Identificação por DNA de agressores sexuais no Rio Grande do Sul : caracterização da melhor sistemática para obtenção de perfil genético autossômico com finalidade de confronto em banco de DNA criminal

Abstract

The numbers of female rape victims are high in Brazil and Rio Grande do Sul (RS). In Division of Forensic Genetics in General Institute of Forenscis (DFG-GIF-RS) more than 2000 cases of sexual assault are awaiting analysis. Because of the close contact between victim and aggressor, DNA traces of the perpetrator are often available and can be used to identify the criminal. In Brazil, recently, the National Bank of Genetic Profiles (NBGP) was established, and becames a very useful tool for cases with no suspect, as it allows the confrontation between DNA profiles obtained from material collected at crime scenes and victims. Thus, this study sought to define, through the analysis of various characteristics of sexual assault cases, priorities for our large DNA backlog, aiming to get autosomal profiles of criminals for insertion into NBGP. For this, we evaluated the rate of obtaining DNA in 272 closed cases by the DFG-IGP-RS, relating to parameters such as: victim's age, relationship between aggressor and victim, post-coital interval, interval between aggression and DNA tests, tests for the presence of sperm and semen, type of DNA extraction, among others. Adding to this, the DNA samples from 31 cases was extracted, first, with no differential protocol (ND). If was obtained Y chromosome profile at first sample and if samples were collected in duplicate, the evidence was extracted using the differential extraction protocol (ED) standardized by the FBI and recommended by the National Public Security Secretariat. Evidence collected in triplicate, were submitted to different types of incubation (n=16 samples). Through the analysis of 272 cases closed, the total rate of autosomal profile detection of the perpetrator (mixture or single source) was 17.3%, with 90% of cases extracted with ND method. During the analysis of pending cases with DE method, the rate was 32.26%, showing an increased efficiency of obtaining profiles for insertion into the NBGP. Regarding the modified incubation protocol, we could not identify a pattern in male DNA detection, requiring more studies. From the data collected from 272 closed cases, we define the most likely case profile to obtain autosomal genetic DNA of the aggressor: post-coital interval less than 24 hours (autosomal profile detected in 27.6%), time elapsed between aggression and analysis less than two years, evidence positive for spermatozoa or prostatic specific antigen, prioritizing the analysis of clothing (autosomal profile detected in 41.6%) compared to vaginal swabs (28.1%). In addition, it was found that recidivist behaviour is more likely amoung unknown perpetrators (48.2%) than known (14.3%). This fact is important when considering the purpose of our study of feeding the NBGP. Due to the recent establishment of NBGP, we do not know a similar study conducted in Brazil, making our study an important landmark for other realities of Brazilian forensic laboratories.