Determination of hprt mutant frequencies in T-lymphocytes from a healthy pediatric population: statistical comparison between newborn, children and adult mutant frequencies, cloning efficiency and age

Abstract

Somatic cell mutant frequencies at the hprt locus of the X-chromosome were measured with the T-lymphocyte cloning assay in a healthy pediatric population. Assays were performed on 49 subjects (29 males and 20 females) ranging in age from 0.08 to 15.2 years. A statistical analysis of the thioguanine-resistant (TG r) mutant frequency ( MF), unselected cloning efficiency ( CE) and age was performed using data obtained in this study and those previously obtained in our laboratory on 66 newborn umbilical cord blood samples and 230 adult blood samples. For statistical comparisons pediatric subjects were divided into 4 groups. Group I included cord blood samples (age 0 years); Group II were subjects between 0 and 5 years; Group III were between 6 and 11 years and Group IV were between 12 and 17 years. The ln MF of Groups I and II were significantly lower than Groups III and IV ( p < 0.05). The mean ln MF for each of Groups I–IV was significantly lower than the adult value. The cloning efficiency for Group I was significantly lower than that for Groups II–IV and adults. The relationships among the ln MF, unselected CE and age were expressed by the equations: ln ( MF) = 0.945 −2.453 CE ( p < 0.001) and ln ( MF) = 0.114 + 0.063 age ( p 0.004). The slope coefficients for unselected CE and age were significantly different from adults ( p < 0.05). Regression analysis of combined data from Groups I–IV and adults were performed using both age and unselected CE as well as terms to reflect differences in their relationships with ln MF in adults and children. The results showed that the intercept and the age coefficients differ significantly for children and adults after adjustment for CE and yielded the following equations: ln ( MF) = 0.548 −1.676 CE + 0.075 age, (Groups I–IV) and ln ( MF) = 2.263 −1.676 CE + 0.014 age (adults). An alternative statistical model using ln (age + 1), ln ( MF) = 0.381 −1.767 CE + 0.673 ln (age + 1), ( p < 0.001), describes the rapid increase in MF with age that levels off in late adolescence. These findings demonstrate the changing influence of age on mutant frequency in the pediatric population as compared to the adult populations. These studies also illustrate that the increase in background somatic mutant frequencies at the hprt locus in T-lymphocytes is not linear from birth to adolescence and is significantly different from that seen in the adult population. This model can be used to determine the possible effects of genotoxic exposures in vivo on MF in children within defined parameters such as age.