DNA Transfer and Contamination

Transcription

1 DNA Transfer and Contamination Simon Ford, Ph.D. LEXIGEN San Francisco Florida Forensic Science DNA Bootcamp Orlando February 25 th, 2017

2 The Transferability of DNA (Cartoons courtesy of William C. Thompson)

3 WCT

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12 Primary, Secondary and Tertiary Transfer Source Evidentiary Item Primary Transfer Source Primary Transfer Vector Secondary Transfer Evidentiary Item Primary Transfer Secondary Transfer Tertiary Transfer WCT

13 1988 to about 2000 Late 1990s to NOW RFLP DQ-alpha Automated STR > 100 ng ng ng As DNA tests become more sensitive, trace levels of DNA that would previously have been below the detection threshold of the test become detectable LEXIGEN

14 DNA tests keep getting more and more sensitive Standard test Test designed to fail if insufficient DNA Get partial profile Tests are getting more and more sensitive Improved chemistry More sensitive genetic analyzers (instruments)... and they are only going to get more sensitive with time Enhanced sensitivity: Post amplification clean-up Increased cycle PCR Consensus approach LEXIGEN

15 Transfer commonsense Wet biological material transfers better than dry material Porous substrates (cotton cloth) are better at absorbing biological material than non-porous substrates (plastic) Mode of transfer Rubbing (friction) > pressure > touch. Ideal secondary transfer: Wet bloodstain on plastic bag rubbed by cotton cloth LEXIGEN

16 The Lukis Anderson Incident

17 Lukis Anderson: Story San Francisco Bay Area murder Minor DNA profile from male victim s fingernails uploaded to CODIS Weak OT+ 1.1 ng total human 0.6 ng male Hit against Lukis Anderson Facing life/death Five months behind bars Also Y-STR (confirmed) Airtight alibi: Drunk (0.40 BAC) and unconscious in hospital at the time Same paramedics who had treated Anderson at liquor store responding to murder two hours later LEXIGEN

18 Lukis Anderson LEXIGEN

19 Lukis Anderson Cal SDIS Hit: Match LEXIGEN

20 Lukis Anderson Cal SDIS Hit: Search LEXIGEN

21 Lukis Anderson CODIS Upload: Blue Loci LEXIGEN

22 Lukis Anderson Y-STR: Yellow Loci LEXIGEN

23 Lukis Anderson: Could not be the perpetrator LEXIGEN

24 Lukis Anderson: Response I m certain they were wearing gloves. We don t have any idea how it specifically happened. But it could be as simple as their bodies and their clothes coming into contact with Mr. Anderson and then also Mr. Kumra Kevin Smith (Santa Clara County DA) Referring to the unique nature of the incidents This is not going to happen on a regular basis Kevin Smith (Santa Clara County DA) We don t have any definitive answers as to what may have caused the transmission of DNA going from one person to another. Once there s a conclusive decision on how that happened, we definitely can look at preventative measures going forward Brian Hubbell (Rural Metro Ambulance Company) LEXIGEN

25 The murder of Meredith Kercher

26 Murder of Meredith Kercher Bra clasp collected from murder room 46 days after death Video evidence shows handled improperly Dropped on floor/picked up Handled alongside other items without changing gloves Knife collected from Sollecito s flat 4 days after death Unclear whether murder weapon Transported in makeshift box Handled by personnel who had just returned from the murder room LEXIGEN

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31 DNA Transfer in the lab Often, but not necessarily, within the same batch Extraction stage is hot spot Extraction control/reagent blank helpful for monitoring for systemic contamination but not so good for inadvertent transfer (sample handling errors) Usually detected due to unexpected profile in: Negative control (reagent blank, amplification neg) Positive control Reference Standard (ladder) Review Log of Unexpected results and Corrective Actions (aka EDNA Log ) LEXIGEN

32 From Corrective Actions 5% 28% 29% 38% LEXIGEN

33 A clean RB does not mean that contamination did not happen The negative control is simply a blank or empty tube run concurrently with the samples in the laboratory beginning at the extraction stage of the analytical process. Consequently, it can only be used as a control for potential reagent contamination. It cannot be used to discount possible contamination, either at the crime scene or in the examination room. The mistaken belief by the trial judges that negative controls somehow provided a guarantee that contamination had not occurred and the application of the reverse burden of proof. Peter Gill: Analysis and implications of the miscarriages of justice of Amanda Knox and Raffaele Sollecito LEXIGEN

34 Key scientific literature on transfer

35 Misleading DNA Evidence: Reasons for Miscarriages of Justice by Peter Gill Elsevier 2014 MUST READ!

36 DNA fingerprints from fingerprints (van Oorschot & Jones, Nature, 1997) DNA recovered from handled items Objects regularly held: Briefcase handle: 75 ng Personal locker handle: 3.7 ng Telephone handset: 10.3 ng Pre-cleaned objects held once: Knife handle: 17.8 ng Mug: 6.8 ng Glass: 34 ng Objects handled by many persons: Last handler usually found, but often earlier handlers too Strongest profile not always last handler Observed transfer from object to handler (previous handlers) Secondary transfer LEXIGEN

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44 Handling Items

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46 Could secondary DNA Transfer Falsely Place Someone at the Scene of a Crime (Cale et al., JFS 2015) Source (S) and Vector (V) shake hands V handles a knife Handle of knife swabbed for DNA; assayed with IdentifilerPlus 24 replications DNA profile of S detected on 15 of 24 knives 63% S was minor and V major on 4 of 24 S and V were equal contributors on 3 of 24 S was major or sole contributor on 5 of 24 S included in a mixture of 3 or more contributors on 3 of 24 LEXIGEN

47 Could secondary DNA Transfer Falsely Place Someone at the Scene of a Crime (Cale et al., JFS 2015) LEXIGEN

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49 The origin of unknown source DNA from touched objects (Buckingham, Harvey & van Oorschott, FSIG 2016)

50 The origin of unknown source DNA from touched objects (Buckingham, Harvey & van Oorschott, FSIG 2016) LEXIGEN

51 Laundry transfer

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55 Flies

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58 Fingerprint Brushes

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62 Habitual Wearer

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65 Secondary detected without vector

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67 Is it primary or secondary transfer?

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70 Shedder status

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75 Levels of transfer

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78 Primary Transfer Secondary Transfer Tertiary Transfer The End WCT