Academic journal article Scandinavian Journal of Work, Environment & Health

Night Work and Breast Cancer Estrogen Receptor Status - Results from the German GENICA Study

Academic journal article Scandinavian Journal of Work, Environment & Health

Night Work and Breast Cancer Estrogen Receptor Status - Results from the German GENICA Study

Article excerpt

Rabstein S, Harth V, Pesch B, Pallapies D, Lotz A, Justenhoven C, Baisch C, Schiffermann M, Haas S, Fischer H-P, Heinze E, Pierl C, Brauch H, Hamann U, Ko Y, Brüning T. Night work and breast cancer estrogen receptor status - results from the German GENICA study. Scand J Work Environ Health. 2013;39(5):448-455. doi:10.5271/sjweh.3360

Objectives The potential mechanisms that link night-shiftwork with breast cancer have been extensively discussed. Exposure to light at night (LAN) depletes melatonin that has oncostatic and anti-estrogenic properties and may lead to a modified expression of estrogen receptor (ER) α. Here, we explored the association between shiftwork and breast cancer in subgroups of patients with ER-positive and -negative tumors.

Methods GENICA (Gene-ENvironment Interaction and breast CAncer) is a population-based case-control study on breast cancer with detailed information on shiftwork from 857 breast cancer cases and 892 controls. ER status was assessed by immunohistochemical staining. Associations between night-shiftwork and ER-positive and -negative breast cancer were analyzed with conditional logistic regression models, adjusted for potential confounders.

Results ER status was assessed for 827 cases and was positive in 653 and negative in 174 breast tumors. Overall, 49 cases and 54 controls were "ever employed" in shiftwork including night shifts for ≥1 year. In total, "ever shiftwork" and "ever night work" were not associated with an elevated risk of ER-positive or -negative breast tumors. Night work for ≥20 years was associated with a significantly elevated risk of ER-negative breast cancer [odds ratio (OR) 4.73, 95% confidence interval (95% CI) 1.22-18.36].

Conclusions Our case-control study suggests that long-term night-shiftwork is associated with an increased risk of ER-negative breast cancers. Further studies on histological subtypes and the analysis of other potentially relevant factors are crucial for discovering putative mechanisms.

Key terms case-control study; circadian disruption; Germany; health; occupation; shiftwork.

Shiftwork causes circadian disruption and was classified as "probably carcinogenic" (Group 2A) to humans by the International Agency for Research on Cancer (IARC) in 2007 (1). Approximately 25% of German employees are currently involved in schedules of shiftwork (2). Six percent of employed German women work in night shifts on a frequent basis (3). The putative mechanisms that link shiftwork and cancer have been extensively discussed for several cancer sites (4), in particular for breast cancer. With an estimated 1.4 million incident breast cancer cases worldwide in 2008 (5) and rising incidences in Western countries during the last decades, the hypothesis of light exposures during night (LAN) work influencing the circadian rhythm and the hormonal balance has emerged as a relevant research topic (6, 7).

Under normal conditions without night work, the endogenous circadian pacemaker located in the suprachiasmatic nuclei (SCN) synchronizes circadian rhythms to the regular 24-hour light-dark cycle. The retinohypothalamic tract mediates information about the light-dark cycle. Via inhibitory neuronal activity, the SCN controls the production of the neurohormone and indoleamine melatonin (N-acetyl-5-methoxytryptamine). During the daytime, the pineal synthesis of melatonin is almost negligible (8).

Shiftwork as well as lifestyle may lead to altered sleep habits, sleep deprivation, and circadian disruption and is thought to interact with the circadian regulation of hormones that may be also relevant to cancer development. This is supported by the observational study of Flynn-Evans and colleagues (9) where blind women with no perception of light showed a reduced breast cancer risk compared to blind women with light perception. Garland and colleagues (10) hypothesized that lack of sunlight on the skin and consequent inadequate vitamin D production might be another carcinogenic mechanism. …

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