From Transrenal DNA to Stem Cell Differentiation: An Unexpected Twist

Abstract

In this issue of Clinical Chemistry, Hung et al. describe the results of a study originally devoted to investigation of the transrenal DNA (Tr-DNA) 1 phenomenon. These authors report on the presence of donorderived, cell-free DNA and cells in the urine of sexmismatched hematopoietic stem cell transplant (HSCT) recipients (1 ).The term Tr-DNA defines DNA molecules that appear in the urine from sources located outside of the urinary system. The investigators who found such molecules in urine for the first time suggested that DNA fragments from cells dying throughout the body appear in the bloodstream as so-called cell-free circulating DNA (cfcDNA) and then cross the kidney barrier into the urine (2 ). Otherwise, it is difficult to explain how fetal DNA appears in the urine of pregnant women or how tumor-specific DNA markers appear in the urine of patients with tumors located outside of the urinary system. This original observation was reproduced in many laboratories (2-7 ). At the same time, several groups could not detect fetal DNA in the urine of pregnant women (8,9 ), raising doubts about the concept of Tr-DNA. The latter results can be explained by the fact that Tr-DNA fragments are shorter than cfcDNA, and shorter amplicons should be used for their detection (4, 7, 10 ), especially in prenatal models in which concentrations of fetal DNA in maternal urine are low. It is also important to remember that urinary DNA from other sources complicates analysis of Tr-DNA. Other sources of urinary DNA include epithelial cells shed from the organs of the urinary system, often lymphocytes and other white blood cells, and cell-free DNA released into the urine after chromatin degradation in cells dying in the kidney and bladder. Bacterial infection is another potential source of urinary DNA; hence urine is widely used for diagnosing sexually transmitted diseases (11, 12 ).As a model for investigation of the Tr-DNA phenomenon, Hung and colleagues studied HSCT recipients, whose plasma contains a significant amount of donor-specific cfcDNA. The primary goal of the study was to find out if there is a correlation between concentrations and sizes of cfcDNA in the plasma and Tr-DNA in the urine. To simplify detection of donor DNA in plasma and urine, the authors selected sexmismatched HSCT recipients. Twenty-one of 22 patients had more than 99% of donor lymphohematopoietic cells in the peripheral blood. To differentiate male and female DNA, a zinc finger-protein gene assay was designed based on the nucleotide difference between the zinc finger-protein genes located on chromosomes X and Y. In addition, Y chromosome-specific SRY sequences were used for male donor/female recipient patients. The contribution of donor-derived cfcDNA in plasma was in the range of 72.8%-79.3%, supporting the earlier conclusion about the predominantly hematopoietic origin of cfcDNA in plasma (13 ). These data indicate that only about 25% of cfcDNA is delivered to the bloodstream by other tissues. However, one should keep in mind that in spite of all preca...