Heterozygosity leading to haploinsufficiency for proteins involved in DNA damage signaling and repair pathways plays a role in tumorigenesis. Animals haploinsufficient for a specified protein have as low tumor development rates as their wild-type counterparts in normal conditions, however, they show much higher tumor incidence rates after DNA is damaged by exogenous stimulations. Mice heterozygous for DNA repair genes have a similar life span to the wild-types when not challenged with mutagens like ionizing radiation, nevertheless, individuals of different genotypes respond differently to the same stress conditions. Predisposition to cancer of individuals heterozygous for a specified gene is age-dependent and results in tumor development not in early stage but in aged one. Population carrying a certain heterozygous gene is close to tumor incidence while that carrying homozygous mutated gene is extremely lower. Apparently heterozygosity when one allele of a gene is inactivated contributes to tumorigenesis. Since one human cell has 20,000-25,000 protein-coding genes, it is quite possible that an individual carries more than one heterozygous gene. We suppose that heterozygosity for two or more genes critical for pathways controlling DNA damage signaling, repair, cell cycle checkpoints, apoptosis, and so on, may enhance tumor initiation.
To verify this hypothesis, cells heterozygous for ATM, BRCA1, and both, were taken as experimental models since the both genes are key factors involved in DNA damage signaling and repair and related with each other. In particular, clinical statistics revealed that breast cancer in AT family is 1.5-14 times higher. In order to minimize the difference in genetic background except for the interested genes, mouse and mouse embryonic fibroblast (MEF) cells were isolated from littermates.
The transformation of MEF cells exposed to 0.5Gy of 1GeV/n Iron was scored by morphological recognition. Transformation frequency of ATM/BRCA1 double heterozygotes was 7.2-, 3.5- and 2.0-fold higher than that of wild-type cells, ATM single heterozygotes and BRCA1 single heterozyogotes, respectively.
Thymocytes are sensitive to radiation and they die in an apoptotic way. The main sub-population of thymocytes (CD4
+CD8
+ cells) was measured by using flow cytometer. The amount of wild-type cells, ATM single heterozygotes, and BRCA1 single heterozygotes decreased 86.5-, 44.0-, and 38.1- times, respectively, in 24 hours after exposed to 5Gy of γ-rays while that of ATM/BRCA1 double heterozygotes decreased only 9.8- times.
γH2AX focus forming reflects DNA double strand breaks. Foci in wild-type cells exposed to 0.5Gy of γ-rays reached to the maximum as rapidly as 15min but it took ATM/BRCA1 double heterozygous cells 30min, which verified that double haploinsufficiency for ATM and BRCA1 had negative impact on DNA damage signaling.
Four kinds of cells showed various kinetics of cell cycle progression although all of them were arrested in G2/M phase after exposed to the same dose of γ-rays. G2/M block was prolonged in ATM single heterozygotes but abrogated in BRCA1 single heterozygotes. Double heterozygotes for ATM/BRCA1 showed a compromised cell-cycle dynamics and the curve was similar to that of wild-type cells.
In summary, haploinsufficiency resulted from heterozygosity for ATM and/or BRCA1 has negative impact on various pathways. It increases cell susceptibility to exogenous stress like ionizing radiation in transformation induction by disturbing cell cycle checkpoint, delaying DNA repair, and suppressing apoptosis. Consequently unrepaired lesions are transmitted to daughter cells, which leads to cell transformation and tumor initiation. Double heterozygosity multiplies the probability of tumor formation.
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