Criminals can’t easily edit their DNA out of forensic databases

Story image for DNA from The Conversation AU

There have been a number of news articles over the last week or so reporting that to avoid being matched to criminal forensic databases, criminals could edit their genomes using cheap, online kits.

What seems to be at the centre of these articles, and giving them a sense of credibility, are some quotes from George Church – a highly respected geneticist from Harvard.

Asked if CRISPR could alter DNA to the extent it would make forensic evidence unusable, Church reportedly told The Telegraph:

We could do that today, easily. A lot of it is done by blood and even if you just get a stem cell transplant you have a new identity.

I could imagine there being an industry.

But is it really so easy? From our perspective there may be some confusion around what is feasible, and what is actually happening now. Let’s unpack some of the issues and think about what would be required to pull off such a feat.

The mainstay of modern DNA identification is short tandem repeat (STR) markers, which are small sections of DNA that vary by length (the number of repeats). Multiple STR markers are used to create a DNA profile.

Most systems now use a panel of 24 DNA markers, but some will allow partial matches of as few as eight or nine markers. It might be possible, in theory, to cheat the system by changing only one of these markers, but in practice a hypothetical DNA-edited criminal would probably want to change several of them.

STR markers are located in the more variable parts of our genome and this may make them more difficult to accurately target with gene editing tools. The easiest way to change your STR profile would probably be to delete some DNA and make the length of that marker shorter.

Technology for reading DNA is getting better, and DNA forensics is currently moving from STR markers to systems that look at more of our DNA and can tell us much more about someone.

In the recent Golden State killer case, so-called “SNP chips” – that measure around 600,000 sites in our genome – were used to make matches to genealogy databases. DNA forensics is a moving field and a future criminal may have to edit much more of their DNA to evade this sort of matching.

But how much of your body would you need to change to avoid detection? Is it just the cells that are used for sampling – for example your cheek cells, your blood cells – or every cell in your body?

As George Church seems to point out, in theory a genetic manipulation to your blood (or another targeted area) could allow a criminal to be excluded as a suspect. In the Golden State killer case, police used “discarded” DNA from the suspect’s trash. To fully evade DNA forensicsyou would therefore likely have to make much more extensive changes (i.e. skin, semen, hair, blood, cheek cells).