Working night shifts has been associated with an increased risk for certain cancers, as well as other health disorders. Indeed, the World Health Organization’s International Agency for Research on Cancer (IARC) has classified night shift work as “probably carcinogenic to humans.”
But why night shift should elevate the risk for cancer has been unclear.
A new study shows that a simulated night shift schedule significantly altered the normal circadian rhythmicity of genes that are involved in cancer hallmark pathways. It also found that this circadian misalignment caused circadian dysregulation of genes involved in key DNA repair pathways.
“Taken together, these findings suggest that night shift schedules throw off the timing of expression of cancer-related genes in a way that reduces the effectiveness of the body’s DNA repair processes when they are most needed,” said co-corresponding author Jason McDermott, a computational scientist with the Pacific Northwest National Laboratory’s biological sciences division in Richland, Wash.
The study was published online in the Journal of Pineal Research.
Study conducted among volunteers
The study was carried out among healthy volunteers who were subjected to simulated night shift or day shift schedules.
The cohort comprised 14 adults between the ages of 22 and 34 years who had normal nighttime sleep schedules. They were randomly assigned (seven in each group) to a simulated day shift schedule that involved 3 days of daytime wakefulness (6 a.m.-10 p.m.), or a simulated night shift schedule involving 3 days of nighttime wakefulness (6 p.m.-10 a.m.).
After the 3 days of simulated shift work, all participants were then kept in a constant routine protocol (used to study humans’ internally generated biological rhythms independent of any external influences). As part of the protocol, they were kept awake for 24 hours in a semi-reclined posture under laboratory conditions with constant light exposure and room temperature and evenly distributed food intake (hourly isocaloric snacks).
Blood samples were collected at 3-hour intervals and used for leukocyte transcriptome analysis and DNA damage assessment.
The authors found that the circadian expression of canonical clock genes was substantially altered by the simulated night shift schedule vs. the day shift schedule. Four genes (CRY1, CRY2, PER2, and NR1D2) lost their normal day-shift rhythmicity following the night shift schedule, and NPAS2 gene expression was not rhythmic during the day shift but exhibited circadian rhythmicity in the simulated night shift condition. Three other genes (NR1D1, PER3, and DBP) were significantly rhythmic during both shifts.
The team also looked at the effect of night shift on circadian rhythmicity in cancer hallmark genes, using a panel of 726 genes. The analysis showed that:
- 257 (35.4%) were rhythmic after at least one of the two simulated shift work conditions.
- 113 (15.6%) were rhythmic in day shift only.
- 96 (13.2%) were rhythmic during night shift only.
- 48 (6.6%) were rhythmic during both shifts.
A subset of 10 (1.4%) genes exhibited a significant phase advance (3.7 to 8.3 hours) or phase delay (2.8 to 7.0 hours) during the night shift vs. the day shift.
Thus, the authors concluded, shift work caused significant disturbances in the rhythmicity of gene expression in cancer hallmark pathways.
Findings also showed that night shift work increases endogenous and exogenous DNA damage. Endogenous DNA damage was generally higher after the night shift compared to the day shift, and across the 24-hour constant routine the percentage of cells with BRCA1 and g H2AX foci was significantly higher for night shift.