AMG Lecture 24

DNA Forensics

All humans, with the exception of monozygotic siblings, have numerous differences in their genomic DNA sequence, and therefore are genotypically distinct. A landmark example demonstrating the utility of DNA fingerprinting methods, came in 1985 when Alec Jeffreys of the University of Leicester used restriction fragment length polymorphisms (RFLPs) as genomic markers to determine that a man confessing to murder, could not, on the basis of a statistical argument, be the same person matching the genotype profile found in the crime scene sample. The police then used Jeffrey’s forensic test to survey a large number of possible suspects in the local village.

PCR is a much more sensitive technique than RFLP, and importantly, PCR assays, performed properly, are more amenable to standardized quality control (QC) protocols than is Southern blotting. By using a minimum of three single locus genetic markers, each of which are highly polymorphic (>90%) and stably inherited (<0.2% mutation rate per generation), it is possible, for example, in paternity cases to assign statistically significant values of inclusion or exclusion.

Two examples of how human genotyping is used in DNA forensics for identification are 1) determining who was at a crime scene based on physical evidence left behind and 2) positive identification of human remains. Both of these applications rely on the ability to compare a known genotype to a sample genotype. For example, samples collected at a crime scene or disaster area are genotyped and compared to that of suscepcted criminals or medical records of missing persons.

Criminal Investigations

DNA forensics in criminal investigations can use multiplex PCR with comparative human samples to assign identity in criminal cases as shown in the example below. These data would indicate high probability that the suspect sexually assaulted victim 2.

With the state of technology for genomic analysis currently available, there are numerous forensic cases where it should be possible to set the record straight by determing lineages, disease states or even identity, by PCR-based SNP analysis. While in the case of criminal DNA forensics, current law supports the prosecution's quest for knowledge, that is not always the case in molecular anthropology as demonstrated by these three popular cases.

Identification of human remains - the Romanovs

Human Origins and Culture

Tracing evolutionary roots to Eve in central Africa

Tracing tribal migrations and effects on cultural isolation

Molecular genetic pathophysiology
The application of molecular genetics to the development of improved disease diagnosis and treatment strategies, is an emerging area of medicine that has been described as molecular genetic pathophysiology. Two such examples of this new technology are;

1) Molecular genetic diagnostics for the purposes of embryo preimplantation decisions

2) Personalized disease treatment strategies called pharmacogenetics

Preimplantation genetic diagnosis (PGD) is a method used to genotype human embryos for known genetic defects prior to uterine implantation. Removal of a single cell at the 8 cell stage is can be done in the laboratory to provide material for PCR analysis. Selected embryos are then re-implanted into the womb.

In this hypothetical example, the Factor VIII gene defect is carried on one of the two X chromosomes on the maternal side. PEP/PCR amplification is used to randomly amplify the entire genome. Amplification of the ZFX and ZFY regions of the X and Y chromosomes, respectively, provides is diagnostic test for embryo gender.

Pharmacogenetics refers to a molecular genetic approach to the personalized prevention, diagnosis and treatment of disease. In this scenario, genotype fingerprinting would be performed early in life to identify gene mutations known to be associated with an increased risk of lung cancer. Early detection methods for lung cancer would include routine PCR-based DNA microchip analysis of lung fluid (sputum) to detect rare cancer cells that would be indicative of disease, and blood samples would be analyzed to detect tumor metastasis.

List two advantages and two potential problems in the use of PCR based assays for DNA Forensics.

Why are three independent genetic markers required to establish inclusion or exclusion parameters in DNA Forensic tests? What would be the theoretical lowest probability required to identify an individual as the one and only suspect in a crime?

What is the principle of PEP-PCR and why is it required for PGD in cases where mulitple genotypes are being assesed?

Currently, what are two technical limitations to using DNA chip-based genotypic fingerprinting to routinely identify genetic predispositions in newborns? What are two ethical considerations that need to be resolved before this will happen even if it is technically possible?


Lecture 24 - DNA Forensics and Pharmacogenomics