Preprocessing and Differential Extraction

Any biological material is a potential source of DNA for analysis. However, the success of DNA typing often depends on the quality and nature of the sample. During an investigation, samples  are often collected on a cotton swab or other solid support. The first step to purify DNA from these samples involves removing the biological material from the solid support, typically by soaking the material in an aqueous buffer. Some samples, such as blood and semen, require a proteinase K digestion at this step for maximum DNA yield and quality. The solid support is removed by placing the entire sample, including the solid support, into a spin basket assembly and centrifuging so that the liquid flows through the spin basket into a collection tube and the solid support remains in the spin basket. The solid support is discarded, and the aqueous DNA-containing fraction undergoes subsequent purification steps to remove PCR inhibitors and other components that may interfere with DNA quantitation and amplification.

Lytic cards are used by many database laboratories as a way of providing long-term storage of blood or buccal samples at room temperature. Chemicals within these cards lyse cells, inactivate bacteria and make DNA available for subsequent amplification. However, these same chemicals can inhibit amplification unless they are first removed by extensive washing procedures. Many Promega PowerPlex® STR Systems are tolerant of these chemicals and allow direct amplification without prior washing . While lytic cards are widely used, they are not the sole solid support available to labs for storing blood or buccal samples. Buccal samples may be obtained with simple cotton or paper swabs. In addition, various non-lytic options exist for storing buccal cells, such as S&S 903 paper. Unlike lytic cards, these supports are not impregnated with chemicals that lyse cells, denature proteins and make DNA available for amplification. For these solid supports, a pretreatment step is required to ensure lysis and release of amplifiable DNA. Buccal swabs pose an additional obstacle in that they are not amenable to punching.

The SwabSolution™ Kit provides a simple and rapid method to generate an extract from buccal swabs that can be added directly to PowerPlex® reactions. The PunchSolution™ Kit is used to pretreat non-lytic card punches prior to direct amplification using PowerPlex® Systems. 

In differential extraction, sperm and epithelial cells are separated. The automated Differex™ System provides an alternative to cumbersome manual methods by automating differential extraction. The Differex™ System works with challenging new and old samples typical of those from sexual assaults. The Differex™ System can be used in combination with the DNA IQ™ System and Slicprep™ 96 Device on robotic platforms to process up to 48 differential extractions in less than 5 hours, including incubation time, and less than 1 hour of hands-on time.