Thursday, February 8, 2018

RELIC DNA IN THE HUMAN GENOME

Yesterday's post listed 7 assertions regarding the role of infectious organisms on the human genome. In the next few blogs we'll look at each assertion, in excerpts from Precision Medicine and the Reinvention of Human Disease. Here's the first:

The majority of the human genome consists of relic DNA derived from ancient invasive organisms.

Nearly half of the human genome is filled with sequences such as LINE and Alu, and DNA transposons, all derived from ancient retroviruses [21]. About 8% of our genome is derived from longer sequences with similarity to known infectious retroviruses, and these longer sequences can usually be recognized by their contained subsequences (e.g., gag, pol, and env genes) and long terminal repeats. The viral sequences in our genomes are the remnants of ancient retroviral infections, and the occasional nonretroviral infection, that were branded into DNA, and subsequently amplified [21–23]. Because much of the endogenous retroviral load in the human genome is due to amplification, and subsequent mutation, it is hard to determine the number of retroviral species that established their niche in the human gene pool, but studies of these viral remains would suggest that we contain species from several dozen families of retroviruses, with an undetermined number of contributions from individual family members [24]. Based on comparisons of the viruses present in different species of primates, it would appear that the most recent acquisition of an endogenous retrovirus occurred in humans between 100,000 and 1 million years ago [25]. Most of the retroviral sequences in our genomes are inactivated due to an accumulation of degenerative mutations collected over the eons, indicating that there has been little or no selective pressure to conserve retroviruses in their pristine sequences.

- Jules Berman

key words: precision medicine, human genome, evolution, infectious diseases, jules j berman, Ph.D., M.D.

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