Non-self DNA on the neck: a 24 hours time-course study

Forensic Science International: Genetics, 2022

Authors

Journal

Forensic Science International: Genetics

DOI

10.1016/j.fsigen.2022.102661

Study Design

Addressed Question

To investigate the persistence and accumulation of non-self DNA on the human neck over the course of 24 h.

Activity Context

Social Contact

Depositor & Contact

Depositor Characteristics

Sex, living arrangement, relationship status.

Criteria for Shedder Status

n.a.

Previous Activities

Participants showered in the morning, used a clean towel, were instructed not to wash the neck until all samples were collected, and not to apply cosmetics before the first sample. They then went about their normal activities. Questionnaire variables included public transport, public spaces, social contacts, cosmetics, and unwashed clothing/items.

Contact Scenario

n.s. No experimentally imposed contact sequence. After a morning shower, participants went about normal daily life. Samples were collected at T0 immediately after showering, T1 after 7–8 h, and T2 after 24 h. Weekday T1 samples were generally collected at the workplace; weekend samples represented predominantly home environments. Cohabiting participants could have contact with partners/children during the day.

Primary Substrate

Primary Substrate Type

Neck

Primary Substrate Material

Skin

Deposit

n.a. No intentional deposit was made.

Delay

n.a. There was no primary-to-secondary transfer sequence.

Secondary Substrate

Secondary Substrate Type

n.a.

Secondary Substrate Material

N/A

Secondary Substrate Contact

n.a.

Further Transfer

n.a.

Sampling

Background DNA on Sampled Surface

Sampled

Sampling Time

0 h, 7–8 h and 24 h after the morning shower, on both a weekday and a weekend day.

Persistence

0–24 h under normal daily-life conditions.

Sampling Method

Moistened cotton swab

Sampling Area

n.s.

Laboratory Analysis

Extraction

Chelex® 100 (Bio-Rad laboratories)

DNA Quantification

PowerQuant® System (Promega)

Input for Profiling

1.0 ng DNA per PCR reaction, 25 µL total reaction volume

Profiling

Amplification: PowerPlex® Fusion 6C System (Promega). Capillary electrophoresis on Applied Biosystems 3500xL Genetic Analyzer. Injection: 24 s at 1.2 kV. Analysis software: GeneMapper ID-X v.1.6. (Life Technologies) with an AT set to 100 RFU.

Reference Samples

Participant reference DNA profiles were obtained from oral mucous samples. 13 of 16 of partners to the participants provided reference samples. .

Profile Interpretation and Mixture Analysis

GeneMapper ID-X v1.6, analytical threshold 100 RFU. Profiles were reviewed by two researchers. Minimum number of contributors was estimated using allele counting. Mixtures were additionally analysed using EuroForMix v3.2.0, including mixture proportions and likelihood ratios. A degradation model was used; Fst = 0.01, drop-in probability = 0.05 and λ = 0.01.

RNA Data Interpretation

n.a.

Results

DNA Quantity

DNA concentration ranged from 0.0004 to 4.69 ng/µL, with a mean of 0.18 ng/µL.

Profile Quality

Overall, 44 single-donor profiles and 73 two to three-person mixtures were detected among the analysed samples. Full and partial donor profiles were obtained from 38% of samples, 53% produced two-person mixtures and 9% three-person mixtures.

Parameter Used for Comparison

Number of non-self alleles, number of unique partner alleles, number of unknown alleles, total allele count, DNA concentration, adjusted average RFU, mixture proportions/contributor proportions; number of mixture contributors, LR values for partner/child hypotheses.

Summary of Results

Non-self DNA accumulated on the neck over 24 h. The number of non-self alleles increased significantly from T0 to T2. Partner/children DNA could already be detected immediately after showering and generally increased throughout the day. Unknown contributors were detected only at T1/T2 and occurred in 11 samples (9%). Mixtures became more frequent over time: at T0, 42% were two-person mixtures; at T1, 57.5% were two-person and 12.5% three-person; at T2, 59% were two-person and 15% three-person. DNA concentration itself did not significantly differ across time points or weekday/weekend, whereas the number of non-self alleles did increase over time.

Raised Questions

The authors identify the need for further investigation of how different living arrangements affect the composition and amount of non-self DNA on the neck, particularly because the “alone,” “flatmates” and “partner & flatmates” groups were small. They also identified the need for a further investigation, by a controlled study, to study how hair length affects non-self DNA present on the neck.

Cautionary Remarks

A limitation of this study is that three participants did not provide their partners reference sample. the study was conducted during COVID-19 restrictions, including social distancing, masking and increased hygiene, which may have reduced environmental/social DNA transfer