Mitochondrial DNA Typing (World of Forensic Science)
The field of forensic science has benefited significantly from the identification, characterization, and basic understanding of the mitochondria. The mitochondrion is a subcellular organelle that is located within the cell and functions to produce energy for various tissues of the body. It contains its own genome distinct from the genome found in the nucleus (nuclear DNA) due to many features, including: how it is inherited; how it is replicated; its copy number; and its size. Mitochondrial DNA is circular, double stranded, and inherited maternally.
Mitochondrial DNA typing is a method used by forensics scientists to match DNA from an unknown sample to a sample collected at a crime scene. It is ideally used in special cases where the DNA is degraded or the source of the sample doesn't contain enough genomic nuclear DNA for analysis. As it is maternally inherited, the DNA from siblings and all maternal relatives should be identical (in the absence of spontaneous mutations). For this reason, the remains of missing persons can be rapidly identified by using mitochondrial DNA analysis of relatives. Additionally, there is generally a lack of recombination, an event that takes place during nuclear DNA cell division in which two stands of DNA cross over and exchange information, thereby creating greater sequence diversity. Therefore, even matriarchal relatives separated by several generations can serve as reference samples. Nuclear DNA samples cannot provide this function, due to multiple recombination events that take place throughout the nuclear DNA genome.
The two genomes are not mutually exclusive, instead they rely on each other for survival. The nuclear DNA can encode roughly 1,000 proteins that are targeted for the mitochondria and play a role in oxidative phosphorylation, or energy production, while the mitochondrial DNA produces energy by producing ATP as well as several other functions. All other DNA typing systems use nuclear DNA analysis.
There are several advantages to studying the mitochondrial DNA of a sample. The application of mitochondrial DNA analysis in forensic sciences stems from characteristics of the mitochondrial DNA genome, including its copy number within the cell, its hypervariable region, its size, and its sequence variations. The mitochondrial genome is roughly 16,569 base pairs in size (compared to the 3 billion base pairs in the nuclear DNA). Whereas nuclear DNA has only two copies of each gene, tightly woven into chromosomes, mitochondrial DNA can be copied 20 times per mitochondrion and there can be hundreds to even thousands of mitochondria per cell. With the mitochondria's role as an energy provider, different tissues contain different amounts of mitochondrial DNA, depending on the energy requirements of the cell. A higher copy number equates to greater sensitivity. This is particularly important if the DNA sample is significantly degraded, or the DNA is present only in a very small quantity. The likelihood of recovering mitochondrial DNA from a small or degraded sample is, therefore, greater in mitochondrial DNA samples compared to nuclear DNA samples since the mitochondrial DNA has a larger copy number.
The low fidelity of DNA repair mechanisms to correct specific mitochondrial DNA mutations has lead to a 50 fold higher mutation rate, and, in turn, a higher rate of evolution. Human identity testing employs these regions where there is hypervariability as a consequence of a higher mutation rate. Two hypervariable (HV1 and HV2) regions are part of a control region. On average, there are roughly 8 nucleotide differences between Caucasians and 15 differences between individuals with African decent in these two hypervariable regions. Mitochondrial DNA typing using HV1 and HV2 can be readily performed by using a mitochondrial DNA-specific polymerase chain reaction and amplification of genomic mitochondrial DNA. This is followed by direct DNA sequencing and identification of sequence variations.
The sample source can often determine which DNA typing system represents the ideal approach. For example, if a hair is left at the scene of the crime, nuclear DNA can only be analyzed if the root is intact. However, mitochondrial DNA can be analyzed from anywhere along the hair follicle, including the shaft. Bones and teeth also contain mitochondrial DNA and can be used in mitochondrial DNA analysis.
There are several disadvantages of using mitochondrial DNA typing in forensics in lieu of nuclear DNA markers. As all individuals of the same maternal lineage are virtually indistinguishable by mitochondrial DNA analysis, identification of the remains of an individual would not be possible without comparing it to maternally-related relatives. Additionally, using mitochondrial DNA analysis to match a suspect to a sample by comparing different genomic locations might reveal a similar profile. Mitochondrial DNA should not be viewed as a unique identifier, since seemingly unrelated individuals might have an unknown shared maternal relative in their distant past. If this is the case, a mistaken match might be suggested. Finally, using a more sophisticated (multi-locus) nuclear DNA analysis will provide far greater discriminatory power.
SEE ALSO DNA fingerprint; DNA profiling; DNA sequences, unique; DNA typing systems.