You’ve all heard the common refrain, “Refrain from smoking lest you develop cancer”. These days, everything we do can be a cause for cancer – eating spicy food, using air conditioners, eating fried food, consuming liquor, passive smoking, being exposed to UV rays and a host of other things as well. And the worst part is, even if you remained a teetotaller, stayed indoors, ate only fruits and generally kept away from popularly categorized carcinogens, you still could develop cancer.
I recently read an article on endogenous retroviruses – something that we, as a race, have been carrying for a few million years now (even before the dawn of “mankind” as we know it). Basically these are sequences of DNA in our somatic and germ cells (read body cells and sperms/ova) which were once infectious retroviruses that plagued our primeval counterparts. For all those unaware of the mechanisms of infection of retroviruses – they are viruses that are able to integrate their genetic material into the genome of the host they infect (I will spare you the excruciating details! :P). So philosophically speaking, your children are part you, part your spouse and part virus. Now imagine this process extrapolated to a million plus years. Throw in some natural selection and other evolutionary pressures and we have now, upto 8% of our DNA as viral DNA.
How can we be sure that the DNA was from viruses and not from our parents? Studies have been conducted on the proteins that are transcribed by these long stretches of DNA that generally do not code for anything in our bodies in vivo and it was found that we exhibit genotypes (but not phenotypes, mostly) for viral proteins. Amazing? Scary? Unbelievable? It gets worse. These stretches of viral DNA are able to jump from one location to another within the DNA sequence itself. They seem to be “searching for a way out” of the evolutionary conundrum. Their integration into the host genome wasn’t meant to last so many generations of DNA replication over so many years. Their parasitic behaviour forces them to look for a way out of this conundrum – they need to be able to hijack the host’s cell machinery and make the cell transcribe their proteins. Add to this the fact that most of our cells are just waiting for an excuse to become neoplastic (cancerous). What’s stopping them is some segments of your DNA called “tumour suppressor genes”.
Imagine now that a retrovirus decides to replace this gene or disrupt it in the process of jumping around your DNA. I don’t need to spell it out for you to develop a prognosis (once that happens). And the probability of this occurrence isn’t zero.