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Heteroplasmy and Forensic Mitochondrial DNA Testing

Heteroplasmy and Forensic Mitochondrial DNA Testing. By Marc Cary. Why Test Mitochondrial DNA?. 1,000 – 10,000 copies per cell Only 16,569 bp Small sample size needed Badly degraded samples work. Sources of mtDNA. Hair Blood Muscle tissue Skin. What is Heteroplasmy?.

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Heteroplasmy and Forensic Mitochondrial DNA Testing

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  1. Heteroplasmy and Forensic Mitochondrial DNA Testing By Marc Cary

  2. Why Test Mitochondrial DNA? • 1,000 – 10,000 copies per cell • Only 16,569 bp • Small sample size needed • Badly degraded samples work

  3. Sources of mtDNA • Hair • Blood • Muscle tissue • Skin

  4. What is Heteroplasmy? • Random mutations that occur in mtDNA • Up to 6 mutations in some individuals • Leads to different versions of mtDNA in an individual • Once thought to be extremely rare

  5. Causes of Heteroplasmy • Molecular events not quite understood • Most probable explanation: mutations in female germ line followed by subsequent differentiation during embryonic development (Table 1) • HVI and HVII (Hypervariable sequences) are typically analyzed for forensic use • Hotspots are prone to heteroplasmy (Table 1)

  6. Causes of Heteroplasmy

  7. Detection of Heteroplasmy • Detection depends upon procedure used to analyze mtDNA • Standardization is needed for mtDNA typing procedures.

  8. Heteroplasmy and Human Hair • Human hair has high frequency of heteroplasmy (stem cell formation) • Example: hair from same individual may have different mtDNA • Although hair contains heteroplasmy, other tissues from same individual may not ie. blood, muscle, skin

  9. Heteroplasmy complicates forensic mtDNA testing • May not always be detected depending on the tissue type and analysis procedure • May be difficult to form a basis of exclusion • In case of non-mutation, sequence frequency is determined form an mtDNA database

  10. Heteroplasmy Benefits Forensic mtDNA Testing • Rarity of a particular heteroplasmy may increase discriminating power

  11. Take Home Messages • An Individual may have different versions of mtDNA • Heteroplasmy may complicate mtDNA testing, but may also make it more efficient • Tissue type is important when analyzing mtDNA • Standardization is needed in mtDNA testing procedures

  12. References • T. Grzybowski, Extremely high levels of human mitochondrial DNA heteroplasmy in single hair roots, Electrophoresis 21 (2000) 548-553 • P. D’Eustachio, High levels of mitochondrial DNA in human hairs by Budowle et al., Forensic Science International 130 (2002) 63-67 • W. Shields, SUNY-Syracuse

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