Whose DNA is it, anyway?
Immunoglobulin, the T cell receptor, and the Shroud of Turin
Below is a brief synopsis of the paper “DNA on the Shroud of Turin: Distinguishing endogenous versus exogenous DNA” recently published on shroud.com. The full- length manuscript is available at http://www.shroud.com/pdfs/kearse2.pdf.
In the 1990s, Garza-Valdes reported in the book “The DNA of God” the cloning and sequencing of human gene segments from blood remnants on the Shroud. The presence of human DNA on the Shroud is sometimes viewed as corroborative evidence that the bloodstains are composed of real blood. Mature red blood cells in the human lack a nucleus and do not contain DNA; any DNA present from bloodstains must come from the white blood cells, which include B and T lymphocytes, or B and T cells for short. It is the DNA present within these two cell types that could prove useful in verifying that (certain) DNA on the Shroud is truly from blood cells and not from contaminating DNA, originating from other sources (see below).
Contamination of handled objects: Touch DNA
A major issue that persists in DNA analysis of objects that have been handled by numerous individuals is contamination. The average person sheds roughly 400,000 skin cells per day, a portion of which contains DNA that may be transferred to handled objects, referred to as touch DNA. DNA may be transferred by direct contact with the cloth, or by touching an object, which then comes in contact with the cloth (or with threads removed from it). A determined skeptic might argue that contaminating DNA is responsible for the previous results, a charge that is somewhat difficult to counter.
The three gene segments that have been isolated and analyzed from the Shroud are: the betaglobin gene (a subunit of hemoglobin), and the amelogenin-X and amelogenin-Y genes (present on the X and Y chromosomes, respectively). Such genes are not exclusive to blood cells, but are present in essentially all cells throughout the body, including skin cells. Thus, analysis and sequencing of such gene segments is unable to distinguish if they originated from DNA from bloodstains present on the cloth, or from contaminating DNA from exogenous sources, i.e. skin cells.
Same technique: Different Target Genes
Previous Shroud DNA studies utilized the polymerase chain reaction (PCR) technique that works by repeated cycling of DNA replication to exponentially amplify DNA. Such methods allow even small gene fragments to be very rapidly and effectively analyzed: a billion DNA copies can be created from a single DNA strand in just three hours time.
Unlike past Shroud studies, which focused on genes that appear identical in all cell types, the suggested experimental approach uses PCR techniques to specifically target lymphocyte receptor genes. B and T lymphocytes are unique in that unlike any other cell type in the body, the DNA encoding their cell surface receptors undergo rearrangement and splicing: immunoglobulin, or antibody for B cells, and the T cell receptor for T cells. As a normal process of B and T cell development, receptor genes separated by large distances on the chromosome are brought close together and edited to create a final product. Although other cell types, including skin cells, contain such genes, they remain in the unrearranged (germline) configuration throughout their lifetime: DNA rearrangement is specific for lymphocytes. Detection of rearranged lymphocyte receptor genes is an established and routine laboratory
procedure, commonly used in the diagnosis of various leukemias and lymphomas. Multiple B cell and T cell receptor genes exist that could be suitable for DNA analysis. Moreover, as many of the genes are present on different chromosomes, this might increase the chances for detection if certain DNA regions are more fragmented than others. Samples taken from several sites on the Shroud, particularly those below the fibers on the surface, would yield the most definitive conclusion. Such an approach may help establish that DNA in the blood areas of the Shroud of Turin originated from white blood cells (lymphocytes) present on the cloth.