How body clocks impact breast cancer:


Breast cancer is a common cancer affecting approximately 1 in 8 women and 1 in 870 men during their lifetime. There are several factors contributing to the risk of breast cancer including age, breast tissue density and particular alleles of genes involved in breast tissue growth.


The focus of this article is to explore the link between the increased incidence of breast cancer and working abnormal shift patterns (i.e. working a mixture of days and nights during a week) which means that for individuals working in this way, the circadian clock is unable to adjust to a shift pattern before it changes again.

Measuring time:

The passage of time is measured by internal body clocks which run on circadian rhythms. These clocks last for approximately 24 hours. In mammals, circadian rhythms are determined by the amount of daylight. Light intensity is detected by the eye, which signals the time of day to a region in the brain called SCN (suprachiasmatic nucleus) found in the hypothalamus.
Function of SCN:

>acts to coordinate all the individual clocks in the body

>synchronises physiological rhythms including hormone release and sleep/wake cycles

>sends information to the pineal gland, which detects light in animals

The Molecular Clock:

>As well as body clocks, there are molecular clocks known to exist in body tissues. For example, in the breast, the expression of hundreds of genes is controlled in a circadian manner.

>The molecular clock consists of genes and proteins, the activity and production of which fluctuate rhythmically throughout the day and night- according to changes in light intensity detected by the SCN.

>Disruption of these rhythms can increase the chance of breast cancer development.

Circadian genes in breast cancer:

It is thought that there is a link between altered circadian clocks and tumorigenesis. Defects in circadian genes in the breast are thought to disrupt cell cycles, resulting in faulty cell division. This can lead to the development of a tumour in the breast, which can mature and proliferate in the presence of hormones such as oestrogen- increasing the susceptibility to cancer.

Shift-work and night-time light exposure:

Constant light exposure during the night disrupts circadian rhythms in the body. Night-time light exposure affects the production of the hormone melatonin (which plays a role in sleep). The production and release of this hormone in the brain is connected to the time of day. Melatonin levels are known to increase during the night.

More importantly, melatonin has been found to have oncostatic activity in breast cancer. This means that melatonin can slow down cancer development. Given that melatonin is secreted in a circadian manner, working during the night reduces the rhythmic secretion of melatonin- which decreases oncostatic activity, thereby increasing the risk of getting breast cancer.

Conclusion:

The increased incidence of breast cancer in workers with irregular shift patterns has serious implications for modern society. Circadian disruption poses a major public health issue, given that it can affect bodily processes. It is now more important than ever to ensure that sleeping patterns are in synchrony with natural day and night cycles in order to coordinate molecular clocks in the body, help the body to function correctly and to reduce breast cancer risk.

 
Body clock

Diagram showing location of SCN

Breast x-ray showing cancer tissue in white



 By Kumaran Rajaratnam.



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