Frequency of acute myeloid leukaemia-associated mouse chromosome 2 deletions in X-ray exposed immature haematopoietic progenitors and stem cells

Abstract

Exposure to ionising radiation can lead to an increased risk of cancer, particularly leukaemia. In radiation-induced acute myeloid leukaemia (rAML), a partial hemizygous deletion of mouse chromosome 2 is a common feature in several susceptible strains. The deletion is an early event detectable 24h after exposure in bone marrow cells using cytogenetic techniques. Expanding clones of bone marrow cells with chromosome 2 deletions can be detected less than a year after exposure to ionising radiation in around half of the irradiated mice. Ultimately, 15-25% of exposed animals develop AML. It is generally assumed that leukaemia originates in an early progenitor cell or haematopoietic stem cell, but it is unknown whether the original chromosome damage occurs at a similar frequency in committed progenitors and stem cells. In this study, we monitored the frequency of chromosome 2 deletions in immature bone marrow cells (Lin(-)) and haematopoietic stem cells/multipotent progenitor cells (LSK) by several techniques, fluorescent in situ hybridisation (FISH) and through use of a reporter gene model, flow cytometry and colony forming units in spleen (CFU-S) following ex vivo or in vivo exposure. We showed that partial chromosome 2 deletions are present in the LSK subpopulation, but cannot be detected in Lin(-) cells and CFU-S12 cells. Furthermore, we transplanted irradiated Lin(-) or LSK cells into host animals to determine whether specific irradiated cell populations acquire an increased proliferative advantage compared to unirradiated cells. Interestingly, the irradiated LSK subpopulation containing cells carrying chromosome 2 deletions does not appear to repopulate as well as the unirradiated population, suggesting that the chromosomal deletion does not provide an advantage for growth and in vivo repopulation, at least at early stages following occurrence.

Keywords: 7-AAD; 7-aminoactinomycin D; BAC-FISH; BMCs; CFU-S12; CMP; CSC; Chromosome deletion; GFP; HSC; IMDM; IR; Iscove's modified Dulbecco's medium; LSK; Lin(−); Lin(−) Sca-1(+) c-Kit(+); MEP; MMP; Mouse model; Myeloid leukaemia; Radiation; Sfpi1(GFP/+); Sfpi1(GFP/GFP); Sfpi1/PU.1; Stem cells; bacterial artificial chromosome-fluorescent in situ hybridisation; bone marrow cells; cancer stem cell; colony forming unit spleen on day 12; common myeloid progenitor; del2; green fluorescent protein; haematopoietic stem cell; heterozygous for the Sfpi1-GFP reporter gene; homozygous for the Sfpi1-GFP reporter gene; interstitial deletion of chromosome 2; ionising radiation; lineage negative immature bone marrow cells; multi-potent progenitor; myeloid-erythroid progenitor; rAML; radiation-induced acute myeloid leukaemia.