Optimization of a DNA extraction protocol from fingerprints for the analysis of nuclear STR and mitochondrial DNA genetic profiles

Most of the cells found in fingerprints belong to the cornified barrier of the epidermis (stratum corneum), lack nuclei and cytoplasmic organelles, and are filled with keratin. Nuclear and mitochondrial DNA is degraded and embedded in the keratin mesh, a very resistant protein that is difficult to remove during DNA extraction. In this work, we studied the possible negative effect of keratin on Polymerase Chain Reaction (PCR) reactions and the influence of keratinase and proteinase K on the extraction of DNA from fingerprints. The role of glycogen in the DNA yield during the precipitation step and the importance of washing the obtained DNA with 70% ethanol were also studied. DNA was extracted from 96 fingerprints corresponding to recent prints and stored for 0, 1, 5, and 18 months from six individuals. No differences were observed in the concentration of extracted DNA or in the number of nuclear Short Tandem Repeat (STR) alleles in the genetic profiles of fingerprints stored during different times. However, sex differences were observed in both the concentration of DNA obtained and the number of nuclear STR alleles detected, being lower in females than in males. In 80% of the fingerprints genetic profiles were obtained with at least half of the STR nuclear markers and, in 50% of the fingerprints genetic profiles were obtained with more than 90% of the markers, which would allow an unambiguous identification of the donor. In all fingerprints where mitochondrial DNA was analyzed, complete sequencing of the HV1 and HV2 regions was possible, which increases the accuracy of the results obtained. The optimized protocol allowed obtaining a complete STR nuclear genetic profile of a 20-year-old palmprint.