Normal and Abnormal Lymphocyte Growth Regulation 
duced that can transmit the cellular ABL gene at 
high efficiency to a wide variety of cell types. 
Using this system, we have been able to select for 
transformed clones that harbor new classes of ac- 
tivating mutations in ABL. 
One new class of mutants shows the loss of se- 
quences downstream of the kinase domain. These 
narturally selected deletion mutants and molecu- 
lar constructs created to mimic their structure are 
weak transforming alleles that require a comple- 
menting oncogene to induce fibroblast cells to 
grow in agar. Their most striking phenotypic 
characteristic is the very low activation of the ty- 
rosine kinase activity of ABL. This is very different 
than all previous transformation alleles of ABL. 
It has previously been shown that all forms of 
the ABL oncogene can synergize for their biologi- 
cal effects with members of the MYC gene family. 
We have recently tested the hypothesis that MYC 
is not only complementary to ABL transformation, 
but essential. Using a series of dominant negative 
mutants to inactivate the effective dosage of MYC 
in both fibroblast and hematopoietic cells, we 
have observed a dramatic decrease in the ability 
of BCR/ABL and viral forms of ABL to transform. 
The dominant negative mutants have no suppres- 
sive effect on other types of oncogenes, including 
MOS. Such essential combinations could be im- 
portant considerations in devising strategies for 
the treatment of certain cancers. 
Regulation of Stem and Progenitor 
Cell Growth 
In collaboration with Naomi Rosenberg of 
Tufts Medical School, we have developed a sys- 
tem for retroviral infection of murine bone 
marrow stem cells with the BCR/ABL oncogene 
and reimplantation into syngeneic hosts. This 
procedure leads to tumors with the characteris- 
tics of human chronic myelogenous leukemia. In- 
terestingly, animals infected with the PI 85 BCR/ 
ABL forms show more-aggressive tumors that 
invade nonhematopoietic organs and show 
shorter latency than animals infected with the 
P210 form. This system should be valuable for 
analyzing new therapies directed at the BCR/ABL 
oncogene and for defining the grov^h regulation 
of primitive hematopoietic stem cells. 
To elucidate the mechanisms that lead to dereg- 
ulated growth of stem cells, we have developed 
an in vitro system to monitor the effects of BCR/ 
ABL on multipotential stem cells. Marrow 
enriched for such cells was infected with P2 1 0 
BCR/ABL expressing retrovirus and plated in agar 
in very low concentrations of growth factors like 
Steel factor and interleukin-3 (IL-3). These low 
concentrations are incapable of stimulating col- 
ony growth but could synergize with BCR/ABL. 
A variety of colony types grew in this assay sys- 
tem, including multipotential cells capable of 
differentiating to form mast cells, macrophages, 
granulocytes, and pre-B cells. Cell lines of these 
different sublineages established from such mul- 
tipotent colonies express a high level of the 
BCR/ABL gene product but are only subtly trans- 
formed in their growth factor requirements and 
nonmalignant when transferred to syngeneic 
animals. 
Reliable test models for the growth of human 
myeloid leukemias are limited to a small subset of 
these leukemias that can be grown into continu- 
ous cell lines in vitro. We have recently estab- 
lished a reproducible procedure for the growth 
of freshly explanted human acute myelogenous 
leukemia and blast crisis specimens from chronic 
myelogenous leukemia patients. Cells from the 
bone marrow of such patients are implanted 
under the kidney capsule of the severe combined 
immunodeficient (SCID) mouse. At this site the 
human cells grow and then migrate to the bone 
marrow and peripheral blood in a pattern quite 
similar to that of the original human disease. 
One long-range goal of our group is to develop 
effective in vitro culture techniques for the prop- 
agation and enrichment of stem and progenitor 
cells for different lineages. Previously we used 
the growth stimulatory properties of the BCR/ 
ABL oncogene in concert with selected bone 
marrow-adherent stromal lines to grow clonal 
lines of B lymphoid progenitor cells that could 
repopulate the B cell lineage of immunodefec- 
tive (SCID) mice. By modifying the culture con- 
ditions, we have now been able to cultivate such 
progenitor cells without the need for co-stimula- 
tion by the oncogene. These populations are very 
effective in reconstituting the B cell lineage in 
vivo. 
We have used these progenitor cells to prepare 
several cDNA libraries in order to search for new 
members of the tyrosine kinase gene family and 
other regulatory genes that may be important in B 
cell development. A new member of the SRC fam- 
ily of tyrosine kinases has been identified that is 
specific to the B cell lineage. 
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