Molecular Genetics of Normal and Leukemic Hematopoiesis 
percent of independent leukemic clones isolated 
from the spleens of mice infected with Friend 
virus. These experiments suggested that />53 is a 
tumor-suppressor gene, a conclusion supported 
by recent experiments demonstrating allelic loss 
and mutation in the p53 gene in a broad spectrum 
and high proportion of human cancers. 
Tlie importance of p53 in the evolution of 
Friend leukemia also became evident from stud- 
ies on transgenic mice that express high levels of 
mutant forms of the p55 protein. In addition to 
displaying an increased spontaneous predisposi- 
tion to a variety of malignancies, these p53 trans- 
genic mice are more susceptible to the late stages 
of Friend leukemia. Thus inactivation of the p53 
gene, either somatically after Friend virus infec- 
tion or as the result of inheritance of dominant 
negative alleles of p53, appears to be a central 
event in the disease. These studies also suggested 
that it is the accumulation of a specific set of mu- 
tational events, rather than the order in which 
they normally occur, that is critical for leukemia 
induction. 
In addition to inactivation of the p53 gene, ac- 
tivation of one of two novel members of the ets 
gene family of DNA-binding transcriptional acti- 
vators occurs during the evolution of Friend leu- 
kemia. Transcription of the Spi-\ gene is acti- 
vated as a result of the integration of Friend 
spleen focus-forming virus (SFFV) in 95 percent 
of erythroleukemic clones induced in adult mice 
by SFFV. In contrast, we showed last year that 
transcription of another ets family member, Fli- 1 , 
is activated by insertion of the replication- 
competent Friend MuLV in 75 percent of erythro- 
leukemia clones induced after infection of new- 
born mice. The members of the ets gene family, 
which include c-ets- 1 and c-ets-2, elk- 1 and elf-1, 
erg, E74, 5p/-l/PU.l, and Fli-\, contain a con- 
served domain of 80-90 amino acids, the ETS do- 
main, which is involved in specific DNA binding, 
as well as a less-conserved transcriptional activa- 
tion domain. 
The strict specificity of the integration sites for 
SFFV and Friend MuLV is intriguing, as Spi-\ and 
Fli-\ are both members of the same gene family 
and both SFFV and Friend MuLV induce erythro- 
leukemias involving activation of the receptor for 
erythropoietin and inactivation of the p53 gene. 
We are currently pursuing the hypothesis that 
Fli- 1 and Spi- 1 are functionally distinct genes, en- 
coding proteins that transactivate a distinct set of 
genes downstream in the leukemogenic pathway. 
These studies involve the analysis of the DNA- 
binding specificity of the Fli-\ and Spi- \ proteins 
as well as the generation of mutant mice carrying 
either gain- or loss-of-function mutations in these 
genes. This work is supported by a grant from the 
National Cancer Institute of Canada. 
The ets gene family was first discovered by ana- 
lyzing the genome of the avian erythroblastosis 
virus E26, which contains a part of a myb fusion 
protein, the \-ets oncogene. Thus at least three 
members of the ets family, \-ets, Spi-1, and Fli-1, 
are involved in the leukemic transformation of 
erythroid cells. This specificity of ets genes for 
the erythroid lineage suggests that members of 
this gene family are involved in the regulation of 
one or more genes critical to erythropoiesis. We 
are interested in identifying these genes to 
gain further insights into both the regulation 
of normal hematopoiesis and the induction of 
leukemia. 
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