Examination of the use of forensic DNA typing from two perspectives: I. Mitochondrial DNA heteroplasmy. II. The role of DNA typing in criminal investigations

Abstract

DNA typing has been used for forensic identity determination for the past ten years. Recently, mitochandrial DNA (mtDNA) sequence analysis of the hypervariable regions of the mtDNA control region has been shown to be a suitable alternative to nuclear DNA testing. The high copy number and maternal mode of inheritance make mtDNA analysis particularly effective when advanced degradation of biological evidence prohibits the detection of nuclear DNA polymorphisms. However, mtDNA heteraplasmy (presence of two or more subpopulations of mtDNA genomes) and segregation of control region heteroplasmic variants among tissues have not been fully explored. This project examined forensic DNA typing from two very different, but related, perspectives. Part one involved implementation of a denaturant gradient gel electrophoresis system (DGGE) to investigate heteroplasmy in the mtDNA control region, specifically hypervariable region 1 (HV1). DGGE was then used to determine the incidence of HV1 heteroplasmy in flood samples from a random population, and to investigate segregation of heteroplasmic variants among fetal and adult tissues. A high incidence of heteroplasmy was found (13.8%, 95%C.L. 9.6-18.0), in addition to contrasting patterns of segregation of heteroplasmic variants among fetal and adult tissues. The majority of heteroplasmy appeared to be inherited, however, there was also evidence for somatic mutations. Additional evidence suggested that control region variants are not selectively neutral. Part two involved examination of the role that DNA typing plays in the investigation and disposition of criminal cases in four jurisdictions in Maryland. A coding instrument was used to abstract information from files of criminal cases using evidence tested by DNA typing, as well as from non-DNA controls (cases with biological evidence where DNA typing was not performed). This study found that DNA typing had an influence not only in the disposition of criminal cases, but also in the plea bargaining and sentencing processes where the guilt of the defendant had already been established. Overall, this dissertation project further defined the strengths and limitations of mtDNA analysis as a forensic tool, enhanced our general understanding of human mitochondrial genetics, and helped to elucidate the impact that DNA testing has on the adjudication of criminal cases.