Tuesday, January 20, 2015

More DNA replication = More cancer incidence?

Some science studies become a part of popular science. It has to do with the simplicity of science message discussing commonly understood science topics.

One such paper was recently published in journal Science. The authors has analyzed 31 human tissues with the regard to tissue-specific stem cell proliferation (division) rate and its relationship to tissue-specific lifetime incidence of cancer

The authors concluded that in 65% of times (i.e. 2/3 of times), the rate of tissue-specific cancer incidence could be explained by simple accumulation of DNA replication errors (mutations), occurring by chance, during stem cell division. Basically, more stem cell divides to repopulate aging or damaged cells more chance it has to acquire cancer-promoting mutation and develop into cancerous tissue.

The idea by itself is very simple and technically speaking, correct. There is no surprise that DNA replication can generate somatic, single nucleotide, non-synonymous mutations leading to cancer.

The novel thing about this paper is the fact the authors found "practical" correlation between tissue turnover rate and cancer rate. However, correlation does not necessary imply causation.

The point is that the authors' calculation are based on two parameters: (1) available data regarding tissue-specific cancer rate in population and (2) tissue-specific stem cell division rate.

However, neither of these parameters are "real" data but statistical or "calculated" assumptions. Especially problematic is a calculation of tissue-specific stem cell division rates.

In fact, tissue turnover rate is not a random process at all but influenced by exogenous factors (age, gut microflora, endogenous retroviruses, etc). For example, the authors speculation about the rates of colorectal versus duodenal cancer incidence could simply be explained by the presence of specific gut microflora in those tissues.

David Usharauli

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