Genetic Analysis of Susceptibility to Radiation-induced Apoptosis of Thymocytes in Mice

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Abstract
Genetic control of thymocyte susceptibility to radiation-induced apoptosis was investigated in BALB/cHeA, STS/A and five other strains of mice by counting pyknotic cells in a selected area of thymic cortex on histological specimens after whole-body X-irradiation. Number of dead cells increased almost linearly with doses (range 0·25–0·75 Gy) in BALB/cHeA and STS/A mice. However, dead cell counts in BALB/cHeA mice were more than twice those in STS/A mice at each dose. Of five other strains of mice, C57BL/6N and B10.BR mice exhibited a sensitive phenotype similar to BALB/cHeA mice, while C3H/HeAMsNrs and NFS/N mice showed a resistant phenotype similar to STS/A mice. A/J mice seemed to be rather resistant. A sex difference was not recognized in BALB/cHeA and STS/A mice. Resistance was dominant over susceptibility in the progenies of reciprocal crosses between the two strains, indicating an autosomal inheritance and no maternal effect. Segregation ratio of susceptible phenotype to resistant one in the backcrosses of female (BALB/cHeA × STS/A)F1 mice with male BALB/cHeA mice was not significantly different from 1:1 and all backcrosses of female (BALB/cHeA × STS/A)F1 mice with male STS/A mice exhibited a resistant phenotype. Results suggested that thymocyte susceptibility to radiation-induced apotosis is controlled by one major autosomal allele.