Spi-1/PU.1 and Fli-1 are both members of the ets family of
transcription factors and were identified as a consequence of their activation in Friend virus-induced erythroleukemia (6, 7, 26,
32). For both genes, our results show that the retrovirus is
integrated in the same region and in an orientation opposite to
transcriptional orientation of the gene as does the Friend MuLV (Fig.
2). The Fli-1 gene is activated in 72% of the non-T, non-b
lymphomas induced by Cas-Br-E MuLV in NIH/Swiss mice
(9). The 10A1 MuLV was also associated with Fli-1
activation in tumors similar to those induced by Cas-Br-E
(31). However, these integrations of Cas-Br-E and 10A1 are
all clustered in exon 1 within 35 nucleotides directly upstream of the
Fli-1 ATG start codon in the same transcriptional orientation as the gene (4, 9, 31). This could indicate the
necessity of a promoter insertion mechanism to activate
Fli-1 in this case due to a weak activating potential of
Cas-Br-E long terminal repeat enhancer (3).
Spi-1/PU.1 is found rearranged in Friend spleen
focus-forming virus-induced erythroleukemia. Friend spleen
focus-forming virus integrations are located 10 kbp upstream of the
first exon of Spi-1/PU.1 (26). In no
other cases were both Fli-1 and
Spi-1/PU.1 genes reported to be activated in
myeloid leukemias. These data confirm the involvement of those two
genes in myeloid leukemogenesis in addition to erythroid leukemogenesis and suggest their importance in normal regulation of myeloid
hematopoiesis. To further characterize the tumors with rearrangements
in Fli-1 and Spi-1/PU.1 genes, we
analyzed the expression of Fli-1, Spi-1/PU.1, and EpoR on Northern
blots performed on total RNA from tumors with rearranged genes. Results
depicted in Fig. 3 clearly demonstrate overexpression of the Fli-1 gene in the three tumors with
rearrangement in that locus compared to the levels from healthy spleen
or from a tumor with nonrearranged genes. In those three same tumors, a
high level of EpoR is also observed (Fig. 3). High levels of EpoR
expression were also observed in Cas-Br-E-induced non-T, non-B
lymphomas that harbored a proviral integration in Fli-1 (8). However, it is possible that the high level of EpoR
observed is correlated with the overexpression of Fli-1, suggesting
that Fli-1 might be involved in the regulation of EpoR expression. Hybridization with a myeloperoxidase cDNA probe did not reveal high
levels of transcription in the three tumors with rearrangements in
Fli-1 gene. A control tumor and the tumor with rearrangement in the Spi-1/PU.1 gene both demonstrate a high
level of expression of myeloperoxidase (Fig. 3), but analysis
of several granulocytic tumors revealed different levels of
myeloperoxidase expression (not shown). This nonuniformity of
expression could be linked to the different stages of differentiation
of the leukemic cells present in each tumor. Indeed,
myeloperoxidase mRNA is detectable by Northern blot analysis
solely in late myeloblastic and promyelocytic stages
(44). Histochemical examination of the three tumors with rearrangements in the Fli-1 gene clearly revealed their
granulocytic origin (not shown). The tumor with rearrangements
in Spi-1 shows a higher expression of the Spi-1
transcript, suggesting an activation by transcriptional enhancement
since the proviral integration is located 10 kbp upstream of the
first exon (Fig. 2). Although only one Graffi MuLV-induced tumor
had rearrangements in the Spi-1/PU.1 locus,
involvement of Spi-1/PU.1 in
granulocytic leukemia is not surprising, since this proto-oncogene
has been shown to take an important part in myeloid cell development
(5, 12, 25, 37). These data strongly suggest that proviral
activation of the Fli-1 and Spi-1 genes resulting
in their deregulated expression plays an important role in the
development of granulocytic leukemia.
We are grateful to Corinne Barat for helpful discussions and
critical review of the manuscript.
This work was supported in part by grant 007072 from the National
Cancer Institute of Canada. C.D. is a recipient of a Cancer Research
Society Inc. studentship.
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