Heritability of susceptibility to ionizing radiation-induced apoptosis of human lymphocyte subpopulations

Abstract

Purpose: To evaluate the heritability of intrinsic radiosensitivity, the induction of apoptosis in lymphocyte subpopulations was determined on samples from related individuals belonging to large kindred families.

Methods and materials: Quiescent lymphocytes from 334 healthy individuals were gamma-irradiated in vitro. Apoptosis was determined 18 h after irradiation by eight-color flow cytometry. Radiosensitivity was quantified from dose-effect curves. Intrafamilial correlations and heritability were computed for 199 father-mother-offspring trios using the programs SOLAR (Sequential Oligogenic Linkage Analysis Routines) and SAGE (Statistical Analysis for Genetic Epidemiology). Segregation analyses were conducted using SAGE.

Results: Marked differential susceptibility of naive and memory T lymphocytes was demonstrated. Also, although age and gender were significant covariates, their effects only accounted for a minor part of the inter-individual variation. Parent-offspring and sib-sib correlations were significant for the radiosensitivity of B cells, T4, and T8 and of effector memory T4 and T8 subpopulations. In the T4-effector memory subpopulation, the phenotype showed correlations most consistent with dominant or additive genetic effects, and the results of the segregation analysis were consistent with the contribution of a bi-allelic dominant locus.

Conclusions: Heritability was demonstrated for the susceptibility to ionizing radiation-induced apoptosis of lymphocyte populations, and the segregation of the T4-effector memory radiosensitivity phenotype was consistent with a simple mendelian transmission model involving one major gene.

Figure 1

Analysis of 10-parameter list mode data for determination of ionizing radiation induced apoptosis (IRIA) in lymphocyte subpopulations. Panels show the progressive gating to obtain B cells (A, B, C and E), and CD4 and CD8 T lymphocytes (A, B, C, D). Panels F and G illustrate definition of naïve (Naïve), central memory (CM), effector memory (EM), and terminal effector (TE) T4- and T8 lymphocytes, respectively. Placement of the CD45RA gate was done separately for CD4 and CD8 T cells, because T4 lymphocytes express lower levels of CD45RA. Panels H and I indicate definition of apoptotic cells within T8- and T4 lymphocytes subpopulations, respectively, whereas Panel J indicates apoptotic B cells. Panel A: FSC vs SSC; panel B: AnnexinV vs HO33258; panel C: CD3 vs AnnexinV; panel D: CD4 vs CD8; panel E: CD45RA vs CD19; panels F and G: CD62L vs CD45RA; panel H, I and J: AnnexinV vs FSC.

Figure 2

Differential sensitivity to ionizing radiation induced apoptosis in T lymphocyte subpopulations.. Eightteen hours after in vitro irradiation (0–6 Gy of ¹³⁷Cs-γ) of quiescent PBL from 33 individuals, apoptosis was determined using 8-color flow cytometry. For the indicated T4- (A) and T8 subpopulations (B), the average surviving fractions and standard deviation (SD) are presented as function of dose. The average proportion of naïve T4- and T8 lymphocytes are presented in the insets. Symbols and lines represent: ●----●, naïve; □---□, central memory; ◆-----◆, effector memory; △---△, terminal effector subpopulations, respectively

Figure 3

Average of the phenotypes of offspring (mean-sib) versus average phenotype of the two parents (mid-parent) with symbols in grey shade levels according to sibship size (from 1 to 7) and the least square regression fit weighted by sibship size.