A research group led by Prof. LIU Yong and ZHU Ling from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences developed a new method for tracing forensic DNA extraction using modified lysis buffer and chitosan-coated magnetic beads.

The results were published in Forensic Science International: Genetics.

Forensic DNA is known as the king evidence in the courtroom. However, with the increasing ability of criminal suspects to evade investigation, the DNA evidence left at the crime scene is very limited and contains a large amount of inhibiting substances. Therefore, how to extract enough DNA from trace evidence samples for genetic marker typing has always been a challenge in forensic DNA analysis and testing.

In this study, the researchers developed novel Chitosan@Beads and a modified lysis buffer.

"It can adsorb negatively charged DNA through electrostatic interactions," said ZHU Cancan, a member of the team.

They used 1% (w/v) of chitosan (75% deacetylation degree) to modify the 50 nm magnetic nanoparticles and obtained the Chitosan@Beads, which theoretically carried positive charge in the pH = 5 of lysis buffer.

To simulate the real crime sense, a set of simulated samples, containing 20 mg/μL of humic acid, pigments, iron ions (Fe²⁺, Fe³⁺), and the coexistence of the above three substances, were prepared. Human bronchial epithelial cells were mixed with 200 μL of the above simulated samples for DNA extraction.

In addition, 400 μL of lysis buffer, 20 μL of proteinase K (10 mg/mL) and 20 μL of Chitosan@Beads solution (20 mg/mL) were used for cell disruption and DNA enrichment. The results showed that the sensitivity of DNA extraction using Chitosan@Beads was superior to commercial reagent kits, with a detection limit of 10 cells.

Furthermore, for complex trace samples from case materials, the STR loci rate of the Chitosan@Beads was 97.9%, demonstrating a higher success rate in short tandem repeat analysis, an important technique used in forensic investigation.

This study has broad prospects for application in the fields of forensic DNA identification, ancestry analysis, martyr remains identification and identification of victims of major disasters.

Preparation of Chitosan@Beads. A: schematic diagram of chitosan-modified magnetic nanobeads; B: Schematic diagram of Chitosan@Beads modification steps. (Image by ZHU Cancan)