metabolism is inhibited and the risk of carcinogenesis increases. Furthermore, lower melatonin levels have been reported to generate estrogen signaling, which releases cancer cells from growth inhibition [9, 10]. In support of these hypotheses, laboratory animal models have demonstrated an increased breast cancer risk with lower melatonin levels resulting from exposure to light at night [10, 11], and epidemiologic studies have indicated an inverse association of urinary melatonin metabolite levels with breast cancer risk [12–14]. Since the IARC evaluation [2], both case–control and cohort studies have been conducted and, following the publication of the relevant IARC monograph [3], several reviews/metaanalyses have been published on shift work and breast cancer risk [15–20]. Kamdar et al. [15] reported a weak association (21 % increase in risk for ever nightshift work and 4 % increase in risk for having worked on night shifts for a period of 8 years or longer); given the substantial heterogeneity of studies in their meta-analysis, the authors concluded that there is weak evidence in support of an association. Ijaz et al. [16] estimated an average & Pagona Lagiou pdlagiou@med.uoa.gr

DOI: 10.1007/s10654-016-0193-z

Cite this paper

@article{Lagiou2016SheddingLO, title={Shedding light on the role of circadian disruption in breast cancer etiology}, author={Pagona Lagiou}, journal={European Journal of Epidemiology}, year={2016}, volume={31}, pages={807-810} }