by Anna Bennett, PhD; Science Writer, Promega
Short Tandem Repeat (STR) analysis is a foundational piece of forensic DNA profiling, offering robust and reliable genetic information for criminal investigations. However, the intricacies of the STR workflow introduce various challenges, potentially compromising the quality and reliability of the results. Here we introduce the “Perfect” STR profile, then explore the typical pitfalls encountered during STR analysis and offer practical solutions. Our aim is to help forensic scientists navigate these challenges to achieve consistent and accurate outcomes.
The “Perfect” STR Profile
STR analysis is comprised of four steps: extraction, quantification, amplification, separation and detection resulting in an STR profile. Each step must be meticulously executed to prevent errors that could skew the results. In an ideal world the resulting STR profile contains the following attributes:
- Complete: The STR profile is complete, meaning all necessary genetic markers are amplified and identified.
- Intra-Locus Balance: The STR profile contains good intra-locus balance, meaning peaks of each genetic marker are consistent within a certain range. No peaks should stand out amongst the range of peaks.
- Intra-Dye Balance: During STR analysis, multiple fluorescent dyes are used to label different sets of STR markers. Each dye emits light at a specific wavelength, which is detected by the capillary electrophoresis (CE) instrument. Different dye channels correspond to different colors (wavelengths) of fluorescence. Good balance between dye channels means that the fluorescent signals from the different dyes are of comparable intensity and quality.
- Peak Morphology: Similar to balanced peak profiles, each peak should have a similar shape.
- Peak Height: Like peak morphology, each STR marker peak should have a peak height within an expected range.
Because there are many variables in STR analysis, a perfect STR profile is difficult to achieve. If you have had trouble with your STR analysis, you are not alone—there are many common problems at each step of the workflow, each with potential solutions to get you closer to the perfect STR profile.
DNA Extraction: Ensuring Purity and Preventing Contamination
During the extraction phase of STR analysis, several common issues can impact the quality of the results. One major problem is the presence of PCR inhibitors, such as hematin or humic acid. Hematin (a compound found in blood samples) and humic acid (an organic compound in soil) inhibit DNA Polymerase activity, resulting in little to zero amplification of a DNA product and reduced or skewed STR profiles, compromising the accuracy and reliability of the analysis. Another frequent issue in STR analysis is ethanol carryover. If DNA samples are not thoroughly dried after the purification process, residual ethanol can remain, negatively affecting subsequent amplification steps.
Careful consideration during the extraction process can effectively prevent these issues. Many extraction kits are specifically designed to remove PCR inhibitors by including additional washing steps to separate inhibitors from the DNA of interest. Additionally, ensuring that DNA samples are completely dried post-extraction is crucial to prevent ethanol carryover, making it especially important not to shorten drying steps in the DNA extraction workflow.
DNA Quantification: Accurate Assessment for Reliable Results
Accurate DNA quantification ensures that appropriate amounts of DNA are used in subsequent steps. However, several common issues can compromise the accuracy of DNA quantification. For example, poor dye calibration can lead to inaccurate measurements of DNA concentration. This miscalibration can skew downstream processes, resulting in either too little or too much DNA used, which can adversely affect the quality of the STR profiles. Another prevalent problem is evaporation, which occurs when quantification plates are not properly sealed. Inadequate sealing allows the samples to evaporate, causing variability in the measured DNA concentrations and leading to inconsistent results.
Poor dye calibration can be addressed by manually inspecting the calibration spectra. This hands-on inspection helps confirm that the dye calibration is accurate and that the fluorescence readings truly reflect the DNA concentrations. For example, significantly diverging signals or irregular peaks are indicators that dye calibration should be repeated. Evaporation of sample can be resolved using recommended adhesive films and ensuring that quantification plates are properly sealed. These adhesive films are specifically designed to provide a secure seal, maintaining the integrity of the samples during analysis. There are readily available kits on the market that you can use to determine the next steps in your analysis—whether you have obtained quality DNA and are ready for STR amplification steps or your sample has some degradation or you need to dilute the DNA before proceeding. An example of such kit can be found here: PowerQuant® System (promega.com).
DNA Amplification: Achieving Optimal Conditions
The amplification step—where the CODIS core loci and other important markers are amplified—must be carried out under optimal conditions to ensure high-quality and reliable STR profiles. However, there are several potential disruptions to this step, starting with inaccurate pipetting. Incorrect volumes of DNA or reagents lead to imbalanced STR profiles because the precise ratios of reagents are crucial for a complete PCR. Another frequent issue is improper mixing of the primer-pair mix. Insufficient mixing can lead to variability in STR profiles, as the primers are not uniformly distributed throughout the solution. Both inconsistencies lead to uneven amplification, resulting in unreliable or variable STR profiles. More specifically, you may see “allelic dropouts”, where insufficient master mix concentration or too much template is used, causing an imbalance in the overall reaction and key genetic markers are not observed in the sample.
Accuracy is key in achieving optimal conditions in the amplification step. Using calibrated pipettes ensures that the correct volumes of DNA and reagents are dispensed, maintaining the necessary balance for effective amplification. Additionally, thoroughly vortexing the primer pair mix before use ensures that the primers are evenly distributed throughout the solution, promoting uniform amplification across all samples. To remove the problem of human error all together, you may consider partially or fully automating this step with an amplification instrument. See one of our previous Forensic Tips on considerations for automated nucleic acid extraction: The Role of Automation in Reducing the SAEK Backlog (forensicmag.com). Nonetheless, implementing solutions to mitigate human error ensures that the amplification process is carried out under optimal conditions and successful amplification steps.
DNA Separation and Detection: Maintaining Quality Control
The final step in the STR analysis workflow—separation and detection—is crucial for generating clear and interpretable genetic profiles. This step relies heavily on the use of fluorescent dyes and formamide to separate and visualize the amplified DNA fragments. One significant issue that can occur during this step is the use of incorrect dye sets. Each dye set is designed to work optimally with specific chemistries, and using non-recommended dye sets can lead to imbalanced dye channels and the appearance of artifacts in the STR profiles. These imbalances and artifacts can obscure true genetic signals and complicate the interpretation of results.
Another issue is the use of degraded formamide. Formamide is essential for denaturing DNA and ensuring proper separation during capillary electrophoresis. However, poor quality or degraded formamide can cause peak broadening and reduce signal intensity. This degradation often results from exposure to air, leading to the formation of formic acid and ammonia, which interfere with the separation process. The compromised formamide quality affects the resolution of the DNA fragments, making it difficult to accurately read the STR profiles.
To address these issues and ensure high-quality DNA separation and detection, it is vital to adhere to the recommended dye sets for each specific chemistry. Using the correct dye sets ensures balanced fluorescence signals across all dye channels and minimizes the risk of artifacts. Additionally, using high-quality, deionized formamide is crucial. It is important to minimize formamide’s exposure to air to prevent degradation and to avoid re-freezing aliquots, as repeated freeze-thaw cycles can further degrade the formamide. Proper storage and handling of formamide helps maintain its quality, ensuring sharp, clear peaks and consistent signal intensity.
Go Forth and Analyze STR Profiles
By addressing common pitfalls and implementing best practices, forensic laboratories can ensure high-quality results that withstand legal scrutiny. As forensic science continues to evolve, maintaining rigorous standards in STR analysis will be essential for upholding the integrity of forensic evidence. To read more about common pitfalls and solutions in STR profiles, check out this comprehensive STR Troubleshooting Guide.