mtDNA investigations after differential lysis

Area of Science:

• Forensic Science
• Genetics
• Molecular Biology

Background:

• Mitochondrial DNA (mtDNA) sequencing is crucial for determining ethnic origin in forensic investigations.
• In a murder case, sperm DNA was the only evidence linking to the suspect.
• Initial mtDNA analysis of sperm yielded sequences identical to the victim's, raising questions about the results' validity.

Purpose of the Study:

• To investigate the cause of identical mtDNA sequences between sperm DNA and the victim's DNA.
• To determine if the observed mtDNA results were coincidental or due to technical limitations in the DNA testing procedure.

Main Methods:

• Differential lysis was used to extract DNA from spermatozoa.
• Mitochondrial DNA HV1 and HV2 regions were sequenced.
• A systematic study was conducted to evaluate mtDNA sequencing success rates from sperm fractions under varying conditions.

Main Results:

• In cases with complete separation of sperm and female cells, no mtDNA sequence could be obtained from the sperm fraction.
• This failure is attributed to the loss of sperm flagellum and mid-piece, along with mitochondria, during the initial lysis step.
• The initial murder case's misleading mtDNA result was caused by a minor carry-over of female cells into the sperm fraction.

Conclusions:

• Standard differential lysis procedures can lead to the loss of sperm mitochondria, rendering mtDNA analysis unreliable.
• Forensic casework requires careful consideration of potential technical limitations in DNA extraction and sequencing.
• The observed mtDNA profile in the murder case was not from the suspect but resulted from female cell contamination in the sperm fraction.