A NEW HYPOTHESIS ON THE ORIGIN OF “TOUCH DNA”

The possibility to detect biological traces belonging to the individual responsible of a crime at the crime scene has always been a fundamental target of forensic investigation. The propensity to leave behind genetic material through contact (“touch DNA”) has been demonstrated to genetically differ among individuals depending upon the specificity of each individual’s skin. So far, these DNA traces have been assumed to originate from the keratinocytes sloughed off the upper epidermal layers, resulting in the rather simplistic definition of two categories of individuals, the so-called “good shedders” and “poor shedders”, depending on the degree of their skin’s propensity to leave DNA traces. The assumption that the propensity to leave behind genetic material reflects the shedding of keratinocytes is conventionally taken for granted, hence has dominated the field despite the lack of any solid scientific evidence. Only recently this assumption on the source of skin-derived “touch DNA” has started to be questioned, prompting experimental investigation. The correct identification of the biological samples under analysis is crucial in forensic investigation in that it represents the pivotal issue attesting that the resulting genetic profiles are fully reliable in terms of weight of the evidence, and to avoid the risk of erroneously considering as significant genetic profiles that, although detected at the crime scene, bear no relevance to the crime itself. The study reported herein was performed to experimentally test the hypothesis that sebaceous fluid may represent an important vector responsible for DNA transfer from skin surface to other surfaces (of objects or other individuals’ bodies) that have been subsequently contacted. Genetic analyses were performed in order to examine primary and secondary DNA transfer and, as to demonstrate the possible origin of “touch DNA”, the presence of fragmented single stranded DNA was investigated by immunohistochemical analysis. Our results show that “touch DNA” secondary transfer is indeed possible from person to person and, in turn, from person to object depending on the specific sebaceous or non sebaceous skin area previously touched. In addition, we demonstrate the presence of fragmented single stranded DNA specifically immunodetected in the vast majority of cells forming the sebaceous gland but not in the epidermis layers, strongly indicating that sebaceous fluid represents an important vector responsible for DNA transfer. In view of our results, forensic investigations need to take into account that the propensity to leave behind genetic material through contact could depend from the individual ability to shed sebaceous fluid on the skin surface. These data shed new light on the field of primary and secondary DNA transfer through contact by showing that propensity to leave behind genetic material depends from the variable activity of sebaceous glands