Use of bleach to eliminate contaminating DNA from the surface of bones and teeth

The extraction of DNA from archaeological or forensic skeletal remains can provide quite powerful data for analysis, but is plagued by a unique set of methodological problems. One of the most important methodological problems to overcome in such analyses is the presence of modern contamination on the surfaces of bones and teeth, which can lead to false positives and erroneous results unless it is removed before DNA extraction is initiated. Ancient DNA (aDNA) researchers and forensic scientists have employed a number of techniques to minimize such contamination. One such technique is the use of bleach (sodium hypochlorite--NaOCl) to "destroy" contaminating DNA. However, a consensus on the optimum concentration of sodium hypochlorite to be used and the amount of time the bone or tooth should be exposed to it has not emerged. The present study systematically approaches the issue by introducing contamination to ancient bones (from approximately 500 BP) and determining which of several sodium hypochlorite treatments best eliminates surface contamination. The elimination of surface contamination from bone requires immersion in at least 3.0% (w/v) sodium hypochlorite (approximately equal parts of commercial bleach and water) for at least 15 min. Endogenous DNA proved to be quite stable to even extreme sodium hypochlorite treatments (6% for 21 h), suggesting that DNA adsorbs to hydroxyapatite in the bone and that this process facilitates the preservation of DNA in ancient skeletal remains.